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Context. Photoevaporation is an important process for protoplanetary disc dispersal, but there has so far been a lack of consensus from simulations over the mass-loss rates and the most important part of the high-energy spectrum involved in driving the wind. Aims. We aim to isolate the origins of these discrepancies through carefully benchmarked hydrodynamic simulations of X-ray photoevaporation with time-dependent thermochemistry calculated on the fly. Methods. We conducted hydrodynamic simulations with PLUTO where the thermochemistry is calculated using PRIZMO. We explored the contribution of certain key microphysical processes and the impact of employing different spectra previously used in literature studies. Results. We find that additional cooling results from the excitation of O by neutral H, which leads to dramatically reduced mass-loss across the disc compared to previous X-ray photoevaporation models, with an integrated rate of ~10−9 M⊙ yr−1. Such rates would allow for longer-lived discs than previously expected from population synthesis. An alternative spectrum with less soft X-ray produces mass-loss rates around a factor of two to three times lower. The chemistry is significantly out of equilibrium, with the survival of H2 into the wind being aided by advection. This leads to H2 becoming the dominant coolant at 10s au, thus stabilising a larger radial temperature gradient across the wind as well as providing a possible wind tracer. [ABSTRACT FROM AUTHOR]

In the past years, many transition disks have been detected via SED modeling or imaging. The disks feature dust cavities in their inner regions with sizes ranging from a few to more than 70 AU. The origin of those structures, however, remains mysterious. We will present how our recent simulation results, which include gap opening by a giant planet and effects of dust evolution, could explain these structures.

The evaporation of molecules from dust grains is crucial to understanding some key aspects of the star- and the planet-formation processes. During the heating phase, the presence of young protostellar objects induces molecules to evaporate from the dust surface into the gas phase, enhancing its chemical complexity. Similarly, in circumstellar discs, the position of the so-called snow lines is determined by evaporation, with important consequences for the formation of planets. The amount of molecules that are desorbed depends on the interaction between the species and the grain surface, which is controlled by the binding energy. Recent theoretical and experimental works point towards a distribution of values for this parameter instead of the single value often employed in astrochemical models.We present a new "multi-binding energy" framework to assess the effects that a distribution of binding energies has on the amount of species bound to the grains. We find that the efficiency of the surface chemistry is significantly influenced by this process, with crucial consequences on the theoretical estimates of the desorbed species. [ABSTRACT FROM AUTHOR]

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The dispersal phase of planet-forming discs via winds driven by irradiation from the central star and/or magnetic fields in the disc itself is likely to play an important role in the formation and evolution of planetary systems. Current theoretical models lack predictive power to adequately constrain observations. We present prizmo , a code for evolving thermochemistry in protoplanetary discs capable of being coupled with hydrodynamical and multifrequency radiative transfer codes. We describe the main features of the code, including gas and surface chemistry, photochemistry, microphysics, and the main cooling and heating processes. The results of a suite of benchmarks, which include photon-dominated regions, slabs illuminated by radiation spectra that include X-ray, and well-established cooling functions evaluated at different temperatures, show good agreement both in terms of chemical and thermal structures. The development of this code is an important step to perform quantitative spectroscopy of disc winds, and ultimately the calculation of line profiles, which is urgently needed to shed light on the nature of observed disc winds. [ABSTRACT FROM AUTHOR]

We predict magnitudes for young planets embedded in transition discs, still affected by extinction due to material in the disc. We focus on Jupiter-sized planets at a late stage of their formation, when the planet has carved a deep gap in the gas and dust distributions and the disc starts to being transparent to the planet flux in the infrared (IR). Column densities are estimated by means of three-dimensional hydrodynamical models, performed for several planet masses. Expected magnitudes are obtained by using typical extinction properties of the disc material and evolutionary models of giant planets. For the simulated cases located at 5.2 au in a disc with a local unperturbed surface density of 127 |$\mathrm{g} \, \mathrm{cm}^{-2}$|⁠ , a 1 M J planet is highly extinct in the J, H , and K bands, with predicted absolute magnitudes ≥ 50 mag. In the L and M bands, extinction decreases, with planet magnitudes between 25 and 35 mag. In the N band, due to the silicate feature on the dust opacities, the expected magnitude increases to ∼40 mag. For a 2 M J planet, the magnitudes in the J, H , and K bands are above 22 mag, while for the L, M , and N bands, the planet magnitudes are between 15 and 20 mag. For the 5 M J planet, extinction does not play a role in any IR band, due to its ability to open deep gaps. Contrast curves are derived for the transition discs in CQ Tau, PDS 70, HL Tau, TW Hya, and HD 163296. Planet mass upper limits are estimated for the known gaps in the last two systems. [ABSTRACT FROM AUTHOR]

From molecular clouds to protoplanetary discs, non-ideal magnetic effects are important in many astrophysical environments. Indeed, in star and disc formation processes, it has become clear that these effects are critical to the evolution of the system. The efficacy of non-ideal effects is, however, determined by the complex interplay between magnetic fields, ionizing radiation, cosmic rays, microphysics, and chemistry. In order to understand these key microphysical parameters, we present a one-dimensional non-ideal magnetohydrodynamics code and apply it to a model of a time-dependent, oblique, magnetic shock wave. By varying the microphysical ingredients of the model, we find that cosmic rays and dust play a major role, and that, despite the uncertainties, the inclusion of microphysics is essential to obtain a realistic outcome in magnetic astrophysical simulations. [ABSTRACT FROM AUTHOR]

Using broadband photometry from the Hubble Space Telescope in combination with Very Large Telescope narrowband Hα observations of the nearby spiral galaxy NGC 300, we explore a method for estimating the escape fractions of hydrogen-ionizing photons from H ii regions within this galaxy. Our goal in this concept study is to evaluate the spectral types of the most massive stars using the broadband data and estimating their ionizing photon output with the help of stellar atmosphere models. A comparison with the Hα flux that gives the amount of ionized gas in the H ii region provides a measure of the escape fraction of ionizing photons from that region. We performed some tests with a number of synthetic young clusters with varying parameters to assess the reliability of the method. However, we found that the derived stellar spectral types and consequently the expected ionizing photon luminosity of a region is highly uncertain. The tests also show that on one hand we tended to overestimate the integrated photon output of a region for young ages and low numbers of stars, and on the other hand we mostly underestimated the combined ionizing luminosity for a large stellar number and older cluster ages. We conclude that the proposed method of using stellar broadband photometry to infer the leakage of ionizing photons from H ii regions is highly uncertain and dominated by the errors of the resulting stellar spectral types. Therefore this method is not suitable. Stellar spectra are needed to reliably determine the stellar types and escape fractions. Studies to this end have been carried out for the Magellanic Clouds. [ABSTRACT FROM AUTHOR]

