A CHEOPS Search for Massive, Long-period Companions to the Warm Jupiter K2-139 b

K2-139 b is a warm Jupiter with an orbital period of 28.4 days, but only three transits of this system have previously been observed–in the long-cadence mode of K2–limiting the precision with which the orbital period can be determined and future transits predicted. We report photometric observations of four transits of K2-139 b with ESA's CHaracterising ExOPlanet Satellite (CHEOPS), conducted with the goal of measuring the orbital obliquity via spot-crossing events. We jointly fit these CHEOPS data alongside the three previously-published transits from the K2 mission, considerably increasing the precision of the ephemeris of K2-139 b. The transit times for this system can now be predicted for the next decade with a 1σ precision less than 10 minutes, compared to over one hour previously, allowing the efficient scheduling of observations with Ariel. We detect no significant deviation from a linear ephemeris, allowing us to exclude the presence of a massive outer planet orbiting with a period less than 150 days, or a brown dwarf with a period less than one year. We also determine the scaled semimajor axis, the impact parameter, and the stellar limb darkening with improved precision. This is driven by the shorter cadence of the CHEOPS observations compared to that of K2, and validates the subexposure technique used for analyzing long-cadence photometry. Finally, we note that the stellar spot configuration has changed from the epoch of the K2 observations; unlike the K2 transits, we detect no evidence of spot-crossing events in the CHEOPS data

Probing the atmosphere of a sub-Jovian planet orbiting a cool dwarf

We derive the 0.01 $\mu$m binned transmission spectrum, between 0.74 and 1.0 $\mu$m, of WASP-80b from low resolution spectra obtained with the FORS2 instrument attached to ESO's Very Large Telescope. The combination of the fact that WASP-80 is an active star, together with instrumental and telluric factors, introduces correlated noise in the observed transit light curves, which we treat quantitatively using Gaussian Processes. Comparison of our results together with those from previous studies, to theoretically calculated models reveals an equilibrium temperature in agreement with the previously measured value of 825K, and a sub-solar metallicity, as well as an atmosphere depleted of molecular species with absorption bands in the IR ($\gg 5\sigma$). Our transmission spectrum alone shows evidence for additional absorption from the potassium core and wing, whereby its presence is detected from analysis of narrow 0.003 $\mu$m bin light curves ($\gg 5\sigma$). Further observations with visible and near-UV filters will be required to expand this spectrum and provide more in-depth knowledge of the atmosphere. These detections are only made possible through an instrument-dependent baseline model and a careful analysis of systematics in the data.Comment: 13 pages, 11 figures, 3 tables. Accepted for publication in MNRA

Power of wavelets in analyses of transit and phase curves in the presence of stellar variability and instrumental noise. I. Method and validation

Context. Stellar photometric variability and instrumental effects, such as cosmic ray hits, data discontinuities, data leaks, instrument aging, and so on, lead to difficulties in the characterisation of exoplanets. Therefore, they can impact the accuracy and precision of the modelling and the detectability of their transits, occultations, and phase curves. Aims. This paper is aimed at improving the transit, occultation, and phase-curve modelling in the presence of strong stellar variability and instrumental noise. To this end, we invoke the wavelet formulation. Methods. We explored the capabilities of the software package Transit and Light Curve Modeller (TLCM). It is able to perform (1) a joint radial-velocity and light-curve fit or (2) a light curve-only fit. It models the transit, occultation, beaming, ellipsoidal, and reflection effects in the light curves (including the gravity-darkening effect). Here, the red noise, stellar variability, and instrumental effects were modelled via wavelets. The wavelet fit was constrained by prescribing that the final white noise level must be equal to the average of the uncertainties of the photometric data points. This helps to avoid overfitting and regularises the noise model. The approach was tested by injecting synthetic light curves into short-cadence Kepler data and modelling them. Results. The method performs well over a certain signal-to-noise (S/N) ratio. We provide limits in terms of the S/N for every studied system parameter that is needed for accurate parameter retrieval. The wavelet approach is able to manage and remove the impact of data discontinuities, cosmic ray events, and long-term stellar variability and instrument ageing, as well as short-term stellar variability, pulsation, and flares (among others). Conclusions. We conclude that precise light-curve models combined with the wavelet method and with well-prescribed constraints on the white noise are able to retrieve the planetary system parameters, even in the presence of strong stellar variability and instrumental noise, including data discontinuities

