An interview with René Liseau

Interview by Bo Reipurth, SFN #337 - January 2021

Back in 1986, you and Göran Sandell wrote a paper on the geometry of anisotropic CO outflows. What was the context in those early days of outflow studies and what did your paper contribute?

Many things that are obvious today were back then, in the infancy of outflow research, not very well understood. That included the geometric form and the 3d orientation of high-velocity flows in molecular clouds and their possible physical relationship with Herbig-Haro objects if any.

Our aim, then, was to develop a simple model for the geometry of the molecular outflows that was easy to use by everyone. A reasonable assumption was that the high-velocity line wings outside the Gaussian line core are due to optically thin emission. The wings would thus sample all molecules in the outflow volumes. As the distances to molecular clouds were not very well known, our model ought ideally to be independent of the distance to the clouds. In addition, as the spatial mapping in molecular lines over large regions in the sky was extremely observing time intensive, we wanted the model to be applicable relatively cheaply also to low-resolution data. We arrived at an analytical solution that allowed the determination of the space velocity vectors from the observation of the radial velocities and the spatial distribution of the bipolar flow. Comparing these to those found for those HH flows for which both observed radial and tangential velocities were available, clearly showed their physical relationship.

Not long after, you published a mm study of the outflows in the NGC 1333 region, providing the most detailed map of the activity in this important region up to that point.

This work was related to the former and was motivated by the need to understand the very early phases of stellar evolution. We discovered that the NGC 1333 region harbors a cluster of outflows and their driving sources, along with their associated HH objects/jets. This offered the opportunity to study an ensemble of forming stars of essentially the same age, in a common environment of the same physical and chemical conditions. Obvious questions included, to what extent there were observable mass dependencies. In addition, from infrared photometric observations, the distribution of the linear polarisation was known. The interpretation was that the polarisation outlined a large scale coherent magnetic field. Were the outflows controlled and aligned by the field?

We obtained detailed maps of the distribution of the line intensity of CO(J=1-0) and its isotopologue 13CO, from which we derived maps of temperature and density. We attempted to model the flow dynamics of individual sources, as the level of contamination in this crowded region was high. This enabled the disentanglement of overlapping flows, the better definition of individual flows, and essentially ruled out the proposed possibility that flows could be diverted by “obstacles" of dense molecular gas.

Based on theoretical arguments and some observational evidence, outflows were thought to be driven by protostellar objects with disks. The outflow orientation in space was assumed to outline the protostellar rotation axis and maybe the most important of the conclusions from this study of the outflow cluster was the implication that stellar spin axes were randomized already very early in protostellar evolution, i.e. on time scales less than 10E5 years. Our results also showed that magnetic fields were not controlling star formation, at least not beyond that point in time. The role of magnetic fields in the star formation process was a hotly debated issue in those days and, in many instances, still is.

In the early 90s, a long time before 2MASS, you and your collaborators carried out a major infrared and mm study of the young embedded population of the little-studied Vela molecular clouds. What did you find?

This work developed during my stay in Italy. I was joining the infrared group at the CNR research institute IFSI in Frascati, near Rome. The group was providing hard- and software for the Long Wavelength Spectrometer (LWS) aboard the ESA Infrared Space Observatory (ISO) and we were in charge of the instrument team's observing program on star formation, once ISO would be in orbit.

The Vela Molecular Ridge, or VMR, is a collection of Giant Molecular Clouds in the outer Galaxy. These GMCs contain more than 10E5 Msun each and three of them, i.e. VMR-A, -C, and -D, are relatively nearby, at a distance of about 700 pc. We argued that detailed studies of the VMR would potentially provide valuable new, complementary insight regarding global star formation on galactic scales.

A valuable tool was the IRAS Point Source Catalogue that contained the flux densities at 12, 25, 60, and 100 μm of 8000 infrared objects in a 250 square degree field toward the VMR along the galactic plane. On the basis of their spectral gradients, we selected those IRAS sources that showed large mid-to-far IR excesses, a characteristic of Class I sources. To find any near-infrared counterparts and to discriminate against background contamination we obtained 1 to 5 μm photometry, scanning the fields around the IRAS sources. In addition, from molecular line observations with the Swedish-ESO Submillimetre Telescope, we obtained radial velocities, associating the objects with the molecular clouds and also identifying the associated very dense gas out of which the objects were forming.

