Abstract
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.
Highlights
Gas giant planets growing in a protoplanetary disk gain their mass via mass accretion from the parent disk until their host star loses its gas disk (e.g., Hayashi et al 1985)
In the following post-processing, we used an image size of 40 × 40 spatial pixels around the central star with a high signal-to-noise ratio (S/N). We found that this size maximized the S/N of PDS 70b
The 50% linewidths (FWHM) of ∼110 km s−1 in the two planets are comparable with a spectral resolution of ∼120 km s−1 in Multi Unit Spectroscopic Explorer (MUSE), and these values could be upper limits
Summary
Gas giant planets growing in a protoplanetary disk gain their mass via mass accretion from the parent disk until their host star loses its gas disk (e.g., Hayashi et al 1985). A part of the gas flow from the outer disk accretes onto gas giant planets while the rest of the flow passes over the planets toward the central star. The relationship between the host star and planets in the mass accretion rate depends on the planetary mass and the disk’s properties (e.g., Lubow et al 1999; Tanigawa & Tanaka 2016). Numerical simulations of disk–planet interactions show that the mass accretion rate of a 1 MJup planet can reach up to ∼90% of the rate of mass accretion from the outer disk, i.e., the planetary-mass accretion rate is an order of magnitude larger than the stellar rate (Lubow & D’Angelo 2006).
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