Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time

Abstract

Using a simulated disc brown dwarf (BD) population, we find that new large area infrared surveys are expected to identify enough BDs covering wide enough mass–age ranges to potentially measure the present day mass function down to ∼0.03 M⊙, and the BD formation history out to 10 Gyr, at a level that will be capable of establishing if BD formation follows star formation. We suggest these capabilities are best realized by spectroscopic calibration of BD properties (Teff, g and [M/H]) which when combined with a measured luminosity and an evolutionary model can give BD mass and age relatively independent of BD atmosphere models. Such calibration requires an empirical understanding of how BD spectra are affected by variations in these properties, and thus the identification and study of ‘benchmark BDs’ whose age and composition can be established independently. We identify the best sources of benchmark BDs as young open cluster members, moving group members, and wide (>1000 au) BD companions to both subgiant stars and high-mass white dwarfs (WDs). To accurately asses the likely number of wide companion BDs available, we have constrained the wide L dwarf companion fraction using the 2-Micron All Sky Survey (2MASS), and find a companion fraction of 2.7+0.7−0.5 per cent for separations of ∼1000–5000 au. This equates to a BD companion fraction of 34+9−6 per cent if one assumes an α∼ 1 companion mass function. Using this BD companion fraction, we simulate populations of wide BD binaries, and estimate that 80+21−14 subgiant–BD binaries, and 50+13−10 benchmark WD–BD binaries could be identified using current and new facilities. The WD–BD binaries should all be identifiable using the Large Area Survey component of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combined with the Sloan Digital Sky Survey. Discovery of the subgiant–BD binaries will require a near-infrared imaging campaign around a large (∼900) sample of Hipparcos subgiants. If identified, spectral studies of these benchmark BD populations could reveal the spectral sensitivities across the Teff, g and [M/H] space probed by new surveys.