Something borrowed, something blue: The nature of blue metal-poor stars inferred from their colours and chemical abundances

Abstract

Blue metal-poor stars (BMPs) are main sequence stars that appear bluer and more luminous than normal turnoff stars. They were originally singled out by using B-V and U-B colour cuts. Early studies found that a larger fraction of field BMP stars were binaries compared to normal halo stars. Thus, BMP stars are ideal field blue straggler candidates for investigating internal stellar evolution processes and binary interaction. In particular, the presence or depletion in lithium in their spectra is a powerful indicator as to their origin. They are either old, halo blue stragglers experiencing internal mixing processes or mass transfer (Li-depletion), or intermediate-age, single stars of possibly extragalactic origin (2.2dex halo plateau Li). However, we note that internal mixing processes can lead to an increased level of Li. Hence, this study combines photometry and spectroscopy to unveil the origin of various BMP stars. We first show how to separate binaries from young blue stars using photometry, metallicity, and lithium. Using a sample of 80 BMP stars (T>6300K), we find that 97% of the BMP binaries have V-Ks_0 < 1.08$\pm 0.03$, while BMP stars that are not binaries lie above this cut in 2/3 of the cases. This cut can help classify stars which lack radial velocities from follow-up observations. We trace the origin of two BMP stars from the photometric sample by conducting a full chemical analysis using new high-resolution and high SN spectra. Based on their radial velocities, Li, alpha, and s- and r-process abundances we show that BPS CS22874-042 is a single star (A(Li)$=2.38\pm 0.10$dex) while with A(Li)$=2.23\pm 0.07$dex CD-48 2445 is a binary, contrary to earlier findings. Our analysis emphasises that field blue stragglers can be segregated from single metal-poor stars, using V-Ks colours with a fraction of single stars polluting the binary sample, but not vice versa.(Abridged)

16 pages, appendix/online table, 7 figures (accepted for publication in A&A)

References in corpus (24)

• Magnetorotationally driven Supernovae as the origin of early galaxy $r$-process elements?
• Determining stellar atmospheric parameters and chemical abundances of FGK stars with iSpec
• A Bitter Pill: The Primordial Lithium Problem Worsens
• Galactic Chemical Evolution and solar s-process abundances: dependence on the 13C-pocket structure
• Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
• First stars VII. Lithium in extremely metal poor dwarfs
• NLTE abundances of Mn in a sample of metal-poor stars
• Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars
• Line lists for the A2PI-X2Sigma+ (red) and {B2Sigma+-X2Sigma} (violet) Systems of CN, 13C14N, and 12C15N, and Application to Astronomical Spectra
• Silver and palladium help unveil the nature of a second r-process
• NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
• The Gaia FGK Benchmark Stars - High resolution spectral library
• Atomic and Molecular Data for Optical Stellar Spectroscopy
• The origin and evolution of the odd-Z iron-peak elements Sc, V, Mn, and Co in the Milky Way stellar disk
• Non-LTE line formation for heavy elements in four very metal-poor stars
• Evolution of the Barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
• How many nucleosynthesis processes exist at low metallicity?
• Lithium abundances in CEMP stars
• Non-LTE Balmer line formation in late-type spectra: Effects of atomic processes involving hydrogen atoms
• First Stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876--032
• The low Sr/Ba ratio on some extremely metal-poor stars
• Possible signature of hypernova nucleosynthesis in a beryllium rich halo dwarf
• A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. III. A 2MASS-Cleaned Version
• Field Blue Stragglers and Related Mass Transfer Issues

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• Titans metal-poor reference stars. I. Accurate effective temperatures and surface gravities for dwarfs and subgiants from 3D non-LTE H$\alpha$ profiles and Gaia parallaxes
• Gaia-Sausage-Enceladus star formation history as revealed by detailed elemental abundances
• Field blue straggler stars: Discovery of white dwarf companions to blue metal-poor stars using UVIT/AstroSat
• One of Everything: The Breakthrough Listen Exotica Catalog
• Chemo-dynamical abundance analysis of the very metal-poor halo star HE 2315-4240