The envelopes of RV Tauri variables

The envelopes of RV Tauri stars are studied with optical spectropolarimetry and infrared photometry. These two observational techniques can provide information about the extended envelopes of objects. Infrared photometry measures the amount and absorption optical depth of dust in the circumstellar envelope of these objects. The polarimetry can probe the geometric distributions of the envelope and photospheres. Since both techniques are useful in furthering the understanding of the envelopes of stars, both have been employed cooperatively to gain insight into the properties of the envelopes and extended photospheres of the RV Tauri variables.

The IR photometry shows that the RV Tauri stars have large circumstellar shells. The data presented confirm past investigations by Lloyd Evans (1985) and Goldsmith et al. (1987). The data show that a strong difference exists between the RV Tauri stars that do and do not vary in mean brightness; the stars that vary in mean brightness have dust envelopes with larger optical depths. Comparison of broad-band polarimetry and the IR colors suggests that a correlation exists between the two. This implies that relatively warm circumstellar dust is important in the formation of the observed polarization.

The spectropolarimetry suggests that the mechanisms that produce the polarization are different for the objects that do and do not show TiO absorption near minimum. The type B objects do not show TiO absorption and show wavelength and temporal variations in the polarization described by sporadic scattering of homogeneous photospheric radiation by localized regions containing small particles, presumably newly formed dust. In the case of AC Her, the localized region is confined to a specific plane. The spectropolarimetry of the type A stars which do develope TiO bands near minima show variations across the TiO absorption bands reminiscent of late-type variables. The apparent connection between the position angle variations of the polarization and the phase of the light curve suggests that the polarigenic process may be due to the scattering of an anisotropic photospheric radiation field by a circumstellar dust shell.