Jay B. Holberg

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

Sirius, the seventh-nearest stellar system, is a visual binary containing the metallic-line A1 V star Sirius A, 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 2.063+/-0.023 Msun and 1.018+/-0.011 Msun for Sirius A and B, respectively. Our precise HST astrometry rules out third bodies orbiting either star in the system, down to masses of ~15-25 Mjup. The location of Sirius B in the H-R 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 in 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 sub-solar metallicity, of about 0.85 Zsun, to fit its location in 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.

TOWARD A NETWORK OF FAINT DA WHITE DWARFS AS HIGH-PRECISION SPECTROPHOTOMETRIC STANDARDS

NASA [NAS5-26555]; NASA Office of Space Science [NNX13AC07G]; Ministerio de Ciencia, Tecnologia e Innovacion Productiva (Argentina) [GS-2013A-Q-8, GS-2013B-Q-22]

CALIBRATION OF THE VOYAGER ULTRAVIOLET SPECTROMETERS AND THE COMPOSITION OF THE HELIOSPHERE NEUTRALS: REASSESSMENT

CNES, Universite Pierre et Marie Curie (UPMC); Centre National de la Recherche Scientifique (CNRS) in France

Probing the Gravitational Dependence of the Fine-Structure Constant from Observations of White Dwarf Stars

Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling, and fundamental physics in the search for variation in the fine structure constant. This variation manifests as shifts in the observed wavelengths of absorption lines, such as quadruply ionized iron (FeV) and quadruply ionized nickel (NiV), when compared to laboratory wavelengths. Berengut et al. (Phys. Rev. Lett. 2013, 111, 010801) demonstrated the validity of such an analysis using high-resolution Space Telescope Imaging Spectrograph (STIS) spectra of G191-B2B. We have made three important improvements by: (a) using three new independent sets of laboratory wavelengths; (b) analysing a sample of objects; and (c) improving the methodology by incorporating robust techniques from previous studies towards quasars (the Many Multiplet method). A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. Here we describe our approach and present preliminary results from nine objects using both FeV and NiV.

Hot DA white dwarf model atmosphere calculations: including improved Ni PI cross-sections

To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages needs to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni {\sc iv}-{\sc vi} bound-bound and bound-free atomic data has on model atmosphere calculations. Models including PICS calculated with {\sc autostructure} show significant flux attenuation of up to

Testing the white dwarf mass-radius relation and comparing optical and far-UV spectroscopic results with Gaia DR2, HST and FUSE

\sim 80

Fundamental Physics from Observations of White Dwarf Stars

\% shortward of 180\AA\, in the EUV region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of this atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including {\sc autostructure} PICS were found to change the abundances of N and O by as much as

The 25 parsec local white dwarf population

\sim 22

Stellar Population Science with LSST

\% compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including {\sc autostructure} PICS caused the abundances of N/O {\sc iv} and {\sc v} to diverge. This is because the increased opacity in the {\sc autostructure} PICS model causes these charge states to form higher in the atmosphere, moreso for N/O {\sc v}. Models using an extended line list caused significant changes to the Ni {\sc iv}-{\sc v} abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using {\sc autostructure} PICS for Ni.

Voyager Ultraviolet Spectrometers calibration and the heliosphere neutrals composition: reassessment

SRGJ acknowledges support from the Science and Technology Facilities Council (STFC, UK). MAB acknowledges support from the Gaia post-launch support programme of the UK Space Agency. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. JBH was partially supported by NSF grant AST-1413537.