Azimuthal asymmetries in the debris disk around HD 61005

Abstract

Context. Debris disks offer valuable insights into the latest stages of circumstellar disk evolution, and can possibly help us to trace the outcomes of planetary formation processes. In the age range 10 to 100 Myr, most of the gas is expected to have been removed from the system, giant planets (if any) must have already been formed, and the formation of terrestrial planets may be on-going. Pluto-sized planetesimals, and their debris released in a collisional cascade, are under their mutual gravitational influence, which may result into non-axisymmetric structures in the debris disk. Aims. High angular resolution observations are required to investigate these effects and constrain the dynamical evolution of debris disks. Furthermore, multi-wavelength observations can provide information about the dust dynamics by probing different grain sizes. Methods. Here we present new VLT/SPHERE and ALMA observations of the debris disk around the 40 Myr-old solar-type star HD 61005. We resolve the disk at unprecedented resolution both in the near-infrared (in scattered and polarized light) and at millimeter wavelengths. We perform a detailed modeling of these observations, including the spectral energy distribution. Results. Thanks to the new observations, we propose a solution for both the radial and azimuthal distribution of the dust grains in the debris disk. We find that the disk has a moderate eccentricity (e ∼ 0.1) and that the dust density is two times larger at the pericenter compared to the apocenter. Conclusions. With no giant planets detected in our observations, we investigate alternative explanations besides planet-disk interactions to interpret the inferred disk morphology. We postulate that the morphology of the disk could be the consequence of a massive collision between ∼1000 kmsized bodies at ∼61 au. If this interpretation holds, it would put stringent constraints on the formation of massive planetesimals at large distances from the star.

Bringing “The Moth” to Light: A Planet-Sculpting Scenario for the Hd 61005 Debris Disk

“…The GPI data in particular highlight this trend, which is also clear in SPHERE data (Olofsson et al 2016). Farther out, the ansae appear as flat shoulders in the radial profiles beyond which there is a break in the profile slope.…”

“…HST STIS optical imaging from Schneider et al (2014) showed a more complete view of the low surface brightness "skirt" of dust stretching between the streamers south of the star, seen previously with NICMOS and ACS. Most recently, Olofsson et al (2016) presented high signal-to-noise ratio (S/N) VLT/SPHERE IRDIS H-and K-band images that further confirm the known features while also showing that the ring brightens with decreasing projected separation and the E ansa remains brighter than the W ansa in polarized intensity. That same work reported Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm data indicatingthat the disk's large grains are confined to a ring with asemimajor axis of ∼66 au.…”

“…We also appear roughly consistent with Steele et al (2016), who combine marginally resolved millimeter-wave images with the disk's SED to infer a dust belt of radius ∼60-70 au. Recent 1.3 mm ALMA data presented by Olofsson et al (2016) also indicated parent bodies with semimajor axes of ∼66 au. Notably, the disk's wings are not detected in the ALMA data despite sufficient angular resolution to do so, which is consistent with our model's distribution of parent bodies in the ring and only dust in the wings.…”

“…To explain the E>W brightness asymmetry, there would need (for some reason) to be more dust grains in the first set of orbits than the second. Our spline function that assigns different weights to orbits having different precession angles f-thereby allowing for particles that have not yet equilibrated secularly (Olofsson et al 2016)-can account for some but not all of the brightness asymmetry. Relaxing our assumption of a single fixed proper eccentricity e p0 should help (i.e., allowing for a locus of orbits in complex eccentricity and inclination space that is not strictly circular; see Figure 7).…”

What Sets the Radial Locations of Warm Debris Disks?

“…Studies of this system's SED that modeled the outer component with a modified blackbody (Kennedy & Wyatt 2010) or with a narrow belt (Ricarte et al 2013) found the warm component to be fairly cold (∼100-125 K) -consistent with the large value of r warm found here (36.1 au). In contrast, Olofsson et al (2016) modeled this SED with a wide outer belt plus a fainter and hotter (∼220 K) warm component. This suggests the outer belt comprises grains with a range of temperatures, which our model accounts for with a warm component placed far from the star.…”