Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars

• Published in: Nature (London) Vol. 529; no. 7585; pp. 181 - 184
• Main Authors: van Saders, Jennifer L., Ceillier, Tugdual, Metcalfe, Travis S., Aguirre, Victor Silva, Pinsonneault, Marc H., García, Rafael A., Mathur, Savita, Davies, Guy R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 14-01-2016; Nature Publishing Group
• Subjects: 639/33/34/861; 639/33/34/867; Astrophysics; Drugs; Humanities and Social Sciences; letter; Magnetic brakes; Magnetism; multidisciplinary; Prescribing; Science; Stars; Stars & galaxies
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature16168

The age of a young to middle-aged star can be determined from how quickly or slowly it rotates, but the relationship breaks down for old stars; models now show that old stars are rotating much more quickly than expected, perhaps because magnetic winds are weaker and therefore brake the rotation less effectively.
Older stars keep on spinning
Stellar ages are notoriously difficult to infer but are central to many phenomena in astrophysics. One system that has been used to establish stellar age is 'gyrochronology', based on the fact that mass and angular momentum loss cause a slowdown in surface rotation as stars age. This empirical chronometer was recently calibrated, and its use has uncovered apparent anomalous slow rotation in older stars. Here Jennifer van Saders
et al
. report stellar evolutionary modelling which confirms the presence of unexpectedly rapid rotation in stars more evolved than the Sun. They demonstrate that dramatically weakened magnetic braking for older stars reproduces both the asteroseismic and cluster data where existing relations cannot. The authors conclude that gyrochronology may be a reliable guide for stellar ages of up to 2.5 billion years, but that older stars do not appear to spin down predictably.
A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation
1
. The technique of ‘gyrochronology’ uses the rotation period of a star to calculate its age
2
,
3
. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age
4
(NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology
5
,
6
. The data for the cluster agree with previous period–age relations
4
, but these relations fail to describe the asteroseismic sample
7
. Here we report stellar evolutionary modelling
5
,
6
,
8
,
9
,
10
, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can—unlike existing models—reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.