The Young Planet DS Tuc Ab Has a Low Obliquity*

Abstract

The abundance of short-period planetary systems with high orbital obliquities relative to the spin of their host stars is often taken as evidence that scattering processes play important roles in the formation and evolution of these systems. More recent studies have suggested that wide binary companions can tilt protoplanetary disks, inducing a high stellar obliquity that form through smooth processes like disk migration. DS Tuc Ab, a transiting planet with an 8.138 day period in the 40 Myr Tucana-Horologium association, likely orbits in the same plane as its now-dissipated protoplanetary disk, enabling us to test these theories of disk physics. Here, we report on Rossiter-McLaughlin observations of one transit of DS Tuc Ab with the Planet Finder Spectrograph on the Magellan Clay Telescope at Las Campanas Observatory. We confirm the previously detected planet by modeling the planet transit and stellar activity signals simultaneously. We test multiple models to describe the stellar activity-induced radial velocity variations over the night of the transit, finding the obliquity to be low: λ = 12 ± 13 degrees, suggesting that this planet likely formed through smooth disk processes and its protoplanetary disk was not significantly torqued by DS Tuc B. The specific stellar activity model chosen affects the results at the ≈ 5 degree level. This is the youngest planet to be observed using this technique; we provide a discussion on best practices to accurately measure the observed signal of similar young planets.

TOI-2076 and TOI-1807: Two Young, Comoving Planetary Systems within 50 pc Identified by TESS that are Ideal Candidates for Further Follow Up

“…In this sense TOI-1807b is the most promising target for mass follow up, as its ultra-short orbital period suggests that the Keplerian signal will be separable from activity at the rotation period of the star. As demonstrated by recent Rossiter-McLaughlin (RM) effect measurements on very young and active stars, it is feasible to measure short-duration RV signals on timescales of hours even in the presence of high-amplitude stellar variability on longer timescales of days (Montet et al 2020;Zhou et al 2020). RV follow up of TOI-2076b/c/d will be more challenging because the planetary orbital periods are comparable to the stellar rotation, and the complexity of the multiplanet RV signal requires a larger number of observations.…”

TESS Hunt for Young and Maturing Exoplanets (THYME). III. A Two-planet System in the 400 Myr Ursa Major Group

“…HD 63433b is the second young small planet with a published measurement of its spin-orbit alignment, after DS Tucb (Montet et al 2020;Zhou et al 2020), and the first in a multiplanet system. Both DS Tucb and HD 63433 b show prograde orbits.…”

“…In particular, measurement of spin-orbit misalignments via the Rossiter-McLaughlin (RM) effect are important for young and multiplanet systems to inform our understanding of their dynamical histories. TESS has already enabled the discovery of young planets around bright stars (Newton et al 2019), facilitating spin-orbit alignment measurements (Montet et al 2020;Zhou et al 2020).…”

Two Young Planetary Systems around Field Stars with Ages between 20 and 320 Myr from TESS

“…R. Newton et al 2020, in preparation;Mann et al 2020;Plavchan et al 2020;Rizzuto et al 2020). These are the first planets around bright young stars that are suitable for in-depth characterizations, enabling the first obliquity measurements of newly formed planets (Montet et al 2020;Zhou et al 2020), as well as atmospheric studies in the near future.…”