TESS asteroseismology of $\beta$ Hydri: a subgiant with a born-again dynamo

TL;DR

This study uses TESS data to analyze the subgiant star β Hydri, revealing its rotation period and magnetic activity cycle, and supports the idea of a rejuvenated dynamo mechanism in evolved stars.

Contribution

It provides the first direct measurement of β Hyi's rotation period and demonstrates that weakened magnetic braking can explain its magnetic activity cycle.

Findings

Rotation period consistent with weakened magnetic braking

Evidence of a 'born-again' dynamo in a subgiant star

Stellar structure changes can reactivate magnetic cycles

Abstract

The solar-type subgiant $\beta$ Hyi has long been studied as an old analog of the Sun. Although the rotation period has never been measured directly, it was estimated to be near 27 days. As a southern hemisphere target it was not monitored by long-term stellar activity surveys, but archival International Ultraviolet Explorer data revealed a 12 year activity cycle. Previous ground-based asteroseismology suggested that the star is slightly more massive and substantially larger and older than the Sun, so the similarity of both the rotation rate and the activity cycle period to solar values is perplexing. We use two months of precise time-series photometry from the Transiting Exoplanet Survey Satellite (TESS) to detect solar-like oscillations in $\beta$ Hyi and determine the fundamental stellar properties from asteroseismic modeling. We also obtain a direct measurement of the rotation period, which was previously estimated from an ultraviolet activity-rotation relation. We then use rotational evolution modeling to predict the rotation period expected from either standard spin-down or weakened magnetic braking (WMB). We conclude that the rotation period of $\beta$ Hyi is consistent with WMB, and that changes in stellar structure on the subgiant branch can reinvigorate the large-scale dynamo and briefly sustain magnetic activity cycles. Our results support the existence of a "born-again" dynamo in evolved subgiants -- previously suggested to explain the cycle in 94 Aqr Aa -- which can best be understood within the WMB scenario.