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TOI-2490b - the most eccentric brown dwarf transiting in the brown dwarf desertWe report the discovery of the most eccentric transiting brown dwarf in the brown dwarf desert, TOI-2490b. The brown dwarf desert is the lack of brown dwarfs around main-sequence stars within <inline-formula><tex-math id=TM0001 notation=LaTeX>$\sim 3$</tex-math></inline-formula> au and is thought to be caused by differences in formation mechanisms between a star and planet. To date, only <inline-formula><tex-math id=TM0002 notation=LaTeX>$\sim 40$</tex-math></inline-formula> transiting brown dwarfs have been confirmed. TOI-2490b is a <inline-formula><tex-math id=TM0003 notation=LaTeX>$73.6\pm 2.4$</tex-math></inline-formula> <inline-formula><tex-math id=TM0004 notation=LaTeX>$M_{\rm J}$</tex-math></inline-formula>, <inline-formula><tex-math id=TM0005 notation=LaTeX>$1.00\pm 0.02$</tex-math></inline-formula> <inline-formula><tex-math id=TM0006 notation=LaTeX>$R_{\rm J}$</tex-math></inline-formula> brown dwarf orbiting a <inline-formula><tex-math id=TM0007 notation=LaTeX>$1.004_{-0.022}^{+0.031}$</tex-math></inline-formula> <inline-formula><tex-math id=TM0008 notation=LaTeX>${\rm M}_{\odot }$</tex-math></inline-formula>, <inline-formula><tex-math id=TM0009 notation=LaTeX>$1.105_{-0.012}^{+0.012}$</tex-math></inline-formula> <inline-formula><tex-math id=TM0010 notation=LaTeX>${\rm R}_{\odot }$</tex-math></inline-formula> sun-like star on a 60.33 d orbit with an eccentricity of <inline-formula><tex-math id=TM0011 notation=LaTeX>$0.77989\pm 0.00049$</tex-math></inline-formula>. The discovery was detected within Transiting Exoplanet Survey Satellite sectors 5 (30 min cadence) and 32 (2 min and 20 s cadence). It was then confirmed with 31 radial velocity measurements with FEROS by the WINE collaboration and photometric observations with the Next Generation Transit Survey. Stellar modelling of the host star estimates an age of <inline-formula><tex-math id=TM0012 notation=LaTeX>$\sim 8$</tex-math></inline-formula> Gyr, which is supported by estimations from kinematics likely placing the object within the thin disc. However, this is not consistent with model brown dwarf isochrones for the system age suggesting an inflated radius. Only one other transiting brown dwarf with an eccentricity higher than 0.6 is currently known in the brown dwarf desert. Demographic studies of brown dwarfs have suggested such high eccentricity is indicative of stellar formation mechanisms.
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The Pre-perihelion Evolution of the Activity of Comet C/2017 K2 (Pan-STARRS) during the Water Ice-line CrossoverComets, relics from the early solar system, consist of dust and ice. The ice sublimates as comets approach the Sun, ejecting dust from their nuclei seen as activity. Different volatiles sublimate at different Sun–comet distances and eject dust of unique sizes, structures, and compositions. In this study, we present new polarimetric observations of Oort cloud comet C/2017 K2 (Pan-STARRS) in R- and I-filter domains before, during, and after its crossover of the water-ice sublimation regime at phase angles of 15.°9, 10.°5, and 20.°0, respectively. Combining multiband optical imaging data covering a wide range of heliocentric distances (∼14‑2.3 au), we aim to characterize the pre-perihelion evolution of cometary activity as well as the properties of its coma dust. Two discontinuous brightening events were observed: at ∼6 au presumably associated with changes in CO-like supervolatile ice activity, and at ∼2.9 au when water ice took over. Particularly, the latter activation is accompanied by changes in coma morphology and color whose trends differ between the inner (∼10<SUP>3</SUP> km) and outer (∼10<SUP>4</SUP> km) parts of the coma. No polarimetric discontinuities on the comet were observed over the inner coma region, all epochs showing phase-angle and wavelength dependencies compatible with those of active comets observed in similar observing geometry. During this period, the underlying dust continuum overwhelmed Hα emission at around 656.3 nm, suggesting less water ice on the comet's surface than expected. We discuss K2's coma environment by combining numerical simulations of light scattered by dust and place the observations within the context of the comet's evolution.
