Context. Directly imaged planets and substellar companions are key targets for the characterization of self-luminous atmospheres. Their photometric appearance at 4-5 mu m is sensitive to the... Show moreContext. Directly imaged planets and substellar companions are key targets for the characterization of self-luminous atmospheres. Their photometric appearance at 4-5 mu m is sensitive to the chemical composition and cloud content of their atmosphere.Aims. We aim to systematically characterize the atmospheres of directly imaged low-mass companions at 4-5 mu m. We want to homogeneously process the data, provide robust flux measurements, and compile a photometric library at thermal wavelengths of these mostly young, low-gravity objects. In this way, we want to find trends related to their spectral type and surface gravity by comparing with isolated brown dwarfs and predictions from atmospheric models.Methods. We used the high-resolution, high-contrast capabilities of NACO at the Very Large Telescope (VLT) to directly image the companions of HIP 65426, PZ Tel, and HD 206893 in the NB4.05 and/or M filters. For the same targets, and additionally beta Pic, we also analyzed six archival VLT/NACO datasets which were taken with the NB3.74, L, NB4.05, and M ' filters. The data processing and photometric extraction of the companions was done with PynPoint while the species toolkit was used to further analyze and interpret the fluxes and colors.Results. We detect for the first time HIP 65426 b, PZ Tel B, and HD 206893 B in the NB4.05 filter, PZ Tel B and HD 206893 B in the M ' filter, and beta Pic b in the NB3.74 filter. We provide calibrated magnitudes and fluxes with a careful analysis of the error budget, both for the new and archival datasets. The L ' -NB4.05 and L ' -M ' colors of the studied sample are all red while the NB4.05-M ' color is blue for beta Pic b, gray for PZ Tel B, and red for HIP 65426 b and HD 206893 B (although typically with low significance). The absolute NB4.05 and M ' fluxes of our sample are all larger than those of field dwarfs with similar spectral types. Finally, the surface gravity of beta Pic b has been constrained tolog g = 4.17(-0.13)(+0.10) textual-formlogg=4.17-0.13+0.10dex from its photometry and dynamical mass.Conclusions. A red color at 3-4 mu m and a blue color at 4-5 mu m might be (partially) caused by H2O and CO absorption, respectively, which are expected to be the most dominant gaseous opacities in hot (T-eff greater than or similar to 1300 K) atmospheres. The red characteristics of beta Pic b, HIP 65426 b, and HD 206893 B at 3-5 mu m, as well as their higher fluxes in NB4.05 and M ' compared to field dwarfs, indicate that cloud densities are enhanced close to the photosphere as a result of their low surface gravity. Show less
Ultra-hot Jupiters offer interesting prospects for expanding our theories on dynamical evolution and the properties of extremely irradiated atmospheres. In this context, we present the analysis of... Show moreUltra-hot Jupiters offer interesting prospects for expanding our theories on dynamical evolution and the properties of extremely irradiated atmospheres. In this context, we present the analysis of new optical spectroscopy for the transiting ultra-hot Jupiter WASP-121b. We first refine the orbital properties of WASP-121b, which is on a nearly polar (obliquity ψ North=88.1±0.25◦ or ψ South=91.11±0.20◦ ) orbit, and exclude a high differential rotation for its fastrotating (P<1.13 days), highly inclined (i North ? =8.1+3.0 −2.6 ◦ or i South ? =171.9+2.5 −3.4 ◦ ) star. We then present a new method that exploits the reloaded Rossiter-McLaughlin technique to separate the contribution of the planetary atmosphere and of the spectrum of the stellar surface along the transit chord. Its application to HARPS transit spectroscopy of WASP121b reveals the absorption signature from metals, likely atomic iron, in the planet atmospheric limb. The width of the signal (14.3±1.2 km s−1 ) can be explained by the rotation of the tidally locked planet. Its blueshift (-5.2±0.5 km s−1 ) could trace strong winds from the dayside to the nightside, or the anisotropic expansion of the planetary thermosphere. Show less
