Accretion Properties of PDS 70b with MUSE

Abstract

We report a new evaluation of the accretion properties of PDS~70b obtained with VLT/MUSE. The main difference from previous studies in Haffert et al. (2019) and Aoyama & Ikoma (2019) is in the mass accretion rate. Simultaneous multiple line observations, such as H$α$ and H$β$, can better constrain the physical properties of an accreting planet. While we clearly detected H$α$ emissions from PDS~70b, no H$β$ emissions were detected. We estimate the line flux of H$β$ with a 3-$σ$ upper limit to be 2.3~$\times$~10$^{-16}$~erg~s$^{-1}$~cm$^{-2}$. The flux ratio $F_{\rm Hβ}$/$F_{\rm Hα}$ for PDS~70b is $<$~0.28. Numerical investigations by Aoyama et al. (2018) suggest that $F_{\rm Hβ}$/$F_{\rm Hα}$ should be close to unity if the extinction is negligible. We attribute the reduction of the flux ratio to the extinction, and estimate the extinction of H$α$ ($A_{\rm Hα}$) for PDS~70b to be $>$~2.0~mag using the interstellar extinction value. %The expected $A_{\rm Hα}$ value in the gap of the protoplanetary disk at the PDS~70b location is 2.4~mag, which is consistent with the estimated extinction. By combining with the H$α$ linewidth and the dereddening line luminosity of H$α$, %we derive the PDS~70b dynamical mass and mass accretion rate to be \hashimotor{12~$\pm$~3~$M_{\rm Jup}$} and $\gtrsim$~5~$\times$~10$^{-7}$~$M_{\rm Jup}$~yr$^{-1}$, respectively. we derive the PDS~70b mass accretion rate to be $\gtrsim$~5~$\times$~10$^{-7}$~$M_{\rm Jup}$~yr$^{-1}$. The PDS~70b mass accretion rate is an order of magnitude larger than that of PDS~70. We found that the filling factor $f_{\rm f}$ (the fractional area of the planetary surface emitting H$α$) is $\gtrsim$0.01, which is similar to the typical stellar value. The small value of $f_{\rm f}$ indicates that the H$α$ emitting areas are localized at the surface of PDS~70b.