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Register a new userResolved Debris Discs Around A Stars in the Herschel DEBRIS Survey
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The majority of debris discs discovered so far have only been detected through infrared excess emission above stellar photospheres. While disc properties can be inferred from unresolved photometry alone under various assumptions for the physical properties of dust grains, there is a degeneracy between disc radius and dust temperature that depends on the grain size distribution and optical properties. By resolving the disc we can measure the actual location of the dust. The launch of Herschel, with an angular resolution superior to previous far-infrared telescopes, allows us to spatially resolve more discs and locate the dust directly. Here we present the nine resolved discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use these data to investigate the disc radii by fitting narrow ring models to images at 70, 100 and 160 {mu}m and by fitting blackbodies to full spectral energy distributions. We do this with the aim of finding an improved way of estimating disc radii for unresolved systems. The ratio between the resolved and blackbody radii varies between 1 and 2.5. This ratio is inversely correlated with luminosity and any remaining discrepancies are most likely explained by differences to the minimum size of grain in the size distribution or differences in composition. We find that three of the systems are well fit by a narrow ring, two systems are borderline cases and the other four likely require wider or multiple rings to fully explain the observations, reflecting the diversity of planetary systems.
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Only 20% of old field stars have detectable debris discs, leaving open the question of what disc, if any, is present around the remaining 80%. Young moving groups allow to probe this population, since discs are expected to have been brighter early on. This paper considers the population of F~stars in the 23~Myr-old BPMG where we find that 9/12 targets possess discs. We also analyse archival ALMA data to derive radii for 4 of the discs, presenting the first image of the 63au radius disc of HD~164249. Comparing the BPMG results to disc samples from $sim45$~Myr and $sim150$~Myr-old moving~groups, and to discs found around field stars, we find the disc incidence rate in young moving~groups is comparable to that of the BPMG and significantly higher than that of field~stars. The BPMG discs tend to be smaller than those around field~stars. However, this difference is not statistically significant due to the small number of targets. Yet, by analysing the fractional luminosity vs disc radius parameter space we find that the fractional luminosities in the populations considered drop by two orders of magnitude within the first 100~Myr. This is much faster than expected by collisional evolution, implying a decay equivalent to $1/text{age}^2$. We attribute this depletion to embedded planets which would be around 170~$M_text{earth}$ to cause a depletion on the appropriate timescale. However, we cannot rule out that different birth environments of nearby young clusters result in brighter debris discs than the progenitors of field~stars which likely formed in a more dense environment.
The new NIKA2 camera at the IRAM 30m radiotelescope was used to observe three known debris disks in order to constrain the SED of their dust emission in the millimeter wavelength domain. We have found that the spectral index between the two NIKA2 bands (1mm and 2mm) is consistent with the Rayleigh-Jeans regime (lambda^{-2}), unlike the steeper spectra (lambda^{-3}) measured in the submillimeter-wavelength domain for two of the three disks $-$ around the stars Vega and HD107146. We provide a succesful proof of concept to model this spectral inversion in using two populations of dust grains, those smaller and those larger than a grain radius a0 of 0.5mm. This is obtained in breaking the slope of the size distribution and the functional form of the absorption coefficient of the standard model at a0. The third disk - around the star HR8799 - does not exhibit this spectral inversion but is also the youngest.
A significant fraction of main-sequence stars are encircled by dusty debris discs, where the short-lived dust particles are replenished through collisions between planetesimals. Most destructive collisions occur when the orbits of smaller bodies are dynamically stirred up, either by the gravitational effect of locally formed Pluto-sized planetesimals (self-stirring scenario), or via secular perturbation caused by an inner giant planet (planetary stirring). The relative importance of these scenarios in debris systems is unknown. Here we present new Herschel Space Observatory imagery of 11 discs selected from the most massive and extended known debris systems. All discs were found to be extended at far-infrared wavelengths, five of them being resolved for the first time. We evaluated the feasibility of the self-stirring scenario by comparing the measured disc sizes with the predictions of the model calculated for the ages of our targets. We concluded that the self-stirring explanation works for seven discs. However, in four cases, the predicted pace of outward propagation of the stirring front, assuming reasonable initial disc masses, was far too low to explain the radial extent of the cold dust. Therefore, for HD 9672, HD 16743, HD 21997, and HD 95086, another explanation is needed. We performed a similar analysis for {ss} Pic and HR 8799, reaching the same conclusion. We argue that planetary stirring is a promising possibility to explain the disk properties in these systems. In HR 8799 and HD 95086 we may already know the potential perturber, since their known outer giant planets could be responsible for the stirring process. Our study demonstrates that among the largest and most massive debris discs self-stirring may not be the only active scenario, and potentially planetary stirring is responsible for destructive collisions and debris dust production in a number of systems.
Millimetre continuum observations of debris discs can provide insights into the physical and dynamical properties of the unseen planetesimals that these discs host. The material properties and collisional models of planetesimals leave their signature on the grain size distribution, which can be traced through the millimetre spectral index. We present 8.8 mm observations of the debris discs HD 48370, CPD 72 2713, HD 131488, and HD 32297 using the Australian Telescope Compact Array (ATCA) as part of the PLanetesimals Around TYpicalPre-main seqUence Stars (PLATYPUS) survey. We detect all four targets with a characteristic beam size of 5 arcseconds and derive a grain size distribution parameter that is consistent with collisional cascade models and theoretical predictions for parent planetesimal bodies where binding is dominated by self-gravity. We combine our sample with 19 other millimetre-wavelength detected debris discs from the literature and calculate a weighted mean grain size power law index which is close to analytical predictions for a classical steady state collisional cascade model. We suggest the possibility of two distributions of q in our debris disc sample; a broad distribution (where q is approximately 3.2 to 3.7) for typical debris discs (gas-poor/non-detection), and a narrow distribution (where q is less than 3.2) for bright gas-rich discs. Or alternatively, we suggest that there exists an observational bias between the grain size distribution parameter and absolute flux which may be attributed to the detection rates of faint debris discs at cm wavelengths.
Numerous nearby FGK dwarfs possess discs of debris generated by collisions among comets. Here we fit the levels of dusty excess observed by Spitzer at 70$umu$m and show that they form a rather smooth distribution. Taking into account the transition of the dust removal process from collisional to Poynting-Robertson drag, all the stars may be empirically fitted by a single population with many low-excess members. Within this ensemble, the Kuiper Belt is inferred to be such a low-dust example, among the last 10% of stars, with a small cometary population. Analogue systems hosting gas giant planets and a modest comet belt should occur for only a few per cent of Sun-like stars, and so terrestrial planets with a comparable cometary impact rate to the Earth may be uncommon. The nearest such analogue system presently known is HD154345 at 18pc, but accounting for survey completeness, a closer example should lie at around 10pc.
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