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Feasibility of meteor surveying from a Venus orbiterMeteor and bolide phenomena caused by the atmospheric ablation of incoming meteoroids are predicted to occur at the planet Venus. Their systematic observation would allow to measure and compare the sub-mm to m meteoroid flux at different locations in the solar system. Using a physical model of atmospheric ablation, we demonstrate that Venus meteors would be brighter, shorter-lived, and appear higher in the atmosphere than Earth meteors. To investigate the feasibility of meteor detection at Venus from an orbiter, we apply the SWARMS survey simulator tool to sets of plausible meteoroid population parameters, atmospheric models and instrument designs suited to the task, such as the Mini-EUSO camera operational on the ISS since 2019. We find that such instrumentation would detect meteors at Venus with a 1.5× to 2.5× higher rate than at Earth. The estimated Venus–Earth detection ratio remains insensitive to variations in the chosen observation orbit and detector characteristics, implying that a meteor survey from Venus orbit is feasible, though contingent on the availability of suitable algorithms and methods for efficient on-board processing and downlinking of the meteor data to Earth. We further show that a hypothetical camera onboard the upcoming EnVision mission to Venus similar to the ISS instrument should detect many times more meteors than needed for an initial characterisation of the large meteoroid population at 0.7 au from the Sun.
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TOpI-2447 b / NGTS-29 b: a 69-day Saturn around a Solar analogueDiscovering transiting exoplanets with relatively long orbital periods (>10 d) is crucial to facilitate the study of cool exoplanet atmospheres (T<SUB>eq</SUB> < 700 K) and to understand exoplanet formation and inward migration further out than typical transiting exoplanets. In order to discover these longer period transiting exoplanets, long-term photometric, and radial velocity campaigns are required. We report the discovery of TOI-2447 b (=NGTS-29 b), a Saturn-mass transiting exoplanet orbiting a bright (T = 10.0) Solar-type star (T<SUB>eff</SUB> = 5730 K). TOI-2447 b was identified as a transiting exoplanet candidate from a single transit event of 1.3 per cent depth and 7.29 h duration in TESS Sector 31 and a prior transit event from 2017 in NGTS data. Four further transit events were observed with NGTS photometry which revealed an orbital period of P = 69.34 d. The transit events establish a radius for TOI-2447 b of $0.865 \pm 0.010\, \rm R_{\rm J}$, while radial velocity measurements give a mass of $0.386 \pm 0.025\, \rm M_{\rm J}$. The equilibrium temperature of the planet is 414 K, making it much cooler than the majority of TESS planet discoveries. We also detect a transit signal in NGTS data not caused by TOI-2447 b, along with transit timing variations and evidence for a ~150 d signal in radial velocity measurements. It is likely that the system hosts additional planets, but further photometry and radial velocity campaigns will be needed to determine their parameters with confidence. TOI-2447 b/NGTS-29 b joins a small but growing population of cool giants that will provide crucial insights into giant planet composition and formation mechanisms.
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Discovery of Magnetically Guided Metal Accretion onto a Polluted White DwarfDynamically active planetary systems orbit a significant fraction of white dwarf stars. These stars often exhibit surface metals accreted from debris disks, which are detected through infrared excess or transiting structures. However, the full journey of a planetesimal from star-grazing orbit to final dissolution in the host star is poorly understood. Here, we report the discovery that the cool metal-polluted star WD 0816–310 has cannibalized heavy elements from a planetary body similar in size to Vesta, and where accretion and horizontal mixing processes have clearly been controlled by the stellar magnetic field. Our observations unveil periodic and synchronized variations in metal line strength and magnetic field intensity, implying a correlation between the local surface density of metals and the magnetic field structure. Specifically, the data point to a likely persistent concentration of metals near a magnetic pole. These findings demonstrate that magnetic fields may play a fundamental role in the final stages of exoplanetary bodies that are recycled into their white dwarf hosts.
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Imaging polarimetry of comet 67 P/Churyumov-Gerasimenko: homogeneous distribution of polarization and its implicationsComet 67P/Churyumov-Gerasimenko (67P) become observable for the first time in 2021 since the Rosetta rendezvous in 2014-2016. Here, we present pre-perihelion polarimetric measurements of 67P from 2021 performed with the Very Large Telescope (VLT), as well as post-perihelion polarimetric measurements from 2015 to 2016 obtained with the VLT and the William Herschel Telescope. This new data covers a phase angle range of ~4<SUP>○</SUP>-50<SUP>○</SUP> and presents polarimetric measurements of unprecedentedly high S/N ratio. Complementing previous measurements, the polarimetric phase curve of 67P resembles that of other Jupiter family comets and high-polarization, dusty comets. Comparing pre- and post-perihelion data sets, we find only a marginal difference between the polarimetric phase curves. In our imaging maps, we detect various linear structures produced by the dust in the inner coma of the comet. Despite this, we find a homogeneous spread of polarization around the photocentre throughout the coma and tail, in contrast to previous studies. Finally, we explore the consequences of image misalignments on both polarimetric maps and aperture polarimetric measurements.
