An interview with Joan Najita

Interview by Bo Reipurth, SFN #339 - March 2021

Your dissertation from 1992 dealt with magnetocentrifugally driven winds from rapidly rotating protostars, with Frank Shu as your advisor. Between 1994 and 1996 you and Frank and other collaborators wrote a very influential series of papers on magnetocentrifugally driven flows. Please tell us about this project.

In 1986, when I started out as a graduate student at UC Berkeley, brilliant people were putting together the basic framework of star formation that we know well today. Molecular outflows and jets were known to accompany the birth of young stars, and their origin was a topic of great debate: what mechanism could account for the high momentum and collimated nature of the flows? A more general question was how stars could continue accreting from a disk and grow to Sun-like masses without running up against an angular momentum barrier that prevented further accretion. Because disk material has high specific angular momentum, stars would have to eject angular momentum in the accretion process in order to continue to accept more mass. The ''X-wind'' picture addressed both questions. It hypothesized that protostars would generate strong stellar magnetic fields, and the coupling of stellar fields to disk rotation would launch powerful winds that remove angular momentum and drive molecular outflows. The idea is so elegant. But it was a really tough problem, especially as a student. It was my great fortune to work with a brilliant, generous advisor. I also learned a lot about the role of persistence in research!

You soon switched to observational problems and wrote a paper with John Carr and Alan Tokunaga on CO overtone emission that provided evidence for rotating disks around young stars.

Having made our X-wind models, Frank and I were hoping to test our new theory by searching for observational evidence of these flows. Al Glassgold had come to Berkeley on sabbatical, and his analysis suggested that the CO overtone transitions could potentially probe the wind acceleration region. Meanwhile, John Carr had already reported CO overtone emission from young stars, and he and Alan Tokunaga were building a new high-resolution infrared spectrometer for the IRTF (CSHELL) to investigate the nature of the emission. We teamed up, hoping to find evidence of X-winds. Instead, we discovered beautiful evidence for rotating disks in the near-stellar environment of young stars. This was back in the day when people still wondered whether disks really existed -- a situation that may be difficult to imagine today. Yes, we knew about the Orion proplyds, which certainly looked like disks, but were they rotationally supported? The search for rotating disks has a celebrated history at millimeter wavelengths, where people were aiming to detect disks hundreds of au in size. But people also wondered whether rotating disks were present on the size scale of the inner Solar System. Observations of FU Ori objects led the way in showing evidence of disk rotation during outburst, but were rotating disks also present around more typical young stars? The CO overtone observations said yes. This project was also the beginning of a long theory collaboration with my wonderful colleague and mentor Al Glassgold (and later with Mate Ádámkovics) on the ionization, thermal, and chemical properties of inner disks. Our work together showed the importance of stellar X-rays for disk ionization and guided the study and interpretation of many disk diagnostics, both molecular (CO, water, OH, simple organics) and atomic (e.g., NeII, NeIII).

In 2003 you, with John Carr and Bob Mathieu, published a highly cited paper demonstrating the importance of CO fundamental emission as a probe of the planet-forming regions of circumstellar disks. What did you learn?

With CSHELL now in operation, we (John, Alan, and I) explored the near-infrared, advantaged by its sensitivity while also hampered by its small spectral window, only ~1000 km/s wide. Observing runs were like a rushed visit to a new continent with blinders on and equipped only with a telephoto lens. Sure, you could see things at high magnification, but only in a tiny field of view. Where would we look first? And how would we put the tiny scenes together to understand the landscape? It was nerve-wracking and exciting, with success depending on imagination, hope, and good luck with the weather! Over several years, we studied not only CO overtone emission but also Br𝛄 emission, which turns out to be a useful way to study magnetospheric accretion in the youngest (i.e., extincted) stars. In the K band, we discovered water emission from disk atmospheres, which we used to demonstrate differential rotation in disks, echoing the earlier work by Kenyon and Hartmann on FU Ori disks. At even longer wavelengths, we (John, Bob Mathieu, and I) made the exciting detection of 4.7 micron CO fundamental emission from disks. The discovery was exciting because -- unlike the CO overtone and near-infrared water emission from disks, which are typically restricted to high accretion rate sources -- CO fundamental emission is very common among T Tauri stars, a result of the high A-value of those transitions. The emission also probes a wider range of disk radii, extending over the terrestrial planet region of disks. It gives us a look at the thermal and dynamical conditions of disks at the radii where many (e.g., Kepler) exoplanets are now known to reside. As a reliable probe of the terrestrial planet region, I imagined that CO fundamental emission would one day help us search for dynamical evidence of forming giant planets through the gaps they create in disks. Moreover, because only a small column density of CO is needed to produce detectable fundamental emission, it seemed that the emission could potentially probe the gas dissipation timescale in the terrestrial planet region; this topic was later taken up in a paper led by Greg Doppmann. I never imagined that CO fundamental emission could also be used to find orbiting giant planets by the molecular emission produced by their circumplanetary disks! Inspired by a hint from the theoretical literature, this topic was pursued in a series of papers with Sean Brittain and John Carr.