Copyright of Cuaternario y Geomorfología is the property of Cuaternario y Geomorfologia and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We present hydrodynamic simulations of stellar wind-magnetosphere interactions in hot Jupiters such as WASP-12b. For fiducial stellar wind rates, we find that a planetary magnetic field of a few G produces a large magnetospheric cavity, which is typically 6-9 planetary radii in size. A bow shock invariably forms ahead of the magnetosphere, but the pre-shock gas is only mildly supersonic (with typical Mach numbers of ≃1.6-1.8) so the shock is weak. This results in a characteristic signature in the ultraviolet (UV) light curve: a broad absorption feature that leads the optical transit by 10-20 per cent in orbital phase. The shapes of our synthetic light curves are consistent with existing observations ofWASP-12b, but the required near-UV optical depth (τ ~ 0.1) can only be achieved if the shocked gas cools rapidly. We further show that radiative cooling is inefficient, so we deem it unlikely that a magnetospheric bow shock is responsible for the observed near-UV absorption. Finally, we apply our model to two other well-studied hot Jupiters (WASP-18b and HD 209458b), and suggest that UV observations of more massive short-period planets (such as WASP-18b) will provide a straightforward test to distinguish between different models of circumplanetary absorption. [ABSTRACT FROM AUTHOR]

Ionizing feedback from massive stars dramatically affects the interstellar medium local to star-forming regions. Numerical simulations are now starting to include enough complexity to produce morphologies and gas properties that are not too dissimilar from observations. The comparison between the density fields produced by hydrodynamical simulations and observations at given wavelengths relies however on photoionization/chemistry and radiative transfer calculations. We present here an implementation of Monte Carlo radiation transport through a Voronoi tessellation in the photoionization and dust radiative transfer code MOCASSIN. We show for the first time a synthetic spectrum and synthetic emission line maps of a hydrodynamical simulation of a molecular cloud affected by massive stellar feedback. We show that the approach on which previous work is based, which remapped hydrodynamical density fields on to Cartesian grids before performing radiative transfer/photoionization calculations, results in significant errors in the temperature and ionization structure of the region. Furthermore, we describe the mathematical process of tracing photon energy packets through a Voronoi tessellation, including optimizations, treating problematic cases and boundary conditions. We perform various benchmarks using both the original version of MOCASSIN and the modified version using the Voronoi tessellation. We show that for uniform grids, or equivalently a cubic lattice of cell generating points, the new Voronoi version gives the same results as the original Cartesian grid version of MOCASSIN for all benchmarks. For non-uniform initial conditions, such as using snapshots from smoothed particle hydrodynamics simulations, we show that the Voronoi version performs better than the Cartesian grid version, resulting in much better resolution in dense regions. [ABSTRACT FROM AUTHOR]

We examine the combined effects of winds and photoionizing radiation from O-type stars on embedded stellar clusters formed in model turbulent molecular clouds covering a range of masses and radii. We find that feedback is able to increase the quantities of dense gas present, but decreases the rate and efficiency of the conversion of gas to stars relative to control simulations in which feedback is absent. Star formation in these calculations often proceeds at a rate substantially slower than the freefall rate in the dense gas. This decoupling is due to the weakening of, and expulsion of gas from, the deepest parts of the clouds' potential wells where most of the star formation occurs in the control simulations. This results in large fractions of the stellar populations in the feedback simulation becoming dissociated from dense gas. However, where star formation does occur in both control and feedback simulations, it does so in dense gas, so the correlation between star formation activity and dense gas is preserved. The overall dynamical effects of feedback on the clusters are minimal, with only small fraction of stars becoming unbound, despite large quantities of gas being expelled from some clouds. This owes to the settling of the stars into virialized and stellar-dominated configurations before the onset of feedback. By contrast, the effects of feedback on the observable properties of the clusters -- their U-, B- and V-band magnitudes -- are strong and sudden. The time-scales on which the clusters become visible and unobscured are short compared with the time-scales which the clouds are actually destroyed. [ABSTRACT FROM AUTHOR]

Integral field unit (IFU) data of the iconic Pillars of Creation in M16 are presented. The ionization structure of the pillars was studied in great detail over almost the entire visible wavelength range, and maps of the relevant physical parameters, e.g. extinction, electron density, electron temperature, line-of-sight velocity of the ionized and neutral gas are shown. In agreement with previous authors, we find that the pillar tips are being ionized and photoevaporated by the massive members of the nearby cluster NGC 6611. They display a stratified ionization structure where the emission lines peak in a descending order according to their ionization energies. The IFU data allowed us to analyse the kinematics of the photoevaporative flow in terms of the stratified ionization structure, and we find that, in agreement with simulations, the photoevaporative flow is traced by a blueshift in the position-velocity profile. The gas kinematics and ionization structure have allowed us to produce a sketch of the 3D geometry of the Pillars, positioning the pillars with respect to the ionizing cluster stars. We use a novel method to detect a previously unknown bipolar outflow at the tip of the middle pillar and suggest that it has an embedded protostar as its driving source. Furthermore we identify a candidate outflow in the leftmost pillar. With the derived physical parameters and ionic abundances, we estimate a mass-loss rate due to the photoevaporative flow of 70 M⊙ Myr1 which yields an expected lifetime of approximately 3 Myr. [ABSTRACT FROM AUTHOR]

Giant molecular clouds (GMCs) are observed to be turbulent, but theory shows that without a driving mechanism turbulence should quickly decay. The question arises by which mechanisms turbulence is driven or sustained. It has been shown that photoionizing feedback from massive stars has an impact on the surrounding GMC and can for example create vast HII bubbles. We therefore address the question of whether turbulence is a consequence of this effect of feedback on the cloud. To investigate this, we analyse the velocity field of simulations of high-mass star-forming regions by studying velocity structure functions and power spectra. We find that clouds whose morphology is strongly affected by photoionizing feedback also show evidence of driving of turbulence by preserving or recovering a Kolmogorov-type velocity field. On the contrary, control run simulations without photoionizing feedback have a velocity distribution that bears the signature of gravitational collapse and of the dissipation of energy, where the initial Kolmogorov-type structure function is erased. [ABSTRACT FROM AUTHOR]

