The Chemical Composition of Carbon‐rich, Very Metal Poor Stars: A New Class of Mildly Carbon Rich Objects without Excess of Neutron‐Capture Elements (original) (raw)
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Extremely Metal‐poor Stars. IV. The Carbon‐rich Objects
The Astrophysical Journal, 1997
Abundances are presented for some 20 elements in three extremely metal-poor, carbon-rich stars. All have [Fe/H] \ [2.5. Based on their high proper motions and spectroscopic gravities two of them have relatively low luminosity : LP 706[7 is on or near the main sequence, while LP 625[44 is a subgiant. The third is the giant CS 22892[052, discovered to be r-processes enriched by Sneden et al. (1994). All three stars have large C and N overabundances, and large enhancements of the heavy neutron-capture elementsÈ D 1È2 dex. In contrast to the r-process signature observed in CS 22892[052, however, both LP 625[44 and LP 706[7 are clearly s-process enriched, suggesting that they may be progenitors of the well-known halo CH giants, the peculiarities of which are believed to result from mass transfer across a binary system containing an asymptotic giant branch star. In both LP 625[44 and LP 706[7 the distribution of sprocess elements is heavily weighted toward higher atomic number. [hs/ls] D 1.5, considerably larger than the values found in the s-process enhanced nearÈmain-sequence stars of the disk populations. [0.5 This implies a much higher neutron exposure per seed nucleus in the Population II objects and identiÐes 13C(a, n)16O as the neutron source. For LP 625[44 radial velocity and Li abundance data are consistent with the binary hypothesis. LP 706[7, however, remains something of an enigma in these respects : it shows no clear evidence for velocity variations, and its Li abundance lies precisely on the Spite Plateau. We estimate that the probability of this occurring in the Ba/CH class of objects is, roughly, [1%. In metal-poor stars the incidence of carbon enrichment appears to increase toward the lowest metallicities, and below [Fe/H] \ [2.5 supersolar values of [C/Fe] are not uncommon. Comparison of the available observational material with the Galactic chemical enrichment model of Timmes et al. (1995) shows that the model produces too little carbon. While the di †erence may result in the sensitivity of carbon production to modeling uncertainties such as the treatment of convection, we also discuss the possible role of a class of carbon producing zero heavy element supernovae and of massive "" hypernovae ÏÏ discussed by Woosley & Weaver (1982, 1995) in explaining this result. The carbon problem is also implicit in the suggestion that the r-process signature seen in CS 22892[052 results from normal supernovae enrichmentÈwhere then does its large carbon overabundance originate ? The models one might invoke to produce carbon overabundances leave black hole remnants in which the layers containing the seed nuclei for the r-process are not available for ejection.
A high-resolution spectral analysis of three carbon-enhanced metal-poor stars
Monthly Notices of the Royal Astronomical Society, 2006
We present results of an analysis of high-resolution spectra (R ∼ 50 000), obtained with the Subaru Telescope High Dispersion Spectrograph, of two carbon-enhanced metal-poor (CEMP) stars selected from the Hamburg/European Southern Observatory prism survey, HE 1305+0007 and HE 1152−0355, and of the classical CH star HD 5223. All these stars have relatively low effective temperatures (4000-4750 K) and high carbon abundances, which result in the presence of very strong molecular carbon bands in their spectra. The stellar atmospheric parameters for these stars indicate that they all have surface gravities consistent with a present location on the red giant branch, and metallicities of [Fe/H] = −2.0 (HE 1305+0007, HD 5223) and [Fe/H] = −1.3 (HE 1152−0355). In addition to their large enhancements of carbon ([C/Fe] = +1.8, +1.6 and +0.6, respectively), all three stars exhibit strong enhancements of the s-process elements relative to iron. HE 1305+0007 exhibits a large enhancement of the third-peak s-process element, lead, with [Pb/Fe] = +2.37, as well as a high abundance of the r-process element europium, [Eu/Fe] = +1.97. The second-peak s-process elements, Ba, La, Ce, Nd and Sm, are found to be more enhanced than the first-peak s-process elements Zr, Sr and Y. Thus, HE 1305+0007 joins the growing class of the so-called 'Lead stars', and also the class of objects that exhibit the presence of both r-and s-process elements, the CEMP-r/s stars. The large enhancements of neutron-capture (n-capture) elements exhibited by HE 1152−0355 and HD 5223 are more consistent with the abundance patterns generally noticed in CH stars, essentially arising from pure s-process nucleosynthesis. The elemental abundance distributions observed in these stars are discussed in light of existing theories of CH star formation, as well as the suggested formation scenarios of the CEMP-r/s group.
