15 research outputs found
The Frequency of Carbon Stars Among Extremely Metal-Poor Stars
We demonstrate that there are systematic scale errors in the [Fe/H] values
determined by the Hamburg/ESO Survey (and by inference by the HK Survey in the
past) for certain extremely metal poor highly C-enhanced giants. The
consequences of these scale errors are that a) the fraction of carbon stars at
extremely low metallicities has been overestimated in several papers in the
recent literature b) the number of extremely metal poor stars known is somewhat
lower than has been quoted in the recent literature c) the yield for extremely
metal poor stars by the HES Survey is somewhat lower than is stated in the
recent literature. A preliminary estimate for the frequency of Carbon stars
among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%;
adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex without
detectable C2 bands raises the fraction to 14 +-4$%.
We rely on the results of an extensive set of homogeneous detailed abundance
analyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES to
establish these claims. We have found that the Fe-metallicity of the cooler
(Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are a
factor of ~10 higher than those obtained via the algorithm used by the HES
project to analyze the moderate resolution follow-up spectra, which is
identical to that used until very recently by the HK Survey. This error in
Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in
the continuum bandpasses of the KP (3933 A line of CaII) and particularly the
HP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strong
molecular bands.Comment: Accepted to the ApJL after a very lengthly duel with the 3
simultaneous referee
The Frequency of Carbon Rich Stars Among Extremely Metal Poor Stars
We demonstrate that there are systematic scale errors in the [Fe/H] values
determined by the Hamburg/ESO Survey (and by the HK Survey by inference) for
certain extremely metal poor (EMP) highly C-enhanced giants. The consequences
of these scale errors are that a) the fraction of carbon stars at extremely low
metallicities has been substantially overestimated in several papers in the
recent literature b) the number of EMP stars known is somewhat lower than has
been quoted in the recent literature c) the yield for EMP stars by the HK and
the HES Survey is somewhat lower than is stated in the recent literature. A
preliminary estimate for the frequency of Carbon stars among the giants in the
HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 \pm 2.9%, and for C-rich giants
with [C/Fe] > +1.0 dex is 14.4 \pm 4%.
Here we present the key results of detailed abundance analyses of 14 C-stars
selected from the HES as candidate EMP stars, of ~50 such stars analyzed. About
80% of such C-stars show highly enhanced Ba as well, with C enhanced by a
factor of about 100, and [Ba/C] roughly Solar. These stars often show prominent
lead lines, The remaining 20% of the C-stars do not show an enhancemement of
the s-process neutron capture elements around the Ba peak. They tend to be the
most metal-poor stars studied. We suggest that all these EMP C-stars are the
remnants of the secondary in a mass transfer binary system where the former
primary was an AGB star, which transferred substantial mass at that
evolutionary stage. (published abstract will be shorter due to space
limitations)Comment: Submitted to the proceedings of the IAU Symposium 228, From Lithium
to Uranium: Elemental Tracers of Early Cosmic Evolution (Paris, 23--27 May
2005), Cambridge University Press, ed: Vanessa Hill, Patrick Francois &
Francesca Prima
Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I
We report broadband UBV and/or BVRI CCD photometry for a total of 1857 stars
in the thick-disk and halo populations of the Galaxy. The majority of our
targets were selected as candidate field horizontal-branch or other A-type
stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the
HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of
additional stars from other samples are also reported.
These data are being used for several purposes. In the case of the FHB/A
candidates they are used to accurately separate the lower-gravity FHB stars
from various higher-gravity A-type stars, a subsample that includes the
so-called Blue Metal Poor stars, halo and thick-disk blue stragglers,
main-sequence A-type dwarfs, and Am and Ap stars. These data are also being
used to derive photometric distance estimates to high-velocity hydrogen clouds
in the Galaxy and for improved measurements of the mass of the Galaxy.
Photometric data for the metal-poor candidates are being used to refine
estimates of stellar metallicity for objects with available medium-resolution
spectroscopy, to obtain distance estimates for kinematic analyses, and to
establish initial estimates of effective temperature for analysis of
high-resolution spectroscopy of the stars for which this information now
exists.Comment: 22 pages, including 3 figures, 5 tables, and two ascii files of full
data, accepted for publication in the Astrophysical Journal (Supplements
Broadband UBVR_CI_C Photometry of Horizontal-Branch and Metal-poor Candidates from the HK and Hamburg/ESO Surveys. I.
We report broadband UBV and/or BVR_CI_C CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists
Carbon Stars in the Hamburg/ESO Survey: Abundances
We have carried out a detailed abundance analysis for a sample of 16 carbon
stars found among candidate extremely metal-poor (EMP) stars from the
Hamburg/ESO Survey. We find that the Fe-metallicities for the cooler C-stars
(Teff ~ 5100K) have been underestimated by a factor of ~10 by the standard HES
survey tools. The results presented here provided crucial supporting data used
by Cohen et al (2006) to derive the frequency of C-stars among EMP stars.
C-enhancement in these EMP C-stars appears to be independent of
Fe-metallicity and approximately constant at ~1/5 the solar C/H. The mostly low
C12/C13 ratios (~4) and the high N abundances in many of these stars suggest
that material which has been through proton burning via the CN cycle comprises
most of the stellar envelope. C-enhancement is associated with strong
enrichment of heavy nuclei beyond the Fe-peak for 12 of the 16 stars. The
remaining C-stars from the HES, which tend to be the most Fe-metal poor, show
no evidence for enhancement of the heavy elements. Very high enhancements of
lead are detected in some of the C-stars with highly enhanced Ba. (We show
that) the s-process is responsible for the enhancement of the heavy elements
for the majority of the C-stars in our sample.
We suggest that both the s-process rich and Ba-normal C-stars result from
phenomena associated with mass transfer in binary systems. This leads directly
to the progression from C-stars to CH stars and then to Ba stars as the
Fe-metallicity increases. (abridged and slightly edited to shorten)Comment: AJ, in press, submitted 13 Dec, 2005, accepted 21 March 200