MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm

Abstract

Context. Directly imaged planets and substellar companions are key targets for the characterization of self-luminous atmospheres. Their photometric appearance at 4–5 μm 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 μm. 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 β 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 β 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 β 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 β Pic b has been constrained to log g = 4.17−0.13+0.10 dex from its photometry and dynamical mass. Conclusions. A red color at 3–4 μm and a blue color at 4–5 μm might be (partially) caused by H2O and CO absorption, respectively, which are expected to be the most dominant gaseous opacities in hot (Teff ≳ 1300 K) atmospheres. The red characteristics of β Pic b, HIP 65426 b, and HD 206893 B at 3–5μm, 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.

MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm

“…From this, the authors conclude that the data is best described by a blackbody spectrum with T eff = 1204 +52 −53 K and R = 2.72 +0.39 −0.34 R J . In this work, we report the first detection of PDS 70 b in the 4-5 µm range as part of the MIRACLES survey (Stolker et al 2020). The object was observed with NACO at the Very Large Telescope (VLT) in Chile and detected with both the Brα and M filters.…”

“…We observed PDS 70 with VLT/NACO (Lenzen et al 2003;Rousset et al 2003) in the NB4.05 (Brα; λ 0 = 4.05 µm, ∆λ = 0.02 µm) and M (λ 0 = 4.78 µm, ∆λ = 0.59 µm) filters (ESO program ID: 0102.C-0649(A)) as part of the MIRACLES survey, which aims at the systematic characterization of directly imaged planets and brown dwarfs at 4-5 µm (Stolker et al 2020). The data were obtained without coronagraph in pupil-stabilized mode, while dithering the star across the detector to sample the thermal background emission.…”

“…This resulted in a total of 1.75 and 2 h of on-source telescope time for NB4.05 and M . A detailed description of the observing strategy for the MIRACLES survey is available in Stolker et al (2020) but a few specifics for the observations of PDS 70 are provided here.…”

“…In general, we followed the processing and calibration procedure that is described in Stolker et al (2020). However, in addition to subtracting the mean background (based on the adjacent data cubes in which the star was dithered to a different detector position) we also applied an additional correction with PCA (Hunziker et al 2018).…”

Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e

“…We consider the 3.3 µm LBT photometry reported in Skemer et al (2012), the L' band and [4.05]-Brα photometry reported in Currie et al (2014), as well as the M' band upper limit of Galicher et al (2011). The photometry was converted from magnitudes to flux using th species 7 toolkit, which has been described in Stolker et al (2020). We decided against including the photometric fluxes in the fit because their relatively low signal-to-noise would add little constraining power to the retrieval, when compared to the spectra, but would double the run-time of our retrievals due to the increased spectral range.…”