Supersaturation and activity-rotation relation in PMS stars: The young cluster h Persei

Abstract

Context. Several studies showed that the magnetic activity of late-type main-sequence (MS) stars is characterized by different regimes and that their activity levels are well described by the Rossby number, Ro, defined as the ratio between the rotational period Prot and the convective turnover time. Very young pre-main-sequence (PMS) stars show, similarly to MS stars, intense magnetic activity. However, they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. Aims. To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Persei, a ∼13 Myr old cluster, that contains both fast and slow rotators. The cluster members have ended their accretion phase and have developed a radiative core. It therefore offers us the opportunity of studying the activity level of intermediate-age PMS stars with different rotational velocities, excluding any interactions with the circumstellar environment. Methods. We constrained the magnetic activity levels of h Per members by measuring their X-ray emission from a Chandra observation, while rotational periods were obtained previously in the framework of the MONITOR project. By cross-correlating these data, we collected a final catalog of 414 h Per members with known rotational period, effective temperature, and mass. In 169 of these, X-ray emission has also been detected. Results. We found that h Per members with 1.0 M⊙< 1.4 M⊙ display different activity regimes: fast rotators clearly show supersaturation, while slower rotators have activity levels compatible to the non-saturated regime. At 13 Myr, h Per is therefore the youngest cluster showing activity-rotation regimes analogous to those of MS stars, indicating that at this age, magnetic field production is most likely regulated by the αΩ type dynamo. Moreover, we observed that supersaturation is better described by Prot than Ro, and that the observed patterns are compatible with the hypothesis of centrifugal stripping. In this scenario we inferred that coronae can produce structures as large as ∼2 R∗ above the stellar surface.