Q: During your 60 year career in Astronomy, you have worked on the full range of stellar evolution, from molecular clouds and star formation, young stars, main sequence stars, to post-main sequence stars (first ascent giants, AGB stars, planetary nebulae, white dwarfs), plus some of your papers consider advanced technological civilizations. Is there a common denominator that has led to this range of explorations?

A: My primary interest in astronomy, beginning when I was in grade school, is life in the Universe. In an autobiography I wrote as I finished sixth grade I said that I wanted to be either an astronomer or a farmer. Connection to life in the Universe was always in the back of my mind in whatever stage of stellar evolution I happened to be researching. However, since this interview is for a star formation newsletter, let’s focus mainly on that.

Q: You spent the first 15 or so years of your research career in radio astronomy. How did that come about?

A: I began my research in astronomy in 1965 as a graduate student at Harvard University. Senior Research Astronomer Alan Maxwell apparently thought I was floundering, so he steered me to the radio astronomical research group of Professor A. Edward Lilley. Ed’s graduate student Ellen Gundermann had just discovered OH maser emission as part of her Ph.D. thesis research and Hoglund & Mezger had announced in Science the first convincing detection of a radio recombination line (n = 110 $\rightarrow$ 109) from Galactic HII regions. So spectral line radio astronomy was hopping and I was glad to become part of it. During the 1970s discovery and study of interstellar and circumstellar molecules was rather akin to discovery and study of extrasolar planets now.

Q: As regards star formation, what was the observational situation at the onset of your career in the mid-1960’s?

A: In 1965 what we knew about star formation came primarily from theory, there was little observational data. Bart Bok (my academic grandfather) found isolated dark clouds (‘Bok globules’) that could be progenitors to stars, and George Herbig had identified numerous T Tauri stars associated with dark clouds, but the interstellar dust grains placed strict limits on what these optical astronomers could hope to accomplish on the actual star formation process. I remember how people would talk of the North America Nebula as a site of recent star formation, but it was only after infrared and radio telescopes were pointed there did we all fully appreciate that current star formation was taking place in the dark ‘Gulf of Mexico’ and not in the famous glowing nebula itself.

Q: As observations of star forming regions began to catch up with and eventually overtake theory, what were some of the problems that you were wrestling with?

A: To answer this question, I took a look at my 1974 ARAA article with Pat Palmer. I was only 30 when we wrote that article on interstellar molecules and recall thinking how thankful I was that Alan Maxwell had steered me to such a blossoming place in modern astronomy.

In the late 1960s the new field of infrared astronomy, along with maser emission from OH and water molecules, enabled astronomers to peer into the depths of dark clouds — some of which were found to be much more massive than any known previously — right at protostars. Maser sizes could be measured with long baseline interferometers that were just coming into their own then.

My 1968 Ph.D. thesis was a study of thermal and maser OH emission; the most significant discovery was of OH maser emission from short-lived energy levels 100s of degrees above zero Kelvin. This was the first detection of interstellar gas for which a hot, dense, environment — such as that near protostars — was essential.

Q: You have been interested in mass flows since the early days of this subject.

A: Tom and Eva Kuiper and I wrote the earliest published paper on an observed, high velocity, molecular outflow, focusing on the Orion molecular cloud behind the Orion Nebula. In 1973 I wrote a paper titled ‘A Model of the Orion Nebula’ that married HII regions and gas flows from massive molecular clouds. This model, that was dubbed by others the ‘blister’ or ‘Champagne’ model, explained, for the first time, the structure of the Orion Nebula. To some degree, HII blisters have replaced the classical idea of HII region Strömgren spheres. My paper was generally well received and it soon earned me an all-expenses paid trip to a memorable meeting at a ski resort in the Austrian Alps. I was greatly impressed by how much beer some astronomers could consume without sliding under the table; Nick Scoville and Eric Becklin come to mind. BTW, the essence of my Orion model was conceived while I was running up a hill alongside the campus of the University of California Berkeley.

Q: One major early question dealt with mass flows involving entire massive molecular clouds, not just portions of them. How did your research relate to this issue?

A: The interior of the Grand Canyon is my favorite place on Earth. So perhaps it is fitting that one of my favorite papers of my research career was hatched while I was hiking in 1973 in the Canyon with my wife. The issue at hand was how to explain the large velocity widths of molecular lines seen coming from giant molecular clouds. Some very smart astronomers, including theorist Peter Goldreich and an observational team led by Arno Penzias and Bob Wilson, apparently preferred overall free-fall collapse of the clouds due to their self-gravity. Peter and his graduate student John Kwan wrote a lovely paper explaining the line profiles in this way.

