Search

Companions at sub-arcsecond separation from young stars are difficult to image. However their presence can be inferred from the perturbations they create in the dust and gas of protoplanetary disks. Here we present a new interpretation of SPHERE polarised observations that reveal the previously detected inner spiral in the disk of HD 100546. The spiral coincides with a newly detected 12CO inner spiral and the previously reported CO emission Doppler-flip, which has been interpreted as the signature of an embedded protoplanet. Comparisons with hydrodynamical models indicate that this Doppler-flip is instead the kinematic counterpart of the spiral, which is likely generated by an inner companion inside the disk cavity., Comment: Accepted to ApJL, 14 pages, 10 figures

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., Comment: 10 pages, 4 figures, accepted

The origin of the inner dust cavities observed in transition discs remains unknown. The segregation of dust and size of the cavity is expected to vary depending on which clearing mechanism dominates grain evolution. We present the results from the Discs Down Under program, an 8.8 mm continuum Australia Telescope Compact Array (ATCA) survey targeting 15 transition discs with large (> 20 au) cavities, and compare the resulting dust emission to Atacama Large millimetre/sub-millimetre Array (ALMA) observations. Our ATCA observations resolve the inner cavity for 8 of the 14 detected discs. We fit the visibilities and reconstruct 1D radial brightness models for 10 sources with a S/N > 5sigma. We find that, for sources with a resolved cavity in both wavebands, the 8.8 mm and sub-mm brightness distributions peak at the same radius from the star. We suggest that a similar cavity size for 8.8 mm and sub-mm dust grains is due to a dust trap induced by the presence of a companion., Comment: 18 pages, 11 figures

The interaction between a YSO stellar magnetic field and its protostellar disc can result in stellar accretional flows and outflows from the inner disc rim. Gas flows with a velocity component perpendicular to disc midplane subject particles to centrifugal acceleration away from the protostar, resulting in particles being catapulted across the face of the disc. The ejected material can produce a "dust fan", which may be dense enough to mimic the appearance of a "puffed-up" inner disc rim. We derive analytic equations for the time dependent disc toroidal field, the disc magnetic twist, the size of the stable toroidal disc region, the jet speed and the disc region of maximal jet flow speed. We show how the observed infrared variability of the pre-transition disc system LRLL~31 can be modelled by a dust ejecta fan from the inner-most regions of the disc whose height is partially dependent on the jet flow speed. The greater the jet flow speed, the higher is the potential dust fan scale height. An increase in mass accretion onto the star tends to increase the height and optical depth of the dust ejection fan, increasing the amount of 1--8~$\mu$m radiation. The subsequent shadow reduces the amount of light falling on the outer disc and decreases the 8-- 40~$\mu$m radiation. A decrease in the accretion rate reverses this scenario, thereby producing the observed "see-saw" infrared variability., Comment: 18 pages, 21 figures

A small subset of young stellar objects (YSOs) exhibit "see-saw" temporal variations in their mid-infrared SED; as the flux short-ward of a fulcrum wavelength ($\lambda_{f}$) increases the flux long-wards of this wavelength decreases (and vice-versa) over timescales of weeks to years. While previous studies have shown that an opaque, axisymmetric occulter of variable height can cause this behaviour in the SED of these objects, the conditions under which a single $\lambda_{f}$ occurs have not previously been determined, nor the factors determining its value. Using radiative transfer modelling, we conduct a parametric study of the exemplar of this class, LRLL 31 to explore this phenomenon, and confirm that the cause of this flux variation is likely due to the change in height of the optically thick inner rim of the accretion disc at the dust sublimation radius, or some other phenomenon which results in a similar appearance. We also determine that a fulcrum wavelength only occurs for high inclinations, where the line of sight intersects the accretion disc. Accepting that the disc of LRLL 31 is highly inclined, the inner rim radius, radial and vertical density profiles are independently varied to gauge what effect this had on $\lambda_{f}$ and its position relative to the silicate feature near $10 \mu$m. While $\lambda_{f}$ is a function of each of these parameters, it is found to be most strongly dependent on the vertical density exponent $\beta$. All other factors being held constant, only for flatter discs ($\beta < 1.2$) did we find a $\lambda_{f}$ beyond the silicate feature., Comment: 12 pages, 11 figures

