Technical Research Narrative with References
For my whole career I have worked on stellar magnetic activity, solar-type stars, and low mass objects (stellar and substellar). The bulk of my observational work has been ground-based high resolution spectroscopy, almost entirely at the Lick and Keck Observatories. It has also involved spacecraft including IUE, HST and Kepler; the first two for UV spectroscopy and the latter for precision photometry. Below is a description of my main lines of research, with references from my publication list (not all papers, reviews, or conference reports are mentioned here). This corpus of work (>200 papers, most with coauthors) has been influential (h-index 80, >25,000 citations).
Initially I worked on ultraviolet spectroscopy of solar/stellar chromospheres (J3-6,J8-10,J17-19), and numerical radiative transfer in model atmospheres (J2,J7,J16,J32). The modeling later expanded to include hot white dwarfs (J12-13,J32), and rotation-activity relations between optical, UV, and X-ray diagnostics (J20-21,J28,J39,J91). My interests extended to other contexts as described below but magnetic activity remained a thread throughout my research. This finally led to my book summarizing the whole field of stellar magnetic activity: “An Introduction to Stellar Magnetic Activity” (2021) that provides an overview of observations, physical explanations, and historical context and development of the field.
From about 1980-1995 I worked substantially on stars in formation. Astronomers knew then that “T Tauri” stars are very young, but didn’t know much about them. They looked like they might have strong magnetic activity, or might still be accreting, or both. In J16 we showed using models that the chromospheric hypothesis could be part of the solution, and in J22 we showed observationally that both are sometimes present. We were the first to clearly identify “spectral veiling” as due to the ultraviolet excess associated with ongoing accretion, and that the accretion is sometimes modulated by stellar rotation. We helped interpret T Tauri spectra and spectral energy distributions (J23,J26,J30,J35-36,J76,J80-81,J91) and estimate accretion rates (J38).
Using an innovated method we provided the first proof that T Tauri stars are covered with kilogauss magnetic fields (J34). This helped solidify the “X-wind” model that explains both the accretion and the narrow fast bipolar jets associated with some T Tauri stars. We found that important emission lines have a narrow component due to the stellar chromosphere and sometimes a broad component due to accretion (J22,J30,J47,J125). This provides another way to assess the accretion status of a newly forming star (J71,J79,J85). We extensively studied the time variability of these lines (as short as a few hours) showing that they must come from very near the star and providing evidence that both winds and accretion are taking place in this region (J37,J44-45,J52,J71,J73,J97). We studied the first example of orbitally-induced accretion that crosses gaps in disks (J55,J57,J108). My expertise in this field led to a number of review articles (R2-6,R8-10,R19,R34)
I worked at various times on direct measurement of stellar magnetic fields. I developed the method mentioned above (J34) based on equivalent widths, and with collaborators improved on another method based on high-resolution line profiles (J24,J27,J29,J40,J42). This work was later extended into the infrared by my students, especially for T Tauri stars. A decade later a postdoc and I developed another new method based on molecular spectra that worked for low mass stars (J102,J105,J115,J117-118). This work showed that contrary to prior expectations, in fully convective stars the magnetic field still depends on rotation and remains strong. It measured fields even stronger than solar-type stars despite M dwarfs not having an important ingredient of the solar dynamo (the tachocline).
From 1993 onward I began work on very low mass objects. I was the lead author in applying the “lithium test” to the Keck search for brown dwarfs (J41). My team was the first to detect the presence of lithium in a faint object (PPL 15 in the Pleiades, identified by Stauffer) leading to the announcement of the successful discovery of brown dwarfs (J46). PPL 15 was fainter than expected; we explained that by increasing the age of the Pleiades. That paper thus contained my development of a new method of determining the ages of young clusters called “lithium dating”. This utilizes the luminosity of the brightest object that still hasn’t destroyed lithium to set an age for a young cluster (J64,R14). It shows that prior reliance on main sequence turnoffs of high mass stars led to ages that were low by up to 50% (due to problems with core convective overshoot). Both the methodology and results have now become standard.
We continued to find new substellar objects (J51,J53,J59-66,J68). I helped propose the new spectral class of “L” dwarfs (J59). We tested the hypotheses that their optical spectra were dominated by alkali resonance lines and dust (J50,J59) and established the first temperature scale for them (J70,J72). We studied binarity in the low mass objects (J65,J77,J88,J94, J101,J103,J111,J114,J116), finding the first spectrocopic (J69) and visual (J63) brown dwarf binaries. We also studied magnetism, fundamental parameters, and ages of low mass objects (J92-93,J99-100,J120, J122,J133-134). I collaborated on work regarding radio, optical, and high energy emission from brown dwarfs and the auroral hypothesis (J75,J98,J109-110,J116,J123). My expertise in these subjects led to a number of review articles (R16,R18,R20,R23,R25-26,R28-29,R31,R36,R37).
I conducted an extensive program on stellar rotation over 3 decades, using Doppler broadening prior to 2010 then photometric periods from Kepler. Those measurements are often embedded in papers on activity or on low mass stars. An unexpected discovery was that very low mass stars can be very rapid rotators (down to a few hours) and yet not show the expected high magnetic activity expected with such short periods (J43). We explained this as due to the extremely low ionization in these very cool atmospheres (J82). This work continued until we understood the rotational behavior of stars above and below the bottom of the main sequence and located the mass below which stellar chromospheres are no longer found (J86,J112,J119,J125,J133).
Building on my previous work on star formation, we also examined the formation of very low mass objects (J74,J83,J85,J87,J96,J113). This helped establish a general mass-dependence of accretion during formation for both stars and brown dwarfs. That and other phenomenology strongly suggests brown dwarfs form in essentially the same way as stars (which was uncertain at first). Objects in star-forming regions that are near the boundary where they might no longer be able to fuse even deuterium began to be discovered. As part of this work (and in the era of Pluto’s demotion from “planethood”) I became involved in the question of “what is a planet?” (R27) and co-authored a review article that still clarifies the terms of this debate (R33).
Through a long relation with the Borucki group at Ames, I worked on two space transit mission proposals before becaming a Co-Investigator on NASA’s Kepler mission (R32,J129,J131). I do not list here a number of exoplanet papers that I was not a significant scientific contributor to. My main role was to anticipate and understand the effects of stellar activity (particularly starspots) on light curves and to help extract the astrophysics the light curves offer (J126,J136-137,J143,J146-147). I have continued to significantly advance that science, which is also useful for ongoing and upcoming photometric space missions. One conclusion is that most work inferring starspot information (other than stellar rotation periods) from differential light curves has been overly naive. One and two spot models are illusory, but the single/double character of light curves is a clear function of rotation. We showed it is more likely diagnostic of starspot lifetimes (J154,J156-159). It appears very difficult to extract solar-type differential rotation but I developed an autocorrelation method that produces significant information about starspot lifetimes as a function of stellar parameters (J160).