Searching for Young Substellar Companions to Gaia Stars
Rickard, Matthew J.
The aim of this thesis is to select young ultra-cool dwarf (UCD, spectral type later than M7) candidates that are likely companions to primary stars within the Tycho-Gaia Astrometric Solution (TGAS) catalogue. The intention is to identify a sample of “benchmark” UCDs for which youth and age constraints can be established through association with well understood primary stars. Candidate UCDs are identified through searches of two large-area optical surveys, the Sloan Digital Sky Survey (SDSS) and the Panoramic Survey Telescope and Rapid Response System (PanSTARRS), and two large-area infrared surveys, the UKIRT Infrared Deep Sky Survey (UKIDSS) and the VISTA Hemisphere Survey (VHS). Potential benchmark pairings are selected using a cone-search around each TGAS star with a search radius of 10,000 AU at the distance of each TGAS star. Photometric (colour and magnitude) requirements are imposed on possible associations through comparison, in colour magnitude diagrams (CMDs), with the location of established parallax samples of young and normal UCDs. A set of complementary approaches has been implemented to identify indications of youth in both the UCD and the primary components, the results of which have been brought together into a prioritisation scheme which has been used to guide and plan follow-up observing runs during the project. These diagnostics included unusual UCD colours in the near-infrared and optical, main-sequence lifetime constraints for the primaries, and primary over-brightness (as a function of [M/H] and Teff) indicative of pre-main sequence evolution. A broad range of database information on the stellar components is also gathered providing additional age limitations. UCD proper motion constraints are determined using multi-epoch astrometry from the search-surveys as well as archival motions from the SuperCOSMOS Science Archive. This extensive search and analysis generated a sample of 1,623 candidate young benchmark associations, twenty-four that already evidence strong youth diagnostics in both UCD and primary star components, fifty that are common-proper-motion associations, thirty where the pairs are in close proximity to each other (< 6 arcseconds), and 348 where the primary candidate has high proper motion leading to expectation that the companion candidate can be rapidly motion-tested. Narrow band near-infrared observations are also presented for ten of the candidate UCD components, in an attempt to search for established H-band morphological indicators of extreme youth. With three of these candidates showing Hs-Hl and J-J3 colours consistent with synthesized predictions using previously known young objects. Finally, high priority followup plans and additional development aimed at near-future Gaia DR2 exploitation are considered as future work.
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