Study of the mass-ratio distribution of spectroscopic binaries - II. The boundaries of the brown-dwarf desert as seen with the APOGEE spectroscopic binaries

Analysis of APOGEE DR12 stellar radial velocities by Troup et al. affirmed the existence of the well-known brown-dwarf desert (BDD). They detected a dearth of spectroscopic binaries (SB) with periods shorter than ∼10-30 d and secondaries with masses in the range of ∼0.01-0.1 M_⊙. We reconsider here their sample of binaries, focusing on 116 systems on the main sequence of the Gaia colour-magnitude diagram, with mostly K-dwarf primaries. Using our recently devised algorithm to analyse the mass-ratio distribution of a sample of SBs we confirm the BDD existence and delineate its boundaries. For the K-dwarf APOGEE 1-25 d binaries, the companion-mass range of the BDD is ∼0.02-$0.2 M_⊙. The mass-ratio distribution of the long-period (25-500 d) binaries does not show any dearth at the q range studied. Instead, their distribution displays a linear increase in log q, implying a tendency towards low-q values. The limits of the BDD do not coincide with the frequently used mass limits of the brown-dwarf population, sometimes defined as 0.013 and 0.08 M_⊙, based on theoretically derived stellar minimum masses for burning deuterium and hydrogen in their cores. Trying to draw the boundaries of the desert, we suggest either a wedged or trapezoidal shape. We discuss briefly different scenarios that can account for the formation of the BDD, in terms of differentiating between stellar secondaries and planets in particular, and compare this desert to the Neptunian desert that can distinguish between Jovian planets and super-Earths of short periods.