Cool Subdwarf Investigations (CSI) I: New Thoughts for the Spectral Types of K and M Subdwarfs

Abstract

Using new spectra of 88 K and M-type subdwarfs, we consider novel methods for assigning their spectral types and take steps toward developing a comprehensive spectral sequence for subdwarf types K3.0 to M6.0. The types are assigned based on the overall morphology of spectra covering 6000Åto 9000Å. The types and sequence presented link the spectral types of cool subdwarfs to their main sequence counterparts, with emphasis on the relatively opacity-free region from 8200--9000Å. When available, supporting abundance, kinematic, and trigonometric parallax information is used to provide more complete portraits of the observed subdwarfs. We find that the CaHn (n $=$ 1--3) and TiO5 indices often used for subdwarf spectral typing are affected in complicated ways by combinations of subdwarfs' temperatures, metallicities, and gravities, and we use model grids to evaluate the trends in all three parameters. Because of the complex interplay of these three characteristics, it is not possible to identify a star as an ``extreme'' subdwarf simply based on very low metallicity, and we suggest that the modifiers ``extreme'' or ``ultra'' only outline locations on spectroscopic indices plots, and should not be used to imply low or very low metallicity stars. In addition, we propose that ``VI'' be used to identify a star as a subdwarf, rather than the confusing ``sd'' prefix, which is also used for hot O and B subdwarfs that are unrelated to the cool subdwarfs discussed in this paper.

arXiv comment84 pages, 35 figures, accepted to AJ

References in corpus (30)

• The Geneva-Copenhagen survey of the Solar neighbourhood: Ages, metallicities, and kinematic properties of 14,000 F and G dwarfs
• Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Southern Sample
• The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
• Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Northern Sample I
• Model Atmospheres for M (Sub)Dwarfs: I. The base model grid
• A Catalog of Northern Stars With Annual Proper Motions Larger Than 0.15 Seconds of Arc (LSPM catalog - North)
• M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale
• Spectroscopic Properties of Cool Stars in the SDSS: An Analysis of Magnetic Activity and a Search for Subdwarfs
• On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars
• Low-Mass Dwarf Template Spectra from the Sloan Digital Sky Survey
• Catalogue of [Fe/H] determinations for FGK stars : 2001 edition
• Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)
• A catalogue of [Fe/H] determinations : 1996 edition
• The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics
• Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs
• The Solar Neighborhood X: New Nearby Stars in the Southern Sky and Accurate Photometric Distance Estimates for Red Dwarfs
• Metallicity measurements using atomic lines in M and K dwarf stars
• The Solar Neighborhood XIII: Parallax Results from the CTIOPI 0.9-m Program -- Stars with mu >= 1"/year (MOTION Sample)
• The Solar Neighborhood VI: New Southern Stars Identified by Optical Spectroscopy
• Optical Spectroscopy of 2MASS Color-Selected Ultracool Subdwarfs
• Accurate M Dwarf Metallicities from Spectral Synthesis: A Critical Test of Model Atmospheres
• Southern Infrared Proper Motion Survey I: Discovery of New High Proper Motion Stars From First Full Hemisphere Scan
• Spectroscopic classification of red high proper motion objects in the Southern Sky
• Discovery of the Coolest Extreme Subdwarf
• The Subdwarf Luminosity Function
• The Solar Neighborhood XV: Discovery of New High Proper Motion Stars with mu >= 0.4"/yr between Declinations -47 degrees and 00 degrees
• The Solar Neighborhood XII: Discovery of New High Proper Motion Stars with 1.0"/yr > mu >= 0.4"/yr between Declinations -90 degrees and -47 degrees
• HST search for M subdwarf binaries
• Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs
• Ages of White Dwarf-Red Subdwarf Systems

Cited by in corpus (53)

