The Near-Ultraviolet Luminosity Function of Young, Early M-Type Dwarf Stars

TL;DR

This study characterizes the evolution of near-ultraviolet emission in early M-type dwarf stars, revealing that their NUV luminosity does not follow simple models and highlighting implications for planetary habitability.

Contribution

It provides the first detailed NUV luminosity function for early M dwarfs, accounting for false positives and detection biases, challenging previous constant star-formation assumptions.

Findings

NUV luminosity function is inconsistent with constant star-formation models.

Approximately 16% of NUV excess detections are false positives.

NUV emission evolution in M dwarfs is complex and not well-described by simple models.

Abstract

Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magneto-hydrodynamic dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lepine & Gaidos (2011) catalog of bright M dwarfs with the GALEX catalog of NUV (1771-2831A) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (> 2.5 sigma) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these "false positives" as ~16%. We constructed a NUV luminosity function that accounted for false positives, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law.