Confirming new white dwarf-ultracool dwarf binary candidates

TL;DR

This study identifies and confirms new white dwarf-ultracool dwarf binary candidates using near-infrared excesses and optical spectroscopy, revealing several potential systems with cool companions and highlighting the need for further radial velocity follow-up.

Contribution

The paper reports the discovery and spectroscopic validation of new white dwarf-ultracool dwarf binary candidates, expanding the known population and characterizing their properties.

Findings

Confirmed 10 white dwarfs with cool companions from 22 candidates.

Identified potential late M and L dwarf companions, including one near the M-L boundary.

Detected photometric variability in one system, suggesting it may be a close binary.

Abstract

We present the results of a study to discover prospective new white dwarf-L dwarf binaries as identified by their near-infrared excesses in the UKIDSS catalogue. We obtained optical spectra to validate the white dwarf nature for 22 of the candidate primary stars, confirming ten as white dwarfs and determining their effective temperatures and gravities. For all ten white dwarfs we determined that the near-infrared excess was indeed indicative of a cool companion. Six of these are suggestive of late M dwarf companions, and three are candidate L dwarf companions, with one straddling the M$-$L boundary. We also present near-infrared spectra of eight additional candidate white dwarf-ultracool dwarf binaries where the white dwarf primary had been previously confirmed. These spectra indicate one candidate at the M$-$L boundary, three potential L dwarf companions, and one suspected M dwarf, which showed photometric variability on a $\sim$6 hour period, suggesting the system may be close. Radial velocity follow up is required to confirm whether these systems are close, or widely separated.