Poster Abstracts

Name/Affiliation:  Beth Biller (University of Edinburgh)

Title:  The binary fraction of the very coolest brown dwarfs

In the last few years, discoveries of the very coolest brown dwarfs {T<800 K} have accelerated, due to results from WISE, UKIDSS, and other IR surveys. These objects close the gap in mass between brown dwarfs and planets -- indeed many of them have planetary masses. Companions around such objects are among the lowest mass imaged to date. Here we present results from a HST WFC3 SNAP programme to obtain IR imaging of newly discovered >T8 dwarfs in order to search for planetary mass companions to these objects. For our sample with typical estimated distances of 10-15 pc, WFC3's 0.13" IR platescale allow us to probe equal magnitude binaries down to separations of 0.2", corresponding to physical separations of 2-3 AU. In order to clearly distinguish cool substellar/planetary mass companions from background stars, we observed in the F127M and the F138M filters, since true cool companions should show deep water absorption features at 1.4 um. Combining our results with other published samples, we present the strongest constraints to date on the binary fraction for >T8 cool brown dwarfs.