EDP Sciences
Free access
Volume 475, Number 3, December I 2007
Page(s) 1027 - 1037
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078196

A&A 475, 1027-1037 (2007)
DOI: 10.1051/0004-6361:20078196

Variations of the high-level Balmer line spectrum of the helium-strong star $\sigma$ Orionis E

M. A. Smith1 and D. A. Bohlender2

1  Department of Physics, Catholic University of America, Washington, DC 20064, USA
    e-mail: msmith@stsci.edu
2  National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 W. Saanich Rd., Victoria, BC V9E 2E7, Canada

(Received 30 June 2007 / Accepted 18 September 2007)

Using the high-level Balmer lines and continuum, we trace the density structure of two magnetospheric disk segments of the prototypical Bp star $\sigma$ Orionis E (B2p) as these segments occult portions of the star during the rotational cycle. High-resolution spectra of the Balmer lines $\ge$H9 and Balmer edge were obtained on seven nights in January-February 2007 at an average sampling of 0.01 cycles. We measured equivalent width variations due to the star occultations by two disk segments 0.4 cycles apart and constructed differential spectra of the migrations of the corresponding absorptions across the Balmer line profiles. We first estimated the rotational and magnetic obliquity angles. We then simulated the observed Balmer jump variation using the model atmosphere codes synspec/circus and evaluated the disk geometry and gas thermodynamics. We find that the two occultations are caused by two disk segments. The first of these transits quickly, indicating that the segment resides in a range of distances, perhaps 2.5-6 R*, from the star. The second consists of a more slowly moving segment situated closer to the surface and causing two semi-resolved absorbing maxima. During its transit this segment brushes across the star's "lower" limb. Judging from the line visibility up to H23-H24 during the occultations, both disk segments have mean densities near 1012 cm-3 and are opaque in the lines and continuum. They have semiheights less than ${\frac 12}$ R*, and their temperatures are near 10 500 K and 12 000 K, respectively. In all, the disks of Bp stars have a much more complicated geometry than has been anticipated, as evidenced by their (sometimes) non-coplanarity, de-centerness, and from star to star, differences in disk height.

Key words: stars: individual: $\sigma$ Orionis E -- stars: magnetic fields -- stars: circumstellar matter -- stars: winds, outflows -- stars: early-type -- ultraviolet: stars

© ESO 2007