Studies on Carbon-Enhanced Metal-Poor (CEMP) stars

Abstract

Chemical composition of metal-poor stars are crucial to develop an understanding of the nature of the earliest stars formed in the universe, the nucleosynthesis events associated with them, as well as, to redefine the models of galaxy formation. Elements heavier than the iron peak are made via two principal processes: the rapid neutron-capture process (r-process) and the slow neutron-capture process (s-process). Insight into the astrophysical sites and the production mechanisms of neutron-capture elements can be obtained by studying chemical composition of stars that exhibit large enhancement of neutroncapture elements such as the Carbon-Enhanced Metal-Poor (CEMP) stars. Among the CEMP stars, CEMP-s stars exhibit the presence of strongly enhanced s-process elements and CEMP-r stars are with strong enhancement of r-process elements. A number of CEMP stars are also known to exhibit enhancement of both r- and s-process elements, the CEMP-r/s stars (Hill et al. 2000, Goswami et al. 2006, Jonsell et al. 2006 etc.). Till now, the upper limit in the metallicity of stars showing double enhancement is [Fe/H] -2 (HE 1305+0007 with [Fe/H] = -2.0, (Goswami et al. 2006)). In spite of several efforts, a physical explanation for the observed double enhancement is still lacking (Qian & Wasserburg 2003; Wanajo et al. 2006). A few CEMP stars are known that show no enhancement of neutron-capture elements, the CEMP-no stars. Identification of an explicit stellar site for s-process nucleosynthesis started with the works of Weigert (1966) and Schwarzschild & H¨arm (1967) on the thermal pulse calculations. Slow neutron-capture elements are now believed to be produced due to partial mixing of protons into the radiative C-rich layers during thermal pulses that initiate the chain of reactions 12C(p; ) 13N( ) 13C( ; n)16O in a narrow mass region of the He intershell during the inter-pulse phases of a low-mass AGB stars. Rapid neutron-capture process elements are thought to be produced during SN explosions or accretion induced collapse. High resolution spectroscopic analyses of CEMP stars have established that the largest group of CEMP stars are s-process rich (CEMP-s) stars and accounts for about 80 per cent of all CEMP stars (Aoki et al. 2007). Chemical composition studies of CEMP stars (Barbuy et al. 2005; Norris et al. 1997a,b, 2002; Aoki et al. 2001, 2002; Goswami et al. 2006, 2010a) also have suggested that a variety of production mechanisms are needed to explain the observed range of elemental abundance patterns in them; however, the binary scenario of CH star formation is currently considered as the most likely formation mechanism also for CEMP-s stars. This idea has gained further support with the demonstration by Lucatello et al. (2005), that the fraction of CEMP-s stars with detected radial velocity variations is consistent with the hypothesis of all being members of binary systems. CH stars characterized by iron deficiency, enhanced carbon and s-process elements are known to be post-mass-transfer binaries (McClure & Woodsworth 1990) in which the companion (primary) has evolved to white dwarf passing through an AGB stage of evolution. The chemical composition of CH stars (secondaries) bear the signature of the nucleosynthesis processes occurring in the companion AGB stars due to mass transfer. Roche-Lobe Overflow (RLOF) and wind accretion are among the suggested mass transfer mechanisms. Recent hydrodynamical simulations have shown in the case of the slow and dense winds, typical of AGB stars, that efficient wind mass transfer is possible through a mechanism called Wind Roche-Lobe Overflow (WRLOF) Mohamed & Podsiadlowski 2007; Abate et al. 2013. CH stars (secondaries) thus form ideal targets for studying the operation of s-process occurring in AGB stars. Chemical abundances of key elements such as Ba, Eu etc. and their abundance ratios could provide insight in this regard. In our studies along this line we have considered a sample of eighty nine faint high latitude carbon stars from the Hamburg/ESO survey (Christlieb et al. 2001) and based on medium resolution spectroscopy found about 33% of the objects to be potential CH star candidates (Chapter 3). Inspite of their usefulness, literature survey shows that detailed chemical composition studies of many of the objects belonging to the CH star catalogue of Bartkevicius (1996) are currently not available. A few studies that exist are either limited by resolution or the wavelength range. The CH star catalogue of Bartkevicius (1996) lists about 261 objects, 17 of which belong to ! Cen globular cluster. Many of the objects listed in this catalogue have no information on binary status. It is worthwhile to compare and examine the abundance patterns of elements observed in the confirmed binaries with their counterparts in objects that have no information on binary status. While long-term radial velocity monitoring are expected to throw light on the binary status, detailed chemical composition studies could also reflect on the binary origin. We have therefore undertaken to carry out chemical composition studies for a selected sample of CH stars from this catalogue using high resolution spectra. Towards this end, we have considered twenty two objects from the catalogue of Bartkevicius (1996) for a detailed chemical composition study (Chapter 4 and Chapter 5). Detailed high resolution spectroscopic analyses for this sample of objects are either not available in the literature or limited by resolution or wavelength range. The sample includes five confirmed binaries, six objects that are known to show radial velocity variability, and for the rest eleven objects, none of these two information is available. In the following text, for convenience, we will refer the objects that are confirmed binaries as group I objects, those with limited radial velocity information as group II objects and the objects for which none of these information are available as group III objects. One of our primary objectives is to estimate the abundances of heavy elements in all the stars of these three groups of objects and critically examine the abundance patterns and abundance ratios if they exhibit characteristic abundance patterns of CH stars. Polarimetric studies of carbon stars by Goswami & Karinkuzhi (2013) include six objects from this sample. Among these, three objects show percentage V-band polarization at a level 0.2% (HD 55496 (pv% 0.18), HD 111721 (pv% 0.22, and HD 164922 (pv% 0.28)) indicating presence of circumstellar dust distribution in non-spherically symmetric envelopes. The other three objects, HD 92545, HD 107574 and HD 126681, show V-band percentage polarization at a level < 0.1%. Among CEMP stars, the group of CEMP-r/s stars show enhancement of both r- and s-process elements (0 < [Ba/Eu] < 0.5, (Beers & Christlieb 2005)). [Ba/Eu] ratio for our programme stars are not falling in this range. The two neutron-capture processes, the s-process and the r-process require entirely different astrophysical environments, different time-scales and neutron flux for their occurrence. While slow neutron-capture elements are believed to be produced in the inter pulse phases of low mass AGB stars, the rapid neutron-capture process requires very high temperatures and neutron flux and are expected to be produced during supernova explosions. To understand the contribution of these two processes to the chemical abundance of the neutron-capture elements we have conducted a parametric model based study. Our study indicates seven objects in our sample to have abundances of heavy elements with major contributions coming from the s-process.