The discovery with the Herschel Space Observatory of bright far-infrared and submm emission from the ejecta of the core-collapse supernova SN 1987A has been interpreted as indicating the presence of some 0.4-0.7 M☉ of dust. We have constructed radiative transfer models of the ejecta to fit optical to far-infrared observations from the literature at epochs between 615 d and 24 years after the explosion, to determine when and where this unexpectedly large amount of dust formed. We find that the observations by day 1153 are consistent with the presence of 3 × 10-3 M☉ of dust. Although this is a larger amount than has previously been considered possible at this epoch, it is still very small compared to the amount present in the remnant after 24 years, and significantly higher dust masses at the earlier epochs are firmly ruled out by the observations, indicating that the majority of the dust must have formed at very late times. By 8515-9200 d after the explosion, 0.6-0.8M☉ of dust is present, and dust grains with radii greater than 2 μm are required to obtain a fit to the observed spectral energy distribution (SED). This suggests that the dust mass increase at late times was caused by accretion on to and coagulation of the dust grains formed at earlier epochs. These findings provide further confirmation that core-collapse supernovae can create large quantities of dust, and indicate that the reason for small dust masses being estimated in many cases is that the vast majority of the dust forms long after most supernovae have been detectable at mid-infrared wavelengths. [ABSTRACT FROM AUTHOR]

Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. Aims. Using X-Shooter broad band - UV to near-infrared - medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. Methods. We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets.We then compared our findings with results for classical T Tauri stars. Results. The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. Conclusions. The sample analyzed here suggests that, at least for some objects, the process responsible of the inner disk clearing allows for a transfer of gas from the outer disk to the inner region. This should proceed at a rate that does not depend on the physical mechanisms that produces the gap seen in the dust emission and results in a gas density in the inner disk similar to that of unperturbed disks around stars of similar mass. [ABSTRACT FROM AUTHOR]

We model the combined effects of photoionization and momentum-driven winds from O-stars on molecular clouds spanning a parameter space of initial conditions. The dynamical effects of the winds are very modest. However, in the lower mass clouds, they influence the morphologies of the H ii regions by creating 10-pc-scale central cavities. The inhomogeneous structures of the model giant molecular clouds (GMCs) make them highly permeable to photons, ionized gas and supernova ejecta, and the leaking of ionized gas in particular strongly affects their evolution, reducing the effectiveness of feedback. Nevertheless, feedback is able to expel large fractions of the mass of the lower escape velocity clouds. Its impact on star formation is more modest, decreasing final star formation efficiencies by 10–20 per cent, and the rate of change of the star formation efficiency per freefall time by about one third. However, the clouds still form stars substantially faster than observed GMCs. [ABSTRACT FROM AUTHOR]

Protoplanetary discs are a natural consequence of the star formation process and as such are ubiquitous around low-mass stars. They are fundamental to planet formation as they hold the reservoir of material from which planets form. Their evolution and final dispersal and the timescales that regulate these process are therefore of particular interest. In this contribution I will review the observational evidence for the dispersal of discs being dominated by two timescales and for the final dispersal to occur quickly and from the inside out. I will discuss the current theoretical models, including X-ray photoevaporation, showing that the latter provides a natural explanation to the observed behaviour and review supporting and contrasting evidence. I will finally introduce a new mechanism based on the interaction between planet formation and photoevaporation that may explain a particular class of transition discs with large inner holes and high accretion rates that are problematic for photoevaporation models and planet formation models alone. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We present the fourth in a series of papers detailing our smoothed particle hydrodynamics study of the effects of ionizing feedback from O-type stars on turbulent star-forming clouds. Here, we study the effects of photoionization on a series of initially partially unbound clouds with masses ranging from 104–106 M⊙ and initial sizes from 2.5–45pc. We find that ionizing feedback profoundly affects the structure of the gas in most of our model clouds, creating large and often well-cleared bubble structures and pillars. However, changes in the structures of the embedded clusters produced are much weaker and not well correlated to the evolution of the gas. We find that in all cases, star formation efficiencies and rates are reduced by feedback and numbers of objects increased, relative to control simulations. We find that local triggered star formation does occur and that there is a good correlation between triggered objects and pillars or bubble walls, but that triggered objects are often spatially mixed with those formed spontaneously. Some triggered objects acquire large enough masses to become ionizing sources themselves, lending support to the concept of propagating star formation. We find scant evidence for spatial age gradients in most simulations, and where we do see them, they are not a good indicator of triggering, as they apply equally to spontaneously formed objects as triggered ones. Overall, we conclude that inferring the global or local effects of feedback on stellar populations from observing a system at a single epoch is very problematic. [ABSTRACT FROM AUTHOR]

We extend our previous smoothed particle hydrodynamics parameter study of the effects of photoionization from O-stars on star-forming clouds to include initially unbound clouds. We generate a set of model clouds in the mass range 104–106 M⊙ with initial virial ratios Ekin/Epot = 2.3, allow them to form stars and study the impact of the photoionizing radiation produced by the massive stars. We find that, on the 3 Myr time-scale before supernovae are expected to begin detonating, the fraction of mass expelled by ionizing feedback is a very strong function of the cloud escape velocities. High-mass clouds are largely unaffected dynamically, while low-mass clouds have large fractions of their gas reserves expelled on this time-scale. However, the fractions of stellar mass unbound are modest and significant portions of the unbound stars are so only because the clouds themselves are initially partially unbound. We find that ionization is much more able to create well-cleared bubbles in the unbound clouds, owing to their intrinsic expansion, but that the presence of such bubbles does not necessarily indicate that a given cloud has been strongly influenced by feedback. We also find, in common with the bound clouds from our earlier work, that many of the systems simulated here are highly porous to photons and supernova ejecta, and that most of them will likely survive their first supernova explosions. [ABSTRACT FROM AUTHOR]

