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1. Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin–Orbit Misalignments Are Common

   https://doi.org/10.3847/1538-3881/acbd34  

   Bowler, Brendan P. ; Tran, Quang H. ; Zhang, Zhoujian ; Morgan, Marvin ; Ashok, Katelyn B. ; Blunt, Sarah ; Bryan, Marta L. ; Evans, Analis E. ; Franson, Kyle ; Huber, Daniel ; et al ( March 2023 , The Astronomical Journal)

   The orientation between a star’s spin axis and a planet’s orbital plane provides valuable information about the system’s formation and dynamical history. For non-transiting planets at wide separations, true stellar obliquities are challenging to measure, but lower limits on spin–orbit orientations can be determined from the difference between the inclination of the star’s rotational axis and the companion’s orbital plane (Δi). We present results of a uniform analysis of rotation periods, stellar inclinations, and obliquities of cool stars (SpT ≳ F5) hosting directly imaged planets and brown dwarf companions. As part of this effort, we have acquired new$v\sin i_{*}$values for 22 host stars with the high-resolution Tull spectrograph at the Harlan J. Smith telescope. Altogether our sample contains 62 host stars with rotation periods, most of which are newly measured using light curves from the Transiting Exoplanet Survey Satellite. Among these, 53 stars have inclinations determined from projected rotational and equatorial velocities, and 21 stars predominantly hosting brown dwarfs have constraints on Δi. Eleven of these (52${}_{-11}^{+10}$% of the sample) are likely misaligned, while the remaining 10 host stars are consistent with spin–orbit alignment. As an ensemble, the minimum obliquity distribution between 10 and 250 au is more consistent with a mixture of isotropic and aligned systems than either extreme scenario alone—pointing to direct cloud collapse, formation within disks bearing primordial alignments and misalignments, or architectures processed by dynamical evolution. This contrasts with stars hosting directly imaged planets, which show a preference for low obliquities. These results reinforce an emerging distinction between the orbits of long-period brown dwarfs and giant planets in terms of their stellar obliquities and orbital eccentricities.

    

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2. Measured spin–orbit alignment of ultra-short-period super-Earth 55 Cancri e

   https://doi.org/10.1038/s41550-022-01837-2  

   Zhao, Lily L. ; Kunovac, Vedad ; Brewer, John M. ; Llama, Joe ; Millholland, Sarah C. ; Hedges, Christina ; Szymkowiak, Andrew E. ; Roettenbacher, Rachael M. ; Cabot, Samuel H. ; Weiss, Sam A. ; et al ( December 2022 , Nature Astronomy)

   A planet’s orbital alignment places important constraints on how a planet formed and consequently evolved. The dominant formation pathway of ultra-short-period planets (P < 1 day) is particularly mysterious as such planets most likely formed further out, and it is not well understood what drove their migration inwards to their current positions. Measuring the orbital alignment is difficult for smaller super-Earth/sub-Neptune planets, which give rise to smaller amplitude signals. Here we present radial velocities across two transits of 55 Cancri (Cnc) e, an ultra-short-period super-Earth, observed with the Extreme Precision Spectrograph. Using the classical Rossiter–McLaughlin method, we measure 55 Cnc e’s sky-projected stellar spin–orbit alignment (that is, the projected angle between the The star 55 Cancri (Cnc) A hosts five known exoplanets with minimum mass estimates ranging from approximately 8M⊕ to 3MJup and periods less than one day to nearly 20 years1–4. Of particular interest has been 55 Cnc e, one of the most massive known ultra-short-period planets (USPs) and the only planet around 55 Cnc found to transit5,6. It has an star’s spin axis and the planet’s orbit normal—will shed light on the formation and evolution of USPs, especially in the case of compact, multiplanet systems. It has been shown that USPs form a statistically distinct popula- tion of planets9 that tend to be misaligned with other planetary orbits in their system10. This suggests that USPs experience a unique migra- tion pathway that brings them close in to their host stars. This inward migration is most likely driven by dissipation due to star–planet tidal interactions that result from either non-zero eccentricities11,12 or plan- etary spin-axis tilts13. orbital period of 0.7365474 +1.3 × 10−6 days, a mass of 7.99 ± 0.33M −1.4 × 10−6 ⊕ and a radius of 1.853 +0.026 R⊕ (refs. 7,8). A precise measure of the −0.027 stellar spin–orbit alignment of 55 Cnc e—the angle between the host planet’s orbital axis and its host star’s spin axis) to be λ = 10 +17∘ with an +14∘ −20∘ unprojected angle of ψ = 23 −12∘. The best-fit Rossiter–McLaughlin model to the Extreme Precision Spectrograph data has a radial velocity semi- amplitude of just 0.41 +0.09 m s−1. The spin–orbit alignment of 55 Cnc e −0.10 favours dynamically gentle migration theories for ultra-short-period planets, namely tidal dissipation through low-eccentricity planet–planet interactions and/or planetary obliquity tides. 

