Recent PhDs

OB associations are gravitationally unbound, low-density groups of young stars delineated by bright OB stars. At the intersection of star formation and Galactic structure, they are important across many areas of astronomy. However, most historical OB associations had their membership defined several decades ago. Recent data shows a lack of kinematic coherence amongst them, alongside many contaminants, which encourages us to revisit them.

I have developed a code that fits a model spectral energy distribution (SED), based on stellar atmosphere and evolutionary models and an extinction map, to an observed SED, based on photometry from several surveys and Gaia parallaxes. This process derives stellar parameters that are used to identify OB stars. I used this to identify OB stars across Cygnus, Auriga and Cassiopeia, identifying thousands of OB stars. I applied clustering algorithms to identify kinematically-coherent OB associations, which I characterized physically and kinematically.

In Cygnus, the OB associations form two groups exhibiting a large-scale expansion pattern. A kinematic traceback revealed that they reached their most compact state ~8 Myr ago. This expansion could either be attributed to feedback or turbulence within their primordial molecular cloud. In Auriga, the OB associations are connected to the surrounding open clusters and star-forming regions. They follow an age gradient, up to ~20 Myr ago, coinciding with the motion of the Perseus spiral arm. In Cassiopeia, the kinematics of OB stars suggest an interesting large-scale pattern similar to that seen in Cygnus, though further investigating suggests it may be due to Galactic rotation. Despite this, the OB stars in Cassiopeia can be traced back to a compact configuration ~17 Myr ago.

This thesis highlights the importance of identifying OB associations with recent data and modern techniques. Not only are they tracers of early stellar evolution, helping to grasp the physical condition driving the expansion of stellar groups, but they can also be used to reconstruct the motion of the spiral arms in the recent past."

Star formation occurs within dense cores within molecular clouds, often associated with filamentary structures. However, there exist isolated instances of star formation, far from nearby forming stars. As a core collapses, a rotationally supported circumstellar disk emerges around a central, gravitating potential with the accretion of gas and dust playing a vital role in regulating the subsequent stellar mass assembly. With recent studies revealing nearly half of all solar-type star systems are multiples, this raises questions about the mechanisms behind their formation. Furthermore, despite numerous discoveries of exoplanets with state-of-the-art space telescopes, the initial stages of planetary formation remain elusive. High-resolution interferometric imaging using ALMA of protoplanetary disks has ubiquitously unveiled intricate substructures, hinting at ongoing planet formation processes. To understand the formation and evolution of stars and their planetary systems, it is essential to better characterize their progenitors, known as "protostars”, particularly the youngest known phases of protostars, so-called Class 0. During the early stages of star formation, when gravitational collapse initiates, the conservation of angular momentum leads to the formation of a rotationally supported disk. However, only a handful of Class 0 protostellar disks, which are highly embedded in gas and dust, have been rigorously detailed so far. Consequently, a more comprehensive analysis of Class 0/I systems is imperative for understanding their formation and evolution. This dissertation aims to address multiple outstanding questions in star formation, beginning with a detailed investigation of an extraordinary triple-source protostellar Class 0 system, L1448 IRS3B. Expanding the focus, BHR7, an isolated Class 0 source, is studied to map the transfer of angular momentum from 1000s of au down to the disk. BHR7 serves as an ideal testbed for non-ideal MHD theory and represents a prototypical isolated Class 0 source, free from contamination by nearby forming stars. Furthermore, a high-resolution survey is conducted in the Perseus region, encompassing 12 known multiple star systems, to determine the most probable formation pathways for each of these sources. Rigorous modeling techniques and statistical tests are employed to disentangle the formation pathways of the protostars. By undertaking these investigations, I aim to enhance our understanding of star formation processes, provide observational constraints on star formation theory, and shed light on the complex formation mechanisms underlying multiple star systems, which are thought to be the early stages of exoplanet progenitors.

Most stars in galaxies are known to form star clusters. Although star clusters of various masses have been observed to date, young massive clusters (YMC) are very important objects in terms of their impact on the surrounding interstellar medium in the form of supernova explosions, stellar winds, and ultraviolet radiation. However, the formation mechanism of YMC is still a mystery. YMC are characterized by high stellar densities, and their formation requires the concentration of massive molecular gas, the raw material for stars, in compact regions. How such a region is created has been completely unknown, but recent observations have observationally suggested that the YMC may be formed by fast collisions of HI gas. In this study, we examine this scenario using MHD  simulations to investigate the origin of the YMC.

As basic equations, we consider magnetohydrodynamic equations that take into account the effects of gas self-gravity, heating and cooling by radiation, and interstellar magnetic fields, as well as the chemical evolution from HI gas to molecular gas. We also introduce the photoionization feedback effect due to radiation when massive stars are formed in the computational domain. The required simulations must be performed on the scale of about 100 pc, which is the scale of gas collisions suggested by observations, and must be resolved to the scale of a star cluster (about 1 pc), which requires massive parallel computations using a supercomputer. Conventional self-gravity solvers are limited to about 10,000 cores due to the nature of their calculation scheme, but in this study, we developed and used a gravity solver with better parallelization efficiency to perform a larger-scale cluster formation simulation. The simulation results show that a massive and compact gas clump, which can be a precursor of YMC, can be formed by the global gravitational collapse of molecular clouds formed in the shock wave compression layer produced by the collision. 

We found that the formed sufficiently compact massive gas clumps have a large escape velocity compared to the sound speed of the HII region, which means that gravity prevents gas evaporation and thus they form stars with a high star formation efficiency and evolve into a YMC.  We also examined the initial condition dependence of YMC formation by fast HI gas collision. Our simulation results showed that the mass of the formed gas clump does not change significantly with the metallicity and that the mass of the formed gas clump becomes larger as the initial density of the colliding gas increases.

Recent observations of molecular clouds show that dense filaments are the sites of present-day star formation. Thus, it is necessary to understand the filament formation process because these filaments provide the initial condition for star for- mation. Theoretical research suggests that shock waves in molecular clouds trigger filament formation. Since several different mechanisms have been proposed for filament formation, the formation mechanism of the observed star-forming filaments requires clarification. In this study, I perform a series of isothermal magnetohy- drodynamics (MHD) simulations of filament formation and identify mechanisms for filament formation. I find that the dominant filament formation mode changes with the velocity of the shock wave triggering the filament formation. Moreover, I show that strong shock waves can naturally create high-line-mass filaments such as those observed in massive star-forming regions in a short time.

Observations show that a massive star cluster formation occurs where the peak of gas column density in a cloud exceeds 10^23 cm−2. I investigate how the initial conditions of massive star formation are realized by performing MHD simulations with gas inflow duration from the boundaries as a controlling parameter. Filaments expand after the duration time for short-duration models, whereas long-duration models lead to star formation by forming massive supercritical filaments. Moreover, when the shock duration is longer than two postshock free-fall times, the peak column density of the compressed layer exceeds 1023 cm−2, and the gravitational collapse of the layer leads that the number of OB stars expected to be formed reaches the order of ten (i.e., massive cluster formation).

The filament width is an important quantity because it determines the fragmen- tation scale by self-gravity. Observations show that the width takes the universal value of 0.1 pc. However, theoretically, the width of the supercritical filaments should contract by self-gravity. Recent studies suggest that massive filaments are bound by the slow shocks that are caused by accretion flows onto the filaments. As the wavefront of such slow shock is known to be unstable (slow shock instability: SSI), the accretion ram pressure can be expected to convert into thermal/turbulent pressure across the shock front that potentially maintains the width. In the scale of dense filaments, ambipolar diffusion (AD) suppresses the SSI at small scales. I investigate the influence of AD on SSI using two-dimensional (2D) MHD simulations. The results demonstrate that the most unstable scale of SSI is approximately five times the length scale of AD calculated using post-shock variables. The filament would be sandwiched between two shocks. I investigate the linear growth of two adjacent slow shocks. The results show that the odd mode is unstable, whereas the even mode is stable. Furthermore, I find that turbulence is driven as a result of the nonlinear evolution of the SSI with AD. I also performed a 3D MHD sim- ulation with self-gravity. For a massive filament of ∼ 70 M⊙ pc−1, the profile is consistent with observations and has a width of 0.06 pc. I propose that the width is maintained by turbulent pressure driven by SSI including AD.