This work presents a study of two Herbig Ae transitional discs, Oph IRS 48 and HD 169142; which both have reported rings in their dust density distributions. We use Keck-II/NIRC2 adaptive optics imaging observations in the L' filter (3.8 micron) to probe the regions of these discs inwards of ~20AU from the star. We introduce our method for investigating these transitional discs, which takes a forward modelling approach: making a model of the disc (using the Monte Carlo radiative transfer code RADMC), convolving it with point-spread functions of calibrator stars, and comparing the convolved models with the observational data. The disc surface density parameters are explored with a Monte Carlo Markov Chain technique. Our analysis recovers emission from both of the discs interior to the well known optically thick walls, modelled as a ring of emission at ~15AU in Oph IRS 48, and ~7AU for HD 169142, and identifies asymmetries in both discs. Given the brightness of the near-symmetric rings compared to the reported companion candidates, we suggest that the reported companion candidates can be interpreted as slightly asymmetric disc emission or illumination., Comment: Accepted at MNRAS. 22 pages, 12 figures

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade., Comment: White paper submitted to the Astro2020 decadal survey

We present an approach that is able to both rapidly assess the dynamical stability of multiple planet systems, and determine whether an exoplanet system would be capable of hosting a dynamically stable Earth-mass companion in its habitable zone. We conduct a suite of numerical simulations using a swarm of massless test particles in the vicinity of the orbit of a massive planet, in order to develop a predictive tool which can be used to achieve these desired outcomes. In this work, we outline both the numerical methods we used to develop the tool, and demonstrate its use. We find that the test particles survive in systems either because they are unperturbed due to being so far removed from the massive planet, or due to being trapped in stable mean motion resonant orbits with the massive planet. The resulting unexcited test particle swarm produces a unique signature in (a,e) space that represents the stable regions within the system. We are able to scale and translate this stability signature, and combine several together in order to conservatively assess the dynamical stability of newly discovered multiple planet systems. We also assess the stability of a system's habitable zone and determine whether an Earth-mass companion could remain on a stable orbit, without the need for exhaustive numerical simulations., Comment: 24 pages, 16 figures

In this work, we hunt for the best places to find exo-Earths in the currently known exoplanet population. While it is still unclear whether Jupiter had a beneficial or detrimental effect on the creation of the right environment for a habitable Earth to develop, we focus on the 51 multiple planet systems that have at least one Jupiter-like planet and aim to identify which would be good candidates to host an exo-Earth. We conduct a series of numerical simulations to identify dynamically stable regions of the habitable zone of the multiple exoplanet systems capable of hosting an Earth-mass planet. We produce a candidate list of 16 systems that could host such a stable exo-Earth in their habitable zone, and for which the induced radial velocity signal of a hypothetical one, two or four Earth-mass planet on the host star would be detectable with the ESPRESSO spectrograph. We find that whilst the gravitational interactions with the massive planet nearest the habitable zone are critical in determining stability, the secular resonant interactions between multiple planets can also have a dramatic influence on the overall stability of the habitable zone., Comment: 20 pages, 12 figures, 6 tables

While the number of exoplanets discovered continues to increase at a rapid rate, we are still to discover any system that truly resembles the Solar system. Existing and near future surveys will likely continue this trend of rapid discovery. To see if these systems are Solar system analogues, we will need to efficiently allocate resources to carry out intensive follow-up observations. We seek to uncover the properties and trends across systems that indicate how much of the habitable zone is stable in each system to provide focus for planet hunters. We study the dynamics of all known single Jovian planetary systems, to assess the dynamical stability of the habitable zone around their host stars. We perform a suite of simulations of all systems where the Jovian planet will interact gravitationally with the habitable zone, and broadly classify these systems. Besides the system's mass ratio (Mpl/Mstar), and the Jovian planet's semi-major axis (apl) and eccentricity (epl), we find that there are no underlying system properties which are observable that indicate the potential for planets to survive within the system's habitable zone. We use Mpl/Mstar, apl and epl to generate a parameter space over which the unstable systems cluster, thus allowing us to predict which systems to exclude from future observational or numerical searches for habitable exoplanets. We also provide a candidate list of 20 systems that have completely stable habitable zones and Jovian planets orbiting beyond the habitable zone as potential first order Solar system analogues., Comment: 14 pages, 10 figures

