Three Years of High-Contrast Imaging of the PDS 70 b and c Exoplanets at Hα with MagAO-X: Evidence of Strong Protoplanet Hα Variability and Circumplanetary Dust

Abstract: We present 3 years of high-contrast imaging of the PDS 70 b and c accreting protoplanets with the new extreme AO system MagAO-X as part of the MaxProtoPlanetS survey of H$\alpha$ protoplanets. In 2023 and 2024 our sharp (25-27 mas FWHM); well AO corrected (20-26% Strehl), deep (2-3.6hr) images detect compact (r~30 mas; r~3 au) circumplanetary disks (CPDs) surrounding both protoplanets. Starlight scattering off the dusty outer edges of these CPDs is the likely source of the bright compact continuum light detected within ~30 mas of both planets in our simultaneously obtained continuum 668 nm filter images. After subtraction of contaminating continuum and PSF residuals with pyKLIP ADI and SDI we obtained high-contrast ASDI H$\alpha$ images of both planets in 2022, 2023 and 2024. We find the H$\alpha$ line flux of planet b fell by (8.1$\pm$1.6)x10$^{-16}$ ergs/s/cm$^2$ a factor of 4.6 drop in flux from 2022 to 2023. In March 2024, planet b continued to be faint with just a slight 1.6x rise to an H$\alpha$ line flux of (3.64$\pm$0.87)x10$^{-16}$ ergs/s/cm$^2$. For c we measure a significant increase of (2.74$\pm$0.51)x10$^{-16}$ ergs/s/cm$^2$ from 2023 to 2024 which is a factor of 2.3x increase. So both protoplanets have recently experienced significant H$\alpha$ variability with ~1 yr sampling. In 2024, planet c is brighter than b: as c is brightening and b generally fading. We also tentatively detect one new point source "CC3" inside the inner disk (~49 mas; at PA~295 deg; 2024) with orbital motion roughly consistent with a ~5.6 au orbit.

High-Contrast Imaging Observations of PDS 70 b and c: Evidence of Variability and Circumplanetary Dust

This essay summarizes three years of high-contrast imaging observations of the PDS 70 b and c exoplanets using the MagAO-X extreme adaptive optics system. The observations, conducted from 2022 to 2024, focus on the Ha emission line variability of these accreting protoplanets, circumplanetary dust detections, and the implications for our understanding of planetary formation within transitional disks.

The study presents substantial variability in the Ha line flux of the PDS 70 b and c exoplanets. PDS 70 b exhibited a notable decrease in Ha line flux, reducing by a factor of 4.6 from 2022 to 2023, reaching just (2.28±0.26) x 10^-16 erg/s/cm^2 by 2023. Although a slight increase was observed in 2024, planet b remained significantly fainter than in previous years. Conversely, PDS 70 c demonstrated an increase in flux by approximately 2.3 times between 2023 and 2024, overtaking PDS 70 b's brightness for the first time in these observations. This is interpreted as compelling evidence of protoplanetary accretion variability, essential for understanding accretion physics and planetary formation processes.

A critical finding of the study is the identification of circumplanetary dust around both exoplanets in 2023 and 2024. The dust is believed to be responsible for the bright scattered light observed near both celestial bodies in the continuum. Circumplanetary dust disks (CPDs) are characterized by a compact structure approximately 3 au in diameter once deprojected, inferred to be a part of the planets' accretion environment. These CPDs have not been previously resolved in scattered light at optical wavelengths, highlighting significant advancements in high-contrast imaging and data reduction techniques provided by the MagAO-X instrumentation.

The MagAO-X system demonstrated improved technical capabilities over time, with advances in adaptive optics corrections leading to increased Strehl ratios and higher image contrasts. These improvements facilitated the identification of these fine structural details in the circumstellar and circumplanetary environment of PDS 70 A. A notable development in this paper is the commissioning of a new BMC 1K actuator DM in 2024, which significantly reduced non-common path aberrations and increased the Ha Strehl ratio to 26%.

The observations provide meaningful insights into the ongoing debate regarding the accretion processes and variability in nascent planetary systems. The contrasting behaviors of the PDS 70 b and c planets, alongside robust imaging capabilities, offer unprecedented opportunities to test theoretical models predicting accretion mechanisms, such as magnetospheric accretion and accretion shocks onto the circumplanetary disk.

In conclusion, the study provides crucial data supporting the existence of circumplanetary disks and the dynamic nature of protoplanetary ha emissions. While the variability observed could be due to the intrinsic episodic nature of accretion processes, it might also reflect transitional phases in planetary growth and disk evolution. Future observations with MagAO-X and complementary instruments will further elucidate the mechanics of these processes and enhance our understanding of planetary formation within transitional disk systems.