New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

Abstract

Context. The first generation of stars formed in the Galaxy left behind the chemical signatures of their nucleosynthesis in the interstellar medium, visible today in the atmospheres of low-mass stars that formed afterwards. Sampling the chemistry of those low-mass provides insight into the first stars. Aims. We aim to increase the samples of stars with extremely low metal abundances, identifying ultra metal-poor stars from spectra with modest spectral resolution and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable metallicities and carbon abundances from such spectra. Methods. We carry out follow-up observations of faint, V>19, metal-poor candidates selected from SDSS spectroscopy and observed with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at GTC. The SDSS and follow-up OSIRIS spectra were analyzed using the FERRE code to derive effective temperatures, surface gravities, metallicities and carbon abundances. In addition, a well-known extremely metal-poor star has been included in our sample to calibrate the analysis methodology. Results. We observed and analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars in our sample have been confirmed as extremely metal-poor stars, in the [Fe/H] < −3.3 regime. We report the recognition of J173403+644632, a carbonenhanced ultra metal-poor dwarf star with [Fe/H] = −4.3 and [C/Fe] = +3.1.Conclusions.

The Pristine survey – VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates

“…The spectral data reduction included bias substraction, flatfielding, and wavelength calibration -using CuNe+CuAr lamps for IDS and ISIS, and He+Ar for EFOSC-, and was performed using the onespec package in IRAF (Tody 1993). At the moderate S/N levels required for this program and at medium-resolution, the contribution of the interstellar medium (ISM) in the Ca H & K area is, in general, not resolved (see e.g., Aguado et al 2016Aguado et al , 2017a. In order to reduce the uncertainties from the spectral analysis, we remove the bluest part of the spectrum most affected by noise, considering only the region redder than 3700Å.…”

“…Beers et al 1985Beers et al , 1992Ryan & Norris 1991;Carney et al 1996), and is still used today in large spectroscopic surveys such as SEGUE, BOSS and LAMOST (see e.g. Caffau et al 2013;Aguado et al 2016Aguado et al , 2017aLi et al 2015Li et al , 2018François et al 2018, and references therein). However, there is additional information present in this spectral range, such as the Mg ib triplet and some weak Fe i and Sr ii lines, and these features can also contribute to the derivation of metallicities, provided that the S/N is high enough to resolve them.…”

“…The former approach has been quite successful and has contributed significantly to the current sample of the most metal-poor stars (e.g. Caffau et al 2013;Aoki et al 2013;Allende Prieto et al 2015;Aguado et al 2016;Li et al 2015;Aguado et al 2017aAguado et al ,b, 2018a, mostly with the help of large spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS, York et al 2000), the Sloan Extension for Galactic Understanding and Exploration (SEGUE, Yanny et al 2009), the Baryonic Oscillations Spectroscopic Survey (BOSS, Eisenstein et al 2011;Dawson et al 2013), and more recently the large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Deng et al 2012). More targeted searches have also been in use for many years, from early efforts using a Ca H & K objective-prism technique, such as the HK survey (Beers et al 1985(Beers et al , 1992 and the Hamburg ESO survey (Christlieb et al 2002), to more recent efforts using targeted narow/medium-band photometry at blue wavelengths, like the SkyMapper survey (Keller et al 2007;Wolf et al 2018;Casagrande et al 2019;Huang et al 2019), and the Pristine survey (Starkenburg et al 2017b).…”

The lowest detected stellar Fe abundance: the halo star SMSS J160540.18−144323.1

“…Targeted efforts by several groups (e.g., Beers et al 1985;Christlieb 2003;Keller et al 2007;Caffau et al 2013;Aguado et al 2017;Starkenburg et al 2017) have led to the discovery of roughly 30 stars with [Fe/H] < −4 1 (Abohalima & Frebel 2018), where the most iron-poor stars in fact only have upper limits. In ⋆ thomasn@mso.anu.edu.au 1 Throughout this discussion we use the 1D LTE abundance values.…”

A collection of model stellar spectra for spectral types B to early-M

“…Both the observed and model spectra retain their original slope; they have not been continuum normalized but simply divided by their mean values in the selected region. Further comparisons of these models with observations can be found, for example, in Aguado et al (2017Aguado et al ( , 2018…”