An infrared search for substellar companions to nearby white dwarfs.

摘要

This paper presents the results of an extensive search for substellar companions to white dwarfs in the solar neighborhood. The work consists of two major components. The first part is a survey conducted with the Keck I 10 meter telescope of 86 white dwarfs. This deep near field search was sensitive to companions orbiting between roughly 50--1100 AU of the primary star. The second part is a wide field survey conducted with the Bok 2.3 meter and Shane 3 meter telescopes of 261 white dwarfs. This latter search was sensitive to companions orbiting between roughly 100--5000 AU. The entire sample has a median distance of around 57 pc.; The J band, centered at 1.25 mum, was chosen as the appropriate wavelength to conduct this search. This atmospheric window lies near the peak in luminosity of substellar objects, commonly called brown dwarfs. The mass limits of the searches are dependent on the age of the white dwarfs in the sample because brown dwarfs cool as they evolve. The exact ages of the target primaries cannot be known because of their uncertain main sequence progenitor lifetimes. However, for white dwarfs about 3 Gyrs old, a reasonably conservative median age estimate, companions more massive than 0.06 M⊙ in the wide field search and more massive than 0.03 M⊙ in the deeper near field search were detectable.; In neither component of the survey were any brown dwarf candidates detected. This implies a brown dwarf companion fraction of less than 0.3% for white dwarfs. In stark contrast, the stellar companion fraction of white dwarfs as measured by this survey is 22%. Moreover, most of the known and suspected stellar companions to white dwarfs are low mass stars whose masses are only slightly greater than the masses of brown dwarfs. This result has strong implications for binary star formation.; Twenty previously undiscovered stellar companions were detected, five of which are confirmed or likely white dwarfs themselves. The remaining fifteen detected companions are confirmed or likely low mass stars.