Complex modulation of rapidly rotating young M dwarfs: adding pieces to the puzzle

Maximilian N. Günther, David A. Berardo, Elsa Ducrot, Catriona A. Murray, Keivan G. Stassun, Katalin Olah, L. G. Bouma, Saul Rappaport, Joshua N. Winn, Adina D. Feinstein, Elisabeth C. Matthews, Daniel Sebastian, Benjamin V. Rackham, Bálint Seli, Amaury H. M. J. Triaud, Edward Gillen, Alan M. Levine, Brice-Olivier Demory, Michaël Gillon, Didier QuelozGeorge Ricker, Roland K. Vanderspek, Sara Seager, David W. Latham, Jon M. Jenkins, C. E. Brasseur, Knicole D. Colón, Tansu Daylan, Laetitia Delrez, Michael Fausnaugh, Lionel J. Garcia, Rahul Jayaraman, Emmanuel Jehin, Martti H. Kristiansen, J. M. Diederik Kruijssen, Peter Philmann Pedersen, Francisco J. Pozuelos, Joseph E. Rodriguez, Bill Wohler, Zhuchang Zhan

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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.
Original languageEnglish
Article number144
Number of pages18
JournalThe Astronomical Journal
Issue number4
Early online date2 Mar 2022
Publication statusPublished - Apr 2022


  • M dwarf stars
  • Periodic variable stars
  • Pre-main sequence stars
  • Starspots
  • Stellar activity
  • Stellar flares
  • Stellar rotation
  • T Tauri stars
  • Variable stars
  • astro-ph.EP
  • astro-ph.SR


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