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Register a new userSeparating extended disc features from the protoplanet in PDS 70 using VLT/SINFONI
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Added by
Valentin Christiaens
Publication date
2019
fields
Physics
and research's language is
English
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Transition discs are prime targets to look for protoplanets and study planet-disc interactions. We present VLT/SINFONI observations of PDS~70, a transition disc with a recently claimed embedded protoplanet. We take advantage of the angular and spectral diversity present in our data for an optimal PSF modeling and subtraction using principal component analysis (PCA). We report the redetection of PDS 70 b, both the front and far side of the outer disc edge, and the detection of several extended features in the annular gap. We compare spectral differential imaging applied before (PCA-SADI), and after (PCA-ASDI) angular differential imaging. Our tests suggest that PCA-SADI better recovers extended features, while PCA-ASDI is more sensitive to point sources. We adapted the negative fake companion (NEGFC) technique to infer the astrometry of the companion, and derived $r = 193.5 pm 4.9 mathrm{mas}$ and PA = 158.7deg $pm$ 3.0deg. We used both NEGFC and ANDROMEDA to infer the $K$-band spectro-photometry of the protoplanet, and found results consistent with recent VLT/SPHERE observations, except for their 2018/02 epoch measurement in the $K2$ filter. Finally, we derived an upper limit of $dot{M_b} < 1.26 times 10^{-7} big[ frac{5 M_{rm Jup}}{M_b} big] big[ frac{R_b}{R_{rm Jup}}big] M_{rm Jup} $ yr$^{-1}$ for the accretion rate of the companion based on an adaptation of PCA-SADI/PCA-ASDI around the Br$gamma$ line.
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We present the first observational evidence for a circumplanetary disk around the protoplanet PDS~70~b, based on a new spectrum in the $K$ band acquired with VLT/SINFONI. We tested three hypotheses to explain the spectrum: Atmospheric emission from the planet with either (1) a single value of extinction or (2) variable extinction, and (3) a combined atmospheric and circumplanetary disk model. Goodness-of-fit indicators favour the third option, suggesting circumplanetary material contributing excess thermal emission --- most prominent at $lambda gtrsim 2.3 mu$m. Inferred accretion rates ($sim 10^{-7.8}$--$10^{-7.3} M_J$ yr$^{-1}$) are compatible with observational constraints based on the H$alpha$ and Br$gamma$ lines. For the planet, we derive an effective temperature of 1500--1600 K, surface gravity $log(g)sim 4.0$, radius $sim 1.6 R_J$, mass $sim 10 M_J$, and possible thick clouds. Models with variable extinction lead to slightly worse fits. However, the amplitude ($Delta A_V gtrsim 3$mag) and timescale of variation ($lesssim$~years) required for the extinction would also suggest circumplanetary material.
The PDS 70 system has been subject to many studies in the past year following the discovery of two accreting planets in the gap of its circumstellar disk. Nevertheless, the mass accretion rate onto the star is still not well known. Here we determined the stellar mass accretion rate and its variability based on TESS and HARPS observations. The stellar light curve shows a strong signal with a $3.03pm0.06$ days period, which we attribute to stellar rotation. Our analysis of the HARPS spectra shows a rotational velocity of $vsin,i=16.0pm0.5,{rm km,s^{-1}}$, indicating that the inclination of the rotation axis is $50pm8$ degrees. This implies that the rotation axes of the star and its circumstellar disk are parallel within the measurement error. We apply magnetospheric accretion models to fit the profiles of the H$alpha$ line and derive mass accretion rates onto the star in the range of $0.6-2.2times10^{-10},{rm M_{odot}yr^{-1}}$, varying over the rotation phase. The measured accretion rates are in agreement with those estimated from NUV fluxes using accretion shock models. The derived accretion rates are higher than expected from the disk mass and planets properties for the low values of the viscous parameter $alpha$ suggested by recent studies, potentially pointing to an additional mass reservoir in the inner disk to feed the accretion, such as a dead zone. We find that the He I $lambda$10830 line shows a blueshifted absorption feature, indicative of a wind. The mass-loss rate estimated from the line depth is consistent with an accretion-driven inner disk MHD wind.
As host to two accreting planets, PDS 70 provides a unique opportunity to probe the chemical complexity of atmosphere-forming material. We present ALMA Band 6 observations of the PDS~70 disk and report the first chemical inventory of the system. With a spatial resolution of 0.4-0.5 ($sim$50 au), 12 species are detected, including CO isotopologues and formaldehyde, small hydrocarbons, HCN and HCO+ isotopologues, and S-bearing molecules. SO and CH3OH are not detected. All lines show a large cavity at the center of the disk, indicative of the deep gap carved by the massive planets. The radial profiles of the line emission are compared to the (sub-)mm continuum and infrared scattered light intensity profiles. Different molecular transitions peak at different radii, revealing the complex interplay between density, temperature and chemistry in setting molecular abundances. Column densities and optical depth profiles are derived for all detected molecules, and upper limits obtained for the non detections. Excitation temperature is obtained for H2CO. Deuteration and nitrogen fractionation profiles from the hydro-cyanide lines show radially increasing fractionation levels. Comparison of the disk chemical inventory to grids of chemical models from the literature strongly suggests a disk molecular layer hosting a carbon to oxygen ratio C/O>1, thus providing for the first time compelling evidence of planets actively accreting high C/O ratio gas at present time.
We present K-band interferometric observations of the PDS 70 protoplanets along with their host star using VLTI/GRAVITY. We obtained K-band spectra and 100 $mu$as precision astrometry of both PDS 70 b and c in two epochs, as well as spatially resolving the hot inner disk around the star. Rejecting unstable orbits, we found a nonzero eccentricity for PDS 70 b of $0.17 pm 0.06$, a near-circular orbit for PDS 70 c, and an orbital configuration that is consistent with the planets migrating into a 2:1 mean motion resonance. Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS 70 b of 10 $M_textrm{Jup}$, while the mass of PDS 70 c was unconstrained. The GRAVITY K-band spectra rules out pure blackbody models for the photospheres of both planets. Instead, the models with the most support from the data are planetary atmospheres that are dusty, but the nature of the dust is unclear. Any circumplanetary dust around these planets is not well constrained by the planets 1-5 $mu$m spectral energy distributions (SEDs) and requires longer wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made the first observations of a circumplanetary environment with sub-au spatial resolution, placing an upper limit of 0.3~au on the size of a bright disk around PDS 70 b.
The recent high spatial/spectral resolution observations have enabled constraining formation mechanisms of giant planets, especially at the final stages. The current interpretation of such observations is that these planets undergo magnetospheric accretion, suggesting the importance of planetary magnetic fields. We explore the properties of accreting, magnetized giant planets surrounded by their circumplanetary disks, using the physical parameters inferred for PDS 70 b/c. We compute the magnetic field strength and the resulting spin rate of giant planets, and find that these planets may possess dipole magnetic fields of either a few 10 G or a few 100 G; the former is the natural outcome of planetary growth and radius evolution, while the resulting spin rate cannot reproduce the observations. For the latter, a consistent picture can be drawn, where strong magnetic fields induced by hot planetary interiors lead both to magnetospheric accretion and to spin-down due to disk locking. We also compute the properties of circumplanetary disks in the vicinity of these planets, taking into account planetary magnetic fields. The resulting surface density becomes very low, compared with the canonical models, implying the importance of radial movement of satellite-forming materials. Our model predicts a positive gradient of the surface density, which invokes the traps for both satellite migration and radially drifting dust particles. This work thus concludes that the final formation stages of giant planets are similar to those of low-mass stars such as brown dwarfs, as suggested by recent studies.
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