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Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time
Using a simulated disc brown dwarf (BD) population, we find that newlarge area infrared surveys are expected to identify enough BDs coveringwide enough mass-age ranges to potentially measure the present day massfunction down to ~0.03Msolar, and the BD formation historyout to 10Gyr, at a level that will be capable of establishing if BDformation follows star formation. We suggest these capabilities are bestrealized by spectroscopic calibration of BD properties (Teff,g and [M/H]) which when combined with a measured luminosity and anevolutionary model can give BD mass and age relatively independent of BDatmosphere models. Such calibration requires an empirical understandingof how BD spectra are affected by variations in these properties, andthus the identification and study of `benchmark BDs' whose age andcomposition can be established independently.We identify the best sources of benchmark BDs as young open clustermembers, moving group members, and wide (>1000au) BD companions toboth subgiant stars and high-mass white dwarfs (WDs). To accuratelyasses the likely number of wide companion BDs available, we haveconstrained the wide L dwarf companion fraction using the 2-Micron AllSky Survey (2MASS), and find a companion fraction of2.7+0.7-0.5percent for separations of~1000-5000au. This equates to a BD companion fraction of34+9-6percent if one assumes an α~ 1companion mass function. Using this BD companion fraction, we simulatepopulations of wide BD binaries, and estimate that80+21-14 subgiant-BD binaries, and50+13-10 benchmark WD-BD binaries could beidentified using current and new facilities. The WD-BD binaries shouldall be identifiable using the Large Area Survey component of the UnitedKingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combinedwith the Sloan Digital Sky Survey. Discovery of the subgiant-BD binarieswill require a near-infrared imaging campaign around a large (~900)sample of Hipparcos subgiants. If identified, spectral studies of thesebenchmark BD populations could reveal the spectral sensitivities acrossthe Teff, g and [M/H] space probed by new surveys.
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