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We initiated a deep-imaging survey of Scorpius-Centaurus A-F stars with predicted warm inner and cold outer belts of debris reminiscent of the architecture of emblematic systems such as HR 8799. We present resolved SPHERE images of a narrow ring of debris around the F5-type star HD 141011 that was observed as part of our survey in 2015, 2016, and 2019. The ring extends up to ~1.1" (~141 au) from the star in the IRDIS and IFS data obtained in 2016 and 2019. The disk is not detected in the 2015 data which are of poorer quality. The disks is best reproduced by models of a noneccentric ring centered on the star with an inclination of $69.1\pm0.9^{\circ}$, a position angle of $-24.6 \pm 1.7^{\circ}$, and a semimajor axis of $127.5\pm3.8$ au. The combination of radial velocity and imaging data excludes brown-dwarf (M>13.6 MJup) companions coplanar with the disk from 0.1 to 0.9 au and from 20 au up to 500 au (90% probability). HD 141011 adds to the growing list of debris disks that are resolved in Sco-Cen. It is one of the faintest disks that are resolved from the ground and has a radial extent and fractional width ($\sim$12.5%) reminiscent of Fomalhaut. Its moderate inclination and large semimajor axis make it a good target for the James Webb Space Telescope and should allow a deeper search for putative companions shaping the dust distribution., 13 pages, 10 figures. Paper II of our Sco-Cen imaging survey. Accepted in A&A

Results are presented from a semicoherent search for continuous gravitational waves from a nearby neutron star candidate, Fomalhaut b, using data collected in the second observing run of Advanced LIGO. The search is based on a hidden Markov model scheme, capable of tracking signal frequency evolution from the star’s secular spin down and stochastic timing noise simultaneously. The scheme is combined with a frequency domain matched filter (F-statistic), calculated coherently over five-day time stretches. The frequency band 100-1000 Hz is searched. After passing the above-threshold candidates through a hierarchy of vetoes, one candidate slightly above the 1% false alarm probability threshold remains for further scrutiny. No strong evidence of continuous waves is found. We present the strain upper limits in the full frequency band searched at 90% confidence level., * The 'upper_limits_full_band.txt' file provides the source data in Figures 5 and 6. The first column is the frequency (Hz). The second column is the strain upper limits in Figure 5 (blue dots). The third and fourth columns are the upper limits on the fiducial ellipticity and r-mode amplitude, in Figures 6(a) and 6(b), respectively. * The 'upper_limits_from_injection.txt' file provides the source data of the upper limits obtained from injections in 22 sub-bands (orange stars in Figure 5). The first column is the frequency (Hz). The second column is the strain upper limits obtained from injections.

The star Fomalhaut hosts a narrow, eccentric debris disc, plus a highly eccentric companion Fomalhaut b. It is often argued that Fomalhaut b cannot have significant mass, otherwise it would quickly perturb the disc. We show that material in internal mean-motion resonances with a massive, coplanar Fomalhaut b would actually be long-term stable, and occupy orbits similar to the observed debris. Furthermore, millimetre dust released in collisions between resonant bodies could reproduce the width, shape and orientation of the observed disc. We first re-examine the possible orbits of Fomalhaut b, assuming that it moves under gravity alone. If Fomalhaut b orbits close to the disc midplane then its orbit crosses the disc, and the two are apsidally aligned. This alignment may hint at an ongoing dynamical interaction. Using the observationally allowed orbits, we then model the interaction between a massive Fomalhaut b and debris. Whilst most debris is unstable in such an extreme configuration, we identify several resonant populations that remain stable for the stellar lifetime, despite crossing the orbit of Fomalhaut b. This debris occupies low-eccentricity orbits similar to the observed debris ring. These resonant bodies would have a clumpy distribution, but dust released in collisions between them would form a narrow, relatively smooth ring similar to observations. We show that if Fomalhaut b has a mass between those of Earth and Jupiter then, far from removing the observed debris, it could actually be sculpting it through resonant interactions., Comment: 24 pages, 11 figures, accepted for publication in MNRAS

Fomalhaut C (LP 876-10) is a low mass M4V star in the intriguing Fomalhaut triple system and, like Fomalhaut A, possesses a debris disc. It is one of very few nearby M-dwarfs known to host a debris disc and of these has by far the lowest stellar mass. We present new resolved observations of the debris disc around Fomalhaut C with the Atacama Large Millimetre Array which allow us to model its properties and investigate the system's unique history. The ring has a radius of 26 au and a narrow full width at half maximum of at most 4.2 au. We find a 3$\sigma$ upper limit on the eccentricity of 0.14, neither confirming nor ruling out previous dynamic interactions with Fomalhaut A that could have affected Fomalhaut C's disc. We detect no $^{12}$CO J=3-2 emission in the system and do not detect the disc in scattered light with HST/STIS or VLT/SPHERE. We find the original Herschel detection to be consistent with our ALMA model's radial size. We place the disc in the context of the wider debris disc population and find that its radius is as expected from previous disc radius-host luminosity trends. Higher signal-to-noise observations of the system would be required to further constrain the disc properties and provide further insight to the history of the Fomalhaut triple system as a whole., Comment: 15 pages, 10 figures, 2 tables, Accepted for publication in MNRAS

This paper shows that the eccentric debris rings seen around the stars Fomalhaut and HD 202628 are narrower than expected in the standard eccentric planet perturbation scenario (sometimes referred to as "pericenter glow"). The standard scenario posits an initially circular and narrow belt of planetesimals at semi-major axis $a$, whose eccentricity is increased to $e_f$ after the gas disc has dispersed by secular perturbations from an eccentric planet, resulting in a belt of width $2ae_f$. In a minor modification of this scenario, narrower belts can arise if the planetesimals are initially eccentric, which could result from earlier planet perturbations during the gas-rich protoplanetary disc phase. However, a primordial eccentricity could alternatively be caused by instabilities that increase the disc eccentricity, without the need for any planets. Whether these scenarios produce detectable eccentric rings within protoplanetary discs is unclear, but they nevertheless predict that narrow eccentric planetesimal rings should exist before the gas in protoplanetary discs is dispersed. PDS 70 is noted as a system hosting an asymmetric protoplanetary disc that may be a progenitor of eccentric debris ring systems., accepted to Royal Society Open Science

