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Register a new userDust-to-gas ratio resurgence in circumstellar disks due to the formation of giant planets: the case of HD 163296
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The amount of dust present in circumstellar disks is expected to steadily decrease with age due to the growth from micron-sized particles to planetesimals and planets. Mature circumstellar disks, however, can be observed to contain significant amounts of dust and possess high dust-to-gas ratios. Using HD 163296 as our case study, we explore how the formation of giant planets in disks can create the conditions for collisionally rejuvenating the dust population, halting or reversing the expected trend. We combine N-body simulations with statistical methods and impact scaling laws to estimate the dynamical and collisional excitation of the planetesimals due to the formation of HD 163296s giant planets. We show that this process creates a violent collisional environment across the disk that can inject collisionally produced second-generation dust into it, significantly contributing to the observed dust-to-gas ratio. The spatial distribution of the dust production can explain the observed local enrichments in HD 163296s inner regions. The results obtained for HD 163296 can be extended to any disk with embedded forming giant planets and may indicate a common evolutionary stage in the life of such circumstellar disks. Furthermore, the dynamical excitation of the planetesimals could result in the release of transient, non-equilibrium gas species like H2O, CO2, NH3 and CO in the disk due to ice sublimation during impacts and, due to the excited planetesimals being supersonic with respect to the gas, could produce bow shocks in the latter that could heat it and cause a broadening of its emission lines.
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We present ALMA observations of the $98.5~mathrm{GHz}$ dust continuum and the $mathrm{^{13}CO}~J = 1 - 0$ and $mathrm{C^{18}O}~J = 1 - 0$ line emissions of the protoplanetary disk associated with HD~142527. The $98.5~mathrm{GHz}$ continuum shows a strong azimuthal-asymmetric distribution similar to that of the previously reported $336~mathrm{GHz}$ continuum, with a peak emission in dust concentrated region in the north. The disk is optically thin in both the $98.5~mathrm{GHz}$ dust continuum and the $mathrm{C^{18}O}~J = 1 - 0$ emissions. We derive the distributions of gas and dust surface densities, $Sigma_mathrm{g}$ and $Sigma_mathrm{d}$, and the dust spectral opacity index, $beta$, in the disk from ALMA Band 3 and Band 7 data. In the analyses, we assume the local thermodynamic equilibrium and the disk temperature to be equal to the peak brightness temperature of $mathrm{^{13}CO}~J = 3 - 2$ with a continuum emission. The gas-to-dust ratio, $mathrm{G/D}$, varies azimuthally with a relation $mathrm{G/D} propto Sigma_mathrm{d}^{-0.53}$, and $beta$ is derived to be $approx 1$ and $approx 1.7$ in the northern and southern regions of the disk, respectively. These results are consistent with the accumulation of larger dust grains in a higher pressure region. In addition, our results show that the peak $Sigma_mathrm{d}$ is located ahead of the peak $Sigma_mathrm{g}$. If the latter corresponds to a vortex of high gas pressure, the results indicate that the dust is trapped ahead of the vortex, as predicted by some theoretical studies.
We investigate the dust and gas distribution in the disk around HD 142527 based on ALMA observations of dust continuum, 13CO(3-2), and C18O(3-2) emission. The disk shows strong azimuthal asymmetry in the dust continuum emission, while gas emission is more symmetric. In this paper, we investigate how gas and dust are distributed in the dust-bright northern part of the disk and in the dust-faint southern part. We construct two axisymmetric disk models. One reproduces the radial profiles of the continuum and the velocity moments 0 and 1 of CO lines in the north and the other reproduces those in the south. We have found that the dust is concentrated in a narrow ring having ~50AU width (in FWHM; w_d=30AU in our parameter definition) located at ~170-200AU from the central star. The dust particles are strongly concentrated in the north. We have found that the dust surface density contrast between the north and south amounts to ~70. Compared to the dust, the gas distribution is more extended in the radial direction. We find that the gas component extends at least from ~100AU to ~250AU from the central star, and there should also be tenuous gas remaining inside and outside of these radii. The azimuthal asymmetry of gas distribution is much smaller than dust. The gas surface density differs only by a factor of ~3-10 between the north and south. Hence, gas-to-dust ratio strongly depends on the location of the disk: ~30 at the location of the peak of dust distribution in the south and ~3 at the location of the peak of dust distribution in the north. Despite large uncertainties, the overall gas-to-dust ratio is inferred to be ~10-30, indicating that the gas depletion may have already been under way.
