Gamma-ray Emission from Nearby Flare Stars and Its Impact on Exoplanetary Atmospheres

Gamma-ray emission from stellar activity, usually generated from relativistic particles, plays an important role in the space weather and habitability of exoplanets. The Sun is so far the only isolated main sequence star detected in gamma-rays, and is particularly bright during powerful flares. Young dwarf stars of K-type and later are far more magnetically active than the Sun, which makes them excellent candidates for gamma-ray searches. With 12 years of Fermi-LAT data, we analyzed the phase-folded light curve of the fast-rotating radio M9 star TVLM 513-46546 and reported a tentative detection of a gamma-ray pulse associated with its rotation. This is the first detection of an isolated main-sequence star in gamma-rays. We also stack Fermi-LAT data during flare events identified from optical and X-ray flare surveys for dwarf stars. This stacking survey indicates an upper limit of gamma-ray emission from the population of flare stars. The stellar gamma-ray emission likely originates from the decay of neutral pions generated by relativistic protons interacting within the stellar atmosphere. These gamma-ray observations can be used to constrain the particle acceleration mechanisms and the resulting stellar energetic particle spectrum. The gamma-ray flux of these flares are likely to be orders of magnitude stronger than typical Solar flares. The stellar energetic particles can contain as much as ~10³⁷ ergs in energy. The estimated magnitude of these flares and the associated stellar winds and coronal mass ejections can be compared to the Solar flares that cause extreme geomagnetic events on Earth. Frequent, repeated events would then likely evaporate parts or all of the exoplanet atmosphere hence decreasing its long-term habitability. This impact will be examined in detail in follow-up studies. We will also search for gamma-rays amongst flaring stars with known exoplanets to determine to what degree this phenomenon would decrease the amount of expected habitable worlds.