A Spitzer IRAC Direct Imaging Substellar Companion Search Around 89 M, L, and T Dwarf Systems

We report observational techniques, results, and Monte Carlo population analyses for a Spitzer IRAC substellar companion search around 89 M, L, and T dwarf systems. For the typical target star distance, the survey is sensitive to T-dwarf/Y-dwarf companions with temperatures down to similar to 600K. This survey therefore represents the most sensitive population study of mid- to wide-separation (greater than similar to 40 AU) substellar companions to low mass stars and brown dwarfs. Based on IRAC color-color identification, and archival ground-based data, we find that no system shows positive evidence of a substellar companion (searchable separation similar to 3 arcsec to similar to 3 arcmin; projected separation similar to 40-2000 AU at the median target distance). Monte Carlo population analyses reveal that, for an example 1000K substellar population, the 40-2000 AU semi-major-axis companion fraction is <= 3.3%. For an example 633K (Y-dwarf) companion population, the companion fraction is less than similar to 20%, for the 200-800 AU semi-major axis range. To the best of our knowledge, this Y-dwarf value represents the first constraints ever placed on the Y-dwarf companion population, for this semi-major axis regime.