Research Program of Gibor Basri
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
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.
Search 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 finding out how common such terrestrial planets are is to
perform a transit experiment in space, looking for the tiny blockage of light
that happens when a planet (regularly) passes between us and its star. NASA's
Kepler Mission will make the first serious attempt to discover rocky planets
with temperatures that would allow oceans. I am a Co-Investigator on the mission
(the Principal Investigator is Bill Borucki at NASA Ames). We hope to launch
in 2009, and this will provide some of NASA's most exciting results of the
next decade if successful.
Photometric Signatures of Activity
My role in the Kepler mission is to understand how stellar activity affects
the brightness of stars and to help assure that it is not fooling us in detecting
terrestrial transits. I will also extract some of the stellar science inherent
in the unprecedented new precision and time-coverage of stellar light-curves
that the mission automatically provides.