A search for radius inflation among active M-dwarfs in Praesepe

Jackson, Richard J; Jeffries, Robin D; Deliyannis, Constantine P; Sun, Qinghui; Douglas, Stephanie T

Abstract

Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5-m telescope, to determine projected radii for fast-rotating, low-mass ($0.15 \leq M/M_{\odot} \leq 0.6$) members of the Praesepe cluster. A maximum likelihood analysis that accounts for observational uncertainties, binarity and censored data, yields marginal evidence for radius inflation -- the average radius of these stars is $6\pm4$ per cent larger at a given luminosity than predicted by commonly-used evolutionary models. This over-radius is smaller (at 2-sigma confidence) than was found for similar stars in the younger Pleiades using a similar analysis; any decline appears due to changes occurring in higher mass ($>0.25 M_{\odot}$) stars. Models incorporating magnetic inhibition of convection predict an over-radius, but do not reproduce this mass dependence unless super-equipartition surface magnetic fields are present at lower masses. Models incorporating flux-blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in over-radius. The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. Our analyses assume random spin-axis orientations; any alignment in Praesepe, as suggested by Kovacs (2018), is strongly disfavoured by the broad distribution of projected radii.

Citation

Jackson, R. J., Jeffries, R. D., Deliyannis, C. P., Sun, Q., & Douglas, S. T. (2019). A search for radius inflation among active M-dwarfs in Praesepe. Monthly Notices of the Royal Astronomical Society, 483(1), 1125-1138. https://doi.org/10.1093/mnras/sty3184

• Journal Article Type: Article
• Acceptance Date: Nov 19, 2018
• Online Publication Date: Nov 28, 2018
• Publication Date: 2019-02
• Publicly Available Date: Apr 11, 2019
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 483
• Issue: 1
• Pages: 1125-1138
• DOI: https://doi.org/10.1093/mnras/sty3184
• Keywords: stars: evolution, stars: low-mass, stars: pre-main-sequence, starspots
• Publisher URL: http://dx.doi.org/10.1093/mnras/sty3184