Star-spots and magnetism: testing the activity paradigm in the Pleiades and M67. Issue 2 (24th September 2022)

• Record Type: Journal Article
• Authors: Cao, Lyra; Pinsonneault, Marc H
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 517:Issue 2(2022)
• Page Start: 2165
• Page End: 2189
• Publication Date: 2022-09-24
• Subjects: stars: activity -- stars: fundamental parameters -- stars: late-type -- stars: magnetic field -- stars: rotation -- star-spots; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2706
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We measure star-spot filling fractions for 240 stars in the Pleiades and M67 open star clusters using APOGEE high-resolution H -band spectra. For this work, we developed a modified spectroscopic pipeline which solves for star-spot filling fraction and star-spot temperature contrast. We exclude binary stars, finding that the large majority of binaries in these clusters (80 per cent) can be identified from Gaia DR3 and APOGEE criteria – important for field star applications. Our data agree well with independent activity proxies, indicating that this technique recovers real star-spot signals. In the Pleiades, filling fractions saturate at a mean level of 0.248 ± 0.005 for active stars with a decline at slower rotation; we present fitting functions as a function of Rossby number. In M67, we recover low mean filling fractions of 0.030 ± 0.008 and 0.003 ± 0.002 for main sequence GK stars and evolved red giants, respectively, confirming that the technique does not produce spurious spot signals in inactive stars. Star-spots also modify the derived spectroscopic effective temperatures and convective overturn time-scales. Effective temperatures for active stars are offset from inactive ones by −109 ± 11 K, in agreement with the Pecaut & Mamajek empirical scale. Star-spot filling fractions at the level measured in active stars changes their inferred overturn time-scale, which biases the derived threshold for saturation. Finally, we identify a population of stars statistically discrepant from mean activity–Rossby relations and present evidence that these are genuine departures from a Rossby scaling. Our technique is applicable to the full APOGEE catalogue, with broad applications to stellar, galactic, and exoplanetary astrophysics.