X-ray and UV radiation in the planet-forming T-Tauri system PDS 70. Signs of accretion and coronal activity

Abstract

Planet formation takes place in protoplanetary discs around young T-Tauri stars. PDS 70 is one of the first confirmed examples of a system where the planets are currently forming in gaps in the disc, and can be directly imaged. One of the main early influences on planet formation is the lifetime of the protoplanetary disk, which is limited by the intense stellar X-ray and UV radiation. Stellar coronal activity and accretion of material onto the star are both potential sources of XUV radiation. Previous Swift observations detected UV emission, which were consistent with a low rate of accretion. We present follow up observations with the XMM-Newton observatory, which observed PDS 70 simultaneously in X-ray and UV in order to determine intensity of XUV radiation in the system, and identify if the source is coronal, accretion, or both. We detect a strong source in both X-ray and UV, with an average X-ray 0.2-12 keV luminosity of 1.37 × 1030 erg s−1, and a possible flare which increased the luminosity to 2.8 × 1030 erg s−1. The UV flux density is in excess of what would be expected from chromospheric emission, and supports the interpretation that PDS 70 has continuing weak accretion less than ∼10−10 M⊙ yr−1. The implications of the detected X-ray and UV radiation are that the disc is likely to be in the final stages of dispersal, and will be completely evaporated in the next million years, bringing an end to the primary planet formation process.

A Magnetically Driven Disk Wind in the Inner Disk of PDS 70*

“…This was suggested by Joyce et al (2020) from analysis of the SWIFT observations, where they predict a mass-loss rate ∼10 −8 M e yr −1 . These XUV measurements were later confirmed with follow-up XMM-Newton observations in Joyce et al (2023). Given this substantial mass-loss rate, the disk would be dispersed in less than ∼1 Myr.…”

The Dynamic, Chimeric Inner Disk of PDS 70

“…where N H is the hydrogen column density inferred from X-ray absorption (assuming a solar-metallicity gas 16 ), m p is the proton mass, κ m is the specific mean opacity of the dust in an optical passband, and Δm is the corresponding dimming in that passband. Joyce et al (2023) obtained 0.2-12 keV X-ray spectra of PDS 70 with the XMM-Newton telescope at six epochs spaced a day apart. They derived a mean N H of (2.2 ± 0.2) × 10 20 cm −2 but there is evidence for variability (χ 2 = 28.7 for 5 degrees of freedom).…”

“…Alternatively, conditions for trapping of occulting dust near the corotation radius, if confirmed, could constrain the strength of the magnetic dipole field, provided the dust particle size and gas density are known (Sanderson et al 2023; see also Zhan et al 2019). Dust size can be determined by reddeningextinction trends, e.g., Figure 5, and gas density can be estimated by X-ray observations (e.g., Joyce et al 2023) or inferred from the accretion rate.…”