The OATMEAL Survey. II. The 3D spin-orbit obliquity of an eccentric transiting brown dwarf in the Ruprecht 147 open cluster

Abstract

We present new analysis of the CWW 89 system as part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. The CWW 89 system is a member of the 2.8 Gyr old Ruprecht 147 (NGC 6774) cluster and features two stars, CWW 89A (EPIC 219388192) and CWW 89B, with the primary hosting a transiting brown dwarf. We use in-transit, highly precise radial velocity measurements with the Keck Planet Finder (KPF) to characterize the Rossiter-McLaughlin (RM) effect and measure the projected spin-orbit obliquity $|λ|=1.4\pm2.5^\circ$ and the full 3D spin-orbit obliquity of the brown dwarf to be $ψ=15.1^{+15.0^\circ}_{-10.9}$. This value of $λ$ implies that the brown dwarf's orbit is prograde and well-aligned with the equator of the host star, continuing the trend of transiting brown dwarfs showing a preference for alignment ($λ\approx 0^\circ$) regardless of the stellar effective temperature. We find that this contrast with the transiting giant planet population, whose spin-orbit alignments depend on host $T_{\rm eff}$, shows an increasingly clear distinction in the formation and orbital migration mechanisms between transiting giant planets and transiting brown dwarfs like CWW 89Ab. For this system in particular, we find it plausible that the brown dwarf may have undergone coplanar high-eccentricity migration influence by CWW 89B.

Figures (5)

• Figure 1: SED photometry of CWW 89A. Blue points are the model values from MIST mist1mist2mist3. Red points are the input data from Gaia gaia_dr3_magnitude_corr, 2MASS 2MASS, and WISE WISE.
• Figure 2: Top: RV orbital solution for CWW 89A using TRES carmichael19 and FIES nowak17 data. Middle: KPF in-transit RV data showing the RV anomaly induced by the Rossiter-McLaughlin Effect. The y-axis units in the top and middle panels are in $\rm m\,s^{-1}$. Bottom: Phase-fold K2 transit photometry of CWW 89A with our EXOFASTv2 transit model overlaid.
• Figure 3: The posterior distribution for the stellar inclination angle $i_\star$ for CWW 89A. We use the $P_{\rm rot}$ measured from nowak17 and the sampling method first presented in masuda2020 and described in bowler2023_inclination to derive an $i_\star=74.4^{+9.7^\circ}_{-18.3}$. We interpret this as aligned with the orbital inclination $i_0$.
• Figure 4: The posterior distribution of $\psi$ from sampling in $\cos{\psi}$ parameter space. Given the skewed nature of this distribution (driven by $v\sin{i_\star} \lessapprox v_{\rm rot}$), a more conservative estimate of the 3D obliquity is $\psi \leq 30.1^\circ$ with the peak of the distribution at $\psi_{\rm peak} = 3.1^\circ$.
• Figure 5: Top:$\lambda$ vs $T_{\rm eff}$ for transiting companions $\gtrsim 0.3\,M_{\rm J}$. The Kraft Break regime in the vertical shaded region is from beyer_white2024, with the traditional 6250K threshold denoted by the dashed line. Brown dwarfs are represented by square symbols. CWW 89Ab is the lowest-mass transiting brown dwarf around a sub-Kraft Break star with its $\lambda$ and $\psi$ measured. Bottom:$\lambda$ vs $q$ ($M_b/M_\star$) for the same sample in the top panel. We highlight the threshold ($q=2\times 10^{-3}$) past which primordial alignment ($\lambda\approx 0^\circ$) dominates the observed distribution rusznak2024. Circled points are systems with inner transiting brown dwarfs and confirmed outer companions (CWW 89 and LP 261-75) or Gaia RUWE values above 1.5. Both CWW 89A and LP 261-75 have RUWE values $<1.4$ despite a confirmed stellar companion. All of the inner brown dwarfs except for the one in LP 261-75 are on eccentric orbits ($e>0.18$), plausibly following a coplanar HEM scenario. The misaligned giant planet system at $q \approx 10^{-2}$ is XO-3b discussed in Section \ref{['sec:hier_trips']}. Data in both panels are from Figure 2 of rusznak2024.