John B. Lester

On the Enhancement of Mass Loss in Cepheids Due to Radial Pulsation

An analytical derivation is presented for computing mass-loss rates of Cepheids by using the method of Castor, Abbott, and Klein 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☉ yr−1, 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.

Properties of the Hα-emitting Circumstellar Regions of Be Stars

Long-baseline interferometric observations obtained with the Navy Prototype Optical Interferometer of the Hα-emitting envelopes of the Be stars η Tau and β 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 Hα observations. The models are adequate for characterizing the angular distribution of the Hα-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 η Tau and β 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 Hα emission and the physical extent of the Hα-emitting circumstellar region. A clear dependence of the net Hα 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 Hα-emitting regions.

THE CIRCUMSTELLAR ENVELOPE OF ¿ TAURI THROUGH OPTICAL INTERFEROMETRY

Abstract : We present optical interferometric observations of the Be star Zeta 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(alpha) 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 +/- 0.21 mas for the angular size of the major axis -62.3 deg +/- 4.4 deg for the position angle and 0.310 +/- 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 Zeta Tau appear to be stable on timescales of years. We also compare our results with the known parameters describing the binary nature of Zeta Tau and we conclude that the envelope surrounds only the primary component and is well within its Roche lobe.

Discovery of a fast wind from a field population II giant star

The He I λ10830 absorption in the metal-deficient field giant star HD 6833, extends to at least −90 km s −1 , a value that is comparable to, or exceeds the escape velocity from the photosphere of the star. This spectrum provides strong observational evidence for mass loss. Because this field giant is a surrogate for red giants in globular clusters, and a velocity of 90 km s −1 is larger than the central escape velocity from most globular clusters, this observation suggests that material lost from globular cluster stars may easily escape from the cluster, and thus resolve the dilemma of the missing interstellar medium in globular clusters

Spherically-symmetric model stellar atmospheres and limb darkening - I. Limb-darkening laws, gravity-darkening coefficients and angular diameter corrections for red giant stars

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.

Testing Mass Loss in Large Magellanic Cloud Cepheids using Infrared and Optical Observations. II. Predictions and Tests of the OGLE-III Fundamental-mode Cepheids

In this paper, we test the hypothesis that Cepheids have infrared excesses due to mass loss. We fit a model using the mass-loss rate and the stellar radius as free parameters to optical observations from the OGLE-III survey and infrared observations from the Two Micron All Sky Survey and SAGE data sets. The sample of Cepheids has predicted minimum mass-loss rates ranging from 0 to 10{sup -8} M{sub sun} yr{sup -1}, where the rates depend on the chosen dust properties. We use the predicted radii to compute the period-radius relation for LMC Cepheids and to estimate the uncertainty caused by the presence of infrared excess for determining angular diameters with the infrared surface brightness technique. Finally, we calculate the linear and nonlinear period-luminosity (P-L) relations for the LMC Cepheids at VIJHK + IRAC wavelengths and find that the P-L relations are consistent with being nonlinear at infrared wavelengths contrary to previous results.