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Search for Stellar Companions of Exoplanet Host Stars with AstraLux/CAHA 2.2 mStellar multiplicity is a key aspect of exoplanet diversity, as the presence of more than one star in a planetary system can have both devastating and positive effects on its formation and evolution. In this paper, we present the results of a Lucky Imaging survey of 212 exoplanet host stars performed with AstraLux at the 2.2 m telescope of the Centro Astronómico Hispano en Andalucía. The survey includes data from seven observing epochs between August 2015 and September 2020, and data for individual targets from four earlier observing epochs. The targets of this survey are nearby, bright, solar-like stars with high proper motions. In total, we detected 46 co-moving companions of 43 exoplanet host stars. Accordingly, this survey shows that the minimum multiplicity rate of exoplanet host stars is $20 \pm 3~{\rm per\ cent}$. In total, 33 binary and 10 hierarchical triple star systems with exoplanets have been identified. All companions were found to have a common proper motion with the observed exoplanet host stars, and with our astrometry we even find evidence of orbital motion for 28 companions. For all targets, we determine the detection limit and explore the detection space for possible additional companions of these stars. Based on the reached detection limit, additional co-moving companions beyond the detected ones can be excluded around all observed exoplanet host stars. The increasing number of exoplanets discovered in multiple stellar systems suggests that the formation of planets in such systems is by no means rare, but common. Therefore, our study highlights the need to consider stellar multiplicity in future studies of exoplanet habitability.
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NGTS-28Ab: a short period transiting brown dwarfWe report the discovery of a brown dwarf orbiting a M1 host star. We first identified the brown dwarf within the Next Generation Transit Survey data, with supporting observations found in TESS sectors 11 and 38. We confirmed the discovery with follow-up photometry from the South African Astronomical Observatory, SPECULOOS-S, and TRAPPIST-S, and radial velocity measurements from HARPS, which allowed us to characterize the system. We find an orbital period of ~1.25 d, a mass of $69.0^{+5.3}_{-4.8}$ M<SUB>J</SUB>, close to the hydrogen burning limit, and a radius of 0.95 ± 0.05 R<SUB>J</SUB>. We determine the age to be >0.5 Gyr, using model isochrones, which is found to be in agreement with spectral energy distribution fitting within errors. NGTS-28Ab is one of the shortest period systems found within the brown dwarf desert, as well as one of the highest mass brown dwarfs that transits an M dwarf. This makes NGTS-28Ab another important discovery within this scarcely populated region.
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Search for stellar companions of exoplanet host stars with AstraLux/CAHA 2.2 mStellar multiplicity is a key aspect of exoplanet diversity, as the presence of more than one star in a planetary system can have both devastating and positive effects on its formation and evolution. In this paper, we present the results of a lucky imaging survey of 212 exoplanet host stars performed with AstraLux at CAHA 2.2 m. The survey includes data from seven observing epochs between August 2015 and September 2020, and data for individual targets from four earlier observing epochs. The targets of this survey are nearby, bright, solar-like stars with high proper motions. In total, we detected 46 co-moving companions of 43 exoplanet host stars. Accordingly, this survey shows that the minimum multiplicity rate of exoplanet host stars is $20 \pm 3~{{\%}}$. In total, 33 binary and ten hierarchical triple star systems with exoplanets have been identified. All companions were found to have a common proper motion with the observed exoplanet host stars, and with our astrometry we even find evidence of orbital motion for 28 companions. For all targets, we determined the detection limit and explore the detection space for possible additional companions of these stars. Based on the reached detection limit, additional co-moving companions beyond the detected ones can be excluded around all observed exoplanet host stars. The increasing number of exoplanets discovered in multiple stellar systems suggests that the formation of planets in such systems is by no means rare, but common. Therefore, our study highlights the need to consider stellar multiplicity in future studies of exoplanet habitability.
