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

   https://doi.org/10.3847/1538-3881/aca408  

   Franson, Kyle ; Bowler, Brendan P. ; Bonavita, Mariangela ; Brandt, Timothy D. ; Chen, Minghan ; Samland, Matthias ; Zhang, Zhoujian ; Lueber, Anna ; Heng, Kevin ; Kitzmann, Daniel ; et al ( January 2023 , The Astronomical Journal)

   Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of${24}_{-4}^{+6}{M}_{\mathrm{Jup}}$, which is 1–2σlower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or 7.9 × 10⁴au) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008)more »atmospheric models and a suite of retrievals. The best-fit grid-based models havef_sed= 2, indicating the presence of clouds,T_eff= 1400 K, and$\log g=4.5\mathrm{dex}$. These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses.

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2. The McDonald Accelerating Stars Survey: Architecture of the Ancient Five-planet Host System Kepler-444

   https://doi.org/10.3847/1538-3881/aca88c  

   Zhang, Zhoujian ; Bowler, Brendan P. ; Dupuy, Trent J. ; Brandt, Timothy D. ; Brandt, G. Mirek ; Cochran, William D. ; Endl, Michael ; MacQueen, Phillip J. ; Kratter, Kaitlin M. ; Isaacson, Howard T. ; et al ( January 2023 , The Astronomical Journal)

   We present the latest and most precise characterization of the architecture for the ancient (≈11 Gyr) Kepler-444 system, which is composed of a K0 primary star (Kepler-444 A) hosting five transiting planets and a tight M-type spectroscopic binary (Kepler-444 BC) with an A–BC projected separation of 66 au. We have measured the system’s relative astrometry using the adaptive optics imaging from Keck/NIRC2 and Kepler-444 A’s radial velocities from the Hobby-Eberly Telescope and reanalyzed relative radial velocities between BC and A from Keck/HIRES. We also include the Hipparcos-Gaia astrometric acceleration and all published astrometry and radial velocities in an updated orbit analysis of BC’s barycenter. These data greatly extend the time baseline of the monitoring and lead to significant updates to BC’s barycentric orbit compared to previous work, including a larger semimajor axis ($a={52.2}_{-2.7}^{+3.3}$au), a smaller eccentricity (e= 0.55 ± 0.05), and a more precise inclination ($i=85.°4_{-0.°4}^{+0.°3}$). We have also derived the first dynamical masses of B and C components. Our results suggest that Kepler-444 A’s protoplanetary disk was likely truncated by BC to a radius of ≈8 au, which resolves the previously noticed tension between Kepler-444 A’s disk mass and planet masses. Kepler-444more »BC’s barycentric orbit is likely aligned with those of A’s five planets, which might be primordial or a consequence of dynamical evolution. The Kepler-444 system demonstrates that compact multiplanet systems residing in hierarchical stellar triples can form at early epochs of the universe and survive their secular evolution throughout cosmic time.

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3. Dynamical Mass of the Exoplanet Host Star HR 8799

   https://doi.org/10.3847/1538-3881/ac3bb5  

   Sepulveda, Aldo G. ; Bowler, Brendan P. ( January 2022 , The Astronomical Journal)

   HR 8799 is a young A5/F0 star hosting four directly imaged giant planets at wide separations (∼16–78 au), which are undergoing orbital motion and have been continuously monitored with adaptive optics imaging since their discovery over a decade ago. We present a dynamical mass of HR 8799 using 130 epochs of relative astrometry of its planets, which include both published measurements and new medium-band 3.1μm observations that we acquired with NIRC2 at Keck Observatory. For the purpose of measuring the host-star mass, each orbiting planet is treated as a massless particle and is fit with a Keplerian orbit using Markov chain Monte Carlo. We then use a Bayesian framework to combine each independent total mass measurement into a cumulative dynamical mass using all four planets. The dynamical mass of HR 8799 is${1.47}_{-0.17}^{+0.12}$M_⊙assuming a uniform stellar mass prior, or${1.46}_{-0.15}^{+0.11}$M_⊙with a weakly informative prior based on spectroscopy. There is a strong covariance between the planets’ eccentricities and the total system mass; when the constraint is limited to low-eccentricity solutions ofe< 0.1, which are motivated by dynamical stability, our mass measurement improves to${1.43}_{-0.07}^{+0.06}$M_⊙. Our dynamical mass and other fundamental measured parameters of HRmore »8799 together with Modules for Experiments in Stellar Astrophysics Isochrones and Stellar Tracks grids yields a bulk metallicity most consistent with [Fe/H] ∼ −0.25–0.00 dex and an age of 10–23 Myr for the system. This implies hot-start masses of 2.7–4.9M_Jupfor HR 8799 b and 4.1–7.0M_Jupfor HR 8799 c, d, and e, assuming they formed at the same time as the host star.

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4. Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ac96ed  

   Chiti, Anirudh ; Simon, Joshua D. ; Frebel, Anna ; Pace, Andrew B. ; Ji, Alexander P. ; Li, Ting S. ( November 2022 , The Astrophysical Journal)

   We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of$M_{1/2}(r_h)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V=${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in linemore »with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

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5. The MASSIVE Survey. XVII. A Triaxial Orbit-based Determination of the Black Hole Mass and Intrinsic Shape of Elliptical Galaxy NGC 2693

   https://doi.org/10.3847/1538-4357/ac58fd  

   Pilawa, Jacob D. ; Liepold, Christopher M. ; Delgado Andrade, Silvana C. ; Walsh, Jonelle L. ; Ma, Chung-Pei ; Quenneville, Matthew E. ; Greene, Jenny E. ; Blakeslee, John P. ( April 2022 , The Astrophysical Journal)

   We present a stellar dynamical mass measurement of a newly detected supermassive black hole (SMBH) at the center of the fast-rotating, massive elliptical galaxy NGC 2693 as part of the MASSIVE survey. We combine high signal-to-noise ratio integral field spectroscopy (IFS) from the Gemini Multi-Object Spectrograph with wide-field data from the Mitchell Spectrograph at McDonald Observatory to extract and model stellar kinematics of NGC 2693 from the central ∼150 pc out to ∼2.5 effective radii. Observations from Hubble Space Telescope WFC3 are used to determine the stellar light distribution. We perform fully triaxial Schwarzschild orbit modeling using the latest TriOS code and a Bayesian search in 6D galaxy model parameter space to determine NGC 2693's SMBH mass (M_BH), stellar mass-to-light ratio, dark matter content, and intrinsic shape. We find$M_{\mathrm{BH}}=\left(1.7\pm 0.4\right)\times {10}^9{M}_{\odot }$and a triaxial intrinsic shape with axis ratiosp=b/a= 0.902 ± 0.009 and$q=c/a={0.721}_{-0.010}^{+0.011}$, triaxiality parameterT= 0.39 ± 0.04. In comparison, the best-fit orbit model in the axisymmetric limit and (cylindrical) Jeans anisotropic model of NGC 2693 prefer$M_{\mathrm{BH}}=\left(2.4\pm 0.6\right)\times {10}^9{M}_{\odot }$and$M_{\mathrm{BH}}=\left(2.9\pm 0.3\right)\times {10}^9{M}_{\odot }$, respectively. Neither model can account for the non-axisymmetric stellar velocity features present inmore »the IFS data.

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