On the hydrogen neutral outflowing disks of B[e] supergiants

¹ Astronomical Institute AV ČR, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: kraus@sunstel.asu.cas.cz
² Observatório do Valongo (UFRJ), Ladeira do Pedro Antonio 43, Rio de Janeiro 20080-090, Brazil e-mail: borges@ov.ufrj.br
³ Observatório Nacional, Rua General José Cristino 77, 20921-400 São Cristovão, Rio de Janeiro, Brazil e-mail: araujo@on.br

Received: 31 August 2006
Accepted: 19 November 2006

Abstract

Context.B[e] supergiants are known to possess geometrically thick dusty disks. Disk-forming wind models in the literature have, however, been found to be insufficient in reproducing the observed dust emission. This problem arises due to the severe assumption that, as for classical Be stars, the near-infrared excess emission originates in the disk. Modeling of the free-free and free-bound emission therefore results in an upper limit for the disk mass loss rate as well as for the disk opacity. Dust condensation in the disk can thus severely be hampered.

Aims.In order to overcome the dust formation problem, and based on our high-resolution optical spectroscopy and model results, we propose a revised scenario for the non-spherical winds of B[e] supergiants: a normal B-type line-driven polar wind and an outflowing disk-forming wind that is neutral in hydrogen at, or very close to the stellar surface.

Methods.We concentrate on the pole-on seen LMC B[e] supergiant R 126 and calculate the line luminosities of the optical [Oi] emission lines and their emergent line profiles with an outflowing disk scenario. In addition, we compute the free-free and free-bound emission from a line-driven polar wind and model the spectral energy distribution in the optical and near-infrared.

Results.Good fits to the [Oi] line luminosities are achieved for an outflowing disk that is neutral in hydrogen right from the stellar surface. Neutral thereby means that hydrogen is ionized by less than 0.1%. Consequently, the free-free and free-bound emission cannot (dominantly) arise from the disk and cannot limit the disk mass loss rate. The hydrogen neutral outflowing disk scenario therefore provides an ideal environment for efficient dust formation. The spectral energy distribution in the optical and near-infrared range can be well fitted with the stellar continuum plus free-free and free-bound emission from the polar line-driven wind. Our modeling further delivers minimum values for yr^-1 and for the density contrast between equatorial and polar wind of ~10.