No Arabic abstract
We report the discovery of K2-287b, a Saturn mass planet orbiting a G-dwarf with a period of $P approx 15$ days. First uncovered as a candidate using K2 campaign 15 data, follow-up photometry and spectroscopy were used to determine a mass of $M_P = 0.317 pm 0.026$ $M_J$, radius $R_P = 0.833 pm 0.013$ $R_J$, period $P = 14.893291 pm 0.000025$ days and eccentricity $e = 0.476 pm 0.026$. The host star is a metal-rich $V=11.410 pm 0.129$ mag G dwarf for which we estimate a mass $M_* = 1.056$ $M_odot$, radius $R_* = 1.07 pm 0.01$ $R_odot$, metallicity [Fe/H] = $0.20 pm 0.05$ and $T_{eff} = 5673 pm 75$ K. This warm eccentric planet with a time-averaged equilibrium temperature of $T_{eq} approx 800$ K adds to the small sample of giant planets orbiting nearby stars whose structure is not expected to be affected by stellar irradiation. Follow-up studies on the K2-287 system could help in constraining theories of migration of planets in close-in orbits.
We report the discovery from K2 of a transiting planet in an 18.25-d, eccentric (0.19$pm$ 0.04) orbit around K2-99, an 11th magnitude subgiant in Virgo. We confirm the planetary nature of the companion with radial velocities, and determine that the star is a metal-rich ([Fe/H] = 0.20$pm$0.05) subgiant, with mass $1.60^{+0.14}_{-0.10}~M_odot$ and radius $3.1pm 0.1~R_odot$. The planet has a mass of $0.97pm0.09~M_{rm Jup}$ and a radius $1.29pm0.05~R_{rm Jup}$. A measured systemic radial acceleration of $-2.12pm0.04~{rm m s^{-1} d^{-1}}$ offers compelling evidence for the existence of a third body in the system, perhaps a brown dwarf orbiting with a period of several hundred days.
We report the discovery and characterization of HD 89345b (K2-234b; EPIC 248777106b), a Saturn-sized planet orbiting a slightly evolved star. HD 89345 is a bright star ($V = 9.3$ mag) observed by the K2 mission with one-minute time sampling. It exhibits solar-like oscillations. We conducted asteroseismology to determine the parameters of the star, finding the mass and radius to be $1.12^{+0.04}_{-0.01}~M_odot$ and $1.657^{+0.020}_{-0.004}~R_odot$, respectively. The star appears to have recently left the main sequence, based on the inferred age, $9.4^{+0.4}_{-1.3}~mathrm{Gyr}$, and the non-detection of mixed modes. The star hosts a warm Saturn ($P = 11.8$~days, $R_p = 6.86 pm 0.14~R_oplus$). Radial-velocity follow-up observations performed with the FIES, HARPS, and HARPS-N spectrographs show that the planet has a mass of $35.7 pm 3.3~M_oplus$. The data also show that the planets orbit is eccentric ($eapprox 0.2$). An investigation of the rotational splitting of the oscillation frequencies of the star yields no conclusive evidence on the stellar inclination angle. We further obtained Rossiter-McLaughlin observations, which result in a broad posterior of the stellar obliquity. The planet seems to conform to the same patterns that have been observed for other sub-Saturns regarding planet mass and multiplicity, orbital eccentricity, and stellar metallicity.
We report the first results from a search for transiting warm Jupiter exoplanets - gas giant planets receiving stellar irradiation below about $10^8$ erg s$^{-1}$ cm$^{-2}$, equivalent to orbital periods beyond about 10 days around Sun-like stars. We have discovered two transiting warm Jupiter exoplanets initially identified as transiting candidates in ${it K2}$ photometry. K2-114b has a mass of $1.85^{+0.23}_{-0.22} M_J$, a radius of $0.942^{+0.032}_{-0.020} R_J$, and an orbital period of 11.4 days. K2-115b has a mass of $0.84^{+0.18}_{-0.20} M_J$, a radius of $1.115^{+0.057}_{-0.061} R_J$, and an orbital period of 20.3 days. Both planets are among the longest period transiting gas giant planets with a measured mass, and they are orbiting relatively old host stars. Both planets are not inflated as their radii are consistent with theoretical expectations. Their position in the planet radius - stellar irradiation diagram is consistent with the scenario where the radius - irradiation correlation levels off below about 10$^8$ erg s$^{-1}$ cm$^{-2}$, suggesting that for warm Jupiters the stellar irradiation does not play a significant role in determining the planet radius. We also report our identification of another ${it K2}$ transiting warm Jupiter candidate, EPIC 212504617, as a false positive.
We report the discovery by the HATSouth survey of HATS-6b, an extrasolar planet transiting a V=15.2 mag, i=13.7 mag M1V star with a mass of 0.57 Msun and a radius of 0.57 Rsun. HATS-6b has a period of P = 3.3253 d, mass of Mp=0.32 Mjup, radius of Rp=1.00 Rjup, and zero-albedo equilibrium temperature of Teq=712.8+-5.1 K. HATS-6 is one of the lowest mass stars known to host a close-in gas giant planet, and its transits are among the deepest of any known transiting planet system. We discuss the follow-up opportunities afforded by this system, noting that despite the faintness of the host star, it is expected to have the highest K-band S/N transmission spectrum among known gas giant planets with Teq < 750 K. In order to characterize the star we present a new set of empirical relations between the density, radius, mass, bolometric magnitude, and V, J, H and K-band bolometric corrections for main sequence stars with M < 0.80 Msun, or spectral types later than K5. These relations are calibrated using eclipsing binary components as well as members of resolved binary systems. We account for intrinsic scatter in the relations in a self-consistent manner. We show that from the transit-based stellar density alone it is possible to measure the mass and radius of a ~0.6 Msun star to ~7% and ~2% precision, respectively. Incorporating additional information, such as the V-K color, or an absolute magnitude, allows the precision to be improved by up to a factor of two.
We present an independent discovery and detailed characterisation of K2-280b, a transiting low density warm sub-Saturn in a 19.9-day moderately eccentric orbit (e = 0.35_{-0.04}^{+0.05}) from K2 campaign 7. A joint analysis of high precision HARPS, HARPS-N, and FIES radial velocity measurements and K2 photometric data indicates that K2-280b has a radius of R_b = 7.50 +/- 0.44 R_Earth and a mass of M_b = 37.1 +/- 5.6 M_Earth, yielding a mean density of 0.48_{-0.10}^{+0.13} g/cm^3. The host star is a mildly evolved G7 star with an effective temperature of T_{eff} = 5500 +/- 100 K, a surface gravity of log(g) = 4.21 +/- 0.05 (cgs), and an iron abundance of [Fe/H] = 0.33 +/- 0.08 dex, and with an inferred mass of M_star = 1.03 +/- 0.03 M_sun and a radius of R_star = 1.28 +/- 0.07 R_sun. We discuss the importance of K2-280b for testing formation scenarios of sub-Saturn planets and the current sample of this intriguing group of planets that are absent in the Solar System.