We present a detailed analysis of HARPS-N radial velocity observations of K2-100, a young and active star in the Praesepe cluster, which hosts a transiting planet with a period of 1.7 days. We model the activity-induced radial velocity variations of the host star with a multi-dimensional Gaussian Process framework and detect a planetary signal of 10.6 ± 3.0 m s −1 which matches the transit ephemeris, and translates to a planet mass of 21.8 ± 6.2 M ⊕ . We perform a suite of validation tests to confirm that our detected signal is genuine. This is the first mass measurement for a transiting planet in a young open cluster. The relatively low density of the planet, 2.04 +0.66 −0.61 g cm −3 , implies that K2-100b retains a significant volatile envelope. We estimate that the planet is losing its atmosphere at a rate of 10 11 − 10 12 g s −1 due to the high level of radiation it receives from its host star.

We report the development of a 4-color simultaneous camera for the 1.52 m Telescopio Carlos Sánchez (TCS) in the Teide Observatory, Canaries, Spain. The new instrument, named MuSCAT2, has a capability of 4-color simultaneous imaging in g (400-550 nm), r (550-700 nm), i (700-820 nm), and z s (820-920 nm) bands. MuSCAT2 equips four 1024×1024 pixel CCDs, having a field of view of 7.4×7.4 arcmin 2 with a pixel scale of 0.44 arcsec per pixel. The principal purpose of MuSCAT2 is to perform high-precision multi-color exoplanet transit photometry. We have demonstrated photometric precisions of 0.057%, 0.050%, 0.060%, and 0.076% as root-mean-square residuals of 60 s binning in g, r, i and z s bands, respectively, for a G0 V star WASP-12 (V = 11.57 ± 0.16). MuSCAT2 has started science operations since January 2018, with over 250 telescope nights per year. MuSCAT2 is expected to become a reference tool for exoplanet transit observations, and will substantially contribute to the follow-up of the TESS and PLATO space missions.

We present near-IR imaging polarimetry of five classical FU Ori-type objects (FU Ori, V1057 Cyg, V1515 Cyg, V1735 Cyg, Z CMa) with a ∼0. 1 resolution observed using HiCIAO+AO188 at Subaru Telescope. We observed scattered light associated with circumstellar dust around four of them (i.e., all but V1515 Cyg). Their polarized intensity distribution shows a variety of morphologies with arms, tails or streams, spikes and fragmented distributions, many of which were reported in our previous paper. The morphologies of these reflection nebulae significantly differ from many other normal young stellar objects (Class I-II objects). These structures are attributed to gravitationally unstable disks, trails of clump ejections, dust blown by a wind or a jet, and a stellar companion. We can consistently explain our results with the scenario that their accretion outbursts (FUor outbursts) are triggered by gravitationally fragmenting disks, and with the hypothesis that many low-mass young stellar objects experience such outbursts.

We report the discovery of TOI 837b and its validation as a transiting planet. We characterize the system using data from the NASA Transiting Exoplanet Survey Satellite mission, the ESA Gaia mission, ground-based photometry from El Sauce and ASTEP400, and spectroscopy from CHIRON, FEROS, and Veloce. We find that TOI 837 is a T=9.9 mag G0/F9 dwarf in the southern open cluster IC 2602. The star and planet are therefore -+ Unified Astronomy Thesaurus concepts: Exoplanets (498); Transits (1711); Exoplanet evolution (491); Stellar ages (1581); Young star clusters (1833)

Context. We report the discovery of TOI 263.01 (TIC 120916706), a transiting substellar object (R = 0.87 R Jup ) orbiting a faint M3.5 V dwarf (V = 18.97) on a 0.56 d orbit.Aims. We set out to determine the nature of the TESS planet candidate TOI 263.01 using ground-based multicolour transit photometry. The host star is faint, which makes RV confirmation challenging, but the large transit depth makes the candidate suitable for validation through multicolour photometry.Methods. Our analysis combines three transits observed simultaneously in r , i , and z s bands using the MuSCAT2 multicolour imager, three LCOGT-observed transit light curves in g , r , and i bands, a TESS light curve from Sector 3, and a low-resolution spectrum for stellar characterisation observed with the ALFOSC spectrograph. We model the light curves with PyTransit using a transit model that includes a physics-based light contamination component that allows us to estimate the contamination from unresolved sources from the multicolour photometry. This allows us to derive the true planet-star radius ratio marginalised over the contamination allowed by the photometry, and, combined with the stellar radius, gives us a reliable estimate of the object's absolute radius. Results. The ground-based photometry strongly excludes contamination from unresolved sources with a significant colour difference to TOI 263. Further, contamination from sources of same stellar type as the host is constrained to levels where the true radius ratio posterior has a median of 0.217 and a 99 percentile of 0.286. The median and maximum radius ratios correspond to absolute planet radii of 0.87 and 1.41 R Jup , respectively, which confirms the substellar nature of the planet candidate. The object is either a giant planet or a brown dwarf (BD) located deep inside the socalled "brown dwarf desert". Both possibilities offer a challenge to current planet/BD formation models and makes TOI 263.01 an object deserving of in-depth follow-up studies.