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Register a new userThe Magellan PFS Planet Search Program: Radial Velocity and Stellar Abundance Analyses of the 360 AU, Metal-Poor Binary Twins HD 133131A & B
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Johanna K. Teske
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We present a new precision radial velocity (RV) dataset that reveals multiple planets orbiting the stars in the $sim$360 AU, G2$+$G2 twin binary HD 133131AB. Our 6 years of high-resolution echelle observations from MIKE and 5 years from PFS on the Magellan telescopes indicate the presence of two eccentric planets around HD 133131A with minimum masses of 1.43$pm$0.03 and 0.63$pm$0.15 $mathcal{M}_{rm J}$ at 1.44$pm$0.005 and 4.79$pm$0.92 AU, respectively. Additional PFS observations of HD 133131B spanning 5 years indicate the presence of one eccentric planet of minimum mass 2.50$pm$0.05 $mathcal{M}_{rm J}$ at 6.40$pm$0.59 AU, making it one of the longest period planets detected with RV to date. These planets are the first to be reported primarily based on data taken with PFS on Magellan, demonstrating the instruments precision and the advantage of long-baseline RV observations. We perform a differential analysis between the Sun and each star, and between the stars themselves, to derive stellar parameters and measure a suite of 21 abundances across a wide range of condensation temperatures. The host stars are old (likely $sim$9.5 Gyr) and metal-poor ([Fe/H]$sim$-0.30), and we detect a $sim$0.03 dex depletion in refractory elements in HD 133131A versus B (with standard errors $sim$0.017). This detection and analysis adds to a small but growing sample of binary twin exoplanet host stars with precise abundances measured, and represents the most metal-poor and likely oldest in that sample. Overall, the planets around HD 133131A and B fall in an unexpected regime in planet mass-host star metallicity space and will serve as an important benchmark for the study of long period giant planets.
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We report the detection of a transiting, dense Neptune planet candidate orbiting the bright ($V=8.6$) K0.5V star HD 95338. Detection of the 55-day periodic signal comes from the analysis of precision radial velocities from the Planet Finder Spectrograph on the Magellan II Telescope. Follow-up observations with HARPS also confirm the presence of the periodic signal in the combined data. HD 95338 was also observed by the Transiting Exoplanet Survey Satellite ({it TESS}) where we identify a clear single transit in the photometry. A Markov Chain Monte Carlo period search on the velocities allows strong constraints on the expected transit time, matching well the epoch calculated from tess{} data, confirming both signals describe the same companion. A joint fit model yields an absolute mass of 42.44$^{+2.22}_{-2.08} M_{oplus}$ and a radius of 3.89$^{+0.19}_{-0.20}$ $R_{oplus}$ which translates to a density of 3.98$^{+0.62}_{-0.64}$ gcm, for the planet. Given the planet mass and radius, structure models suggest it is composed of a mixture of ammonia, water, and methane. HD 95338,b is one of the most dense Neptune planets yet detected, indicating a heavy element enrichment of $sim$90% ($sim38, M_{oplus}$). This system presents a unique opportunity for future follow-up observations that can further constrain structure models of cool gas giant planets.
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The star Kepler-1625 recently attracted considerable attention when an analysis of the stellar photometric time series from the Kepler mission was interpreted as showing evidence of a large exomoon around the transiting Jupiter-sized planet candidate Kepler-1625b. We aim to detect the radial velocity (RV) signal imposed by Kepler-1625b (and its putative moon) on the host star or, as the case may be, determine an upper limit on the mass of the transiting object. We took a total of 22 spectra of Kepler-1625 using CARMENES, 20 of which were useful. Observations were spread over a total of seven nights between October 2017 and October 2018, covering $125%$ of one full orbit of Kepler-1625b. We used the automatic Spectral Radial Velocity Analyser (SERVAL) pipeline to deduce the stellar RVs and uncertainties. Then we fitted the RV curve model of a single planet on a Keplerian orbit to the observed RVs using a $chi^2$ minimisation procedure. We derive upper limits on the mass of Kepler-1625b under the assumption of a single planet on a circular orbit. In this scenario, the $1,sigma$, $2,sigma$, and $3,sigma$ confidence upper limits for the mass of Kepler-1625b are $2.90,M_{rm J}$, $7.15,M_{rm J}$, and $11.60,M_{rm J}$, respectively. We present strong evidence for the planetary nature of Kepler-1625b, making it the 10th most long-period confirmed planet known today. Our data does not answer the question about a second, possibly more short-period planet that could be responsible for the observed transit timing variation of Kepler-1625b.
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We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The $J$, $H$, $K1$, and $K2$ spectra from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 K to 1800 K for effective temperature and from 3.0 to 5.0 for log($g$), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of sub-micron dust particles, similar to dereddening approaches applied to the peculiar red field L-dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L-dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from four years of astrometric monitoring is consistent with a $sim$30-year period, orbital inclination of 147$^{circ}$, and semimajor axis of 10 au, well within the estimated disk inner radius of $sim$50 au. As one of very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion-disk interaction and provide a benchmark for substellar and planetary atmospheric study.
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