The apparent detection of an exoplanet orbiting Fomalhaut was announced in 2008. However, subsequent observations of Fomalhaut b raised questions about its status: Unlike other exoplanets, it is bright in the optical and nondetected in the infrared, and its orbit appears to cross the debris ring around the star without the expected gravitational perturbations. We revisit previously published data and analyze additional Hubble Space Telescope (HST) data, finding that the source is likely on a radial trajectory and has faded and become extended. Dynamical and collisional modeling of a recently produced dust cloud yields results consistent with the observations. Fomalhaut b appears to be a directly imaged catastrophic collision between two large planetesimals in an extrasolar planetary system. Similar events should be very rare in quiescent planetary systems of the age of Fomalhaut, suggesting that we are possibly witnessing the effects of gravitational stirring due to the orbital evolution of hypothetical planet(s) around the star., 12 pages, 9 figures, accepted to PNAS

Proxima Centauri is known to host an earth-like planet in its habitable zone; very recently a second candidate planet was proposed based on radial velocities. At quadrature, the expected projected separation of this new candidate is larger than 1 arcsec, making it a potentially interesting target for direct imaging. While difficult, identification of the optical counterpart of this planet would allow detailed characterization of the closest planetary system. We searched for a counterpart in SPHERE images acquired during four years through the SHINE survey. In order to account for the large orbital motion of the planet, we used a method that assumes the circular orbit obtained from radial velocities and exploits the sequence of observations acquired close to quadrature in the orbit. We checked this with a more general approach that considers keplerian motion, K-stacker. We did not obtain a clear detection. The best candidate has S/N=6.1 in the combined image. A statistical test suggests that the probability that this detection is due to random fluctuation of noise is < 1% but this result depends on the assumption that distribution of noise is uniform over the image. The position of this candidate and the orientation of its orbital plane fit well with observations in the ALMA 12m array image. However, the astrometric signal expected from the orbit of the candidate we detected is 3-sigma away from the astrometric motion of Proxima as measured from early Gaia data. This, together with the unexpectedly high flux associated with our direct imaging detection, means we cannot confirm that our candidate is indeed Proxima c. On the other hand, if confirmed, this would be the first observation in imaging of a planet discovered from radial velocities and the second one (after Fomalhaut b) of reflecting circumplanetary material. Further confirmation observations should be done as soon as possible., 14 pages, 5 figures. Accepted for publication on Astronomy and Astrophysics

Recent analysis suggests that the faint optical point source observed around Fomalhaut from 2004-2014 (Fomalhaut b) is gradually fading and expanding, supporting the case that it may be a dispersing dust cloud resulting from the sudden disruption of a planetesimal. These types of disruptions may arise from catastrophic collisions of planetesimals, which are perturbed from their original orbits in the Fomalhaut dust ring by nearby giant planets. However, disruptions can also occur when the planetesimals pass within the tidal disruption field of the planet(s) that perturbed them in the first place, similar to the Shoemaker-Levy event observed in the Solar System. Given that a gravitationally focusing giant planet has a much larger interaction cross-section than a planetesimal, tidal disruption events can match or outnumber planetesimal collision events in realistic regions of parameter space. Intriguingly, the Fomalhaut dust cloud offers an opportunity to directly distinguish between these scenarios. A tidal disruption scenario leads to a very specific prediction of ephemerides for the planet causing the event. At a most probable mass of 66 Mearth, a semi-major axis of 117 AU, and a system age of 400-500 Myr, this planet would be readily detectable with the James Webb Space Telescope. The presence or absence of this planet at the specific, predicted position is therefore a distinctive indicator of whether the dispersing cloud originated from a collision of two planetesimals or from the disruption of a planetesimal in the tidal field of a giant planet., Comment: 6 pages, 3 figures, accepted for publication in A&A

We present constraints on the presence of substellar companions to the nearby ($d\sim7.6$ pc), young ($440\pm40$ Myr) K4Ve star TW Piscis Austrini, the wide ($\sim$0.3 pc) companion to the A4V star Fomalhaut. We combined absolute astrometry from Hipparcos and Gaia with literature radial velocity measurements and dedicated high-contrast imaging observations obtained with Keck/NIRC2 to achieve sensitivity to brown dwarf and planetary-mass companions ($\gtrsim2$ M$_{\rm Jup}$) over many decades of orbital period ($\lesssim 10^3$ yr). The significant astrometric acceleration measured between the Hipparcos and Gaia catalogues, reported previously in the literature, cannot be explained by the orbital motion of TW PsA around the barycenter of the Fomalhaut triple system. Instead, we find that it is consistent with the reflex motion induced by an orbiting substellar companion. The combination of astrometry, radial velocities, and a deep $L^{\prime}$ imaging dataset leads to a constraint on the companion mass of $1.2_{-0.6}^{+0.7}$ M$_{\rm Jup}$. However, the period of the companion is poorly constrained, with a highly multi-modal period posterior distribution due to aliasing with the 24.25 year baseline between Hipparcos and Gaia. If confirmed through continued astrometric or spectroscopic monitoring, or via direct detection, the companion to TW PsA would represent a choice target for detailed atmospheric characterization with high-contrast instruments on the upcoming James Webb Space Telescope and Wide Field Infrared Survey Telescope., Accepted for publication in the Astronomical Journal. 15 pages, 11 figures

