Calibrating M-Dwarf Metallicities Using Molecular Indices: Extension to Low-metallicity Stars

• Published in: Publications of the Astronomical Society of the Pacific 2009-02, Vol.121 (876), p.117-124
• Main Authors: Woolf, Vincent M., Lépine, Sébastien, Wallerstein, George
• Format: Article
• Language: English
• Subjects: Astronomy; Calibration; Dwarf stars; Earth, ocean, space; Exact sciences and technology; Main sequence stars; Metal abundance; Molecular spectra; Spectral index; Stars; Stellar spectra; Subdwarf stars; Telescopes
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/597433

We have calculated Fe and Ti abundances in 12 low-metallicity main sequence M stars using high-resolution spectra. These subdwarf and extreme-subdwarf stars allow us to extend our calibration of a method to determine cool-dwarf-star metallicities using molecular band strength indices from low-resolution spectra. Our calibration can now be used to determine metallicity to within± 0.3 dex ± 0.3     dex for stars with [Fe/H] between-1.5 - 1.5 and+0.05 + 0.05 and temperatures between 3500 and 4000 K. We also report a method to estimate temperatures for M dwarfs using equivalent width measurements of the infrared Ca II triplet and the K I line at 7699 Å. Our metallicity measurements show that the recently proposed classification system for low-mass stars (dwarfs, subdwarfs, extreme subdwarfs, and ultrasubdwarfs) does represent a metallicity sequence, with the ultrasubdwarfs the most metal-poor stars.