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The UV and X-ray activity of the M dwarfs within 10 pc of the Sun [ Erratum: 2014MNRAS.442..343S ]
Stelzer, B.; Marino, A.; Micela, G.; López-Santiago, J.; Liefke, C.
AA(INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy; ), AB(Dipartimento di Fisica e Astronomia, G. Galilei, vicolo dell'Osservatorio 3, I-35122 Padova, Italy), AC(INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy), AD(Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain), AE(Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, D-69120 Heidelberg, Germany)
Monthly Notices of the Royal Astronomical Society, Volume 431, Issue 3, p.2063-2079 (MNRAS Homepage)
Publication Date:
Astronomy Keywords:
stars: activity, stars: chromospheres, stars: coronae, stars: late-type, ultraviolet: stars, X-rays: stars
Abstract Copyright:
2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Bibliographic Code:


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. We 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 per cent completeness). Archival ROSAT, XMM-Newton and GALEX data are combined with published spectroscopic studies of Halpha emission and rotation to obtain a broad picture of stellar activity on M dwarfs. We make use of synthetic model spectra to determine the relative contributions of photospheric and chromospheric emission to the ultraviolet flux. We also analyse the same diagnostics for a comparison sample of young M dwarfs in the TW Hya association (˜10 Myr). We 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 Halpha, near-ultraviolet, far-ultraviolet and X-ray emission are connected via a power-law dependence. We present here for the first time such flux-flux relations involving broad-band ultraviolet emission for M dwarfs. Activity indices are defined as flux ratio between the activity diagnostic and the bolometric flux of the star in analogy to the Ca II R'HK index. For given spectral type, these indices display a spread of 2-3 dex which is largest for M4 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 Halpha, 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 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.

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