(Abridged)

In my thesis work, I studied the impact of the initial conditions on star- and disk-formation processes in different environments, using two young star-forming regions as benchmarks: The high-mass star-forming region IC1396, a H II region, and the Coronet cluster, a star-forming region without massive stars but with high stellar density. Both regions are well-known but lacked a global and cohesive study, so my work is centered on a systematic study of both clusters and their stellar population. I combined several techniques and observational data to characterize the structure of both regions, studying the kinematics, cluster structure, and properties of the cluster members. I also explored the effects of feedback and stellar density to derive the star formation history of both regions. I used optical and NIR imaging together with Herschel and Spitzer maps, optical spectra, millimeter data, continuum and molecular line observations, and Gaia data. The star formation history of the IC1396 region is complex. Its structure revealed four subclusters or populations with different proper motions, which also differ to some extent in age. Their members are not uniformly spatially distributed over the region. I also found that the region appears to be expanding. The combination of kinematics, age, and evolutionary state differences between groups of sources within the same cloud points out that the region has suffered multiple episodes of star formation over time, likely resulting from various mechanisms. The star formation history of the Coronet cluster is not as dramatic, although our study is restricted to the small cluster in the R CrA region. I found that even at this scale, the feedback from the low-mass stars appears to affect the cloud and the initial conditions of further star formation. The impact of the feedback by nearby intermediate-mass stars (2-3 M☉) prevents the collapse of some regions with high-density peaks, while further star formation is found at larger distances. The kinematics of the members revealed that there are no significant differences in the proper motions and radial velocity of the objects, so the entire cluster is part of the same filament. Gas kinematics also revealed that the gas is not gravitationally bound in the central part of the Coronet. This thesis demonstrates the power of using multiwavelength and multi-tracer data, including kinematics, to characterize the properties of young stellar clusters as a whole, to identify their members at different depths, and to complete the picture of star formation history in regions with different properties. 

Publication: https://ui.adsabs.harvard.edu/abs/2023A%26A...669A..22P/abstract

This thesis presents the development and implementation of a machine learning classification of massive Young Stellar Objects (YSOs) in two Local Group galaxies, NGC6822 and M33. Using archival near- and far-IR data, point sources in both galaxies are classified into multiple stellar classes using a Probabilistic Random Forest classifier (PRF) trained on objects of known types. The spatial distributions of all classes are discussed. YSOs are classified with a high level of confidence (up to 97 percent) in both galaxies. In NGC6822, 125 YSOs are confirmed and 199 are newly identified. All major star forming regions (SFRs) in NGC6822 are recovered and, additionally smaller SFRs are newly identified. In M33 4985 YSOs were identified across the disk of M33 and, applying a density-based clustering analysis 68 SFRs were identified primarily in the galaxy’s spiral arms. SFRs associated with known Hii regions were recovered, with ∼91 percent of SFRs spatially coincident with giant molecular clouds identified in the literature. Using photometric measurements, as well as SFRs in NGC6822 with an established evolutionary sequence as a benchmark, I employed a novel approach combining, into one metric, ratios of [Hα]/[24µm] and [250µm]/[500µm] to estimate the relative evolutionary status of all M33 SFRs. By comparing the YSOs identified in M33 with model grids for mass determination, a star formation rate is estimated for the first time from direct YSO counts; (1.42 ± 0.16M⊙yr−1) that is lower than that of the more massive Milky Way as expected. This project for the first time identifies massive YSOs on galactic scales in a Local Group spiral galaxy, extending such analysis beyond the nearby star-forming dwarf galaxies (LMC, SMC and NGC6822). The techniques developed offer an invaluable tool for classifying large data sets.

(Abridged)

V582 Aur is an FU Orionis-type object (FUor), which is currently in an outbursting state but shows photometric variations due to an orbiting dust clump. I studied the optical–infrared photometric variability of the system in order to examine its evolution. The new data presented in this thesis imply that changing extinction is the dominant physical mechanism behind the light variations, and suggest the viscous spreading of the dust particles along the orbit. The long-term measurements, as well as an accretion disk modeling hint at a general fading of V582 Aur, suggesting that the source will reach the quiescent level in ∼80 yr.  For a sample of objects in the Chamaeleon I star forming region, we arranged ground-based optical and near-infrared photometric, and high-resolution spectroscopic (VLT/ESPRESSO, 2.2/FEROS) measurements contemporaneously with the TESS space telescope observations in order to examine the accretion process, analyze the effects of hot/cold spots, and identify the signs of line-of-sight obscuration by circumstellar matter. For one particular object, the CR Cha single T Tauri star, I was able to study the accretion process on timescales from hours to a decade by including earlier data from the AAT/UCLES and the HARPS instruments. These show that the accretion variations increase on timescales from hours to days/weeks, after which it saturates, and the overall accretion variability is within the factor of ∼3 on timescales of a decade. The near-infrared JHK-band light curves reveal an interesting pattern in the color-magnitude diagrams: they show that CR Cha becomes redder as it brightens. This unusual pattern may be explained by a disk model, which states that this behaviour may be caused by the changing accretion rate, or by changes in the inner disk structure. For an other object, the VW Cha multiple system, four high-resolution spectra were taken besides the TESS and the ground-based photometry. Spectra were obtained in both fainter and brighter photometric states of the system, allowing me to examine the origin of a photometric brightening event. The results show that this brightening event can be explained by increased accretion. In addition, the new spectroscopic data suggest that the primary component of VW Cha is a spectroscopic binary with a speculated orbital period around 10 days.

Thesis: https://sites.google.com/view/gabriellazsidi/research/phd-thesis 

A large scale magnetic field permeates the interstellar medium (ISM) of our Galaxy. The ISM magnetic field strength is notoriously difficult to measure, and for this reason there is a long-standing debate about its dynamical importance in the star formation process. The magnetic field strength can be directly measured, with the Zeeman effect, only in a limited number of cases. For this reason, indirect methods have been developed for estimating the magnetic field strength. The most widely accessible methods are based on dust polarization. Dust polarization probes directly the plane-of-the-sky magnetic field morphology, but not its strength. These indirect magnetic field strength estimation methods are based on the energy balance of incompressible turbulence. Observations, however, indicate that turbulence in the ISM is highly compressible, hence the assumptions of the existing methods are incompatible with the observations. In this thesis, we propose a novel method for estimating the magnetic field strength from dust polarization based on the energetics of compressible turbulence. We assess the accuracy of the proposed method with synthetic data produced from a suite of numerical simulations. We find that with the proposed method an accuracy better than a factor of two can be achieved.

My dissertation focuses on the effect of magnetic fields on disk and core evolution during star-formation. We investigate the fragmentation scales of gravitational instability of a rotationally-supported self-gravitating protostellar disk using linear perturbation analysis in the presence of two nonideal magnetohydrodynamic (MHD) effects: Ohmic dissipation and ambipolar diffusion. Our results show that molecular clouds exhibit a preferred lengthscale for collapse that depends on mass-to-flux ratio, magnetic diffusivities, and the Toomre-Q parameter. In addition, the influence of the magnetic field on the preferred mass for collapse leads to a modified threshold for the fragmentation mass, as opposed to a Jeans mass, that might lead to giant planet formation in the early embedded phase. Furthermore, we apply the nonideal MHD threshold for fragmentation scales to fit the data of prestellar core lifetimes and as well as the number of enclosed cores formed in a clump, as found with the observations of Herschel and Submillimeter Array (SMA), respectively. Our results show that the trend found in the observed lifetime and fragmentation mass cannot be explained in a purely hydrodynamic scenario. Our best-fit model exhibits B ~ n0.43, which signifies a means to indirectly infer the effect of the ambipolar diffusion on mildly supercritical dense regions of molecular clouds. We also develop a semi-analytic formalism of episodic mass accretion (therefore episodic luminosity) from a disk to star, which provides a good match to the observed luminosity distribution of protostars. In contrast, neither a constant nor a time-dependent but smoothly varying mass accretion rate is able to do so. Our analytic work provides insight into global MHD simulations of protoplanetary disks that we carry out using the FEOSAD numerical code. Our numerical results demonstrate the long-term evolution of disks, including the formation and evolution of clumps, and especially the episodic nature of accretion, which might explain the origin of observed knots in the molecular jet outflows. 

