Imaging of Jovian-Mass Protoplanets in the PDS 70 System
The recent study titled "Keck/NIRC2 $L$'-Band Imaging of Jovian-Mass Accreting Protoplanets around PDS 70" presents $L$'-band observations of the PDS 70 planetary system using the Keck/NIRC2 equipped with a new infrared pyramid wavefront sensor. The investigation reports the detection of two accreting gas giant planets, PDS 70 b and PDS 70 c, alongside the circumstantial disk structures within the system.
The researchers applied advanced imaging techniques to distinguish the planets from the surrounding circumstellar disk, successfully resolving both planets in the $L$'-band with improved astrometric and photometric precision compared to previous works. The authors derived semi-major axes of $20{+3}_{-4}$ au for PDS 70 b and $34{+12}_{-6}$ au for PDS 70 c, suggesting orbits with minimal inclination relative to the circumstellar disk, indicative of dynamical stability and potential mean-motion resonances.
The spectral energy distribution (SED) fit for PDS 70 b provides well-defined constraints, particularly enhancing the understanding of the planet's infrared characteristics beyond $K$-band. The analysis infers a photospheric radius of 2-3 $R_{Jup}$ for PDS 70 b, attributing the unexpectedly large size to either significant atmospheric opacity or an obscuring circumplanetary dust shell. For PDS 70 c, while less constrained, the luminosity determination still offers essential insights into its characteristics. The researchers employed multiple atmospheric models, finding that a single blackbody spectrum often yielded satisfactory fits to the observed data, though acknowledging the potential model insufficiencies for these active, young planets.
Utilizing the evolutionary framework of \citet{GinzburgChiang2019}, which models accreting protoplanets post run-away growth but prior to complete atmospheric dissipation, the study estimates the mass and accretion rates for the planets. For PDS 70 b, a mass range of 2-4 $M_{Jup}$ and an average accretion rate between $3 \times 10{-7}$ and $8 \times 10{-7} M_{Jup}/yr$ is suggested. PDS 70 c presents values of 1-3 $M_{Jup}$ and accretion between $1 \times 10{-7}$ and $5 \times 10{-7} M_{Jup}/yr$. These values suggest that these are among the least massive directly imaged giant planets known, with substantial ongoing accretion indicative of late-stage formation. The paper discusses potential deviations from standard accretion and luminosity estimation methods, suggesting $\beta$ coefficients greater than 1 could align other observational mass estimates with their findings.
Concerning the planets' atmospheres, the research indicates that dust, possibly delivered through active accretion pathways, could influence optical properties significantly, leading to an SED without prominent molecular absorption features typically expected at these effective temperatures. This hypothesis aligns with the relatively large radii observed and presents an exciting avenue for further theoretical and observational studies.
In summary, the paper enhances the understanding of protoplanetary detection and characterization methodologies by utilizing advanced infrared imaging techniques and evolves the discourse on Jovian planet formation and the accreting phase. Future work should consider additional infrared measurements and further dynamical orbit analysis to solidify these initial findings and elucidate the potential of protoplanetary atmospheres to deviate from traditional expectations due to accretion processes.