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Register a new userMIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4-5 $mu$m. II. Constraints on the mass and radius of the enshrouded planet PDS 70 b
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The circumstellar disk of PDS 70 hosts two forming planets, which are actively accreting gas from their environment. In this work, we report the first detection of PDS 70 b in the Br$alpha$ and $M$ filters with VLT/NACO, a tentative detection of PDS 70 c in Br$alpha$, and a reanalysis of archival NACO $L$ and SPHERE $H23$ and $K12$ imaging data. The near side of the disk is also resolved with the Br$alpha$ and $M$ filters, indicating that scattered light is non-negligible at these wavelengths. The spectral energy distribution of PDS 70 b is well described by blackbody emission, for which we constrain the photospheric temperature and photospheric radius to $T_mathrm{eff}=1193 pm 20$ K and $R=3.0 pm 0.2$ $R_mathrm{J}$. The relatively low bolometric luminosity, $log(L/L_odot) = -3.79 pm 0.02$, in combination with the large radius, is not compatible with standard structure models of fully convective objects. With predictions from such models, and adopting a recent estimate of the accretion rate, we derive a planetary mass and radius in the range of $M_mathrm{p}approx 0.5-1.5$ $M_mathrm{J}$ and $R_mathrm{p}approx 1-2.5$ $R_mathrm{J}$, independently of the age and post-formation entropy of the planet. The blackbody emission, large photospheric radius, and the discrepancy between the photospheric and planetary radius suggests that infrared observations probe an extended, dusty environment around the planet, which obscures the view on its molecular composition. Finally, we derive a rough upper limit on the temperature and radius of potential excess emission from a circumplanetary disk, $T_mathrm{eff}lesssim256$ K and $Rlesssim245$ $R_mathrm{J}$, but we do find weak evidence that the current data favors a model with a single blackbody component.
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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. We aim at systematically characterizing the atmospheres of directly imaged low-mass companions at 4-5 $mu$m. We want to homogeneously process the data 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. We have used the high-resolution, high-contrast capabilities of NACO at the 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 have also analyzed six archival VLT/NACO datasets which were taken with the NB3.74, $L$, NB4.05, and $M$ filters. 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 to $log{g} = 4.17_{-0.13}^{+0.10}$ dex from its photometry and dynamical mass. A red color at 3-4 $mu$m and a blue color at 4-5 $mu$m might be (partially) caused by H$_2$O and CO absorption, respectively. 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.
The theory of remote sensing shows that observing a planet at multiple phase angles ($alpha$) is a powerful strategy to characterize its atmosphere. Here, we analyse how the information contained in reflected-starlight spectra of exoplanets depends on the phase angle, and the potential of multi-phase measurements to better constrain the atmospheric properties and the planet radius ($R_p$). We simulate spectra (500-900 nm) at $alpha$=37$^circ$, 85$^circ$ and 123$^circ$ with spectral resolution $R$~125-225 and signal-to-noise ratio $S/N$=10. Assuming a H$_2$-He atmosphere, we use a seven-parameter model that includes the atmospheric methane abundance ($f_{CH_4}$), the optical properties of a cloud layer and $R_p$. All these parameters are assumed unknown a priori and explored with an MCMC retrieval method. We find that no single-phase observation can robustly identify whether the atmosphere has clouds or not. A single-phase observation at $alpha$=123$^circ$ and $S/N$=10 can constrain $R_p$ with a maximum error of 35%, regardless of the cloud coverage. Combining small (37$^circ$) and large (123$^circ$) phase angles is a generally effective strategy to break multiple parameter degeneracies. This enables to determine the presence or absence of a cloud and its main properties, $f_{CH_4}$ and $R_p$ in all the explored scenarios. Other strategies, such as doubling $S/N$ to 20 for a single-phase observation or combining small (37$^circ$) and moderate (85$^circ$) phase angles, fail to achieve this. We show that the improvements in multi-phase retrievals are associated with the shape of the scattering phase function of the cloud aerosols and that the improvement is more modest for isotropically-scattering aerosols. We finally discuss that misidentifying the background gas in the retrievals of super-Earth observations leads to a systematic underestimate of the absorbing gas abundance.
