Multitechnique testing of the viscous decretion disk model

I. The stable and tenuous disk of the late-type Be star β CMi^⋆,⋆⋆

¹ Astronomical Institute of Charles University, Charles University in Prague, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
e-mail: robertklement@gmail.com
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
³ European Organisation for Astronomical Research in the Southern Hemisphere, Casilla 19001, Santiago 19, Chile
⁴ Ritter Observatory, Department of Physics & Astronomy, University of Toledo, Toledo, OH 43606, USA
⁵ Department of Physics, Central Michigan University, Mount Pleasant, MI 48859, USA
⁶ US Naval Observatory, Flagstaff Station, 10391 W. Naval Observatory Rd., Flagstaff, AZ 86001, USA
⁷ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Casilla 5030, Valparaíso, Chile

Received: 15 May 2015
Accepted: 27 August 2015

Abstract

Context. The viscous decretion disk (VDD) model is able to explain most of the currently observable properties of the circumstellar disks of Be stars. However, more stringent tests, focusing on reproducing multitechnique observations of individual targets via physical modeling, are needed to study the predictions of the VDD model under specific circumstances. In the case of nearby, bright Be star β CMi, these circumstances are a very stable low-density disk and a late-type (B8Ve) central star.

Aims. The aim is to test the VDD model thoroughly, exploiting the full diagnostic potential of individual types of observations, in particular, to constrain the poorly known structure of the outer disk if possible, and to test truncation effects caused by a possible binary companion using radio observations.

Methods. We use the Monte Carlo radiative transfer code HDUST to produce model observables, which we compare with a very large set of multitechnique and multiwavelength observations that include ultraviolet and optical spectra, photometry covering the interval between optical and radio wavelengths, optical polarimetry, and optical and near-IR (spectro)interferometry.

Results. A parametric VDD model with radial density exponent of n = 3.5, which is the canonical value for isothermal flaring disks, is found to explain observables typically formed in the inner disk, while observables originating in the more extended parts favor a shallower, n = 3.0, density falloff. Theoretical consequences of this finding are discussed and the outcomes are compared with the predictions of a fully self-consistent VDD model. Modeling of radio observations allowed for the first determination of the physical extent of a Be disk (35⁺¹⁰_-5 stellar radii), which might be caused by a binary companion. Finally, polarization data allowed for an indirect measurement of the rotation rate of the star, which was found to be W ≳ 0.98, i.e., very close to critical.