Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Activity, Binarity and Sub-subgiants
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| Format: | Preprint |
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2025
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| _version_ | 1866917980538929152 |
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| author | Dixon, Don Stassun, Keivan G. Mathieu, Robert D. Tayar, Jamie Cao, Lyra |
| author_facet | Dixon, Don Stassun, Keivan G. Mathieu, Robert D. Tayar, Jamie Cao, Lyra |
| contents | Red Giant Branch (RGB) stars are overwhelmingly observed to rotate very slowly compared to main-sequence stars, but a few percent of them show rapid rotation and high activity, often as a result of tidal synchronizationn or other angular momentum transfer events. In this paper we build upon previous work using a sample of 7,286 RGB stars from APOGEE DR17 with measurable rotation. We derive an updated NUV excess vs $v \sin{i}$ rotation-activity relation that is consistent with our previous published version, but reduces uncertainty through the inclusion of a linear [M/H] correction term. We find that both single stars and binary stars generally follow our rotation-activity relation, but single stars seemingly saturate at $P_{\text{rot}}$/$\sin{i}$ $\sim$10 days while binary stars show no sign of saturation, suggesting they are able to carry substantially stronger magnetic fields. Our analysis reveals Sub-subgiant stars (SSGs) to be the most active giant binaries, with rotation synchronized to orbits with periods $\lesssim$ 20 days. Given their unusually high level of activity compared to other short-period synchronized giants we suspect the SSGs are most commonly overactive RS CVn stars. Using estimates of critical rotation we identify a handful giants rotating near break-up and determine tidal spin up to this level of rotation is highly unlikely and instead suggest planetary engulfment or stellar mergers in a fashion generally proposed for FK Comae stars. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2504_05561 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Activity, Binarity and Sub-subgiants Dixon, Don Stassun, Keivan G. Mathieu, Robert D. Tayar, Jamie Cao, Lyra Solar and Stellar Astrophysics Astrophysics of Galaxies Red Giant Branch (RGB) stars are overwhelmingly observed to rotate very slowly compared to main-sequence stars, but a few percent of them show rapid rotation and high activity, often as a result of tidal synchronizationn or other angular momentum transfer events. In this paper we build upon previous work using a sample of 7,286 RGB stars from APOGEE DR17 with measurable rotation. We derive an updated NUV excess vs $v \sin{i}$ rotation-activity relation that is consistent with our previous published version, but reduces uncertainty through the inclusion of a linear [M/H] correction term. We find that both single stars and binary stars generally follow our rotation-activity relation, but single stars seemingly saturate at $P_{\text{rot}}$/$\sin{i}$ $\sim$10 days while binary stars show no sign of saturation, suggesting they are able to carry substantially stronger magnetic fields. Our analysis reveals Sub-subgiant stars (SSGs) to be the most active giant binaries, with rotation synchronized to orbits with periods $\lesssim$ 20 days. Given their unusually high level of activity compared to other short-period synchronized giants we suspect the SSGs are most commonly overactive RS CVn stars. Using estimates of critical rotation we identify a handful giants rotating near break-up and determine tidal spin up to this level of rotation is highly unlikely and instead suggest planetary engulfment or stellar mergers in a fashion generally proposed for FK Comae stars. |
| title | Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Activity, Binarity and Sub-subgiants |
| topic | Solar and Stellar Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2504.05561 |