Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.1705n&link_type=abstract
American Astronomical Society, AAS Meeting #212, #17.05; Bulletin of the American Astronomical Society, Vol. 40, p.211
Astronomy and Astrophysics
Astronomy
Scientific paper
If magnetic activity in outer stellar atmospheres is due to an interplay between rotation and subsurface convection, as generally presumed, then we would not expect to observe indicators of activity in stars with effective temperature greater than about 8300 K. Xray and ultraviolet emission from these stars must be due either to a different mechanism or to an unresolved, active binary companion. Due to their poor spatial resolution, xray instruments have been especially susceptible to source confusion. Near ultraviolet spectra of stars hotter than this putative dividing line are dominated by photospheric continuum. We therefore used the Far Ultraviolet Spectroscopic Explorer (FUSE) to obtain spectra of the O VI emission lines, which are indicators of sub-coronal temperatures and lie at a wavelength at which the photospheric continuum of early A stars is relatively weak. We observed stars spanning the temperature range from 7720 to 10,000 K. Of these, 11 out of 14 showed O VI emission lines, including 6 with T > 8300 K. At face value, this suggests that activity does not fall off at the convective/radiative boundary. However, the emission lines are narrower than expected from these rapidly-rotating stars, suggesting that the emission could could come from unresolved companion late-type stars. Furthermore, the O VI emission level is consistent with that expected from an unseen active K or M dwarf binary companinon, and the high x-ray to O VI flux ratios indicate that this must be the case. Our results are therefore consistent with earlier studies that show a rapid drop-off in activity at the radiative/convective boundary expected around 8300 K. We have also been analyzing ultraviolet spectra of nearby A stars to search for evidence of circumstellar material. We will discuss our findings based on the entire FUSE sample of nearby, main-sequence A stars observed with FUSE.
Cheng Kwang-Peng
Neff James E.
Simon Theodore
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