THE LACK OF BLUE SUPERGIANTS IN NGC 7419, A RED SUPERGIANT-RICH GALACTIC OPEN CLUSTER WITH RAPIDLY ROTATING STARS

According to previous studies based on photometry alone, NGC 7419 reveals a surprisingly low ratio of blue to red supergiants: only one blue supergiant (BSG) along with a record number of five red supergiants (RSGs). However, for a cluster like NGC 7419 with solar metallicity, one expects twice as many BSGs as RSGs. To verify the small ratio of BSGs to RSGs, we have obtained spectroscopic observations of the seven most luminous blue member stars using the 1.6 m telescope of the Mount Megantic Observatory. (The RSGs have already been studied spectroscopically.) To classify the stars, we have developed a system especially adapted for these heavily reddened stars in a spectral region from 8400 to 8920 A, near the hydrogen Paschen series limit. This classification system is based on standard stars of known MK spectral type extending over O9–B5 and all luminosity classes and is linked through a grid of synthetic spectra to the atmospheric physical parameters Teff and log g. We also include Be stars. Among the seven blue stars observed in NGC 7419, four have red spectra that are dominated by absorption lines and three by emission lines. The spectral types for the former are B2.5 II–III, B2.5 III, B0 III, and B4 III (e), while those for the latter are Be, B1 III–Ve, and Be, respectively. The average heliocentric radial velocity of these stars is -66 ± 6 km s-1, compatible with the value of -74 ± 9 km s-1 measured for the five RSG members. A distance of 1.7 ± 0.4 kpc for this cluster was estimated using the blue-star spectral types, in agreement with the value of 2.3 ± 0.3 kpc found by Beauchamp and coworkers, based on isochrone fitting in the color-magnitude diagram. With no BSG stars detected spectroscopically, we confirm the low number, in this case absence, of BSGs in this cluster. The high fraction of Be stars detected by us and others among the bright blue member stars could be explained by an average rotational velocity for the stars in NGC 7419, which is significantly higher than in other clusters of similar age and metallicity. Since higher stellar rotation rates shorten the BSG phase, we suggest that this explains why the evolved stars in NGC 7419 have become RSGs. Thus, NGC 7419 is an exceptional case, since high stellar rotation normally tends to occur at lower metallicity.

Limb darkening in spherical stellar atmospheres

Context. Stellar limb darkening, I(μ = cos θ), is an important constraint for microlensing, eclipsing binary, planetary transit, and interferometric observations, but is generally treated as a parameterized curve, such as a linear-plus-square-root law. Many analyses assume limb-darkening coefficients computed from model stellar atmospheres. However, previous studies, using I(μ) from planeparallel models, have found that fits to the flux-normalized curves pass through a fixed point, a common μ location on the stellar disk, for all values of Teff ,l ogg and wavelength. Aims. We study this fixed μ-point to determine if it is a property of the model stellar atmospheres or a property of the limb-darkening laws. Furthermore, we use this limb-darkening law as a tool to probe properties of stellar atmospheres for comparison to limbdarkening observations. Methods. Intensities computed with plane-parallel and spherically-symmetric Atlas models (characterized by the three fundamental

Determining parameters of cool giant stars by modeling spectrophotometric and interferometric observations using the SAtlas program (Research Note)

Context. Optical interferometry is a powerful tool for observing the intensity structure and angular diameter of stars. When combined with spectroscopy and/or spectrophotometry, interferometry provides a powerful constraint for model stellar atmospheres. Aims. The purpose of this work is to test the robustness of the spherically symmetric version of the Atlas stellar atmosphere program, SAtlas, using interferometric and spectrophotometric observations. Methods. Cubes (three dimensional grids) of model stellar atmospheres, with dimensions of luminosity, mass, and radius, are computed to fit observations for three evolved giant stars, ψ Phoenicis, γ Sagittae, and α Ceti. The best-fit parameters are compared with previous results. Results. The best-fit angular diameters and values of χ 2 are consistent with predictions using Phoenix and plane-parallel Atlas models. The predicted effective temperatures, using SAtlas, are about 100 to 200 K lower, and the predicted luminosities are also lower due to the differences in effective temperatures. Conclusions. It is shown that the SAtlas program is a robust tool for computing models of extended stellar atmospheres that are consistent with observations. The best-fit parameters are consistent with predictions using Phoenix models, and the fit to the interferometric data for ψ Phe differs slightly, although both agree within the uncertainty of the interferometric observations.

satlas: spherical versions of the atlas stellar atmosphere program

Context. The current stellar atmosphere programs still cannot match some fundamental observations of the brightest stars, and with new techniques, such as optical interferometry, providing new data for these stars, additional development of stellar atmosphere codes is required. Aims. To modify the open-source model atmosphere program atlas to treat spherical geometry, creating a test-bed stellar atmosphere code for stars with extended atmospheres. Methods. The plane-parallel atlas has been changed by introducing the necessary spherical modifications in the pressure structure, in the radiative transfer and in the temperature correction. Results. Several test models show that the spherical program matches the plane-parallel models in the high surface gravity regime, and matches spherical models computed by phoenix and by marcs in the low gravity case.