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Register a new userMIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4-5 $mu$m. I. Photometric analysis of $beta$ Pic b, HIP 65426 b, PZ Tel B and HD 206893 B
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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.
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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.
HIP 65426 b is a recently discovered exoplanet imaged during the course of the SPHERE-SHINE survey. Here we present new $L$ and $M$ observations of the planet from the NACO instrument at the VLT from the NACO-ISPY survey, as well as a new $Y-H$ spectrum and $K$-band photometry from SPHERE-SHINE. Using these data, we confirm the nature of the companion as a warm, dusty planet with a mid-L spectral type. From comparison of its SED with the BT-Settl atmospheric models, we derive a best-fit effective temperature of $T_{text{eff}}=1618pm7$ K, surface gravity $log g=3.78^{+0.04}_{-0.03}$ and radius $R=1.17pm0.04$ $R_{text{J}}$ (statistical uncertainties only). Using the DUSTY and COND isochrones we estimate a mass of $8pm1$ $M_{text{J}}$. Combining the astrometric measurements from our new datasets and from the literature, we show the first indications of orbital motion of the companion (2.6$sigma$ significance) and derive preliminary orbital constraints. We find a highly inclined orbit ($i=107^{+13}_{-10}$ deg) with an orbital period of $800^{+1200}_{-400}$ yr. We also report SPHERE sparse aperture masking observations that investigate the possibility that HIP 65426 b was scattered onto its current orbit by an additional companion at a smaller orbital separation. From this data we rule out the presence of brown dwarf companions with masses greater than 16 $M_{text{J}}$ at separations larger than 3 AU, significantly narrowing the parameter space for such a companion.
A low-mass companion to the two-solar mass star HIP65426 has recently been detected by SPHERE at around 100 au from its host. Explaining the presence of super-Jovian planets at large separations, as revealed by direct imaging, is currently an open question. We want to derive statistical constraints on the mass and initial entropy of HIP65426b and to explore possible formation pathways of directly imaged objects within the core-accretion paradigm, focusing on HIP65426b. Constraints on the planets mass and post-formation entropy are derived from its age and luminosity combined with cooling models. For the first time, the results of population synthesis are also used to inform the results. Then, a formation model that includes N-body dynamics with several embryos per disc is used to study possible formation histories and the properties of possible additional companions. Finally, the outcomes of two- and three-planet scattering in the post-disc phase are analysed, taking tides into account. The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hot population and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Core formation at small separations from the star followed by outward scattering and runaway accretion at a few hundred AU succeeds in reproducing the mass and separation of HIP65426b. Alternatively, systems having two or more giant planets close enough to be on an unstable orbit at disc dispersal are likely to end up with one planet on a wide HIP65426b-like orbit with a relatively high eccentricity (>~ 0.5). If this scattering scenario explains its formation, HIP65426b is predicted to have a high eccentricity and to be accompanied by one or several roughly Jovian-mass planets at smaller semi-major axes, which also could have a high eccentricity. This could be tested by further direct-imaging as well as radial-velocity observations.
We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The $J$, $H$, $K1$, and $K2$ spectra from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 K to 1800 K for effective temperature and from 3.0 to 5.0 for log($g$), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of sub-micron dust particles, similar to dereddening approaches applied to the peculiar red field L-dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L-dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from four years of astrometric monitoring is consistent with a $sim$30-year period, orbital inclination of 147$^{circ}$, and semimajor axis of 10 au, well within the estimated disk inner radius of $sim$50 au. As one of very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion-disk interaction and provide a benchmark for substellar and planetary atmospheric study.
The determination of the fundamental properties (mass, separation, age, gravity and atmospheric properties) of brown dwarf companions allows us to infer crucial informations on their formation and evolution mechanisms. Spectroscopy of substellar companions is available to date only for a limited number of objects (and mostly at very low resolution, R<50) because of technical limitations, i.e., contrast and angular resolution. We present medium resolution (R=350), coronagraphic long-slit spectroscopic observations with SPHERE of two substellar companions, HD 1160 B and HD 19467 B. We found that HD 1160 B has a peculiar spectrum that cannot be fitted by spectra in current spectral libraries. A good fit is possible only considering separately the Y+J and the H spectral band. The spectral type is between M5 and M7. We also estimated a T_eff of 2800-2900 K and a log(g) of 3.5-4.0 dex. The low surface gravity seems to favour young age (10-20 Myr) and low mass (~20 M Jup ) for this object. HD 19467 B is instead a fully evolved object with a T_eff of ~1000 K and log g of ~5.0 dex. Its spectral type is T6+/-1.
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