Refined estimation of the metallicity and carbon abundances for stars observed as part of the JINA (Joint Institute for Nuclear Astrophysics)-suppported SEGUE project

Abstract

A proposal is made to identify and analyze up to several thousand carbon-enhanced stars in the Milky Way galaxy, based on medium-resolution stellar spectroscopy and broadband ugriz photometry obtained with the Sloan Digital Sky Survey (both the former survey now known as SDSS-I and its extension, known as SDSS-II, which includes the project SEGUE: Sloan Extension for Galactic Understanding and Exploration). Based on techniques that have either already been developed, or are presently being refined, these data are sufficient to derive estimates of both [Fe/H] and [C/Fe] for all stars in this sample. Accurate radial velocities will also enable detailed investigations of the kinematics of these stars. These data will be used to constrain (1) the frequency of carbon enhancement among stars as a function of declining metallicity, which provides information on the nature of the IMF for early-generation stars, (2) the distribution of carbon enhancement among low-metallicity stars, e.g., is it continuous or multi-modal?, and (3) the separation of carbon-enhanced, metal-poor stars into (at least) the two main categories that are presently recognized, those that exhibit the presence of s-process elements, and those that do not, based on the detection (or not) of the strong barium and strontium features that are associated with production in the s-process. It is further planned to obtain a substantial database of high-quality high-resolution spectroscopy for the brightest and most interesting subset of this sample, including stars of very low metallicity, or stars that are peculiar in other respects. A subset of these stars will form the basis for long-term radial-velocity monitoring, which we will initiate with this proposal. Accomplishment of this objective will enable detailed studies of (1) the diverse patterns of elemental abundances associated with the carbon-enhancement phenomenon, which apparently includes both production and mass transfer from Asymptotic Giant Branch progenitors, as well as possible production at extremely low metallicity by either low-luminosity Type II supernovae or massive, rapidly rotating progenitors with [Fe/H] < -6.0, (2) the variation in these abundance patterns across the H-R diagram, in particular from the main-sequence turnoff to the tip of the giant branch, which provides information on important stellar mixing processes, (3) the abundance ratios of an important subset of these stars that is associated with the presence of elements apparently produced in both the r- and s-processes, and (4) determination of the binarity (or lack thereof) and orbital parameters for the stars shown to be binaries, knowledge that is required in order to suggest definitive origins of these stars. (AU)