A Transient Transit Signature Associated with the Young Star RIK-210

Abstract

We find transienttransit-like dimming events within the K2 time series photometry of the young star RIK-210 in the Upper Scorpius OB association. These dimming events are variable in depth, duration, and morphology. High spatial resolution imaging revealed thatthe star is singleand radial velocity monitoring indicated that the dimming events cannot be due to an eclipsing stellar or brown dwarf companion. Archival and follow-up photometry suggest the dimming events are transient in nature. The variable morphology of the dimming events suggests they are not due to a singlespherical body. The ingress of each dimming event is always shallower than egress, as one would expect for an orbiting body with a leading tail. The dimming events are periodic and synchronous with the stellar rotation. However, we argue it is unlikely the dimming events could be attributed to anything on the stellar surface based on the observed depths and durations. Variable obscuration by a protoplanetary disk is unlikely on the basis that the star is not actively accreting and lacks the infrared excess associated with an inner disk. Rather, we explore the possibilities that the dimming events are due to magnetospheric clouds, a transiting protoplanet surrounded by circumplanetary dust and debris, eccentric orbiting bodies undergoing periodic tidal disruption, or an extended field of dust or debris near the corotation radius.

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

“…As discussed in Sections 7, 8.2, and in David et al (2017), the transient short-duration flux dips found for four of our stars cannot be produced by cool spots on the stellar photosphere or by any other plausible phenomenon associated with the surface of the star. The equality of the period for the narrow flux dip and the sinusoidal waveform (which we assume is due to spots) therefore places the material responsible for the narrow flux dips at the Keplerian co-rotation radius.…”

“…These characteristics seem better matched to some type of dusty debris, orbiting a young planet located at or near the Keplerian co-rotation radius. A detailed examination of the best studied of this group (RIK-210) and a comparison of those data to a variety of physical models is provided in David et al (2017).…”

“…Table 5 provides our estimated stellar radii and compares the predicted maximum dip durations to the longest duration flux dips for each star. The uncertainties in the stellar radius estimates, and hence in the durations, from propagating uncertainties in the distance and age and in the stellar properties are likely 15%-20% (see, for example, similar calculations for individual planet-hosting stars by Mann et al 2016 andDavid et al 2017). Because radii two to three times as large as we estimate would be required for the dip durations to be consistent with observations for several of our stars (including three of four of the transient dip class), we conclude that radius inflation due to magnetic effects (e.g., MacDonald & Mullan 2017) cannot explain these discrepancies and instead that the transiting/occulting body must have size comparable to the star.…”

“…In David et al (2017), we provide a detailed examination of the K2 light curve for RIK-210 and an analysis of synoptic spectra for it, which we obtained with Keck HIRES. Radial velocities at 15 epochs exclude any EB scenario to explain the narrow flux dips.…”

“…What is not obvious is the mechanism for creation of the dusty cloud that would cause the transits for our stars. Several possibilities are explored in David et al (2017).…”