With continued improvement in telescope sensitivity and observational techniques, the search for rocky planets in stellar habitable zones is entering an exciting era. With so many exoplanetary systems available for follow-up observations to find potentially habitable planets, one needs to prioritise the ever-growing list of candidates. We aim to determine which of the known planetary systems are dynamically capable of hosting rocky planets in their habitable zones, with the goal of helping to focus future planet search programs. We perform an extensive suite of numerical simulations to identify regions in the habitable zones of single Jovian planet systems where Earth mass planets could maintain stable orbits, specifically focusing on the systems in the Catalog of Earth-like Exoplanet Survey Targets (CELESTA). We find that small, Earth-mass planets can maintain stable orbits in cases where the habitable zone is largely, or partially, unperturbed by a nearby Jovian, and that mutual gravitational interactions and resonant mechanisms are capable of producing stable orbits even in habitable zones that are significantly or completely disrupted by a Jovian. Our results yield a list of 13 single Jovian planet systems in CELESTA that are not only capable of supporting an Earth-mass planet on stable orbits in their habitable zone, but for which we are also able to constrain the orbits of the Earth-mass planet such that the induced radial velocity signals would be detectable with next generation instruments., Comment: 15 pages, 12 figures, Accepted for publication by MNRAS

Planet formation is thought to occur in discs around young stars by the aggregation of small dust grains into much larger objects. The growth from grains to pebbles and from planetesimals to planets is now fairly well understood. The intermediate stage has however been found to be hindered by the radial-drift and fragmentation barriers. We identify a powerful mechanism in which dust overcomes both barriers. Its key ingredients are i) backreaction from the dust onto the gas, ii) grain growth and fragmentation, and iii) large-scale gradients. The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped. We show that these self-induced dust traps are robust: they develop for a wide range of disc structures, fragmentation thresholds and initial dust-to-gas ratios. They are favored locations for pebbles to grow into planetesimals, thus opening new paths towards the formation of planets., Comment: 15 pages, 16 figures. Added a 1-page erratum correcting an inconsequential error in Appendix C. A higher resolution version of the paper, as well as the online videos, are available at https://cral.univ-lyon1.fr/labo/perso/jean-francois.gonzalez/Papers/SIDT/

We model the behaviour of dust grains entrained by photoevaporation-driven winds from protoplanetary discs assuming a non-rotating, plane-parallel disc. We obtain an analytic expression for the maximum entrainable grain size in extreme-UV radiation-driven winds, which we demonstrate to be proportional to the mass loss rate of the disc. When compared with our hydrodynamic simulations, the model reproduces almost all of the wind properties for the gas and dust. In typical turbulent discs, the entrained grain sizes in the wind are smaller than the theoretical maximum everywhere but the inner disc due to dust settling., Comment: 11 pages, 9 figures, 1 table, 1 animation. Accepted for publication in MNRAS

Observations of debris disks offer a window into the physical and dynamical properties of planetesimals in extrasolar systems through the size distribution of dust grains. In particular, the millimeter spectral index of thermal dust emission encodes information on the grain size distribution. We have made new VLA observations of a sample of seven nearby debris disks at 9 mm, with 3" resolution and $\sim5$ $\mu$Jy/beam rms. We combine these with archival ATCA observations of eight additional debris disks observed at 7 mm, together with up-to-date observations of all disks at (sub)millimeter wavelengths from the literature to place tight constraints on the millimeter spectral indices and thus grain size distributions. The analysis gives a weighted mean for the slope of the power law grain size distribution, $n(a)\propto a^{-q}$, of $\langle q \rangle = 3.36\pm0.02$, with a possible trend of decreasing $q$ for later spectral type stars. We compare our results to a range of theoretical models of collisional cascades, from the standard self-similar, steady-state size distribution ($q=3.5$) to solutions that incorporate more realistic physics such as alternative velocity distributions and material strengths, the possibility of a cutoff at small dust sizes from radiation pressure, as well as results from detailed dynamical calculations of specific disks. Such effects can lead to size distributions consistent with the data, and plausibly the observed scatter in spectral indices. For the AU Mic system, the VLA observations show clear evidence of a highly variable stellar emission component; this stellar activity obviates the need to invoke the presence of an asteroid belt to explain the previously reported compact millimeter source in this system., Comment: 34 pages, 5 figures

