An interview with Steve Beckwith

Interview by Bo Reipurth, SFN #346 - October 2021

Your PhD in 1978 dealt with observations of interstellar molecular hydrogen emission. Who was your advisor, and what were your main results?

My advisor was Gerry Neugebauer, an early pioneer of infrared astronomy. Gerry ran a lab where we were all expected to build instruments to use on the telescopes at Mount Wilson and Palomar Observatories for our research. Gerry was in the physics department, as were his students, and we were regarded as interlopers by the astronomers, most of whom used instruments built by other people. But it seemed clear to me that building new observational capabilities was the path to discovery, and the whole process was enormously appealing: designing and using my own instruments on large telescopes to explore the skies. It attracted me to astronomy.

Gary Grasdalen and Dick Joyce reported the first near-infrared observations of molecular hydrogen emission from the Orion Nebula in 1976. It was that novel discovery that got me interested to follow up with new equipment. I converted an old optical Ebert-Fastie spectrometer to work in the 2 μm window and used it on the 100 inch Hooker telescope at Mount Wilson in the same year. Eric Persson, Eric Becklin, along with some of my lab mates and I made the first follow up observations of Orion that winter and discovered that the emission was much more extended than the initial reports. It looked bipolar and centered somewhere near the BN/KL objects in the dark nebula. Our subsequent work with that instrument revealed molecular hydrogen emission from a wide variety of objects including star forming regions like the Orion Nebula, T Tauri stars, and even planetary nebulae.

The H₂ emission comes from high temperature regions behind shock fronts in the molecular clouds surrounding the young objects like BN/KL. Hot molecular hydrogen is one of several indicators of violent mass outflow from the regions near these young stars as part of a process that allows them to accrete material as they grow and also shed the angular momentum of the infalling gas. Modern images of this hot gas show incredibly well organized structures including sharply collimated bipolar jets as well as shocked trajectories of bullet-shaped ejecta along with broad shock fronts.

You and your collaborators observed HL Tau in 1984 with near-infrared speckle interferometry and discovered a solar system-size halo around it. What was known then about the HL Tau environment and what did you conclude from these data?

HL Tau was one of a handful of young stars with variable light emission and complicated patterns of light that indicated structure in the dust clouds surrounding the stars. Ben Zuckerman, Mike Skrutskie, Mel Dyck and I looked at HL Tau by scanning the image across a slit photometer at high speed and taking discrete Fourier transforms of the photometric time series to measure the one-dimensional speckle patterns. This was early speckle interferometry at infrared wavelengths with single detector photometers. These patterns contained enough information to tell us the spatial extent of the light from the star in different directions and at different wavelengths.

The spatial extent and blue (in the infrared) color of the infrared light from HL Tau told us that we were looking at scattered light from small particles surrounding the star. The dust distribution is almost twice as extended in one direction as in the perpendicular direction. We could estimate the amount of dust needed to produce the scattered light, and it was enough to block the starlight. But the star was relatively unobscured, so we concluded the dust must be confined to an extended region that was compact in at least one dimension not in the line of sight unlike a spherical cloud. The simplest stable configuration is a disk where the dust particles orbit the star in a plane tilted to the line of sight. Our analysis of the amount of material needed to scatter the light in the disk showed enough to build a planetary system like our own. It was one of the very first indications to support the idea that the Solar System resulted from a disk, a very old notion first put forward by Immanuel Kant in the 18$^{\rm th}$ century.

A few years later you presented the first mm interferometric observations of HL Tau.

Anneila Sargent and I were old friends from graduate school; we both chose non-traditional paths in astronomy, she in millimeter wave astronomy, and I in infrared astronomy. We were chatting about the speckle interferometry results on HL Tau and realized that millimeter wave observations might detect both the gas and the dust and even reveal an image of the distribution. She was playing a key role in making the very new Owens Valley Millimeter Interferometer into an observatory, and she offered to look at HL Tau with that new interferometer. Sure enough we saw both CO emission from the gas and continuum emission from the dust in the disk surrounding the star. It is another example of how discoveries in astronomy are enabled by using new instruments.