Fomalhaut is a nearby stellar system and has been found to be a triple based on astrometric observations. With new radial velocity and astrometric data, we study the association between Fomalhaut A, B, and C in a Bayesian framework finding that the system is gravitationally bound or at least associated. Based on simulations of the system, we find that Fomalhaut C can be easily destabilized through combined perturbations from the Galactic tide and stellar encounters. Considering that observing the disruption of a triple is probably rare in the solar neighbourhood, we conclude that Fomalhaut C is a so-called "gravitational pair" of Fomalhaut A and B. Like the Cooper pair mechanism in superconductors, this phenomena only appears once the orbital energy of a component becomes comparable with the energy fluctuations caused by the environment. Based on our simulations we find (1) an upper limit of 8 km/s velocity difference is appropriate when selecting binary candidates and (2) an empirical formula for the escape radius, which is more appropriate than tidal radius when measuring the stability of wide binaries., 9 pages, 7 figures, MNRAS in press

Context. Vega and Fomalhaut display a thermal emission brightness that could possibly arise from hot dust near the stars, an inner extension of their planetary debris disks. An idea has been suggested that nanometer-sized dust particles are kept in the vicinity of the stars by electromagnetic forces. This resembles the trapping that model calculations show in the corotating magnetic field in the inner heliosphere within approximately 0.2 AU from the Sun. Aims. The aim of this work is to study whether the trapping of dust due to electromagnetic forces acting on charged dust near the Sun can occur around Vega and Fomalhaut and what are the conditions for trapping. Methods. We studied the dust trajectories with numerical calculations of the full equation of motion, as well as by using the guiding center approximation. We assumed a constant dust charge and a Parker-type magnetic field, which we estimated for the two stars. Results. We find no bound trajectories of charged particles around Vega or Fomalhaut as long as the radiation pressure force exceeds the gravitational force, that is, for particles smaller than 1 μm. A trapping zone could exist inside of 0.02 AU for Vega and 0.025 AU for Fomalhaut, but only for those particles with radiation pressure force smaller than gravitational force. In comparison to the Sun, the trapping conditions would occur closer to the stars because their faster rotation leads to a more closely wound-up magnetic field spiral. We also show that plasma corotation can be consistent with trapping. Our model calculations show that the charged particles are accelerated to stellar wind velocity very quickly, pass 1 AU after approximately three days, and are further ejected outward where they pass the debris disks at high velocity. We find this for particles with a surface charge-to-mass ratio larger than 10−6 elementary charges per proton mass for both negatively and positively charged dust and independent of the strength of the radiation pressure force. Based on charging assumptions, this would correspond to dust of sizes 100 nm and smaller.

International audience; We present the first scattered-light images of two debris disks around the F8 star HD104860 and the F0V star HD192758, respectively ∼45 and ∼67pc away. We detected these systems in the F110W and F160W filters through our reanalysis of archival Hubble Space Telescope (HST) NICMOS data with modern starlight-subtraction techniques. Our image of HD104860 confirms the morphology previously observed by Herschel in thermal emission with a well-defined ring at a radius of ∼114au inclined by ∼58°. Although the outer edge profile is consistent with dynamical evolution models, the sharp inner edge suggests sculpting by unseen perturbers. Our images of HD192758 reveal a disk at radius ∼95au inclined by ∼59°, never resolved so far. These disks have low scattering albedos of 10% and 13%, respectively, inconsistent with water ice grain compositions. They are reminiscent of several other disks with similar inclination and scattering albedos: Fomalhaut, HD92945, HD202628, and HD207129. They are also very distinct from brighter disks in the same inclination bin, which point to different compositions between these two populations. Varying scattering albedo values can be explained by different grain porosities, chemical compositions, or grain size distributions, which may indicate distinct formation mechanisms or dynamical processes at work in these systems. Finally, these faint disks with large infrared excesses may be representative of an underlying population of systems with low albedo values. Searches with more sensitive instruments on HST or on the James Webb Space Telescope and using state-of-the art starlight-subtraction methods may help discover more of such faint systems.

Fomalhaut b was long thought to shape the eccentric debris belt in the Fomalhaut system, but its orbit was found to be too eccentric for it to be the dominant belt-shaping perturber. This indicates that Fomalhaut b is Earth-sized at most and that the belt-shaping perturber, hereafter named Fomalhaut c, remains to be discovered. In addition, since its orbit more or less crosses that of Fomalhaut b, it also indicates that the current configuration of the system is transient and was reached recently. In this talk, we show that this current configuration can be explained if Fomalhaut c is Saturn- to Neptune-sized, and Fomalhaut b originates from a mean-motion resonance with Fomalhaut c., Comment: 4 pages, 2 figures

Imaging a star's companion at multiple epochs over a short orbital arc provides only four of the six coordinates required for a unique orbital solution. Probability distributions of possible solutions are commonly generated by Monte Carlo (MCMC) analysis, but these are biased by priors and may not probe the full parameter space. We suggest alternative methods to characterise possible orbits, which compliment the MCMC technique. Firstly the allowed ranges of orbital elements are prior-independent, and we provide means to calculate these ranges without numerical analyses. Hence several interesting constraints (including whether a companion even can be bound, its minimum possible semi-major axis and its minimum eccentricity) may be quickly computed using our relations as soon as orbital motion is detected. We also suggest an alternative to posterior probability distributions as a means to present possible orbital elements, namely contour plots of elements as functions of line of sight coordinates. These plots are prior-independent, readily show degeneracies between elements and allow readers to extract orbital solutions themselves. This approach is particularly useful when there are other constraints on the geometry, for example if a companion's orbit is assumed to be aligned with a disc. As examples we apply our methods to several imaged sub-stellar companions including Fomalhaut b, and for the latter object we show how different origin hypotheses affect its possible orbital solutions. We also examine visual companions of A- and G-type main sequence stars in the Washington Double Star Catalogue, and show that $\gtrsim50$ per cent must be unbound., Accepted for publication in MNRAS

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., 15 pages with 6 figures and 4 tables, paper in press; Monthly Notices Royal Astronomical Society, 2015

