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1. Coherent radio bursts from known M-dwarf planet-host YZ Ceti

   https://doi.org/10.1038/s41550-023-01914-0  

   Pineda, J. Sebastian ; Villadsen, Jackie ( April 2023 , Nature Astronomy)

   Observing magnetic star–planet interactions (SPIs) offers promise for determining the magnetic fields of exoplanets. Models of sub-Alfvénic SPIs predict that terrestrial planets in close-in orbits around M dwarfs can induce detectable stellar radio emission, manifesting as bursts of strongly polarized coherent radiation observable at specific planet orbital positions. Here we present 2–4 GHz detections of coherent radio bursts on the slowly rotating M dwarf YZ Ceti, which hosts a compact system of terrestrial planets, the innermost of which orbits with a two-day period. Two coherent bursts occur at similar orbital phases of YZ Ceti b, suggestive of an enhanced probability of bursts near that orbital phase. We model the system’s magnetospheric environment in the context of sub-Alfvénic SPIs and determine that YZ Ceti b can plausibly power the observed flux densities of the radio detections. However, we cannot rule out stellar magnetic activity without a well-characterized rate of non-planet-induced coherent radio bursts on slow rotators. YZ Ceti is therefore a candidate radio SPI system, with unique promise as a target for long-term monitoring.
2. Variability Timescales of Hα on Active Mid-to-late M dwarfs

   https://doi.org/10.3847/1538-4357/ac5738  

   Medina, Amber A. ; Charbonneau, David ; Winters, Jennifer G. ; Irwin, Jonathan ; Mink, Jessica ( April 2022 , The Astrophysical Journal)

   We present a study of the variation timescales of the chromospheric activity indicator Hαon a sample of 13 fully convective, active mid-to-late M stars with masses between 0.1 and 0.3 solar masses. Our goal was to determine the dominant variability timescale and, by inference, a possible mechanism responsible for the variation. We gathered 10 or more high-resolution spectra each of 10 stars using the TRES spectrograph at times chosen to span all phases of stellar rotation, as determined from photometric data from the MEarth Observatories. All stars varied in their Hαemission. For nine of these stars, we found no correlation between Hαand rotational phase, indicating that constant emission from fixed magnetic structures, such as star spots and plage, are unlikely to be the dominant source of Hαemission variability. In contrast, one star, G 7–34, shows a clear relationship between Hαand stellar rotational phase. Intriguingly, we found that this star is a member of the AB Doradus moving group and hence has the young age of 149 Myr. High-cadence spectroscopic observations of three additional stars revealed that they are variable on timescales ranging from 20 to 45 minutes, which we posit may be due to flaring behavior. For one star,more »GJ 1111, simultaneous TESS photometry and spectroscopic monitoring show an increase in Hαemission with increased photometric brightness. We conclude that low-energy flares are able to produce variation in Hαon the timescales we observe and thus may be the dominant source of Hαvariability on active fully convective M dwarfs.

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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. The Hot Neptune WASP-166 b with ESPRESSO – I. Refining the planetary architecture and stellar variability

   https://doi.org/10.1093/mnras/stac2178  

   Doyle, L. ; Cegla, H. M. ; Bryant, E. ; Bayliss, D. ; Lafarga, M. ; Anderson, D. R. ; Allart, R. ; Bourrier, V. ; Brogi, M. ; Buchschacher, N. ; et al ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   In this paper, we present high-resolution spectroscopic transit observations from ESPRESSO of the super-Neptune WASP-166 b. In addition to spectroscopic ESPRESSO data, we analyse photometric data from TESS of six WASP-166 b transits along with simultaneous NGTS observations of the ESPRESSO runs. These observations were used to fit for the planetary parameters as well as assessing the level of stellar activity (e.g. spot crossings, flares) present during the ESPRESSO observations. We utilize the reloaded Rossiter McLaughlin (RRM) technique to spatially resolve the stellar surface, characterizing the centre-to-limb convection-induced variations, and to refine the star–planet obliquity. We find WASP-166 b has a projected obliquity of $\lambda = -15.52^{+2.85}_{-2.76}\, ^{\circ }$ and vsin (i) = 4.97 ± 0.09 km s−1 which is consistent with the literature. We were able to characterize centre-to-limb convective variations as a result of granulation on the surface of the star on the order of a few km s−1 for the first time. We modelled the centre-to-limb convective variations using a linear, quadratic, and cubic model with the cubic being preferred. In addition, by modelling the differential rotation and centre-to-limb convective variations simultaneously, we were able to retrieve a potential antisolar differential rotational shear (α ∼ −0.5) and stellar inclination (i* either 42.03$^{+9.13}_{-9.60}\, ^{\circ }$ ormore »133.64$^{+8.42}_{-7.98}\, ^{\circ }$ if the star is pointing towards or away from us). Finally, we investigate how the shape of the cross-correlation functions change as a function of limb angle and compare our results to magnetohydrodynamic simulations.

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5. Surveying nearby brown dwarfs with HGCA: direct imaging discovery of a faint, high-mass brown dwarf orbiting HD 176535 A

   https://doi.org/10.1093/mnras/stad1315  

   Li, Yiting ; Brandt, Timothy D. ; Brandt, G. Mirek ; An, Qier ; Franson, Kyle ; Dupuy, Trent J. ; Chen, Minghan ; Bowens-Rubin, Rachel ; Lewis, Briley L. ; Bowler, Brendan P. ; et al ( May 2023 , Monthly Notices of the Royal Astronomical Society)

   Brown dwarfs with well-measured masses, ages, and luminosities provide direct benchmark tests of substellar formation and evolutionary models. We report the first results from a direct imaging survey aiming to find and characterize substellar companions to nearby accelerating stars with the assistance of the Hipparcos–Gaia Catalog of Accelerations (HGCA). In this paper, we present a joint high-contrast imaging and astrometric discovery of a substellar companion to HD 176535 A, a K3.5V main-sequence star aged approximately $3.59_{-1.15}^{+0.87}$ Gyr at a distance of 36.99 ± 0.03 pc. In advance of our high-contrast imaging observations, we combined precision High Accuracy Radial velocity Planet Searcher (HARPS) Radial Velocities (RVs) and HGCA astrometry to predict the potential companion’s location and mass. We thereafter acquired two nights of KeckAO/NIRC2 direct imaging observations in the L′ band, which revealed a companion with a contrast of $\Delta L^{\prime }_p = 9.20\pm 0.06$ mag at a projected separation of ≈0.35 arcsec (≈13 au) from the host star. We revise our orbital fit by incorporating our dual-epoch relative astrometry using the open-source Markov chain Monte Carlo orbit fitting code orvara. We obtain a dynamical mass of $65.9_{-1.7}^{+2.0} M_{\rm Jup}$ that places HD 176535 B firmly in the brown dwarf regime. HD 176535 Bmore »is a new benchmark dwarf useful for constraining the evolutionary and atmospheric models of high-mass brown dwarfs. We found a luminosity of $\rm log(\mathit{ L}_{bol}/L_{\odot }) = -5.26\pm 0.07$ and a model-dependent effective temperature of 980 ± 35 K for HD 176535 B. We infer HD 176535 B to be a T dwarf from its mass, age, and luminosity. Our dynamical mass suggests that some substellar evolutionary models may be underestimating luminosity for high-mass T dwarfs. Given its angular separation and luminosity, HD 176535 B would make a promising candidate for Aperture Masking Interferometry with JWST and GRAVITY/Keck Planet Imager and Characterizer, and further spectroscopic characterization with instruments like the CHARIS/SCExAO/Subaru integral field spectrograph.

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