Polaris: Mass and Multiplicity

Polaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide 18 physical companion, the F-type dwarf Polaris B. Polaris itself is a single-lined spectroscopic binary with an orbital period of 30 years Kamper 1996. By combining Hipparcos measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al. 2000 have predicted the astrometric orbit of the close companion. 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. Based on the Wielen et al. orbit, the Hipparcos parallax, and our measurement of the separation 0.176 or- 0.002, we find a preliminary mass of 5.0 or- 1.5 Mcircle dot solar for the Cepheid and 1.38 or- 0.61 Mcircle dot solar for the close companion. These values will be refined by additional HST observations scheduled for the next 3 years. We have also obtained a Chandra ACIS-I image of the Polaris field. Two distant companions C and D are not X-rays sources and hence are not young enough to be physical companions of the Cepheid. There is one additional stellar X-ray source in the field, located 253 from Polaris A, which is a possible companion. Further investigation of such a distant companion is valuable to confirm the full extent of the system.