Thesis:  https://ir.lib.uwo.ca/etd/8728/

The knowledge of interstellar dust grain properties is crucial to understanding numerous physical processes in galaxies. Mid-infrared (MIR) spectra contain most of the solid-state resonance features of interstellar grains, including the unidentified infrared bands (UIBs) ranging from 3.3 to 17 microns. They arise from a statistical mixing of polycyclic aromatic hydrocarbons (PAHs) of different sizes and structures. 

The spectral features of galaxies in the MIR have been widely observed by the Spitzer (IRS; 5.2 - 38 microns) and the AKARI (IRC; 2.5 - 5 microns) satellites. Carefully combining the spectra from both instruments and inter-calibrating the different modules have allowed us to analyze all the MIR features jointly. For that purpose, I have developed a new spectral decomposition software, MILES, applying the hierarchical Bayesian (HB) inference to model heterogeneous samples of nearby galaxies and galactic regions simultaneously. The HB approach addresses the numerous degeneracies of the model and extracts the maximum information from the data, accounting for various sources of uncertainties without over-interpreting the observations. In particular, I have performed a case study of M82 to analyze the available archived data consistently and interpret the spatial variations of MIR features with the physical conditions of interstellar PAHs (ionization, size distribution, dehydrogenation, etc.). This work is apt for the JWST observations, which have a spectroscopic capability (NIRSpec: 0.6 - 5 microns; MIRI: 5 - 27 microns) covering the entire spectral range of the UIBs, along with unprecedented angular resolution and sensitivity.

In this thesis, I use the adaptive mesh code arepo to model the dense Interstellar Medium and star formation in low metallicity dwarf galaxies with the aim of better understanding how these extreme galactic environments affect star formation. In chapter 3, the first paper, we model a suite of four isolated hydrodynamical dwarf galaxies with low metallicity and UV-field strengths in order to better understand their evolution and star formation. Our resolution gives sub-parsec cell radii at number densities of 104 cm-3, allowing us to resolve star-forming gas clouds. The models include a time-dependent, non-equilibrium chemical network that tracks more individual species directly during the simulation than previous models by our group, a total of 9, with 8 more arising from post-processing. The chemical network includes the effects of self-shielding and an ambient UV-field. We include self-gravity and directly model the gravitational collapse of gas into star-forming clumps and cores, and their subsequent accretion using sink particles. The metallicity and UV-field are independently varied between 1% and 10% of solar neighbourhood values. We find that reducing the metallicity and UV-field by a factor of 10 has no effect on star formation, with minimal effect on the cold, dense star-forming gas within the galaxies. Secondly, in chapter 4, we investigate the difference between the fiducial 10% metallicity and 10% UV-field strength hydrodynamical model and the same model but including a primordial magnetic seed field of 10-12 G. We found that an increase in star formation had arisen and through thorough investigation found an error in the code which was still under investigation at the time of writing. Finally, in chapter 5, we re-model the 10% and 1% metallicity hydrodynamical models at a higher resolution giving sub-parsec cell radii at number densities of 100 cm-3. In these extremely high-resolution models, we include the effects of photoionisation feedback from massive stars and perform radiative transfer calculations to produce synthetic observations of the velocity-integrated intensity, WCO, of various dense CO regions. Analysis of the conversion factor, XCO, betweenWCO and H2 number density for the regions finds an average XCO of 2 x 1021 cm-2 (K km s-1)-1 at both metallicities. This shows that XCO is not related to metallicity at low Z. A factor of 10 reduction in metallicity between the two models does not produce a factor of 10 increase in XCO as some literature has predicted.

The angular momentum of a molecular cloud core plays a key role in star formation, since it is directly related to the outflow and the jet emanating from the new-born star and it eventually results in the formation of the protoplanetary disk. However, the origin of the core rotation and its time evolution are not well understood. Recent observations reveal that molecular clouds exhibit a ubiquity of filamentary structures and that star forming cores are associated with the densest filaments. Since these results suggest that dense cores form primarily in filaments, the mechanism of core formation from filament fragmentation should explain the distribution of the angular momentum of these cores. In this thesis, I investigate the origin and evolution of the angular momentum of molecular cloud cores formed through filament fragmentation process. 

First, I semi-analytically derive the relation between the turbulent velocity field and resultant core angular momentum in filamentary molecular clouds. I  show that the sub sonic (or transonic) Kolmogorov  turbulent velocity field model can reproduce the observed property of the angular momentum of cores. I conclude that the origin of the angular momentum of cores is sub sonic (or transonic) Kolmogorov  turbulent velocity field. Recent theoretical research shows that the filamentary structures are formed when the molecular cloud is swept by the shock wave. I also analyse the results of filament formation simulations and show that the the line mass and velocity power spectrum along the filaments follow Kolmogorov turbulence. Therefore, I conclude that filament formation scenario can explain the origin of the angular momentum of cores.

Finally, I perform the three-dimensional simulations to investigate the time evolution of angular momentum of molecular cloud core formed through filament fragmentation process. As a result, I find that the angular momenta of cores change only by 30% in their formation process. I also find that most of the cores rotate perpendicular to the filament axis. In addition, I analyze the internal angular momentum structure of cores. Although the cores gain various angular momentum from the initial turbulent velocity fluctuations in the filament, the angular momentum profile in a core converges to the self-similar solution. I also show that the degree of complexity of the angular momentum structure in a core decreases over time. Moreover, I perform synthetic observation and show that the angular momentum profile measured from the mean velocity map is compatible with the observations. The present result provides a convenient test for the theory of core formation in the filament with the observed velocity field in the filaments and angular momentum structures in the cores. 

Young stellar objects (YSOs) having ages typically t=10^5~10^6 yrs are named as Pre-Main Sequence (PMS) stars. The members of this group are located above the Main Sequence in Hertzsprung-Russell (HR) diagram. Depending on the mass of the stars, PMS stars are divided into different classes, which exhibit different types of variability. My thesis was devoted to studying eruptive variables, i.e. FUor, EXor, UXor, and "intermediate" type objects.

Despite all the great progress made in the field of eruptive variables, there are still many open questions. Even the origin of the outbursts is a matter of debate. While the reason for the UXor type variability is more or less clear, the mechanism responsible for the FUor and EXor type outbursts is still a controversial issue. It is uncertain whether the same mechanism is responsible for both types of outbursts or whether they occur differently. Maybe both classes present the same phenomena in different evolutionary stages. Another problem is the undoubted classification of the objects as FUor, EXor, or UXor. Many recently discovered examples of eruptive stars somehow filled the gap between long-term (FUor) and short-term (EXor) outburst manifestations. These newly discovered objects display a kind of intermediate timescale of outburst decay and other characteristics in comparison to classical FUors and EXors. It is not excluded that the same object at the same time can represent both, for example, FUor and UXor type variability.

The proper investigation of these kinds of variables is very important. These stars are among the most intriguing PMS objects since the identification of their origin could potentially lead to a consistent picture of the early phases of stellar evolution. They could be used in studies of the evolutionary processes of circumstellar disks.

For the further discoveries of probable new types of eruptive YSOs and PMS objects, firstly, we need a comprehensive, long-term, multi-wavelength monitoring of eruptive variables, including photometric and spectroscopic investigations. Nowadays, many space-based telescopes are operating, carrying out surveys in different wavelengths. Indeed, large-field monitoring facilities enlarge our arsenal of observational data, but, despite that, we are reliant that the ground-based telescopes still have great potential. In this case, the observational material of the individual objects is extremely important, particularly if observations are systematic and include photometry and spectroscopy.