In recent decades, thousands of substellar companions have been discovered with both indirect and direct methods of detection. In this paper, we focus our attention on substellar companions detected with the direct imaging technique, with the primary goal of investigating their close surroundings and looking for additional companions and satellites, as well as disks and rings. Any such discovery would shed light on many unresolved questions, particularly with regard to their possible formation mechanisms. To reveal bound features of directly imaged companions we need to suppress the contribution from the source itself. Therefore, we developed a method based on the negative fake companion (NEGFC) technique that first estimates the position in the field of view (FoV) and the flux of the imaged companion, then subtracts a rescaled model point spread function (PSF) from the imaged companion. Next it performs techniques, such as angular differential imaging (ADI), to further remove quasi-static patterns of the star. We applied the method to the sample of substellar objects observed with SPHERE during the SHINE GTO survey. Among the 27 planets and brown dwarfs we analyzed, we detected a possible point source close to DH Tau B. This candidate companion was detected in four different SPHERE observations, with an estimated mass of $sim 1$ Mtextsubscript{Jup}, and a mass ratio with respect to the brown dwarf of $1/10$. This binary system, if confirmed, would be the first of its kind, opening up interesting questions for the formation mechanism, evolution, and frequency of such pairs. In order to address the latter, the residuals and contrasts reached for 25 companions in the sample of substellar objects observed with SPHERE were derived. If the DH Tau Bb companion is real, the binary fraction obtained is $sim 7%$, which is in good agreement with the results obtained for field brown dwarfs.
Aims: We aim to characterize the orbital and atmospheric properties of PDS 70 b, which was first identified on May 2015 in the course of the SHINE survey with SPHERE, the extreme adaptive-optics instrument at the VLT. Methods: We obtained new deep SPHERE/IRDIS imaging and SPHERE/IFS spectroscopic observations of PDS 70 b. The astrometric baseline now covers 6 years which allows us to perform an orbital analysis. For the first time, we present spectrophotometry of the young planet which covers almost the entire near-infrared range (0.96 to 3.8 micrometer). We use different atmospheric models covering a large parameter space in temperature, log(g), chemical composition, and cloud properties to characterize the properties of the atmosphere of PDS 70 b. Results: PDS 70 b is most likely orbiting the star on a circular and disk coplanar orbit at ~22 au inside the gap of the disk. We find a range of models that can describe the spectrophotometric data reasonably well in the temperature range between 1000-1600 K and log(g) no larger than 3.5 dex. The planet radius covers a relatively large range between 1.4 and 3.7 R_jupiter with the larger radii being higher than expected from planet evolution models for the age of the planet of 5.4 Myr. Conclusions: This study provides a comprehensive dataset on the orbital motion of PDS 70 b, indicating a circular orbit and a motion coplanar with the disk. The first detailed spectral energy distribution of PDS 70 b indicates a temperature typical for young giant planets. The detailed atmospheric analysis indicates that a circumplanetary disk may contribute to the total planet flux.
Abbreviated. By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association. Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The subtraction of the stellar point spread function (PSF) is based on reference star differential imaging (RDI) using the other targets in the survey in combination with principal component analysis. We report the discovery of YSES 2b, a planetary-mass companion to the K1 star YSES 2 (TYC 8984-2245-1). The primary has a Gaia EDR3 distance of 110 pc, and we derive a revised mass of $1.1,M_odot$ and an age of approximately 14 Myr. We detect the companion in two observing epochs southwest of the star at a position angle of 205$^circ$ and with a separation of $sim1.05$, which translates to a minimum physical separation of 115 au at the distance of the system. We derive a photometric planet mass of $6.3^{+1.6}_{-0.9},M_mathrm{Jup}$ using AMES-COND and AMES-dusty evolutionary models; this mass corresponds to a mass ratio of $q=(0.5pm0.1)$% with the primary. This is the lowest mass ratio of a direct imaging planet around a solar-type star to date. We discuss potential formation mechanisms and find that the current position of the planet is compatible with formation by disk gravitational instability, but its mass is lower than expected from numerical simulations. Formation via core accretion must have occurred closer to the star, yet we do not find evidence that supports the required outward migration, such as via scattering off another undiscovered companion in the system. YSES 2b is an ideal target for follow-up observations to further the understanding of the physical and chemical formation mechanisms of wide-orbit Jovian planets.
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