Spectroscopic Properties of Cool Stars in the Sloan Digital Sky Survey: An Analysis of Magnetic Activity and a Search for Subdwarfs

By Andrew A. West, S. Hawley, Lucianne M. Walkowicz, K. Covey, N. Silvestri, S. Raymond, H. Harris, J. Munn, P. McGehee, Željko Ivezic´ and 1 more. 2004

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We present a spectroscopic analysis of nearly 8000 late-type dwarfs in the Sloan Digital Sky Survey. Using the Hα emission line as an activity indicator, we investigate the fraction of active stars as a function of spectral type and find a peak near type M8, confirming previous results. In contrast to past findings, we find that not all M7–M8 stars are active. We show that this may be a selection effect of the distance distributions of previous samples, since the active stars appear to be concentrated near the Galactic plane. We also examine the activity strength (ratio of the luminosity emitted in Hα to the bolometric luminosity) for each star and find that the mean activity strength is constant over the range M0–M5 and declines at later types. The decline begins at a slightly earlier spectral type than previously found. We explore the effect that activity has on the broadband photometric colors and find no significant differences between active and inactive stars. We also carry out a search for subdwarfs using spectroscopic metallicity indicators and find 60 subdwarf candidates. Several of these candidates are near the extreme subdwarf boundary. The spectroscopic subdwarf candidates are redder by ∼0.2 mag in g-r compared with disk dwarfs at the same r-i color.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Simulations of Dynamo Action in Fully Convective Stars (2007, background): “Indeed, the fraction of stars that show such emission increases markedly after the transition to full convection, reaching a maximum around spectral type M8 (West et al. 2004).”
ERRATUM: “THE LUMINOSITY AND MASS FUNCTIONS OF LOW-MASS STARS IN THE GALACTIC DISK. II. THE FIELD” (2010, AJ, 139, 2679) (2010, background): “…resource for statistical investigations of the properties of low-mass stars, including their magnetic activity and chromospheric properties (West et al. 2004, 2008), flare characteristics (Kowalski et al. 2009), and their use as tracers of Galactic structure (GS) and kinematics (Bochanski…”
CARMENES input catalogue of M dwarfs. I. Low-resolution spectroscopy with CAFOS (2015, background): “M dwarfs with late spectral types tend to show the Hα line in (strong) emission more often than earlier stars (see e.g. Hawley et al. 1996, West et al. 2004, or Reiners et al. 2013).”
A NEARBY M STAR WITH THREE TRANSITING SUPER-EARTHS DISCOVERED BY K2 (2015, background): “The strongest line in the series, Hα at 6563 Å, is classically used to asses the activity of M dwarfs and as a crude indicator of age (West et al. 2004, 2008).”
A CATALOG OF ROTATION AND ACTIVITY IN EARLY-M STARS (2012, background): “Despite the changes in stellar structure, strong magnetic activity also appears in very-low mass stars that are fully convective (Hawley et al. 1996; West et al. 2004).”
CONSTRAINING THE AGE–ACTIVITY RELATION FOR COOL STARS: THE SLOAN DIGITAL SKY SURVEY DATA RELEASE 5 LOW-MASS STAR SPECTROSCOPIC SAMPLE (2007, background): “The u − g and g − r colors were not shown because of the large influence that metallicity has on the g − r colors (West et al. 2004; Bochanski et al. 2007a) and the large uncertainties in the u-band fluxes for most of the sample stars.”
THE ULTRAVIOLET RADIATION ENVIRONMENT AROUND M DWARF EXOPLANET HOST STARS (2012, background): “M dwarfs display variability on many timescales and flare activity occurs with greater frequency and amplitude than on main-sequence G and K dwarfs (Hawley et al. 1996; West et al. 2004; Welsh et al. 2007; Walkowicz et al. 2011).”
Large-scale magnetic topologies of late M dwarfs⋆ (2008, background): “However, many fullyconvective stars are known to show various signs of activ- ity such as radio, Balmer line, and X-ray emissions (e.g., Joy & Humason 1949; Lovell et al. 1963; Delfosse et al. 1998; Mohanty & Basri 2003; West et al. 2004).”
Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, background): “Yet, many M dwarfs are known to be active, and these stars follow the usual rotation-activity relation (e.g., Delfosse et al. 1998; Reiners & Basri 2007; Kiraga & Stepien 2007; West et al. 2004).”
THE Hα EMISSION OF NEARBY M DWARFS AND ITS RELATION TO STELLAR ROTATION (2016, background): “West et al. (2004) and Kruse et al. (2010) show a similar decrease in LHα/Lbol for spectral types M6 and later, and similar levels of mean LHα/Lbol.”
A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO (2009, background): “A large fraction of ultracool dwarfs show strong chromospheric emission (e.g., West et al. 2004) probably due to magnetic fields that are sustained by a stellar dynamo (Reiners & Basri 2007).”
Biosignatures from Earth-Like Planets Around M Dwarfs (2005, unknown): “ West et al. (2004) found that LH￻/Lbol varied by over two orders of magnitude among the active stars, with the variation being about an order of magnitude at a given spectral type.”
The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars (2006, background): “It is actually hard to find nearby VLMS without Hα emission between M5 and M9 (West et al. 2004), and all but 2 of our 14 targets in that range exhibit it (but at different levels).”
THE SLOAN DIGITAL SKY SURVEY DATA RELEASE 7 SPECTROSCOPIC M DWARF CATALOG. I. DATA (2011, methodology): “We include the g − r colors but warn that they are likely metallicity dependent (West et al. 2004; Bochanski et al. 2007b; see also Section 3.3 below).”
Low-Mass Dwarf Template Spectra from the Sloan Digital Sky Survey (2006, methodology): “SDSS has already proven to be an excellent source of low-mass stellar spectroscopy (Hawley et al. 2002; Raymond et al. 2003; West et al. 2004; Silvestri et al. 2006).”
Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs (2008, background): “…L and T dwarfs have relative luminosities two orders of magnitude lower than observed for saturated activity in earlier type M dwarfs (Neuhäser et al. 1999; Fleming et al. 2003; Gizis et al. 2000; Mohanty & Basri 2003; Stelzer 2004; West et al. 2004; Stelzer et al. 2006a; Schmidt et al. 2007).”
The Habitability of Planets Orbiting M-dwarf Stars (2016, background): “…that are magnetically active has been found to increase monotonically with later spectral type—likely due to the longer activity lifetimes of late-type M dwarfs (West et al., 2008)—and reaches a peak at around a spectral type of M7 (Hawley et al., 1996a; Gizis et al., 2000; West et al., 2004).”
Stellar SEDs from 0.3 to 2.5 μm: Tracing the Stellar Locus and Searching for Color Outliers in the SDSS and 2MASS (2007, methodology): “…SDSS spectra, the Hammer has since been modified to allow it to process spectra in a variety of formats, and also incorporates routines written by West et al. (2004) to identify M-type subdwarfs and diagnose stellar magnetic activity; the IDL code for the Hammer can be downloaded from…”
COUNTING LOW-MASS STARS IN INTEGRATED LIGHT (2011, background): “Activity occurs in≈ 60% of such stars, and increases to ≈ 75% for M8V stars (West et al. 2004).”