Consequently, in 2015 our group started a new observational program on the 2.6m. and 1m. telescopes of Byurakan Astrophysical Observatory (BAO). Several spectra were obtained with the 6m. telescope of the Special Astrophysical Observatory. The program was devoted to optical investigation of young eruptive objects. The aim was to re-observe selected young stars, which traditionally show odd and peculiar variations, thus obtaining new observational material on them. Also, we intended to find new possible candidates for different classes of eruptive variables. This thesis encompasses the systematic monitoring (both spectroscopy and photometry) of five stars V1318 Cyg, V1686 Cyg, V565 Mon, PV Cep, V350 Cep from September 2015 to December 2020.

In recent years, thousands of exoplanets have been discovered around a variety of stellar hosts. The disks of gas and dust surrounding young stars are the location and source of material for planet formation. The properties of these protoplanetary disks, therefore directly affect the planetary systems that may form. However, the details of the planet formation process are still unclear. In this dissertation, I constrain planet formation mechanisms by measuring the properties of protoplanetary disks, focusing on mass, dust grain growth, and dust settling. I use physically-motivated models and an Artificial Neural Network along with a Markov Chain Monte Carlo (MCMC) fitting procedure to obtain these and other disk properties. This dissertation compiles the largest sample to date of consistently-modeled protoplanetary disks, probing how disk properties vary with host mass and age.

The occurrence of planetary companions increases as stellar mass decreases. Thus, brown dwarfs (BDs), with smaller masses than pre-main-sequence stars, may commonly host planets. Studying properties of BD disks and comparing them to pre- main-sequence star disks is therefore important for constraining their planet-forming potential. I present spectral energy distribution (SED) models for BD and pre-main- sequence star disks in four star-forming regions. The SEDs consist of archival photometry data spanning optical to millimeter wavelengths. I model the BD disk SEDs using physically-motivated radiative transfer code; pre-main-sequence star SEDs are modeled using a newly-developed MCMC fitting procedure that allows for a more complete analysis of the disk properties. I compare disk masses and dust settling in these two disk categories to gauge how host mass affects these properties.

Typical disk lifetimes are a few tens of millions of years; planet formation likely occurs within the first few million years or less. Comparing how disk properties vary between star-forming regions of different ages can help pinpoint the timeline for planet formation. I present SED models for BDs in four star-forming regions and pre-main-sequence stars in eleven star-forming regions. I obtain the disk masses, dust grain sizes, and amount of dust settling in the disks and discuss the differences and similarities of these properties across regions of varying age.

How molecular clouds fragment and create the dense structures which go on to eventually form stars is an open question. In my thesis, I numerically investigate various aspects of fragmentation and structure formation in young molecular clouds based on the SILCC-Zoom and SILCC deep-zoom simulations. The SILCC-Zoom simulations follow the self-consistent formation of molecular clouds in a few hundred parsec sized region of a stratified galactic disc, which include (self-) gravity, magnetic fields, supernova driven turbulence, as well as a non-equilibrium chemical network and treatment of the interstellar radiation field, with resolutions of ∼ 0.1 parsec. The SILCC deep-zoom simulations are an extension of the cloud scale SILCC-Zoom simulations and allow us to resolve structures with a maximum resolution of 0.0078 parsec ( ∼ 1600 AU). 

In my work, I identify 3D volumes inside the simulated clouds as structures using dendrograms and analyze their behaviour. By considering the energetic balance of cloud scale sub-structures, I find that our molecular clouds are dominated by the interplay of turbulence and self-gravity - with self-gravity becoming dynamically dominant only over time. This supports the gravo-turbulent scenario of structure formation. By tracing the morphology of cloud scale structures, I evaluate the clouds to be sheet-like on larger scales, likely tracing the shells of bubbles driven by supernovae. I estimate the effect of magnetic fields in molecular clouds and their atomic envelopes and find that magnetic fields alter the nature of fragmentation at low densities, slow down the formation of denser structures, but do not seem to be dynamically important in the further evolution of these potentially star forming sub-structures.

I extend the study of energetics and morphology to sub-pc scale structures using the novel SILCC deep-zoom simulations. I find different methods of forming filaments - fragmentation of mostly self-gravitating structures, as well as shock compression. Moreover, I find that in both pathways gravitationally bound, spheroidal cores emerge at ∼ 0.1 parsec scales and are embedded inside gravitationally dominated filaments.

Methanol masers at 6.7 GHz are the brightest of class II methanol masers and have been found almost exclusively towards massive star forming regions. These masers can thus be used as an ideal tool to probe the early phases of massive star formation. The primary goal of my thesis was to investigate the evolutionary stage of the young stellar objects that excite 6.7 GHz methanol masers. Even though there have been several studies in this regard, they were either limited by small sample size or lack of data in the far-infrared. This work has made use of the entire sample from the Methanol Multibeam Survey (MMB) -- the largest unbiased Galactic plane survey for 6.7 GHz methanol masers, FIR data from the Herschel Infrared Galactic plane survey (Hi-GAL) and millimetre wave spectroscopic data from the MALT90 survey. We investigated the evolutionary states of 6.7 GHz maser hosts from two perspectives: (1) studying the physical properties of the methanol maser sources (2) probing the chemical environments of maser hosts. For the first case, we obtained the spectral energy distributions (SEDs) from 870 to 70 μm for 320 6.7 GHz methanol maser sources, and used the best-fit parameters of the SED fits to derive the maser clump properties. A comparison of the mass–luminosity diagram of the sample with evolutionary tracks from the turbulent core model suggests that most methanol masers are associated with massive young stellar objects, with over 90 percent in early evolutionary stages where they are accreting matter. However, there also appears to be a small population of sources that are likely to be associated with intermediate- or low-mass stars, suggesting that the association between high-mass star formation and methanol maser emission is not exclusive.  

We also studied the chemical properties of the sources associated with the masers using the molecular line observations from the MALT90 survey. This study was carried out for a sample of 68 out of the 320 methanol masers of the first study, with the selection based on data availability and the signal-to-noise ratio of the molecular lines. We used the line intensities and abundances of four molecular transitions: N2H+(1-0), HCN (1-0), HNC (1-0) and HCO+(1-0) since they are bright and are good tracers of dense gas. The molecular spectra were modelled using radiative transfer under the assumption of local thermodynamic equilibrium (LTE). The excitation temperatures and column densities were compared to models that solve for time dependent astrochemistry in star forming cores. The molecular abundances and integrated line intensities agree well with the typical values found towards high-mass star forming regions. The HCN/HNC, N2H+/ HCO+, HNC/ HCO+ and N2H+/ HNC ratios of column density and integrated intensity suggest that methanol masers are at an earlier evolutionary state than HII regions, but more evolved than the quiescent phase -- much in agreement with previous dust continuum studies. My thesis work thus gives strong evidence that along a timeline for massive star formation, the 6.7 GHz methanol maser phase originates in massive young stellar objects that are more evolved than infrared dark clouds, and is quenched by the time the sources evolve into ultracompact HII regions. 

Magnetic fields have a significant impact on the internal structure and atmospheric properties of low-mass stars. In particular, during the T Tauri phase, magnetic fields play a fundamental role in the star-disk interaction, in the stellar mass accretion process, and in regulating the angular momentum evolution of the system. Although magnetic fields have been measured for some T Tauri stars, little is known about the magnetic fields in protostars and older pre-main sequence sources. In this thesis, I present the hitherto largest and most comprehensive study of surface magnetic field strength of low-mass young stellar sources. I used iSHELL, a high-spectral resolution $R\sim 50,000$ near-infrared spectrograph at IRTF to observe over 100 young stars in the $K$-band from different star-forming regions and young associations. Combining high-quality observations with a detailed magnetic radiative transfer code, I derive magnetic field strengths, temperatures, gravities, infrared veiling, and projected rotational velocities for 107 young sources with ages of ~< 0.5 Myr to over 100 Myr and with masses between ~0.3 Msun and ~1.3 Msun. 