• A Survey of Stellar Families: Multiplicity of Solar-Type Stars
• The Solar Neighborhood XXXVII: The Mass-Luminosity Relation for Main Sequence M Dwarfs
• The Solar Neighborhood XLV. The Stellar Multiplicity Rate of M Dwarfs Within 25 pc
• Prospecting in late-type dwarfs: a calibration of infrared and visible spectroscopic metallicities of late-K and M dwarfs spanning 1.5 dex
• The Solar Neighborhood XXXV: Distances to 1404 M Dwarf Systems Within 25 pc in the Southern Sky
• The Solar Neighborhood. XXXIII. Parallax Results from the CTIOPI 0.9m Program: Trigonometric Parallaxes of Nearby Low-Mass Active and Young Systems
• The Solar Neighborhood XLIV: RECONS Discoveries within 10 Parsecs
• Exploring the age dependent properties of M and L dwarfs using Gaia and SDSS
• What are little worlds made of? Stellar abundances and the building blocks of planets
• The Solar Neighborhood XXIV. Parallax Results from the CTIOPI 0.9-m Program: Stars with $\mu$ $\ge$ 1\farcs0 yr${}^{-1}$ (MOTION Sample) and Subdwarfs
• The Benchmark Ultracool Subdwarf HD 114762B: A Test of Low-Metallicity Atmospheric and Evolutionary Models
• Discovery of a T dwarf + white dwarf binary system
• A high resolution spectroscopic atlas of M subdwarfs - Effective temperature and metallicity
• Radii of 88 M Subdwarfs and Updated Radius Relations for Low-Metallicity M Dwarf Stars
• Trigonometric parallaxes of ten ultracool subdwarfs
• A 3D Search for Companions to 12 Nearby M-Dwarfs
• Primeval very low-mass stars and brown dwarfs - IV. New L subdwarfs, Gaia astrometry, population properties, and a blue brown dwarf binary
• A blue depression in the optical spectra of M dwarfs
• The Solar Neighborhood XXIII CCD Photometric Distance Estimates of SCR Targets -- 77 M Dwarf Systems within 25 Parsecs
• The Solar Neighborhood XXXI: Discovery of an Unusual Red+White Dwarf Binary at ~25 Parsecs via Astrometry and UV Imaging
• The Solar Neighborhood. XVIII. Discovery of new proper motion stars with 0.40 arcsec yr^-1 > mu >= 0.18 arcsec yr^-1 between declinations -90 degrees and -47 degrees
• New ultracool subdwarfs identified in large-scale surveys using Virtual Observatory tools: Part I: UKIDSS LAS DR5 vs SDSS DR7
• CCD Parallaxes for 309 Late-type Dwarfs and Subdwarfs
• A spectroscopic and proper motion search of Sloan digital Sky Survey: red subdwarfs in binary systems
• GTC Osiris spectroscopic identification of a faint L subdwarf in the UKIRT Infrared Deep Sky Survey
• The Solar Neighborhood XXIX: The Habitable Real Estate of our Nearest Stellar Neighbors
• Cool Subdwarf Investigations II: Multiplicity
• A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics
• New ultracool subdwarfs identified in large-scale surveys using Virtual Observatory tools: II. SDSS DR7 vs UKIDSS LAS DR6, SDSS DR7 vs UKIDSS LAS DR8, SDSS DR9 vs UKIDSS LAS DR10, and SDSS DR7 vs 2MASS
• Characterization of the Very Low Mass Secondary in the GJ 660.1AB System
• Cool dwarfs stars from the Torino Observatory Parallax Program
• Lucky Imaging of M subdwarfs
• Chemical Properties of the Local Galactic Disk and Halo. I. Fundamental Properties of 1,544 Nearby, High Proper-Motion M dwarfs and subdwarfs
• M subdwarf research. I. Identification, modified classification system, and sample construction
• Spectral energy distribution of M-subdwarfs: A study of their atmospheric properties
• Spectral classification and HR diagram of pre-main sequence stars in NGC6530
• Metallicity, temperature, and gravity scales of M subdwarfs
• Cool Subdwarf Investigations III: Dynamical Masses of Low Metallicity Subdwarfs
• The Solar Neighborhood XXV: Discovery of New Proper Motion Stars with 0.40 "/yr > mu > 0.18 "/yr between Declinations -47 degrees and 00 degrees
• Using autoencoders and deep transfer learning to determine the stellar parameters of 286 CARMENES M dwarfs
• Primeval very low-mass stars and brown dwarfs -- VII. The discovery of the first wide M + L extreme subdwarf binary
• An ancient F-type subdwarf from the halo crossing the Galactic plane
• The Solar Neighborhood XLII. Parallax Results from the CTIOPI 0.9-m Program --- Identifying New Nearby Subdwarfs Using Tangential Velocities and Locations on the H-R Diagram
• The Closest M-Dwarf Quadruple System to the Sun
• M Subdwarf Research. II. Atmospheric Parameters and Kinematics
• Collisions of Terrestrial Worlds: The Occurrence of Extreme Mid-Infrared Excesses around Low-Mass Field Stars
• Spectro-photometric characterization of high proper motion sources from WISE
• M dwarfs and the fraction of high carbon-to-oxygen stars in the solar neighbourhood
• Low-mass and sub-stellar eclipsing binaries in stellar clusters
• M Subdwarf Research III. Spectroscopic Diagnostics for Breaking Parameter Degeneracy
• Metal-Depleted Brown Dwarfs
• Benchmark brown dwarfs -- I. A blue M2 + T5 wide binary and a probable young [M4 + M4] + [T7 + T8] hierarchical quadruple
• Optical and Near-Infrared Spectroscopy of the L Subdwarf SDSS J125637.13-022452.4