We present ALMA mosaic observations at 1.3mm (223 GHz) of the Fomalhaut system with a sensitivity of 14μJy/beam. These observations provide the first millimeter map of the continuum dust emission from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt an MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of 136.3±0.9au and width of 13.5±1.8au. We determine a best-fit eccentricity of 0.12±0.01. Assuming a size distribution power-law index of q=3.46±0.09, we constrain the dust absorptivity power-law index β to be 0.9<β<1.5. The geometry of the disk is robustly constrained with inclination 65°.6±0°. 3, position angle 337°.9±0°. 3, and argument of periastron 22°.5±4°.3. Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with Hubble Space Telescope, SCUBA, and ALMA. However, we cannot rule out structures 10 au in size or that only affect smaller grains. The central star is clearly detected with a flux density of 0.75±0.02mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths.
The origin of trace hydrogen in white dwarfs (WDs) with He-dominated atmospheres is a longstanding problem, one that cannot satisfactorily be explained by the historically-favoured hypothesis of accretion from the interstellar medium. Here we explore the possibility that the gradual accretion of exo-Oort cloud comets, which are a rich source of H, contributes to the apparent increase of trace H with WD cooling age. We determine how often remnant exo-Oort clouds, freshly excited from post-main-sequence stellar mass loss, dynamically inject comets inside the WD's Roche radius. We improve upon previous studies by considering a representative range of single WD masses (0.52 − 1.00M ⊙ ) and incorporating different cloud architectures, giant branch stellar mass loss, stellar flybys, Galactic tides and a realistic escape ellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyr ages of WD cooling. We find that ∼ 10 −5 of the material in an exo-Oort cloud is typically amassed onto the WD, and that the H deposits accumulate even as the cloud dissipates. This accumulation may account for the relatively large amount of trace H, 10 22 − 10 25 g, that is determined frequently among WDs with cooling ages 1 Gyr. Our results also reaffirm the notion that exo-Oort cloud comets are not the primary agents of the metal budgets observed in polluted WD atmospheres.
The Large Binocular Telescope Interferometer (LBTI) enables nulling interferometric observations across the N band (8 to 13 µm) to suppress a star's bright light and probe for faint circumstellar emission. We present and statistically analyze the results from the LBTI/HOSTS (Hunt for Observable Signatures of Terrestrial Systems) survey for exozodiacal dust. By comparing our measurements to model predictions based on the Solar zodiacal dust in the N band, we estimate a 1 σ median sensitivity of 23 zodis for early type stars and 48 zodis for Sun-like stars, where 1 zodi is the surface density of habitable zone (HZ) dust in the Solar system. Of the 38 stars observed, 10 show significant excess. A clear correlation of our detections with the presence of cold dust in the systems was found, but none with the stellar spectral type or age. The majority of Sun-like stars have relatively low HZ dust levels (best-fit median: 3 zodis, 1 σ upper limit: 9 zodis, 95% confidence: 27 zodis based on our N band measurements), while ∼20% are significantly more dusty. The Solar system's HZ dust content is consistent with being typical. Our median HZ dust level would not be a major limitation to the direct imaging search for Earth-like exoplanets, but more precise constraints are still required, in particular
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