Abundances of carbon-enhanced metal-poor stars as constraints on their formation

Abstract

Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] = −2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V − K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12 C/ 13 C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (∼10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres

“…We refer to these chemically peculiar stars as carbon-enhanced metal-poor (CEMP) stars. Slightly different definitions of CEMP stars have been proposed (see for example Hansen et al 2016, and references therein), but we prefer to keep the "classical" definition, for the reasons explained in Bonifacio et al (2015). As CEMP stars make up a considerable fraction of metal-poor stars, it is important to understand how they acquired their enhanced carbon abundance.…”

Carbon-enhanced metal-poor stars in the SDSS–APOGEE data base

“…The latter scenario is more likely as a result of the instability of the Roche lobe in thermally pulsing AGB stars and the typically larger spatial separations of these binary systems (Paczyński 1965, Abate et al 2013. Similarly, Herwig (2005), Placco et al (2013), and Hansen et al (2016c) have shown the abundance profiles of the CEMP-s stars are consistent with enrichment from an AGB companion. The difference between these similarly natured objects currently exists only in an arbitrary cut in metallicity.…”

Abundance analysis of a CEMP-no star in the Carina dwarf spheroidal galaxy

“…ALW-8 is in the RGB phase where the primordial carbon abundance has been reduced due to the CN cycling and the presently observed abundance will be lower. While the moderate [C/N] ratio of −0.2 dex alone would suggest that its surface composition has not been significantly altered by internal mixing during stellar evolution (Spite et al 2005;Hansen et al 2016a), other tracers of mixing paint a different picture. Amongst these are the strong depletion in lithium and the carbon isotopic ratio, where Spite et al (2006) suggest an upper limit of 12 C/ 13 C < 10 as a proxy for effective deep mixing.…”

“…Amongst these, Carbon-enhanced metal-poor (CEMP) stars (defined by [Fe/H] < −2.0 and [C/Fe] >0.7) are of great interest since the origin of the carbon overabundance is versatile and closely related to the formation and enrichment history of the Send offprint requests to: A. Susmitha; e-mail: susmitha@iiap.res.in; a.koch1@lancaster.ac.uk; sivarani@iiap.res.in Based on observations collected at the European Southern Observatory at Paranal, Chile; Large Programme proposal 171.B-0520. Table A.1 is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.ustrasbg.fr/cgi-bin/qcat?J/A+A/ host stellar system (Aoki et al 2007;Masseron et al 2010;Carollo et al 2014;Bonifacio et al 2015;Hansen et al 2016a;Koch et al 2016;Lee et al 2017). These stars are primarily categorized based on the presence and absence of neutron capture elements as CEMP-s, CEMP-r, or CEMP-r/s, versus CEMP-no, respectively.…”

“…arxiv_carina (e.g., Umeda & Nomoto 2003;Meynet et al 2006;Kobayashi et al 2011;Ishigaki et al 2014). In this regard, Yoon et al (2016) and Hansen et al (2016a) suggest that probably more than one class of first-generation progenitors is required to account for the abundance patterns of CEMP-no stars.…”