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.

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Last modified July 26 2005.