For my whole career I have worked on stellar magnetic activity and stellar atmospheres. Initially this work was on ultraviolet spectroscopy of stellar chromospheres, and computer modeling of semi-empirical model atmospheres and radiative transfer. It later expanded to include hot white dwarfs, rotation-activity relations and relations between diagnostics at different temperatures. My observational work has involved spacecraft including IUE, Hubble, and Kepler, and my ground-based work has been high resolution spectroscopy almost entirely at the Lick and Keck Observatories.
Later I began to work on direct measurement of stellar magnetic fields. I developed a new method based on equivalent widths, and with collaborators improved on another method based on high-resolution spectroscopy of line profiles. This work was extended into the infrared by my students, especially for T Tauri stars. A while later a postdoc and I developed another new method based on molecular spectra that could be used for low mass stars. This work showed that contrary to some previous theories, in fully convective stars the magnetic field still depends on rotation and remains strong.
I conducted an extensive program on stellar rotation over 3 decades, using Doppler broadening prior to 2010 then photometric periods from Kepler. After the discovery of brown dwarfs we proposed the new spectral class of L dwarfs, tested the hypotheses that their optical spectra were dominated by alkali resonance lines and dust, and established the first temperature scale for them. We discovered and studied the death of stellar chromospheres at the bottom of the main sequence, and the relation of that to rotational histories of very low mass stars. I collaborated on work regarding radio and optical emission from brown dwarfs and the auroral hypothesis.
Building on a long relation with the group at Ames, I became a Co-Investigator on NASA’s Kepler mission. It revolutionized the field of exoplanets by gathering ~200,000 precision stellar light curves and searching them for planetary transits. It also greatly expanded the field of asteroseismology. Bill Borucki (PI for the mission) was awarded the Shaw Prize in Astronomy in 2015. 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. I have continued to significantly advance that science, which is also useful for ongoing and upcoming photometric space missions.
I have authored a book (Dec. 2021) that summarizes the whole field of stellar magnetic activity. It provides an overview of observations, physical explanations, historical context and development, current progress, and possible future directions. It is for advanced undergraduate or graduate students, or scientists who would like an overview of this subject. Click the image below for the publisher’s site; the detailed table of contents can be found here.
For a more detailed technical description of my whole research program, with references, go here.