A Swift view of X-ray and UV radiation in the planet-forming T-Tauri system PDS 70

Abstract

PDS 70 is a $\sim$5 Myr old star with a gas and dust disc in which several proto-planets have been discovered. We present the first UV detection of the system along with X-ray observations taken with the \textit{Neil Gehrels Swift Observatory} satellite. PDS 70 has an X-ray flux of 3.4$\times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 0.3-10.0 keV range, and UV flux (U band) of 3.5$\times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ . At the distance of 113.4 pc determined from Gaia DR2 this gives luminosities of 5.2$\times 10^{29}$ erg s$^{-1}$ and 5.4$\times 10^{29}$ erg s$^{-1}$ respectively. The X-ray luminosity is consistent with coronal emission from a rapidly rotating star close to the log $\frac{L_{\mathrm{X}}}{L_{\mathrm{bol}}} \sim -3$ saturation limit. We find the UV luminosity is much lower than would be expected if the star were still accreting disc material and suggest that the observed UV emission is coronal in origin.