The time-scale over which and the modality by which young stellar objects (YSOs) disperse their circumstellar discs dramatically influence the eventual formation and evolution of planetary systems. By means of extensive radiative transfer modelling, we have developed a new set of diagnostic diagrams in the infrared colour–colour plane (K − [24] versus K − [8]), to aid with the classification of the evolutionary stage of YSOs from photometric observations. Our diagrams allow the differentiation of sources with unevolved (primordial) discs from those evolving according to different clearing scenarios (e.g. homologous depletion versus inside-out dispersal), as well as from sources that have already lost their disc. Classification of over 1500 sources in 15 nearby star-forming regions reveals that approximately 39 per cent of the sources lie in the primordial disc region, whereas between 31 and 32 per cent disperse from the inside-out and up to 22 per cent of the sources have already lost their disc. Less than 2 per cent of the objects in our sample lie in the homogeneous draining regime. Time-scales for the transition phase are estimated to be typically a few 105 yr independent of stellar mass. Therefore, regardless of spectral type, we conclude that currently available infrared photometric surveys point to fast (of the order of 10 per cent of the global disc lifetime) inside-out clearing as the preferred mode of disc dispersal. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a detailed study of star formation occurring in bound star-forming clouds under the influence of internal ionizing feedback from massive stars across a spectrum of cloud properties. We infer which objects are triggered by comparing our feedback simulations with control simulations in which no feedback was present. We find that feedback always results in a lower star formation efficiency and usually but not always results in a larger number of stars or clusters. Cluster mass functions are not strongly affected by feedback, but stellar mass functions are biased towards lower masses. Ionization also affects the geometrical distribution of stars in ways that are robust against projection effects, but may make the stellar associations more or less subclustered depending on the background cloud environment. We observe a prominent pillar in one simulation which is the remains of an accretion flow feeding the central ionizing cluster of its host cloud and suggest that this may be a general formation mechanism for pillars such as those observed in M16. We find that the association of stars with structures in the gas such as shells or pillars is a good but by no means foolproof indication that those stars have been triggered and we conclude overall that it is very difficult to deduce which objects have been induced to form and which formed spontaneously simply from observing the system at a single time. [ABSTRACT FROM AUTHOR]

Aims. Grain growth has been suggested as one possible explanation for the diminished dust optical depths in the inner regions of protoplanetary "transition" disks. In this work, we directly test this hypothesis in the context of current models of grain growth and transport. Methods. A set of dust evolution models with different disk shapes, masses, turbulence parameters, and drift efficiencies is combined with radiative transfer calculations in order to derive theoretical spectral energy distributions (SEDs) and images. Results. We find that grain growth and transport effects can indeed produce dips in the infrared SED, as typically found in observations of transition disks. Our models achieve the necessary reduction of mass in small dust by producing larger grains, yet not large enough to be fragmenting efficiently. However, this population of large grains is still detectable at millimeter wavelengths. Even if perfect sticking is assumed and radial drift is neglected, a large population of dust grains is left behind because the time scales on which they are swept up by the larger grains are too long. This mechanism thus fails to reproduce the large emission cavities observed in recent millimeter-wave interferometric images of accreting transition disks. [ABSTRACT FROM AUTHOR]

Context. The [Ne II] line 12.81 μm was proposed to be a good tracer of gas in the environments of proto-planetary disks; its origin is explained by different mechanisms: jets in outflows, photo-evaporative disk winds driven by stellar X-rays/EUV or by the X-ray irradiated proto-planetary disk atmosphere. Previous Spitzer studies gave hints toward the neon emitting mechanism by exploring correlations between the line luminosity and properties of the star-disk system. These studies concluded that the origin of the emission is likely related to accretion and outflows, with some influence from X-rays. Aims. We provide direct constraints on the origin of the [Ne II] emission using high-spatial and spectral resolution observations that allow us to study the kinematics of the emitting gas. In addition we compare the [Ne II] line with optical forbidden lines. Methods. We obtained high-resolution ground-based observations with VISIR-VLT for 15 stars and UVES-VLT for three of them. The stars were chosen for having bright neon emission lines detected with Spitzer/IRS. The velocity shifts and profiles are used to disentangle the different emitting mechanisms producing the [Ne II] line. A comparison between results from this study and previous high-resolution studies is also presented. Results. The [Ne II] line was detected in seven stars, among them the first confirmed detection of [Ne II] in a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate an origin in a jet. In two stars, the small shifts and asymmetric profiles indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to edge-on, shows a spatially unresolved line centered at the stellar rest velocity, although cross-dispersion centroids move within 10 AU from one side of the star to the other as a function of wavelength. The line profile is symmetric with wings extending up to ∼±80 km s-1. The origin of the [Ne II] line is unclear and could either be due to the bipolar jet or to the disk. For the stars with VLT-UVES observations, in several cases, the optical forbidden line profiles and shifts are very similar to the profile of the [Ne II] line, suggesting that the lines are emitted in the same region. A general trend observed with VISIR is a lower line flux when compared with the fluxes obtained with Spitzer. We found no correlation between the line full-width at half maximum and the line peak velocity. The [Ne II] line remains undetected in a large part of the sample, an indication that the emission detected with Spitzer in those stars is likely extended. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a multiwavelength study of three star-forming regions, spanning the age range 1-14 Myr, located between the 30 Doradus complex and supernova SN 1987A in the Large Magellanic Cloud (LMC). We reliably identify about 1000 pre-main-sequence (PMS) star candidates actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (). Our measurements represent the largest data set of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical of PMS stars in the LMC is higher than for galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and estimates, the difference in between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disc dispersal and find indications that typically higher are to be expected in low-metallicity environments. However, many issues of this scenario need to be clarified by future observations and modelling. We also find that, in the mass range 1-2 M⊙, of PMS stars in the LMC increases with stellar mass as , with b≈ 1, i.e. slower than the second power law found for galactic PMS stars in the same mass regime. [ABSTRACT FROM AUTHOR]

Context. The global size and spatial distribution of dust is an important ingredient in the structure and evolution of protoplanetary disks and in the formation of larger bodies, such as planetesimals. Aims. We aim to derive simple equations that explain the global evolution of the dust surface density profile and the upper limit of the grain size distribution and which can readily be used for further modeling or for interpreting of observational data. Methods. We have developed a simple model that follows the upper end of the dust size distribution and the evolution of the dust surface density profile. This model is calibrated with state-of-the-art simulations of dust evolution, which treat dust growth, fragmentation, and transport in viscously evolving gas disks. Results. We find very good agreement between the full dust-evolution code and the toy model presented in this paper. We derive analytical profiles that describe the dust-to-gas ratios and the dust surface density profiles well in protoplanetary disks, as well as the radial flux by solid material "rain out", which is crucial for triggering any gravity assisted formation of planetesimals. We show that fragmentation is the dominating effect in the inner regions of the disk leading to a dust surface density exponent of -1.5, while the outer regions at later times can become drift-dominated, yielding a dust surface density exponent of -0.75. Our results show that radial drift is not efficient in fragmenting dust grains. This supports the theory that small dust grains are resupplied by fragmentation due to the turbulent state of the disk. [ABSTRACT FROM AUTHOR]

