Carbon-Enhanced Metal-Poor Stars in SDSS/Segue. I. Carbon Abundance Estimation and Frequency of Cemp Stars

Abstract

We describe a method for the determination of stellar [C/Fe] abundance ratios using low-resolution (R = 2000) stellar spectra from the Sloan Digital Sky Survey (SDSS) and its Galactic sub-survey, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). By means of a star-by-star comparison with a set of SDSS/SEGUE spectra with available estimates of [C/Fe] based on published high-resolution analyses, we demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N ≥ 15 Å −1 to a precision better than 0.35 dex for stars with atmospheric parameters in the range T eff = [4400, 6700] K, log g = [1.0, 5.0], [Fe/H] = [−4.0, +0.5], and [C/Fe] = [−0.25, +3.5]. Using the measured carbon-to-iron abundance ratios obtained by this technique, we derive the frequency of carbon-enhanced stars ([C/Fe] ≥ +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars and other samples from the literature. We find that the differential frequency slowly rises from almost zero to about 14% at [Fe/H] ∼ -2.4, followed by a sudden increase, by about a factor of three, to 39% from [Fe/H] ∼ -2.4 to [Fe/H] ∼ -3.7. Although the number of stars known with [Fe/H] < −4.0 remains small, the frequency of carbon-enhanced metal-poor (CEMP) stars below this value is around 75%. We also examine how the cumulative frequency of CEMP stars varies across different luminosity classes. The giant sample exhibits a cumulative CEMP frequency of 32% for [Fe/H] ≤ −2.5, 31% for [Fe/H] ≤ −3.0, and 33% for [Fe/H] ≤ −3.5; a roughly constant value. For the main-sequence turnoff stars, we obtain a lower cumulative CEMP frequency, around 10% for [Fe/H] ≤ −2.5, presumably due to the difficulty of identifying CEMP stars among warmer turnoff stars with weak CH G-bands. The dwarf population displays a large change in the cumulative frequency for CEMP stars below [Fe/H] = -2.5, jumping from 15% for [Fe/H] ≤ −2.5 to about 75% for [Fe/H] ≤ −3.0. When we impose a restriction with respect to distance from the Galactic midplane (|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low metallicity ([Fe/H] < −2.5), but rather, decreases, due to the dilution of C-rich material in stars that have undergone mixing with CNOprocessed material from their interiors. The frequency of CEMP stars near the main-sequence turnoff, which are not expected to have experienced mixing, increases for [Fe/H] ≤ −3.0. The general rise in the global CEMP frequency at low metallicity is likely due to the transition from the inner-halo to the outer-halo stellar populations with declining metallicity and increasing distance from the plane.

The early days of the Sculptor dwarf spheroidal galaxy

“…Skúladóttir et al (2015) notice that the fraction of CEMP-no stars in Sculptor starts deviating from the expected number derived from the statistics in the Milky Way halo below [Fe/H] ≤ −3. With a ∼30% fraction of CEMP stars among the Milky Way halo EMPS (see also Yong et al 2013;Lee et al 2013;Placco et al 2014), they argue that we should have observed three CEMP stars in Sculptor in this metallicity range. Among the classical dwarfs, only in Sextans and Draco have such stars been detected.…”

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

“…We estimated T eff , log g, and [Fe/H] using the n-SSPP, a modified version of the SEGUE Stellar Parameter Pipeline (SSPP; see e.g., Lee et al 2008Lee et al , 2013, for a detailed description of the methods employed). The n-SSPP uses lowresolution optical spectra (typically covering the wavelength range 380−550 nm), which we have for our programme stars, and photometric information (V 0 , (B − V) 0 , J 0 , and (J − K) 0 , corrected for extinction and reddening based on the Schlegel et al 1998 dust maps), to determine first-pass estimates for each parameter; the [Fe/H] estimates are particularly important.…”

“…The handful of stars known with [Fe/H] < −5.0 all exhibit large carbonicities (Lee et al 2013;Placco et al 2014b;Frebel & Norris 2015), including the most iron-poor star presently known, SMSS J031300.36-670839.3, with [Fe/H] < −7.5 (Keller et al 2014;Bessell et al 2015). A definitive interpretation of this increasing frequency has not yet been found, but it has been argued (e.g., Carollo et al 2012) that CEMP stars are more frequently associated with the outer-halo population of the Galaxy than with the inner-halo population, suggesting differences in the nucleosynthetic production sites of carbon in these components.…”

TOPoS

“…A carbon-enhanced metal-poor (CEMP) star has traditionally been defined as a star having [C/Fe] > +1 (Beers & Christlieb 2005), although [C/Fe] ≥ +0.7 has been used (e.g. Aoki et al 2007;Carollo et al 2012Carollo et al , 2014Norris et al 2013;Lee et al 2013;Skúladóttir et al 2015). In our opinion the latter criterion should not be used, since it may lead to ambiguous classifications.…”