Orbiting Clouds of Material at the Keplerian Co-rotation Radius of Rapidly Rotating Low-mass WTTs in Upper Sco

Abstract

Using K2 data, we identified 23 very-low-mass members of the ρ Oph and Upper Scorpius star-forming region as having periodic photometric variability not easily explained by well-established physical mechanisms such as star spots, eclipsing binaries, or pulsation. All of these unusual stars are mid-to-late M dwarfs without evidence of active accretion, and with photometric periods generally <1 day. Often the unusual light-curve signature takes the form of narrow flux dips; when we also have rotation periods from star spots, the two periods agree, suggesting that the flux dips are due to material orbiting the star at the Keplerian co-rotation radius. We sometimes see "statechanges" in the phased light-curve morphologies where ∼25% of the waveform changes shape on timescales less than a day; often, the "state-change" takes place immediately after a strong flare. For the group of stars with these sudden light-curve morphology shifts, we attribute their flux dips as most probably arising from eclipses of warm coronal gas clouds, analagous to the slingshot prominences postulated to explain transient Hα absorption features in AB Doradus and other rapidly rotating late-type stars. For another group of stars with somewhat longer periods, we find the short-duration flux dips to be highly variable on both short and long timescales, with generally asymmetric flux-dip profiles. We believe that these flux dips are due to particulate clouds possibly associated with a close-in planet or resulting from a recent collisional event.

More Rapidly Rotating PMS M Dwarfs with Light Curves Suggestive of Orbiting Clouds of Material

“…We had reached a similar conclusion previously for the M dwarfs observed with K2 in the Pleiades (Stauffer et al 2017). The conclusion that about half of the stars with scallop-shell light curves are in binaries is likely just a product of two correlations: (a) the fact that the physical mechanism producing the scallop-shell light curves is likely linked to very rapid rotation; and (b) the fact that components of binaries tend to be the most rapidly rotating M dwarfs in Upper Sco.…”

“…Models to explain the scallop-shell light curves probably require there to be a torus of gas and dust at the Keplerian co-rotation radius and that our line-of-sight pass through that torus (Townsend & Owocki 2005;D'Angelo et al 2017;Farihi et al 2017;Stauffer et al 2017). Therefore, the percentage of stars with scallop-shell light curve morphologies is a lower limit to the fraction of stars having these tori; for plausible torus scale heights, the majority of the rapidly rotating young dM stars may be so affected.…”

“…Inevitably, the quality and quantity of those light curves has also resulted in identification of stars whose photometric variability is not easily associated with any of the previously known physical mechanisms and which offer the possibility for exploring new physics. In Stauffer et al (2017; hereafter S17), we identified 23 stars observed by K2 in the ∼8 Myr old Upper Scorpius association as having photometric variability not easily attributable to the mechanisms usually associated with low mass, pre-main sequence (PMS) stars. All of these stars are mid to late M dwarfs with no obvious IR excess nor other indication of ongoing accretion; most are very rapid rotators with periods often less than 1 day.…”

M Dwarf Rotation from the K2 Young Clusters to the Field. I. A Mass–Rotation Correlation at 10 Myr

“…Notably, these V K S -values are an assortment of real V-band measurements compared to 2MASS K S , and transformations from other color bands (mainly from SDSS g-K S and r−K S ). A detailed description of the Upper Sco data will be presented in a forthcoming paper (L. Rebull et al 2017, in preparation), but the analysis is quite similar to that of the K2 Pleiades rotation rates (see Stauffer et al 2017). These data are plotted in Figure 1, with the reddening corrections described above.…”

Rotation of Late-type Stars in Praesepe with K2

“…All six stars were mid-M dwarfs with very short ( < P 0.7 days) periods. We identified another 19 rapidly rotating, mid-to-late-M dwarfs in the 8 Myr old Upper Sco association whose K2 phased LCs seemed to show more structure than could be explained by spots (Stauffer et al 2017). We have attributed the LC structure to warm clouds of coronal gas orbiting the stars near their Keplerian co-rotation radius, following models by Jardine & van Ballegooijen (2005) and Townsend & Owocki (2005).…”