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Long-baseline interferometric observations obtained with the Navy Prototype Optical Interferometer of the H-emitting envelopes of the Be stars Tau and CMi are presented. For compatibility with the previously published interferometric results in the literature of other Be stars, circularly symmetric and elliptical Gaussian models were fitted to the calibrated H observations. The models are adequate for characterizing the angular distribution of the H-emitting circumstellar material associated with these Be stars. To study the correlations between the various model parameters and the stellar properties, the model parameters for Tau and CMi were combined with data for other Be stars from the literature. After accounting for the different distances to the sources and stellar continuum flux levels, it was possible to study the relationship between the net H emission and the physical extent of the H-emitting circumstellar region. A clear dependence of the net H emission on the linear size of the emitting region is demonstrated, and these results are consistent with an optically thick line emission that is directly proportional to the effective area of the emitting disk. Within the small sample of stars considered in this analysis, no clear dependence on the spectral type or stellar rotation is found, although the results do suggest that hotter stars might have more extended H-emitting regions.

We present optical interferometric observations of the Be star Tauri obtained using the Navy Prototype Optical Interferometer (NPOI). The multichannel capability of the NPOI allows a high-quality internal calibration of the squared visibilities corresponding to the H emission from the circumstellar environment. The observations suggest a strong departure from circular symmetry and thus are described by an elliptical Gaussian model. We use a nonlinear least-squares fit to the data to obtain the likeliest parameters, and the corresponding uncertainties are determined using a Monte Carlo simulation. We obtain 3:14 AE 0:21 mas for the angular size of the major axis, À62: 3 AE 4: 4 for the position angle, and 0:310 AE 0:072 for the axial ratio. By comparing our results with those already in the literature, we conclude that the model parameters describing the general characteristics of the circumstellar envelope of Tau appear to be stable on timescales of years. We also compare our results with the known parameters describing the binary nature of Tau, and we conclude that the envelope surrounds only the primary component and is well within its Roche lobe.

An analytical derivation is presented for computing mass-loss rates of Cepheids by using the method of Castor, Abbott, & Klein (1975) modified to include a term for momentum input from pulsation and shocks generated in the atmosphere. Using this derivation, mass-loss rates of Cepheids are determined as a function of stellar parameters. When applied to a set of known Cepheids, the calculated mass-loss rates range from 10^{-10} to 10^{-7}M_{Sun}/yr, larger than if the winds were driven by radiation alone. Infrared excesses based on the predicted mass-loss rates are compared to observations from optical interferometry and IRAS, and predictions are made for Spitzer observations. The mass-loss rates are consistent with the observations, within the uncertainties of each. The rate of period change of Cepheids is discussed and shown to relate to mass loss, albeit the dependence is very weak. There is also a correlation between the large mass-loss rates and the Cepheids with slowest absolute rate of period change due to evolution through the instability strip. The enhanced mass loss helps illuminate the issue of infrared excess and the mass discrepancy found in Cepheids.Comment: 46 pages, 12 figures, 6 tables, ApJ accepte

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