Debris discs around M stars: non-existence versus non-detection

TL;DR

This study uses survival models to explore why debris discs are rarely observed around M stars, suggesting detection limitations and dust grain properties influence observed infrared excesses and disc survival.

Contribution

It introduces a new survival model framework to analyze debris disc existence around M stars, accounting for dust properties and observational constraints.

Findings

Debris discs around M stars may be undetected due to small semi-major axes and dust grain properties.

Low-mass debris discs could be common but faint, escaping current detection.

Infrared excesses in AU Mic are consistent with its young age and specific dust conditions.

Abstract

Motivated by the reported dearth of debris discs around M stars, we use survival models to study the occurrence of planetesimal discs around them. These survival models describe a planetesimal disc with a small number of parameters, determine if it may survive a series of dynamical processes and compute the associated infrared excess. For the WISE satellite, we demonstrate that the dearth of debris discs around M stars may be attributed to the small semi-major axes generally probed if either: 1. the dust grains behave like blackbodies emitting at a peak wavelength coincident with the observed one; 2. or the grains are hotter than predicted by their blackbody temperatures and emit at peak wavelengths that are shorter than the observed one. At these small distances from the M star, planetesimals are unlikely to survive or persist for time scales of 300 Myr or longer if the disc is too massive. Conversely, our survival models allow for the existence of a large population of low-mass debris discs that are too faint to be detected with current instruments. However, our interpretation becomes less clear and large infrared excesses are allowed if only one of these scenarios holds: 3. the dust grains are hotter than blackbody and predominantly emit at the observed wavelength; 4. or are blackbody in nature and emit at peak wavelengths longer than the observed one. Both scenarios imply that the parent planetesimals reside at larger distances from the star than inferred if the dust grains behaved like blackbodies. In all scenarios, we show that the infrared excesses detected at 22 and 70 microns from AU Mic are easily reconciled with its young age. We elucidate the conditions under which stellar wind drag may be neglected when considering dust populations around M stars. The WISE satellite should be capable of detecting debris discs around young M stars with ages on the order of 10 Myr.