Variations of the high-level Balmer line spectrum of the helium-strong star σ Orionis E

Abstract

Using the high-level Balmer lines and continuum, we trace the density structure of two magnetospheric disk segments of the prototypical Bp star σ Orionis E (B2p) as these segments occult portions of the star during the rotational cycle. High-resolution spectra of the Balmer lines ≥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 10 12 cm −3 and are opaque in the lines and continuum. They have semiheights less than 1 2 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.

Polarimetric Observations of Σ Orionis E

“…The resulting blob density corresponds to an electron optical depth of τ = 0.13, which does not violate the optically thin assumption. It is worth mentioning that our estimate for the electron density in the blobs, 10 12 e − cm −3 , is in good agreement with the values derived by Groote & Hunger (1982) and Smith & Bohlender (2007) using the Inglis-Teller formula. From the best-fitting parameters one can calculate the mass of each model component.…”

The Discovery of a Strong Magnetic Field and Corotating Magnetosphere in the Helium-Weak Star Hd 176582

“…For parameters provided in Table 3, and estimated values of Ṁ = 10 −10 M ⊙ yr −1 and v ∞ = 800 km s −1 for a star like HD 176582 (Babel & Montmerle 1997a), η * ≈ 2.3 × 10 5 and the weak wind of HD 176582 can be expected to be very strongly confined. It is therefore not surprising that the star displays the same phenomena observed in other hot, strongly magnetic Bp stars such as σ Ori E (Townsend, Owocki, & Groote 2005;Smith & Bohlender 2007;Oksala et al 2011), δ Ori C (Leone et al 2010), and 36 Lyn (Wade et al 2006;Smith et al 2006) also known to have magnetically confined winds. Examination of Figs.…”

“…This includes variable Hα emission or absorption (Short & Bolton 1994;Townsend, Owocki, & Groote 2005;Wade et al 2006;Leone et al 2010), modulated satellite UV resonance lines (e.g. Shore & Brown 1990) and variations in high-level Balmer lines (Groote & Hunger 1976; david.bohlender@nrc-cnrc.gc.ca Smith & Bohlender 2007). The prototype of the heliumstrong stars, σ Ori E, is the best studied example (e.g.…”

HR 5907: Discovery of the most rapidly rotating magnetic early B-type star by the MiMeS Collaboration★†

“…In comparison to other He‐strong stars such as σ Ori E (e.g. Landstreet & Borra 1978; Reiners et al 2000; Smith & Bohlender 2007) or HR 7355 (Rivinius et al 2010) we find that HR 5907 shows relatively weak variability in its helium lines, of ∼20 per cent in EW.…”