The HAT-P-13 Exoplanetary System: Evidence for Spin-Orbit Alignment and a Third Companion

TL;DR

This study presents new measurements of the HAT-P-13 system, showing that the planet's orbit is well-aligned with the star's spin, and provides evidence for a third companion, refining our understanding of its dynamics.

Contribution

It provides the first evidence of spin-orbit alignment in HAT-P-13 and suggests the presence of an additional third body influencing the system's dynamics.

Findings

Planet b's orbit is well-aligned with stellar spin (lambda ≈ 0°).

Refined orbit of planet b is slightly eccentric (e ≈ 0.014).

Evidence for a third companion from radial velocity trend.

Abstract

We present new radial-velocity measurements of HAT-P-13, a star with two previously known companions: a transiting giant planet "b" with an orbital period of 3 days, and a more massive object "c" on a 1.2 yr, highly eccentric orbit. For this system, dynamical considerations would lead to constraints on planet b's interior structure, if it could be shown that the orbits are coplanar and apsidally locked. By modeling the Rossiter-McLaughlin effect, we show that planet b's orbital angular momentum vector and the stellar spin vector are well-aligned on the sky (lambda = -0.9 +/- 8.5 deg), suggesting that the planetary orbits are also well-aligned. The refined orbital solution favors a slightly eccentric orbit for planet b (e = 0.0142_{-0.0044}^{+0.0052}), although it is not clear whether it is apsidally locked with c's orbit (\Delta\omega = 48_{-38}^{+25} deg). We find a long-term trend in the star's radial velocity and interpret it as evidence for an additional body "d", which may be another planet or a low-mass star. The next inferior conjunction of c, when a transit may happen, is expected on JD 2,455,315.2 +/- 1.9 (centered on UT 17h 2010 April 28).