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Michael D. Albrow (STScI, University of Canterbury)
During the last decade, considerable advances have been made towards the determination of Cepheid dynamical masses in systems where the companion is a B-type main sequence star. (See the series of papers by N. Evans et al. for details.) Such determinations have involved two stages. First, intensive optical spectroscopic observations of the primary Cepheid star are carried out over a number of years to quantify its pulsational and orbital motion. Second, ultraviolet spectroscopic observations from HST (and previously IUE) are used to measure the radial velocity of the hot companion star at key orbital phases. These methods have proven relatively successful, but in a number of the observed systems (e.g. S Mus) the B-star \gamma-velocity has not matched that of the Cepheid, prompting suspicion of a third body being present in the system and casting doubts over the derived Cepheid mass. Resolution of this problem will only come from a larger program of observations of the secondaries.
Through numerical simulations, I will show that in some cases such measurements of the companion-star radial velocities can be made with sufficient precision from the ground, enabling more intensive campaigns than can readily be carried out with HST. The new method requires high-resolution, high-signal-to-noise optical spectroscopy and employs the TODCOR algorithm (Zucker & Mazeh 1994) for simultaneous measurement of the radial velocities of Cepheid and companion. The first measurements of Cepheid-companion radial velocities using this technique will be presented.
The author(s) of this abstract have provided an email address for comments about the abstract: albrow@stsci.edu