Context. Several diagnostics ranging from the radio to the X-ray band are suitable for investigating the magnetic activity of late-type stars. Empirical connections between the emission at different wavelengths place constraints on the nature and efficiency of the emission mechanism and the physical conditions in different atmospheric layers. The activity of ultracool dwarfs, at the low-mass end of the main-sequence, is poorly understood. Aims. We perform a multi-wavelength study of one of the nearest M9 dwarfs, DENIS-P J104814.7-395606 (4 pc), to examine its position within the group of magnetically active ultracool dwarfs, and, in general, advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Methods. We obtained an XMM-Newton observation of DENIS-P J104814.7-395606 and a broad-band spectrum from the ultraviolet to the near-infrared with X-Shooter. From this dataset, we derive the X-ray properties, stellar parameters, kinematics, and the emission-line spectrum tracing chromospheric activity. We integrate these data by compiling the activity parameters of ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation provides the first X-ray detection of DENIS-P J104814.7-395606 (log Lx = 25.1), as well as the first measurement of its V band brightness (V = 17.35 mag). The flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS 1048-3956 are compatible with optically thick plasma in local thermal equilibrium (LTE) at low, nearly photospheric temperature or optically thin LTE plasma at 20 000 K. Describing the decrements with case B recombination requires different emitting regions for Hα and the higher Balmer lines. The high observed Hα/Hβ flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behaviors. [ABSTRACT FROM AUTHOR]

ABSTRACT We present an analysis of multi-epoch observations of the Type II-P supernova SN 2004et. New and archival optical spectra of SN 2004et are used to study the evolution of the Hα and [O i] 6300-Å line profiles between days 259 and 646. Mid-infrared imaging with Michelle on Gemini-North and with all three instruments of the Spitzer Space Telescope was carried out between 2004 and 2010, supplemented by archival Spitzer data. We include Spitzer'warm' mission photometry at 3.6 and 4.5 μm obtained on days 1779, 1931 and 2151, along with ground-based and Hubble Space Telescope ( HST) optical and near-infrared observations obtained between days 79 and 1803. Multiwavelength light curves are presented, as well as optical-infrared spectral energy distributions (SEDs) for multiple epochs. Starting from about day 300, the optical light curves provide evidence for an increasing amount of circumstellar extinction attributable to newly formed dust, with the additional extinction reaching 0.8-1.5 mag in the V band by day 690. The overall SEDs were fitted with multiple blackbody components, in order to investigate the luminosity evolution of the supernova, and then with Monte Carlo radiative transfer models using smooth or clumpy dust distributions, in order to estimate how much new dust condensed in the ejecta. The luminosity evolution was consistent with the decay of 56Co in the ejecta up until about day 690, after which an additional emission source is required, in agreement with the findings of Kotak et al. Clumped dust density distributions consisting of 20 per cent amorphous carbons and 80 per cent silicates by mass were able to match the observed optical and infrared SEDs, with dust masses that increased from 8 × 10−5 M⊙ on day 300 to 1.5 × 10−3 M⊙ on day 690, still significantly lower than the values needed for core-collapse supernovae to make a significant contribution to the dust enrichment of galaxies. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a 3D photoionization model of the planetary nebula NGC 6302, one of the most complex and enigmatic objects of its kind. Its highly bipolar geometry and dense massive disc, coupled with the very wide range of ions present, from neutral species up to Si8 +, make it one of the ultimate challenges to nebular photoionization modelling. Our mocassin model is composed of an extremely dense geometrically thin circumstellar disc and a large pair of diffuse bipolar lobes, a combination which was necessary to reproduce the observed emission-line spectrum. The masses of these components, 2.2 M⊙ and 2.5 M⊙, respectively, give a total nebular mass of 4.7 M⊙, of which 1.8 M⊙ (39 per cent) is ionized. Discrepancies between our model fit and the observations are attributed to complex density inhomogeneities in the nebula. The potential to resolve such discrepancies with more complex models is confirmed by exploring a range of models introducing small-scale structures. Compared to solar abundances helium is enhanced by 50 per cent, carbon is slightly subsolar, oxygen is solar, and nitrogen is enhanced by a factor of 6. These all imply a significant third dredge-up coupled with hot-bottom burning CN-cycle conversion of dredged-up carbon to nitrogen. Aluminium is also depleted by a factor of 100, consistent with depletion by dust grains. The central star of NGC 6302 is partly obscured by the opaque circumstellar disc, which is seen almost edge-on, and as such its properties are not well constrained. However, emission from a number of high-ionization 'coronal' lines provides a strong constraint on the form of the high-energy ionizing flux. We model emission from the central star using a series of stellar model atmospheres, the properties of which are constrained from fits to the high-ionization nebular emission lines. Using a solar abundance stellar atmosphere we are unable to fit all of the observed line fluxes, but a substantially better fit was obtained using a 220 000 K hydrogen-deficient stellar atmosphere with log g= 7.0 and L★= 14 300 L⊙. The H-deficient nature of the central star atmosphere suggests that it has undergone some sort of late thermal pulse, and fits to evolutionary tracks imply a central star mass of 0.73-0.82 M⊙. Time-scales for these evolutionary tracks suggest the object left the top of the asymptotic giant branch ∼2100 years ago, in good agreement with studies of the recent mass-loss event that formed one pair of the bipolar lobes. Based on the modelled nebular mass and central star mass we estimate the initial mass of the central star to be 5.5 M⊙, in approximate agreement with that derived from evolutionary tracks. [ABSTRACT FROM AUTHOR]

We present a study of the variation of spatial structure of stellar populations within dwarf galaxies as a function of the population age. We use deep Hubble Space Telescope/Advanced Camera for Surveys imaging of nearby dwarf galaxies in order to resolve individual stars and create composite colour-magnitude diagrams (CMDs) for each galaxy. Using the obtained CMDs, we select blue helium burning stars, which can be unambiguously age-dated by comparing the absolute magnitude of individual stars with stellar isochrones. Additionally, we select a very young () population of OB stars for a subset of the galaxies based on the tip of the young main sequence. By selecting stars in different age ranges, we can then study how the spatial distribution of these stars evolves with time. We find, in agreement with previous studies, that stars are born within galaxies with a high degree of substructure which is made up of a continuous distribution of clusters, groups and associations from parsec to hundreds of parsec scales. These structures disperse on time-scales of tens to hundreds of Myr, which we quantify using the two-point correlation function and the Q-parameter developed by Cartwright & Whitworth. On galactic scales, we can place lower limits on the time it takes to remove the original structure (i.e. structure survives for at least this long), t, which varies between (NGC 2366) and (DDO 165). This is similar to what we have found previously for the Small Magellanic Cloud () and the Large Magellanic Cloud (). We do not find any strong correlations between t and the luminosity of the host galaxy. [ABSTRACT FROM AUTHOR]

