Is there more than meets the eye? Presence and role of sub-micron grains in debris discs

P Thebault,Q Kral

doi:10.1051/0004-6361/201935341

Abstract

Context. The presence of sub-micron grains has been inferred in several debris discs, usually because of a blue colour of the spectrum in scattered light or a pronounced silicate band around 10 μm, even though these particles should be blown out by stellar radiation pressure on very short timescales. So far, no fully satisfying explanation has been found for this apparent paradox. Aims. We investigate the possibility that the observed abundances of sub-micron grains could be naturally produced in bright debris discs, where the high collisional activity produces them at a rate high enough to partially compensate for their rapid removal. We also investigate to what extent this potential presence of small grains can affect our understanding of some debris disc characteristics. Methods. We used a numerical collisional code to follow the collisional evolution of a debris disc down to sub-micron grains far below the limiting blow-out size sblow. We considered compact astrosilicates and explored different configurations: A and G stars, cold and warm discs, bright and very bright systems. We then produced synthetic spectra and spectral energy distributions, where we identified and quantified the signature of unbound sub-micron grains. Results. We find that in bright discs (fractional luminosity ≳10−3) around A stars, the number of sub-micron grains is always high enough to leave detectable signatures in scattered light where the disc colour becomes blue, and also in the mid-IR (10 ≲ λ ≲ 20 μm), where they boost the disc luminosity by at least a factor of 2 and induce a pronounced silicate solid-state band around 10 μm. We also show that with this additional contribution of sub-micron grains, the spectral energy distribution can mimic that of two debris belts separated by a factor of ~2 in radial distance. For G stars, the effect of s ≤ sblow grains remains limited in the spectra although they dominate the geometrical cross section of the system. We also find that for all considered cases, the halo of small (bound and unbound) grains that extends far beyond the main disc contributes to ~50% of the flux up to λ ~ 50 μm wavelengths.

Highlights

An important fraction of main-sequence stars are known to be surrounded by debris discs, which are detected by the luminosity excess that is produced by small dust particles
We see the well-known waviness in the particle size distribution (PSD) at the lower end of the bound-grain size domain, which is due to the discontinuity at the s = sblow frontier (Campo Bagatin et al 1994; Thebault et al 2003; Thebault & Augereau 2007)
Close to the two orders of magnitude, so that even if the PSD increases again for grains in the 0.1 μm range, the total geometrical cross section of the disc is largely dominated by bound grains

Summary

Introduction

An important fraction of main-sequence stars are known to be surrounded by debris discs, which are detected by the luminosity excess that is produced by small dust particles. For a canonical collisional equilibrium size-distribution in dN ∝ s−3.5ds, the geometrical cross section of the system is dominated by the smallest grains, of size smin, in the cascade These smallest grains should dominate the disc luminosity at all wavelengths shorter than ∼2πsmin, that is, typically from the visible up to the near- to mid-IR. At these wavelengths, observations are expected to be mostly sensitive to grains of sizes smin ∼ sblow, where sblow is the limiting size below which particles are blown out by stellar radiation pressure. Even though detailed numerical collisional models have shown that a realistic grain size distribution can significantly depart from the idealized dN ∝ s−3.5ds power law, they all confirmed that the geometrical cross section is probably still dominated by grains close to the sblow value (Thebault et al 2003; Krivov et al 2006; Thebault & Augereau 2007)

Results

Discussion

Conclusion