NaI/KI scattering observations in circumstellar envelopes: constraints on ionization and mass-loss rates

Abstract

International audienceWe investigate KI or NaI fluorescent emission in circumstellar shells to study their ionization and mass-loss rates. First-time KI detections around the mira-type stars W Hya and R Hya are presented. Together with the previously analysed cases of α Ori, α Her and o Cet (Mauron and Caux 1992A&A...265..711M) and other observations, this gives an extended sample of 10 envelopes which includes μ Cep, CE Tau, β Peg, ρ Per and g Her. A few non-detections are also considered. In order to compare observed with expected values of KI intensities, the ionization model of Glassgold and Huggins (1986ApJ...306..605G) is used, and the relevant parameters such as distance d, mass-loss rate ˙(M), gas temperature, fractional electron abundance x_e_ and stellar photoionizing rates Goplus _have to be known. Using data found in the literature, we estimate these parameters in detail for each case. The values of x_e_ are obtained with a hypothesized simple rule based on the circumstellar abundance of molecules and grains which could lock electron donors, giving either x_e_~3x10^-4^ or ~2x10^-5^. The Goplus _values depend on the rare ultraviolet spectrophotometric data available for red giants. The kinetic temperature was assumed to be 30K at the probed impact parameters r_obs_~0.5 to 5x10^16^cm. Despite real uncertainties, the predicted KI intensities with our best estimates of the parameters are in very reasonable agreement with observations in 8 cases in 10. The largest discrepancy concerns the red supergiant μ Cep (M2Ia): our KI data suggest that, similar to α Ori, CO and dust are incompletely formed; for this object we favor ˙(M)=5x10^-6^Msun_/yr. The wind of β Peg is the second case for which a larger mass-loss and/or a larger x_e_ than primarily believed is suggested, but confirming observations are needed. Finally there is no indication that K or Na might be depleted in silicate grains