The Thermal and Ionization Structure of the Flow from Near-Main-Sequence B stars

J.P. Cassinelli[1], D.H. Cohen[2] and J.J. MacFarlane[3]

[1] University of Wisconsin, Madison WI, USA
[2] University of Delaware, Newark DE, USA
[3] Prism Computational Sciences, Madison WI, USA

Non-supergiant B stars (here called BV stars) offer several advantages for studies of the ionization and thermal structure of winds. The winds are sufficiently dense that significant emission is produced at X-ray and EUV energies and wind properties can be deduced from UV spectra. Yet the winds are optically thin enough that we can observe all layers in the wind at all of these wavelengths. During this decade we have obtained very high quality space data regarding bright BV stars, and these show several interesting and unexpected properties of the stars. In contrast with the O stars the X-ray luminosities do not in general follow the L_{x}/L_{bol} ~ 10^{-7} law, but instead decreases sharply to 10^{-9} at about spectral class B2 V. Nevertheless, the X-ray emission measure for the later B stars can have a very strong effect on physical conditions such as the ionization and temperature structure in the wind. For the early B stars, there are well known stars such as tau Sco which have hard X-ray emission and red shifted O VI absorption. It is suggested that the hard X-ray emission is due to the formation of stagnating infalling clumps in the winds, and the O VI is also affected by the clump motion.

Perhaps the biggest surprise from space observations came from the EUVE satellite measurements of of the two giant stars epsilon CMa (B2II) and beta CMa (B1 II-III). These are two of the very brightest EUV objects in sky between 500 and 700 angstroms. In part because of their location in the nearly evacuated ISM tunnel in the Canis Majoris direction, but also because the stars have a photospheric fluxes that exceed those predicted by plane parallel model atmospheres by about an order of magnitude. The departure of the fluxes from models provides new insight regarding the thermal structure of the outer atmospheres. In the case of epsilon CMa, there are emission lines of a wide range of ionization appearing in the EUV spectrum. The strong line of He II 304 Å gives rise to the O III 374 Å line via the Bowen fluorescence mechanism, and the X-ray producing regions produce lines of iron ranging in ionization from Fe X to Fe XVI. From an analysis of the stellar flux distribution and the observed emission lines, we now have a better understanding of the energy budget in the envelope of this star than we do for any other early type star.