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Published July 2011 | Published
Journal Article Open

Stellar Activity in the Broadband Ultraviolet


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 stars within 50 pc as a calibration sample 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 Å) excess flux is roughly proportional to R'_(HK). We also detect a correlation between near-UV (NUV, 1780-2830 Å) flux and activity or age, but the effect is much more subtle, particularly for stars older 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.

Additional Information

© 2011 The American Astronomical Society. Received 2011 January 4; accepted 2011 May 3; published 2011 June 9. We thank the referee for many insightful comments and suggestions, Ted Wyder and Patrick Morrissey for addressing our questions about the GALEX data, and Chad Schafer and John Carpenter for their advice on the statistical analysis. This research was supported in part by NASA grant NNX08AH95G to L.H. Facilities: GALEX, HIPPARCOS, KPNO:CFT

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