Astronomy and Astrophysics, 2005
We have carried out a new determination of abundances in the very metal-poor CH/CN strong stars CS 22948-27 and CS 29497-34, using high-resolution spectra obtained with the HARPS spectrograph at the 3.6 m telescope of ESO, La Silla, that covers the range λλ 4000−6900 Å at a resolution of R = 100 000. Both stars are found to be long period binaries. It is confirmed that the abundance patterns show an enhancement of the α-elements (like Mg, Ca), of the proton capture elements (like Na and Al) and a strong enrichment in "r" and "s" process elements, where the s-enrichment is probably due to a mass transfer episode from a companion in its AGB phase. The possible origins of the abundance pattern and especially of the strong enhancement of both "s" and "r" elements are discussed.
Carbon‐enhanced Metal‐poor Stars. I. Chemical Compositions of 26 Stars
The Astrophysical Journal, 2007
The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C 2 molecular bands are determined based on high-resolution spectroscopy. We define carbon-enhanced stars taking account of the carbon abundance ratio ([C/Fe]) and the evolutionary status, which is a slight modification over previous definitions. Twenty two stars in our sample satisfy our modified definition for Carbon-Enhanced Metal-Poor (CEMP) stars. In addition, we measure Na abundances for nine other carbon-enhanced stars for which abundances of other elements have been previously reported. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained: (1) All but one of the 37 stars with [Fe/H] ≥ −2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (−1.2 < [Ba/Fe]<+3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in asymptotic giant branch (AGB) stars, where the main s-process occurs. The correlation between the barium abundance ratio and that of carbon plus nitrogen ([(C+N)/Fe]) is relatively weak, because of the large excesses of nitrogen in some extremely metal-poor stars. (3) The majority of the Ba-enhanced stars have −1.
The Origins of Two Classes of Carbon‐enhanced, Metal‐poor Stars
The Astrophysical Journal, 2005
We have compiled composition, luminosity, and binarity information for carbonenhanced, metal-poor (CEMP) stars reported by recent studies. We divided the CEMP star sample into two classes, having high and low abundances, respectively, of the sprocess elements, and consider the abundances of several isotopes, in particular 12 C, 13 C, and 14 N, as well as the likely evolutionary stages of each star. Despite the fact that objects in both groups were selected from the same surveys (primarily the HK survey), without a-priori knowledge of their s-process element abundances, we identify the following remarkable difference between the two classes: s-element-rich CEMP (CEMP-s) stars occupy a wide range of evolutionary states, but do not have a strongly evolved 13 C/ 14 N ratio, whereas s-element-normal CEMP stars (CEMP-no) are found only high up the first-ascent giant branch, and possess 13 C/ 14 N ratios approaching the CN-cycle equilibrium value. We argue that these observational constraints can be accommodated by the following scenarios. CEMP-s stars acquire their distinctive surface compositions during their lifetimes when mass is transferred from an AGB companion that has recently synthesised 12 C and s-process elements. Such mass-accreting stars can be enriched at almost any stage of their evolution, and hence will be found throughout the HR diagram. Dilution of transferred surface material as the accretor ascends the giant branch, and its surface convective zone deepens, may reduce the number of such stars whose surfaces remain C-rich at high luminosities. Many, but not necessarily all, such stars should currently be in binary systems. Li-preserving CEMP-s stars may require a different explanation. In contrast, a CEMP-no star is proposed to have formed from gas that was enriched in 12 C from the triple-alpha process in a previous generation of stars, some of which has been converted to 13 C and 14 N during the present star's giant-branch evolution. The binary fraction of such stars should be the same as that of non-carbon-enhanced, metal-poor stars.
A holistic approach to carbon-enhanced metal-poor stars
Astronomy and Astrophysics, 2010
Context. Carbon-Enhanced Metal-Poor (CEMP) stars are known to be the direct witnesses of the nucleosynthesis of the first low-and intermediate-mass stars, because most have been polluted by a now-extinct AGB star. Aims. By considering the various CEMP subclasses separately, we try to derive, from the specific signatures imprinted on the abundances, parameters (such as metallicity, mass, temperature, and neutron source) characterizing AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion. Methods. To put CEMP stars in a broad context, we collect abundances for about 180 stars of various metallicities (from solar down to [Fe/H]=-4), luminosity classes (dwarfs and giants), and abundance patterns (C-rich and poor, Ba-rich and poor, etc), from our own sample and from literature. Results. First, we show that there are CEMP stars which share the properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This strongly points at the operation of the 13 C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s-and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2 nd and 3 rd s-process peaks bears instead the signature of the 22 Ne neutron source. Adding the fact that CEMP-rs stars exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. We also note that abundances are not affected by the evolution of the CEMP-rs star itself (especially by the first dredge-up). This implies that mixing must have occurred while the star was on the main sequence and that a large amount of matter must have been accreted so as to trigger thermohaline mixing. Finally, we argue that most CEMP-no stars (with no overabundances for the neutron-capture elements) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H] < −3.2). This trend is not predicted by any of the current AGB models.