We investigate dust entrainment by photoevaporative winds in protoplanetary discs using dusty smoothed particle hydrodrodynamics (SPH). We use unequal-mass particles to resolve more than five orders of magnitude in disc/outflow density and a one-fluid formulation to efficiently simulate an equivalent magnitude range in drag stopping time. We find that only micron sized dust grains and smaller can be entrained in EUV driven winds. The maximum grain size is set by dust settling in the disc rather than aerodynamic drag in the wind. More generally, there is a linear relationship between the base flow density and the maximum entrainable grain size in the wind. A pileup of micron sized dust grains can occur in the upper atmosphere at critical radii in the disc as grains decouple from the low-density wind. Entrainment is a strong function of location in the disc, resulting in a size sorting of grains in the outflow---the largest grain being carried out between $10$--$20\,$AU. The peak dust density for each grain size occurs at the inner edge of its own entrainment region., Comment: 20 pages, 13 figures, Accepted for publication in MNRAS

Infrared observations provide the dust composition in the protoplanetary discs surface layers, but can not probe the dust chemistry in the midplane, where planet formation occurs. Meteorites show that dynamics was important in determining the dust distribution in the Solar Nebula and needs to be considered if we are to understand the global chemistry in discs. 1D radial condensation sequences can only simulate one disc layer at a time and cannot describe the global chemistry or the complexity of meteorites. To address these limitations, we compute for the first time the two dimensional distribution of condensates in the inner Solar Nebula using a thermodynamic equilibrium model, and derive timescales for vertical settling and radial migration of dust. We find two enstatite-rich zones within 1 AU from the young Sun: a band ~0.1 AU thick in the upper optically-thin layer of the disc interior to 0.8 AU, and in the optically-thick disc midplane out to ~0.4 AU. The two enstatite-rich zones support recent evidence that Mercury and enstatite chondrites shared a bulk material with similar composition. Our results are also consistent with infrared observation of protoplanetary disc which show emission of enstatite-rich dust in the inner surface of discs. The resulting chemistry and dynamics suggests that the formation of the bulk material of enstatite chondrites occurred in the inner surface layer of the disc, within 0.4~AU. We also propose a simple alternative scenario in which gas fractionation and vertical settling of the condensates lead to an enstatite-chondritic bulk material., Comment: Astroph version. Accepted in MNRAS on 2015 December 23. Received 2015 December 16; in original form 2015 May 04

Protoplanetary discs are now routinely observed and exoplanets, after the numerous indirect discoveries, are starting to be directly imaged. To better understand the planet formation process, the next step is the detection of forming planets or of signposts of young planets still in their disc, such as gaps. A spectacular example is the ALMA science verification image of HL Tau showing numerous gaps and rings in its disc. To study the observability of planet gaps, we ran 3D hydrodynamical simulations of a gas and dust disc containing a 5 M J gap-opening planet and characterised the spatial distribution of migrating, growing and fragmenting dust grains. We then computed the corresponding synthetic images for ALMA. For a value of the dust fragmentation threshold of 15 m s --1 for the collisional velocity, we identify for the first time a self-induced dust pile up in simulations taking fragmentation into account. This feature, in addition to the easily detected planet gap, causes a second apparent gap that could be mistaken for the signature of a second planet. It is therefore essential to be cautious in the interpretation of gap detections., Comment: Accepted for publication in MNRAS Letters. 5 pages, 6 figures. MNRAS, Oxford University press, 2015, in press

We present observations of Epsilon Eridani from the Submillimeter Array (SMA) at 1.3 millimeters and from the Australia Telescope Compact Array (ATCA) at 7 millimeters that reach an angular resolution of ~4" (13 AU). These first millimeter interferometer observations of Epsilon Eridani, which hosts the closest debris disk to the Sun, reveal two distinct emission components: (1) the well-known outer dust belt, which, although patchy, is clearly resolved in the radial direction, and (2) an unresolved source coincident with the position of the star. We use direct model-fitting of the millimeter visibilities to constrain the basic properties of these two components. A simple Gaussian shape for the outer belt fit to the SMA data results in a radial location of $64.4^{+2.4}_{-3.0}$ AU and FWHM of $20.2^{+6.0}_{-8.2}$ AU (fractional width $\Delta R/R = 0.3$. Similar results are obtained taking a power law radial emission profile for the belt, though the power law index cannot be usefully constrained. Within the noise obtained (0.2 mJy/beam), these data are consistent with an axisymmetric belt model and show no significant azimuthal structure that might be introduced by unseen planets in the system. These data also limit any stellocentric offset of the belt to $<9$ AU, which disfavors the presence of giant planets on highly eccentric ($>0.1$) and wide (10's of AU) orbits. The flux density of the unresolved central component exceeds predictions for the stellar photosphere at these long wavelengths, by a marginally significant amount at 1.3 millimeters but by a factor of a few at 7 millimeters (with brightness temperature $13000 \pm 1600$ K for a source size of the optical stellar radius). We attribute this excess emission to ionized plasma from a stellar corona or chromosphere., Comment: 27 pages, 6 figures