Deep spectroscopy of the planetary nebula NGC 6153 shows that its heavy element abundances derived from optical recombination lines (ORLs) are 10 times higher than those derived from collisionally excited lines (CELs), and points to the existence of H-deficient inclusions embedded in the diffuse nebula. In this study, we have constructed chemically homogeneous and bi-abundance three-dimensional photoionization models, using the Monte Carlo photoionization code . We attempt to reproduce the multiwaveband spectroscopic and imaging observations of NGC 6153, and investigate the nature and origin of the postulated H-deficient inclusions, as well as their impacts on the empirical nebular analyses assuming a uniform chemical composition. Our results show that chemically homogeneous models yield small electron temperature fluctuations and fail to reproduce the strengths of ORLs from C, N, O and Ne ions. In contrast, bi-abundance models incorporating a small amount of metal-rich inclusions ( per cent of the total nebular mass) are able to match all the observations within the measurement uncertainties. The metal-rich clumps, cooled down to a very low temperature ( K) by ionic infrared fine-structure lines, dominate the emission of heavy element ORLs, but contribute almost nil to the emission of most CELs. We find that the abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform chemical composition, are about 30 per cent lower than the corresponding average values of the whole nebula, including the contribution from the H-deficient inclusions. Ironically, in the presence of H-deficient inclusions, the traditional standard analysis of the optical helium recombination lines, assuming a chemically homogeneous nebula, overestimates the helium abundance by 40 per cent. [ABSTRACT FROM AUTHOR]

SN 2008S erupted in early 2008 in the grand design spiral galaxy NGC 6946. The progenitor was detected by Prieto et al. in Spitzer Space Telescope images taken over the four years prior to the explosion, but was not detected in deep optical images, from which they inferred a self-obscured object with a mass of about . We obtained Spitzer observations of SN 2008S 5 days after its discovery, as well as coordinated Gemini and Spitzer optical and infrared observations 6 months after its outburst. We have constructed radiative transfer dust models for the object before and after the outburst, using the same density distribution of pre-existing amorphous carbon grains for all epochs and taking light travel time effects into account. We rule out silicate grains as a significant component of the dust around SN 2008S. The inner radius of the dust shell moved outwards from its pre-outburst value of 85 au to a post-outburst value of 1250 au, attributable to grain vaporization by the light flash from SN 2008S. Although this caused the circumstellar extinction to decrease from before the outburst to 0.8 after the outburst, we estimate that less than 2 per cent of the overall circumstellar dust mass was destroyed. The total mass-loss rate from the progenitor star is estimated to have been . The derived dust-mass-loss rate of implies a total dust injection into the interstellar medium of up to over the suggested duration of the self-obscured phase. We consider the potential contribution of objects like SN 2008S to the dust enrichment of galaxies. [ABSTRACT FROM AUTHOR]

The lifetime of protoplanetary discs is intimately linked to the mechanism responsible for their dispersal. Since the formation of planets within a disc must operate within the time frame of disc dispersal, it is crucial to establish what is the dominant process that disperses the gaseous component of discs around young stars. Planet formation itself as well as photoevaporation by energetic radiation from the central young stellar object has been proposed as plausible dispersal mechanisms. There is, however, still no consensus as what the dominant process may be. In this paper, we use the different metallicity dependence of X-ray photoevaporation and planet formation to discriminate between these two processes. We study the effects of metallicity, Z, on the dispersal time-scale, tphot, in the context of a photoevaporation model, by means of detailed thermal calculations of a disc in hydrostatic equilibrium irradiated by extreme ultraviolet and X-ray radiation from the central source. Our models show for a pure photoevaporation model. By means of analytical estimates, we derive instead a much stronger negative power dependence on metallicity of the disc lifetime for a dispersal model based on planet formation. A census of disc fractions in lower metallicity regions should therefore be able to distinguish between the two models. A recent study by Yasui et al. in low-metallicity clusters of the extreme outer Galaxy ( and dust-to-gas ratio of ∼0.001) provides preliminary observational evidence for shorter disc lifetimes at lower metallicities, in agreement with the predictions of a pure photoevaporation model. While we do not exclude that planet formation may indeed be the cause of some of the observed discs with inner holes, these observational findings and the models and analysis presented in this work are consistent with X-ray photoevaporation as the dominant disc dispersal mechanism. We finally develop an analytical framework to study the effects of metallicity-dependent photoevaporation on the formation of gas giants in the core accretion scenario. We show that accounting for this effect strengthens the conclusion that planet formation is favoured at higher metallicity. We find, however, that the metallicity dependence of photoevaporation only plays a secondary role in this scenario, with the strongest effect being the positive correlation between the rate of core formation and the density of solids in the disc. [ABSTRACT FROM AUTHOR]

We present the first radiation-hydrodynamic model of a protoplanetary disc irradiated with an X-ray extreme ultraviolet (X-EUV) spectrum. In a model where the total ionizing luminosity is divided equally between X-ray and EUV luminosity, we find a photoevaporation rate of , which is two orders of magnitude greater than the case of EUV photoevaporation alone. Thus, it is clear that the X-rays are the dominant driving mechanism for photoevaporation. This can be understood inasmuch as X-rays are capable of penetrating much larger columns and can thus effect heating in denser regions and at larger radius than the EUV. The radial extent of the launching region of the X-ray-heated wind is 1–70 au compared with the pure EUV case where the launch region is concentrated around a few au. When we couple our wind mass-loss rates with models for the disc's viscous evolution, we find that, as in the pure EUV case, there is a photoevaporative switch, such that an inner hole develops at ∼1 au at the point when the accretion rate in the disc drops below the wind mass-loss rate. At this point, the remaining disc material is quickly removed in the final 15–20 per cent of the disc's lifetime. This is consistent with the 105 yr transitional time-scale estimated from observations of T Tauri stars. We however note several key differences to previous EUV-driven photoevaporation models. The two orders of magnitude higher photoevaporation rate is now consistent with the average accretion rate observed in young stars and will cut the disc off in its prime. Moreover, the extended mass-loss profile subjects the disc to a significant period (∼20 per cent of the disc's lifetime) of ‘photoevaporation-starved accretion’. We also caution that although our mass-loss rates are high compared to some accretion rates observed in young stars, our model has a rather large X-ray luminosity of ; further modelling is required in order to investigate the evolutionary implications of the large observed spread of X-ray luminosities in T Tauri stars. [ABSTRACT FROM AUTHOR]