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NGTS-28Ab: A short period transiting brown dwarfWe report the discovery of a brown dwarf orbiting a M1 host star. We first identified the brown dwarf within the Next Generation Transit Survey data, with supporting observations found in TESS sectors 11 and 38. We confirmed the discovery with follow-up photometry from the South African Astronomical Observatory, SPECULOOS-S, and TRAPPIST-S, and radial velocity measurements from HARPS, which allowed us to characterise the system. We find an orbital period of ~1.25 d, a mass of $69.0^{+5.3}_{-4.8}$ M<SUB>J</SUB>, close to the Hydrogen burning limit, and a radius of 0.95 ± 0.05 R<SUB>J</SUB>. We determine the age to be >0.5 Gyr, using model isochrones, which is found to be in agreement with SED fitting within errors. NGTS-28Ab is one of the shortest period systems found within the brown dwarf desert, as well as one of the highest mass brown dwarfs that transits an M dwarf. This makes NGTS-28Ab another important discovery within this scarcely populated region.
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Evidence of Eta Aquariid outbursts recorded in the classic Maya hieroglyphic script using orbital integrationsNo firm evidence has existed that the ancient Maya civilization recorded specific occurrences of meteor showers or outbursts in the corpus of Maya hieroglyphic inscriptions. In fact, there has been no evidence of any pre-Hispanic civilization in the Western Hemisphere recording any observations of any meteor showers on any specific dates. <P />The authors numerically integrated meteoroid-sized particles released by Comet Halley as early as 1404 BC to identify years within the Maya Classic Period, AD 250-909, when Eta Aquariid outbursts might have occurred. Outbursts determined by computer model were then compared to specific events in the Maya record to see if any correlation existed between the date of the event and the date of the outburst. The model was validated by successfully explaining several outbursts around the same epoch in the Chinese record. Some outbursts observed by the Maya were due to recent revolutions of Comet Halley, within a few centuries, and some to resonant behavior in older Halley trails, of the order of a thousand years. Examples were found of several different Jovian mean motion resonances as well as the 1:3 Saturnian resonance that have controlled the dynamical evolution of meteoroids in apparently observed outbursts.
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Orbital dynamics of highly probable but rare Orionid outbursts possibly observed by the ancient MayaUsing orbital integrations of particles ejected from Comet Halley's passages between 1404 and 240 BC, the authors investigate possible outbursts of the Orionids (twin shower of the Eta Aquariids) that may have been observed in the Western hemisphere. In an earlier orbital integration study, the authors determined there was a high probability linking probable outbursts of the Eta Aquariid meteor shower with certain events recorded in inscriptions during the Maya Classic Period, AD 250-900. This prior examination was the first scientific inquiry of its kind into ancient meteor outbursts possibly recorded in the Western hemisphere where previously no pre-Columbian observations had existed. In the current paper, the aim is to describe orbital dynamics of rare but probable Orionid outbursts that would have occurred on or near applicable dates recorded in the Classic Maya inscriptions. Specifically, significant probable outbursts are found in AD 417 and 585 out of 30 possible target years. The driving mechanisms for outbursts in those two years are Jovian 1:6 and 1:7 mean motion resonances acting to maintain compact structures within the Orionid stream for over 1 kyr. Furthermore, an Orionid outburst in AD 585 recorded by China is confirmed.
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Photoionization Models of the Inner Gaseous Disk of the Herbig Be Star BD+65 1637We attempt to constrain the physical properties of the inner, gaseous disk of the Herbig Be star BD+65 1637 using non-LTE, circumstellar disk codes and observed spectra (3700-10500 Å) from the ESPaDOnS instrument on the Canada-France-Hawaii Telescope. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. We model optical and near-infrared emission lines that are thought to form in this region using standard techniques that have been successful in modeling the spectra of classical Be stars. By comparing synthetic line profiles of hydrogen, helium, iron, and calcium with the observed line profiles, we try to constrain the geometry, density structure, and kinematics of the gaseous disk. Reasonable matches have been found for all line profiles individually; however, no disk density model based on a single power law for the equatorial density was able to simultaneously fit all of the observed emission lines. Among the emission lines, the metal lines, especially the Ca II IR triplet, seem to require higher disk densities than the other lines. Excluding the Ca II lines, a model in which the equatorial disk density falls as 10<SUP>-10</SUP> (R<SUB>*</SUB>/R)<SUP>3</SUP> g cm<SUP>-3</SUP> seen at an inclination of 45° for a 50 R<SUB>*</SUB> disk provides reasonable matches to the overall line shapes and strengths. The Ca II lines seem to require a shallower drop-off as 10<SUP>-10</SUP> (R<SUB>*</SUB>/R)<SUP>2</SUP> g cm<SUP>-3</SUP> to match their strength. More complex disk density models are likely required to refine the match to the BD+65 1637 spectrum.
