Infrared excesses in early-type stars - Gamma Cassiopeiae

Spectrophotometry of the classical Be star Gamma Cas (1-4 microns, with about 2% spectral resolution) is presented. These data, together with existing broad-band observations, are accurately described by simple isothermal LTE models for the IR excess which differ from most previously published work in three ways: (1) hydrogenic bound-free emission is included; (2) the attenuation of the star by the shell is included; and (3) no assumption is made that the shell contribution is negligible in some bandpass. It is demonstrated that the bulk of the IR excess consists of hydrogenic bound-free and free-free emission from a shell of hot ionized hydrogen gas, although a small thermal component cannot be ruled out. The bound-free emission is strong, and the Balmer, Paschen, and Brackett discontinuities are correctly represented by the shell model with physical parameters as follows: a shell temperature of approximately 18,000 K, an optical depth (at 1 micron) of about 0.5, an electron density of approximately 1 trillion per cu cm, and a size of about 2 trillion cm. Phantom shells (i.e., ones which do not alter the observed spectrum of the underlying star) are discussed.