The changing transit shape of TOI-3884 b

Abstract

TOI-3884 b is a sub-Saturn transiting a fully convective M-dwarf. Observations indicate that the transit shape is chromatic and asymmetric as a result of persistent starspot crossings. This, along with the lack of photometric variability of the host star, indicates that the rotational axis of the star is tilted along our line of sight and the planet-occulted starspot is located close to the stellar pole. We acquired photometric transits over a period of three years with the Swiss 1.2-meter Euler telescope to track changes in the starspot configuration and detect any signs of decay or growth. The shape of the transit changes over time, and so far no two observations match perfectly. We conclude that the observed variability is likely not caused by changes in the temperature and size of the spot, but due to a slight (5.64 $\pm$ 0.64$^{\circ}$) misalignment between the spot center and the stellar pole, i.e., a small spin-spot angle ($Θ$). In addition, we were able to obtain precise measurements of the sky-projected spin-orbit angle ($λ$) of 37.3 $\pm$ 1.5\degree, and the true spin-orbit angle ($ψ$) of 54.3 $\pm$ 1.4\degree. The precise alignment measurements along with future atmospheric characterisation with the James Webb Space Telescope will be vital for understanding the formation and evolution of close-in, massive planets around fully convective stars.

Figures (8)

• Figure 1: Phase-folded EulerCam light curves of TOI-3884 b. The top panel shows the normalised flux, corrected for instrumental systematics, with the best-fit PyTranSpot models, obtained using the second (or the spin-spot misalignment) approach over-laid on top. The lower panels displays the model residuals.
• Figure 2: Posteriors for spot size and contrast for TOI-3884. The top panel shows a violin plot with spot size and posterior distribution for each observation obtained using the first (or spot evolution) approach. The lower plot displays the spot contrast. The horizontal lines show the median spot size and contrast obtained using the second (or the spin-spot misalignment) approach, with dashed lines showing the 1$\sigma$ limits.
• Figure 3: The posterior position of the spot centre on the stellar surface, as inferred from the second approach analysis for each transit observation, is shown with shaded regions indicating the $1\sigma$ and $2\sigma$ credible intervals, using the same colour coding as in Figs. \ref{['fig:lightcurves_approach_2']} and \ref{['figure:spot_size_temp']}. The transit path for the best-fit model is represented by horizontal lines, while the planet, depicted as a circle, contains an arrow indicating its direction of movement. The best-fit spherical circle for the spin-spot misalignment model is shown with a dashed gray line, with coloured dots representing the positions of the transit observations. The coloured circles centred on these points represent the spot. The black dot in the spherical circle indicates the position at the reference time, while the black diagonal cross marks the position of the spin axis. An orange arrow represents CCW movement, while a blue arrow represents CW movement.
• Figure 4: Same as Fig. \ref{['fig:lightcurves_approach_2']} but with best-fit PyTranSpot models obtained using the first (or spot evolution) approach.
• Figure 5: Modelling of the spot center’s position in latitude and longitude from the second approach analysis (i.e. the spin-spot misalignment approach) for each transit observation using the spin-spot misalignment model (Sect. \ref{['subsection:obliquity']}). A thousand random samples from the posterior distribution for each direction, CW (light blue) and CCW (light orange, are shown. The best-fit model for a rotational period of $\sim10.8$ days in the CCW direction is represented by an orange line.