Chemical composition of very metal-poor stars with carbon-excesses

Abstract

Recent surveys of metal-poor stars have discovered a number of carbon-rich ([C/Fe]>+1) objects. To investigate the origin of the carbon-excess is an important issue to understand the nucleosynthesis processes in the early Galaxy. For detailed abundance studies, high resolution spectra of 11 carbon-rich stars were obtained with Subaru/HDS. 9 of them show large excesses of s-process elements (e.g., Ba), which would be yields of AGB stars in the early Galaxy. The abundance patterns of heavy elements (from Sr to Pb) in these stars give a strong constraint on modeling of s-process at very low metallicity. Another object with normal Ba-abundance, CS29498-043, has turned out to be extremely metal-poor ([Fe/H]=-3.75) but shows large excesses of Mg and Si. This chemical nature should originate from supernovae in which relatively little material escaped from the iron core.

Introduction

• A number of carbon-rich objects have been found by surveys of very metal-poor stars (e.g., the HK-survey: Beers et al. 1992, AJ, 103, 1987). The fraction of carbon-rich stars is highest in the most metal-poor range.

• Some of them are known to show large excesses of s-process (slow neutron-capture process) elements. These are believed to originate from nucleosynthesis in AGB stars (evolved low-mass stars).

• Other carbon-rich stars show abundances of s-process elements similar to those in non-carbon-rich stars. Origin of these objects is unknown.

Carbon abundances ([C/Fe]) as a function of [Fe/H]. Though there should be bias in our sample (carbon-rich stars are easily identified), the fraction of carbon-rich stars is obviously high at low metallicity. HE0107-5240 ([Fe/H]=-5.3), recently found by Christlieb et al. (2002, Nature,419, 886), shows extremely high [C/Fe].

Observations

• Most objects were discovered by the survey of metal-poor stars called ‘HK-survey’.

• Observations were made with HDS for 11 objects from July 2000 to July 2001.

• Spectral resolution is 50,000 or 90,000, and typical S/N ratios are 100 per resolution element.

Spectra of two subgiants with [Fe/H]~-2.5. LP625-44 is carbon-rich ([C/Fe]~+2), while HD140283 has normal carbon abundance ([C/Fe]~0). We can find the carbon-rich nature by the CH G-band.

Classification of carbon-rich objects:

Criterion 1: s-Process elements (e.g., Ba)

9 objects are Ba-rich ([Ba/Fe]>1.0)

Spectra of two carbon-rich ([C/Fe]~+2) stars. Ba II line of LP625-44 is much stronger than that of CS22957-027.

Other neutron-capture elements also appear in the spectrum of LP625-44.

Criterion 2: alpha-elements (e.g., Mg)

The Ba-normal object CS29498-043 shows large enhancements of Mg and Si.

Abundance patterns of stars with excesses of s-process elements:

• One of the heaviest stable elements, Pb, was detected in 8 of the 9 s-process-rich objects.

• The s-process produces 3 abundance peaks at Sr (Z=38), Ba (56), and Pb (82). The Pb/Ba ratio (as well as the Sr/Ba ratio) is a strong constraint on the modeling of the s-process.

• 9 of the 11 objects show large overabundances of heavy neutron-capture elements (e.g., Ba), which are attributed to s-process.

• Very large Pb production is predicted by recent models of metal-deficient AGB stars, which suscessfully explain some objects. However, there exist objects with low Pb/Ba ratios in our sample, which cannot be explained by small modification of the above models. New models of the s-process at low metallicity are required.

Ba abundance ratio as a function of metallicity ([Fe/H]). Carbon-rich stars are shown with error bars.

Abundance ratios of Pb/Ba as a function of metallicity ([Fe/H]). Lines indicate prediction by recent AGB models with different parameters (e.g., Goriely & Mowlavi 2000, A&A 362, 599).

Discovery of the α-element-enhanced star CS29498-043

• One of the two stars with normal Ba-abundance, CS29498-043, shows very large overabundance of Mg (and Si), while its Fe abundance is extremely low.

• The abundance pattern of this object is similar to the previously known α-element-rich star CS22949-037.

• Since production of α-elements is not expected from models of the low-mass star evolution, the chemical nature of these objects should originate from supernovae. The low abundances of iron-peak elements suggest that little material escaped from the iron core in the explosion.

• An observational program to search for stars with these extreme abundance characteristics is ongoing using Subaru/HDS.

Mg abundance ratios as a function of [Fe/H]. Carbon-rich stars are shown with error bars

Elemental Abundances, relative to other stars with similar metallicity, of the two α-element-rich stars CS29498-043 and CS22949-037. These stars show large excesses of C, N, Mg and SI relative to iron.