Research Interests
Judy Cohen is currently completing a large survey of extremely metal
poor stars in the halo of our galaxy. These are among the oldest
stars in the Galaxy, and represent the local equivalent of the
high redshift universe. The goal is to isolate a new, large
sample of stars with extremely low metallicities, [Fe/H] less than or equal to
roughly -3 dex, using a new sample of candidates from the Hamburg/ESO Stellar Survey
(HES), with particular emphasis on the determination of the abundances of
the elements relative to H, and on the implications of the elemental
abundance patterns in terms of nucleosynthetic processes and stellar
sources.
The most metal deficient stars in the Galaxy provide critical evidence on
the early epochs of our Galaxy, the onset of chemical evolution of the
Galaxy, the possible stellar sources which produced many elements
at very early epochs
(very massive stars and SNII), the age of the Galaxy, the relationship
between the halo field stars and the galactic globular clusters, the
possible role
of the stars that formed the first generation of metals in re-ionizing
the Universe, etc. This work is being carried out in collaboration
with Andy McWilliam, Steve Shectman and Ian Thompson of the Carnegie
Observatories and with Norbert Christlieb of Hamburg.
Prior to this, she was the head of the Caltech Faint Galaxy Redshift Survey
team, a small group dedicated to exploring the properties of faint field
galaxies to redshift 1 and beyond. Major efforts in the Hubble Deep Field
and in a second deep field produced a redshift survey with more than
1500 galaxies, a description of the spectral energy distributions
of these galaxies, a luminosity function for such galaxies as a function
of spectral type and of redshift, a study of the merger rate and of
clustering as a function of redhsift, etc. This effort was her
dominant research interest from about 1998 to 2002, and arose
from a desire to utilize the
Low Resolution Imaging Spectrograph, one of the three first light
instruments now installed and working on the Keck telescope on Mauna
Kea in Hawaii. Bev Oke, now retired, and
Judy designed and built the spectrograph over the past 5 years,
and she, with the help of a talented team of engineers,
tuned it up to its optimum potential.
Judy is also continuing to work in the area of the abundances of stars in the
galactic globular clusters, the star-to-star differences of such seen among
the light elements C, N, O, Na, Al and Mg, in particular. The exploration
of this interesting and complex set of problems
began with her key papers in the late 1970s and early 1980s, and
still continues to date, most recently in collaboration with Hubble Fellow Evan Kirby,
whom she mentors. She also collaborates with the Ohio State, Carnegie, and VLT
bulge microlensing groups in an effort to characterize and understand
these rare events.
Recently she has become the leader of a small group trying to use the
Palomar Transient Factory to study the Galactic halo. The initial
emphasis will be on finding RR Lyrae variables (the work of postdoctoral
fellow Branimir Sesar) far out in the halo. We expect to find about
10,000 RR Lyr variables at distances out to 80 kpc,
and obtain estimates for their metallicities from
precision light curves, then derive the smoothed spatial density
of the Milky Way halo as well as look for substructure, streams etc.
Once multi-color images are obtained with the PTF, a search for
new Galactic satellite galaxies will commence.
The possibilities with Keck are endless, and its sure to be both
fun and surprising to work on these and many other areas of
astronomy.
Some recent references (links are to astro-ph):
Abundances in Very Metal Poor Dwarf Stars, Cohen,
Christlieb, McWilliam, Shectman,
Thompson, Wasserburg, Ivans, Dehn. Karlsson & Melendez,
2004, ApJ, 612, 1107
Caltech Faint Galaxy Redshift Survey XVI:
The Luminosity Function for Galaxies in the Region of the HDF-North
to z = 1.5, J.G.Cohen, ApJ, 2002, ApJ, 567, 672
C and N Abundances in Stars At the Base of the Red Giant
Branch in M15,
Judith G. Cohen, Michael M. Briley and Peter B. Stetson, 2005, AJ, 130, 1177
Abundances in a Large Sample of Stars in M3 and M13, J. Cohen & J. Melendez,
2005, AJ, 129, 303
Outer Versus Inner Halo Globular Clusters: NGC 7492 Abundances,
J. Cohen & J. Melendez, 2005, AJ, 129, 1607
J.B.Oke, J.G.Cohen, M.Carr, A.Dingizian, F.Harris, R.Lucinio,
S.Labrecque, W.Schaal, and S.Southard Jr.,
Proc. SPIE, 2198, 178, 1994, ``The Low Resolution Imaging Spectrometer
for the Keck Telescope.''
A Puzzle Involving Galactic Bulge
Microlensing Events, J.G.Cohen, A. Gould, I.B.Thompson et al, 2010, ApJ, 711, L48.
For a more complete list of publications, you may want to check out the biblography page.
Home | Research Interests | Selected Bibliography |
Ay 123 Class | Links
Last modified July 26 2005. jlc@astro.caltech.edu