The B Supergiant Discontinuous Drop in X-ray Luminosity at Spectra Type B1

Details The B Supergiant Discontinuous Drop in X-ray Luminosity at Spectra Type B1 The B Supergiant Discontinuous Drop in X-ray Luminosity at Spectra Type B1

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...211.8005w&link_type=abstract

American Astronomical Society, AAS Meeting #211, #80.05; Bulletin of the American Astronomical Society, Vol. 39, p.871

Astronomy and Astrophysics

Astronomy

Scientific paper

At spectral type B1, the stellar winds of B supergiants (SGs) display a discontinuous drop in their observed terminal velocities which is referred to as the "bi-stability jump". Similarly, Einstein satellite observations of B SG also revealed evidence for a discontinuous drop in their observed X-ray luminosities at B1. Since the X-ray emission is believed to arise from stellar wind shock structures, B SGs provide an excellent laboratory for testing the still uncertain relationships between the radiation forces, wind properties, and the resultant X-ray emission. This interesting coincidence at spectral type B1 has not been explored in depth mostly due to the insufficient number of B SGs with high S/N X-ray observations. To explore the significance and interconnection of the X-ray and wind discontinuous drops, we obtained high S/N XMM-Newton EPIC observations of a sample of B SGs in the vicinity of spectral type B1. The good sensitivity of EPIC allows us to carry out a thorough X-ray spectral analysis of these stars and extract well constrained X-ray parameters. Our observations confirmed the B1 discontinuous drop in X-ray luminosity by a factor of 4, and this X-ray drop is found to coincide with the same effective temperature of the wind bi-stability jump. Although current shock X-ray emission models are found to be roughly consistent with predicting the observed drop in the X-ray luminosity, our detailed X-ray analyses have revealed several discrepancies with these models. For example, since shock produced X-ray temperatures scale with the flow velocity squared, we expected to see a dramatic drop in the spectral hardness across the discontinuity due to the drop in wind speed, but this is not observed. We discuss the significance of our X-ray results with regards to understanding the wind-dynamics of B SGs and suggest alternative X-ray source models.

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