Finally, less than 1% of the initial target list met the various selection criteria. The results for these formed the basis of our discussion, addressing star formation history, star formation rates, star formation efficiencies, and the Initial Mass Function. We found that star formation in the VMR is consistent with “normal” and globally quiescent, non-violent processes. Based on the observed luminosity functions, we argued that the VMR is producing an IMF basically consistent with that of Salpeter for the solar neighborhood.

You also led large teams in using the Infrared Space Observatory (ISO) to study the far-infrared emission of H2O towards HH54 and more recently, you and your collaborators detected O2 towards ρ Oph cloud with Odin and Herschel.

In an influential paper published in 1987, Goldsmith & Langer presented calculations of cooling functions for molecules in the dense interstellar medium. Based on gas-phase chemistry, an important result was the predicted high abundances of H2O and O2 and their large cooling rates, significantly impacting the thermal energy balance of dense molecular clouds.

The telluric atmosphere is opaque in the spectral lines of these molecules and meaningful observations will have to be made from space. For the observation of H2O and O2, two dedicated space missions, SWAS and Odin, were built in the USA and in Europe, respectively, and launched into orbit around the Earth. Astonishingly, both missions returned unexpectedly low interstellar concentrations of these molecules in the ISM. Whereas weak H2O emission was detected in several regions in the sky, O2 was firstly and only detected by Odin toward the ρ Oph cloud. It became evident that the pure gas-phase models had to be revised and modern models include networks of chemical reactions on the surfaces of the accompanying dust grains.

The Odin detection of O2 was later confirmed with HIFI aboard Herschel also in other transitions. Based on models by astrochemists, we interpreted the low O2 abundance as being due to a transitional effect that occurred during a brief period of protostellar evolution. That allowed the timing on a “chemical clock”, providing a lower limit to the age of the cloud. Obtaining reliable cloud ages is difficult in general.

In the only other source where O2 had been detected (also by our Herschel O2 team), i.e. the OMC, the findings were ascribed to shock excited H2O chemistry. Several years earlier we had reported the first detection of thermal H2O emission from a Herbig-Haro object. The spectroscopic ISO-LWS data for HH54 were consistent with molecular excitation by a relatively slow, continuous shock with velocities of order 10 km/s. This is in contrast to an abrupt jump shock, where the physical conditions change discontinuously directly behind the front. In the absence of strong ultraviolet radiation fields, a non-zero concentration of ions is maintained by cosmic rays and C-shocks are smoothed by the coupling of the neutrals to the ions, indicating the presence of magnetic fields at the 10-ish μG level to which the ions are tied. In molecular clouds, these fields are directly observed through Zeeman-split spectral lines.

I am very happy to let you know that the Swedish led Odin, a project in collaboration with Canada, Finland, and France and launched in February 2001, had a design lifetime of 2 years, but is at the time of writing (December 2020) still operational. To this day, Odin performs regular aeronomical observations of the Earth’s atmosphere.

In 2006 you analyzed a large trove of Infrared Space Observatory LWS spectra of several hundred YSOs and their outflows. What did you find?

We were focussing on the prominent fine-structure lines of neutral oxygen and ionized carbon that were falling into the waveband of the LWS. Being forbidden, these FIR transitions are collisionally excited and were generally believed to be optically thin. In addition, being intrinsically bright and unaffected by extinction, these lines were frequently used as diagnostic cooling lines of both the galactic and, especially, the extragalactic ISM. We analyzed the

[O I] 63 µm, [O I] 145 µm and [C II] 157 µm lines from several hundred galactic sources to get a solid statistical basis for the understanding of the cooling mechanisms of PDRs and outflow generated shocks.

The flux ratio of the oxygen lines is particularly useful for testing the low optical depth assumption. To our big surprise, the major fraction of the observed ratios were clearly deviating from the optical thin value. Similar results were found later for the [C II] 157 µm line.

We analyzed the line opacity issue in quite some depth, examining several physical processes, including [O I] 3P0→1 masing, that might be responsible for the observed phenomena. A firm conclusion was that observations of these lines require a very thorough analysis, and therefore may not be the cheap diagnostic tool as was widely believed.