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Constraints on the structure and seasonal variations of Triton's atmosphere from the 5 October 2017 stellar occultation and previous observationsContext. A stellar occultation by Neptune's main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. <BR /> Aims: We aimed at constraining Triton's atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. <BR /> Methods: We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range ~8 km to ~190 km, corresponding to pressure levels from 9 µbar down to a few nanobars. <BR /> Results: (i) A pressure of 1.18 ± 0.03 µbar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 µbar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude. <P />Light curves are only available at the CDS via anonymous ftp to <A href=http://cdsarc.u-strasbg.fr>cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href=http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/659/A136>http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/659/A136</A>
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The hazard from fragmenting cometsComet disintegration proceeds through both sublimation and discrete splitting events. The cross-sectional area of material ejected by a comet may, within days, become many times greater than that of the Earth, making encounters with such debris much more likely than collisions with the nucleus itself. The hierarchic fragmentation and sublimation of a large comet in a short-period orbit may yield many hundreds of such short-lived clusters. We model this evolution with a view to assessing the probability of an encounter that might have significant terrestrial effects, through atmospheric dusting or multiple impacts. Such an encounter may have contributed to the large animal extinctions and sudden climatic cooling of 12 900 yr ago, and the near-simultaneous collapse of civilisations around 2350 BC.
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Influence of aerosols, clouds, and sunglint on polarization spectra of EarthshineContext. Ground-based observations of the Earthshine, I.e., the light scattered by Earth to the Moon, and then reflected back to Earth, simulate space observations of our planet and represent a powerful benchmark for the studies of Earth-like planets. Earthshine spectra are strongly linearly polarized, owing to scattering by molecules and small particles in the atmosphere of the Earth and surface reflection, and may allow us to measure global atmospheric and surface properties of planet Earth. <BR /> Aims: We aim to interpret already published spectropolarimetric observations of the Earthshine by comparing them with new radiative transfer model simulations including a fully realistic three-dimensional (3D) surface-atmosphere model for planet Earth. <BR /> Methods: We used the highly advanced Monte Carlo radiative transfer model MYSTIC to simulate polarized radiative transfer in the atmosphere of the Earth without approximations regarding the geometry, taking into account the polarization from surface reflection and multiple scattering by molecules, aerosol particles, cloud droplets, and ice crystals. <BR /> Results: We have shown that Earth spectropolarimetry is highly sensitive to all these input parameters, and we have presented simulations of a fully realistic Earth atmosphere-surface model including 3D cloud fields and two-dimensional (2D) surface property maps. Our modeling results show that scattering in high ice water clouds and reflection from the ocean surface are crucial to explain the continuum polarization at longer wavelengths as has been reported in Earthshine observations taken at the Very Large Telescope in 2011 (3.8% and 6.6% at 800 nm, depending on which part of Earth was visible from the Moon at the time of the observations). We found that the relatively high degree of polarization of 6.6% can be attributed to light reflected by the ocean surface in the sunglint region. High ice-water clouds reduce the amount of absorption in the O<SUB>2</SUB>A band and thus explain the weak O<SUB>2</SUB>A band feature in the observations.
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Extra-Terrestrial MeteorsAll planets and satellites of our solar system are subject to a continuous rain of material, ranging in size from specks of dust to objects the size of boulders. Upon impact, these objects deposit their kinetic energy into the incident surface or atmosphere and affect the environment of the target body in ways not yet well understood. Recent high-profile events - impact flashes on Jupiter and the encounter of comet C/Siding Spring with Mars - brought the study of ``extraterrestrial meteors'' and their effects into the fore. Here we review the history, status and future prospects of meteor studies on planets other than the Earth. Would bright meteors appear in the atmosphere of Mars and what are the long-term effects on the planet's atmosphere? How do high-speed impacts of particulate matter on the airless surfaces of Mercury and the Moon affect their environment and those of the countless other bodies like them? These are some of the questions we attempt to answer in this Chapter.
