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N. R. Evans (SAO), G. Schaefer, H. Bond (STScI), G. Bono (INAF Rome), M. Karovska (SAO), E. Nelan (STScI), D. Sasselov (Harvard)
Polaris, the nearest and brightest classical Cepheid, is a member of a triple system. Its wide visual companion, Polaris~B, is well known, but Polaris itself is also a single-lined spectroscopic binary with an orbital period of 30 years (Kamper, 1996, JRASC, 90, 140). By combining Hipparcos\/ measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al.\ (2000, A&A, 360, 399) have predicted the astrometric orbit of the close companion. They obtain two possible solutions, with prograde and retrograde senses of revolution. Using the Hubble Space Telescope\/ and the Advanced Camera for Surveys' High-Resolution Channel with an ultraviolet (F220W) filter, we have now directly detected the close companion. We will present details of this detection, including its clear support for one of the two orbit solutions, the luminosity and implied spectral type of the companion, and the first purely dynamical mass ever obtained for a Cepheid. We plan to propose future Hubble\/ observations to refine the dynamical mass determination as a direct test of Cepheid theory. Support for this work was provided by grants HST-GO-10593.01-A and NAS8-03060.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.