Stellar Obliquities of Young Systems, Atmospheres Undergoing Contraction and Escape (SOYSAUCE): a likely aligned orbit for the 3 Myr planet TIDYE-1 b

TL;DR

This study measures the obliquity of the 3 Myr transiting planet TIDYE-1 b (IRAS 04125+2902 b) using the Rossiter–McLaughlin effect, despite lacking a pre-transit baseline. By combining MAROON-X RVs with multi-band photometry from TESS and LCOGT and employing external priors on stellar activity, the authors fit a global RM/transit model that yields $|\lambda|=11.8^{+5.9}_{-5.0}$° and $\psi=15.2^{+7.3}_{-5.7}$°, indicating an aligned or near-aligned orbit. Relaxing priors broadens the obliquity constraints to $|\lambda|=14.4^{+6.2}_{-5.5}$°, $\psi=17.3^{+6.9}_{-6.0}$°, with a tail permitting larger misalignments due to degeneracies from limited ingress coverage. The results support a scenario in which the planet remains aligned with the stellar spin even if the outer disk is misaligned, emphasizing the value of an expanded sample of young systems and the SOYSAUCE survey’s goals to test dynamical evolution and atmospheric signatures in such systems via RM/DT data.

Abstract

Despite the wide range of planet-star (mis)alignments in the mature population of transiting exoplanets, the small number of known young transiting planets are nearly all aligned with the rotation axes of their host stars, as determined by the sky-projected obliquity angle. The small number of young systems with measured obliquities limits statistical conclusions. Here we determine the sky-projected obliquity ($λ$) of the 3 Myr transiting planet with a misaligned outer protoplanetary disk, TIDYE-1 b (IRAS 04125+2902 b), using the Rossiter-McLaughlin (RM) effect. Our dataset lacks a pre-transit baseline and ingress, complicating a blind RM fit. Instead, we use contemporaneous spectra and photometry from a mass-measurement campaign to model the stellar activity trend across the transit and provide an external prior on the velocity baseline. We determine $|λ|=11.8^{+5.9\,\circ}_{-5.0}$. Combined with the published rotational velocity of the star, we find a true three-dimensional obliquity of $ψ=15.2^{+7.3\,\circ}_{-5.7}$. Our result is consistent with an aligned orbit, suggesting the planet remains aligned to its star even though the outer disk is misaligned, though additional RM observations are needed to exclude the low-probability tail of misaligned ($>30^{\circ}$) scenarios present in our posterior.

Figures (4)

• Figure 1: Simultaneous photometric observations of the transit of IRAS 04125+2902 b in TESS (top), LCOGT (left), and MuSCAT3 (right). In all observations, the 15-minute binned data is shown in purple with the raw data shown as the background colored points. The best-fit transit and stellar variability model is shown as the brown line in every filter.
• Figure 2: MAROON-X relative radial velocities from the red (red squares; top) and blue (blue circles; middle) channels. The best-fit RM and stellar RV trend model for each channel is shown as the opaque dotted red and solid blue lines, with 50 random sample fits pulled from the posterior shown as the translucent dotted red and solid blue lines. The bottom plot shows the residuals compared to the best-fit model for each channel, with a 5-minute shift applied to the blue channel RVs for clarity. The red and blue channel fits agree with one another, with the blue RV trend line showing a slightly steeper slope, as expected.
• Figure 3: Corner plot of a subset of the RM parameters. The contours show the 1, 2, and 3$\sigma$ levels. Only 99% of the distribution is shown for clarity. The sky-projected obliquity angle ($\lambda$) is correlated with the impact parameter ($b$) but seems to be minimally impacted by the the RV trend line parameters ($\gamma$ and $\dot\gamma$), with the exemption of the largest $\lambda$ values (with a near-flat RM signal near egress) corresponding to a tighter RV trend line solution. Remaining, unshown parameters are similarly gaussian distributed with little correlation between parameters.
• Figure 4: Distribution of sky-projected obliquity angles ($|\lambda|$) for transiting planets across system ages southworth2011. This only considers planets with masses below 10$M_J$ (or no measured mass), $T_{\text{eff}}$$<6250$ K, $\sigma_\lambda<25^\circ$, and those with literature ages exoplanetArchive. All points are colored by the planet radius. Young systems tend to be aligned but the difference is not significant. TIDYE-1 b (IRAS 04125+2902 b; star) is consistent with the few other $<0.1$ Gyr systems. The background shading represents rough approximations of the timescales for processes of interest for the SOYSAUCE survey.