Astrometric Accelerations as Dynamical Beacons: Discovery and Characterization of HIP 21152 B, the First T-dwarf Companion in the Hyades*

Franson, Kyle; Bowler, Brendan P.; Bonavita, M.; Brandt, Timothy D.; Chen, Minghan; Samland, M.; Zhang, Zhoujian; Lueber, Anna; Heng, Kevin; Kitzmann, Daniel; Wolf, Trevor; Jones, Brandon A.; Tran, Quang H.; Gagliuffi, Daniella Bardalez; Biller, Beth; Chilcote, Jeffrey; Crepp, Justin R.; Dupuy, Trent J.; Faherty, Jacqueline K.; Fontanive, C.; Groff, Tyler D.; Gratton, R.; Guyon, Olivier; Jensen-Clem, Rebecca; Jovanović, Nemanja; Kasdin, N. Jeremy; Lozi, Julien; Magnier, E. A.; Mužić, Koraljka; Sanghi, Aniket; Theissen, Christopher A.

doi:10.3847/1538-3881/aca408

Cited by 23 publications

(24 citation statements)

References 205 publications

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“…Finally, in addition to being only the second directly imaged exoplanet whose presence was hinted at using the RV method, our precise dynamical mass for HD 206893c means that this effort potentially represents the first discovery via direct imaging of a bona fide exoplanet that partially relies on precise measurements of the host star astrometry from the Gaia mission. This complements numerous other recent discoveries of substellar companions (e.g., Bonavita et al 2022;Kuzuhara et al 2022;Franson et al 2023Franson et al , 2022Currie et al, in prep. ) whose discovery also used Gaia astrometry.…”

Section: Discussionsupporting

confidence: 85%

Direct discovery of the inner exoplanet in the HD 206893 system

Hinkley¹,

Lacour²,

Marleau³

et al. 2023

A&A

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Aims. HD 206893 is a nearby debris disk star that hosts a previously identified brown dwarf companion with an orbital separation of ∼10 au. Long-term precise radial velocity (RV) monitoring, as well as anomalies in the system proper motion, has suggested the presence of an additional, inner companion in the system. Methods. Using information from ongoing precision RV measurements with the HARPS spectrograph, as well as Gaia host star astrometry, we have undertaken a multi-epoch search for the purported additional planet using the VLTI/GRAVITY instrument. Results. We report a high-significance detection over three epochs of the companion HD 206893c, which shows clear evidence for Keplerian orbital motion. Our astrometry with ∼50−100 μarcsec precision afforded by GRAVITY allows us to derive a dynamical mass of 12.7$ ^{+1.2}_{-1.0} $MJup and an orbital separation of 3.53$ ^{+0.08}_{-0.06} $ au for HD 206893c. Our fits to the orbits of both companions in the system use both Gaia astrometry and RVs to also provide a precise dynamical estimate of the previously uncertain mass of the B component, and therefore allow us to derive an age of 155 ± 15 Myr for the system. We find that theoretical atmospheric and evolutionary models that incorporate deuterium burning for HD 206893c, parameterized by cloudy atmosphere models as well as a “hybrid sequence” (encompassing a transition from cloudy to cloud-free), provide a good simultaneous fit to the luminosity of both HD 206893B and c. Thus, accounting for both deuterium burning and clouds is crucial to understanding the luminosity evolution of HD 206893c. Conclusions. In addition to using long-term RV information, this effort is an early example of a direct imaging discovery of a bona fide exoplanet that was guided in part by Gaia astrometry. Utilizing Gaia astrometry is expected to be one of the primary techniques going forward for identifying and characterizing additional directly imaged planets. In addition, HD 206893c is an example of an object narrowly straddling the deuterium-burning limit but unambiguously undergoing deuterium burning. Additional discoveries like this may therefore help clarify the discrimination between a brown dwarf and an extrasolar planet. Lastly, this discovery is another example of the power of optical interferometry to directly detect and characterize extrasolar planets where they form, at ice-line orbital separations of 2−4 au.