In recent years, gas has been observed in an increasing number of debris discs, though its nature remains to be determined. Here, we analyse CO molecular excitation in optically thin debris discs, and search ALMA Cycle-0 data for CO J=3-2 emission in the Fomalhaut ring. No significant line emission is observed; we set a 3-$\sigma$ upper limit on the integrated line flux of 0.16 Jy km s$^{-1}$. We show a significant dependency of the CO excitation on the density of collisional partners $n$, on the gas kinetic temperature $T_k$ and on the ambient radiation field $J$, suggesting that assumptions widely used for protoplanetary discs (e.g. LTE) do not necessarily apply to their low density debris counterparts. When applied to the Fomalhaut ring, we consider a primordial origin scenario where H$_2$ dominates collisional excitation of CO, and a secondary origin scenario dominated by e$^-$ and H$_2$O. In either scenario, we obtain a strict upper limit on the CO mass of 4.9 $\times$ 10$^{-4}$ M$_{\oplus}$. This arises in the non-LTE regime, where the excitation of the molecule is determined solely by the well-known radiation field. In the secondary scenario, assuming any CO present to be in steady state allows us to set an upper limit of $\sim$55% on the CO/H$_2$O ice ratio in the parent planetesimals. This could drop to $\sim$3% if LTE applies, covering the range observed in Solar System comets (0.4-30%). Finally, in light of our analysis, we present prospects for CO detection and characterisation in debris discs with ALMA., Comment: Accepted for publication in MNRAS - 14 pages, 8 figures

Extended gaps in the debris disks of both Vega and Fomalhaut have been observed. These structures have been attributed to tidal perturbations by multiple super-Jupiter gas giant planets. Within the current observational limits, however, no such massive planets have been detected. Here we propose a less stringent `lone-planet' scenario to account for the observed structure with a single eccentric gas giant and suggest that clearing of these wide gaps is induced by its sweeping secular resonance. During the depletion of the disk gas, the planet's secular resonance propagates inward and clears a wide gap over an extended region of the disk. Although some residual intermediate-size planetesimals may remain in the gap, their surface density is too low to either produce super-Earths or lead to sufficiently frequent disruptive collisions to generate any observable dusty signatures. The main advantage of this lone-planet sweeping-secular-resonance model over the previous multiple gas giant tidal truncation scenario is the relaxed requirement on the number of gas giants. The observationally inferred upper mass limit can also be satisfied provided the hypothetical planet has a significant eccentricity. A significant fraction of solar or more massive stars bear gas giant planets with significant eccentricities. If these planets acquired their present-day kinematic properties prior to the depletion of their natal disks, their sweeping secular resonance would effectively impede the retention of neighboring planets and planetesimals over a wide range of orbital semi-major axes., Comment: 20 pages, 12 figures. Accepted for publication in ApJ

Fomalhaut A is among the most well-studied nearby stars and has been discovered to possess a putative planetary object as well as a remarkable eccentric dust belt. This eccentric dust belt has often been interpreted as the dynamical signature of one or more planets that elude direct detection. However, the system also contains two other stellar companions residing ~100,000 AU from Fomalhaut A. We have designed a new symplectic integration algorithm to model the evolution of Fomalhaut A's planetary dust belt in concert with the dynamical evolution of its stellar companions to determine if these companions are likely to have generated the dust belt's morphology. Using our numerical simulations, we find that close encounters between Fomalhaut A and B are expected, with a ~25% probability that the two stars have passed within at least 400 AU of each other at some point. Although the outcomes of such encounter histories are extremely varied, these close encounters nearly always excite the eccentricity of Fomalhaut A's dust belt and occasionally yield morphologies very similar to the observed belt. With these results, we argue that close encounters with Fomalhaut A's stellar companions should be considered a plausible mechanism to explain its eccentric belt, especially in the absence of detected planets capable of sculpting the belt's morphology. More broadly, we can also conclude from this work that very wide binary stars may often generate asymmetries in the stellar debris disks they host., Accepted to MNRAS, 22 pages, 15 figures, 2 appendices

We present the experimental phase functions of three types of millimeter-sized dust grains consisting of enstatite, quartz, and volcanic material from Mount Etna, respectively. The three grains present similar sizes but different absorbing properties. The measurements are performed at 527 nm covering the scattering angle range from 3° to 170°. The measured phase functions show two well-defined regions: (i) soft forward peaks and (ii) a continuous increase with the scattering angle at side- and back-scattering regions. This behavior at side- and back-scattering regions is in agreement with the observed phase functions of the Fomalhaut and HR 4796A dust rings. Further computations and measurements (including polarization) for millimeter-sized grains are needed to draw some conclusions about the fluffy or compact structure of the dust grains.

In a previous work (Pichardo et al. 2005), we studied stable configurations for circumstellar discs in eccentric binary systems. We searched for "invariant loops": closed curves (analogous to stable periodic orbits in time-independent potentials) that change shape with the binary orbital phase, as test particles in them move under the influence of the binary potential. This approach allows us to identify stable configurations when pressure forces are unimportant, and dissipation acts only to prevent gas clouds from colliding with one another. We now extend this work to study the main geometrical properties of circumbinary discs. We have studied more than 100 cases with a range in eccentricity 0 .le. e .le. 0.9, and mass ratio 0.1 .le. q .le. 0.9. Although gas dynamics may impose further restrictions, our study sets lower stable bounds for the size of the central hole in a simple and computationally cheap way, with a relation that depends on the eccentricity and mass ratio of the central binary. We extend our previous studies and focus on an important component of these systems: circumbinary discs. The radii for stable orbits that can host gas in circumbinary discs are sharply constrained as a function of the binary's eccentricity. The circumbinary disc configurations are almost circular, with eccentricity e_d < 0.15, but if the mass ratio is unequal the disk is offset from the center of mass of the system. We compare our results with other models, and with observations of specific systems like GG Tauri A, UY Aurigae, HD 98800 B, and Fomalhaut, restricting the plausible parameters for the binary., 12 pages, 10 figures and 6 tables. MNRAS, accepted

We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 $\mu$Jy/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 a 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 $136.3\pm0.9$ AU and width of $13.5\pm1.8$ AU. We determine a best-fit eccentricity of $0.12\pm0.01$. Assuming a size distribution power law index of $q=3.46\pm 0.09$, we constrain the dust absorptivity power law index $\beta$ to be $0.9, Comment: 15 pages, 8 figures

Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4$\sigma$ detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived ($0.65-42 \times10^{-7}$ M$_{\oplus}$) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO$_2$ mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that $(49\pm 27)$% of the detected flux originates from a region near the eccentric belt's pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts., Comment: Accepted for publication in ApJ. 17 pages, 8 figures

Fomalhaut b is a directly imaged object in the debris disk of the star Fomalhaut. It has been hypothesized to be a planet, however there are issues with the observed colours of the object that do not fit planetary models. An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut's disk. We test if Fomalhaut b could be a neutron star using X-ray observations with Chandra's HRC-I instrument in the energy range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on the X-ray flux of Fomalhaut b of F_X < 1.3e-14 erg/cm/s^2 in the energy range 0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper limit of L_X < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b's X-ray non-detection constrains the parameter space for a possible neutron star significantly, implying surface temperatures lower than 91000 K and distances closer than 13.3 pc to the solar system. In addition we find that reflected starlight from the central star fits the available optical detections of Fomalhaut b; a smaller planet with a large ring system might explain such a scenario., 7 pages

To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from October 2015 through December 2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 +\- 9.7 milli-mag/yr in the Swift wavebands, with amounts of 21.0 +\- 4.5 mmag in the groundbased B measurements, 14.0 +\- 4.5 mmag in V, and 13.0 +\- 4.5 in R, and a rate of 5.0 +\- 1.2 mmag/yr averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at >= 3 sigma by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous SED models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with T_{eff} = 7000 - 7100 K and A_V ~ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 microns, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (i.e., R_V >= 5, but not flat across all the bands) compared with the extinction law for the general ISM (R_V = 3.1), suggesting that the dimming arises from circumstellar material., accepted by the Astrophysical Journal; acknowledgements revised 9/1/2017

Fomalhaut harbours a moderately eccentric dust belt with a planet candidate (Fom b) imaged near its inner edge. MCMC-based orbital determination of Fom b shows that the orbit is highly eccentric (e≃0.9), nearly apsidally aligned with the belt. We study the secular interaction between the planet and the dust ring. We show that only if it is a small mass object, Fom b can perturb the belt without destroying it. But Fom b's perturbing action inevitably drives the belt to high eccentricity and apsidal misalignment. This behaviour is due to the planet's high eccentricity.This dynamical outcome contradicts both observations and orbital determination. We conclude that another, more massive and less eccentric planet (Fom c) is required to dynamically control the belt. We show that Fom b is likely to have been formerly trapped in mean-motion resonance with Fom c and that subsequent eccentricity increase caused it to cross Fom c's orbit and to jump on its present day orbit via a scattering event.

High contrast imaging observations with the Hubble Space Telescope show that the nearby star Fomalhaut is surrounded by a dusty debris belt and a candidate planet, Fomalhaut b, located just inside the edge of the belt. Fomalhaut b has unexpected characteristics, such as a relatively blue spectrum, leading to the hypothesis that the detected object is a low-mass planet hosting a giant planetary dust ring or cloud seen in reflected light. Here we present new HST/STIS observations made in 2010 and 2012 that authenticate the existence of Fomalhaut b. Our MCMC analysis of four epochs of astrometry spread over eight years indicate that the orbit has a~170 AU and e~0.85. Fomalhaut b's orbit is apsidally aligned with the main belt, and periapse is located approximately ~30 AU south of the star. We also show the existence of a ~50 AU wide azimuthal dust depletion in the dust belt. These new findings provide a revised picture of Fomalhaut as a dynamically complex system, where the orbit of Fomalhaut b and the belt structure signify the presence of additional massive planets orbiting the star.

Context. The formation of planetary systems is a common, yet complex mechanism. Numerous stars have been identified to possess a debris disk, a proto-planetary disk or a planetary system. The understanding of such formation process requires the study of debris disks. These targets are substantial and particularly suitable for optical and infrared observations. Sparse Aperture masking (SAM) is a high angular resolution technique strongly contributing to probe the region from 30 to 200 mas around the stars. This area is usually unreachable with classical imaging, and the technique also remains highly competitive compared to vortex coronagraphy. Aims. We aim to study debris disks with aperture masking to probe the close environment of the stars. Our goal is either to find low mass companions, or to set detection limits. Methods. We observed eight stars presenting debris disks ( $\beta$ Pictoris, AU Microscopii, 49 Ceti, $\eta$ Telescopii, Fomalhaut, g Lupi, HD181327 and HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close companions were detected using closure phase information under 0.5 of separation from the parent stars. We obtained magnitude detection limits that we converted to Jupiter masses detection limits using theoretical isochrones from evolutionary models. Conclusions. We derived upper mass limits on the presence of companions in the area of few times the diffraction limit of the telescope around each target star., Comment: 7 pages, All magnitude detection limits maps are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)

This is a single-factor analysis of the variance of the average quasimonochromatic magnitudes of Fomalhaut, α PsA, in the 325-1080 nm spectral range for different observation seasons collected in the Pulkovo Spectrophotometric Data Base (PSDB). The overall variance of the data combined over several seasons is decomposed into components characterizing the randomness and variability factors separately. The influence of the variability factor on the variance of the average magnitudes is found to be significant with a confidence coefficient exceeding 95%. The estimated amplitude of the variability is 0m.02-0m.03. It is proposed that the main causes of the variability are photospheric pulsations (changes in the temperature by ~40 K and in the radius by ~0.5%) and variations in the scattering of radiation by dust in the circumstellar disk owing to the large scale inhomogeneity of the disk. Data from the Hipparcos catalog indicate a variability of the star in the Vt band with an amplitude of ~0m.017 and a period of 20.6 days. It is suggested that the disk surrounding the star contains a region with an elevated concentration of particles which periodically eclipses the star as it undergoes Kepler rotation. Another explanation is also proposed: transit of an unknown planet across the star’s disk. The existence of a planet at a distance of ~0.2 a.u. will require further confirmation.

