2 research outputs found
Obliquity Constraints on an Extrasolar Planetary-Mass Companion
We place the first constraints on the obliquity of a planetary-mass companion outside of the solar system. Our target is the directly imaged system 2MASS J01225093–2439505 (2M0122), which consists of a 120 Myr 0.4 M⊙ star hosting a 12–27 M_J companion at 50 au. We constrain all three of the system's angular-momentum vectors: how the companion spin axis, the stellar spin axis, and the orbit normal are inclined relative to our line of sight. To accomplish this, we measure projected rotation rates (v sin i) for both the star and the companion using new near-infrared high-resolution spectra with NIRSPEC at Keck Observatory. We combine these with a new stellar photometric rotation period from TESS and a published companion rotation period from Hubble Space Telescope to obtain spin-axis inclinations for both objects. We also fitted multiple epochs of astrometry, including a new observation with NIRC2/Keck, to measure 2M0122b's orbital inclination. The three line-of-sight inclinations place limits on the true de-projected companion obliquity and stellar obliquity. We find that while the stellar obliquity marginally prefers alignment, the companion obliquity tentatively favors misalignment. We evaluate possible origin scenarios. While collisions, secular spin–orbit resonances, and Kozai–Lidov oscillations are unlikely, formation by gravitational instability in a gravito-turbulent disk—the scenario favored for brown dwarf companions to stars—appears promising
Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin-Orbit Misalignments are Common
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 (). 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 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
. Eleven of these (52% of the sample) are likely
misaligned, while the remaining ten host stars are consistent with spin-orbit
alignment. As an ensemble, the minimum obliquity distribution between 10-250 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.Comment: AJ, accepte