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Section: Discussionsupporting

confidence: 85%

Direct discovery of the inner exoplanet in the HD 206893 system

Hinkley¹,

Lacour²,

Marleau³

et al. 2023

A&A

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“…Results from this study reinforce an emerging distinction between imaged giant planets and brown dwarf companions in terms of their orbits (Bowler et al 2020a), demographics (Nielsen et al 2019;Vigan et al 2021), and mass functions (Wagner et al 2019): brown dwarf companions are most consistent with formation in a stellar-like pathway from fragmenting turbulent clouds than within aligned disks. In the future, we expect that new discoveries from dynamically informed surveys (e.g., Franson et al 2023;Hinkley et al 2022), continued astrometric orbit monitoring of known systems (e.g., Zurlo et al 2022), precision astrometry from VLTI/GRAVITY (Nowak et al 2020;Wang et al 2021), and RVs of companions from instruments such as the Keck Planet Imager and Characterizer (Mawet et al 2016) will be especially fruitful to build a larger sample of systems well-characterized spin-orbit constraints. In addition, spectro-interferometry capable of spatially resolving offsets from stellar rotation will enable true obliquity measurements for imaged planets orbiting early-type stars (Kraus et al 2022).…”

Section: Discussionmentioning

confidence: 99%

“…Two exceptions are made to this spectral type cut. HIP 21152 is an F5 member of the Hyades with a recently discovered brown dwarf companion (Bonavita et al 2022;Franson et al 2023;Kuzuhara et al 2022). Its light-curve periodogram shows significant peaks that are not integer harmonics of its strongest signal, and are therefore not likely to be caused by rotational modulation.…”

Section: Sample Of Substellar Hostsmentioning

confidence: 99%

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

Bowler

Tran

Zhang

et al. 2023

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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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“…This technique leverages the precision of the HIPPARCOS (ESA 1997;van Leeuwen 2007) and Gaia (Gaia Collaboration 2021) astrometric catalogs to detect the small deviations of the position and motion of the star through space as it orbits the barycenter of the system, caused by the gravitational influence of an orbiting companion. Previous detections using this technique have been limited to companions within the stellar or brown dwarf regime (e.g., Currie et al 2020;Chilcote et al 2021;Kuzuhara et al 2022;Franson et al 2023b); however, this technique is sensitive enough to detect the signal induced by a planetary mass companion around the closest, youngest stars.…”

Section: Targeted Imaging Searchesmentioning

confidence: 99%

Direct imaging discovery of a super-Jovian around the young Sun-like star AF Leporis

Rosa

Nielsen

Wahhaj

et al. 2023

A&A

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Context. Expanding the sample of directly imaged companions to nearby, young stars that are amenable to detailed astrometric and spectroscopic studies is critical for the continued development and validation of theories of their evolution and atmospheric processes. Aims. The recent release of the Gaia astrometric catalog allows us to efficiently search for these elusive companions by targeting those stars that exhibit the astrometric reflex motion induced by an orbiting companion. The nearby (27 pc), young (24 Myr) star AF Leporis (AF Lep) was targeted because of its significant astrometric acceleration measured between the HIPPARCOS and Gaia astrometric catalogs, consistent with a wide-orbit planetary-mass companion detectable with high-contrast imaging. Methods. We used the SPHERE instrument on the VLT to search for faint substellar companions in the immediate vicinity of AF Lep. We used observations of a nearby star interleaved with those of AF Lep to efficiently subtract the residual point spread function. This provided sensitivity to faint planetary-mass companions within 1″ (~30 au) of the star. Results. We detected the companion AF Lep b at a separation of 339 mas (9 au) from the host star, at almost the exact location predicted by the astrometric acceleration, and within the inner edge of its unresolved debris disk. The measured K-band contrast and the age of the star yield a model-dependent mass of between 4 and 6 MJup, consistent with the mass derived from an orbital fit to the absolute and relative astrometry of 4.3−1.2+2.9 MJup. The near-infrared spectral energy distribution of the planet is consistent with an object at the L−T spectral type transition, but under-luminous with respect to field-gravity objects. Conclusions. AF Lep b joins a growing number of substellar companions imaged around stars in the young β Pictoris moving group. With a mass of between 3 and 7 MJup, it occupies a gap in this isochronal sequence between hotter, more massive companions, such as PZ Tel B and β Pic b, and the cooler 51 Eri b, which is sufficiently cool for methane to form within its photosphere. Lying at the transition between these two classes of objects, AF Lep b will undoubtedly become a benchmark for studies of atmospheric composition and processes, as well as an anchor for models of the formation and evolution of substellar and planetary-mass companions.

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Astrometric Accelerations as Dynamical Beacons: Discovery and Characterization of HIP 21152 B, the First T-dwarf Companion in the Hyades*

Cited by 23 publications

References 205 publications

Direct discovery of the inner exoplanet in the HD 206893 system

Direct discovery of the inner exoplanet in the HD 206893 system

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

Direct imaging discovery of a super-Jovian around the young Sun-like star AF Leporis

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