The Rossiter–McLaughlin effect revolutions: an ultra-short period planet and a warm mini-Neptune on perpendicular orbits

Abstract

Comparisons of the alignment of exoplanets with a common host star and each other can be used to distinguish among concurrent evolution scenarios for the star and the planets. However, multi-planet systems usually host mini-Neptunes and super-Earths, whose sizes make orbital architecture measurements challenging. We introduce the Rossiter-McLaughlin effect Revolutions (RMR) technique, which can access the spin-orbit angle of small exoplanets by exploiting the full extent of information contained in spectral transit time series. We validated the technique through its application to published HARPS-N data of the mini-Neptune HD 3167c (P = 29.8 days), refining its high sky-projected spin-orbit angle (−108.9−5.5+5.4°), and we applied it to new ESPRESSO observations of the super-Earth HD 3167 b (P = 0.96 days), revealing an aligned orbit (−6.6−7.9+6.6°). Surprisingly different variations in the contrast of the stellar lines occulted by the two planets can be reconciled by assuming a latitudinal dependence of the stellar line shape. In this scenario, a joint fit to both datasets constrains the inclination of the star (111.6−3.3+3.1°) and the 3D spin-orbit angles of HD 3167b (29.5−9.4+7.2°) and HD 3167c (107.7−4.9+5.1°). The projected spin-orbit angles do not depend on the model for the line contrast variations, and so, with a mutual inclination of 102.3−8.0+7.4°, we can conclude that the two planets are on perpendicular orbits. This could be explained by HD 3167b being strongly coupled to the star and retaining its primordial alignment, whereas HD 3167c would have been brought to a nearly polar orbit via secular gravitational interactions with an outer companion. Follow-up observations of the system and simulations of its dynamical evolution are required to search for this companion and explore the likelihood of this scenario. HD 3167 b (R = 1.7 REarth) is the smallest exoplanet with a confirmed spectroscopic Rossiter-McLaughlin signal. The RMR technique opens the way to determining the orbital architectures of the super-Earth and Earth-sized planet populations.

Stellar obliquities in exoplanetary systems

“…9. Such a data-stacking analysis can be useful in the presence of correlated noise (Johnson et al 2014) or a low signal-to-noise ratio (Hjorth et al 2021;Bourrier et al 2021). Another approach to detecting small RM signals, employing Gaussian Processes, was presented by Kunovac Hodžić et al (2021a).…”

An Aligned Orbit for the Young Planet V1298 Tau b

“…We highlight V1298 Tau b as the red-bordered star, and also show as larger symbols the four systems with published spin-orbit misalignment measurements for more than one planet: V1298 Tau (stars), HD 63433 (upward triangles), K2-290 (squares), and HD 3167 (downward triangles). Note that K2-290 is consistent with coplanarity, as the uncertainty on the measurement of λ for the inner planet is large (Hjorth et al 2021), while HD 3167 is highly non-coplanar (Bourrier et al 2021). Bottom: spin-orbit misalignments as a function of age; young planets are toward the center, older planets toward the edge.…”

A CHEOPS-enhanced view of the HD 3167 system

“…Compared to the HARPS-N DRS version 3.8, the new version of the reduction pipeline, along with the performed optimizations, provide smaller night-to-night variations, estimated to be 0.5 m/s rms 3 https://archive.stsci.edu/hst/wfc3 compared to 0.8 m/s, and a better stability of the RVs on the long-term, due to a careful selection of Thorium lines used for calibrating the instrument. The new pipeline further extracts RVs from cross-correlation functions computed with improved binary masks (here the G9 mask closer in type to HD 3167), built with weights more representative of the RV information content of each spectral line (Bourrier et al 2021). We rejected the observation '2018-01-08T20-30-55.308' because it was not possible to correct for the color effect induced by Earth atmospheric diffusion.…”

“…Merging the data from all four instruments, we obtain a time series of 434 RV data points. To prevent biases induced by the Rossiter-McLaughlin signals from the planets b and c (Dalal et al 2019;Bourrier et al 2021), we discarded the 111 data points observed during their transits. The filtered RV data set that we included in our analysis represents a total of 323 data points (39 HARPS points, 102 APF/Levy points, 55 Keck/HIRES points, and 127 HARPS-N points) covering more than 5.3 years (∼ 1940 days).…”

“…The intermediate planet d is not transiting, but is nonetheless expected to have a low mutual inclination with planet c based on dynamical calculations (Dalal et al 2019). The orbital architecture of the HD 3167 system is particularly intriguing, because the orbital plane of its innermost planet b is close to the stellar equatorial plane and perpendicular to the orbital planes of the outer planets d and c, on polar orbits around the star (Dalal et al 2019, Bourrier et al 2021.…”