# Is there more than meets the eye? Presence and role of submicron grains in debris discs

**Authors:** Philippe Thebault, Quentin Kral  
**arXiv:** [1904.05395](https://arxiv.org/abs/1904.05395)  
**Date:** 2019-04-10  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1904.05395v1/

## Key points

- Debris discs contain submicron grains despite being blown out by radiation pressure
- High collisional activity in bright debris discs can produce submicron grains at a high enough rate to compensate for their removal
- A code was used to study the evolution of debris discs down to submicron grains
- Bright discs around A stars always have detectable signatures of submicron grains, including scattered-light and mid-IR emissions
- The additional contribution of submicron grains can mimic the SED of two debris belts separated by a factor of 2 in radial distance
- For G stars, the effect of submicron grains remains limited in spectra despite dominating the system's geometrical cross section
- Small grain halos contribute to about 50% of flux up to $\lambda\sim50\mu$m wavelengths
- Observed abundances of submicron grains in debris discs could be naturally produced in bright debris discs and affect our understanding of some debris disc characteristics.

## Summary

Debris discs are known to contain submicron grains despite the fact that these particles should be blown out by stellar radiation pressure on very short timescales. They explore the possibility that the observed abundances of submicron grains could be "naturally" produced in bright debris discs, where high collisional activity produces them at a rate high enough to partially compensate for their rapid removal. The authors use a code following the collisional evolution of a debris disc down to submicron grains far below the limiting blow-out size $s_{blow}$ and explore different configurations such as A and G stars, cold and warm discs, "bright" and "very bright" systems. The study finds that in bright discs (fractional luminosity $>10^{-3}$) around A stars, there is always a high-enough amount of submicron grains to leave detectable signatures. These signatures include scattered-light where the disc's color becomes blue, and mid-IR ($10<\lambda<20\mu$m), where it boosts the disc's luminosity by at least a factor of 2 and induces a pronounced silicate solid-state band around $10\mu$m. Additionally, with this additional contribution of submicron grains, the SED can mimic that of two debris belts separated by a factor of 2 in radial distance. For G stars however, the effect of $s<s_{blow}$ grains remains limited in spectra despite dominating the system's geometrical cross section. Furthermore, for all considered cases small (bound and unbound) grain halos extending far beyond the main disc contribute to $\sim50$% of flux up to $\lambda\sim50\mu$m wavelengths. Overall, this study suggests that observed abundances of submicron grains in debris discs could be naturally produced in bright debris discs; their presence can affect our understanding of some debris disc characteristics.