But I was unhappy with this model and, while walking in the Canyon, hit upon some problems with free-fall collapse. At that time I was a visiting researcher at UC Berkeley and was working with Neal Evans who was Charlie Townes’ graduate student. Neal and I published a 1974 paper ‘Models of Massive Molecular Clouds’ that contained three principal conclusions: (1) if massive molecular clouds were collapsing at all, then it was at velocities much less than free-fall, (2) turbulence was the likely mechanism that was slowing the collapse and (3) ‘the clouds are very nonuniform and resemble a piece of Swiss cheese with many holes and little cheese’.

Conclusion #1 was accepted quickly by the star formation community. The same could not be said for conclusion #2. The unusual citation history of our paper illustrates the prevailing ideas in the field over the next 50 years, right up to the present day. For decades the favored mechanism to slow cloud collapse was interstellar magnetism and our paper was hardly cited. But turbulence made a comeback and the situation is still hotly debated today. As regards conclusion #3, we stated that cloud structure could be ‘checked by complete maps of $^{13}$CO extending to positions far from the center of a cloud’. It took some years for the spatial resolution of radio telescopes to match the angular scale present in giant molecular clouds. But now we know that, typically, molecular clouds are filamentary where dense filaments fill not much of the volume of a cloud. Our paper continues to be cited regularly 50 years after publication.

Q: In the late 1970s and early 1980s you drifted away from the study of interstellar molecular clouds and away from radio astronomy. Why?

A: Some first-rate astronomers have spent their entire careers focused on one area of astronomy. But after a decade or so of radio study of interstellar molecules and star formation my attention began to wane and I first shifted into millimeter wavelength study of mass loss from red giant stars and planetary nebulae and then into high-resolution (speckle) infrared astronomy as it was just beginning.

Another decade passed and I became interested in searching for brown dwarfs in the infrared. In the late 1980s Eric Becklin and I found the first identified L-type dwarf object GD 165B (which might or might not be the first brown dwarf to be discovered). For a brief while we thought we had also found a brown dwarf in orbit around the white dwarf G29-38. But James Graham and colleagues soon demonstrated convincingly that the excess infrared emission was due to heated dust rather than a brown dwarf companion. At the time I was sorely disappointed. But now I’m at peace, because the infrared excess at G29-38 was the very first salvo in an ongoing story that has by now revealed the presence of extensive planetary systems in orbit around at least 25% of all white dwarfs.

In addition to white dwarf planetary systems, my principal interests during the past two decades have been infrared investigation of dusty (and gaseous) debris disks at nearby main sequence stars, direct high-resolution infrared imaging of extrasolar planets, and identification of young stars near the Sun — such stars are an essential component of all direct imaging searches for recently born planets. Since these young stars have ages from 10 to 100 Myr they represent the very last stage of star and planet formation and, thus, I’ve come almost full circle back to my roots in astronomy.

Q: Please elaborate on the topic of young stars near Earth.

A: The early history of the field of young stars near Earth is given in my 2004 ARAA article written with Inseok Song. Here I’ll mention a few important early highlights along with a few discoveries I’ve been involved with during the past 20 years.

As mentioned earlier, George Herbig and others had identified numerous T Tauri stars associated with dark clouds. But these clouds are at least 130 pc or so from Earth. In 1978 Herbig called attention to one T Tauri star, TW Hya, located much closer to Earth, but for years no one, including Herbig, seemed to know what to do with this star — just what was it doing there?

Around 1990, a Brazilian group led by Carlos Torres identified four other T Tauri stars in the general vicinity of TW Hya and they argued that, therefore, it was not likely to be a ‘runaway’ star. They considered the existence of a T Tauri association as more probable. Papers in 1997 and 1999, led by my PhD students Joel Kastner and Richard Webb, conclusively demonstrated the existence of a 10 Myr old stellar association — now called the TW Hya Association — only 60 pc or so from Earth.

During the years that followed, the Brazilian team and my colleagues and I carried out a friendly competition to identify other young moving groups (stellar associations) near the Sun. As noted earlier, young nearby stars are an essential component of all direct infrared imaging searches for extrasolar planets and will remain so in the coming era of giant ground-based telescopes.