The [Ne ii] fine-structure emission line at 12.8 μm has been detected in several young stellar objects spectra. This line is thought to be produced by X-ray irradiation of the warm protoplanetary disc atmospheres; however, the observational correlation between [Ne ii] luminosities and measured X-ray luminosities shows a large scatter. Such spread limits the utility of this line as a probe of the gaseous phase of discs, as several authors have suggested pollution by outflows as a probable cause of the observed scatter. In this work, we explore the possibility that the large variations in the observed [Ne ii] luminosity may be caused instead by different star–disc parameters. In particular we study the effects that the hardness of the irradiating source and the structure (flaring) of the disc have on the luminosity and spectral profile of the [Ne ii] 12.8 μm line. We find that varying these parameters can indeed cause up to an order of magnitude variation in the emission luminosities which may explain the scatter observed, although our models predict somewhat smaller luminosities than those recently reported by other authors who observed the line with the Spitzer Space Telescope. Our models also show that the hardness of the spectrum has only a limited (undetectable) effect on the line profiles, while changes in the flaring power of the disc significantly affect the size of the [Ne ii] emission region and, as a consequence, its line profile. In particular, we suggest that broad-line profiles centred on the stellar radial velocity may be indicative of flat discs seen at large inclination angles. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation of the effect of multiple ionization sources in H ii regions on the total elemental abundances derived from the analysis of collisionally excited emission lines. We focus on empirical methods based on direct temperature measurements that are commonly employed in cases when the temperature of the nebular gas can be determined from the ratio of nebular to auroral lines of (e.g.) doubly ionized oxygen. We find that direct temperature methods that employ a two-temperature zone approach (DT2T methods) are very robust against the spatial distribution of sources. Errors smaller than 0.15 dex are estimated for regions where the metallicity is twice solar and errors below 0.05 dex for solar metallicities and below. The biases introduced by the spatial distribution of the ionization sources are thus much smaller for DT2T methods than for strong line methods, previously investigated by Ercolano, Bastian & Stasińska. Our findings are in agreement with the recent study of H ii regions in NGC 300 by Bresolin et al. [ABSTRACT FROM AUTHOR]

We re-examine the recent suggestion of a high fraction of transition discs (i.e. those with a cleared inner hole) in M stars, motivated by the fact that we expect that, for M stars, even discs without inner holes should exhibit very weak excess shortwards of around . Our analysis of spectral energy distribution models suggests that this indeed means that M stars where a detectable excess begins at around may be misclassified as transition discs when in fact they have optically thick dust extending in to the dust sublimation radius. Consequently, we estimate that the transition disc fraction among M stars in the Coronet cluster is per cent (rather than the recently claimed value of 50 per cent). This revised figure would imply that the transition disc fraction is not after all markedly higher in later type stars. We suggest that, for M stars, transition discs can only be readily identified if they have emission that is close to photospheric out to . [ABSTRACT FROM AUTHOR]

The projected distribution of stars in the Small Magellanic Cloud (SMC) from the Magellanic Clouds Photometric Survey is analysed. Stars of different ages are selected via criteria based on V magnitude and colour, and the degree of ‘grouping’ as a function of age is studied. We quantify the degree of structure using the two-point correlation function and a method based on the Minimum Spanning Tree and find that the overall structure of the SMC is evolving from a high degree of substructure at young ages (∼10 Myr) to a smooth radial density profile. This transition is gradual and at ∼75 Myr the distribution is statistically indistinguishable from the background SMC distribution. This time-scale corresponds to approximately the dynamical crossing time of stars in the SMC. The spatial positions of the star clusters in the SMC show a similar evolution of spatial distribution with age. Our analysis suggests that stars form with a high degree of (fractal) substructure, probably imprinted by the turbulent nature of the gas from which they form, which is erased by random motions in the galactic potential on a time-scale of a galactic crossing time. [ABSTRACT FROM AUTHOR]

We have analysed deep optical spectra of the ‘born-again’ planetary nebula Abell 58 and its hydrogen-deficient knot, surrounding V605 Aql, which underwent a nova-like eruption in 1919. Our analysis shows that the extinction towards the central knot is much higher than previously thought, with . The outer nebula is less reddened, with . We find that the outer nebula has a Ne/O ratio higher than the average PN value. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O iii] nebular-to-auroral transition ratio gives a temperature of 20 800 K, while the ratio of the [N ii] nebular and auroral lines gives Te= 15 200 K. The helium line ratios λ5876/λ4471 and λ6678/λ4471 imply temperatures of 350 and 550 K, respectively. Weakly temperature-sensitive O ii recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (ADF) for O2+ reaching 89 – the second highest known value after that found for the hydrogen-deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionized material. Although the central star is carbon-rich (C/O > 1), the knot is found to be oxygen-rich, a situation not predicted by the single-star ‘born-again’ theory of its formation. We compare the known properties of Abell 58 to those of Abell 30, Sakurai's Object and several novae and nova remnants. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and the presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models. [ABSTRACT FROM AUTHOR]

We present a new fast algorithm which allows the simulation of ionizing radiation emitted from a point source to be included in high-resolution three-dimensional smoothed particle hydrodynamics simulations of star cluster formation. We employ a Strömgren volume technique in which we use the densities of particles near the line-of-sight between the source and a given target particle to locate the ionization front in the direction of the target. Along with one-dimensional tests, we present fully three-dimensional comparisons of our code with the three-dimensional Monte Carlo radiative transfer code,mocassin, and show that we achieve good agreement, even in the case of highly complex density fields. [ABSTRACT FROM AUTHOR]

Star formation within galaxies appears on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the local group spiral galaxy M33, based on a new implementation of the minimum spanning tree method. Unlike previous studies which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (∼20 pc) to kpc scales. Once the groups are identified, we study their properties, for example, size and luminosity distributions, and compare them with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star cluster spatial scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power law with an index, −2, the same as has been found for the luminosity and mass functions (MFs) of young star clusters, as well as the MF of GMCs. Additionally, the groups follow a similar mass–radius relation as GMCs. The size distribution of the groups is best described by a lognormal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group. We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a lognormal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ∼4 kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming interstellar medium, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of H ii regions, GMCs, and star-forming groups are all highly correlated. [ABSTRACT FROM AUTHOR]

