Chemical analysis of carbon stars in the Local Group

被引:36
作者
Abia, C. [1 ]
de laverny, P. [2 ]
Wahlin, R. [3 ]
机构
[1] Univ Granada, Dpto Fis Teor & Cosmos, E-18071 Granada, Spain
[2] Observ Cote Azur, Dpt Cassiopee, UMR6 202, F-06304 Nice 4, France
[3] Dept Space Phys & Astron, S-75120 Uppsala, Sweden
关键词
stars : abundances; stars : carbon; galaxies : dwarf; stars : AGB and post AGB; galaxies : Local Group;
D O I
10.1051/0004-6361:20079114
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
Aims. We present new results of our ongoing chemical study of carbon stars in Local Group galaxies to test the critical dependence of s-process nucleosynthesis on the stellar metallicity. Methods. We collected optical spectra with the VLT/UVES instrument of two carbon stars found in the Carina Dwarf Spheroidal (dSph) galaxy, namely ALW-C6 and ALW-C7. We performed a full chemical analysis using the new generation of hydrostatic, spherically symmetric carbon-rich model atmospheres and the spectral synthesis method in LTE. Results. The luminosities, atmosphere parameters and chemical composition of ALW-C6 and ALW-C7 are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature ( external pollution in a binary stellar system) cannot be definitively excluded. Our chemical analysis shows that the metallicity of both stars agree with the average metallicity ([Fe/H] similar to -1.8 dex) previously derived for this satellite galaxy from the analysis of both low resolution spectra of RGB stars and the observed colour magnitude diagrams. ALW-C6 and ALW-C7 present strong s-element enhancements, [s/Fe] = + 1.6, + 1.5, respectively. These enhancements and the derived s-process indexes [ls/Fe], [hs/Fe] and [hs/ls] are compatible with theoretical s-process nucleosynthesis predictions in low mass AGB stars (similar to 1.5 M-circle dot) on the basis that the C-13(alpha, n) O-16 is the main source of neutrons. Furthermore, the analysis of C-2 and CN bands reveals a large carbon enhancement (C/O similar to 7 and 5, respectively), much larger than the values typically found in galactic AGB carbon stars ( C/O similar to 1-2). This is also in agreement with the theoretical prediction that AGB carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. However, theoretical low-mass AGB models apparently fail to simultaneously fit the observed s-element and carbon enhancements. On the other hand, Zr is found to be less enhanced in ALW-C7 compared to the other elements belonging to the same s-peak. Although the abundance errors are large, the fact that in this star the abundance of Ti ( which has a similar condensation temperature to Zr) seems also to be lower than those of others metals, may indicate the existence of some depletion into dust-grains in its photosphere.
引用
收藏
页码:161 / 168
页数:8
相关论文
共 63 条
[1]   The 85KR s-process branching and the mass of carbon stars [J].
Abia, C ;
Busso, M ;
Gallino, R ;
Domínguez, I ;
Straniero, O ;
Isern, J .
ASTROPHYSICAL JOURNAL, 2001, 559 (02) :1117-1134
[2]  
ABIA C, 1993, ASTRON ASTROPHYS, V272, P455
[3]  
Abia C., 2002, APJ, V579, P817, DOI DOI 10.1086/342924
[4]  
Alksnis A, 1998, ASTRON ASTROPHYS, V338, P209
[5]   A subaru/high dispersion spectrograph study of lead (Pb) abundances in eight s-process element-rich, metal-poor stars [J].
Aoki, W ;
Ryan, SG ;
Norris, JE ;
Beers, TC ;
Ando, H ;
Tsangarides, S .
ASTROPHYSICAL JOURNAL, 2002, 580 (02) :1149-1158
[6]   Carbon-enhanced metal-poor stars. I. Chemical compositions of 26 stars [J].
Aoki, Wako ;
Beers, Timothy C. ;
Christlieb, Norbert ;
Norris, John E. ;
Ryan, Sean G. ;
Tsangarides, Stelios .
ASTROPHYSICAL JOURNAL, 2007, 655 (01) :492-521
[7]   Line formation in solar granulation -: VI.: [Cl], Cl, CH and C2 lines and the photospheric C abundance [J].
Asplund, M ;
Grevesse, N ;
Sauval, AJ ;
Prieto, CA ;
Blomme, R .
ASTRONOMY & ASTROPHYSICS, 2005, 431 (02) :693-705
[8]  
Asplund M., 2005, ASP C SER, V336
[9]  
AZZOPARDI M, 1986, ASTRON ASTROPHYS, V161, P232
[10]   New analysis of the two carbon-rich stars CS 22948-27 and CS 29497-34: Binarity and neutron capture elements [J].
Barbuy, B ;
Spite, M ;
Spite, F ;
Hill, V ;
Cayrel, R ;
Plez, B ;
Petitjean, P .
ASTRONOMY & ASTROPHYSICS, 2005, 429 (03) :1031-1042