In this work, I performed the first survey of magnetic field strength in Class I and Flat Spectrum sources. I found that the magnetic field strength of Class I sources ranges from 0.5~kG to 4.1~kG with a median strength of 1.7 kG. The distribution of magnetic fields for the Class I sources is statistically indistinguishable from the magnetic fields of the Class II sources (or Classical T Tauri stars). Thus, no evolution in magnetic field strength is detected between the two classes. I also found that the gravities of Class I and II sources are statistically different, although a significant overlap exists. When combined with stellar evolutionary models, these results mean that about half of the Class I sources have ages of <= 0.6 Myr and are likely in the protostellar phase, while the other half of  Class I sources have gravities and ages consistent with Class II sources (or T Tauri stars). In a separate study, I discovered that T~Tauri~North is not in the same evolutionary stage as most T Tauri stars. Instead, its lower gravity, and thus earlier age ~< 0.6 Myr, suggest that the iconic TTau N source is a protostar ejected from the embedded southern binary companion shortly after its formation.

In a series of studies, I discovered that infrared temperatures of Class II sources are almost always lower than their optical temperatures. Moreover, the observed temperature differences correlate with the magnetic field strengths of the stars and increase for hotter sources. I attribute this phenomenon to magnetically induced spots on the surface of the highly magnetic young stars. Since low-mass young stars contract isothermally as they descend the Hayashi track, an almost one-to-one correlation between temperature and stellar mass can be established. The discovery of an optical-infrared temperature difference necessarily implies that masses derived from optical temperatures are almost always higher than masses derived from infrared observations. By using independent mass measurements for a sub-sample of source, I found that K-band infrared temperatures produce more precise and accurate stellar masses than optical temperatures when combined with magnetic stellar evolutionary models.

Analyzing the full sample of 107 young sources, I found that the magnetic field strength of sources less massive than M* ~0.9 Msun remains strong at a ~2 kG level during the first ~100 Myr. However, stars more massive than M* ~0.9 Msun often have magnetic field strengths below our detection limit of ~0.3 kG by the age of ~100 Myr. This suggests a change in the magnetic dynamo operating in these stars. Furthermore, by placing the stars in the theoretical HR diagram and overplotting stellar interior models, I found that the development of a radiative core has no effect on the measured magnetic field strength of the stars. The only substantial change in the magnetic field strength of the stars occurs when the convective layers in the stars thin to less than ~35% in radial depth or below ~10% in stellar mass.

With the aim of characterizing the role played by magnetic fields in the formation of young protostars, several recent studies have revealed unprecedented features toward high angular resolution ALMA dust polarization observations of Class 0 protostellar cores. In this thesis, I present observations of polarized dust emission that allow us to investigate the physical processes involved in the Radiative Alignment Torques (RATs) acting on dust grains from the core to disk scales, which align the angular momentum of grains with magnetic field. We find that the dust polarization is enhanced along the cavity walls of bipolar outflows, which are subject to high irradiation from the reprocessed radiation field emanating from the center of the protostar. In addition, highly polarized dust thermal emission has been detected in region most likely linked with the infalling envelope, in the form of filamentary structure being potential magnetized accretion streamer. Notably, we propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present in these cavities have grown larger than what is typically expected in young protostellar cores. To approach an estimation of the efficiency of dust alignment in protostars, we gathered a large sample of ALMA dust polarization observations of Class 0 protostars in order to perform a statistical analysis examining the trend between the dispersion of polarization position angles and the fractional polarization. We report a significant correlation between these two quantities, whose power-law index differs significantly from the one observed by Planck in star-forming clouds, confirming the different nature for the disorganized component of magnetic fields at the scales of protostellar envelopes. The grain alignment efficiency, is surprisingly constant across three orders of magnitude in envelope column density. Synthetic observations of non-ideal magneto-hydrodynamic simulations of protostellar cores implementing RATs, show that the ALMA values of grain alignment efficiency lie among those predicted by a perfect alignment of grains, and are significantly higher than the ones obtained with RATs. Ultimately, our results suggest dust alignment mechanism(s) are efficient at producing polarized dust emission in the local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. We performed further detailed modeling of the protostellar inner envelope physical conditions, alongside tentative comparisons between ALMA molecular line observations of UV-sensitive chemical tracers and dust polarization observations. We found that indeed, grains with super-paramagnetic inclusions, significant irradiation conditions (qualitatively comforted by the chemical observations), and large grains (10 micron) of compact structure are necessary to reproduce the observed grain alignment efficiency. However, further studies leading to a better characterization of dust grain characteristics, and additional grain alignment mechanisms, will be required to investigate deeper the cause of strong polarized dust emission located in regions of the envelope where alignment conditions are not favorable.

In this thesis, I present the results of a number of studies using high-resolution observations from world-class telescopes. The aim of these studies is to investigate protostellar jets and their role in star formation. In a case study of the bipolar jet from the T Tauri star DO Tau, we observe significant asymmetries in the morphology and kinematics of the jet and counterjet. The collimation of the jet supports the idea that magnetic fields collimate the jet, rather than pressure from the infalling envelope. If magnetic fields are responsible for jet collimation, then they may also drive jet launching. By measuring the radial velocities across the jet, we can calculate an upper limit on the jet launching radii. Our results support an X-wind or narrow disk wind model. Jet axis wiggling is also observed and is consistent with jet precession, which may be caused by an unseen companion in the disk or by launching the disk wind. In a study of four Class 0/I jets (HH 1, HH 34, HH 46 and HH 111) using high-resolution HST images, we were able to detect the inner knots of the red-shifted lobes in all four sources in more detail than previous studies. We compare these images to archival data to measure the proper motions in each jet. Jet axis wiggling is observed in three of these sources and the wiggling pattern in the HH 111 jet is consistent with jet precession and the presence of a companion in the disk. We also measure the extinction in each jet, which is quite high close to the star but decreases further out along each jet. Extinction measurements are important in the interpretation of emission-line ratios, which reveal plasma conditions along the jet and hence the mass and momentum transfer. These two case studies illustrate the power of high-resolution observations in differentiating between models of jet launching, and also reveal that protostellar jet trajectories could be a useful tool in identifying newly forming substellar companions close to the star. Finally, a survey of over 100 stars was conducted using high-resolution X-Shooter spectra. This study examines He I line profiles which vary between the two star forming regions examined in the sample, suggesting a trend with age. We also find that the maximum absorption velocity appears to be correlated with the source inclination and with the accretion rates of the sources. Our study confirms the results of past works and supports the idea that these winds are accretion powered. This survey gives context to our two case studies, by examining the link between accretion and ejection. High resolution observations are critical to advance understanding of the role of protostellar jets in star formation, as illustrated by the contribution of this thesis. The recently launched JWST facility marks the start of an era which will see exciting progress in this field, as its near-IR instruments peer deep into the embedded accretion-ejection engine."

The modern picture of the star formation scenario is far from explaining all the physical mechanism and observational features of forming stars. One of the most compelling unsolved questions is how low-mass stars collect their masses. The goal of this Thesis is trying to contribute in answering this question, through the observations of low-mass star forming systems in their prestellar, protostellar and pre-main sequence phase. Low-mass stars form from the collapse of gravitationally bound cores (or prestellar cores), which result from the fragmentation process of the Giant Molecular Clouds (GMCs), in an evolutionary sequence that leads to the formation of a protostellar object (the so-called Class 0/I young stellar objects, YSOs) accreting in mass from a dense circumstellar disk. YSOs gradually evolve in the optically visible pre-main sequence (PMS) stars following the dissipation of their envelope and disk (Class II or T-Tauri stars, and Class III YSOs). Although the general picture is rather well defined, still several questions remain unsolved, especially during the first stages of the evolution from cloud cores to protostars, when sources are so cold and/or deeply embedded in their parental cloud that can be observed only at infrared (IR) and sub-millimetric wavelengths. In the work presented in this Thesis, two key aspects of the star formation process will be approached to contribute in understanding how low-mass stars reach their final masses. Firstly, the distribution and physical properties of prestellar cores of the Serpens/Aquila star forming cloud will be investigated, through the analysis of far-infrared (FIR, 70-500 μm) images acquired by the Herschel Space Observatory in the framework of the Gould Belt key program. In particular, the prestellar core mass distribution (core mass function, CMF) will be derived and compared with the stellar initial mass function (IMF) to understand if the present distribution of stellar masses is already defined prior to collapse. This will lead to a complete census of the starless cores population of the cloud, to understand whether they are gravitational bound and will eventually collapse forming stars. As a second part of the work, a spectroscopic survey in the near-IR (1 − 2.4 μm) of a sample of Class I and II YSOs in the NGC 1333 young (< 1 Myr) cluster will be presented, conducted with the ESO/VLT-KMOS facility. The aim of this survey is to investigate the inner star-disk interaction region (i.e. < 1 AU), where accretion from the disk onto the protostar, and the consequent ejection of matter in the form of jets, are taking place. In particular, stellar parameters as visual extinction, stellar mass, radius and luminosity are computed, and the disk accretion luminosity obtained from the luminosity of HI emission lines (e.g. Paβ and Brγ). This will lead to the determination of the mass accretion rates of the NGC 1333 YSOs population, and to a comparison with the results relative to older (2 − 3 Myr old) star-forming regions as Lupus and Chamaeleon I. The main original results of this Thesis are the following: i) it demonstrates that the CMF is not universal but depends on the star-forming region properties, and its shape is not always compatible with the one of the IMF; ii) the determination of the accretion and stellar properties of a population of embedded YSOs, conducted here for the first time, has demonstrated that Class I sources have larger mass accretion rates than Class II objects, but not large enough to justify the observed masses, assuming a steady-accretion framework.

