Stellar Evidence that the Sun May be in Transition

Friday, July 1, 2016

The study of Sun-like stars helps us to put the Sun in context. HAO Newkirk Fellow Ricky Egeland has co-authored a new publication with Space Science Institute scientist Travis Metcalfe and Carnegie Fellow Jennifer van Saders. The research proposes that the Sun may be undergoing a fundamental shift that occurs in all middle-aged stars (Metcalfe et al. 2016, ApJ Letters, accepted). This work (link to article) was recently featured in a popular science article on the Forbes magazine web site.

chromospheric activity and rotation in field dwarfs and subgiants image
Figure1: Relationship between chromospheric activity and rotation in field dwarfs and subgiants. Younger, high activity stars are in the top portion of the diagram, while older, low activity stars are towards thebottom. The Sun falls into the Ro~2 band where magnetic braking is drastically reduced, indicated by the cusps in the colored model curves.

The story begins with the recent observational evidence by the Kepler spacecraft (van Saders et al. 2016, Nature) that finds that older main-sequence stars are rotating faster than expected according to the well known Skumanich spin-down law, made famous by retired HAO scientist Andy Skumanich. The observations indicate that magnetic braking operates at a dramatically reduced efficiency beyond a critical Rossby number (Ro~2), the ratio of rotation period to the convective time scale. The new paper by Metcalfe, Egeland, and van Saders identifies the counterpart of this transition in chromospheric activity measurements and develops a new evolutionary scenario that explains the cessation of magnetic braking. The authors discuss the implications for understanding the solar cycle in the stellar context. Previous searches for stellar magnetic activity cycles have not found cycles in G-type stars (like the Sun) that are rotating near the solar period of 26 days or slower—those stars are all in a flat activity state. The new paper suggests that this transition in dynamo behavior could be related to the cessation of magnetic braking, whereby the magnetic field on larger spatial scales responsible for braking are suppressed. The Sun's Maunder Minimum, a prolonged period of low activity from 1645–1715, may be a manifestation of a gradual transition into a permanent flat-activity state, as are the older Sun-like stars.

Arxiv preprint link: http://arxiv.org/abs/1606.01926