Since the 1980's it has been becoming increasingly clear that the Solar System's irregular satellites are collisionally evolved. We derive a general model for the collisional evolution of an irregular satellite swarm and apply it to the Solar System and extrasolar planets. Our model reproduces the Solar System's complement of observed irregulars well, and suggests that the competition between grain-grain collisions and Poynting-Robertson (PR) drag helps set the fate of the dust. Because swarm collision rates decrease over time the main dust sink can change with time, and may help unravel the accretion history of synchronously rotating regular satellites that show brightness asymmetries. Some level of dust must be present on AU scales around the Solar System's giant planets, which we predict may be at detectable levels. We also predict whether dust produced by extrasolar circumplanetary swarms can be detected. The coronagraphic instruments on JWST will have the ability to detect the dust generated by these swarms, which are most detectable around planets that orbit at tens of AU from the youngest stars. Because the collisional decay of swarms is relatively insensitive to planet mass, swarms can be much brighter than their host planets and allow discovery of Neptune-mass planets that would otherwise remain invisible. This dust may have already been detected. The observations of the planet Fomalhaut b can be explained as scattered light from dust produced by the collisional decay of an irregular satellite swarm around a 10 Earth-mass planet. Such a swarm comprises about 5 Lunar masses worth of irregular satellites. Finally, we consider what happens if Fomalhaut b passes through Fomalhaut's main debris ring, which allows the circumplanetary swarm to be replenished through collisions with ring planetesimals. (abridged)

Long-period comets observed in our solar system are believed to originate from the Oort cloud, which is estimated to extend from roughly a few thousand to $10^5$ AU from the Sun. Despite many theoretical arguments for its existence, no direct observations of the cloud have been reported. Here, we explore the possibility of measuring Oort clouds around other stars through their emission at submillimeter wavelengths. Observations with the 545 and 857 GHz bands of the Planck satellite are well matched to the expected temperatures of Oort cloud bodies (on the order of 10 K). By correlating the Planck maps with catalogs of stars observed by the Gaia mission, we are able to constrain interesting regions of the exo-Oort cloud parameter space, placing limits on the total mass and the minimum size of grains in the cloud. We compare our measurements with known debris disk systems -- in the case of Vega and Fomalhaut we find a significant excess that is in agreement with measurements from Herschel. We use the measurements around Fomalhaut to constrain a possible exo-Oort cloud of that system. We explore an observed excess around the brightest and nearest stars in our sample as arising from possible exo-Oort clouds or other extended sources of thermal emission. We argue that future CMB surveys and targeted observations with far-infrared and millimeter wavelength telescopes have the potential to detect exo-Oort clouds or other extended sources of thermal emission beyond $\sim 1000$ AU from the parent stars., 19 pages, 12 figures, submitted to ApJ

We present the first spatially resolved mid-infrared (37.1 $��$m) image of the Fomalhaut debris disk. We use PSF fitting and subtraction to distinctly measure the flux from the unresolved component and the debris disk. We measure an infrared excess in the point source of $0.9 \pm 0.2$ Jy, consistent with emission from warm dust in an inner disk structure (Su et al. 2016), and inconsistent with a stellar wind origin. We cannot confirm or rule out the presence of a pileup ring (Su et al. 2016) near the star. In the cold region, the 37 $��$m imaging is sensitive to emission from small, blowout grains, which is an excellent probe of the dust production rate from planetesimal collisions. Under the assumptions that the dust grains are icy aggregates and the debris disk is in steady state, this result is consistent with the dust production rates predicted by Kenyon & Bromley (2008) from theoretical models of icy planet formation. We find a dust luminosity of $(7.9 \pm 0.8) \times 10^{-4}$ L$_\odot$ and a dust mass of 8 -- 16 lunar masses, depending on grain porosity, with $\sim 1$ lunar mass in grains with radius 1 $��$m -- 1 mm. If the grains are icy and highly porous, meter-sized objects must be invoked to explain the far-IR, submm, and mm emission. If the grains are composed of astronomical silicates, there is a dearth of blowout grains (Pawellek et al. 2014) and the mass loss rate is well below the predicted dust production values., 8 pages, 8 figures, 2 tables, accepted for publication in ApJ

We present a new 'collisional grooming' algorithm that enables us to model images of debris disks where the collision time is less than the Poynting-Robertson (PR) time for the dominant grain size. Our algorithm uses the output of a collisionless disk simulation to iteratively solve the mass flux equation for the density distribution of a collisional disk containing planets in three dimensions. The algorithm can be run on a single processor in approx1 hr. Our preliminary models of disks with resonant ring structures caused by terrestrial mass planets show that the collision rate for background particles in a ring structure is enhanced by a factor of a few compared to the rest of the disk, and that dust grains in or near resonance have even higher collision rates. We show how collisions can alter the morphology of a resonant ring structure by reducing the sharpness of a resonant ring's inner edge and by smearing out azimuthal structure. We implement a simple prescription for particle fragmentation and show how PR drag and fragmentation sort particles by size, producing smaller dust grains at smaller circumstellar distances. This mechanism could cause a disk to look different at different wavelengths, and may explain themore » warm component of dust interior to Fomalhaut's outer dust ring seen in the resolved 24 mum Spitzer image of this system.« less

Disk fragmentation resulting from the gravitational instability has been proposed as an efficient mechanism for forming giant planets. We use the planet Fomalhaut b, the triple-planetary system HR 8799, and the potential protoplanet associated with HL Tau to test the viability of this mechanism. We choose the above systems since they harbor planets with masses and orbital characteristics favored by the fragmentation mechanism. We do not claim that these planets must have formed as the result of fragmentation, rather the reverse: if planets can form from disk fragmentation, then these systems are consistent with what we should expect to see. We use the orbital characteristics of these recently discovered planets, along with a new technique to more accurately determine the disk cooling times, to place both lower and upper limits on the disk surface density--and thus mass--required to form these objects by disk fragmentation. Our cooling times are over an order of magnitude shorter than those of Rafikov (2005),which makes disk fragmentation more feasible for these objects. We find that the required mass interior to the planet's orbital radius is ~0.1 Msun for Fomalhaut b, the protoplanet orbiting HL Tau, and the outermost planet of HR 8799. The two inner planets of HR 8799 probably could not have formed in situ by disk fragmentation., 5 pages, 1 figure, accepted for publication in ApJL

