Evidence for Variable Accretion onto PDS 70 c and Implications for Protoplanet Detections

Abstract: Understanding the processes of planet formation and accretion in young systems is essential to unraveling the initial conditions of planetary systems. The PDS 70 system, which hosts two directly imaged protoplanets, provides a unique laboratory for studying these phenomena, particularly through H-alpha emission a commonly used accretion tracer. We present multi-epoch observations and examine the variability in accretion signatures within this system, focusing on PDS 70 b and c. Using Hubble Space Telescope narrowband H-alpha imaging from 2020 and 2024, we achieve high signal-to-noise ratio detections of these planets and reveal significant changes in H-alpha flux. For PDS 70 c, the H-alpha flux more than doubled between 2020 and 2024. The trend is consistent with the one identified in recently published MagAO-X data, further confirming that PDS 70 c has become significantly brighter in H between 2023 March and 2024 May. The observed variability suggests dynamic accretion processes, possibly modulated by circumplanetary disk properties or transient accretion bursts. High-amplitude variability in PDS 70 c motivates simultaneous monitoring of multiple accretion tracers to probe the mechanisms of mass growth of gas giant planets. We quantify the impact of variability on the detectability of protoplanets in imaging surveys and emphasize the need for continued and regular monitoring to accurately assess the occurrence and characteristics of young, forming planets.

Analysis of Accretion Variability in PDS 70 and Its Impact on Protoplanet Detection

The study presented in this paper explores the dynamic accretion processes occurring in the PDS 70 system, known for hosting directly imaged protoplanets in its protoplanetary disk. Utilizing multi-epoch Hubble Space Telescope observations, the authors investigate variations in the H$\alpha$ emission of these planets, revealing significant changes in the observed flux. Specifically, the H$\alpha$ flux of PDS 70 c more than doubled from 2020 to 2024, indicating notable variability in accretion.

Key Results

1. H$\alpha$ Flux Variability: The study reports a substantial increase in the H$\alpha$ flux of PDS 70 c, consistent with trends noted in recent observations with the MagAO-X instrument. From 2023 to 2024, the continued increase in brightness aligns with data showing an overall rise in H$\alpha$ luminosity for PDS 70 c. This variability suggests dynamic accretion processes potentially modulated by circumplanetary disk properties or transient accretion bursts.
2. Astrometric Consistency and New Detections: The positions of PDS 70 planets were confirmed to align with predictions from recent orbital solutions, reaffirming the accuracy of previous models. Moreover, the detection of scattered light from the outer disk provides a more comprehensive view of the system.
3. Impact on Protoplanet Detection: The variability in accretion signatures impacts the detectability of protoplanets. Given the high contrast needed for direct imaging detections, fluctuations in brightness due to accretion can significantly alter the visibility of these planets. This highlights the necessity for continuous monitoring to accurately assess the occurrence and nature of young forming planets.

Implications and Future Directions

This research has critical implications for our understanding of planet formation and the interpretation of direct imaging surveys. The observed variability in the accretion process underscores the dynamic nature of planetary formation environments and the potential challenges in interpreting sporadic data points from imaging campaigns. Regular monitoring using multiple accretion tracers can provide insights into mass growth mechanisms and the geometry of accretion processes associated with forming gas giants.

The findings also prompt reconsideration of the assumptions in mass accretion models. The wide range of estimated mass accretion rates indicates that current models may not fully account for variability. Therefore, refining these models to incorporate dynamic accretion scenarios could enhance the accuracy of mass accretion rate calculations.

Future studies could benefit from an integrated approach involving space- and ground-based telescopes to mitigate calibration discrepancies and confirm short-term variability. Such collaborative efforts, coupled with advancements in imaging technologies, might improve detection capabilities and provide a clearer understanding of the intrinsic properties of protoplanetary systems.

In conclusion, the study provides compelling evidence of variable accretion within the PDS 70 system and its significant implications for the detectability and analysis of forming planets. This research represents an important contribution to the field and underscores the value of continual monitoring to decipher the complexities of planetary formation environments.