A Spectroscopic Analysis of the Ultraviolet Evolution of K Stars

A haiku about this work: K dwarfs are awesome HST has been staring Cool science in store! Efforts to discover and characterize habitable zone planets have primarily focused on Sun-like stars and M dwarfs. Yet the intermediate K stars provide an appealing compromise between these two extremes that has been relatively unexplored. Compared to solar-type stars, K stars are more abundant, maintain longer main-sequence lifetimes, and their planets are more suitable to observations based on the mass and radius ratios of the planet to star. While M stars also excel in these regards, they have been observed to have frequent flares with ultraviolet (UV) energies over 100 times greater than anything observed in the recent history of our Sun. Therefore, K stars may provide "super-habitable" environments for exoplanets that are more easily detectable and characterizable. Understanding the UV radiation incident on planets is pivotal in determining the habitability of any planets orbiting these stars, as well as our ability to observe any potential biosignatures in their atmospheres. With this aim, we observed a sample of K stars in the near-UV and far-UV at three representative ages using the Cosmic Origins Spectrograph on the Hubble Space Telescope (HST) to spectroscopically analyze the UV evolution of K stars. Here, we present the first results of this HST program. We will discuss the evolution of both the continuum and line emission from K stars from ages 40 Myr to ~Gyrs, representing key periods in planetary atmospheric formation.

This work was funded by the NASA FINESST award, number 19-ASTRO20-0081.