Abstract
ABSTRACT Dippers are a common class of young variable star exhibiting day-long dimmings with depths of up to several tens of per cent. A standard explanation is that dippers host nearly edge-on (id ≈ 70°) protoplanetary discs that allow close-in (<1 au) dust lifted slightly out of the mid-plane to partially occult the star. The identification of a face-on dipper disc and growing evidence of inner disc misalignments brings this scenario into question. Thus, we uniformly (re)derive the inclinations of 24 dipper discs resolved with (sub-)mm interferometry from ALMA. We find that dipper disc inclinations are consistent with an isotropic distribution over id ≈ 0−75°, above which the occurrence rate declines (likely an observational selection effect due to optically thick disc mid-planes blocking their host stars). These findings indicate that the dipper phenomenon is unrelated to the outer (>10 au) disc resolved by ALMA and that inner disc misalignments may be common during the protoplanetary phase. More than one mechanism may contribute to the dipper phenomenon, including accretion-driven warps and ‘broken’ discs caused by inclined (sub-)stellar or planetary companions.
Highlights
Photometric variability is a hallmark of young ( 10 Myr) stars and studies of this variability provide insight into the physical processes underpinning early stellar evolution and planet formation
Our sample consists of all known dippers in the ρ Ophiuchi (ρ Oph) and Upper Scorpius (Upper Sco) star-forming regions that have been identified by their K2 Campaign 2 (K2/C2) light curves and have discs resolved by Atacama Large sub-millimeter/Millimeter Array (ALMA)
Dippers are a common class of young variable star often assumed to host protoplanetary discs viewed nearly-edge on, such that dusty structures lifted slightly out of the mid-plane partially occult the star as the disc rotates, producing the characteristic dimming events seen in their optical light curves
Summary
C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. In our previous work (Ansdell et al 2016a), we used archival ALMA data of three dippers to show that their outer discs ranged from face-on to edge-on. This hinted towards significantly misaligned inner disc components and/or the need for other dipper mechanisms. A notable example is the dipper J1604, which hosts a face-on transition disc resolved by ALMA (Ansdell et al 2016a) and variable shadows seen by VLT/SPHERE (Pinilla et al 2018a), suggesting a highly misaligned (∼70–90◦) and dynamic inner disc component. We uniformly (re)analyse resolved (sub-)mm ALMA data for two dozen dipper discs in an effort to robustly infer the distribution of their outer disc inclinations.
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