Dixon, Don; Stassun, Keivan G.; Mathieu, Robert D.; Tayar, Jamie; Cao, Lyra. “Rotationally driven ultraviolet emission of red giant stars. II. Metallicity, activity, binarity, and subsubgiants.” Astronomical Journal 169 (2025): 309. https://doi.org/10.3847/1538-3881/adc92a.
Most red giant branch (RGB) stars rotate very slowly compared to younger, main-sequence stars. However, a small percentage of red giants spin quickly and show high levels of magnetic activity. This is often due to processes like tidal synchronization in binary systems or other events that transfer angular momentum.
This study analyzes 7,286 red giants from the APOGEE DR17 survey, all with detectable rotation, to better understand the connection between a star’s rotation speed and its activity level. By adding a correction based on a star’s metallicity (measured as [M/H]), the updated relation between ultraviolet (NUV) excess and rotation activity improves on earlier models by reducing uncertainty.
The results show that both single stars and binary stars generally follow the same rotation–activity trend. However, there is a key difference: single stars appear to reach a saturation point in activity when their rotation periods drop to about 10 days. In contrast, binary stars do not show this saturation, suggesting they can generate much stronger magnetic fields.
The study also highlights a type of binary star known as subsubgiant stars (SSGs) as particularly active. These stars tend to have their rotation locked to very short orbital periods (less than ~20 days), and their activity levels suggest they may be a type of star known as RS CVn stars, known for being unusually active.
Finally, the analysis identifies a few red giants rotating near their critical speed—so fast that they are close to breaking apart. This extreme rotation is unlikely to be caused by tidal forces in binary systems. Instead, it is likely the result of more dramatic events like planetary engulfment or stellar mergers, similar to the processes thought to form FK Comae stars.
Figure 1. Kiel diagram of queried giants from APOGEE DR17. Hexagonal histograms represent the ∼6.5 × 105 unique systems in the full catalog. The marginal distributions on the top and right of the figure depict the full sampling of Teff and log g for DR17. Our initial sample of log g < 3.5 dex and > 0 km s−1 is plotted as a 2D histogram with 100 bins per axis going from blue to yellow with increasing bin count. The gray background highlights the Teff range where we remove stars not consistent with the giant branch. Partial evolutionary tracks with equal mass spacing are given from 1 M⊙ to 2.5 M⊙.