Following the recent advances in astrometry of Gaia DR2, I use its photometric and kinematic data to explore the structure of the star -orming region associated with the molecular cloud of Perseus. Apart from the two well-known clusters, IC 348 and NGC 1333, I present five new clusters, which contain between 30 and 300 members, named Autochthe, Alcaeus, Mestor, Electryon and Heleus. I construct reliable membership lists for the seven clusters and of the dispersed population of the complex.

I investigate the youth of individual sources within the clusters, and outside, using a combination of youth indicators from Gaia, WISE, Spitzer, and Planck data. These give the relative ages of the clusters and yield lists of young sources (τ_age ~ 5Myr). Based on this work, IC 348 spans ages from 1 Myr up to 5 Myr. Autochthe and NGC 1333 are the youngest clusters at 1 Myr. These three clusters constitute the star-forming centers of the cloud. Heleus and Mestor have average ages of about 4 Myr while Alcaeus and Electryon are at about 5 Myr. NGC1333, Autochthe and IC348 have the highest disc fractions (66±14%, 58±30% and 41±7%) while the rest four clusters have fractions below ∼30%.

Finally I construct the system Mass Function (MF) for all seven clusters by determining the masses of the cluster members using 2MASS photometry. For 0.08 < m/M⊙ < 1.4 the MF slopes are in agreement for the seven clusters and range between 0.5 and 1.3. These are by and large smaller than the Kroupa (2001) slope of 1.3.

The region of Perseus exhibits a bimodal nature in proper motion, spatial distribution, and age span. This bimodality is supported by a star-formation scenario where the seven clusters have formed from two kinematically distinct sub-clouds.

(Abridged) The general goal of this thesis is to study the first stages of the star formation process in multiple systems. For that, we analyze two complementary aspects of multiple star formation: the simultaneous formation of protostars in a cluster, and the formation of an individual binary system. 

In the first place, we present multiwavelength (0.7 - 5 cm), multiepoch (1994 - 2015) VLA observations toward the region enclosing the bright far-IR sources FIR 3 (HOPS 370) and FIR 4 (HOPS 108) in the Orion Molecular Cloud (OMC) 2, in Orion. We report the detection of 10 radio sources, seven of them identified as young stellar objects (YSOs). We image a well-collimated radio jet with a thermal free-free core (VLA 11) associated with the Class I intermediate-mass protostar HOPS 370. The jet presents several knots (VLA 12N, 12C, 12S) of non-thermal radio emission (likely synchrotron from shock-accelerated relativistic electrons) at distances of ∼ 7500 - 12500 au from the protostar, in a region where other shock tracers have been previously identified. We show that these knots are moving away from the HOPS 370 protostar at ∼ 100 km/s. The Class 0 protostar HOPS 108, which we detect as an independent, kinematically decoupled radio source, falls in the path of these non-thermal radio knots. These results favor the previously proposed scenario where the formation of HOPS 108 has been triggered by the impact of the HOPS 370 outflow with a dense clump. However, HOPS 108 presents an apparent proper motion velocity of ∼30 km/s, similar to that of other runaway stars in Orion, whose origin is puzzling.

Later, we extend our study to the whole field of view of those observations, a 12.5'-long portion of the Integral Shaped Filament (ISF) which contains the whole OMC-2 region and the southernmost 2.5' of the OMC-3 region. We report 47 additional radio sources, 23 of which are presented here for the first time. For these 47 radio sources, we report the positions, flux densities, and spectral indices. We discuss the nature of the 47 reported radio sources by analyzing their radio spectral index, morphology, emission at other wavelength ranges, position relative to the ISF, and association with dust clumps. We conclude that, out of the 57 total radio sources in the field of view, 41 (72%) are very likely associated with the star-forming region, 6 are extragalactic background sources, and we are unsure about the association with the star-forming region of the remaining 10 sources. Of the 41 sources associated with the star-forming region, 36 are tracing the position of YSOs, 3 are tracing non-thermal emission from jet knots, one is tracing the shock-ionized wall of an outflow cavity, and one could be either a YSO or a jet knot. We unambiguously identify the radio continuum emission of the driving sources of the 9 previously reported molecular outflows in our field of view. In four of these sources, we clearly resolve the elongated morphology characteristic of radio jets. We detect thermal free-free emission from two Class III YSOs that may be tracing their associated photoevaporating disks. We find signs of variability in 13 radio sources, seven of which are also variable at shorter wavelengths (near-IR/optical/X-ray). We find 7 pairs of radio sources with a projected separation < 3000 au, that are potential binary systems; taking into account the additional sources detected at other wavelengths, we then identify a total of 17 potential multiple systems in our field of view within this separation range.

Finally, we study the close binary system SVS 13, with components separated 90 au. We detect at least two circumstellar disks of dust and gas, and one circumbinary disk with prominent spiral arms, that appears to be in the earliest stages of formation, associated with this protostellar system. Dust emission appears more intense and compact toward component VLA 4B, while VLA 4A seems to be associated with a larger amount of dust and with stronger molecular transitions. We are able to estimate rotational temperatures and molecular column densities, indicating warm temperatures and rich chemistry. Molecular transitions typical of hot corinos are detected toward both VLA 4A and VLA 4B. From the observed dust emission and the kinematical information, we estimate the orientation of the system, the stellar masses and the mass of their associated disks. Our analysis of the proper motions and the kinematics of the disks suggest that up to four stellar objects, one of them a visible star, could be present within a region of size <100 au. In summary, SVS 13 seems to be an excellent test-bed to test numerical simulations of the earliest stages in the formation of binary and multiple systems. The thesis is is available here.

Orion is the prominent constellation in the northern winter night sky, containing a rich, complex, and very active star-forming region, comprising giant molecular clouds (GMCs) and generations of young stars with ages up to 20 Myr. Additionally, Orion contains the closest massive star-forming region to Earth (d ∼ 1300 light-years), making it one of the best-studied regions in the sky. Nevertheless, many fundamental questions remain: Why is the molecular gas distributed the way it is? How detailed do we know the young stellar population? Why does the star formation rate (SFR) vary by an order of magnitude inside the same cloud, namely Orion A? Is there a causal relation between the different clouds and stellar groups in Orion? The major observational handicaps in the past have been the lack of large deep near-infrared surveys, and critically, the lack of three-dimensional (3D) information. However, only the third dimension will bring an unambiguous understanding of the location, extent, shape, and space motion of the observed gas and stellar structures, and a more complete understanding of how nature forms stars.

