Abstract
Context. Near- and mid-infrared interferometric observations have revealed populations of hot and warm dust grains populating the inner regions of extrasolar planetary systems. These are known as exozodiacal dust clouds, or exozodis, reflecting the similarity with the solar system’s zodiacal cloud. Radiative transfer models have constrained the dust to be dominated by tiny submicron-sized, carbon-rich grains that are accumulated very close to the sublimation radius. The origin of this dust is an unsolved issue. Aims. We explore two exozodiacal dust production mechanisms, first re-investigating the Poynting-Robertson drag pile-up scenario, and then elaborating on the less explored but promising exocometary dust delivery scenario. Methods. We developed a new, versatile numerical model that calculates the dust dynamics, with non-orbit-averaged equations for the grains close to the star. The model includes dust sublimation and incorporates a radiative transfer code for direct comparison to the observations. We consider in this study four stellar types, three dust compositions, and we assume a parent belt at 50 au. Results. In the case of the Poynting-Robertson drag pile-up scenario, we find that it is impossible to produce long-lived submicron-sized grains close to the star. The inward drifting grains fill in the region between the parent belt and the sublimation distance, producing an unrealistically strong mid-infrared excess compared to the near-infrared excess. The dust pile-up at the sublimation radius is by far insufficient to boost the near-IR flux of the exozodi to the point where it dominates over the mid-infrared excess. In the case of the exocometary dust delivery scenario, we find that a narrow ring can form close to the sublimation zone, populated with large grains from several tens to several hundreds of micrometers in radius. Although not perfect, this scenario provides a better match to the observations, especially if the grains are carbon-rich. We also find that the number of active exocomets required to sustain the observed dust level is reasonable. Conclusions. We conclude that the hot exozodiacal dust detected by near-infrared interferometry is unlikely to result from inward grain migration by Poynting-Robertson drag from a distant parent belt, but could instead have an exocometary origin.
Highlights
Hot exozodiacal dust clouds have been detected by means of interferometric observations in the near-infrared, around about 25 main sequence stars (Absil et al 2013; Ertel et al 2014, 2016; Kral et al 2017; Nuñez et al 2017)
We developed a Python implemented version of the GRaTeR radiative transfer code (Augereau et al 1999) that allows us to calculate thermal emission and scattered light maps at any wavelength from the 2D (r, s) maps, as well as spectral energy distributions (SED) of the exozodis in order to directly compare our numerical results with the observations
PR-drag pile-up scenario We consider a setup similar to the one explored by Kobayashi et al (2011) and Van Lieshout et al (2014), with a population of small grains assumed to be released by collisions in a Kuiper belt-like ring, whose evolution is followed, taking into account PR-drag and sublimation near the star, until the grains leave the system either by total sublimation, by falling onto the star, or by dynamical ejection
Summary
Hot exozodiacal dust clouds (exozodis) have been detected by means of interferometric observations in the near-infrared (near-IR, H- or K-band), around about 25 main sequence stars (Absil et al 2013; Ertel et al 2014, 2016; Kral et al 2017; Nuñez et al 2017). We employ a different approach to compute τ, described in detail in Appendix C It combines density profiles derived from the limited number of test grains for which the dynamics have been calculated accurately, and timescale estimates for a broader range of grain sizes, to produce 2D (r, s) density and optical depth maps. 3. PR-drag pile-up scenario We consider a setup similar to the one explored by Kobayashi et al (2011) and Van Lieshout et al (2014), with a population of small grains assumed to be released by collisions in a Kuiper belt-like ring (parent bodies located at r0 = 50 au), whose evolution is followed, taking into account PR-drag and sublimation near the star, until the grains leave the system either by total sublimation, by falling onto the star, or by dynamical ejection.
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