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The cloudbow of planet Earth observed in polarisationContext. Scattering processes in the atmospheres of planets cause characteristic features that can be particularly well observed in polarisation. For planet Earth, both molecular scattering (Rayleigh) and scattering by small particles (Mie) imprint specific signatures in its phase curve. Polarised phase curves allow us to infer physical and chemical properties of the atmosphere like the composition of the gaseous and liquid components, droplet sizes, and refraction indices. <BR /> Aims: An unequivocal prediction of a liquid-water-loaded atmosphere is the existence of a rainbow feature at a scattering angle of around 138-144°. Earthshine allows us to observe the primary rainbow in linear polarisation. <BR /> Methods: We observed polarisation spectra of Earthshine using FORS2 at the Very Large Telescope for phase angles from 33° to 65° (Sun-Earth-Moon angle). The spectra were used to derive the degree of polarisation in the B, V, R, and I passbands and the phase curve from 33° to 136°. The new observations extend to the smallest phases that can be observed from the ground. <BR /> Results: The degree of polarisation of planet Earth is increasing for decreasing phase angles downwards of 45° in the B, V, R, and I passbands. From comparison of the phase curve observed with models of an Earth-type atmosphere we are able to determine the refractive index of water and to constrain the mean water droplet sizes to 6-7μm. Furthermore, we can retrieve the mean cloud fraction of liquid water clouds to 0.3, and the mean optical depth of the water clouds to values between 10 and 20. <BR /> Conclusions: Our observations allow us to discern two fundamentally different scattering mechanisms of the atmosphere of planet Earth: molecular and particle scattering. The physical and chemical properties can be retrieved with high fidelity through suitable inversion of the phase curve. Observations of polarimetric phase curves of planets beyond the Solar System shall be extremely valuable for a thorough characterisation of their atmospheres.
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Modelling the inner debris disc of HR 8799In many ways, the HR 8799 planetary system strongly resembles our own. It features four giant planets and two debris belts, analogues to the Asteroid and Edgeworth-Kuiper belts. Here, we present the results of dynamical simulations of HR8799's inner debris belt, to study its structure and collisional environment. Our results suggest that HR 8799's inner belt is highly structured, with gaps between regions of dynamical stability. The belt is likely constrained between sharp inner and outer edges, located at ∼6 and ∼8 au, respectively. Its inner edge coincides with a broad gap cleared by the 4:1 mean-motion resonance with HR 8799e. Within the belt, planetesimals are undergoing a process of collisional attrition like that observed in the Asteroid belt. However, whilst the mean collision velocity in the Asteroid belt exceeds 5 km s<SUP>-1</SUP>, the majority of collisions within HR 8799's inner belt occur with velocities of order 1.2 km s<SUP>-1</SUP>, or less. Despite this, they remain sufficiently energetic to be destructive - giving a source for the warm dust detected in the system. Interior to the inner belt, test particles remain dynamically unstirred, aside from narrow bands excited by distant high-order resonances with HR 8799e. This lack of stirring is consistent with earlier thermal modelling of HR 8799's infrared excess, which predicted little dust inside 6 au. The inner system is sufficiently stable and unstirred that the formation of telluric planets is feasible, although such planets would doubtless be subject to a punitive impact regime, given the intense collisional grinding required in the inner belt to generate the observed infrared excess.
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A polarimetric study of asteroids: fitting phase-polarization curvesBy considering all published asteroid linear polarization data available in the literature, it is possible to obtain updated phase-polarization curves for several tens of objects. In a separate paper, we have produced new calibrations of different relations between the geometric albedo and several polarimetric parameters, based on an analysis of a limited sample of asteroids for which the albedo is known with sufficient accuracy. In this paper, we present the main polarization parameters and corresponding albedos for a larger data set of asteroids which we did not use for calibration purposes. We find a good agreement between the albedo values computed using different polarization parameters. Conversely, in the case of the so-called Barbarian asteroids the situation is rather unclear. Moreover, we present an updated analysis of the distributions of different polarimetric parameters, including the so-called inversion angle and the solar phase angle corresponding to the extreme value of negative polarization, and study their mutual relations. We find that the above parameters can be used to clearly distinguish some unusual classes of asteroids. Polarimetric parameters are known to be related to physical properties of asteroid surfaces which are difficult to infer by means of other observing techniques. By using a much larger data set, in our analysis we confirm and extend some results obtained in the past by other authors, and we explore more systematically some features that had been mostly unexplored before, mainly concerning the morphology of the negative polarization branch.
