Multitechnique testing of the viscous decretion disk model I. The stable\n and tenuous disk of the late-type Be star $\\beta$ CMi

Abstract

The viscous decretion disk (VDD) model is able to explain most of the\ncurrently observable properties of the circumstellar disks of Be stars.\nHowever, more stringent tests, focusing on reproducing multitechnique\nobservations of individual targets via physical modeling, are needed to study\nthe predictions of the VDD model under specific circumstances. In the case of\nnearby, bright Be star $\\beta$ CMi, these circumstances are a very stable\nlow-density disk and a late-type (B8Ve) central star. The aim is to test the\nVDD model thoroughly, exploiting the full diagnostic potential of individual\ntypes of observations, in particular, to constrain the poorly known structure\nof the outer disk if possible, and to test truncation effects caused by a\npossible binary companion using radio observations. We use the Monte Carlo\nradiative transfer code HDUST to produce model observables, which we compare\nwith a very large set of multitechnique and multiwavelength observations that\ninclude ultraviolet and optical spectra, photometry covering the interval\nbetween optical and radio wavelengths, optical polarimetry, and optical and\nnear-IR (spectro)interferometry. Due to the absence of large scale variability,\ndata from different epochs can be combined into a single dataset. A parametric\nVDD model with radial density exponent of $n$ = 3.5, which is the canonical\nvalue for isothermal flaring disks, is found to explain observables typically\nformed in the inner disk, while observables originating in the more extended\nparts favor a shallower, $n$ = 3.0, density falloff. Modeling of radio\nobservations allowed for the first determination of the physical extent of a Be\ndisk (35$^{+10}_{-5}$ stellar radii), which might be caused by a binary\ncompanion. Finally, polarization data allowed for an indirect measurement of\nthe rotation rate of the star, which was found to be $W \\gtrsim 0.98$, i.e.,\nvery close to critical.\n