Another highly cited study was a statistical investigation of the properties of transition objects.

The launch of the Spitzer Space Telescope opened another new window on the planet formation region of disks. It also led to the opportunity to carry out a scientific investigation with my long-time colleague and friend Steve Strom. Steve and I were both intrigued by the new Spitzer results on transition disks, a topic that Steve and his colleagues had written about (presciently) in 1989 as objects that might be in the process of forming planets. The new, detailed spectral energy distributions of transition disks measured with Spitzer were now hotly debated as those of either systems that have formed giant planets or, alternatively, disks that are photoevaporating away. On a lark, I happened to plot measured disk accretion rates against disk masses for T Tauri stars in Taurus and noticed a locus of discrepant objects with unusually low accretion rates for their disk masses; the locus turned out to be populated by transition disks! Their properties -- low disk accretion rates and high disk masses -- were in fact those expected theoretically for disks that are forming giant planets. The simple plot was a new (demographic) way to examine the nature of transition objects, and it strongly favored the giant planet interpretation. The recent discovery of orbiting, potentially planetary objects in the central cavities of transition objects like PDS 70 appears to bear out this interpretation. The exploration of gaseous disk atmospheres also advanced considerably with the launch of Spitzer. The audacious attempt, led by John Carr, at high (~300) signal-to-noise spectroscopy with IRS led to the thrilling discovery that T Tauri disks commonly show emission from water and organic molecules, the building blocks of life, in the terrestrial planet region. Our discovery eventually led to a more extensive study (with Colette Salyk, Klaus Pontoppidan, Ewine van Dishoeck, and Geoff Blake) of a larger sample of T Tauri stars. The Spitzer data provided a rare insight into the planet formation status of disks. One day I tried to predict the result of water sequestration beyond the snowline due to the formation of planetesimals and larger bodies that are large enough to stop migrating and drop out of the accretion flow. It seemed plausible that this process would dehydrate the inner disk and raise its C/O ratio. With planet formation anticipated to advance quicker in more massive disks, we might expect the chemical signs of a higher C/O ratio in the inner regions of more massive disks. Plotting up our results, I was surprised to find the expected increasing trend of HCN/water emission strength with millimeter continuum emission! By combining the observations with our models of gaseous disk atmospheres, we showed, in a series of papers from 2011 to 2018, that the correlation could be the chemical fingerprint of the growth of large bodies (kilometer-size and larger) in disks -- evidence that planet formation begins much earlier than previously thought. This idea found surprising confirmation a few years later in a study that took a very different tack.

In a 2014 paper, you and Scott Kenyon explored the time-scale and efficiency of planet formation. Please elaborate on your methodology.

