White dwarfs in the UKIDSS Large Area Survey: The Substellar Companion Fraction

TL;DR

This study uses near-infrared data from UKIDSS to identify and estimate the frequency of unresolved substellar companions and debris disks around white dwarfs, revealing new candidate systems and binary fractions.

Contribution

It introduces a method to detect unresolved substellar companions and debris disks around white dwarfs using UKIDSS near-infrared photometry, and estimates their occurrence rates.

Findings

Identified 14-16 white dwarf + very low mass stellar systems.

Found 9-11 white dwarf + brown dwarf systems.

Estimated binary fractions: WD+dL > 0.4%, WD+dT > 0.2%, WD+BD > 0.5%.

Abstract

We present a near-infrared photometric search for unresolved substellar companions and debris disks around white dwarfs in the UKIRT Infrared Deep Sky Survey Large Area Survey. We cross-correlate the SDSS DR4 and McCook & Sion catalogues of white dwarfs with the UKIDSS DR8 producing 3109 and 163 unique matches respectively. Cooling models are fitted to the optical photometry of a subsample of DA white dwarfs and extended to the near-infrared. A comparison is then made with the observed photometry to identify those stars with a near-infrared excess consistent with the presence of a cool companion or debris disk. Where present, we have estimated the approximate spectral type of any putative companion, or an upper limit on the temperature of a debris disk. In total we identify 14-16 new candidate white dwarf + very low mass stellar systems, 9-11 candidate white dwarf + brown dwarf systems, and 3 candidate white dwarf + debris disks. We place lower limits on the unresolved (<2") companions to all DA white dwarfs and thus assess the sensitivity of UKIDSS to such objects. We use this result to estimate unresolved binary fractions of f(WD+dL)>0.4+/-0.3%, f(WD+dT)>0.2% and f(WD+BD)>0.5+/-0.3%.