Complex Modulation of Rapidly Rotating Young M Dwarfs: Adding Pieces to the Puzzle

Kavli Affiliate: George Ricker

| First 5 Authors: Maximilian N. G√ľnther, David A. Berardo, Elsa Ducrot, Catriona A. Murray, Keivan G. Stassun

| Summary:

New sets of young M dwarfs with complex, sharp-peaked, and strictly periodic
photometric modulations have recently been discovered with Kepler/K2 (scallop
shells) and TESS (complex rotators). All are part of star-forming associations,
are distinct from other variable stars, and likely belong to a unified class.
Suggested hypotheses include star spots, accreting dust disks, co-rotating
clouds of material, magnetically constrained material, spots and misaligned
disks, and pulsations. Here, we provide a comprehensive overview and add new
observational constraints with TESS and SPECULOOS Southern Observatory (SSO)
photometry. We scrutinize all hypotheses from three new angles: (1) we
investigate each scenario’s occurrence rates via young star catalogs; (2) we
study the features’ longevity using over one year of combined data; and (3) we
probe the expected color dependency with multi-color photometry. In this
process, we also revisit the stellar parameters accounting for activity
effects, study stellar flares as activity indicators over year-long time
scales, and develop toy models to simulate typical morphologies. We rule out
most hypotheses, and only (i) co-rotating material clouds and (ii) spots and
misaligned disks remain feasible – with caveats. For (i), co-rotating dust
might not be stable enough, while co-rotating gas alone likely cannot cause
percentage-scale features; and (ii) would require misaligned disks around most
young M dwarfs. We thus suggest a unified hypothesis, a superposition of
large-amplitude spot modulations and sharp transits of co-rotating gas clouds.
While the complex rotators’ mystery remains, these new observations add
valuable pieces to the puzzle going forward.

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