An empirical metallicity tracer in CEMP and C-normal stars

Abstract

Deriving the metallicity, [Fe/H], in low-resolution spectra of carbon-enhanced metal-poor (CEMP) stars is a tedious task that, owing to the large number of line blends, often leads to uncertainties on [Fe/H] exceeding 0.25dex. The CEMP stars increase in number with decreasing [Fe/H] and some of these are known to be bona fide second generation halo stars. Hence, knowing their [Fe/H] is important for tracing the formation and chemical evolution of the Galaxy. Here, we aim to improve the [Fe/H] measurements in low-resolution spectra by avoiding issues related to blends. We improve our chemical tagging in such spectra at low metallicities. We developed an empirical way of deriving [Fe/H] in CEMP (and C-normal) stars that relates the equivalent width (EW) of strong lines, which remain detectable in lower-resolution, metal-poor spectra. The best [Fe/H] tracers are found to be Cr I and Ni I, which both show strong transitions in spectral regions that are free of molecular bands (between ~5200-6800A, a region accessible to most surveys). We derive different relations for dwarfs and giants. The relations are valid in the ranges ~-3<[Fe/H]<-0.5 and 10<EW<800mA (Cr) or [Fe/H]>-3.2 and EW>5mA (Ni), depending on the element and line as well as the stellar evolutionary stage. The empirical relations are valid for both CEMP and C-normal stars and have been proven to be accurate tracers in a sample of ~400 stars (mainly giants). The metallicities are accurate to within ~0.2 depending on the sample and resolution, and the empirical relations are robust to within 0.05-0.1dex. Our relations will improve the metallicity determination in future surveys, which will encounter a large number of CEMP stars, and will greatly speed up the process of determining [Fe/H] as the EWs only need to be measured in two or three lines in relatively clean regions compared to dealing with numerous blended Fe lines. Abrigded.

arXiv comment12 pages, 15 figures, 7 pages appendix. Accepted by A&A

References in corpus (43)

• Gaia Data Release 2. Summary of the contents and survey properties
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
• An absolutely calibrated effective temperature scale from the InfraRed Flux Method
• Determining stellar atmospheric parameters and chemical abundances of FGK stars with iSpec
• Carbon Enhanced Metal-Poor Stars. I. Chemical Compositions of 26 Stars
• The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample
• A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
• The Rise of the s-Process in the Galaxy
• The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5
• An extremely primitive halo star
• The Binary Frequency Among Carbon-Enhanced, s-Process Rich, Metal-Poor Stars
• Galactic Chemical Evolution and solar s-process abundances: dependence on the 13C-pocket structure
• Gravitational fragmentation in turbulent primordial gas and the initial mass function of Population III stars
• The s process in asymptotic giant branch stars of low metallicity and the composition of carbon-enhanced metal-poor stars
• $s$-process production in rotating massive stars at solar and low metallicities
• First stars XII. Abundances in extremely metal-poor turnoff stars,and comparison with the giants
• A holistic approach to carbon-enhanced metal-poor stars
• The intermediate neutron-capture process and carbon-enhanced metal-poor stars
• TOPoS: II. On the bimodality of carbon abundance in CEMP stars. Implications on the early chemical evolution of galaxies
• The role of binaries in the enrichment of the early Galactic halo. III. Carbon-enhanced metal-poor stars -- CEMP-s stars
• The Hamburg/ESO R-process Enhanced Star survey (HERES) III. HE 0338-3945 and the formation of the r+s stars
• An Elemental Assay of Very, Extremely, and Ultra Metal-Poor Stars
• Carbon-enhanced metal-poor stars: the most pristine objects?
• Silver and palladium help unveil the nature of a second r-process
• Are C-rich ultra iron poor stars also He-rich?
• Descendants of the first stars: the distinct chemical signature of second generation stars
• SP_Ace: a new code to derive stellar parameters and elemental abundances
• The formation of the Milky Way halo and its dwarf satellites, a NLTE-1D abundance analysis. I. Homogeneous set of atmospheric parameters
• Lithium abundances in CEMP stars
• Observational Constraints on First-Star Nucleosynthesis. II. Spectroscopy of an Ultra Metal-Poor CEMP-no Star
• Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba
• Abundances of carbon-enhanced metal-poor stars as constraints on their formation
• Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars
• Carbon and oxygen in metal-poor halo stars
• The Most Metal-Poor Stars. V. The CEMP-no Stars in 3D and Non-LTE
• Follow-up observations of extremely metal-poor stars identified from SDSS
• An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres I. Formation of the G-band in metal-poor dwarf stars
• Understanding the orbital periods of CEMP-s stars
• Constraints on CEMP-no progenitors from nuclear astrophysics
• ATHOS: On-the-fly stellar parameter determination of FGK stars based on flux ratios from optical spectra
• Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST
• Metal Abundance Calibration of the Ca II Triplet Lines in RR Lyrae Stars

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