ORCID Profile
0000-0002-3726-4881
Current Organisations
University of California Santa Cruz
,
University of Hawai'i at Manoa Institute for Astronomy
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Publisher: American Astronomical Society
Date: 17-03-2023
Abstract: 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 (Δ i ). 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 s le 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 Δ i . Eleven of these (52 − 11 + 10 % of the s le) 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.
Publisher: American Astronomical Society
Date: 05-01-2023
Abstract: We analyzed 20 s cadence Transiting Exoplanet Survey Satellite time-series photometry of the exoplanet host star HR 8799 collected in Sector 56. The litude spectrum shows Gamma Doradus ( γ Dor) pulsations consistent with previous space-based photometry from MOST. Assuming that HR 8799 is a representative of γ Dor stars in the Kepler s le, the dominant dipole mode at 1.98 cycles day −1 implies a core rotation period of ∼0.7 day, which combined with v sin i and stellar radius measurements would result in a preliminary stellar inclination of ∼28° assuming rigid rotation. We find no significant residual photometric variation after removing the pulsation signal aside from a ∼9 days trend that is likely a systematic effect or an artifact from performing aggressive frequency subtraction in the presence of red noise.
Publisher: American Astronomical Society
Date: 10-2022
Abstract: 51 Eri is well known for hosting a directly imaged giant planet and for its membership to the β Pictoris moving group. Using 2 minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS), we detect multiperiodic variability in 51 Eri that is consistent with pulsations of Gamma Doradus ( γ Dor) stars. We identify the most significant pulsation modes (with frequencies between ∼0.5 and 3.9 cycles day −1 and litudes ranging between ∼1 and 2 mmag) as dipole and quadrupole gravity modes, as well as Rossby modes, as previously observed in Kepler γ Dor stars. Our results demonstrate that previously reported variability attributed to stellar rotation is instead likely due to γ Dor pulsations. Using the mean frequency of the ℓ = 1 gravity modes, together with empirical trends of the Kepler γ Dor population, we estimate a plausible stellar core rotation period of 0.9 − 0.1 + 0.3 days for 51 Eri. We find no significant evidence for transiting companions around 51 Eri in the residual light curve. The detection of γ Dor pulsations presented here, together with follow-up observations and modeling, may enable the determination of an asteroseismic age for this benchmark system. Future TESS observations would allow a constraint on the stellar core rotation rate, which in turn traces the surface rotation rate, and thus would help clarify whether or not the stellar equatorial plane and orbit of 51 Eri b are coplanar.
Publisher: American Astronomical Society
Date: 09-11-2020
Location: United States of America
Location: United States of America
No related grants have been discovered for Zhoujian Zhang.