Elemental abundances and classification of carbon-enhanced metal-poor stars

Abstract

We present a detailed study of carbon-enhanced metal-poor (CEMP) stars, based on high-resolution spectroscopic observations of a sample of 18 stars. The stellar spectra for this sample were obtained at the 4.2 m William Herschel Telescope in 2001 and 2002, using the Utrecht Echelle Spectrograph, at a resolving power R ∼ 52 000 and S /N ∼ 40, covering the wavelength range λλ3700−5700 Å. The atmospheric parameters determined for this sample indicate temperatures ranging from 4750 K to 7100 K, log g from 1.5 to 4.3, and metallicities −3.0 ≤ [Fe/H] ≤ −1.7. Elemental abundances for C, Na, Mg, Sc, Ti, Cr, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu, Gd, Dy are determined. Abundances for an additional 109 stars were taken from the literature and combined with the data of our sample. The literature sample reveals a lack of reliable abundance estimates for species that might be associated with the r-process elements for about 67% of CEMP stars, preventing a complete understanding of this class of stars, since [Ba/Eu] ratios are used to classify them. Although eight stars in our observed sample are also found in the literature sample, Eu abundances or limits are determined for four of these stars for the first time. From the observed correlations between C, Ba, and Eu, we argue that the CEMP-r/s class has the same astronomical origin as CEMP-s stars, highlighting the need for a more complete understanding of Eu production.

The first stars: CEMP-no stars and signatures of spinstars

“…The standard models for producing r-elements shows a very low scatter of the [Sr/Ba] ratio (about 0.2 dex) according to Cescutti et al (2013). The models of binary mass transfer from an AGB star to a lower mass companion (Herwig 2004;Karakas & Lattanzio 2007) account for the stars very well with [Sr/Ba] between −2 and 0, as confirmed by several authors (Masseron et al 2010;Allen et al 2012). This is the domain of CEMP-s stars, which in addition are all likely to be binary stars (Lucatello et al 2005).…”

“…We examine here the properties of CEMP-no stars, typically in the range of [Fe/H] = −2.5 to −7.1, where they occur, and compare them to model data of AGB and spinstars, when available. We use the data on spectroscopic determinations of chemical abundances collected by Masseron et al (2010), Allen et al (2012), and completed by recent observations by Norris et al (2013). When different values of the abundance parameters are given by these three groups, we use the most recent ones, although the small differences between authors have no consequences on the results.…”

“…We further discuss HE 1327-2326 in Sect. 7.2 We also include four other stars: CS 22945-017, CS 22956-028, CS 31080-095, and HE 1410+0213, cataloged as CEMPno stars by Masseron et al (2010) and as Ba-low (with 0 < [Ba/Fe] < 1) by Allen et al (2012). These stars are indicated by "b-" in the last column of Table 1 and labeled with an additional horizontal bar over the representative points in all figures.…”

Possible discovery of the r-process characteristics in the abundances of metal-rich barium stars

“…However, the inhomogeneity of the interstellar medium is not expected in such a environment with [Fe/H] > 0. Allen et al (2012) proposed CEMP-r/s stars have the same origin as CEMP-s stars. In other words, the additional r-process site is not needed for the formation of CEMP-r/s stars.…”

“…Although their abundance patterns were fitted very well using a mixed mechanism such as pre-enrichment, we still think that the most plausible formation mechanism for these stars is the s-process pollution. That we cannot explain them well using the s-process nucleosynthesis alone is maybe due to our incomplete knowledge on the production of Nd and other relevant elements by the s-process in metalrich and super metal-rich environments (see details in Allen et al 2012).…”