An interview with Douglas Whittet

Interview by Bo Reipurth, SFN #353 - May 2022

Your PhD in 1975 dealt with interstellar extinction in the southern Milky Way. What led you to this subject?

Pure chance. I did my undergraduate degree in physics at Queens College, Dundee, and had made many friends in the area, so there was incentive to continue living there. St.~Andrews was the only local option for graduate study in astronomy, and interstellar extinction was the only available project! It turned out to be a lucky happenstance. Within a few months I was on my way to Chile with my advisor, Ian van Breda, to observe at Cerro Tololo --- a life changing experience. The aim of the project was to search for variations in the spectral dependence of extinction that might correlate with the environment, accomplished by comparing observations of reddened and unreddened stars of matching spectral type. We found clear evidence for evolution in the optical properties of dust from the general Milky Way field to denser dark clouds. Earlier work had focused largely on the ``nuisance value" of dust --- how best to correct for its presence in data acquired to address different questions. It was just beginning to emerge as an important ingredient of the cosmos in its own right, so it was an opportune time to get involved, and my subsequent career has focused almost entirely on dust-related topics.

A few years later you and Ian van Breda explored the correlation of the interstellar extinction law with the wavelength of maximum polarization.

I have always found polarization to be one of the most fascinating aspects of research on dust -- somehow, these tiny grains line up systematically such that their longest axes tend to be parallel, and the resultant polarization maps the magnetic fields of entire galaxies! Polarization also elucidates the physical properties of the grains -- most obviously shape, but also size and refractive index. The work with Ian was part of a program to study the evolution of grain size with the environment, using both extinction and polarization data to show that grains grow inside dark clouds, primarily as the result of grain-grain coagulation -- in essence, the first step toward planet formation.

In a highly cited study from 1992, you investigated the influence of the environment on the size distribution of the aligned polarizing grains in different star forming regions.

The pioneering studies of interstellar polarization in the 1970s by K. Serkowski and others were based almost entirely on observations in the visible. Extension of the available coverage into the infrared and (later) the ultraviolet provided new insights, and our 1992 paper was part of a larger project to collect and interpret such data. New subtleties were beginning to emerge in how grains respond to the magnetic field in different environments according to their size and composition. The correlation of peak polarization with grain size was found to depend not only on general coagulation of the particles in dense environments (which also affects extinction) but also on size-dependent alignment efficiency (independent of extinction). In dense clouds, the small grains tend to be poorly aligned, so most of the polarization is produced by larger ones. These observations would later provide a test bed for new developments in alignment theory, lending support to radiative torques as the primary mechanism for grain alignment in dense regions. The process is most efficient when a particle absorbs a photon of wavelength comparable with its dimensions; so in an environment shielded from UV photons but relatively transparent in the infrared, only the large ones tend to align.

In 1983 you published a Letter in Nature on the 3 μm ice feature towards several stars obscured by the Taurus clouds, followed up a few years later by a larger survey. What were the key results?

It seems obvious with hindsight, but at that time no one had demonstrated that grains acquire ice mantles in quiescent regions of dark clouds. Up till then, detections of (e.g.) the strong 3 μm water-ice absorption band were almost entirely limited to deeply embedded YSOs such as the Becklin-Neugebauer object. I recall reading a 1980 paper by Jay Elias on an infrared survey of the Taurus dark cloud, in which many of the sources were identified as reddened background field stars, and it occurred to me that they would be ideal targets to address this issue. Checking the calendar, I realized that the next proposal deadline for observing time at Mauna Kea was only a few days away, so my colleagues and I threw together a proposal, and by some miracle it was accepted.

The spectra demonstrated that ice is ubiquitous in this dark cloud (subsequent observations of other clouds confirmed this to be a general result). The abundance of ice correlates strongly with optical depth into the cloud, and exhibits a “threshold” effect, suggesting that a certain level of shielding from the external radiation field is necessary before ice begins to accumulate. Ices such as CO that are more volatile than H₂O have higher thresholds. These results place important constraints on models for both the physics and the chemistry of dark clouds.

With the launch of the Infrared Space Observatory the study of interstellar ices was revolutionized. In 1996 you led a large team reviewing the progress in this field. What were the key contributions of ISO to the study of ices?

The impact of ISO was immense. Prior to its launch, observations of the necessary sensitivity and resolution were largely limited to ground-based observations of features accessible through telluric windows. The instruments carried by ISO opened up the entire spectral range containing the vibrational features of solids, and yielded spectral resolving powers sufficient to detect diagnostic structure in the profiles. Such a structure enables us to distinguish, for example, between amorphous and crystalline solids, and between “polar” (H₂O-dominated) and “non-polar” (CO-dominated) phases in the ices. We thus gained not only an extended inventory of the ice composition but also insight into cycles of growth in progressively denser environments, from predominantly hydrogenated molecules (H₂O, NH₃, etc.) in regions of moderate density to ices dominated by freeze-out from the gas and composed mostly of CO in dense cores. Species that form by surface reactions that begin with CO are especially important as they control the complexity of the subsequent chemistry in regions of active star formation. Oxidation to CO₂ is a chemical dead-end, whereas hydrogenation to H₂CO and CH₃OH opens pathways to more complex organic molecules. Results from ISO and subsequent studies with Spitzer led to refinements of astrochemical models and clarified the conditions that lead to greater chemical complexity.

In a 2008 study you investigated how polarization data can be used to discriminate between grain alignment mechanisms in dense regions. What did you conclude?

