A Semiempirical Determination of the Wind Velocity Structure for the Hybrid-Chromosphere Star alpha Trianguli Australis

Details A Semiempirical Determination of the Wind Velocity Structure for the Hybrid-Chromosphere Star alpha Trianguli Australis A Semiempirical Determination of the Wind Velocity Structure for the Hybrid-Chromosphere Star alpha Trianguli Australis

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...452..407h&link_type=abstract

Astrophysical Journal v.452, p.407

Astronomy and Astrophysics

Astronomy

30

Line: Formation, Stars: Chromospheres, Stars: Individual Constellation Name: Alpha Trianguli Australis, Stars: Late-Type, Stars: Mass Loss, Ultraviolet: Stars

Scientific paper

We have used the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope to study the wind of the hybrid-chromosphere star α TrA (K4 II). The stellar wind produces significant absorption at negative radial velocities in the chromospheric Mg II resonance lines (h and k). Spectra obtained with the GHRS echelle high-resolution grating (4A 85,000) on 1993 February 10 and 1994 May 1 reveal complex interstellar absorption in the Mg II emission 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 flux near line center, which supports the assignment of the low-velocity absorption components to interstellar absorption rather than to a chromospheric self-reversal. For the 1993 February observation, the parameters for our simple wind model are as follows: terminal velocity V(∞) = 100 km s-1, turbulent velocity Vturb = 24 km s-1, Mṡ ˜ 1.8 × 1010 Msun yr-1, for a fixed value of the velocity-law parameter β = 1 and fixed stellar radius of R* = 97 Rsun, assuming Mg II is the dominant ionization state in the flow. Our analysis of the 1994 May observation resulted in similar values for these parameters, and the mass-loss rate could be as small as Mṡ ˜ 1.6 × 10-10 Msun yr-1. The value of β is uncertain (≥ 0.3) and if β ˜ 3.5 as found from the recent analysis of the ζ Aurigae systems, Mṡ could be larger by a factor of 3-4. A comparison of our result with numerical solutions to the momentum and conservation equations reveals that the derived velocity distribution lies within a limited region of parameter space where there is a large nonthermal pressure on the plasma close to the base of the wind consistent with previous wind models for α TrA. Our best model fit to the two interstellar absorption components indicates a total hydrogen column density toward α TrA of NHI = 2 × 1019 cm-2.

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