A global analysis ofSpitzerand new HARPS data confirms the loneliness and metal-richness of GJ 436 b

Abstract

Context. GJ 436b is one of the few transiting warm Neptunes for which a detailed characterisation of the atmosphere is possible, whereas its non-negligible orbital eccentricity calls for further investigation. Independent analyses of several individual datasets obtained with Spitzer have led to contradicting results attributed to the different techniques used to treat the instrumental effects. Aims. We aim at investigating these previous controversial results and developing our knowledge of the system based on the full Spitzer photometry dataset combined with new Doppler measurements obtained with the HARPS spectrograph. We also want to search for additional planets. Methods. We optimise aperture photometry techniques and the photometric deconvolution algorithm DECPHOT to improve the data reduction of the Spitzer photometry spanning wavelengths from 3-24 µm. Adding the high-precision HARPS radial velocity data, we undertake a Bayesian global analysis of the system considering both instrumental and stellar effects on the flux variation.Results. We present a refined radius estimate of R P = 4.10 ± 0.16 R ⊕ , mass M P = 25.4 ± 2.1 M ⊕ , and eccentricity e = 0.162 ± 0.004 for GJ 436b. Our measured transit depths remain constant in time and wavelength, in disagreement with the results of previous studies. In addition, we find that the post-occultation flare-like structure at 3.6 µm that led to divergent results on the occultation depth measurement is spurious. We obtain occultation depths at 3.6, 5.8, and 8.0 µm that are shallower than in previous works, in particular at 3.6 µm. However, these depths still appear consistent with a metal-rich atmosphere depleted in methane and enhanced in CO/CO 2 , although perhaps less than previously thought. We could not detect a significant orbital modulation in the 8 µm phase curve. We find no evidence of a potential planetary companion, stellar activity, or a stellar spin-orbit misalignment. Conclusions. Recent theoretical models invoking high-metallicity atmospheres for warm Neptunes are a reasonable match to our results, but we encourage new modelling efforts based on our revised data. Future observations covering a wide wavelength range of GJ 436b and other Neptune-class exoplanets will further illuminate their atmosphere properties, whilst future accurate radial velocity measurements might explain the eccentricity.

Variability in the super-Earth 55 Cnc e

“…The number of grid points is determined at the beginning of the MCMC by ensuring that at least 5 valid photometric measurements are located in each mesh box. Similar to Lanotte et al (2014), we find that the PRF's FWHM along the x and y axes evolve with time and allows further improvement on the systematics correction. We thus combine the BLISS algorithm to a linear function of the FWHM along each axis.…”

“…Indeed, in our 2012 work, we had not yet noticed the strong dependency of the measured fluxes on the measured widths of the PRF in the x and/or y directions. The improvement brought by the addition of the PRF FWHM in the baseline model is described in detail in Lanotte et al (2014) for the case of GJ 436. In 2012, our modelling of the Spitzer systematics was based uniquely on polynomials of the measured x and y position of the PRF centre.…”

The HARPS-N Rocky Planet Search

“…Knutson et al 2008). The second had as variables the PSF widths in the x-and/or the y-direction, its inclusion in the baseline model strongly increasing the quality of the fit for Warm Spitzer photometry (see also Lanotte et al 2014). The third, optional, function was a polynomial of the logarithm of time + a slope to represent a sharp decrease of the detector response at the start of the AOR ("ramp" effect, Knutson et al 2008).…”

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b

“…The density and velocity structures of the gas in the exosphere depend on three free model parameters: the escape rate of neutral hydrogen (Ṁ H 0 in g s −1 , at the distance of the semi-major axis), the photoionization rate per atom (Γ ion in s −1 , at the distance of the semi-major axis), and the velocity of the planetary wind at the Roche lobe (v p wind , in km s −1 ). We accounted for the effect of the orbital eccentricity (e = 0.16, Lanotte et al 2014) that causes the escape rate and photoionization rate to vary in time with the inverse distance to the star squared. The effect of radiation pressure and photoionization on neutral hydrogen particles was calculated taking self-shielding within the exospheric cloud into account.…”