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

Abstract

New sets of young M dwarfs with complex, sharp-peaked, and strictly periodic photometric modulations have recently been discovered with Kepler/K2 and TESS data. All of these targets are part of young star-forming associations. Suggested explanations range from accretion of dust disks to corotating clouds of material to stellar spots getting periodically occulted by spin-orbit-misaligned dust disks. Here we provide a comprehensive overview of all aspects of these hypotheses, and add more observational constraints in an effort to understand these objects with photometry from TESS and the SPECULOOS Southern Observatory (SSO). We scrutinize the hypotheses from three different angles:(1) we investigate the occurrence rates of these scenarios through existing young star catalogs; (2) we study the longevity of these features using over one year of combined photometry from TESS and SSO; and (3) we probe the expected color dependency with multi-color photometry from SSO. 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 imitate typical morphologies. We identify which parts of the hypotheses hold true or are challenged by these new observations. So far, none of the hypotheses stand out as a definite answer, and each come with limitations. While the mystery of these complex rotators remains, we here add valuable observational pieces to the puzzle for all studies going forward.

Rotation Distributions around the Kraft Break with TESS and Kepler: The Influences of Age, Metallicity, and Binarity

“…Stars observed in the southern continuous viewing zone (SCVZ), however, have a combined observing baseline of ∼2 yr from the 13 southern hemisphere observing sectors and extended mission observations. Although the combined baseline of these observations should enable studies of rotation signals that span multiple TESS observing sectors, attempts at measuring rotation periods longer than the length of a single sector (27.4 days) have been unsuccessful with standard rotation period measurement methods (e.g., Günther et al 2020;Martins et al 2020), although there has been some success with machine-learning-based methods of extracting rotation signals (e.g., with neural networks; Lu et al 2020;Claytor et al 2022). These prior studies using traditional methods of measuring rotation periods from light curves have suggested that challenges in measuring rotation signals across multiple sectors in TESS are likely due to scattered light at the start and end of observing sectors and signals from the orbital motion of the spacecraft around Earth.…”

The Puzzling Story of Flare Inactive Ultra Fast Rotating M dwarfs. I. Exploring their Magnetic Fields

“…Zhan et al ( 2019) conclude the highly structured nature of the rotational modulation of UPM J0113-5939 is a result of a dusty ring surrounding the star. Currently the nature of young M dwarfs with complex, sharp peaked and periodic rotational modulation is unknown with several scenarios to explain the origins including: spots only; accreting dust disks; co-rotating clouds of material; magnetically constrained material; spots, and a spin-orbitmisaligned disk, and spots and pulsations (see Stauffer et al 2017;Günther et al 2020a;Koen 2021, for more details).…”

Magnitude-squared coherence: A powerful tool for disentangling Doppler planet discoveries from stellar activity

“…Spectral power and coherence at high-order harmonics may result from the star hosting multiple large spots or spot groups (e.g. Rodono et al 1986;Günther et al 2020;Perger et al 2021;Perugini et al 2021). In the next section, we will investigate coherent stellar signals that do not appear to be associated with the dominant rotation period, but which may be artifacts of differential rotation, giant cells, or supergranulation.…”