Day 1394

Thursday.

1804.01236
On the nature of the black-body stars
Serenelli, Rohrmann, Fukugita

Study the physical nature of the black-body (BB) stars recently discovered.  Use sets of white dwarf (WD) model atmospheres with pure-He, pure-H, and H/He mixtures to compute synthetic spectra and compare theoretical expectations with observations of the BB stars.  Find that the spectral properties of the BB stars in the UV and the optical are reproduced to high-accuracy by He dominated atmospheres possibly with trace amounts of H below the spectroscopic detection limit, -8 <= log (H/He) <= -6, and typical logG=9 values corresponding to WDs of 0.6 Msun.  Pure He models with LogG=8 also provide a good match from 12000 down to T_eff=8500K, but fail to simultaneously fit the UV and optical colors for the cooler stars in the sample.  However, pure-He models with logG=9 show a better match with the data, implying that BB stars could be massive WDs, around ~1.2 Msun.  Find the the emerging spectrum has a Planck distribution shifted towards the blue compared to the expected shape based on the model effective temperature.  The BB temperatures determined from colors overestimated the actual T_eff of these stars by 400K and up to 1000K, depending on temperature and log(H/He).  Finally show that precision of end-of-mission Gaia parallaxes should allow disentangling whether the BB stars have typical WD masses or represent a massive, peculiar population.


1804.01318
The standard model of cosmology: a skeptic's guide
Scott

The status of the standard cosmo model, also known as LCDM is described.  With some simple assumptions, this model fits a wide range of data, with just 6 or 7 free parameters.  One should be skeptical about this claim, since it implies that we now ave an astonishingly good picture of the statistical properties of the LS Universe.  However, the successes of the model cannot be denied, including more than 1000 sigma worth of detection of CMB anisotropy power.  The model is older than most modern astrophysicists seem to appreciate, and has not fundamentally changed for more than a quarter of a century.  Tensions and anomalies are often discussed, and while we should of course be open to the possibility of new physics, we should also be skeptical of the importance of 2-3 sigma differences between data sets until they become more significant.  Still, today's SMC is surely not the full story and we should be looking for extension or new ingredients to the model, guided through by a skeptical outlook.


1804.01427
A Gaia DR2 mock stellar catalog
Rybizki, et al

Present a mock catalog of MW stars, matching in volume and depth the content of the Gaia data release 2 (GDR2).  Generated the catalog using Galaxia, a tool to sample stars from Besancon Galactic model, together with a realistic 3D dust extinction map.  The catalog mimics the complete GDR2 data model and contains most of the entries in the Gaia source catalog: 5 parameter astrometry, 3-band photometry, radial velocities, stellar parameters, and associated scaled nominal uncertainty estimates.  In addition, supplemented the catalog with extinctions and photometry for non-Gaia bands.  This catalog can be used to prepare GDR2 queries in a realistic runtime environment, and it can serve as a Galactic model against which to compare the actual GDR2 data in the space of observables.  The catalog is hosted through the virtual observatory GAVO's Heidelberg data center service and thus can be queried using ADQL as for GDR2 data.