Major Themes in My Research Program
T Tauri Stars (very young stars)
We have been able to find and study stars in the process of formation. These are guides to the processes that formed our own Solar System. We use primarily high-resolution spectroscopy to diagnose the physical conditions in these systems. We are particularly known for our time-resolved studies of these stars; they actually change from night to night and this helps us understand what is going on in the near-stellar region.
Accretion Disks/ Star-Disk Interaction
Even before Hubble Space Telescope pictures of actual disks around newly forming stars, we had deduced their presence and developed means of understanding their properties. My particular interest is in the region where the stellar magnetic field interrupts the disk, and material takes its final plunge to the stellar surface.
Formation of red and brown dwarfs
Most stars are half the mass of our Sun or less. Below about 0.075 solar masses, the objects are considered “substellar” because they never acheive a stable luminosity (although they do experience some fusion). We have been studying the question of whether these objects form as stars do, or through another mechanism (as some have suggested).
Brown and Red Dwarfs (substellar/very low-mass stars)
Lithium Brown Dwarfs (discovery; lithium dating)
Until 1995, the existence of substellar objects was predicted but not confirmed. It was thought they might be a component of the “dark matter” (they aren’t). I successfully applied the “lithium test” to discover the first young brown dwarf in 1995 (in the same year, an old brown dwarf too cool to be a star was also announced). In the process, I developed a technique to use lithium depletion to determine ages of young star clusters, and showed that previous estimates were too young. This method is now widely used. Hundreds of brown dwarfs are now known, and there are perhaps 1/10 as many of them as there are stars. We helped suggest and participated in the development of the “L” spectral type (the first new stellar classification since the early 20th century).
Brown and Red Dwarf properties
Brown dwarfs have only been known since 1995. Red dwarfs (stars less than half our Sun’s mass) have been known for a long time, but they are very faint and did not receive the same attention as brighter stars. Using the 10-m telescope, I have applied high-resolution spectroscopy to these objects to diagnose their physical properties and move our level of understanding closer to that for the more massive stars.
Binary Brown and Red Dwarfs
Many stars come with companions. We have been studying the question of how this depends on stellar mass (low-mass stars have fewer companions) and what the binary systems look like (low-mass stars are in tighter systems). We discovered both the first brown dwarf spectroscopic (close) binary and the first brown dwarf wide (imaged) binary systems.
Stellar Magnetic Activity
Rotation and Activity
Our Sun exhibits magnetic activity in the form of sunspots, active regions, flares, and the hot corona. It turns out to be a relatively weak example of such phenomena – young stars are far more active (as it once was). The level of activity turns out to depend on the stellar rotation rate (fast rotation cranks up the magnetic dynamo), but magnetic activity acts to slow a star down. The study of this subject has long been my interest, and I have contributed to many of the basic conclusions. We are now extending these studies to red and brown dwarfs, where surprising new effects are showing up.
It is hard to measure magnetic fields on stars directly, so most of what we know comes from understanding how to use indirect proxies for the fields. I have done a lot of work in various aspects of how to accomplish this.
Measurement of Magnetic Fields
It is hard to measure magnetic fields on stars directly, but not impossible. I have been one of the leaders in this field, and have developed two new methods as well as improved upon existing ones. We recently have been able to extend direct measurements to very low-mass objects.
Searching for Earth-sized (Exo-) Planets
No extrasolar planets were known before 1995. Now well over 200 have been found, but none are small enough to be like the Earth and have a solid surface. Our best hope for determining the frequency of terrestrial planets is to perform a transit experiment in space. NASA’s Kepler Mission is the first serious attempt to do this. I was a Co-Investigator on the mission (the Principal Investigator is Bill Borucki at NASA Ames). It launched in March 2009, and has found over 3500 planet candidates. My role in the mission was to understand the photometric variability caused by stellar activity, and to extract some of the treasure-trove of collateral stellar science.
Photometric Signatures of Activity
I have used the Kepler mission is to understand how stellar activity affects the brightness of stars. I have helped extract some of the stellar science inherent in the unprecedented new precision and time-coverage of stellar light-curves that the mission automatically provides. Among the topics I’m studying are the rotation and differential rotation of stars, starspot coverage and evolution, and using photometric “flicker” to measure stellar gravity.