I started the thesis by revisiting and updating previously published catalogs of young stellar objects (YSOs) for the Orion A GMC, using the seeing limited near-infrared ESO-VISTA survey, the VIenna Survey In OrioN (VISION). The updated YSO catalog enabled a study of the 2D distribution of the YSOs relative to the dense parts of the Orion A cloud, revealing that different YSO classes show different projected distributions relative to regions of high dust column-density, depending on their evolutionary class. The YSO catalog was then combined with the groundbreaking Gaia astrometry and the average parallaxes and proper motions of the YSOs were used as proxies for cloud 3D properties, on the basis that these YSOs are close to their parental molecular clouds and that they share on average the same radial velocities as the gas. The YSO average parallaxes revealed the true 3D shape of the Orion A GMC. I found that the cloud is about 70◦ inclined relative to the plane of the sky, hence about twice as long (∼90 pc) as previously assumed. Moreover, it has a peculiar “bent head”, which is the location of the massive Orion Nebula Cluster. This new view of the Orion A cloud added new puzzle pieces to our understanding of the region, suggesting the existence of an external feedback source that could have shaped the head of Orion A and additionally enhanced the SFR, and triggered massive star formation.

To test the hypothesis of a feedback-driven star formation history, I studied the 3D motions of the whole southern Orion molecular cloud complex. I found that, surprisingly, the studied molecular clouds show a coherent expanding motion at the 100-pc scale with an average velocity of about 5 ± 3 km/s. One possible explanation for this expansion is the existence of a powerful feedback event, named Orion-BB event (Orion big-blast event), that likely took place about 6 Myr ago. We suggest that this event shaped and pushed the gas and triggered most of the current star formation in the complex. The observational results in this thesis bring a new view of the Orion complex, one where the feedback of previous generations of massive stars (ionizing radiation, winds, supernovae) likely played a fundamental role in shaping the gas distribution and influencing the SFR. The results also validate a technique where a careful selection of YSOs can be used as a proxy for the distance and proper motion of their parental molecular cloud.

Protoplanetary disks are the essential link between molecular clouds and planetary systems. Protoplanetary disk evolution determines the resulting planetary systems. In this dissertation, I focus on tracing evolution in populations of disks by analyzing large samples of disks. This dissertation is made possible by new observations and physically-motivated models. The main processes that I use to study disk evolution are the dust growth and evolution that occur in the outer disk and the accretion of gas from the inner disk onto the star.

Lynds 1641 (L1641) is a star-forming region in the Orion Molecular Cloud A, and it has great potential as a laboratory for protoplanetary disk evolution. I present observations of disks in L1641 from the Herschel Space Observatory and the Atacama Large Millimeter Array (ALMA). The far-infrared Herschel data are sensitive to micron-sized dust grains in the outer disk atmosphere, and the radio ALMA data trace the millimeter-sized dust grains in the disk midplane. I use accretion disk models to show that the far-infrared data are consistent with disks that show signs of dust evolution, even in this young (~1.5 Myr) region. I compare the L1641 millimeter data to other surveys and show that L1641 is at a stage of evolution between young protostellar systems and more evolved disks where planet formation is already well-underway. 

Accretion of material onto a star is an important mechanism in protoplanetary disk dispersal and heating. The classical magnetospheric accretion paradigm is well-understood and established for low-mass stars. However, higher-mass stars may not have the magnetic field strength for magnetospheric accretion to occur. I present a large survey of intermediate-mass systems with near-infrared spectra. I use the accretion-tracing Brgamma line to find trends with system properties and find a break in the accretion rate—stellar mass relationship, which may indicate a break in the accretion mechanism. Additionally, I use magnetospheric accretion models to reproduce the observations to determine the accretion properties in a subset of objects, finding that these models can reproduce fast-moving emission. 

A golden age of interferometry is upon us, allowing observations at smaller scales in greater detail than ever before. In a few fields has this had the huge impact as that of planet formation and the study of young stars. State of the art high angular resolution observations provide invaluable insights into a host of physical processes from accretion and sublimation to disk winds and other outflows. In this thesis, I present the wide-ranging works of my PhD, encompassing both instrumentation and observational science. Instrumentational activities stem from the development of new generation baseline solutions at CHARA to the commissioning of a new observing mode on MIRC-X, allowing for the first ever J band interferometric observations of a young stellar object ever published. The science results find direct evidence of a dusty wind emanating from the innermost regions of the young object SU Aurigae in addition to exquisite image reconstruction revealing inclination induced asymmetries. Additionally, I find evidence of viscous heating of the inner disk of outbursting star FU Orionis as I derive the temperature gradient to unparalleled precision. While it is difficult to draw one overall conclusion from the varied works of this thesis, the results described here are a testament to the uniqueness of young stellar systems and provide vital information on some the most ubiquitous processes in astrophysics. The instrumentational developments also open up exciting opportunities for future science in the ever-growing field of optical interferometry.

The interaction between interstellar dust, ice and gas plays a major role for the chemistry in regions where stars and planets form. Different reactions occur in the gas and on the ice mantles of dust grains in these regions, and consequently, the mutual exchange of matter between the two phases is what regulates the chemical evolution of newborn stars and planets. Methanol (CH3OH) is a key molecule in this process, as it predominantly forms through the sequential addition of hydrogen atoms to condensed CO molecules. Once present on the ice mantles, it is considered a fundamental precursor of more complex interstellar species. 

To determine the importance of the various chemical processes governing this interplay, I conduct multi-wavelength observational studies and obtain relative abundances of solid- and gas-phase molecules, particularly of methanol. This methodology allows to calculate gas-to-ice ratios and directly access the efficiencies of condensation and desorption processes. The selected targets are the cold protostellar envelopes of low-mass stars belonging to three nearby star-forming regions, with distinct physical conditions and histories. By comparing these, it is possible to test the dependencies of the chemical evolution of protostars on the large-scale environment from which they form. 

The calculated CH3OH gas-to-ice ratios of the order of ~10^-3-10^-4 validate previous experimental and theoretical predictions, consistent with a considerably efficient non-thermal desorption mechanism in cold envelopes. Similarities in the gas-to-ice ratios in different nearby star-forming regions suggest that the CH3OH-mediated chemistry in the outer protostellar envelopes is relatively independent on variations of the physical conditions. This might explain the ubiquitous presence of methanol in a variety of interstellar and circumstellar environments. The combination of millimetric and infrared observations presented in this thesis has proven to be an essential tool to cast light onto the small-scale variations in the ice chemistry and its relation to the physics on large scales in star-forming regions. Thereby, these studies serve as a critical pathfinder for future work with the James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array to constrain further the routes to the formation of complex molecules during the embedded stages of star formation.  

Thesis: https://nbi.ku.dk/english/theses/phd-theses/giulia-perotti/Giulia_Perotti.pdf

In this Thesis we aim to answer a long-standing astrophysical problem, quantifying the timescales of the evolutionary phases characterising the high-mass star formation process. Understanding the details of the formation of massive stars (i.e. M>8-10 Msun) is not trivial, since these objects are rare and at a relatively large distance. They also form and evolve very quickly and almost their entire formation takes place deeply embedded in their parental clumps. During the evolution, the chemical composition of massive clumps can be heavily affected by the changes in density and temperature induced by the presence of massive young stellar objects. Chemical tracers that show a relation between their observed abundances and the different phases of the star formation process are commonly called chemical clocks. In this Thesis, through the comparison of observations of a large sample of massive clumps in different evolutionary stages, and accurate time-dependent chemical models, we estimate the timescales of the different phases over the entire star formation process. In addition, we provide relevant information on the reliability of crucial chemical clocks, both for the early and the late stages, confirming that the chemistry is a powerful tool to establish a timeline for the high-mass star formation process.

