A search for radius inflation among active M-dwarfs in Praesepe. (arXiv:1811.11232v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Jackson_R/0/1/0/all/0/1">R.J. Jackson</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Jeffries_R/0/1/0/all/0/1">R.D. Jeffries</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Deliyannis_C/0/1/0/all/0/1">Constantine P. Deliyannis</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Sun_Q/0/1/0/all/0/1">Qinghui Sun</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Douglas_S/0/1/0/all/0/1">Stephanie T. Douglas</a> (3) ((1) Astrophysics Group, Keele University, (2) Department of Astronomy, Indiana University, (3) Harvard-Smithsonian Center for Astrophysics)
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 $6pm4$ 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.
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 $6pm4$ 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.
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