${\hat{I}}^2$ Pictoris' inner disk in polarized light and new orbital parameters for ${\hat{I}}^2$ Pictoris b

Abstract

We present $H$-band observations of ${\hat{I}}^2$ Pic with the Gemini Planet Imager's (GPI's) polarimetry mode that reveal the debris disk between ~0.3" (~6 AU) and ~1.7" (~33 AU), while simultaneously detecting ${\hat{I}}^2$ Pic $b$. The polarized disk image was fit with a dust density model combined with a Henyey-Greenstein scattering phase function. The best fit model indicates a disk inclined to the line of sight ($\ddot{I}•=85.27\hat{A}{°}_{-0.19}^{+0.26}$) with a position angle $\hat{I}{¸}_{PA}=30.35\hat{A}{°}_{-0.28}^{+0.29}$ (slightly offset from the main outer disk, $\hat{I}{¸}_{PA}\approx 29\hat{A}°$), that extends from an inner disk radius of $23.6_{-0.6}^{+0.9}$ AU to well outside GPI's field of view. In addition, we present an updated orbit for ${\hat{I}}^2$ Pic $b$ based on new astrometric measurements taken in GPI's spectroscopic mode spanning 14 months. The planet has a semi-major axis of $a=9.2_{-0.4}^{+1.5}$AU, with an eccentricity $e\le 0.26$. The position angle of the ascending node is $\hat{I}\text{c◯}=31.75\hat{A}°\pm 0.15$, offset from both the outer main disk and the inner disk seen in the GPI image. The orbital fit constrains the stellar mass of ${\hat{I}}^2$ Pic to $1.60\pm 0.05M_{\odot }$. Dynamical sculpting by ${\hat{I}}^2$ Pic $b$ cannot easily account for the following three aspects of the inferred disk properties: 1) the modeled inner radius of the disk is farther out than expected if caused by ${\hat{I}}^2$ Pic b; 2) the mutual inclination of the inner disk and ${\hat{I}}^2$ Pic $b$ is $4\hat{A}°$, when it is expected to be closer to zero; and 3) the aspect ratio of the disk ($h_0=0.137_{-0.006}^{+0.005}$) is larger than expected from interactions with ${\hat{I}}^2$ Pic $b$ or self-stirring by the disk's parent bodies.