Polaris: Mass and multiplicity

Nancy Remage Evans, Gail Schaefer, Howard E. Bond, Edmund Nelan, Giuseppe Bono, Margarita Karovska, Scott Wolk, Dimitar Sasselov, Edward Guinan, Scott Engle, Eric Schlegel, Brian Mason

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


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 0.002), we find a preliminary mass of 5.0 1.5 M for the Cepheid and 1.38 0.61 M 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.

Original languageEnglish (US)
Pages (from-to)102-104
Number of pages3
JournalProceedings of the International Astronomical Union
Issue numberS240
StatePublished - Aug 2006

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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