An Ultraviolet Investigation of Activity on Exoplanet Host Stars

TL;DR

This study uses GALEX UV data to investigate whether close-in exoplanets influence host star activity, finding tentative evidence of increased UV activity in stars with close-in planets, but no strong correlations with system parameters.

Contribution

First UV-based statistical analysis of star-planet interactions, providing new insights into stellar activity related to exoplanet proximity using GALEX data.

Findings

Tentative UV activity enhancement in stars with close-in planets.

No strong correlation between activity and planet mass or orbital distance.

Need for time-resolved studies due to variability in UV luminosity.

Abstract

Using the far-UV (FUV) and near-UV (NUV) photometry from the NASA Galaxy Evolution Explorer (GALEX), we searched for evidence of increased stellar activity due to tidal and/or magnetic star-planet interactions (SPI) in the 272 of known FGK planetary hosts observed by GALEX. With the increased sensitivity of GALEX, we are able probe systems with lower activity levels and at larger distances than what has been done to date with X-ray satellites. We compared samples of stars with close-in planets (a < 0.1 AU) to those with far-out planets (a > 0.5 AU) and looked for correlations of excess activity with other system parameters. This statistical investigation found no clear correlations with a, M_p, nor M_p/a, in contrast to some X-ray and Ca II studies. However, there is tentative evidence (at a level of 1.8-\sigma) that stars with RV-detected close-in planets are more FUV-active than stars with far-out planets, in agreement with several published X-ray and Ca II results. The case is strengthened to a level of significance of 2.3-\sigma\ when transit-detected close-in planets are included. This is most likely because the RV-selected sample of stars is significantly less active than the field population of comparable stars, while the transit-selected sample is similarly active. Given the factor of 2-3 scatter in fractional FUV luminosity for a given stellar effective temperature, it is necessary to conduct a time-resolved study of the planet hosts in order to better characterize their UV variability and generate a firmer statistical result.