Wednesday, September 16, 2015

New Orbital Parameters for β Pictoris b

β Pictoris' inner disk in polarized light and new orbital parameters for β Pictoris b


Millar-Blanchaer et al


We present H-band observations of β 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 β 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 (ϕ=85.27deg+0.26−0.19) with a position angle θPA=30.35deg+0.29−0.28 (slightly offset from the main outer disk, θPA≈29deg), that extends from an inner disk radius of 23.6+0.9−0.6 AU to well outside GPI's field of view. In addition, we present an updated orbit for β 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+1.5−0.4AU, with an eccentricity e≤0.26. The position angle of the ascending node is Ω=31.75deg±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 β Pic to 1.60±0.05M⊙. Dynamical sculpting by β 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 β Pic b; 2) the mutual inclination of the inner disk and β Pic b is 4deg, when it is expected to be closer to zero; and 3) the aspect ratio of the disk (h0=0.137+0.005−0.006) is larger than expected from interactions with β Pic b or self-stirring by the disk's parent bodies.

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