There is much evidence that planet formation is occurring in the disk around the Herbig Be star HD100546. To learn more about the processes occurring in this disk we conducted high resolution imaging at 43/45 GHz with the Australia Telescope Compact Array (ATCA). Multiple array configurations were used, providing a best spatial resolution of $\sim$ 0.15 arcsec, or 15 AU at HD100546's distance of $\sim$ 100 pc. Significant structure is revealed, but its precise form is dependent on the $u-v$ plane sampling used for the image reconstruction. At a resolution of $\leq$ 30 AU we detected an inner gap in the disk with a radius of $\sim$ 25 AU and a position angle approximately along the known disk major axis. With different weighting, and an achieved resolution of $\sim$ 15 AU, emission appears at the centre and the disk takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of $\sim$ 25 AU. The position angle of the disk major axis and its inclination from face-on are determined to be $140^{\circ}\pm5^{\circ}$ and $40^{\circ}\pm5^{\circ}$ respectively. The $\sim$ 25 AU gap radius is confirmed by a null in the real part of the binned visibilities at 320$\pm$10 k$\lambda$, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free-free emission from a stellar or disk wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 AU., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society

A pathway to the formation of planetesimals, and eventually giant planets, may occur in concentrations of dust grains trapped in pressure maxima. Dramatic crescent-shaped dust concentrations have been seen in recent radio images at sub-mm wavelengths. These disk asymmetries could represent the initial phases of planet formation in the dust trap scenario, provided that grain sizes are spatially segregated. A testable prediction of azimuthal dust trapping is that progressively larger grains should be more sharply confined and furthermore the trapped grains should follow a distribution that is markedly different from the gas. However, gas tracers such as CO and the infrared emission from small grains are both very optically thick where the submm continuum originates, so observations have been unable to test the trapping predictions or to identify compact concentrations of larger grains required for planet formation by core-accretion. Here we report multifrequency observations of HD142527, from 34GHz to 700GHz, that reveal a compact concentration of ~cm-sized grains, with a few Earth masses, embedded in a large-scale crescent of ~mm-sized particles. The emission peaks at wavelengths shorter than ~1mm are optically thick and trace the temperature structure resulting from shadows cast by the inner regions. Given this temperature structure, we infer that the largest dust grains are concentrated in the 34 GHz clump. We conclude that dust trapping is efficient for approximately cm-sized grains and leads to enhanced concentrations, while the smaller grains largely reflect the gas distribution., Comment: Submitted to ApJ

In a series of papers, we present a comprehensive analytic study of the global motion of growing dust grains in protoplanetary discs, addressing both the radial drift and the vertical settling of the particles. Here we study how the radial drift of dust particles is affected by grain growth. In a first step, toy models in which grain growth can either be constant, accelerate or decelerate are introduced. The equations of motion are analytically integrable and therefore the grains dynamics is easy to understand. The radial motion of growing grains is governed by the relative efficiency of the growth and migration processes which is expressed by the dimensionless parameter Lambda, as well as the exponents for the gas surface density and temperature profiles, denoted p and q respectively. When Lambda is of order unity, growth and migration are strongly coupled, providing the most efficient radial drift. For the toy models considered, grains pile up when -p+q+1/2<0. Importantly, we show the existence of a second process which can help discs to retain their solid materials. For accelerating growth, grains end up their migration at a finite radius, thus avoiding being accreted onto the central star., Comment: 12 pages, 9 figures. Accepted for publication in MNRAS. v2: typos corrected

