The Sirius System and Its Astrophysical Puzzles: Hubble Space Telescope and Ground-based Astrometry^∗

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 M 2.063 0.023   and M 1.018 0.011   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-M 25 Jup . 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 Z 0.85  , 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.

Relativistic deflection of background starlight measures the mass of a nearby white dwarf star

“…The vast majority of WD masses cannot be measured directly, but have to be inferred from model-dependent determinations of their surface gravities and estimates of their radii from parallax determinations and photometric or spectroscopic flux measurements (44), or from gravitational redshifts in cases where the true radial velocity of the WD is known from measurements of a companion star (45). The number of WDs whose masses and radii have been directly measured with sufficient precision to test theoretical MRRs includes just three WDs in nearby wide visual binaries [Sirius B (46), Procyon B (47) and 40 Eri B (43)] and about 10 WDs in short-period eclipsing binaries (44). However the stars in the latter group have undergone common-envelope events and have therefore not evolved in the same way as isolated single stars.…”

MESAS: Measuring the Emission of Stellar Atmospheres at Submillimeter/millimeter Wavelengths

“…Fundamental stellar parameters of Sirius A are taken from measured constraints on the angular diameter and bolometric flux (Davis et al 2011) together with the trigonometric parallax and dynamical mass (Bond et al 2017) from its orbit about Sirius B. For the model, we adopt an effective temperature of T ef f = 9843 K, a surface gravity of log(g)=4.28, and a radius of 1.71 R .…”

The MESAS Project: Long-wavelength Follow-up Observations of Sirius A

“…Sirius A is a 225 -250 Myr A1Vm star (Adelman 2004;Liebert et al 2005;Bond et al 2017) that has no observed circumstellar debris. Located only 2.64 ± 0.01 pc (van Leeuwen 2007) from our Solar System, this bright star is an excellent target to observe and model the submm-cm stellar emission of A-type stars.…”