An Interferometric Study of the Fomalhaut Inner Debris Disk. Ii. Keck Nuller Mid-Infrared Observations

Abstract

We report on high contrast mid-infrared observations of Fomalhaut obtained with the Keck Interferometer Nuller (KIN) showing a small resolved excess over the level expected from the stellar photosphere. The measured null excess has a mean value of 0.35% ± 0.10% between 8 and 11 µm and increases from 8 to 13 microns. Given the small field of view of the instrument, the source of this marginal excess must be contained within 2AU of Fomalhaut. This result is reminiscent of previous VLTI Kband (≃ 2µm) observations, which implied the presence of a ∼ 0.88% excess, and argued that thermal emission from hot dusty grains located within 6 AU from Fomalhaut was the most plausible explanation. Using a parametric 2D radiative transfer code and a Bayesian analysis, we examine different dust disk structures to reproduce both the near and midinfrared data simultaneously. While not a definitive explanation of the hot excess of Fomalhaut, our model suggests that the most likely inner few AU disk geometry consists of a two-component structure, with two different and spatially distinct grain populations. The 2 to 11 microns data are consistent with an inner hot ring of very small (≃ 10 to 300 nm) carbon-rich grains concentrating around 0.1 AU. The second dust populationinferred from the KIN data at longer mid infrared wavelengths -consists of larger grains (size of a few microns to a few tens of microns) located further out in a colder region where regular astronomical silicates could survive, with an inner edge around 0.4 AU to 1 AU. From a dynamical point of view, the presence of the inner concentration of sub-micron sized grains is surprising, as such grains should be expelled from the inner planetary system by radiation pressure within only a few years. This could either point to some inordinate replenishment rates (e.g. many grazing comets coming from an outer reservoir) or to the existence of some braking mechanism preventing the grains from moving out.Subject headings: instrumentation: interferometers-infrared: stars-stars: circumstellar matter-stars: individual (Fomalhaut) High accuracy long baseline interferometric observations can provide the resolution and contrast

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.…”

“…There are two additional problems with this scenario, which also serve as clues for solving the hot exozodiacal dust mystery. (1) Hot exozodiacal dust is thought to consist mostly of blowout grains with sizes around 0.01-0.1 µm (Akeson et al 2009;Defrère et al 2011;Mennesson et al 2013;Lebreton et al 2013), while P-R drag only transports bound grains to the sublimation zone, the smallest of which have sizes of about 1 µm. (2) The dust distribution resulting from the balance between P-R drag and collisions yields an SED with a positive slope in the infrared domain, while observations find negative slopes (e.g., Akeson et al 2009;Acke et al 2012), and the pile-up of dust is too inefficient to have an effect on the slope of the SED 15 .…”

“…(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.…”

A near-infrared interferometric survey of debris-disc stars

“…Figure 1 shows that if all the circumstellar emission was located at the sublimation radius of micron-size carbonaceous grains, its diameter would only be about 5 mas and would therefore not be fully resolved. However, all the models that we have developed so far to reproduce resolved near-infrared detections of exozodiacal dust (Defrère et al 2011;Mennesson et al 2013;Lebreton et al 2013) have shown that most of the disc's thermal emission is produced at the sublimation distance of submicron grains, i.e., a region about 12 mas in diameter in the case of tet Per. We therefore expect that any dusty disc around tet Per would be fully resolved with FLUOR on the shortest baseline of the CHARA array.…”

Exozodiacal clouds: hot and warm dust around main sequence stars

“…The sky transmission maps of the KIN and LBTI mid-IR nulling experiments are, on the other hand, much more complex in shape, and depend sensibly on observing parameters such as the observing date/time, and the parallactic and hour angles [e.g. 77,113]. The above 1D approach (Equation (4)) is therefore not appropriate, and it is necessary to multiply the synthetic 2D disc image in the sky plane with the 2D transmission map to obtain a synthetic null value.…”