The EBLM project XVI. Moderate spin-orbit misalignment of the low mass eclipsing binary EBLM J0021-16

TL;DR

EBLM J0021-16 is a 5.97-day eclipsing binary comprising a $1.05\,M_\odot$ G-dwarf and a $0.19\,M_\odot$ M-dwarf. By combining CORALIE radial velocities, TESS photometry, and a measured stellar rotation period of $P_{\rm rot}=7.042\pm0.061$ days, the study performs a joint RV–photometry Rossiter–McLaughlin analysis to determine a sky-projected obliquity of $|\lambda|=2.0\pm1.1^{\circ}$ and a true obliquity of $\psi=28.9\pm2.1^{\circ}$. The orbit is nearly circular ($e=0.000534\pm0.000095$) and not synchronized with the primary’s rotation, challenging simple tidal realignment expectations and hinting at possible tertiary influence or complex rotation dynamics. The M-dwarf companion is measured with sub-percent precision ($M_B=0.1877\pm0.0011\,M_\odot$, $R_B=0.2214\pm0.0019\,R_\odot$), with a radius inflation of $\sim5.8\%$ relative to MIST models, contributing to the broader radius-inflation puzzle for low-mass stars. Overall, the work provides a rare 3D obliquity for an eclipsing binary, tests tidal evolution theories, and demonstrates the value of joint RM analyses in the EBLM program.

Abstract

Thousands of tight ($<1$ AU) main sequence binaries have been discovered, but it is uncertain how they formed. There is likely too much angular momentum in a collapsing, fragmenting protostellar cloud to form such binaries in situ, suggesting some post processing. One probe of a binary's dynamical history is the angle between the stellar spin and orbital axes -- its obliquity. The classical method for determining stellar obliquity is the Rossiter-McLaughlin effect. It has been applied to over 100 hot Jupiters, but less than a dozen stellar binaries. In this paper, we present the Rossiter-McLaughlin measurement of EBLM J0021-16, a $0.19M_\odot$ M-dwarf eclipsing a $1.05M_\odot$ G-dwarf on a 5.97 day, almost-circular orbit. We combine CORALIE spectroscopy with TESS photometry and a measured primary star rotation period of 7.04 days, according to star spot modulation. We show that the orbital axis is misaligned with the primary star's spin axis, with a true 3D obliquity of $ψ=28.9\pm2.1^{\circ}$. EBLM J0021-16, being neither spin-orbit aligned nor synchronized, yet with an almost circular orbit, is a curious case for tidal evolution in tight binaries. It becomes one of a handful of eclipsing binaries with true obliquity measurements. Finally, we derive the M-dwarf's mass and radius to a fractional precision better than 1\%. The radius of the M-dwarf is inflated by 6\% ($7.4σ$) with respect to stellar models, consistent with many other M-dwarfs in the literature.

Figures (10)

• Figure 1: Radial velocity observations of EBLM J0021-16, phase-folded on the best fitting orbital period. The CORALIE data are shown by the black dots, with error bars on the order of m/s and are invisible at this scale. The red line shows the PyMC3 best fit model with the RM fit as well. The RM here is not visible because it happens on the order of m/s, which cannot be seen in the Keplerian, which is on the order of km/s.
• Figure 2: Relationship between the CCF bisector span and RV residuals from a Keplerian + linear drift model. The RM data are excluded. As noted in Triaud2017, there is clear anti-correlation between the RV residuals and the bisector. This implies the presence of stellar activity, which is also seen in the star spot modulation in Fig. \ref{['fig:PDCSAP vs SAP Transit']}.
• Figure 3: PDCSAP and the SAP flux data (black points) with the Wotan fit (red line) that was used to de-trend the data. The zoomed-in graph in the bottom right corner of the PDCSAP plot shows an artifact that resembles another transit. The zoomed-in graph on the SAP plot shows no artifact over the same period. This suggests that the artifact came from data processing and is not an astrophysical effect. This figure also shows the sinusoidal pattern in the light curve as a result of star spots, which allowed Sethi2024 to measure the rotation period of J0021$-$16
• Figure 4: SED (top) and evolutionary track (bottom) of EBLM J0021-16 found by Exofastv2. In the bottom graph, the black line represents the evolutionary track for the best fit stellar mass. The black dot represents the $\log g$ and $T_{\rm eff}$ from the Exofastv2 fit. The red asterisk represents the predicted value by the evolutionary track. The MIST model indicates an age of $8.5 \pm 2.8$ Gyr.
• Figure 5: Raw (top) and detrended (middle) light curves of EBLM J0021-16. The bottom left plot shows the phase-folded primary eclipse and the MCMC fit, and the bottom right graph shows the phase-folded secondary eclipse and the MCMC fit.