On a sabbatical visit to the CfA, I had the opportunity to reconnect with my friend and colleague Scott Kenyon and catch up on new results. Exoplanet surveys carried out with the Kepler Space Telescope and microlensing were reporting that super-Earths and Neptunes were very common as companions to Sun-like stars. The news was exciting, but also troubling because complete surveys of protoplanetary disk masses, also just reported from the CfA by Sean Andrews and colleagues, seemed to show that most disks had only limited reservoirs of dust from which to build these planets. Curious about what this meant, Scott and I dug deeper and inventoried the solids known to have been "left behind'' -- in exoplanet populations and parent bodies of debris disks -- and compared these with the inventory of small solids in protoplanetary disks. The comparison showed that the amount of solids needed to explain the known exoplanets and debris disks surpass the small solids present in Class II disks. That is, the objects that we had been calling "protoplanetary'' disks as if they were just about to begin the planet formation process, are actually highly evolved, having already converted most of their solids into planetesimals and possibly planets. In other words, planet formation really begins before the Class II phase, in the Class I phase, as the star assembles its mass. Planet formation and star formation are intertwined processes, twin stories that unfold on the same timescale, sharing the same disk resources. Although it was unconventional at the time, the idea that planet formation is well underway in the Class II phase seemed quite plausible because of our earlier disk chemistry results on the HCN/water ratio in disks. This perspective was confirmed in breathtaking fashion a couple of years later, with the famous ALMA image of HL Tau. A very young Class II object, its disk showed a delicate sequence of nested gaps and rings that attest to its evolved nature. Such gaps and rings, which are expected to accompany the birth of orbiting planets, are now known to be common features of large disks. While the ALMA results sweep away our old conceptions and make plain that Class II disks are hardly the primordial structures we once imagined them to be, it's also amusing, and satisfying, that such a dramatic discovery could be foreshadowed by simple analyses, like inventories of disk and exoplanet solids or emission feature ratios from mid-infrared spectra of disks.

Recently you and Ted Bergin have studied protoplanetary disk sizes as a probe into the mechanisms of angular momentum transport. What did you conclude?

When Ted and I attended a KITP workshop a couple of years ago on MHD processes in disks, it was clear that the old paradigm of accretion in Class II disks had undergone a major phase transition: theorists described how the old picture of disk angular momentum transport via the magnetorotational instability (MRI) could not account for the observed accretion rates of T Tauri stars, a consequence of insufficient disk ionization. Moreover, observers reported that T Tauri disks lacked the signature of turbulence expected from an MRI-active disk. The MRI was, therefore, out the window, and angular momentum removal by magnetized disk winds was the new favored mechanism. People were actively looking for evidence of disk winds to support this picture. Wondering how we could test the new paradigm in a different way, I thought about how the old and new pictures make different predictions for the evolution of the sizes of gaseous disks. When angular momentum is redistributed within the disk, as in the MRI, disks must spread as some material acquires high specific angular momentum, which allows accretion to occur. If angular momentum is removed from the disk, as in a disk wind, disks need not grow in size but can in fact shrink with time. The basic difference between these two pictures suggested a simple way to gain new insight into how disk accretion works: if disks grow in size from the Class I phase to the Class II phase, that would be evidence for “in-disk” angular momentum transport (i.e., redistribution within the disk), but if disks stay the same or shrink in size, that would be consistent with angular momentum removal in a wind. Class II gas disk sizes have, of course, been measured for decades, and luckily, Class I gas disk sizes were becoming more readily available at that time. So we compared the disk sizes, and the Class I disks appeared markedly smaller, suggesting that some “in-disk” angular momentum transport mechanism plays an important role in the Class II phase. Since the MRI is now apparently untenable, the nature of that mechanism is currently unknown, a new mystery! My colleagues and I recently reported observations that may help to identify the mechanism: spectroscopic evidence for a supersonic “surface accretion flow'' in a Class I disk that carries a T Tauri-like accretion rate. Such flows are predicted in simulations of magnetized disks.

At the Protostar and Planet V conference back in 2005 you gave a review of gaseous inner disks. What have been the key developments in this area since then, and what are the critical issues still remaining?

Behind that review article was a simple hope: that the early discoveries of CO and water emission from inner disks would open a new window onto disk evolution and we would, as a community, eventually develop a toolkit of atomic and molecular diagnostics of the planet formation region of disks. We've made a lot of progress, first in the near-infrared (e.g., with CSHELL) and the UV (with HST), and more recently in the mid-infrared (with Spitzer and ground-based spectrometers). The diagnostics have been used to investigate well-known questions, as well as topics never imagined. They've been used to measure disk truncation radii created by stellar magnetospheres and orbiting planets, probe the gas dissipation timescale of disks, find evidence for early planetesimal formation and orbiting circumplanetary disks, and more. I imagine we're about to learn a lot more with JWST, which will be a lot of fun. But even beyond that, there's so much ahead! One thing I love about studying planet formation and disks right now is that there is so much we don't know. All of the questions aren't known, much less their solutions. There is tremendous opportunity and freedom to discover these questions and so many tools (dynamics, chemistry, spectroscopy, demographics, etc.) with which to address them.