Chemical Abundances of Stars in the Halo (CASH)

A long-term observing project with the Hobby-Eberly-Telescope


Goal of the project

The goal of this project is a comprehensive, multi-year spectroscopic survey of ~1000 very metal-poor stars with [Fe/H] < -2.0. We are employing the University of Texas Hobby-Eberly-Telescope located at McDonald Observatory.
This program has among its primary goals the identification of large numbers of metal-poor stars that are alpha-element rich (and -poor), carbon-rich with and without accompanying s-process overabundances, and highly r-process-enhanced.
The selection of the targets is such that, for the first time, the absolute frequencies of abundance anomalies based on statistically significant samples of metal-poor stars will be obtained.
Such information is required for an improvement in our understanding of the nature and interplay of the nucleosynthetic processes, and to help in identifying their astrophysical sites in the early Galaxy.
This survey will represent the largest such high-resolution effort of metal-poor halo stars ever conducted, and thus will be one of the "legacy results" to come from the Hobby-Eberly Telescope.
We expect that this survey will provide the fundamental spectroscopic database capable of resolving numerous fundamental questions concerning the origin of elements in the first generations of stars.


The Team

Anna Frebel, Carlos Allende Prieto, Matthew D. Shetrone, Jaehyon Rhee, Ian U. Roederer, Christopher Sneden, Timothy C. Beers, John J. Cowan, Volker Bromm, David Lambert & the Hobby-Eberly-Telescope

Current Status of project

We have observed ~200 stars as of September '07


Targets


There are already many thousands of SEGUE stars with estimated metallicities (based on low-resolution spectroscopy from that survey) [Fe/H] < -2.0 at present the number with g < 15.5 is roughly 200, with more being added all the time as the SEGUE survey continues (for another two years). The HK-II survey of Rhee (2001) is based on the machine-scanned HK survey plates, and has identified numerous candidate stars with [Fe/H] < -2.0. A dedicated effort has been made over the course of the past few years to obtain low-resolution spectroscopic confirmation of their metal-poor status using facilities at the McDonald Observatory and elsewhere. The HK-II candidates are of particular interest for our proposed work, as they range in brightness from B = 12 to 15. We can meet our intended S/N target for HK-II stars with total integration times from 10 to 30 minutes. Over 100 such candidates are immediately available that are reachable from the HET. Further targets will be identified within the next year.

Here are some papers and useful websites about our targets.

SDSS and SEGUE

HKI survey

HKI survey II

HKII survey

Hamburg/ESO survey (HES bright)


Results

We already have already submitted one paper to ApJ
"Chemical Abundances of Stars in the Halo (CASH) Project.
I. The Lithium-, r-, and s-Enhanced Metal-Poor Giant"
by Ian U. Roederer, Anna Frebel, Matthew D. Shetrone, Carlos Allende Prieto, Jaehyon Rhee, Roberto Gallino, Sara Bisterzo, Christopher Sneden, Timothy C. Beers, John J. Cowan


At the Frank N. Bash Symposium 2007 we will present three posters
1. "The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) Project. I. The Lithium-, r-, s-Enriched Metal-Poor Giant HKII 17435-00532"
by Frebel, Roederer, Shetrone, Allende Prieto et al.

Abstract:
We present a abundance analysis of HKII 17435-00532, a HKII object for which HET observations have been obtained as part of the CASH project. The high Li abundance observed in this evolved star (red horinzontal branch or giant) can be explained by self-enrichment. The r+s enhancement is due to either mass transfer from a binary companion or was already present in the birthcloud of the star. Despite the current non-detection of radial velocity variations (over the period of several months) it is likely that HKII 17435-00532 is in a long-period system, similar to other Li-enriched, evolved stars.


2. "The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) Project. II. Sample Selection and Results from the First Year"
by Frebel, Allende Prieto, Roederer, Shetrone, Sneden et al.

Abstract:
We present the results from the first year of HET observations as part of the University of Texas Long Term Project to discover and analyze new metal-poor Galactic halo stars. Thus far, ~200 objects are observed. Data reduction and stellar parameter determination, as well as our analysis procedure are described. An outlook over the results and future observations is also given.


3. "The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) Project. III. Abundance analysis of two bright Hamburg/ESO Survey Stars"
by Davies, Frebel, Allende Prieto, Sneden, Roederer et al.

Abstract:
We present an abundance analysis of two newly discovered stars from the Hamburg/ESO survey for which HET observations have been obtained as part of the CASH project. One star (with [Fe/H]=-2.8) appears to be enhanced in Ca and Mg beyond the usual alpha-element enhancement found in metal-poor stars. The other object (with [Fe/H]=-2.5) has a typical abundance pattern for stars in that metallicity range. The neutron-capture element Ba was detected in both stars, while only upper limit could be measured for a number of other neutron-capture elements. A discussion of the chemical signatures of these stars as well as a comparison with objects found in the literature is also given.


Frank N. Bash Symposium 2007 - Austin, Texas



last updated: Oct 12, 2007 by A. Frebel, anna at astro dot as dot utexas dot edu
and S. Kang, sjkang at mail dot utexas dot edu
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