First stars X. The nature of three unevolved carbon-enhanced metal-poor stars
Astronomy and Astrophysics, 2006
We seek to understand the nature of the progenitors of three main-sequence turnoff Carbon-Enhanced Metal-Poor (CEMP) stars, CS 31080−095, CS 22958−042, and CS 29528−041, based on a detailed abundance analysis. From high-resolution VLT/UVES spectra (R ∼ 43, 000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as 12 C/ 13 C isotopic ratios. All three stars have −3.30 ≤ [Fe/H] ≤ −2.85 and moderate to high CNO abundances. CS 22958−042 is one of the most carbon-rich CEMP stars known ([C/Fe] = +3.2), while CS 29528−041 (one of the few N-enhanced metal-poor stars known) is one of the most nitrogen rich ([N/Fe] = +3.0). Oxygen is very high in CS 31080−095 ([O/Fe] = +2.35) and in CS 22958−042 ([O/Fe] = +1.35). All three stars exhibit [Sr/Fe] < 0; Ba is not detected in CS 22958−042 ([Ba/Fe] < −0.53), but it is moderately enhanced ([Ba/Fe] ∼ 1) in the other two stars. CS 22958−042 displays one of the largest sodium overabundances yet found in CEMP stars ([Na/Fe] = +2.8). CS 22958−042 has 12 C/ 13 C = 9, similar to most other CEMP stars without enhanced neutroncapture elements, while 12 C/ 13 C ≥ 40 in CS 31080−095. CS 31080−095 and CS 29528−041 have A(Li) ∼ 1.7, below the Spite Plateau, while Li is not detected in CS 22958−042.
C,N and key heavy elements in metal-poor and very metal-poor carbon-enhanced stars
Proceedings of the International Astronomical Union, 2005
A new class of carbon-rich stars was revealed by large surveys of very metal-poor objects, the carbon-enhanced metal-poor stars (CEMPs). This carbon enhancement is reminiscent of that found in classical CH stars, which despite being halo stars, are not as metal-poor as CEMPs. Although a mass-transfer scenario similar to that formerly at work in CH stars could account for the abundance pattern of CEMPs, differences arise for some key heavy elements. Moreover, statistical studies find 14% of metal-poor C-rich stars among very metal-poor stars. Thus, this important population of stars represents a precious testimony for nucleosynthesis and chemical evolution at early stages of the Galaxy. We have started a detailed analysis of a large sample of both CH and metal-poor C-rich stars. Here we present results concerning the chemical composition obtained via high resolution and high signal-to-noise VLT-UVES spectra, with special emphasis on the challenges encountered during the abundance analysis. The discussion also includes preliminary results of our ongoing radial velocity monitoring programme which aims at evaluating the relevance of the binary scenario.
Carbon-enhanced metal-poor stars in the early galaxy
2005
Very metal-deficient stars that exhibit enhancements of their carbon abundances are of crucial importance for understanding a number of issues -the nature of stellar evolution among the first generations of stars, the shape of the Initial Mass Function, and the relationship between carbon enhancement and neutron-capture processes, in particular the astrophysical s-process. One recent discovery from objective-prism surveys dedicated to the discovery of metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars increases with declining metallicity, reaching roughly 25% for [Fe/H] < −2.5.
Carbon-enhanced metal-poor stars enriched in s-process and r-process elements
Journal of Astrophysics and Astronomy
We present an ongoing project consisting of analysis of a sample of twenty-five metal-poor stars, most of them carbon-enriched and thus tagged carbon-enhanced metal-poor (CEMP) stars, observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma), the UVES spectrograph on VLT (ESO Chile), or the HIRES spectrograph on KECK (Hawaii). This sample consists of CEMP-s stars, which are CEMP stars enriched in slow-neutron-capture (s-process) elements, as well as CEMP-rs stars enriched with both s-process and rapid-neutron-capture (r-process) elements. We also included an r-process-enriched star for comparison purposes. The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process (intermediate nucleosynthesis process), whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We aim at understanding whether the i-process and its putative site can reproduce the abundance pattern measured in CEMP-rs stars.