We aim to derive a simple analytic model to understand the essential properties of vertically settling growing dust grains in laminar protoplanetary discs. Separating the vertical dynamics from the motion in the disc midplane, we integrate the equations of motion for both a linear and an exponential grain growth rate. Numerical integrations are performed for more complex growth models. We find that the settling efficiency depends on the value of the dimensionless parameter gamma, which characterises the relative efficiency of grain growth with respect to the gas drag. Since gamma is expected to be of order as the initial dust-to-gas ratio in the disc (of order 10^-2), grain growth enhances the energy dissipation of the dust particles and improve the settling efficiency in protoplanetary discs. This behaviour is mostly independent of the growth model considered as well as of the radial drift of the particles., Comment: 8 pages, 9 figures. Accepted for publication in MNRAS. v2: typos corrected and minor changes

Among the numerous known extrasolar planets, only a handful have been imaged directly so far, at large orbital radii and in rather evolved systems. The Atacama Large Millimeter/submillimeter Array (ALMA) will have the capacity to observe these wide planetary systems at a younger age, thus bringing a better understanding of the planet formation process. Here we explore the ability of ALMA to detect the gaps carved by planets on wide orbits., Comment: 2 pages, 2 figures, to appear in the Proceedings of IAU Symp. 299: Exploring the Formation and Evolution of Planetary Systems (Victoria, Canada)

A number of objects in primitive meteorites have oxygen isotopic compositions that place them on a distinct, mass-independent fractionation line with a slope of one on a three-isotope plot. The most popular model for describing how this fractionation arose assumes that CO self-shielding produced 16O-rich CO and 16O-poor H2O, where the H2O subsequently combined with interstellar dust to form relatively 16O-poor solids within the Solar Nebula. Another model for creating the different reservoirs of 16O-rich gas and 16O-poor solids suggests that these reservoirs were produced by Galactic chemical evolution (GCE) if the Solar System dust component was somewhat younger than the gas component and both components were lying on the line of slope one in the O three-isotope plot. We argue that GCE is not the cause of mass-independent fractionation of the oxygen isotopes in the Solar System. The GCE scenario is in contradiction with observations of the 18O/17O ratios in nearby molecular clouds and young stellar objects. It is very unlikely for GCE to produce a line of slope one when considering the effect of incomplete mixing of stellar ejecta in the interstellar medium. Furthermore, the assumption that the Solar System dust was younger than the gas requires unusual timescales or the existence of an important stardust component that is not theoretically expected to occur nor has been identified to date., Comment: 28 pages, 3 figures, accepted for publication on The Astrophysical Journal

The composition of the most primitive Solar System condensates, such as calcium-aluminum-rich inclusions (CAI) and micron-sized corundum grains, show that short-lived radionuclides (SLR), e.g., 26Al, were present in the early Solar System. Their abundances require a local origin, which however is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range roughly 7-11 Msun. These stars evolve through core H, He, and C burning. After core C burning they go through a "Super"-asymptotic giant branch (Super-AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (~ 10^8 K), and strong stellar winds driving the H-rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super-AGB stars are mostly O-Ne white dwarfs. Our Super-AGB models produce 26Al/27Al yield ratios ~ 0.02 - 0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the Solar Nebula of the order of 1 part of Super-AGB mass per several 10^2 to several 10^3 of Solar Nebula mass, resulting in associated changes in the O composition in the range 3 permil to 20 permil. This is in agreement with observations of the O isotopic ratios in primitive Solar System condensated, which do not carry the signature of a stellar pollutor. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super-AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super-AGB production of SLR heavier than iron, such as 107Pd., Comment: 23 pages, 5 figures, 6 tables, accepted for publication on Meteoritics and Planetary Science

As gas flowed from the solar accretion disk or solar nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner solar nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, "infall" atmosphere around the inner solar nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased. In the first hundred thousand years after the formation of the solar nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) - the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 to 100,000 years after the formation of the solar nebula. It is possible that conditions were conducive to RMN/CAI formation over an eighty thousand year timescale. The "infall" atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems. The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner solar nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event., Comment: 51 pages, 22 figures

