Sculpting Eccentric Debris Disks with Eccentric Gas Rings

Abstract

Many debris disks seen in scattered light have shapes that imply their dust grains trace highly eccentric, apsidally aligned orbits. Apsidal alignment is surprising, especially for dust. Even when born from an apse-aligned ring of parent bodies, dust grains have their periastra dispersed in all directions by stellar radiation pressure. The periastra cannot be re-oriented by planets within the short dust lifetimes at the bottom of the collisional cascade. We propose that what re-aligns dust orbits is drag exerted by second-generation gas. Gas is largely immune to radiation pressure, and when released by photodesorption or collisions within an eccentric ring of parent bodies should occupy a similarly eccentric, apse-aligned ring. Dust grains launched onto misaligned orbits cross the eccentric gas ring supersonically and can become dragged into alignment within collisional lifetimes. The resultant dust configurations, viewed nearly but not exactly edge-on, with periastra pointing away from the observer, appear moth-like, with kinked wings and even doubled pairs of wings, explaining otherwise mysterious features in HD 61005 ("The Moth") and HD 32297, including their central bulbs when we account for strong forward scattering from irregularly shaped particles. Around these systems we predict gas at Kuiper-belt-like distances to move on highly elliptical streamlines that owe their elongation, ultimately, to highly eccentric planets. Unresolved issues and an alternative explanation for apsidal alignment are outlined.

The Gemini Planet Imager View of the HD 32297 Debris Disk

“…Resolved images of the HD 32297 debris disk revealed two spatially distinct components: a parent body belt and an extended outer halo. The halo, which was the first component detected in scattered light (Kalas 2005), extends to at least 1800 au (Schneider et al 2014) and displays an unusually curved morphology that may be indicative of interaction with the interstellar medium (Debes et al 2009), with an undetected planet (Lee & Chiang 2016) or with the gas component of the disk Lin & Chiang (2019), or of a recent collision in the disk as proposed by Mazoyer et al (2014) to explain a similar structure in the HD 15115 disk. Either way, the halo is thought to be populated by the smallest dust grains produced by collisions in the parent belt and that are subsequently placed in high-eccentricity orbits through radiative forces.…”

“…A planet could however be respon-sible for the inner dust depletion (inside of 30-50 au) without introducing measurable local perturbation. In addition, Lee & Chiang (2016) proposed that an interior planet on an inclined orbit is responsible for the "double wing" in the extended outer halo (Schneider et al 2014), although this could also arise from interaction with secondary gas (Lin & Chiang 2019). Current observations of the HD 32297 system are thus inconclusive regarding the presence and structure of its planetary system.…”

Debris Disk Results from the Gemini Planet Imager Exoplanet Survey's Polarimetric Imaging Campaign

“…Future analyses may reveal offsets in other GPIES rings, too, but the high inclinations of many of them complicate this by concealing the ansae and prohibiting measurements along the projected minor axis. A case in point is HD 32297, which has been interpreted as an elliptical disk with its major axis nearly parallel to our line of sight that leads to almost no projected offset (Lee & Chiang 2016;Lin & Chiang 2019).…”

“…Illustrating the importance of knowing the morphology are model explanations for some of the more extreme debris disk morphologies (e.g., "moths," "double wings," "needles"; Esposito et al 2016;Lee & Chiang 2016;Lin & Chiang 2019) that require the dust rings to be at least moderately eccentric. The eccentricity is typically ascribed to the gravitational influence of a nearby substellar companion.…”

The unexpected narrowness of eccentric debris rings: a sign of eccentricity during the protoplanetary disc phase

“…While many of these images suggest the influence of unseen planets (more systems than might be surmised based on a simple expectation of eccentric rings, [ 7 ]), a major limitation is that these images trace small dust. This dust is subject to strong radiation and stellar wind forces, and possibly gas drag which, in addition to opening the possibility of entirely different sculpting scenarios [ 18 , 19 ], makes connecting the observed structure to the orbits of the underlying parent planetesimals difficult. Ideally, inferences of unseen planets would be made at longer wavelengths, where the typical grain sizes are large enough to be immune to non-gravitational perturbations, and the observed structure more reasonably assumed to be representative of the planetesimal orbits.…”