The Dynamical Mass of the Bright Cepheid Polaris

Details The Dynamical Mass of the Bright Cepheid Polaris The Dynamical Mass of the Bright Cepheid Polaris

Jul 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005hst..prop10593e&link_type=abstract

HST Proposal ID #10593

Astronomy and Astrophysics

Astronomy

Hst Proposal Id #10593 Cool Stars

Scientific paper

Cepheid variables are of central importance in Galactic and extragalactic astronomy. They are the primary standard candles for measuring extragalactic distances, and they provide critical tests of stellar-evolution theory. Surprisingly, however, there is not a single Cepheid with a purely dynamical measurement of its mass. Polaris {alpha UMi} is the nearest and brightest of all Cepheids. It offers the unique opportunity to measure the dynamical mass of a Cepheid, because it is in a binary system for which a single-lined spectroscopic orbit is already available. We show that the binary should be easily resolved in the UV using ACS/HRC, thus providing the first direct detection of the companion. In the present proposal we request one HST orbit in order to make this detection and measure the separation. We show that this initial detection, combined with the HST/FGS parallax {see below}, will provide a mass accurate to 0.9 Msun. Only HST's combination of high spatial resolution and UV sensitivity can achieve this result. We plan to continue the program in future cycles, leading to rapid refinement of the dynamical mass measurement of the Cepheid. The parallax is a key ingredient in the mass determination. In an ongoing multi-year program {GO-9888, GO-10113}, we are using the FGS to improve significantly upon the Hipparcos parallax of Polaris. The ACS imaging proposed here will thus provide extremely valuable astrophysical information from a very modest additional investment of observing time.

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