A Magnetically Torqued Disk Model for Be Stars

Abstract

Despite extensive study, the mechanisms by which Be star disks acquire high densities and angular momentum while displaying variability on many timescales are still far from clear. In this paper, we discuss how magnetic torquing may help explain disk formation with the observed quasi-Keplerian (as opposed to expanding) velocity structure and their variability. We focus on the effects of the rapid rotation of Be stars, considering the regime where centrifugal forces provide the dominant radial support of the disk material. Using a kinematic description of the angular velocity, v ðrÞ, in the disk and a parametric model of an aligned field with a strength BðrÞ, we develop analytic expressions for the disk properties that allow us to estimate the stellar surface field strength necessary to create such a disk for a range of stars on the main sequence. The fields required to form a disk are compared with the bounds previously derived from photospheric limiting conditions. The model explains why disks are most common for main-sequence stars at about spectral class B2 V. The earlier type stars with very fast and high-density winds would require unacceptably strong surface fields (>10 3 G) to form torqued disks, while the late B stars (with their low mass-loss rates) tend to form disks that produce only small fluxes in the dominant Be diagnostics. For stars at B2 V the average surface field required is about 300 G. The predicted disks provide an intrinsic polarization and a flux at H comparable to observations. The radial extent of our dense quasi-Keplerian disks is compatible with typical estimates. We also discuss whether the effect on field containment of the time-dependent accumulation of matter in the flux tubes/disk can help explain some of the observed variability of Be star disks.

Evidence for Different Disk Mass Distributions between Early- and Late-type Be Stars in the BeSOS Survey

“…The material is expelled from the central star and placed in a thin equatorial disk with Keplerian rotation (Meilland et al 2007). Different mechanisms such as rapid rotation (Porter 1996;Townsend et al 2004;Domiciano de Souza et al 2003;Frémat et al 2005), mass loss from the stellar wind (Stee & de Araujo 1994;Bjorkman & Cassinelli 1993;Curé 2004;Silaj et al 2014a), binarity (Okazaki et al 2002;Romero et al 2007;Oudmaijer & Parr 2010), magnetic fields (Donati et al 2001;Cassinelli et al 2002;Neiner et al 2003), and stellar pulsations (Rivinius et al 2003) have been proposed to explain how the star loses enough mass to form the CE and how this material is placed in orbit, but it seems that more than one mechanism is required to reproduce the observations. Such mechanisms must continually supply enough angular momentum from the star to form and to maintain the disk.…”

X‐ray emission from massive stars with magnetic fields

“…Overall, considering the analysis of X-ray observations of magnetic O stars, it appears that only one star, θ 1 Ori C, displays properties that are fully compatible with the predictions of the MCWS model. Cassinelli et al (2002) and Brown et al (2008) studied the case of fast rotating magnetic massive stars, specifically addressing the formation of disks in classical Be-type stars. They showed that magnetic torquing and channeling of wind flow from intermediate latitudes of the stellar surface can, for plausible field strengths, create a dense disk a few stellar radii in extent.…”

Magnetic Channeling of Radiatively Driven Hot-Star Winds

“…Let us next examine the nature of magnetic channeling for the winds from rotating hot stars, with particular emphasis on whether a large-scale magnetic field could spin-up the stellar wind outflow into a "Magnetically Torqued Disk" (MTD), as advocated by Cassinelli et al [21]. As noted above, MHD simulations [24,25] indicate that a dipole magnetic field can confine the flow within closed loops that extend out to about the Alfven radius, R A ≈ η 1/4 * R * .…”

“…The next section applies MHD simulations of a Magnetically Confined Wind Shock (MCWS) model developed [18,19] to explain X-ray emission observed by Rosat [20] from the magnetic O7V star θ 1 Ori C. We then discuss the role of magnetic fields in spinning up the wind outflow from a rotating star, emphasizing that this does not produce the Magnetically Torqued Disk (MTD) proposed [21] as a mechanism for producing the orbiting, Keplerian disks inferred from the characteristic Balmer line emission in Be stars. We next show however that the very strong magnetic fields of Bp stars can lead to a Rigidly Rotating Magnetosphere (RRM) [22], with rigid-body disks or clouds that can explain quite well the observed emission in Bp stars like σ Ori E. We further show that the eventual centrifugal breakout of such material can lead to strong heating from magnetic reconnnection, which thus could explain the very hard X-ray flares seen from this star.…”