Rotation, activity and lithium in NGC 6475

James, David J.; Jeffries, Robin D.

Abstract

Radial and rotational velocities, chromospheric activity and lithium abundances are presented for an X-ray-selected sample of stars in the young (220 Myr) open cluster NGC 6475. Low-mass members of the cluster have been identified on the basis of photometric and spectroscopic criteria. The observations show that the rapid spin-down seen amongst solar-type stars in the Pleiades is incomplete at 220 Myr, as there are F, G and K stars in NGC 6475 with υ sin i > 10 km s−1. Peak rotation rates for G stars are 12–14 km s−1 and are robust to uncertain inclination angles, because the magnetic activity of these stars is less than the saturation value observed for fast rotators (υ sin i > 15 km s−1) in the Pleiades. Two mid-K stars are found with υ sin i ∼25 km s−1 and with saturated magnetic activity levels, indicating that spin-down time-scales are mass dependent, increasing from ∼ 20 Myr for early-G stars to > 75 Myr for mid-K stars. We have compared our rotation data, for the most rapidly rotating G and K stars in NGC 6475 and in young open clusters, with published rotational evolution models incorporating magnetic dynamo saturation at high rotation rates ( > Ωsat). Models with interior angular momentum transport are unable to simultaneously fit the rapidly rotating stars of the α Per, Pleiades and NGC 6475 clusters. Likely solutions are a plausible increase in the age of the Pleiades to 100 Myr, the incorporation of centrifugal wind driving in the angular momentum loss treatment, or ad hoc differences in initial conditions. If an increase in the age of the Pleiades is assumed, we find that a mass dependent Ωsat is required, such that dynamo saturation occurs for G and K stars at a Rossby number of ∼ 0.4. This value is incompatible with the faster saturation thresholds inferred from coronal X-rays, but Ωsat can be increased if solid body rotation is enforced or centrifugal wind driving is included. Comparisons with such models are favourable for G stars but less so for K stars. This may result from neglecting the evolving moment of inertia at early ages.

Citation

James, D. J., & Jeffries, R. D. (1997). Rotation, activity and lithium in NGC 6475. Monthly Notices of the Royal Astronomical Society, 292(2), 252–272. https://doi.org/10.1093/mnras/292.2.252

• Journal Article Type: Article
• Acceptance Date: Jun 27, 1997
• Publication Date: 1997-12
• Deposit Date: Feb 15, 2024
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 292
• Issue: 2
• Pages: 252–272
• DOI: https://doi.org/10.1093/mnras/292.2.252
• Publisher URL: https://academic.oup.com/mnras/article/292/2/252/1044931