A SEMIEMPIRICAL DETERMINATION OF THE WIND VELOCITY STRUCTURE FOR THE HYBRID-CHROMOSPHERE STAR ALPHA-TRIANGULI-AUSTRALIS

We have used the Goddard High-Resolution Spectrograph (GHRS) on the Hu bble Space Telescope to study the wind of the hybrid-chromosphere star alpha TrA (K4 II). The stellar wind produces significant absorption a t negative radial velocities in the chromospheric Mg II resonance line s (h and k). Spectra obtained with the GHRS echelle high-resolution gr ating (lambda/Delta lambda approximate to 85,000) on 1993 February 10 and 1994 May 1 reveal complex interstellar absorption in the Mg II emi ssion lines and a high-velocity wind absorption feature centered near -95 km s(-1). The 1993 February observation shows an asymmetry of the Mg II emission cores, corresponding to an apparent redshift of 6.0 +/- 1.5 km s(-1). We construct a simple wind model that explains several of the key observational features. The scattering of the Mg II h and k photons in a geometrically extended region dominates the observed flu x near line center, which supports the assignment of the low-velocity absorption components to interstellar absorption rather than to a chro mospheric self-reversal. For the 1993 February observation, the parame ters for our simple wind model are as follows: terminal velocity V(inf inity) = 100 km s(-1) turbulent velocity V-turb = 24 km s(-1), M simil ar to 1.8 x 10-(10) M. yr(-1), for a fixed value of the velocity-law p arameter beta = 1 and fixed stellar radius of R = 97 R., assuming Mg II is the dominant ionization state in the flow. Our analysis of the 1 994 May observation resulted in similar values for these parameters, a nd the mass-loss rate could be as small as M similar to 1.6 x 10-(10) M. yr(-1). The value of beta is uncertain (greater than or equal to 0. 3) and if beta similar to 3.5 as found from the recent analysis of the zeta Aurigae systems, M could be larger by a factor of 3-4. A compari son of our result with numerical solutions to the momentum and conserv ation equations reveals that the derived velocity distribution lies wi thin a limited region of parameter space where there is a large nonthe rmal pressure on the plasma close to the base of the wind consistent w ith previous wind models for alpha TrA. Our best model fit to the two interstellar absorption components indicates a total hydrogen column d ensity toward alpha TrA of N-HI = 2 x 10(19) cm(-2).