Constraining the reflective properties of WASP-178 b using CHEOPS photometry

Context. Multiwavelength photometry of the secondary eclipses of extrasolar planets is able to disentangle the reflected and thermally emitted light radiated from the planetary dayside. Based on this, we can measure the planetary geometric albedo Ag, which is an indicator of the presence of clouds in the atmosphere, and the recirculation efficiency ϵ, which quantifies the energy transport within the atmosphere. Aims. We measure Ag and ϵ for the planet WASP-178 b, a highly irradiated giant planet with an estimated equilibrium temperature of 2450 K. Methods. We analyzed archival spectra and the light curves collected by CHEOPS and TESS to characterize the host WASP-178, refine the ephemeris of the system, and measure the eclipse depth in the passbands of the two telescopes. Results. We measured a marginally significant eclipse depth of 70 ± 40 ppm in the TESS passband, and a statistically significant depth of 70 ± 20 ppm in the CHEOPS passband. Conclusions. Combining the eclipse-depth measurement in the CHEOPS (λeff = 6300 Å) and TESS (λeff = 8000 Å) passbands, we constrained the dayside brightness temperature of WASP-178 b in the 2250–2800 K interval. The geometric albedo 0.10.7 makes WASP-178 b an interesting laboratory for testing the current heat-recirculation models

Spectroscopic follow-up of TESS candidates with KESPRINT 1.5 - 3-m telescopes network

We report on the spectroscopic follow-up of TESS planetary candidates with a network of 2-3 meter telescopes located in Ondrejov, CZ, Tautenburg, DE, McDonald observatory, US and SMARTS telescope, CL, which use spectrographs with high resolving power. We coordinate our observing campaigns within the KESPRINT consortium and we significantly contribute to validation and characterization of mostly gas giant planets but not only. We briefly present involved observatories and their current observing campaigns

A transiting M-dwarf showing beaming effect in the field of Ruprecht 147

We report the discovery and characterization of an eclipsing M5Vdwarf star, orbiting a slightly evolved F7V main sequence star. In contrast to previous claims in the literature, we confirm that the system does not belong to the galactic open cluster Ruprecht 147. We determine its fundamental parameters combining K2 time-series data with spectroscopic observations from the McDonald Observatory, FIES@NOT, and HIRES@KECK. The very precise photometric data from the K2 mission allows us to measure variations caused by the beaming effect (relativistic doppler boosting), ellipsoidal variation, reflection, and the secondary eclipse. We determined the radial velocity using spectroscopic observations and compare it to the radial velocity determined from the beaming effect observed in the photometric data. The M5V star has a radius of 0.200+0.007−0.008  R⊙ and a mass of 0.187+0.012−0.013  M⊙. The primary star has a radius of 1.518+0.038−0.049 R⊙ and a mass of 1.008+0.081−0.097 M⊙. The orbital period is 5.441995 ± 0.000007 d. The system is one of the few eclipsing systems with observed beaming effect and spectroscopic radial velocity measurements and it can be used as a test case for the modelling of the beaming effect. Current and forthcoming space missions such as TESS and PLATO might benefit from the analysis of the beaming effect to estimate the mass of transiting companions without the need for radial velocity follow up observations, provided that the systematic sources of noise affecting this method are well understood.Funding for the K2 mission is provided by the NASA Science Mission directorate. HJD acknowledges support by grant ESP2015-65712-C5-4-R of the Spanish Secretary of State for R&D&i (MINECO). ME and WDC were supported by NASA grant NNX16AE70G to The University of Texas at Austin

Evidence of apsidal motion and a possible co-moving companion star detected in the WASP-19 system

