Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...304..365z&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 304, May 1, 1986, p. 365-370.
Astronomy and Astrophysics
Astrophysics
19
Absorption Spectra, Emission Spectra, Helium, Late Stars, Stellar Spectrophotometry, X Ray Spectra, Dwarf Stars, Giant Stars, Spectral Line Width, Stellar Coronas, Stellar Luminosity, Stellar Magnetic Fields, Supergiant Stars
Scientific paper
Measurements of He I 10830 A absorption equivalent widths and fractional X-ray luminosities L(x)/L(bo) are presented for a large sample of late-type stars. A correlation of increasing 10830 A strength with increasing L(x)/L(bo) is evident in dwarfs, giants, and supergiants later than and including spectral type F7. No equivalent correlation is apparent in main-sequence stars earlier than dF7 (B-V of about 0.5). The data suggest that coronal X-ray irradiation constitutes an important source of He I excitation in late-type stars. The fact that X-ray luminosity depends exponentially on 10830 A equivalent width is attributed to the production of X-rays in active regions covering only a fraction of the stellar surface. It is conjectured that the absence of 10830 A X-ray correlation in early dF stars is due to the increased importance of electron collisional excitation of He I. This effect is discussed in light of evidence that the nature of magnetic activity changes significantly across the color range spanned by dF stars; a change related directly to the increasing depth of stellar convection with decreasing effective temperature.
Zarro Dominic M.
Zirin Harold
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