Fomalhaut and epsilon Eridani are two young, nearby stars that possess extended debris disks whose structures suggest the presence of perturbing planetary objects. With its high sensitivity and stable point spread function, Spitzer/IRAC is uniquely capable of detecting cool, Jupiter-like planetary companions whose peak emission is predicted to occur near 4.5 um. We report on deep IRAC imaging of these two stars, taken at 3.6 and 4.5 um using subarray mode and in all four channels in wider-field full array mode. Observations acquired at two different telescope roll angles allowed faint surrounding objects to be separated from the stellar diffraction pattern. No companion candidates were detected at the reported position of Fomalhaut b with 3 sigma model-dependent mass upper limits of 3 MJ (for an age of 200 Myr). Around epsilon Eridani we instead set a limit of 4 and

We present ATCA continuum observations at a wavelength of 6.8 mm of five debris disks: $\beta$ Pictoris, q$^1$ Eridani, HD 107146, HD 181327, and HD 95086. These observations provide the detection at the longest wavelengths obtained to date for all these debris disks. By combining our 6.8 mm data with previous detections at shorter sub-millimeter/millimeter wavelengths we measure the long wavelength spectral index of these sources. We then use previous estimates for the temperature of the emitting dust to derive the spectral index of the dust emissivity. Under the assumption that all the detected flux comes from dust only, we constrain the slope of the solid size distribution, assumed to be a power-law. The values that we infer for the slope of the size distribution range between about 3.36 and 3.50. We compare our findings with the case of the Fomalhaut debris disk and use these results to test the predictions of collisional cascades of planetesimal belts., Comment: 7 pages, 3 figures, accepted for publication on ApJ

Imaging companions to main-sequence stars often allows to detect a projected orbital motion. MCMC has become very popular in for fitting their orbits. Some of these companions appear to move on very eccentric, possibly unbound orbits. This is the case for the exoplanet Fomalhaut b and the brown dwarf companion PZ Tel B. For such orbits, standard MCMC codes assuming only bound orbits may be inappropriate. We develop a new MCMC implementation able to handle bound and unbound orbits as well in a continuous manner, and we apply it to the cases of Fomalhaut b and PZ Tel B. This code is based on universal Keplerian variables and Stumpff functions formalism. We present two versions of this code, the second one using a different set of angular variables designed to avoid degeneracies arising when the projected orbital motion is quasi-radial, as it is the case for PZ Tel B. We also present additional observations of PZ Tel B. The code is applied to Fomalhaut b and PZ Tel B. Concerning Fomalhaut b, we confirm previous results, but we show that open orbital solutions are also possible. The eccentricity distribution nevertheless peaks around ~0.9 in the bound regime. We present a first successful orbital fit of PZ Tel B, showing in particular that the eccentricity distribution presents a sharp peak very close to e=1, meaning a quasi-parabolic orbit. It was recently suggested that unseen inner companions may lead orbital fitting algorithms to artificially give high eccentricities. We show that this caveat is unlikely to apply to Fomalhaut b. Concerning PZ Tel B, an inner ~12 MJup companion would mimic a e=1 orbit despite a real eccentricity around 0.7, but a dynamical analysis reveals that such a system would not be stable. We conclude that our orbital fit is robust., Comment: Accepted in Astronomy and Astrophysics

The planet candidate Fomalhaut b is bright in optical light but undetected in longer wavelengths, requiring a large, reflective dust cloud. The most recent observations find an extremely eccentric orbit (e ~ 0.8), indicating that Fomalhaut b cannot be the planet that is constraining the system's eccentric debris ring. An irregular satellite swarm around a super-Earth has been proposed, however, explaining the well-constrained debris ring requires an additional planet on an orbit that crosses that of the putative super-Earth. This paper expands upon a second theory: Fomalhaut b is a transient dust cloud produced by a catastrophic collision between planetesimals in the disk. We perform collisional probability simulations of the Fomalhaut debris disk based on the structure of our Kuiper belt, finding that the catastrophic disruption rate of d ~ 100 km bodies in the high-eccentricity scattering component is several per decade. This model paints a picture of the Fomalhaut system as having recently (within ~10-100 Myr) experienced a dynamical instability within its planetary system, which scattered a massive number of planetesimals onto large, high eccentricity orbits similar to that of Fom b. If Fomalhaut b is indeed a dust cloud produced by such a collision, we should soon see another appear, while Fomalhaut b will expand until it is either resolved or becomes too faint to be seen., accepted to ApJL

We have imaged the circumstellar disk of Fomalhaut at 350 microns wavelength, using SHARC II at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9 arcsec) has been enhanced by a factor of three using the HiRes deconvolution procedure. We find that at this wavelength and signal to noise ratio (~ 12), the observed morphology is that of a simple inclined ring (i ~ 70 deg), with little or no other apparent structure; this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 micron data with Spitzer Space Telescope images at 24, 70, and 160 microns in order to estimate the 2-dimensional spatial variation of relative column density ("tau map") using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ring-like morphology, but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~ 0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map., 13 pages, 3 Postscript figures. To be published in ApJ Letters (accepted 12/23/04)

An investigation of the stability of the transfer function of the European Southern Observatory's Very Large Telescope Interferometer has been carried out through observations of Fomalhaut over a wide range in hour angle. No significant variation in the transfer function was found for the zenith angle range 5-70 degrees. The projected baseline varied between 139.7 m and 49.8 m during the observations and, as an integral part of the determination of the transfer function, a new accurate limb-darkened angular diameter for Fomalhaut has been established. This has led to improved values for the emergent flux, effective temperature, radius and luminosity., Comment: 6 pages, 4 figures, uses an.cls, accepted for publication in Astron. Nacht

Radiation pressure force, Poynting–Robertson effect, and collisions are important to determine the size distribution of dust in circumstellar debris disks with the two former parameters depending on the light-scattering properties of grains. We here present Mie and discrete-dipole approximation (DDA) calculations to describe the optical properties of dust particles around Pictoris, Vega, and Fomalhaut in order to study the influence of the radiation pressure force. We find that the differences between Mie and DDA calculations are lower than 30% for all porosities. Therefore, Mie calculations can be used to determine the cut-off limits which contribute to the size distribution for the different systems.

