TOI-1333Ab is on a well-aligned orbit. An aligned hot Jupiter around an F-type star with a mutually inclined stellar companion

Abstract

Spin-orbit obliquity measurements of hot-Jupiter systems constrain giant planet migration and tidal evolution. In binary systems, combining stellar obliquities with the orbit-orbit angle ($γ$) between the planetary and stellar companion orbits provides further insight into the dynamical influence of stellar companions. Here we aim to determine the projected obliquity ($λ$) of the hot Jupiter TOI-1333Ab ($P\approx4.72$ d, $M_{\rm p}\approx2.4$ M$_{\rm J}$) and place the system in the context of hot-Jupiter migration and tidal realignment in binary systems. We analysed spectroscopic observations obtained during planetary transit to model the Rossiter-McLaughlin effect and derive the projected obliquity. We combined this measurement with published system parameters and constraints on the wide stellar companion orbit to assess plausible migration scenarios. We measure a projected obliquity of $λ=-5 \pm 10^\circ$, showing that TOI-1333Ab is well aligned with the stellar spin axis of its F-type host star. The low obliquity and its modest eccentricity ($e=0.073^{+0.092}_{-0.052}$) are consistent with either disc-driven migration or high-eccentricity migration followed by efficient tidal circularisation and realignment. With an effective temperature of $6274\pm94$ K, the host star lies above the canonical Kraft break where the systems are frequently misaligned. Despite this, we find the system to be well aligned. In comparison with other planetary systems in binaries, TOI-1333 occupies a relatively isolated region in projected obliquity-orbit-orbit angle ($γ=81.5\pm1.1^\circ$) space, making it a valuable system for studying the interplay between migration, tides, and stellar companions.

Figures (5)

• Figure 1: TESS light curve of TOI-1333 folded on the transit of TOI-1333Ab. The black dots demarcate observations taken with a cadence of 30 min. The grey points indicate the 2 min. cadence data. The best-fitting transit model is shown as the solid white line, and the residuals (in parts per thousand) are shown in the bottom.
• Figure 2: FIES RVs of TOI-1333A after subtracting the best-fitting Keplerian. The solid line denotes the best-fitting model for the RM effect, and the shaded areas represent the 1- and 2-$\sigma$ confidence intervals.
• Figure 3: Projected orbit-orbit angle ($\gamma$) against the projected obliquity ($\lambda$) based on the table by Rice2024. Markers are coloured according to the effective temperature of the host star with blue (red) denoting stars cooler (hotter) than $6105$ K, as found to be appropriate for binary systems by Wang2025. The triangles demarcate triple star systems, and TOI-1333 is indicated with a square. As in Rice2024 systems with either $\sigma_\lambda>25^\circ$ or $\sigma_\gamma>25^\circ$ are shown with low opacity.
• Figure 4: Posterior distributions from our MCMC showing the correlation between $\lambda$, $v\sin i_\star$, and $b$. The red curves in the histogram denote the priors applied.
• Figure 5: TESS light curves of TOI-1333 showing the rotational modulation, which cannot necessarily be attributed to TOI-1333A. The colour-coding is the same as in Figure \ref{['fig:lc']}. The GP model used for detrending is shown as the white line with a black outline. Transits were removed by subtracting the best-fitting transit model.