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Upcoming Colloquium

Advancing Stellar Activity and Binary Evolution Science with Rapidly Rotating RGB Stars
Dr. Don Dixon, Vanderbilt University

April 21, 2025
11:00am in HSCI-105

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The rotational dependence of dynamo-powered stellar activity has been thoroughly investigated for main-sequence stars, but much less so for stars evolved onto the Red Giant Branch (RGB). In addition to being bigger and brighter, giant stars are generally observed to have negligible rotation rates compared to main-sequence stars and consequently tend to have comparatively weak dynamo action and global magnetic fields. This is generally expected due to angular momentum being carried away by stellar winds before ascending the RGB, but if a companion star orbits in close enough proximity, an RGB star can be spun up by tidal synchronization or mass accretion events. As a result, rotationally active giants are unique systems for furthering the understanding of evolved stellar activity and binary evolution processes that impart angular momentum for rotational spin-up. To characterize giant branch rotational activity, we derive an empirical rotation-activity relation using projected rotational velocities (vsini) and excess near-ultraviolet emission produced by magnetic heating of the chromosphere (Eζ(NUV)). We find our relation is similar in structure to those fitted to main-sequence stars, which may suggest a similar dynamo mechanism. Additionally, we find that the most active giants have near-ultraviolet emission that exceeds expectations given their rotation rates and that they are all in synchronized binaries with orbital periods < 20 days. Lastly, we find tentative evidence that giants approaching breakup rotational velocities are likely formed from planetary engulfment or stellar merger processes, both of which are still not well understood.

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