A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

Authors:

Kral et al

Abstract:

This paper presents a self-consistent model for the evolution of gas produced in the debris disc of β Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous α model. We present new gas observations of β Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity α > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in β Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.

Magellen Adaptive Optics Observe β Pictoris b

MAGELLAN ADAPTIVE OPTICS FIRST-LIGHT OBSERVATIONS OF THE EXOPLANET β PIC b. II. 3–5 μm DIRECT IMAGING WITH MagAO+Clio, AND THE EMPIRICAL BOLOMETRIC LUMINOSITY OF A SELF-LUMINOUS GIANT PLANET

Authors:

Morzinski et al

Abstract:

Young giant exoplanets are a unique laboratory for understanding cool, low-gravity atmospheres. A quintessential example is the massive extrasolar planet β Pic b, which is 9 AU from and embedded in the debris disk of the young nearby A6V star β Pictoris. We observed the system with first light of the Magellan Adaptive Optics (MagAO) system. In Paper I we presented the first CCD detection of this planet with MagAO+VisAO. Here we present four MagAO+Clio images of β Pic b at 3.1 μm, 3.3 μm, L', and ${M}^{\prime },$ including the first observation in the fundamental CH4 band. To remove systematic errors from the spectral energy distribution (SED), we re-calibrate the literature photometry and combine it with our own data, for a total of 22 independent measurements at 16 passbands from 0.99 to 4.8 μm. Atmosphere models demonstrate the planet is cloudy but are degenerate in effective temperature and radius. The measured SED now covers >80% of the planet's energy, so we approach the bolometric luminosity empirically. We calculate the luminosity by extending the measured SED with a blackbody and integrating to find log(${{\mathcal{L}}}_{\mathrm{bol}}$/${{\mathcal{L}}}_{\odot }$) $\;=\;-3.78\pm 0.03.$ From our bolometric luminosity and an age of 23 ± 3 Myr, hot-start evolutionary tracks give a mass of 12.7 ± 0.3 ${{\mathcal{M}}}_{\mathrm{Jup}}$, radius of 1.45 ± 0.02 ${{\mathcal{R}}}_{\mathrm{Jup}}$, and Teff of 1708 ± 23 K (model-dependent errors not included). Our empirically determined luminosity is in agreement with values from atmospheric models (typically $-3.8$ dex), but brighter than values from the field-dwarf bolometric correction (typically $-3.9$ dex), illustrating the limitations in comparing young exoplanets to old brown dwarfs.

New Orbital Parameters for β Pictoris b

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

Authors:

Millar-Blanchaer et al

Abstract:

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.

Modeling Collisions of Planetesimals and Dust in the β Pictoris Debris Disk

A SMACK Model of Colliding Planetesimals and Dust in the β Pictoris Debris Disk: Thermal Radiation and Scattered Light

Authors:

Nesvold et al

Abstract:

We present a new model of the β Pictoris disk-and-planet system that simulates both the planetesimal collisions and the dynamics of the resulting dust grains, allowing us to model features and asymmetries in both thermal and scattered light images of the disk. Given the observed inclination and eccentricity of the β Pictoris b planet, the model neatly ties together several features of the disk: the central hole in the submillimeter images, the two-disk "x"-pattern seen in scattered light, the "wing-tilt" asymmetry, and possibly even the clumpy gas seen by ALMA. We also find that most of the dust in the β Pictoris system is likely produced outside the ring at 60-100 AU. Instead of a birth ring, this disk has a "stirring ring" at 60-100 AU where the high-velocity collisions produced by the secular wave launched by the planet are concentrated. The two-disk x-pattern arises because collisions occur more frequently at the peaks and troughs of the secular wave. The perturbations of the disk in this region create an azimuthally and vertically asymmetric spatial distribution of collisions, which could yield an azimuthal clump of gas without invoking resonances or an additional planet.