Both the gas and continuum measurements confirmed that there was enough solid material to be a pre-planetary disk. The gas velocities were low indicating gas bound to the star as opposed to streaming out as part of an outflow.

The next year, we used an improved version of the interferometer in five different configurations to make a map of the gas emission at better angular resolution and at several different velocities demonstrating an elongated disk-like structure with a velocity field consistent with Keplerian rotation. The continuum emission while unresolved was at much higher signal-to-noise and reaffirmed our belief that the solid matter was sufficient to create planets like those in the Solar System.

In 1990 came your famous paper with Anneila Sargent and collaborators, cited more than 1500 times, which describes a survey for circumstellar disks around young stars. What were the key results?

Once we understood the power of millimeter continuum radiation to reveal the presence of solid particles orbiting these young stars, we proposed to use a bolometer on a large single dish millimeter telescope built in Spain by the Max-Planck-Institut für Radioastronomie to look at a large number of stars with much greater sensitivity than we had for HL Tau. We looked at 86 young T Tauri stars and detected emission from 42% of them, an astonishingly large fraction. We estimated the dust mass using the same kind of analysis we had used for HL Tau and concluded that most had sufficient mass to create planetary systems like our own. All had enough mass to create terrestrial-mass planets.

This was the first evidence that the precursors to planetary systems commonly occurred around young, solar-mass stars. It implied that if disks evolved to planetary systems, extrasolar planetary systems were very common in the Galaxy. It was five years before the first extrasolar planet would be detected by Michel Mayor and Didier Queloz who won the 2019 Nobel Prize in Physics for their discovery.

We now know from the large number of extrasolar planetary systems detected by the Kepler satellite and other facilities that our early conclusion was correct.

A year later you and Anneila followed up with a study of particle emissivity in circumstellar disks.

We knew that a next step was to find evidence that the dust was actually building planets. In those days, there were not many ways to do that. But we already had a hint by comparing the 1.3 mm and 2.7 mm continuum results from the survey observations that the emissivity of the dust at these long wavelengths was flatter (it fell more slowly from 1.3 mm to 2.7 mm) than expected for interstellar dust particles that are much, much smaller than the millimeter wavelengths. So we used another bolometer detector on the Caltech millimeter dish in Hawaii and set out to measure the millimeter colors of the dust particles around 29 of the brightest of the disks.

We discovered that the particle emissivity was generally much flatter than expected for small spherical particles. This finding implied that the particles orbiting the young stars were different from typical interstellar dust particles. The natural interpretation was to say the particles were larger; in other words, there was evidence for the growth of large solids in these disks. That was exciting because it was evidence for the early stages of planet formation.

In a subsequent study with Anneila you discussed CO transitions towards the disks of HL Tau and two other young stars in terms of temperature vs mass distributions. What did you conclude?

CO is often optically thick and therefore not very sensitive to the total mass. We decided to observe several isotopes of CO going from the common ¹²C¹⁶O to the rarer isotopes ¹³C¹⁶O and ¹²C¹⁸O in an effort to find optically thin emission. We found, however, that the lower rotational transitions in all these isotopes were optically thick, and the emission depended only on the temperature of the gas and the size of the regions.

Those temperatures were higher than what we expected for gas at large distances from the stars: the disks were more than 400 AU in extent. That result puzzled us. Shortly thereafter, Eugene Chiang and Peter Goldreich provided what is now the definitive explanation by showing how the optical light from the star is absorbed at large distances by dust well above the mid-plane, but the infrared radiation is trapped by the vertical structure in the disks (a kind of greenhouse effect), and the temperatures we inferred were just to be expected if there was an extended disk with some vertical structure.

For 'Protostars and Planets IV' you, with Thomas Henning and Yoshitsugu Nakagawa, wrote a review on dust properties and assembly of large particles in protoplanetary disks. What do you see as the key developments in the 20 years since then?

I remember that review as the last work I intended to do on circumstellar disks. Bob O'Dell's images of disk silhouettes around young stars in Orion put to rest any doubts about the existence of dust disks of the right size and shape to be pre-planetary. We and others had done about as well with the existing technology as possible, and it seemed to me that the next major advance would have to come from much more capable facilities.