We run numerical simulations to study the accretion of gas and dust grains onto gas giant planets embedded into massive protoplanetary discs. The outcome is found to depend on the disc cooling rate, planet mass, grain size and irradiative feedback from the planet. If radiative cooling is efficient, planets accrete both gas and pebbles rapidly, open a gap and usually become massive brown dwarfs. In the inefficient cooling case, gas is too hot to accrete onto the planet but pebble accretion continues and the planets migrate inward rapidly. Radiative feedback from the planet tends to suppress gas accretion. Our simulations predict that metal enrichment of planets by dust grain accretion inversely correlates with the final planet mass, in accordance with the observed trend in the inferred bulk composition of Solar System and exosolar giant planets. To account for observations, however, as much as ~30-50% of the dust mass should be in the form of large grains.
HD$,$169142 is an excellent target to investigate signs of planet-disk interaction due to the previous evidence of gap structures. We performed J-band (~1.2{mu}m) polarized intensity imaging of HD169142 with VLT/SPHERE. We observe polarized scattered light down to 0.16 (~19 au) and find an inner gap with a significantly reduced scattered light flux. We confirm the previously detected double ring structure peaking at 0.18 (~21 au) and 0.56 (~66 au), and marginally detect a faint third gap at 0.70-0.73 (~82-85 au). We explore dust evolution models in a disk perturbed by two giant planets, as well as models with a parameterized dust size distribution. The dust evolution model is able to reproduce the ring locations and gap widths in polarized intensity, but fails to reproduce their depths. It, however, gives a good match with the ALMA dust continuum image at 1.3 mm. Models with a parameterized dust size distribution better reproduce the gap depth in scattered light, suggesting that dust filtration at the outer edges of the gaps is less effective. The pile-up of millimeter grains in a dust trap and the continuous distribution of small grains throughout the gap likely require a more efficient dust fragmentation and dust diffusion in the dust trap. Alternatively, turbulence or charging effects might lead to a reservoir of small grains at the surface layer that is not affected by the dust growth and fragmentation cycle dominating the dense disk midplane. The exploration of models shows that extracting planet properties such as mass from observed gap profiles is highly degenerate.
The Rosseland mean opacity of dust in protoplanetary disks is often calculated assuming the interstellar medium (ISM) size distribution and a constant dust-to-gas ratio. However, the dust size distribution and the dust-to-gas ratio in protoplanetary disks are distinct from those of the ISM. Here, we use simple dust evolution models that incorporate grain growth and transport to calculate the time evolution of mean opacity of dust grains as a function of distance from the star. Dust dynamics and size distribution are sensitive to the assumed value of the turbulence strength $alpha_{rm t}$ and the velocity at which grains fragment $v_{rm frag}$. For moderate-to-low turbulence strengths of $alpha_{rm t} lesssim 10^{-3}$ and substantial differences in $v_{rm frag}$ for icy and ice-free grains, we find a spatially non-uniform dust-to-gas ratio and grain size distribution that deviate significantly from the ISM values, in agreement with previous studies. The effect of a non-uniform dust-to-gas ratio on the Rosseland mean opacity dominates over that of the size distribution. This spatially varying -- that is non-monotonic -- dust-to-gas ratio creates a region in the protoplanetary disk that is optimal for producing hydrogen-rich planets, potentially explaining the apparent peak in gas giant planet occurrence rate at intermediate distances. The enhanced dust-to-gas ratio within the ice line also suppresses gas accretion rates onto sub-Neptune cores, thus stifling their tendency to undergo runaway gas accretion within disk lifetimes. Finally, our work corroborates the idea that low mass cores with large primordial gaseous envelopes (`super-puffs) originate beyond the ice line.
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