The OATMEAL Survey. III. An Aligned Transiting Warm Brown Dwarf and Evidence for Quiescent Brown Dwarf Migration

TL;DR

This study measures the sky-projected orbital obliquity of HIP 33609 b, a $\sim$68 $M_J$ brown dwarf on a $\sim$39-day, highly eccentric orbit around a hot $T_eff \sim 10{,}300$ K A-type star, using Doppler Tomography on in-transit spectra. A global EXOFASTv2 fit combining Gaia parallax, SED, and TESS/RV data yields $|\lambda| = 12.7 \pm 1.3^\circ$ and a companion mass $M_P \approx 67.9^{+7.3}_{-7.2} M_J$, with $P \approx 39.4718$ d and $e \approx 0.557$, indicating a dynamically active, yet tidally detached system. The authors argue that fragmentation followed by coplanar high-eccentricity migration best explains the low obliquity, while noting ongoing uncertainty among migration pathways. Comparisons with warm Jupiters suggest brown dwarfs may migrate quiescently in isolated environments, a scenario testable by searching for nearby planets or additional companions. This work extends the BD obliquity census and links formation and migration histories to the observed spin-orbit architectures.

Abstract

We present the first measurement of the sky-projected orbital obliquity of a benchmark transiting brown dwarf host, HIP 33609, as a part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. HIP 33609 b is a highly eccentric, 68 $M_{\rm J}$ brown dwarf orbiting a 10,300 K, A-type star with an orbital period of 39 days. Its host star is a known member of the 150 Myr old MELANGE-6 moving group, making it an excellent laboratory for testing sub-stellar evolutionary models. Using in-transit spectra collected by the Planet Finder Spectrograph (PFS) on the Magellan II Clay 6.5 m telescope, we measured a sky-projected orbital obliquity of $|λ|= 12.7 \pm 1.3$°. The mass of the brown dwarf is most consistent with a stellar-like fragmentation formation history followed by a period of migration. Given the high eccentricity ($e=0.557$) but low orbital obliquity of the brown dwarf, we claim that coplanar high eccentricity tidal migration seems to be the most plausible pathway, however, it remains difficult to conclusively rule out other migration mechanisms. The low orbital obliquity for HIP 33609 is consistent with previous measurements of high mass-ratio companions, and bears a striking resemblance to the obliquity distribution of transiting warm Jupiters. We suggest brown dwarfs may follow a dynamically quiescent migration pathway, consistent with them forming in isolated conditions.

Figures (4)

• Figure 1: Doppler shadow analysis of HIP 33609 b. The left, middle, and right panels show the reduced data, best-fit model, and residuals (data minus model), respectively. The horizontal lines indicate the times of ingress and egress, while the vertical lines mark the projected stellar rotational velocity limits, $\pm v \sin i$. The color scale represents the fractional variation in flux.
• Figure 2: The sky-projected stellar obliquity vs. host star $T_{\rm eff}$ for transiting companions ranging $0.7<M_b<80$$M_{\rm J}$. The vertical dashed line at $6250$ K marks the Kraft break Kraft:1967. Giant planets planets (gray) transition from low to high stellar obliquity at the Kraft break, whereas BDs (blue) exhibit low stellar obliquity regardless of their host star's effective temperature. HIP 33609 b stands out as the hottest stellar obliquity measurement to date and is denoted by the blue diamond. All data for this figure were retrieved using TEPCat Southworth2011.
• Figure 3: The sky-projected stellar obliquity vs. the mass ratio of the system for transiting companions ranging $0.7<M_b<80$$M_{\rm J}$. Giant planets are shown as gray circles whereas BDs are blue circles. Our new measurement for HIP 33609 b is highlighted as the blue diamond. HIP 33609 b's stellar obliquity is consistent with the Rusznak2025 framework wherein high mass-ratio companions form isolated with low stellar obliquities.
• Figure 4: The sky-projected stellar obliquity vs. host star $T_{\rm eff}$ for transiting companions ranging $0.7<M_b<80$$M_{\rm J}$. Warm Jupiters are plotted in color as triangles and BDs as circles. Hot Jupiters are the gray background points. Warm Jupiters and BDs are colored based on their eccentricities. Systems with a detected outer stellar companion are highlighted with a red outline. Warm Jupiters and BDs orbiting single stars exhibit low stellar obliquities regardless of their host star's effective temperature and orbital eccentricity suggesting a potentially shared, quiescent migration pathway (see §\ref{['sec:populations']}).