HAZMAT III: The UV Evolution of Mid- to Late-M Stars with GALEX

TL;DR

This study investigates the UV radiation evolution of low-mass M stars, revealing that lower mass stars retain higher UV activity longer than early M stars, impacting planetary habitability assessments.

Contribution

It extends previous UV evolution studies to include lower mass M stars and provides new insights into their UV activity over time.

Findings

Lower mass M stars retain high UV activity up to billions of years.

UV flux decreases less for lower mass M stars compared to early M stars.

FUV/NUV ratio remains relatively constant for mid- and late-type M stars across ages.

Abstract

Low-mass stars are currently the most promising targets for detecting and characterizing habitable planets in the solar neighborhood. However, the ultraviolet (UV) radiation emitted by such stars can erode and modify planetary atmospheres over time, drastically affecting their habitability. Thus knowledge of the UV evolution of low-mass stars is critical for interpreting the evolutionary history of any orbiting planets. Shkolnik & Barman (2014) used photometry from the Galaxy Evolution Explorer (GALEX) to show how UV emission evolves for early type M stars ($>$0.35 $M_\odot$). In this paper, we extend their work to include both a larger sample of low-mass stars with known ages as well as M stars with lower masses. We find clear evidence that mid- and late-type M stars (0.08-0.35 $M_\odot$) do not follow the same UV evolutionary trend as early-Ms. Lower mass M stars retain high levels of UV activity up to field ages, with only a factor of 4 decrease on average in GALEX NUV and FUV flux density between young ($<$50 Myr) and old ($\sim$5 Gyr) stars, compared to a factor of 11 and 31 for early-Ms in NUV and FUV, respectively. We also find that the FUV/NUV flux density ratio, which can affect the photochemistry of important planetary biosignatures, is mass and age-dependent for early Ms, but remains relatively constant for the mid- and late-type Ms in our sample.