# A re-investigation of debris disc halos

**Authors:** Philippe Thebault, Johan Olofsson, Quentin Kral  
**arXiv:** [2303.17434](https://arxiv.org/abs/2303.17434)  
**Date:** 2023-03-30  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2303.17434v1/

## Key points

- Debris disc halos are wide regions of small grains pushed into high-eccentricity orbits by radiation pressure
- Effects of small unbound grains, non-isotropic scattering phase functions, and finite instrument resolution on scattered light surface brightness profiles were previously unexplored
- Unbound grains contribute significantly to the halo's luminosity in scattered light and can flatten the radial profile
- Realistic size-dependent scattering phase functions result in shallower surface brightness profiles
- Roughly half of observationally-derived halo profiles found in literature are compatible with new results, while remaining systems likely shaped by additional processes
- Wide halos remain detectable up to far-infrared wavelengths in thermal emission, but strongly decrease in sub-millimeter to millimeter ranges while still representing a few percent of a system's luminosity at 1mm wavelength
- Presence of a halo can affect determining a disc's radius from its spectral energy distribution for unresolved systems
- Two ALMA-detected halos for HD 32297 and HD 61005 may require additional mechanisms beyond natural behavior at long wavelengths

## Summary

This study focuses on the phenomenon of debris disc halos, which are wide regions of small grains produced in the ring of parent bodies and pushed into high-eccentricity orbits by radiation pressure. The authors revisit this phenomenon and address two previously unexplored issues: the effects of small unbound grains, non-isotropic scattering phase functions, and finite instrument resolution on scattered light surface brightness profiles; and how the halo manifests at longer wavelengths. They find that unbound grains contribute significantly to the halo's luminosity in scattered light and can flatten the radial profile. Realistic size-dependent scattering phase functions also have an effect, resulting in shallower surface brightness profiles. For edge-on discs, not resolving the vertical profile can also flatten the projected surface brightness. The authors show that roughly half of the observationally-derived halo profiles found in literature are compatible with their new results, while remaining systems are likely shaped by additional processes. They propose that future observational studies should fit characteristics of parent body belts and halos separately. In thermal emission, wide halos remain detectable up to far-infrared wavelengths, with exceptions in certain domains. The halo accounts for more than half of a system's total flux up to certain wavelengths but strongly decreases in sub-millimeter to millimeter ranges while still representing a few percent of a system's luminosity at 1mm wavelength. For unresolved systems, the presence of a halo can affect determining a disc's radius from its spectral energy distribution. The study concludes by discussing whether two ALMA-detected halos for HD 32297 and HD 61005 can be explained without invoking additional mechanisms beyond natural behavior at long wavelengths. The authors note that these halos appear relatively steep compared to their numerical results and suggest that additional mechanisms may be at play in these systems' outer regions.