Nearby, young, moving groups listed in the 2004 ARAA article include the four most important: TW Hya, Tuc/Hor, AB Dor, and beta Pic. Since then other associations have been found or proposed; of these I’ve written about Carina-Near, Octans-Near, and Argus, as well as $\chi^{1}$ For, at $\sim$ 40 Myr the second youngest open cluster located within $\sim$ 100 pc of the Sun. $\chi^{1}$ For appears to be closely related to the major Tuc/Hor and Columba Associations that are spread out over much of the southern sky.

Q: You mentioned the importance of young nearby stars for direct imaging searches for exoplanets. Have you been involved in such searches?

A: It seems as if every four years or so I’ve been fortunate to be involved with a notable discovery at these young stars. In 2004 our French-American team reported the first object of planetary mass (2M1207b) to be imaged in orbit around an object not our Sun. In 2008 a team with a substantial UCLA connection obtained the first image of a multiplanet extrasolar planetary system (at HR 8799). In 2012 my former graduate student Carl Melis led our team as we watched a massive extrasolar zodiacal-cloud analog mysteriously vanish in a space of only 1.5 years. And in 2015 I was part of the team that used the Gemini Planet Imager to discover a Jupiter-like planet at a projected separation of 13 au from the star 51 Eri, a member of the beta Pic moving group. In more recent years the major discoveries I have been involved with entail evolved stars (white dwarfs) rather than youthful stars

Q: You have written a number of ARAA articles. Have these tracked your changing astronomical interests?

A: I’ve had 4 ARAA articles between 1974 and 2004 and am not anxious to write any more; they are a lot of work! I’m happy to say that the first one, on interstellar molecules, has had some legs and is still being cited. My 1980 article on mass loss from red giants appeared shortly before the launch of IRAS, which revolutionized our knowledge of the field, and soon made my article obsolete. The third ARAA article, on dusty circumstellar disks, appeared in 2001 and was in large part a tribute to IRAS. The article appeared shortly before the launch of Spitzer whose results transformed the field of debris disks and again made my article mostly obsolete — a bit of déjà vu. As noted earlier, the 2004 article is about young stars near the Sun.

Q: In a 2012 paper, published with Inseok Song, you discuss two 30-40 Myr old stars with gas-rich circumstellar disks and suggest they are evidence for cometary collisions in massive Kuiper Belts. Are these pathological cases, or examples of a more general phenomenon?

A: A 1995 Nature paper I wrote with Thierry Forveille and my former student Joel Kastner reported (among other things) CO rotational emission from the A-type star 49 Ceti. As regards CO emission, for 16 years 49 Ceti was unique and its age, and the nature of the emission, mysterious. Some researchers suggested an age of $<$ 10 Myr and a star in transition between a protoplanetary and a debris disk.

In our 2012 paper, Inseok and I figured out that 49 Ceti is $\sim$ 40 Myr old; thus too old to still retain a protoplanetary disk. We suggested a model where 49 Ceti and the A-type star HD 21997, found in 2011 to have CO emission, are surrounded by massive comet clouds that contain growing Pluto-like objects that dynamically excite CO- and CO$_2$-rich comets into collisions that release the CO and CO$_2$ (that is then photodissociated to CO). The CO is then photodissociated to C and the C is photoionized; the electrons so released can excite the CO rotational levels. In the years that followed, ALMA observations by other astronomers revealed other examples of gas in debris disks and included the importance of atomic species, e.g., C, in shielding CO against rapid photodissociation.

Q: Following retirement from UCLA, what have been your astronomical interests?

As we grow older we evolve and so have my principal interests; these now focus on evolved stars, specifically planetary systems around white dwarfs. All but one of the various techniques now used for study of extrasolar planets — direct imaging, transits, precision radial velocities, astrometry, and microlensing — were conceived of theoretically before they were employed observationally. The one exception is spectroscopy of white dwarf stars.

The surprising realization that this technique could be used to discover extrasolar planetary systems came 90 or so years after the first relevant observations; these were the pre-1920 spectroscopy of the white dwarf van Maanen 2, by Adriaan van Maanen. White dwarf planetary systems inform us about various realms largely inaccessible by study of main sequence stars. Perhaps most important is precise information of the bulk elemental composition of rocky exoplanets — from white dwarf studies we now know that, typically, rocky exoplanet composition is earth-like.

My interest in life in the Universe, especially technological life, has not evolved but has remained steadfast. The title of my 2002 article in Mercury magazine — “Why SETI Will Fail” — speaks for itself. But the arguments given there, that technological life is very uncommon, has not stopped me from looking for it. Twenty years later, in 2022, my article “Infrared and optical detectability of Dyson spheres at white dwarf stars” was published in MNRAS.