Since then, the Atacama Large Millimeter Array came on line and revolutionized the study of these disks. ALMA routinely synthesizes images that resolve the substructure in the disks, including gaps and rings and all sorts of interesting structures at millimeter and submillimeter wavelengths. The molecules provide good maps of the velocity fields, and the continuum emission shows where the solid material is and provides a good estimate of the mass. The data are unbelievably rich. It is clear now that these disks create planets.

At about the time of that review, the first observations of extrasolar planets were coming out. In the intervening years, the detection of exoplanets has become a cottage industry revealing the ubiquity, variety, and remarkable robustness of planetary systems around stars in the Galaxy.

I did not imagine then the enormous progress that would be made in this field during the next 20 years, especially the last decade. It is tremendously gratifying to see how many young scientists have taken the early work we did and advanced our understanding well beyond what we thought possible in the 1990's.

You have been investing much time and effort in running large organizations: Director of the Max-Planck- Institut für Astronomie (MPIA), Director of the Space Telescope Science Institute (STScI), Vice President for Research for the UC system, and now Director of Berkeley's Space Sciences Lab (SSL). What have been some of the personal highlights of these efforts?

My move into leadership positions happened by accident. I learned in graduate school to think of scientific achievement largely as individual efforts. Indeed, all of the usual reward systems -- hiring and promotion, tenure, prizes and awards, membership in prestigious societies -- rely on the assessment of an individual's contribution, especially for creativity, originality, and scientific taste; people who went into management or administration were often denigrated by my mentors, at least in private. And it was certainly true when I was a professor at Cornell that my achievements were assessed for my individual contributions.

The move from Cornell to the MPIA came about because my interest was in building instrumentation, and the cost of infrared detectors by the end of the 1980's had grown beyond what typical NSF grants would fund. The MPIA offered me an order of magnitude more monetary and infrastructure support than I could get at Cornell. The Max Planck society also stressed individual achievement centered on its directors who were given enormous resources.

When I got to Heidelberg, I saw an institute full of scientists whose performance was well under what I would have expected from the number and the quality of the people. It seemed to me that the real power I had as a Director was to change the organization to give many more people the ability to succeed, which basically meant dedicating some of my time to supporting other people's science. So I set about to do that, and we tripled the output and impact of the MPIA during the seven years I was there, the vast majority of contributions coming from other scientists. I found that personally very gratifying.

STScI was a well-oiled machine. The few organizational problems were baked into the culture, and I did not make many improvements, although I was able to get new funding for an infrared camera that would prove crucial to winning support for continuing Hubble's lifetime. A few years after I arrived, however, NASA decided to cancel the final servicing mission to the Hubble Space Telescope putting an end to its science, and I had the opportunity to get that decision reversed. Helping to reverse that decision 16 years ago enabled far more important research in astronomy than I could ever hope to accomplish as an individual.

The UC Vice President's job was an incredible opportunity to learn about research across all fields of research not just astronomy. I had a chance to open up a number of new opportunities for researchers around the system in areas as diverse as cancer research and digital humanities. I acquired a deep interest and some expertise in the biosciences and got to grapple with issues where research collides with societal values (look up NAGPRA). That work gave me a perspective on how we need to see our research in the context of a larger society.

As Director of Berkeley's Space Sciences Lab, my personal evolution has come full circle. All of my efforts go toward supporting the research of other people, mostly young people. It is tremendously gratifying. Over the last three years, SSL has tripled its research spending on campus, and we are poised to step up to larger projects and new directions in instrumentation such as using constellations of small satellites to replace enterprise missions. Ironically, Berkeley has no way of rewarding me for this effort, because as a professor of astronomy their systems only recognize individual research contributions that I am no longer making.

The pride in making other people successful now has replaced the pride I used to take in my individual accomplishments. I encourage anyone who is thinking about going down the same path to recognize that personal satisfaction in science can come in many ways and not to be dissuaded from taking on leadership roles if they think their talents will let them help gifted people contribute more than they otherwise would have without the efforts of leadership.