Hubble Space Telescope Optical Imaging of the Eroding Debris Disk HD 61005

TL;DR

This study uses Hubble Space Telescope optical imaging to analyze the asymmetric, swept-back debris disk HD 61005, suggesting interaction with a low-density interstellar cloud influences its morphology, with implications for other debris disks.

Contribution

First detailed polarization imaging of HD 61005 revealing its asymmetric structure and proposing a new explanation involving low-density interstellar medium interaction.

Findings

Disk shows asymmetric, swept-back morphology.

Grains are predominantly sub-micron sized with no strong color gradient.

Interstellar medium interaction likely causes observed asymmetries.

Abstract

We present Hubble Space Telescope optical coronagraphic polarization imaging observations of the dusty debris disk HD 61005. The scattered light intensity image and polarization structure reveal a highly inclined disk with a clear asymmetric, swept back component, suggestive of significant interaction with the ambient interstellar medium. The combination of our new data with the published 1.1 micron discovery image shows that the grains are blue scattering with no strong color gradient as a function of radius, implying predominantly sub-micron sized grains. We investigate possible explanations that could account for the observed swept back, asymmetric morphology. Previous work has suggested that HD 61005 may be interacting with a cold, unusually dense interstellar cloud. However, limits on the intervening interstellar gas column density from an optical spectrum of HD 61005 in the Na I D lines render this possibility unlikely. Instead, HD 61005 may be embedded in a more typical warm, low-density cloud that introduces secular perturbations to dust grain orbits. This mechanism can significantly distort the ensemble disk structure within a typical cloud crossing time. For a counterintuitive relative flow direction--parallel to the disk midplane--we find that the structures generated by these distortions can very roughly approximate the HD 61005 morphology. Future observational studies constraining the direction of the relative interstellar medium flow will thus provide an important constraint for future modeling. Independent of the interpretation for HD 61005, we expect that interstellar gas drag likely plays a role in producing asymmetries observed in other debris disk systems, such as HD 15115 and Delta-Velorum.