THE EPSILON-AURIGAE SECONDARY - A HYDROSTATICALLY SUPPORTED DISK

Epsilon Aurigae is an F supergiant in a spectroscopic binary system th at undergoes a flat-bottomed partial eclipse of 2 yr duration every 27 yr. The spectrum appears to be single-lined, aside from extra absorpt ion features detected during and shortly after eclipse. Eclipse charac teristics indicate that the secondary is a very elongated object 5-10 AU in dimension parallel to its orbit. Orbital characteristics and the spectral properties of the primary are consistent with two different models of the system, with the secondary having a mass of either simil ar to 15 M(.) or similar to 4 M(.). We have modeled the cool, dark sec ondary in the epsilon Aurigae system as a geometrically thin circumste llar disk of gas and dust (surrounding one or two unseen stars, at or near its center), which is rotationally supported about its short axis and pressure supported perpendicular to its midplane. We assume that the midplane of the disk is coplanar with the orbital plane of the sys tem. The gross features of the eclipse light curve observed at any sin gle wavelength are easily reproduced, using a variety of disk scale he ights and optical depths, provided that we are viewing within similar to 3 degrees of the symmetry plane of the system. Central holes in the disk only affect the eclipse profile for models with low optical dept h (and correspondingly large pressure scale height). The observed gray ness of the eclipse in the visible and near-IR implies that the partic les in the disk are significantly larger than those in the typical ISM . Either particles of radius less than or similar to 5 mu m are almost completely absent, or the disk must be very opaque. If the disk is ve ry opaque, then the observed eclipse depth implies a small scale heigh t for the disk, equal to roughly 3% of the disk's radius at the outer edge. This is a factor of similar to 1.5-2 smaller than the value expe cted for the low-mass model from hydrostatic balance with the disk tem perature measured in the thermal-IR, suggesting that the high-mass mod el of the system is correct and/or the dust particles have settled int o a thinner disk than the pressure-supported gas. We have also constru cted a quasi-hydrodynamic model of the expansion of the material in th e outermost edge of the disk secondary in response to the heating that it receives as it rotates into view of the luminous primary. Light cu rves computed using this model reproduce the basic features of absorpt ion lines, which are observed to be deepest subsequent to the middle o f the continuum eclipse and to persist after fourth contact.