Day 1152

Thursday.

1609.04010
Constraining the frequency of free-floating planets from a synthesis of microlensing, radial velocity, and direct imaging survey results
Clinton, Gaudi

A microlensing survey by Sumi+2011 exhibits an overabundance of short-timescale events (STEs; t_E<2 days) relative to what is expected from known stellar populations and a smooth power-law extrapolation down to the brown dwarf regime.  This excess has been interpreted as a population of ~Jupiter-mass objects that outnumber MS stars by nearly 2x; however the microlensing data alone cannot distinguish between events due to wide-separation (a>~10AU) and free-floating planets.  Assuming these STEs are indeed due to planetary-mass objects, aim to constrain the fraction of these events that can be explained by bound by wide-separation planets.  Fit the observed timescale distribution with a lens mass function comprised of brown dwarfs, MS stars, and stellar remnants, finding and thus corroborating the initial identification of an excess of STEs.  Then include population of bound planets that are expected not to show signatures of the primary lens (host) in their microlensing light curves and that are also consistent with results from representative microlensing, radial velocity, and direct imaging surveys.  Find that bound planets alone cannot explain the entire STE excess without violating the constraints from the surveys considered and thus some fraction of these events must be due to free-floating planets, if the model for bound planets holds.  Estimate a median fraction of STEs due to free-floating planets to be f=0.67 (0.23-0.85 at 95% confidence) when assuming "hot-start" planet evolutionary models and f=0.58 ().14-0.83 at 95% confidence) for "cold-start" models.  Assuming a delta-function distribution of free-floating planets of mass m_p=2 M_Jup yields a number of free-floating planets per MS star of N=1.4 (0.48-1.8 at 95% confidence) in the "hot-start" use and N=1.2 (0.29-1.8 at 95% confidence) in the "cold-start" case.


1609.04303
Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
Lindgren et al

Gaia DR1 contains astrometric results for more than 1 billion stars with m<20.7 based on observations collected by the Gaia satellite during the first 14 months of its operational phase.  Give a brief overview of the astrometric content of the data release and of the model assumptions, data processing, and validation of the results.  For stars in common with the Hipparcos and Tycho-2 catalogues, complete astrometric single-star solutions are obtained by incorporating positional information from the earlier catalogues.  For other stars only their positions are obtained by neglecting their proper motions and parallaxes.  The results are validated by an analysis of the residuals, through special validation runs, and by comparison with external data.  Results.  For about 22 million of the brighter stars (down to m~11.5) obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better.  For these stars, systematic errors depending e.g. on position and color are at a level of 0.3 mas.  For the remaining stars, obtain positions at epoch J2015.0 accurate to ~10mas.  Positions and proper motions are given in a reference fame that is aligned with the International Celecitial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03 mas/yr.  The Hipparcos reference frame is found to rotate with respect to the Gia DR1 frame at a rate of 0.24 mas/yr.  Based on less than a quarter of the nominal mission length and on very provisional and incomplete calibrations, the quality and completeness of the astrometric data in Gaia DR1 are far from what is expected for the final mission products.  The results nevertheless represent a huge improvement n the available fundamental stellar data and practical definition of the optical reference frame.