Stellar Activity in the Broad-Band Ultraviolet

TL;DR

This study utilizes GALEX ultraviolet photometry to develop new activity indicators for stars, establishing correlations between UV flux, optical activity measures, and stellar age, despite significant observational scatter.

Contribution

It introduces preliminary relations linking UV flux to stellar activity and age, expanding the use of UV data for stellar activity studies beyond previous limitations.

Findings

FUV excess flux roughly proportional to R'_HK

Detected correlation between NUV flux and activity/age

Large scatter (~0.2 mag, ~0.18 dex, ~0.4 dex) in UV flux and age predictions

Abstract

The completion of the GALEX All-Sky Survey in the ultraviolet allows activity measurements to be acquired for many more stars than is possible with the limited sensitivity of ROSAT or the limited sky coverage of Chandra, XMM, or spectroscopic surveys for line emission in the optical or ultraviolet. We have explored the use of GALEX photometry as an activity indicator, using as a calibration sample stars within 50 pc, representing the field, and in selected nearby associations, representing the youngest stages of stellar evolution. We present preliminary relations between UV flux and the optical activity indicator R'_HK and between UV flux and age. We demonstrate that far-UV (FUV, 1350-1780{\AA}) excess flux is roughly proportional to R'_HK. We also detect a correlation between near-UV (NUV, 1780-2830{\AA}) flux and activity or age, but the effect is much more subtle, particularly for stars older than than ~0.5-1 Gyr. Both the FUV and NUV relations show large scatter, ~0.2 mag when predicting UV flux, ~0.18 dex when predicting R'_HK, and ~0.4 dex when predicting age. This scatter appears to be evenly split between observational errors in current state-of-the-art data and long-term activity variability in the sample stars.