Early work on grain alignment in dense clouds relied on broadband observations, but we and others subsequently obtained spectropolarimetric data corresponding to the infrared absorption features of silicates and ices in the same lines of sight. The results showed that silicate dust is the primary source of polarization in diffuse regions of the ISM, and that the aligned silicate grains acquire icy mantles in molecular clouds. Study of the alignment status of the icy grains provides an important test because many of the proposed alignment mechanisms fail in dense regions where dust and gas temperatures tend to equilibrate (classical alignment theory invokes a heat engine driven by the temperature difference between them). The detection of polarization corresponding to the CO ice feature was especially significant as it shows that alignment is effective in low-temperature environments (< 17 K, the sublimation temperature for CO).

We combined broadband and spectral data to investigate the systematics of alignment efficiency in both quiescent and protostellar regions of dense clouds. Efficient alignment requires that the grains spin at angular speeds far higher than those resulting from random collisions with a gas in thermal equilibrium. Possible causes of this “supra-thermal” spin include impulses from the release of binding energy at surface sites of molecule formation and radiative torques that transfer angular momentum from photons to grains as a function of their size and shape. Our results support the radiative torques model as the primary mechanism in dense clouds. The relatively high levels of alignment observed in protostellar regions can be attributed to radiation from the YSOs themselves, whereas alignment in quiescent regions is driven by penetration of the external radiation field.

The third edition of your well-known book ‘Dust in the Galactic Environment’ is just about to be published. What do you see as the main developments in interstellar dust studies in recent years?

The second edition was published in 2003, placing it between ISO and Spitzer in terms of relevant mission chronology; the third edition will appear in mid 2022, just as the James Webb Space Telescope is expected to begin full operation. Developments in our understanding of cosmic dust in the intervening years are such that the new edition is in large part a different book. Advances have been driven not only by data from observational facilities such as Spitzer, Herschel and Planck but also by parallel developments in laboratory studies of cosmic and terrestrial analog materials and in numerical modeling of all aspects of dust. I'll summarize a few highlights, in no particular order.

The Planck mission enabled polarized far-infrared emission from aligned grains to be mapped in detail over the entire sky. Results may be compared with the “scattershot” view provided by observations of polarized starlight, which depends on the distribution of suitable targets. Because the far infrared data lack distance information, other observations must be folded in to provide a 3D picture of the magnetic field distribution in the Milky Way. Spectacular results have been published in a series of papers by the Planck Collaboration team. On smaller scales, studies of individual prestellar and protostellar clouds are also obviously important because the magnetic fields control core collapse and subsequent star formation. We now have a better understanding of grain alignment in dense clouds, as already mentioned, and this is relevant to interpretation of the data --- if grains fail to align, no polarization is produced! So detection of polarized radiation that clearly emanates from deep within a molecular cloud is highly significant as it enables us to trace the internal magnetic field.

Our understanding of the nature and evolution of the ices that accumulate on dust in dense clouds has evolved dramatically. Results from Spitzer built on those from ISO, in tandem with laboratory work designed to support these missions, and this progression is set to continue as observations enabled by the enhanced capabilities of the JWST build on results from ISO and Spitzer. Some prior discoveries have already been mentioned: we have gained insight into not only the composition and structure of the ices but also how they evolve with changing conditions as clouds collapse and begin to form stars. As they are warmed, the ices undergo selective sublimation, segregation and annealing. Some ice-mantled grains accrete directly into icy planetesimals in protoplanetary disks, whilst others sublimate and recondense. Significantly, all such volatiles retain an isotopic signature of their low-temperature origins, in the form of greatly enhanced D/H ratios.

And this leads to another point. Studies of interstellar matter and primitive solar-system materials seem to have converged in recent decades, to the benefit of both fields. Enhanced D/H ratios and other isotopic anomalies are detected in reservoirs that range from comets and asteroids to the Earth's ocean water, suggestive of an astrochemical heritage. The presence of presolar grains in meteorites has been known since the 1980s, but more recent refinements in laboratory techniques have greatly enhanced our ability to detect and study them. These particles are the oldest materials ever studied in a terrestrial laboratory! They carry isotopic signatures of the stars that produced their elements, and chemical memories of the conditions in which they condensed. The results open new windows on nucleosynthesis processes and yield direct evidence on the nature of at least some fraction of the interstellar grain population. Meanwhile, rendezvous missions such as Stardust and Rosetta have provided new information on the nature of cometary dust and ices, including the first detection of an amino acid in a comet.

You have had a life-long interest in astrobiology and headed the New York Center for Astrobiology, and you have also written a textbook on Origins of Life.

Many decades ago, as a student with ambitions to be an “astronomer”, I was advised to focus my studies on physics and mathematics, as nothing else in science was deemed to be of much relevance. It has been fascinating to observe how astronomy has evolved since then, from a rather esoteric field to an interdisciplinary one in which the “astro” prefix is routinely attached to chemistry and biology, as well as to physics. This trend appears to have been instigated by a rapid growth in the inventory of known interstellar molecules, and sustained by the discovery of exoplanets, especially earth-analogs, with the possibility to detect biosignatures in their atmospheres. Simultaneously, our understanding of the processes that forged our own planetary system are being clarified by research programs that range from analyses of extinct radionuclides in meteorites to cometary and asteroidal rendezvous missions and high-resolution spatial and spectral imaging of protoplanetary disks around potential early solar analogs. Research on dust is emblematic of this trend: as the bearer of elements and compounds from which planetary systems are made, it is pertinent to all aspects of science, so this was a natural connection for me.