Variable Accretion onto Protoplanet Host Star PDS 70

Abstract

The PDS 70 system has been subject to many studies in the past year following the discovery of two accreting planets in the gap of its circumstellar disk. Nevertheless, the mass accretion rate onto the star is still not well known. Here we determined the stellar mass accretion rate and its variability based on TESS and HARPS observations. The stellar light curve shows a strong signal with a $3.03\pm 0.06$ days period, which we attribute to stellar rotation. Our analysis of the HARPS spectra shows a rotational velocity of $v\sin i=16.0\pm 0.5\mathrm{km}{\mathrm{s}}^{-1}$, indicating that the inclination of the rotation axis is $50\pm 8$ degrees. This implies that the rotation axes of the star and its circumstellar disk are parallel within the measurement error. We apply magnetospheric accretion models to fit the profiles of the H$\alpha$ line and derive mass accretion rates onto the star in the range of $0.6-2.2\times 10^{-10}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, varying over the rotation phase. The measured accretion rates are in agreement with those estimated from NUV fluxes using accretion shock models. The derived accretion rates are higher than expected from the disk mass and planets properties for the low values of the viscous parameter $\alpha$ suggested by recent studies, potentially pointing to an additional mass reservoir in the inner disk to feed the accretion, such as a dead zone. We find that the He I $\lambda$10830 line shows a blueshifted absorption feature, indicative of a wind. The mass-loss rate estimated from the line depth is consistent with an accretion-driven inner disk MHD wind.

Accepted for publication in The Astrophysical Journal; 13 pages; 8 figures

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