The Wind‐ISM Interaction of α Tauri

Ultraviolet spectra ofTau (K5 III) obtained by the Hubble Space Telescope (HST) show many emission lines affectedbybroad absorptionfrom thestrongwindof thisred giant star. For the Mgii handklines there isalsoa narrow absorptionfeature in the midst of the windabsorptionthathas beeninterpreted as being fromTau's windYinterstellar medium (ISM) interaction region (i.e., its ''astrosphere''). We try to reproduce this absorption using hydrodynamic models of theTau astrosphere, which show that stellar wind material heated, compressed, and decelerated at the wind's termination shock (TS) can produce significant absorption at about the right velocity. By experimenting with different model input parameters, we findthatthe parameter that the absorptionismostsensitive to isthe ISM pressure, which determines the location of and therefore the density at the TS. However, the models underestimate both the amountofdecelerationatthe TSand the amountof absorptionfor realistic input parameters. We demonstrate thatthese problems can in principle be resolved by modeling the TS as a radiative shock. However, a cooling timescale short enough to affect the postshock flow is only attainable ifTau's wind speed is increased from the 27Y30 km s � 1 values derivedfromfitstowindabsorptiontoatleast35kms � 1 . The modelsalsoseemto require averyhighISM pressure of P/k � 30;000 cm � 3 K toinducedensitiesattheTShighenoughtoyieldsufficient radiativecooling.Thispressureis at least a factor of 2 higher than other estimates of ISM thermal pressure within the Local Bubble. Subject headingg circumstellar matter — stars: individual (� Tauri) — stars: winds, outflows — ultraviolet: stars