We present ATCA results of a 3 and 7 mm continuum survey of 20 T Tauri stars in the Chamaeleon and Lupus star forming regions. This survey aims to identify protoplanetary discs with signs of grain growth. We detected 90% of the sources at 3 and 7 mm, and determined the spectral slopes, dust opacity indices and dust disc masses. We also present temporal monitoring results of a small sub-set of sources at 7, 15 mm and 3+6 cm to investigate grain growth to cm sizes and constrain emission mechanisms in these sources. Additionally, we investigated the potential correlation between grain growth signatures in the infrared (10 \mu m silicate feature) and millimetre (1-3 mm spectral slope, {\alpha}). Eleven sources at 3 and 7 mm have dominant thermal dust emission up to 7 mm, with 7 of these having a 1-3 mm dust opacity index less than unity, suggesting grain growth up to at least mm sizes. The Chamaeleon sources observed at 15 mm and beyond show the presence of excess emission from an ionised wind and/or chromo- spheric emission. Long-timescale monitoring at 7 mm indicated that cm-sized pebbles are present in at least four sources. Short-timescale monitoring at 15 mm suggests the excess emission is from thermal free-free emission. Finally, a weak correlation was found between the strength of the 10 \mum feature and {\alpha}, suggesting simultaneous dust evolution of the inner and outer parts of the disc. This survey shows that grain growth up to cm-sized pebbles and the presence of excess emission at 15 mm and beyond are common in these systems, and that temporal monitoring is required to disentangle these emission mechanisms., Comment: 31 pages, 13 figures, 11 tables, accepted for publication in MNRAS

Context. To form metre-sized pre-planetesimals in protoplanetary discs, growing grains have to decouple from the gas before they are accreted onto the central star during their phase of fast radial migration and thus overcome the so-called "radial-drift barrier" (often inaccurately referred to as the "metre-size barrier"). Aims. To predict the outcome of the radial motion of dust grains in protoplanetary discs whose surface density and temperature follow power-law profiles, with exponent p and q respectively. We investigate both the Epstein and the Stokes drag regimes which govern the motion of the dust. Methods. We analytically integrate the equations of motion obtained from perturbation analysis. We compare these results with those from direct numerical integration of the equations of motion. Then, using data from observed discs, we predict the fate of dust grains in real discs. Results. When a dust grain reaches the inner regions of the disc, the acceleration due to the increase of the pressure gradient is counterbalanced by the increase of the gas drag. We find that most grains in the Epstein (resp. the Stokes) regime survive their radial migration if-p+q+1/2 \leq0 (resp. if q\leq 2/3). The majority of observed discs satisfies both-p+q+ 1/2 \leq0 and q\leq 2/3: a large fraction of both their small and large grains remain in the disc, for them the radial drift barrier does not exist., Comment: Accepted for publication in A&A

We present the properties of an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) mapped at 11 pc resolution in the CO(1-0) line. We identify clouds as regions of connected CO emission, and find that the distributions of cloud sizes, fluxes and masses are sensitive to the choice of decomposition parameters. In all cases, however, the luminosity function of CO clouds is steeper than dN/dL \propto L^{-2}, suggesting that a substantial fraction of mass is in low-mass clouds. A correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. We argue that the correlation between virial mass and CO luminosity is the result of comparing two covariant quantities, with the correlation appearing tighter on larger scales where a size-linewidth relation holds. The virial parameter (the ratio of a cloud's kinetic to self-gravitational energy) shows a wide range of values and exhibits no clear trends with the CO luminosity or the likelihood of hosting young stellar object (YSO) candidates, casting further doubt on the assumption of virialization for molecular clouds in the LMC. Higher CO luminosity increases the likelihood of a cloud harboring a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming the close link between giant molecular clouds and massive star formation., Comment: Accepted by ApJS; 22 pages in emulateapj format; full-resolution version and data tables available at http://mmwave.astro.illinois.edu/magma/

Here we report on the Women in Astronomy Workshop (http://asawomeninastronomy.org/meetings/wia2011/), which was held on 13 May 2011 in Sydney, Australia. The workshop was organised by the Astronomical Society of Australia's Chapter on Women in Astronomy, to discuss some of the issues that face women in astronomy and make recommendations to help support the success of women in Australian astronomy but came to broader conclusions that have value for the whole astronomical community. The workshop consisted of four sessions, with presentations by invited speakers on demographics, leadership, varied career paths, and how institutions & individuals can help. The workshop ended with a discussion panel that summarised the day's debate and presented a list of recommendations for the Australian astronomical community (both individuals and institutions) that are provided in this report., Comment: 9 pages