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  1. ABSTRACT

    We explore the prospects for Twinkle to determine the atmospheric composition of the nearby terrestrial-like planet LTT 1445 Ab, including the possibility of detecting the potential biosignature ammonia (NH3). At a distance of 6.9 pc, this system is the second closest known transiting system and will be observed through transmission spectroscopy with the upcoming Twinkle mission. Although LTT 1445 Ab has been suggested to be a candidate for a Hycean world, constraints on the interior composition based on its mass and radius suggests that the planet lacks a substantial water layer, and thus the proposed Hycean scenario is disfavoured. We use PETITRADTRANS and a Twinkle simulator to simulate transmission spectra for the more likely scenario of a cold Haber world for which NH3 is considered to be a biosignature. We study the detectability under different scenarios: varying hydrogen fraction, concentration of ammonia, and cloud coverage. We find that ammonia can be detected at an ∼3σ level for optimal (non-cloudy) conditions with 25 transits and a volume mixing ration of 4.0 ppm of NH3. We provide examples of retrieval analysis to constrain potential NH3 and H2O in the atmosphere. Our study illustrates the potential of Twinkle to characterize atmospheres of potentially habitable exoplanets.

     
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  2. ABSTRACT

    We analyse high-cadence data from the Transiting Exoplanet Survey Satellite (TESS) of the ambiguous nuclear transient (ANT) ASASSN-18el. The optical changing-look phenomenon in ASASSN-18el has been argued to be due to either a drastic change in the accretion rate of the existing active galactic nucleus (AGN) or the result of a tidal disruption event (TDE). Throughout the TESS observations, short-time-scale stochastic variability is seen, consistent with an AGN. We are able to fit the TESS light curve with a damped-random-walk (DRW) model and recover a rest-frame variability amplitude of $\hat{\sigma } = 0.93 \pm 0.02$ mJy and a rest-frame time-scale of $\tau _{DRW} = 20^{+15}_{-6}$ d. We find that the estimated τDRW for ASASSN-18el is broadly consistent with an apparent relationship between the DRW time-scale and central supermassive black hole mass. The large-amplitude stochastic variability of ASASSN-18el, particularly during late stages of the flare, suggests that the origin of this ANT is likely due to extreme AGN activity rather than a TDE.

     
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  3. Abstract The recent discoveries of WD J091405.30+191412.25 (WD J0914 hereafter), a white dwarf (WD) likely accreting material from an ice-giant planet, and WD 1856+534 b (WD 1856 b hereafter), a Jupiter-sized planet transiting a WD, are the first direct evidence of giant planets orbiting WDs. However, for both systems, the observations indicate that the planets’ current orbital distances would have put them inside the stellar envelope during the red-giant phase, implying that the planets must have migrated to their current orbits after their host stars became WDs. Furthermore, WD J0914 is a very hot WD with a short cooling time that indicates a fast migration mechanism. Here, we demonstrate that the Eccentric Kozai–Lidov Mechanism, combined with stellar evolution and tidal effects, can naturally produce the observed orbital configurations, assuming that the WDs have distant stellar companions. Indeed, WD 1856 is part of a stellar triple system, being a distant companion to a stellar binary. We provide constraints for the orbital and physical characteristics for the potential stellar companion of WD J0914 and determine the initial orbital parameters of the WD 1856 system. 
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  6. Abstract We report the analysis of microlensing event OGLE-2017-BLG-1038, observed by the Optical Gravitational Lensing Experiment, Korean Microlensing Telescope Network, and Spitzer telescopes. The event is caused by a giant source star in the Galactic Bulge passing over a large resonant binary-lens caustic. The availability of space-based data allows the full set of physical parameters to be calculated. However, there exists an eightfold degeneracy in the parallax measurement. The four best solutions correspond to very-low-mass binaries near ( M 1 = 170 − 50 + 40 M J and M 2 = 110 − 30 + 20 M J ), or well below ( M 1 = 22.5 − 0.4 + 0.7 M J and M 2 = 13.3 − 0.3 + 0.4 M J ) the boundary between stars and brown dwarfs. A conventional analysis, with scaled uncertainties for Spitzer data, implies a very-low-mass brown-dwarf binary lens at a distance of 2 kpc. Compensating for systematic Spitzer errors using a Gaussian process model suggests that a higher mass M-dwarf binary at 6 kpc is equally likely. A Bayesian comparison based on a galactic model favors the larger-mass solutions. We demonstrate how this degeneracy can be resolved within the next 10 years through infrared adaptive-optics imaging with a 40 m class telescope. 
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  8. We complete the analysis of all 2018 prime-field microlensing planets identified by the Korea Microlensing Telescope Network (KMTNet) Anomaly Finder. Among the ten previously unpublished events with clear planetary solutions, eight are either unambiguously planetary or are very likely to be planetary in nature: OGLE-2018-BLG-1126, KMT-2018-BLG-2004, OGLE-2018-BLG-1647, OGLE-2018-BLG-1367, OGLE-2018-BLG-1544, OGLE-2018-BLG-0932, OGLE-2018-BLG-1212, and KMT-2018-BLG-2718. Combined with the four previously published new Anomaly Finder events and 12 previously published (or in preparation) planets that were discovered by eye, this makes a total of 24 2018 prime-field planets discovered or recovered by Anomaly Finder. Together with a paper in preparation on 2018 subprime planets, this work lays the basis for the first statistical analysis of the planet mass-ratio function based on planets identified in KMTNet data. By systematically applying the heuristic analysis to each event, we identified the small modification in their formalism that is needed to unify the so-called close-wide and inner-outer degeneracies. 
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