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The PDS 70 system, hosting two planets within its disk, is an ideal target for examining the effect of planets on dust accumulation, growth, and ongoing planet formation. Here, we present high-resolution (0.″07 = 8 au) dust continuum observations of the PDS 70 disk in ALMA Band 3 (3.0 mm). While previous Band 7 observations showed a dust ring with slight asymmetry, our Band 3 observations reveal a more prominent asymmetric peak in the northwest direction, where the intensity is 2.5 times higher than in other directions and the spectral index is at the local minimum with α _SED ∼ 2.2. This indicates that a substantial amount of dust is accumulated both radially and azimuthally in the peak. We also detect point-source emission around the stellar position in the Band 3 image, which is likely to be free–free emission. We constrain the eccentricity of the outer ring to be e < 0.04 from the position of the central star and the outer ring. From the comparison with numerical simulations, we constrain the mass of PDS 70c to be less than 4.9 ${M}_{{\rm{Jupiter}}}$ if the gas turbulence strength α _turb = 10 ^−3 . Then, we discuss the formation mechanism of the disk structures and further planet formation scenarios in the disk.

PDS 70 is a protoplanetary system that hosts two actively accreting gas giants, namely, PDS 70b and PDS 70 c. The system has a ∼60–100 au dusty ring that has been resolved by the Atacama Large Millimeter/Submillimeter Array (ALMA), along with circumplanetary disks around the two gas giants. Here, we report the first Karl G. Jansky Very Large Array (JVLA) Q - (40–48 GHz), Ka - (29–37 GHz), K - (18–26 GHz), and X - (8–12 GHz) bands' continuum observations, and the complementary ALMA Bands 3 (∼98 GHz) and 4 (∼145 GHz) observations towards PDS 70. The dusty ring appears azimuthally asymmetric in our ALMA images. We obtained firm detections at Ka and K bands without spatially resolving the source; we obtained a marginal detection at Q band, and no detection at X band. The spectral indices ( α ) are 5 ± 1 at 33–44 GHz and 0.6 ± 0.2 at 22–33 GHz. At 10–22 GHz, the conservative lower limit of α is 1.7. The 33–44 GHz flux density is likely dominated by the optically thin thermal emission of grown dust with ≳1 mm maximum grain sizes, which may be associated with the azimuthally asymmetric substructure induced by planet–disk interaction. Since PDS 70 was not detected at X band, we found it hard to explain the low spectral index at 22–33 GHz only with free–free emission. Hence, we attribute the dominant emission at 22–33 GHz to the emission of spinning nano-meter-sized dust particles, while free–free emission may partly contribute to emission at this frequency range. In some protoplanetary disks, the emission of spinning nano-meter-sized dust particles may resemble the 20–50 GHz excess in the spectra of millimeter-sized dust. The finding of strong continuum emission of spinning nano-meter-sized particles can complicate the procedure of constraining the properties of grown dust. Future high resolution, multifrequency JVLA/Next Generation Very Large Array and Square Kilometer Array observations may shed light on this issue.

The dust size in protoplanetary disks is a crucial parameter for understanding planet formation, while the observational constraints on dust size distribution have large uncertainties. In this study, we present a new method to constrain the dust size distribution from the dust spatial distribution, utilizing the fact that larger dust grains are more spatially localized. We analyze Atacama Large Millimeter/submillimeter Array Band 6 (1.25 mm) and Band 4 (2.14 mm) high-resolution images and constrain the dust size distribution in the two rings of the HD 163296 disk. We find that the outer ring at 100 au appears narrower at longer wavelengths, while the inner ring at 67 au appears to have similar widths across the two wavelengths. We model dust rings trapped at gas pressure maxima, where the dust grains follow a power-law size distribution, and the dust grains of a specific size follow a Gaussian spatial distribution, with the width depending on the grain size. By comparing the observations with the models, we constrain the maximum dust size ${a}_{\max }$ and the exponent of the dust size distribution p . We constrain $0.9\ \mathrm{mm}\lt {a}_{\max }\lt 5\ \mathrm{mm}$ and p < 3.3 in the inner ring, and ${a}_{\max }\gt 3\,\times \,{10}^{1}\ \mathrm{mm}$ and 3.4 < p < 3.7 in the outer ring. The larger maximum dust size in the outer ring implies a spatial dependency in dust growth, potentially influencing the formation location of the planetesimals. We further discuss the turbulence strength α derived from the constrained dust spatial distribution, assuming equilibrium between turbulent diffusion and the accumulation of dust grains.