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The Sirius System and Its Astrophysical Puzzles: Hubble Space Telescope and Ground-based Astrometry<sup>∗</sup>

Howard E. BondDepartment of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA; [email protected]Gail SchaeferThe CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USARonald L. GillilandCenter for Exoplanets and Habitable Worlds, Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USAJ. B. HolbergLunar & Planetary Laboratory, University of Arizona, 1541 E. University Boulevard, Tucson, AZ 85721, USABrian D. MasonU.S. Naval Observatory, 3450 Massachusetts Avenue, Washington, DC 20392, USAIrving W. LindenbladU.S. Naval Observatory, 3450 Massachusetts Avenue, Washington, DC 20392, USAMiranda Seitz-McLeeseU.S. Naval Observatory, 3450 Massachusetts Avenue, Washington, DC 20392, USAW. David ArnettSteward Observatory, 933 N. Cherry Ave., University of Arizona, Tucson, AZ 85721, USAP. DemarqueDepartment of Astronomy, Yale University, Box 208101, New Haven, CT 06520, USAF. SpadaLeibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482, Potsdam, GermanyPatrick YoungSchool of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USAM. A. BarstowDepartment of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UKM. R. BurleighDepartment of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UKDonald H. GudehusDepartment of Physics & Astronomy, Georgia State University, Atlanta, GA 30303, USA

2017en

ABI

Annotatsiya

Abstract Sirius, the seventh-nearest stellar system, is a visual binary containing the metallic-line A1 V star Sirius A, the brightest star in the sky, orbited in a 50.13 year period by Sirius B, the brightest and nearest white dwarf (WD). Using images obtained over nearly two decades with the Hubble Space Telescope ( HST ), along with photographic observations covering almost 20 years and nearly 2300 historical measurements dating back to the 19th century, we determine precise orbital elements for the visual binary. Combined with the parallax and the motion of the A component, these elements yield dynamical masses of and for Sirius A and B, respectively. Our precise HST astrometry rules out third bodies orbiting either star in the system, down to masses of ∼15– . The location of Sirius B in the Hertzsprung–Russell diagram is in excellent agreement with theoretical cooling tracks for WDs of its dynamical mass, and implies a cooling age of ∼126 Myr. The position of Sirius B on the mass–radius plane is also consistent with WD theory, assuming a carbon–oxygen core. Including the pre-WD evolutionary timescale of the assumed progenitor, the total age of Sirius B is about 228 ± 10 Myr. We calculated evolutionary tracks for stars with the dynamical mass of Sirius A, using two independent codes. We find it necessary to assume a slightly subsolar metallicity, of about , to fit its location on the luminosity–radius plane. The age of Sirius A based on these models is about 237–247 Myr, with uncertainties of ±15 Myr, consistent with that of the WD companion. We discuss astrophysical puzzles presented by the Sirius system, including the probability that the two stars must have interacted in the past, even though there is no direct evidence for this and the orbital eccentricity remains high.

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DOI: 10.3847/1538-4357/aa6af8

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