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.