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3. Stellar Companions to TESS Objects of Interest: A Test of Planet–Companion Alignment

   https://doi.org/10.3847/1538-3881/ac53a7  

   Behmard, Aida ; Dai, Fei ; Howard, Andrew W. ( March 2022 , The Astronomical Journal)

   We present a catalog of stellar companions to host stars of Transiting Exoplanet Survey Satellite Objects of Interest (TOIs) identified from a marginalized likelihood ratio test that incorporates astrometric data from the Gaia Early Data Release 3 catalog (EDR3). The likelihood ratio is computed using a probabilistic model that incorporates parallax and proper-motion covariances and marginalizes the distances and 3D velocities of stars in order to identify comoving stellar pairs. We find 172 comoving companions to 170 non-false-positive TOI hosts, consisting of 168 systems with two stars and 2 systems with three stars. Among the 170 TOI hosts, 54 harbor confirmed planets that span a wide range of system architectures. We conduct an investigation of the mutual inclinations between the stellar companion and planetary orbits using Gaia EDR3, which is possible because transiting exoplanets must orbit within the line of sight; thus, stellar companion kinematics can constrain mutual inclinations. While the statistical significance of the current sample is weak, we find that${73}_{-20}^{+14}\%$of systems with Kepler-like architectures (R_P≤ 4R_⊕anda< 1 au) appear to favor a nonisotropic orientation between the planetary and companion orbits with a typical mutual inclinationαof 35° ± 24°. In contrast,${65}_{-35}^{+20}$% of systems with close-in giants (P< 10 days andR_P> 4R_⊕) favor a perpendicular geometry (α= 89° ± 21°) between the planet and companion. Moreover, the close-in giants with large stellar obliquities (planet–host misalignment) are also those that favor significant planet–companion misalignment.

    

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4. High-eccentricity Migration with Disk-induced Spin–Orbit Misalignment: A Preference for Perpendicular Hot Jupiters

   https://doi.org/10.3847/2041-8213/acaea6  

   Vick, Michelle ; Su, Yubo ; Lai, Dong ( January 2023 , The Astrophysical Journal Letters)

   High-eccentricity migration is a likely formation mechanism for many observed hot Jupiters, particularly those with a large misalignment between the stellar spin axis and orbital angular momentum axis of the planet. In one version of high-eccentricity migration, an inclined stellar companion excites von Zeipel–Lidov–Kozai (ZLK) eccentricity oscillations of a cold Jupiter, and tidal dissipation causes the planet’s orbit to shrink and circularize. Throughout this process, the stellar spin can evolve chaotically, resulting in highly misaligned hot Jupiters (HJs). Previous population studies of this migration mechanism have assumed that the stellar spin is aligned with the planetary orbital angular momentum when the companion begins to induce ZLK oscillations. However, in the presence of a binary companion, the star’s obliquity may be significantly excited during the dissipation of its protoplanetary disk. We calculate the stellar obliquities produced in the protoplanetary disk phase and use these to perform an updated population synthesis of ZLK-driven high-eccentricity migration with an F-type host star. We find that the resulting obliquity distribution of HJ systems is predominantly retrograde with a broad peak near 90°. The distribution we obtain has intriguing similarities to the recently observed preponderance of perpendicular planets close to their host stars.

    

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5. An Aligned Orbit for the Young Planet V1298 Tau b

   https://doi.org/10.3847/1538-3881/ac6271  

   Johnson, Marshall C. ; David, Trevor J. ; Petigura, Erik A. ; Isaacson, Howard T. ; Van Zandt, Judah ; Ilyin, Ilya ; Strassmeier, Klaus ; Mallonn, Matthias ; Zhou, George ; Mann, Andrew W. ; et al ( May 2022 , The Astronomical Journal)

   The alignment of planetary orbits with respect to the stellar rotation preserves information on their dynamical histories. Measuring this angle for young planets helps illuminate the mechanisms that create misaligned orbits for older planets, as different processes could operate over timescales ranging from a few megayears to a gigayear. We present spectroscopic transit observations of the young exoplanet V1298 Tau b; we update the age of V1298 Tau to be 28 ± 4 Myr based on Gaia EDR3 measurements. We observed a partial transit with Keck/HIRES and LBT/PEPSI, and detected the radial velocity anomaly due to the Rossiter–McLaughlin effect. V1298 Tau b has a prograde, well-aligned orbit, with$\lambda =4_{-10}^{+7}$deg. By combining the spectroscopically measured$v\sin i_{\star }$and the photometrically measured rotation period of the host star we also find that the orbit is aligned in 3D,$\psi =8_{-7}^{+4}$deg. Finally, we combine our obliquity constraints with a previous measurement for the interior planet V1298 Tau c to constrain the mutual inclination between the two planets to bei_mut= 0° ± 19°. This measurements adds to the growing number of well-aligned planets at young ages, hinting that misalignments may be generated over timescales of longer than tens of megayears. The number of measurements, however, is still small, and this population may not be representative of the older planets that have been observed to date. We also present the derivation of the relationship betweeni_mut,λ, andifor the two planets.

    

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