Ice and dust play a key role in building the Solar System. During their life cycle, these primitive components are exposed to complex physical and chemical processes. Some of the earliest remnants from the formation of the early Solar nebula still remain in comets providing a way to probe these initial building blocks.  As comets travel toward the sun, ices sublimate revealing much about their composition and history.  However, the survival of ices in comets is poorly constrained.  The comet 49P/Arend-Rigaux is a low-activity periodic comet and was suspected of losing its volatiles (ices) over time.  Over several apparitions small tail and jet-like features were observed.  Using dust dynamical models I determine the grain properties and the outgassing duration of these different displays of activity.  By modeling the ice sublimation over time I show there is a clear decrease in activity over 30 years providing a strong example of a comet transitioning to a dormant state.   Outside the Solar System, initial conditions promoting ice formation can be studied within small dense molecular cores where cold surfaces of dust grains become chemical factories for simple and complex ice molecules to form.   However it is unclear if complex organic molecule (COM) formation requires energetic UV radiation from newborn stars.   To test this, I measure the CO and CH3OH abundances for the first time with lines of sight toward background stars through molecular cores.   I find a large abundance of CH3OH ice and a high conversion rate from CO into CH3OH during the pre-stellar phase, signifying that COMs can indeed form in cold environments and account for COMs observed at later stages of star formation.  To constrain the local density of hydrogen (H and H2) in the cores where COMs can form, I create very high spatial resolution extinction maps and transform them into three dimensions using an inverse-Abel transformation.  Only a small fraction (~<2%) of the volume of the cores have sufficient density for CH3OH and thus presumably other COMs to form.  This work is in preparation for large-scale ice maps that will be obtained with the slitless spectroscopy mode of JWST-NIRCAM.  I present simulations testing the feasibility of the observations and projected science return.

(Abridged) Magnetic fields are important in the process of accretion disk formation and evolution. In the millimeter and submillimeter regime, the main way to infer the magnetic field is by observing polarized emission from dust grains that have been magnetically aligned. In this thesis, we are aimed to better understand the massive star formation process paying special attention to the role of the magnetic field. To do this, we will carry out a multi-scale analysis with a double approach, theoretical and observational. Specifically on one hand, we will investigate the accretion process through an accretion disk around a high-mass star. Our goal is to understand whether massive stars can be formed in a similar way to low-mass stars, that is, accreting material from the envelope to the protostar through an accretion disk that would avoid the radiation pressure problem. To do this, we modeled the 1.14 mm image taken with the “Atacama Large (Sub) Millimeter Array” (ALMA) with high angular resolution towards the accretion disk around GGD 27–MM1. To this end, we will use the models developed by D’Alessio et al. (2006) and which have been successfully applied to protoplanetary disks around low-mass stars. On the other hand, we will study the fragmentation process in an infrared dark cloud (IRDC G14.225- 0.506). We intend to understand the role of the magnetic field in this fragmentation process. To do this, we will study polarized emission observations taken with the “Caltech Submillimeter Observatory” (CSO) towards two physically identical hubs but with different levels of fragmentation.

This thesis is focusing on characterizing components of young protostellar systems, most notably their jets and disks. Using observations with the ALMA and VLA interferometers, we observed the environments where the first stages of star and planet formation occur. We revealed information on crucial chemical tracers of various protostellar systems components. With a particular focus on molecular jets, I show differentiation in chemical composition between the fast jet and the low-velocity outflow. For the first time, I was able to compare dust masses of young disks with older disks. By comparing this information with masses of the extrasolar planets detected so far, I showed that the solid cores of gas giants must form in the first 0.1 Myr of stellar life. That is an important time constraint that pushes the onset of planet formation earlier and highlights the importance of characterization of the youngest protostars in understanding the origin of Solar System and Earth.

Thesis: https://home.strw.leidenuniv.nl/~tychoniec/thesis.html

The interstellar medium is composed by approximately 1% of dust in terms of mass. Surprisingly, this tiny amount of dust already plays a very important role in stellar formation. The dynamics of dust grains may differ from that of the gas particles, leading to local variations in concentration. However, very few studies have focused on the gas and dust differential dynamics during star formation. My thesis aims to fill this gap and is divided into four parts. In the first part, I develop a module dealing efficiently with dust dynamics that can simultaneously include multiple grain species intended to the multidimensional adaptive grid code RAMSES (Teyssier 2002). I then carefully test my module by comparing my results with known analytical solutions. I also show that my implementation is robust, fast and accurate. Then I perform star formation simulations that consider multiple dust species. This study establishes that a decoupling between the dust and the gas appears for grains of sizes larger or equivalent to a hundred micrometers. I also find that this decoupling depends strongly on the initial properties of the prestellar core. Then, I develop an analytical formalism, similar to the non-ideal magnetohydrodynamics but that includes the dynamics of charged grains. This formalism allows to highlight different coupling regimes between the grains, the magnetic field and the gas as a function of the grain size, its charge and its environment. In parallel, I investigate the dynamics of dust in the weakly ionized zones of protoplanetary disks in order to study the formation of chondrules. Chondrules are dust grains found in most meteorites and are key to understand the formation of disks and planets.

Understanding how stars form is one of the fundamental questions which astronomy aims to answer. Currently, it is well accepted that the majority of stars form in groups and that their predominant mechanism of formation is the core-collapse. However, several mechanisms have been suggested to explain the formation of substellar objects and their contribution is still under debate.

The main goal of this thesis is to determine the initial mass function, the mass distribution of stars at birth time, in different associations and star-forming regions. The mass function constitutes a fundamental observational parameter to constrain stellar and substellar formation theories since different formation mechanisms predict a different fraction of stellar and substellar objects. We used the Gaia Data Release 2 catalogue together with ground-based observations from the COSMIC-DANCe project to look for high probability members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. We applied this method to the 30 Myr open cluster IC 4665 and the 1-10 Myr star-forming region Upper Scorpius (USC) and rho Ophiuchi (rho Oph). We found very rich populations of substellar objects which largely exceed the numbers predicted by core-collapse models. In USC, where our sensitivity is best, we found a large number of free-floating planets and we suggest that ejection from planetary systems must have a similar contribution as core-collapse in their formation.

Age is a fundamental parameter to study the formation and evolution of stars and is essential to accurately convert luminosities to masses. For that, we also presented a strategy to study the dynamical traceback age of young local associations through an orbital traceback analysis. We applied this method to determine the age of the beta Pictoris moving group and in the future, we plan to apply it to other regions such as USC.

The members we identified with the membership analysis are excellent targets for follow-up studies such as a search for discs, exoplanets, characterisation of brown dwarfs, and free-floating planets. I this thesis, we presented a search for discs hosted by members of IC 4665 and we found six excellent candidates to be imaged with ALMA or the JWST. The tools we developed, are ready to be used in other regions such as USC and rho Oph, where we expect to find a larger number of disc-host stars.

Our position in the Milky Way, buried within its disk, makes it extraordinarily difficult to piece together the structure of our home Galaxy. We know the Milky Way is a barred spiral but many questions, including the precise number, location, and prominence of spiral features remain debated. Towards the goal of better understanding star formation and the structure of our Milky Way, we present four avenues of research developed to map the molecular gas and dust in the Galaxy. Specifically, using a combination of extraordinarily elongated gaseous filaments, numerical simulations of Milky Way analogs, 3D dust mapping of our solar neighborhood, and 4D spatial-kinematic views of individual star-forming regions, we are beginning to build new models of our Milky Way's interstellar medium both locally and towards the inner Galaxy.

Towards the inner Galaxy, we systematically characterize the physical properties of the largest-scale filaments in the interstellar medium. We find that the diversity in their physical properties likely reflects different formation mechanisms and evolutionary histories, with the longest and densest filaments most likely to trace the Galaxy's gross spiral structure in position-position-velocity space. By producing synthetic observations of comparable filaments forming in an AREPO simulation of a Milky Way-like galaxy, we find that while large-scale filaments preferentially form in the mid-plane of the galaxy, additional physics (stellar feedback, magnetic fields) is needed to reproduce the range of observations.

Within the solar neighborhood, we use 3D dust mapping techniques in combination with stellar distances from Gaia DR2 to produce the largest uniform catalog of accurate distances to local molecular clouds. Comparison with "gold-standard" maser distances obtained from VLBI observations indicate agreement to within 10%, with no systematic offsets out to 2.5 kpc. Using this new catalog, we present the discovery of a 2.7 kpc long coherent arrangement of stellar nurseries, which undulates about the Galactic plane with an amplitude of 160 pc and appears to be the Local Arm of our Galaxy nearby. Extensions of the 3D dust mapping technique applied to a single cloud in this structure demonstrate that 3D spatial views of dust can be knitted together with kinematic information from gas to create 4D views of the local interstellar medium. Ultimately, we plan to build on these and complementary techniques to produce an integrated 3D model of our Milky Way's stars, gas, and dust out to 6 kpc in the coming years.