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Description
M dwarfs are the most numerous stars in the galaxy. They are characterized by strong magnetic activity. The ensuing high-energy emission is crucial for the evolution of their planets and the eventual presence of life on them. The authors systematically study the X-ray and ultraviolet emission of a subsample of M dwarfs from a recent proper-motion survey, selecting all M dwarfs within 10 pc to obtain a nearly volume-limited sample (~90% completeness). Archival ROSAT, XMM-Newton and GALEX data are combined with published spectroscopic studies of H-alpha emission and rotation to obtain a broad picture of stellar activity on M dwarfs. The authors make use of synthetic model spectra to determine the relative contributions of photospheric and chromospheric emission to the ultraviolet flux. They also analyze the same diagnostics for a comparison sample of young M dwarfs in the TW Hya association (~10 Myr old). The authors find that generally the emission in the GALEX bands is dominated by the chromosphere but the photospheric component is not negligible in early-M field dwarfs. The surface fluxes for the H-alpha, near-ultraviolet, far-ultraviolet and X-ray emission are connected via a power-law dependence. The authors present in the reference paper for the first time such flux-flux relations involving broad-band ultraviolet emission for M dwarfs. Activity indices are defined as the flux ratios between the activity diagnostics and the bolometric flux of the star in analogy to the Ca II R'(HK) index. For a given spectral type, these indices display a spread of 2-3 dex which is largest for M4-type stars. Strikingly, at mid-M spectral types, the spread of rotation rates is also at its highest level. The mean activity index for fast rotators, likely representing the saturation level, decreases from X-rays over the FUV to the NUV band and H-alpha, i.e. the fractional radiation output increases with atmospheric height. The comparison to the ultraviolet and X-ray properties of TW Hya members shows a drop of nearly three orders of magnitude for the luminosity in these bands between ~10 Myr and a few Gyr age. A few young field dwarfs (<1 Gyr) in the 10-pc sample bridge the gap indicating that the drop in magnetic activity with age is a continuous process. The slope of the age decay is steeper for the X-ray than for the UV luminosity. This sample is based on the All-Sky Catalog of bright M dwarfs published by Lepine & Gaidos (2011, AJ, 142, 138, <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/AJ/142/138">CDS Cat. J/AJ/142/138</a>, available at the HEASARC as the MDWARFASC table). The authors selected all 163 stars from this reference that are within 10pc. Four of these stars that were discovered to be actually late K-type stars were removed from this initial sample, leaving a final sample of 159 stars. This table was created by the HEASARC in July 2017 based upon <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/MNRAS/431/2063">CDS Catalog J/MNRAS/431/2063</a> files table1.dat and table2.dat. The positions of the stars were not explicitly given in these tables, but were taken by the HEASARC from the All-Sky Catalog of Bright M Dwarfs published by Lepine & Gaidos (2011, AJ, 142, 138, <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/AJ/142/138">CDS Cat. J/AJ/142/138</a>, available at the HEASARC as the MDWARFASC table). The version of Table 2 used by the HEASARC is the corrected one given in the Erratum (Stelzer et al. 2014) rather than the version given in the original paper (Stelzer et al. 2013). This is a service provided by NASA HEASARC .
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