* Astronomy

Title: The companion candidate near Fomalhaut - a background neutron star?
Author: Ralph Neuhaeuser (AIU Jena), Markus Hohle (LMU Muenchen), Christian Ginski (U Leiden), Janos Schmidt, Valeri Hambaryan (AIU Jena), Tobias Schmidt (U Hamburg)

The directly detected planetary mass companion candidate close to the young, nearby star Fomalhaut is a subject of intense discussion. While the detection of common proper motion led to the interpretation as Jovian-mass companion, later non-detections in the infrared raised doubts. Recent astrometric measurements indicate a belt crossing or highly eccentric orbit for the object, if a companion, making the planetary interpretation potentially even more problematic. In this study we discuss the possibility of Fomalhaut\,b being a background object with a high proper motion. By analysing the available photometric and astrometric data of the object, we show that they are fully consistent with a neutron star: Neutron stars are faint, hot (blue), and fast moving. Neutron stars with an effective temperature of the whole surface area being 112,000 K to 126,500 K (with small to negligible extinction) at a distance of roughly 11 pc (best fit) would be consistent with all observables, namely with the photometric detections in the optical, with the upper limits in the infrared and X-rays, as well as with the astrometry (consistent with a distances of 11 pc or more and high proper motion as typical for neutron stars) as well as with non-detection of pulsation (not beamed). We consider the probability of finding an unrelated object or even a neutron star nearby and mostly co-aligned in proper motion with Fomalhaut A and come to the conclusion that this is definitely well possible.

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Title: An interferometric study of the Fomalhaut inner debris disk. III. Detailed models of the exozodiacal disk and its origin
Authors: J. Lebreton, R. van Lieshout, J.-C. Augereau, O. Absil, B. Mennesson, M. Kama, C. Dominik, A. Bonsor, J. Vandeportal, H. Beust, D. Defrère, S. Ertel, V. Faramaz, P. Hinz, Q. Kral, A.-M. Lagrange, W. Liu, P. Thébault

Debris disks are extrasolar analogs to the solar system planetesimal belts. The star Fomalhaut harbors a cold debris belt at 140 AU as well as evidence of a warm dust component, which is suspected of being a bright analog to the solar system's zodiacal dust.
Interferometric observations obtained with the VLTI and the KIN have identified near- and mid-infrared excesses attributed to hot and warm exozodiacal dust in the inner few AU of the star. We performed parametric modelling of the exozodiacal disk using the GRaTeR radiative transfer code to reproduce the interferometric data, complemented by mid- to far-infrared measurements. A detailed treatment of sublimation temperatures was introduced to explore the hot population at the sublimation rim. We then used an analytical approach to successively testing several source mechanisms.
A good fit to the data is found by two distinct dust populations: (1) very small, hence unbound, hot dust grains confined in a narrow region at the sublimation rim of carbonaceous material; (2) bound grains at 2 AU that are protected from sublimation and have a higher mass despite their fainter flux level. We propose that the hot dust is produced by the release of small carbon grains following the disruption of aggregates that originate from 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 the observed amount of dust must be triggered by intense dynamical activity.
Fomalhaut may be representative of exozodis that are currently being surveyed 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 from a warm counterpart residing near the ice line.

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Title: Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT
Authors: Kate Y. L. Su, G. H. Rieke, R. Malhotra, K. R. Stapelfeldt, A. M. Hughes, A. Bonsor, D. J. Wilner, Z. Balog, D. M. Watson, M. W. Werner, K. A. Misselt

Vega and Fomalhaut, are similar in terms of mass, ages, and global debris disk properties; therefore, they are often referred as "debris disk twins". We present Spitzer 10-35 um spectroscopic data centred at both stars, and identify warm, unresolved excess emission in the close vicinity of Vega for the first time. The properties of the warm excess in Vega are further characterised with ancillary photometry in the mid infrared and resolved images in the far-infrared and submillimetre wavelengths. The Vega warm excess shares many similar properties with the one found around Fomalhaut. The emission shortward of ~30 um from both warm components is well described as a blackbody emission of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show such an unresolved warm dust using the same approach. These warm components may be analogous to the solar system's zodiacal dust cloud, but of far greater. The dust temperature and tentative detections in the submillimetre suggest the warm excess arises from dust associated with a planetesimal ring located near the water-frost line and presumably created by processes occurring at similar locations in other debris systems as well. We also review the properties of the 2 um hot excess around Vega and Fomalhaut, showing that the dust responsible for the hot excess is not spatially associated with the dust we detected in the warm belt. We suggest it may arise from hot nano grains trapped in the magnetic field of the star. Finally, the separation between the warm and cold belt is rather large with an orbital ratio >~10 in all four systems. In light of the current upper limits on the masses of planetary objects and the large gap, we discuss the possible implications for their underlying planetary architecture, and suggest that multiple, low-mass planets likely reside between the two belts in Vega and Fomalhaut.

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Title: Fomalhaut b: Independent Analysis of the Hubble Space Telescope Public Archive Data
Authors: Raphael Galicher, Christian Marois, Ben Zuckerman, Bruce Macintosh

The nature and even the existence of a putative planet-mass companion ("Fomalhaut b") to Fomalhaut has been debated since 2008. In the present paper we reanalyse the multi-epoch Hubble Space Telescope (HST) optical images on which the discovery claim was based. We confirm that the HST images do reveal an object in orbit around Fomalhaut but the detailed results from our analysis differ in some ways from previous discussions. In particular, we do not confirm flux variability over a two-year interval at 0.6-micron wavelength, we detect Fomalhaut b for the first time at the short wavelength of 0.43microns, we find that the HST image of Fomalhaut b at 0.8m icrons may be extended beyond the PSF, and we cannot determine from our astrometry if Fomalhaut b will cross or not the dust ring. The optical through mid-infrared spectral energy distribution (SED) of Fomalhaut b cannot be explained as due to direct or scattered radiation from a massive planet. We consider two models to explain the SED: (1) a large circumplanetary disk around a massive, but unseen, planet and (2) the aftermath of a collision during the past 100 years of two Kuiper Belt-like objects of radii about 50 km.

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