Investigating coronal saturation and supersaturation in fast-rotating M-dwarf stars
Abstract
At fast rotation rates, the coronal activity of G- and K-type stars has been observed to 'saturate' and then decline again at even faster rotation rates - a phenomenon dubbed 'supersaturation'. In this paper, we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547 and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit increasing coronal activity with decreasing Rossby number NR, the ratio of period to convective turnover time, and that activity saturates at LX/Lbol≃ 10-3 for log NR < -0.8. However, supersaturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with log NR < -1.8, where supersaturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of supersaturation; P ≤ 0.3 d for K-dwarfs and perhaps P ≤ 0.2 d for M-dwarfs. These observations favour the 'centrifugal stripping' model for supersaturation, where coronal structures are forced open or become radiatively unstable as the Keplerian corotation radius moves inside the X-ray-emitting coronal volume.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2011
- DOI:
- 10.1111/j.1365-2966.2010.17848.x
- arXiv:
- arXiv:1010.2152
- Bibcode:
- 2011MNRAS.411.2099J
- Keywords:
-
- stars: activity;
- stars: coronae;
- stars: rotation;
- open clusters and associations: individual: NGC 2547;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for Monthly Notices of the Royal Astronomical Society. 15 pages, 7 figures (electronic tables from the author)