Investigating a Possible Spectral Signature of the Wind-ISM Interaction Region of Alpha Tau

Abstract

Ultraviolet spectra from the GHRS instrument on board the Hubble Space Telescope reveal the presence of a mysterious absorption feature in the Mg II h & k lines of the nearby (d=20.0 pc) K5 III star Alpha Tau. The narrow absorption looks like an interstellar absorption feature but it is in the wrong location based on our knowledge of the local ISM flow vector. Since the absorption is close to the rest frame of the star, it has been interpreted as being from the interaction region between Alpha Tau's massive, cool wind and the interstellar medium, i.e., Alpha Tau's "astrosphere". We compute hydrodynamic models of the Alpha Tau astrosphere in order to see if the models can reproduce the Mg II absorption feature. These models do predict that stellar wind material heated, decelerated, and compressed after passing through a termination shock a few thousand AU from the star should produce a Mg II absorption feature with about the right width at roughly the right velocity. However, our first models underestimate the Mg II column density by an order of magnitude. A much larger parameter search is necessary to see whether the observed Mg II absorption can be reproduced by acceptable changes to the adopted stellar wind and ISM properties.