Stellar Obliquity of the Ultra-Short-Period Planet System HD 93963

TL;DR

This study measures the sky-projected obliquity of the transiting mini-Neptune HD 93963 Ac using the Rossiter-McLaughlin effect, revealing a near-aligned orbit and contributing to understanding of ultra-short-period planetary system architectures.

Contribution

First obliquity measurement of HD 93963 Ac, adding to the small sample of systems with ultra-short-period planets and known stellar obliquities, and providing insights into system alignment.

Findings

HD 93963 Ac has a sky-projected obliquity of 14^{+17}_{-19} degrees.

The inner planets are likely well aligned with the stellar spin.

HD 93963 system is similar to other coplanar ultra-short-period systems.

Abstract

We report an observation of the Rossiter-McLaughlin (RM) effect of the transiting planet HD 93963 Ac, a mini-Neptune planet orbiting a G0-type star with an orbital period of $P_{\rm{c}} = 3.65\,\mathrm{d}$, accompanied by an inner super-Earth planet with $P_{\rm{b}} = 1.04\,\mathrm{d}$. We observed a full transit of planet c on 2024 May 3rd UT with Keck/KPF. The observed RM effect has an amplitude of $\sim 1\,\mathrm{m\,s}^{-1}$ and implies a sky-projected obliquity of $\lambda = 14^{+17}_{-19}$ degrees for HD 93963 Ac. Our dynamical analysis suggests that the two inner planets are likely well aligned with the stellar spin, to within a few degrees, thus allowing both to transit. Along with WASP-47, 55 Cnc, and HD 3167, HD 93963 is the fourth planetary system with an ultra-short-period planet and obliquity measurement(s) of any planet(s) in the system. HD 93963, WASP-47, and 55 Cnc favor largely coplanar orbital architectures, whereas HD 3167 has been reported to have a large mutual inclination ($\sim$100$^\circ$) between its transiting planets b and c. In this configuration, the probability that both planets transit is low. Moreover, one planet would quickly evolve to be non-transiting due to nodal precession. Future missions such as ESO/PLATO should detect the resulting transit duration variations. We encourage additional obliquity measurements of the HD 3167 system to better constrain its orbital architecture.