# HAZMAT. V. The Ultraviolet and X-ray Evolution of K Stars

## TL;DR

This study provides the first detailed analysis of the ultraviolet and X-ray evolution of K stars, revealing they emit significantly less high-energy radiation than M stars, which impacts planetary habitability assessments.

## Contribution

It offers the first comprehensive investigation into the UV and X-ray evolution of K stars, filling a gap in stellar activity research relevant to exoplanet habitability.

## Key findings

- K stars have 5-50 times lower UV/X-ray flux than early-M stars.
- K dwarf activity decreases earlier than M dwarfs.
- Habitable zones around K stars receive much less high-energy radiation.

## Abstract

Knowing the high-energy radiation environment of a star over a planet's formation and evolutionary period is critical in determining if that planet is potentially habitable and if any biosignatures could be detected, as UV radiation can severely change or destroy a planet's atmosphere. Current efforts for finding a potentially habitable planet are focused on M stars, yet K stars may offer more habitable conditions due to decreased stellar activity and more distant and wider habitable zones (HZ). While M star activity evolution has been observed photometrically and spectroscopically, there has been no dedicated investigation of K-star UV evolution. We present the first comprehensive study of the near-UV, far-UV, and X-ray evolution of K stars. We used members of young moving groups and clusters ranging in age from 10 - 625 Myr combined with field stars and their archived GALEX UV and ROSAT X-ray data to determine how the UV and X-ray radiation evolve. We find that the UV and X-ray flux incident on a HZ planet is 5 - 50 times lower than that of HZ planets around early-M stars and 50 - 1000 times lower than those around late-M stars, due to both an intrinsic decrease in K dwarf stellar activity occurring earlier than for M dwarfs and the more distant location of the K dwarf HZ.