Nancy Remage Evans et al. 2008 The Astronomical Journal 136 1137 doi:10.1088/0004-6256/136/3/1137
DIRECT DETECTION OF THE CLOSE COMPANION OF POLARIS WITH THE HUBBLE SPACE TELESCOPE *
FREE ISSUENancy Remage Evans1, Gail H. Schaefer2, Howard E. Bond2, Giuseppe Bono3,4, Margarita Karovska1, Edmund Nelan2, Dimitar Sasselov5 and Brian D. Mason6
Show affiliationsPolaris, the nearest and brightest classical Cepheid, is a single-lined spectroscopic binary with an orbital period of 30 yr. Using the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) at a wavelength of ~2255 Å, we have directly detected the faint companion at a separation of 0
17. A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction. By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris Aa of 4.5+2.2 –1.4 M 
—the first purely dynamical mass determined for any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of 1.26+0.14 –0.07 M , consistent with an inferred spectral type of F6 V and with a flux difference of 5.4 mag observed at 2255 Å. The magnitude difference at the V band is estimated to be 7.2 mag. Continued HST observations will significantly reduce the mass errors, which are currently still too large to provide critical constraints on the roles of convective overshoot, mass loss, rotation, and opacities in the evolution of intermediate-mass stars. Our astrometry, combined with two centuries of archival measurements, also confirms that the well-known, more distant (18'') visual companion, Polaris B, has a nearly common proper motion with that of the Aa, Ab pair. This is consistent with orbital motion in a long-period bound system. The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa, Ab.
- Footnote
- Keywords
astrometry; binaries: visual; Cepheids; stars: fundamental parameters; stars: individual (Polaris)
- Dates
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Issue 3 (2008 September)
Received 2008 May 8, accepted for publication 2008 June 24
Published 2008 August 11
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Direct Detection of the Close Companion of Polaris with the Hubble Space Telescope
Nancy Remage Evans et al. 2008 The Astronomical Journal 136 1137
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