Bridging the gap on tight separation brown dwarf binaries

TL;DR

This paper develops a spectral index method to identify tight separation brown dwarf binaries, expanding the understanding of their formation and binary fraction at small separations.

Contribution

It introduces a novel spectral index approach and confirms new tight separation brown dwarf binary candidates from a large spectral library.

Findings

Identified 12 new binary candidates.

Confirmed 2 previously reported candidates.

Ruled out 2 previously reported candidates.

Abstract

Multiplicity is a key statistic for understanding the formation of very low mass (VLM) stars and brown dwarfs. Currently, the separation distribution of VLM binaries remains poorly constrained at small separations (<1 AU), leading to uncertainty in the overall binary fraction. We approach this problem by searching for late M/early L plus T dwarf spectral binaries whose combined light spectra exhibit distinct peculiarities, making their identification independent of separation. We define a set of spectral indices designed to identify these systems, and use a spectral template fitting method to confirm and characterize spectral binary (SB) candidates from a library of 738 spectra from the SpeX Prism Spectral Libraries. We present twelve new binary candidates, confirm two previously reported candidates and rule out other two previously reported candidates. All of our candidates have primary and secondary spectral types between M7-L7 and L8-T8 respectively. We find that blue L dwarfs and subdwarfs are contaminants in our sample and propose a method for segregating these sources. If confirmed by follow-up observations, these systems may potentially add to the growing list of tight separation binaries, giving further insight into brown dwarf formation scenarios.