2024ApJ...972..163L

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.