Accretion Properties of PDS 70b with MUSE

Abstract

We report a new evaluation of the accretion properties of PDS 70b obtained with the Very Large Telescope/Multi Unit Spectroscopic Explorer. The main difference from the previous studies of Haffert et al. and Aoyama & Ikoma is in the mass accretion rate. Simultaneous multiple line observations, such as Hα and Hβ, can better constrain the physical properties of an accreting planet. While we clearly detected Hα emissions from PDS 70b, no Hβ emissions were detected. We estimate the line flux of Hβ with a 3σ upper limit to be 2.3 × 10⁻¹⁶ erg s⁻¹ cm⁻². The flux ratio F_(Hβ) /F_(Hα) for PDS 70b is <0.28. Numerical investigations by Aoyama et al. suggest that F_(Hβ) /F_(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 Hα (A_(Hα) ) for PDS 70b to be >2.0 mag using the interstellar extinction value. By combining with the Hα linewidth and the dereddening line luminosity of Hα, we derive the PDS 70b mass accretion rate to be ≳5 × 10⁻⁷ M_(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_f (the fractional area of the planetary surface emitting Hα) is ≳0.01, which is similar to the typical stellar value. The small value of f_f indicates that the Hα emitting areas are localized at the surface of PDS 70b.