An interferometric study of the Fomalhaut inner debris disk

Abstract

Context. Debris disks are thought to be extrasolar analogs to the solar system planetesimal belts. The star Fomalhaut harbors a cold debris belt at 140 AU comparable to the Edgeworth-Kuiper belt, as well as evidence of a warm dust component, unresolved by singledish telescopes, which is suspected of being a bright analog to the solar system's zodiacal dust. Aims. Interferometric observations obtained with the VLTI/VINCI instrument and the Keck Interferometer Nuller have identified nearand mid-infrared excesses attributed respectively to hot and warm exozodiacal dust residing in the inner few AU of the Fomalhaut environment. We aim to characterize the properties of this double inner dust belt and to unveil its origin. Methods. We performed parametric modeling of the exozodiacal disk ("exozodi") using the GRaTeR radiative transfer code to reproduce the interferometric data, complemented by mid-to far-infrared photometric measurements from Spitzer and Herschel . A detailed treatment of sublimation temperatures was introduced to explore the hot population at the size-dependent sublimation rim. We then used an analytical approach to successively testing several source mechanisms for the dust and suspected parent bodies. Results. A good fit to the multiwavelength data is found by two distinct dust populations: (1) a population of very small (0.01 to 0.5 μm), hence unbound, hot dust grains confined in a narrow region (∼0.1-0.3 AU) at the sublimation rim of carbonaceous material; (2) a population of bound grains at ∼2 AU that is protected from sublimation and has a higher mass despite its fainter flux level. We propose that the hot dust is produced by the release of small carbon grains following the disruption of dust aggregates that originate in the warm component. A mechanism, such as gas braking, is required to further confine the small grains for a long enough time. In situ dust production could hardly be ensured for the age of the star, so we conclude that the observed amount of dust is triggered by intense dynamical activity. Conclusions. Fomalhaut may be representative of exozodis that are currently being surveyed at near and mid-infrared wavelengths worldwide. We propose a framework for reconciling the "hot exozodi phenomenon" with theoretical constraints: the hot component of Fomalhaut is likely the "tip of the iceberg" since it could originate in the more massive, but fainter, warm dust component residing near the ice line. This inner disk exhibits interesting morphology and can be considered a prime target for future exoplanet research.

Near-infrared emission from sublimating dust in collisionally active debris disks

“…This means that not only the cross-section, but also the mass is dominated by barely bound grains in the innermost parts of the disk. Interestingly, such steep size distributions are also invoked to explain NIR interferometric observations of hot exozodiacal dust (Defrère et al 2011;Mennesson et al 2013;Lebreton et al 2013). The drop in the size distribution from β ≈ 0.5 to β ≈ 1 is also much more pronounced in the inner disk than in the parent belt.…”

“…Closer to the star, however, the pace at which material is processed by collisions is much higher, so that the lifetime of a debris belt in collisional equilibrium is much shorter there (Dominik & Decin 2003;Wyatt et al 2007). For this reason, hot exozodiacal dust cannot be explained by in situ planetesimal belts (Wyatt et al 2007;Lebreton et al 2013) 1 , and a different mechanism is needed to replenish it, the lifetime of the dust needs to be extended by some process, or both.…”

“…(e) For η Lep, 110 Her, 10 Tau, ξ Boo, and κ CrB, the possibility that the observed NIR excess is due to a low-mass companion within the field-of-view cannot be excluded . Morphological models of exozodiacal dust disks, constrained by the NIR observations, indicate that the hot dust is concentrated in a sharply peaked ring, whose inner boundary is determined by dust sublimation (Defrère et al 2011;Mennesson et al 2013;Lebreton et al 2013). The process of dust sublimation may therefore play an important role in shaping exozodiacal clouds.…”

Effects of disc asymmetries on astrometric measurements

“…Kral et al 2013). The grain temperature T s is worked out solving the thermal equilibrium of grains using the code GRaTeR (Augereau et al 1999;Lebreton et al 2013). Indeed, for small grains the black-body assumption is far from being accurate and, since we are interested in the whole wavelength range for astrometric measurements, we assess the infrared emission from the dust properly through Eq.…”

“…These regions have not yet been investigated, with the exception of a few interferometric observations (e.g. Fomalhaut, Lebreton et al 2013) and photometric light curves (Rodriguez et al 2016). We are thus constrained to rely on models by extrapolating below 1 arcsec (assuming systems close to the Earth) observations that are made further than 1 arcsec from the parent star.…”