EDP Sciences
Free access
Issue
A&A
Volume 446, Number 1, January IV 2006
Page(s) 279 - 293
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20053685


A&A 446, 279-293 (2006)
DOI: 10.1051/0004-6361:20053685

Physical parameters and wind properties of galactic early B supergiants

P. A. Crowther1, D. J. Lennon2 and N. R. Walborn3

1  Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Shefffield, S3 7RH, UK
    e-mail: Paul.Crowther@sheffield.ac.uk
2  Isaac Newton Group, Apartado 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain
3  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA

(Received 23 June 2005 / Accepted 12 September 2005)

Abstract
We present optical studies of the physical and wind properties, plus CNO chemical abundances, of 25 O9.5-B3 Galactic supergiants. We employ non-LTE, line blanketed, extended model atmospheres, which provide a modest downward revision in the effective temperature scale of early B supergiants of up to 1-2 kK relative to previous non-blanketed results. The so-called "bistability jump" at B1 ( $T_{\rm eff} \sim 21$ kK) from Lamers et al. is rather a more gradual trend (with large scatter) from $v_{\infty}/v_{\rm esc}\sim3.4$ for B0-0.5 supergiants above 24 kK to $v_{\infty}/v_{\rm esc}\sim 2.5$ for B0.7-1 supergiants with 20 kK $\leq$ $T_{\rm eff}$ $\leq$ 24 kK, and $v_{\infty}/v_{\rm esc}\sim 1.9$ for B1.5-3 supergiants below 20 kK. This, in part, explains the break in observed UV spectral characteristics between B0.5 and B0.7 subtypes as discussed by Walborn et al. We compare derived (homogeneous) wind densities with recent results for Magellanic Cloud B supergiants and generally confirm theoretical expectations for stronger winds amongst Galactic supergiants. However, winds are substantially weaker than predictions from current radiatively driven wind theory, especially at mid-B subtypes, a problem which is exacerbated if winds are already clumped in the H$\alpha$ line forming region. In general, CNO elemental abundances reveal strongly processed material at the surface of Galactic B supergiants, with mean N/C and N/O abundances 10 and 5 times higher than the Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating the lowest degree of processing in our sample, and HD 152236 (B1.5 Ia+) the highest.


Key words: stars: early-type -- stars: fundamental parameters -- stars: abundances -- stars: evolution -- stars: mass-loss

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