We report in this paper the direct interferometric measurement of the angular diameter of five nearby Vega-like stars: α PsA, β Leo, β Pic, � Eri and τ Cet. The near-infrared (K and H bands) observations were conducted at the VLTI during the commissioning period with the VINCI instrument and three different baselines ranging from 66 m to 140 m. The five stellar photospheres are resolved and we derive their angular diameters with a 1 to 2% accuracy, except for β Pic (14%). We discuss the detectability and the influence of a possible small amount of warm circumstellar dust on our measurements. In addition, we have used the stellar evolution code CESAM (Morel 1997) to compare the computed fundamental parameters to the observed values (linear diameter, luminosity, temperature and chemical abundance). As a result of the simulation, the age of the stars is inferred and found to be in good agreement with previous estimates from various other methods.

We have conducted K band interferometric observations of four nearby main-sequence Vega-like stars at the VLTI with very long baselines. The very high resolution allowed us to probe the innermost region of the disks, where planets are supposed to be formed. The diameters of three bright and nearby prototypes β Pictoris, Fomalhaut (α PsA) and ∊ Eridani as well as τ Ceti have been measured with VINCI, the VLTI commissioning instrument, with a high accuracy. The derived diameters were used to constrain their age with help of the evolution code CESAM. The precision achieved with VINCI allowed us to discuss the shape of their photosphere and the possible detection of warm circumstellar material within the narrow interferometric field of view.

We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find $\sim$ 20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

New submillimeter images of the cold dust emission around the nearby main-sequence star Fomalhaut are presented. Observations at a wavelength of 450 ?m, where the telescope beam size is equivalent to a resolution of 50 AU, reveal that Fomalhaut is encircled by a significantly nonaxisymmetric inclined ring. Smooth axisymmetric models of the ring images suggest the existence of a least one "clump" with an estimated flux of about 5% of the total from the disk, thus implying a clump mass of 0.075 lunar masses. At the resolution of the data, this clump could instead be a ring arc. The most plausible explanation is that this feature is produced by dust trapped in a resonance with a large planet. The observed structures around Fomalhaut and other Vega-excess stars qualitatively resemble features seen in numerical simulations with a gas giant perturber.

This paper presents a model for the outcome of collisions between planetesimals in a debris disk and assesses the impact of collisional processes on the structure and size distribution of the disk. The model is presented by its application to Fomalhaut's collisionally replenished dust disk; a recent 450 micron image of this disk shows a clump embedded within it with a flux ~5 per cent of the total. The following conclusions are drawn: (i) SED modelling is consistent with Fomalhaut's disk having a collisional cascade size distribution extending from bodies 0.2 m in diameter down to 7 micron-sized dust. (ii) Collisional lifetime arguments imply that the cascade starts with planetesimals 1.5-4 km in diameter. Any larger bodies must be predominantly primordial. (iii) Constraints on the timescale for the ignition of the cascade are consistent with these primordial planetesimals having a distribution that extends up to 1000km, resulting in a disk mass of 5-10 times the minimum mass solar nebula. (iv) The debris disk is expected to be intrinsically clumpy, since planetesimal collisions result in dust clumps. The intrinsic clumpiness of Fomalhaut's disk is below current detection limits, but could be detectable by future observatories such as the ALMA, and could provide the only way of determining the primordial planetesimal population. (v) The observed clump could have originated in a collision between two runaway planetesimals, both larger than 1400 km diameter. It is unlikely that we should witness such an event unless both the formation of these runaways and the ignition of the collisional cascade occurred within the last ~10 Myr. (vi) Another explanation for Fomalhaut's clump is that ~5 per cent of the planetesimals in the ring are trapped in 1:2 resonance with a planet orbiting at 80 AU., Comment: 21 pages, 13 figures, accepted by MNRAS

We consider the ability of three models - impacts, captures, and collisional cascades - to account for a bright cloud of dust in Fomalhaut b. Our analysis is based on a novel approach to the power-law size distribution of solid particles central to each model. When impacts produce debris with (i) little material in the largest remnant and (ii) a steep size distribution, the debris has enough cross-sectional area to match observations of Fomalhaut b. However, published numerical experiments of impacts between 100 km objects suggest this outcome is unlikely. If collisional processes maintain a steep size distribution over a broad range of particle sizes (300 microns to 10 km), Earth-mass planets can capture enough material over 1-100 Myr to produce a detectable cloud of dust. Otherwise, capture fails. When young planets are surrounded by massive clouds or disks of satellites, a collisional cascade is the simplest mechanism for dust production in Fomalhaut b. Several tests using HST or JWST data - including measuring the expansion/elongation of Fomalhaut b, looking for trails of small particles along Fomalhaut b's orbit, and obtaining low resolution spectroscopy - can discriminate among these models., 47 pages of text, 2 tables, and 14 figures; ApJ, accepted

Dust in debris discs is constantly replenished by collisions between larger objects. In this paper, we investigate a method to detect these collisions. We generate models based on recent results on the Fomalhaut debris disc, where we simulate a background star transiting behind the disc, due to the proper motion of Fomalhaut. By simulating the expanding dust clouds caused by the collisions in the debris disc, we investigate whether it is possible to observe changes in the brightness of the background star. We conclude that in the case of the Fomalhaut debris disc, changes in the optical depth can be observed, with values of the optical depth ranging from $10^{-0.5}$ for the densest dust clouds to $10^{-8}$ for the most diffuse clouds with respect to the background optical depth of $\sim1.2\times10^{-3}$., 19 pages, 15 figures, accepted for publication in MNRAS