Star–disc (mis-)alignment in Rho Oph and Upper Sco: insights from spatially resolved disc systems with K2 rotation periods

Abstract

The discovery of close in, giant planets (hot Jupiters) with orbital angular momentum vectors misaligned with respect to the rotation axis of their host stars presents problems for planet formation theories in which planets form in discs with angular momentum vectors aligned with that of the star. Violent, high eccentricity migration mechanisms purported to elevate planetary orbits above the natal disc plane predict populations of proto-hot Jupiters which have not been observed with Kepler. Alternative theories invoking primordial star-disc misalignments have recently received more attention. Here, the relative alignment between stars and their protoplanetary discs is assessed for the first time for a sample of 20 pre-main-sequence stars. Recently published rotation periods derived from high quality, long duration, high cadence K2 light curves for members of the ρ Ophiuchus and Upper Scorpius star forming regions are matched with high angular resolution observations of spatially resolved discs and projected rotational velocities to determine stellar rotation axis inclination angles which are then compared to the disc inclinations. Ten of the fifteen systems for which the stellar inclination could be estimated are consistent with star-disc alignment while five systems indicate potential misalignments between the star and its disc. The potential for chance misalignment of aligned systems due to projection effects and characteristic measurement uncertainties is also investigated. While the observed frequency of apparent star-disc misalignments could be reproduced by a simulated test population in which 100% of systems are truly aligned, the distribution of the scale of inferred misalignment angles could not.

Stellar obliquities in exoplanetary systems

“…These internally misaligned or "broken" disks might form planets with large mutual inclinations, setting the starting conditions for some of the obliquityexcitation processes discussed in the next section. The obliquities of stars with resolved debris disks have been investigated, usually using the projected rotation velocity method, and have generally been found to be lower than ≈30 • (Watson et al 2011;Greaves et al 2014;Davies 2019).…”

Are inner disc misalignments common? ALMA reveals an isotropic outer disc inclination distribution for young dipper stars

“…Velocity perturbations in HCO + emission have been seen for the dipper AA Tau (Loomis et al 2017), and both kinematic signatures in the gas (Mayama et al 2018) and variable shadows in scattered light (Takami et al 2014;Pinilla et al 2018b) have been observed for EPIC 204638512 (J1604). For the latter, Davies (2019) also derive a stellar inclination that is highly misaligned with the outer disc.…”

The distribution of mutual inclinations arising from the stellar quadrupole moment

“…In concert, observational characterization of the spin vectors of planet-hosting stars indicate that substantial values of stellar obliquity, the angle between the stellar spin pole and the planet orbit, are widespread (Winn et al 2010;Albrecht et al 2012;Winn & Fabrycky 2015;Winn et al 2017). Their origins remain uncertain, but viable pathways exist toward misalignments arising within the earliest 10-100 Myr in many cases (Batygin 2012;Spalding & Batygin 2014;Davies 2019). Thus, a significant fraction of planetary systems likely experienced an early epoch during which they felt the gravitational influence of a rapidly-rotating, tilted star.…”