Context. Love numbers measure the reaction of a celestial body to perturbing forces, such as the centrifugal force caused by rotation, or tidal forces resulting from the interaction with a companion body. These parameters are related to the interior density profile. The non-point mass nature of the host star and a planet orbiting around each other contributes to the periastron precession. The rate of this precession is characterized mainly by the second-order Love number, which offers an opportunity to determine its value. When it is known, the planetary interior structure can be studied with one additional constraint beyond the mass, radius, and orbital parameters. Aims. We aim to re-determine the orbital period, eccentricity, and argument of the periastron for WASP-19Ab, along with a study of its periastron precession rate. We calculated the planetary Love number from the observed periastron precession rate, based on the assumption of the stellar Love number from stellar evolutionary models. Methods. We collected all available radial velocity (RV) data, along with the transit and occultation times from the previous investigations of the system. We supplemented the data set with 19 new RV data points of the host star WASP-19A obtained by HARPS. Here, we summarize the technique for modeling the RV observations and the photometric transit timing variations (TTVs) to determine the rate of periastron precession in this system for the first time. Results. We excluded the presence of a second possible planet up to a period of ~4200 d and with a radial velocity amplitude bigger than ≃ 1 m s−1. We show that a constant period is not able to reproduce the observed radial velocities. We also investigated and excluded the possibility of tidal decay and long-term acceleration in the system. However, the inclusion of a small periastron precession term did indeed improve the quality of the fit. We measured the periastron precession rate to be 233−35+25″d−1. By assuming synchronous rotation for the planet, it indicates a k2 Love number of 0.20−0.03+0.02 for WASP-19Ab. Conclusions. The derived k2,p value of the planet has the same order of magnitude as the estimated fluid Love number of other Jupiter-sized exoplanets (WASP-18Ab, WASP-103b, and WASP-121b). A low value of k2,p indicates a higher concentration of mass toward the planetary nucleus

The Rapid Outbursting Star GM Cep: An EX-or in Tr 37?

We present optical, IR and millimeter observations of the solar-type star 13-277, also known as GM Cep, in the 4 Myr-old cluster Tr 37. GM Cep experiences rapid magnitude variations of more than 2 mag at optical wavelengths. We explore the causes of the variability, which seem to be dominated by strong increases in the accretion, being similar to EX-or episodes. The star shows high, variable accretion rates (up to ~10$^{-6}$ Msun/yr), signs of powerful winds, and it is a very fast rotator (Vsini~43 km/s). Its strong mid-IR excesses reveal a very flared disk and/or a remnant envelope, most likely out of hydrostatic equilibrium. The 1.3 millimeter fluxes suggest a relatively massive disk (Mdisk~0.1 Msun). Nevertheless, the millimeter mass is not enough to sustain increased accretion episodes over large timescales, unless the mass is underestimated due to significant grain growth. We finally explore the possibility of GM Cep having a binary companion, which could trigger disk instabilities producing the enhanced accretion episodes.Comment: 43 pages, including 10 figures, ApJ in pres

EPIC 219388192 b - an inhabitant of the brown dwarf desert in the Ruprecht 147 open cluster

We report the discovery of EPIC 219388192 b, a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147, the oldest nearby open cluster association, which was photometrically monitored by K2 during its Campaign 7. We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system. EPIC 219388192 b has a radius of $R_\mathrm{b}$=$0.937\pm0.042$~$\mathrm{R_{Jup}}$ and mass of $M_\mathrm{b}$=$36.50\pm0.09$~$\mathrm{M_{Jup}}$, yielding a mean density of $59.0\pm8.1$~$\mathrm{g\,cm^{-3}}$. The host star is nearly a Solar twin with mass $M_\star$=$0.99\pm0.05$~$\mathrm{M_{\odot}}$, radius $R_\star$=$1.01\pm0.04$~$\mathrm{R_{\odot}}$, effective temperature $\mathrm{T_{eff}}$=$5850\pm85$~K and iron abundance [Fe/H]=$0.03\pm0.08$~dex. Its age, spectroscopic distance, and reddening are consistent with those of Ruprecht-147, corroborating its cluster membership. EPIC 219388192 b is the first brown dwarf with precise determinations of mass, radius and age, and serves as benchmark for evolutionary models in the sub-stellar regime.Comment: 13 pages, 11 figures, 4 tables, submitted to AAS Journal