GHRS observations of cool, low-gravity stars. V. The outer atmosphere and wind of the nearby K supergiant lambda Velorum

UV spectra of lambda Velorum taken with the Goddard High Resolution Spectro graph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at lambda = 3.6 cm a re used to obtain an independent check on the wind velocity and mass-loss r ate inferred from the UV observations. Parameters of the chromospheric stru cture are estimated from measurements of UV line widths, positions, and flu xes and from the UV continuum flux distribution. The ratios of optically th in C II] emission lines indicate a mean chromospheric electron density of l og N-e approximate to 8.9 +/- 0.2 cm(-3). The profiles of these lines indic ate a chromospheric turbulence (upsilon(0) approximate to 25-36 km s(-1)), which greatly exceeds that seen in either the photosphere or wind. The cent roids of optically thin emission lines of Fe II and of the emission wings o f self-reversed Fe II lines indicate that they are formed in plasma approxi mately at rest with respect to the photosphere of the star. This suggests t hat the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increase s with height in the chromosphere from a well-defined temperature minimum o f 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] p rovides evidence of material at higher than chromospheric temperatures in t he outer atmosphere of this noncoronal star. The photon-scattering wind pro duces self-reversals in the strong chromospheric emission lines, which allo w us to probe the velocity held of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus hei ght in the wind, and therefore directly map the wind acceleration. The widt h and shape of these; self-absorptions reflect a wind turbulence of approxi mate to 9-21 km s(-1). We further characterize the wind by comparing the ob servations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple mode ls of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter beta similar to 0.9, a terminal velocity of 29-33 km s(-1), and a mass-los s rate similar to 3 x 10(-9) M. yr(-1). Modeling of the 3.6 cm radio flux o bserved in 1997 suggests a more slowly accelerating wind (higher beta) and/ or a higher mass-loss rate than inferred from the UV line profiles. These d ifferences may be due to temporal variations in the wind or from limitation s in one or both of the models. The discrepancy is currently under investig ation.