Accurate, Empirical Radii and Masses of Planets with Gaia Parallaxes

Accurate, Empirical Radii and Masses of Planets with Gaia Parallaxes

Authors:

Stassun et al

Abstract:

We present new, empirical measurements of the radii of 132 stars that host transiting planets. These stellar radii are determined using only direct observables---the bolometric flux at Earth, the stellar effective temperature, and the parallax newly provided by the Gaia first data release---and thus are virtually model independent, extinction being the only free parameter. We also determine each star's mass using our newly determined radius and the stellar density, itself a virtually model independent quantity from the previously published transit analysis. The newly determined stellar radii and masses are in turn used to re-determine the transiting planet radii and masses, once again using only direct observables. The uncertainties on the stellar radii and masses are typically 8% and 30%, respectively, and the resulting uncertainties on the planet radii and masses are 9% and 22%, respectively. These accuracies are generally larger than the previously published model-dependent precisions of 5% and 6% on the planet radii and masses, respectively, but the newly determined values are purely empirical. We additionally report stellar radii for 366 stars that host radial-velocity (non-transiting) planets, with a typical achieved accuracy in the radii of 2%. Most importantly, the stellar bolometric fluxes and angular radii reported here---with typical accuracies of 1.7% and 1.8%, respectively---will serve as a fundamental data set to permit the re-determination of the planet radii and masses with the {\it Gaia\} second data release to 3% and 5% accuracy, comparable to or better than currently published precisions, but in an entirely empirical fashion.

WISE J085510.83-071442.5: a new Parallax Measurement for the Coldest Known Brown Dwarf

A New Parallax Measurement for the Coldest Known Brown Dwarf

Authors:

Luhman et al

Abstract:

WISE J085510.83-071442.5 was recently discovered as the coldest known brown dwarf based on four epochs of images from the Wide-field Infrared Survey Explorer and the Spitzer Space Telescope. We have improved the accuracy of its parallax measurement by obtaining two additional epochs of Spitzer astrometry. We derive a parallactic distance of 2.31+/-0.08 pc, which continues to support its rank as the fourth closest known system to the Sun when compared to WISE J104915.57-531906.1 AB (2.02+/-0.02 pc) and Wolf 359 (2.386+/-0.012 pc). The new constraint on the absolute magnitude at 4.5um indicates an effective temperature of 235-260 K based on four sets of theoretical models. We also show the updated positions of WISE J085510.83-071442.5 in two color-magnitude diagrams. Whereas Faherty and coworkers cited its location in MW2 versus J-W2 as evidence of water clouds, we find that those data can be explained instead by cloudless models that employ non-equilibrium chemistry.

Exoplanet Hunt Around Very Cool Stars and Brown Dwarfs Produces First Parallax Results

Astrometric planet search around southern ultracool dwarfs I: First results including parallaxes of 20 M8-L2 dwarfs

Authors:

Sahlmann et al

Abstract:

Extrasolar planet searches targeting very low-mass stars and brown dwarfs are hampered by intrinsic or instrumental limitations. Time series of astrometric measurements with precisions better than one milli-arcsecond can yield new evidence on the planet occurrence around these objects. We present first results of an astrometric search for planets around 20 nearby dwarf stars with spectral types M8-L2. Over a timespan of two years, we obtained I-band images of the target fields with the FORS2 camera at the Very Large Telescope. Using background stars as references, we monitored the targets' astrometric trajectories, which allowed us to measure parallax and proper motions, set limits on the presence of planets, and to discover the orbital motions of two binary systems. We determined trigonometric parallaxes with an average accuracy of 0.09 mas (~0.2 %) resulting in a reference sample for the study of ultracool dwarfs at the M/L transition, whose members are located at distances of 9.5-40 pc. This sample contains two newly discovered tight binaries (DE0630-18 and DE0823-49) and one previously known wide binary (DE1520-44). Only one target shows I-band variability greater than 5 mmag r.m.s. We derived planet exclusion limits that set an upper limit of 9 % to the occurrence of giant planets with masses greater than 5 MJup in intermediate-separation (0.01-0.8 AU) orbits around M8-L2 dwarfs. We demonstrated that astrometric observations with an accuracy of 120 micro-arcsec over two years are feasible from the ground and can be used for a planet search survey. The detection of two tight very low-mass binaries showed that our search strategy is efficient and may lead to the detection of planetary-mass companions through follow-up observations.