M-dwarf binaries as tracers of star and brown dwarf formation

TL;DR

This paper models M-dwarf binary separations using a dynamical population synthesis approach, suggesting a possible overlap between star-like and brown dwarf-like formation modes, but data remain inconclusive.

Contribution

It introduces a DPS model combining star-like and brown dwarf-like populations to explain M-dwarf binary separation distributions, highlighting the need for further observations.

Findings

The model reproduces the observed peak in late M-dwarf separations.

Early M-dwarfs show no such peak, aligning with some observational data.

Current data cannot definitively confirm the continuity between brown dwarfs and stars.

Abstract

The separation distribution for M-dwarf binaries in the ASTRALUX survey is narrower and peaking at smaller separations than the distribution for solar-type binaries. This is often interpreted to mean that M-dwarfs constitute a continuous transition from brown dwarfs (BDs) to stars. Here a prediction for the M-dwarf separation distribution is presented, using a dynamical population synthesis (DPS) model in which "star-like" binaries with late-type primaries ($\lesssim1.5 M_{\rm sun}$) follow universal initial distribution functions and are dynamically processed in their birth embedded clusters. A separate "BD-like" population has both its own distribution functions for binaries and initial mass function (IMF), which overlaps in mass with the IMF for stars. Combining these two formation modes results in a peak on top of a wider separation distribution for late M-dwarfs consistent with the late ASTRALUX sample. The DPS separation distribution for early M-dwarfs shows no such peak and is in agreement with the M-dwarfs in Multiples (MinMS) data. We note that the latter survey is potentially in tension with the early ASTRALUX data. Concluding, the ASTRALUX and MinMS data are unable to unambiguously distinguish whether or not BDs are a continuous extension of the stellar IMF. Future observational efforts are needed to fully answer this interesting question. The DPS model predicts that binaries outside the sensitivity range of the ASTRALUX survey remain to be detected. For application to future data, we present a means to observationally measure the overlap of the putative BD-like branch and the stellar branch. We discuss the meaning of universal star formation and distribution functions.