Spatially resolved studies of star-forming regions show that the assumption of spherical geometry is not realistic in most cases, with a major complication posed by the gas being ionized by multiple non-centrally located stars or star clusters. Geometrical effects including the spatial configuration of ionizing sources affect the temperature and ionization structure of these regions. We try to isolate the effects of multiple non-centrally located stars via the construction of 3D photoionization models using the 3D Monte Carlo photoionization codemocassin with very simple gas density distributions, but various spatial configurations for the ionization sources. Our first aim is to study the resulting temperature structure of the gas and investigate the behaviour of temperature fluctuations within the ionized region. We show that geometry affects the temperature structures in our models differently according to metallicity. For the geometries and stellar populations considered in our study, at intermediate and high metallicities, models with ionizing sources distributed in the full volume, whose Strömgren spheres rarely overlap, show smaller temperature fluctuation than their central ionization counterparts, with fully overlapping concentric Strömgren spheres. The reverse is true at low metallicities. Finally, the true temperature fluctuations due to the stellar distribution (as opposed to the large-scale temperature gradients due to other gas properties) are small in all cases and not a significant cause of error in metallicity studies. Emission-line spectra from H ii regions are often used to study the metallicity of star-forming regions, as well as providing a constraint for temperatures and luminosities of the ionizing sources. Empirical metallicity diagnostics must often be calibrated with the aid of photoionization models. However, most studies so far have been carried out by assuming spherical or plane-parallel geometries, with major limitations on allowed gas and dust density distributions and with the spatial distribution of multiple, non-centrally located ionizing sources not being accounted for. We compare integrated emission-line spectra from our models and quantify any systematic errors caused by the simplifying assumption of a single, central location for all ionizing sources. We find that the dependence of the metallicity indicators on the ionization parameter causes a clear bias due to the fact that models with a fully distributed configuration of stars always display lower ionization parameters than their fully concentrated counterparts. The errors found imply that the geometrical distribution of ionization sources may partly account for the large scatter in metallicities derived using model-calibrated empirical methods. [ABSTRACT FROM AUTHOR]

We present a study of the effects of clumping on the emergent spectral energy distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse radiation source distributed throughout the medium. These models are appropriate for Type II SNe older than a few hundred days, when the energy input is dominated by γ-rays from 56Co decay. The fully 3D radiation transport problem is solved using a Monte Carlo code,mocassin, and we present a set of models aimed at investigating the sensitivity of the SEDs to various clumping parameters. We find that, contrary to the predictions of analytical prescriptions, the combination of an optical and infrared (IR) observational data set is sufficient to constrain dust masses even in the case where optically thick clumps are present. Using both smoothly varying and clumped grain density distributions, we obtain new estimates for the mass of dust condensed by the Type II SN 1987A by fitting the optical and IR spectrophotometric data of Wooden et al. at two epochs (day 615 and day 775). When using amorphous carbon grains, our best fits to the observational data imply that about 2.0 × 10−4 M⊙ of dust had condensed in the envelope of SN 1987A by day 615 and between 2.0 × 10−4 and 4.2 × 10−3 M⊙ by day 775. We find that the absence of a silicate emission or absorption feature in the observed mid-IR spectra implies that no more than 15 per cent of the dust formed around SN 1987A can have been in the form of silicate particles. Our models require larger dust masses for the case of graphite grains, namely between 4.2 × 10−4 and 6.6 × 10−4 M⊙ at day 615 and between 4.5 × 10−4 and 7.5 × 10−4 M⊙ at day 775. From our numerical models, we derive dust masses for SN 1987A that are comparable to previous analytic clumped graphite grain mass estimates, and at least two orders of magnitude below the 0.1–0.3 M⊙ that have been predicted to condense as dust grains in primordial core-collapse SN ejecta. This low condensation efficiency for SN 1987A is in contrast to the case of SN 2003gd, for which a dust condensation efficiency as large as 0.12 has recently been estimated. [ABSTRACT FROM AUTHOR]

We present coefficients for the calculation of the continuous emission spectra of H i, He i and He ii due to electron–ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the threshold of hydrogen , and for temperatures . The emission coefficients for He i are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1 per cent in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionization/plasma codes and should aid with the interpretation of spectra from the very cold ionized gas phase inferred to exist in a number of gaseous clouds. [ABSTRACT FROM AUTHOR]

We have constructed a 3D photoionization model of a planetary nebula (PN) similar in structure to NGC 7009 with its outer pair of knots (also known as FLIERs – fast, low-ionization emission regions). The work is motivated by the fact that the strong [N ii]λ6583 line emission from FLIERs in many PNe has been attributed to a significant local overabundance of nitrogen. We explore the possibility that the apparent enhanced nitrogen abundance previously reported in the FLIERs may be due to ionization effects. The model is constrained by the results obtained by Gonçalves et al. from the analysis of both Hubble Space Telescope ( HST) [O iii] and [N ii] images, and long-slit spectra of NGC 7009. Our model is indeed able to reproduce the main spectroscopic and imaging characteristics of the bright inner rim of NGC 7009 and its outer pairs of knots, assuming homogeneous elemental abundances throughout the nebula, for nitrogen as well as all the other elements included in the model. We also study the effects of a narrow slit on our non-spherically symmetric density distribution, via the convolution of the model results with the profile of the long slit used to obtain the spectroscopic observations that constrained our model. This effect significantly enhances the [N ii]/Hβ emission, more in the FLIERs than in the inner rim. Because of the fact that the (N+/N)/(O+/O) ratio predicted by our models is 0.60 for the rim and is 0.72 for the knots, so clearly in disagreement with the assumption of the ionization correction factor (icf) method, the icfs will be underestimated by the empirical scheme, in both components, rim and knots, but more so in the knots. This effect is partly responsible for the apparent inhomogeneous N abundance empirically derived. The differences in the above ratio in these two components of the nebula may be due to a number of effects including charge exchange – as pointed out previously by other authors – and the difference in the ionization potentials of the relevant species – which makes this ratio extremely sensitive to the shape of the local radiation field. Because of the latter, a realistic density distribution is essential to the modelling of a non-spherical object, if useful information is to be extracted from spatially resolved observations, as in the case of NGC 7009. [ABSTRACT FROM AUTHOR]

We present the first 3D Monte Carlo (MC) photoionization code to include a fully self-consistent treatment of dust radiative transfer (RT) within a photoionized region. This is the latest development (version 2.0) of the gas-only photoionization code mocassin and employs a stochastic approach to the transport of radiation, allowing both the primary and secondary components of the radiation field to be treated self-consistently, whilst accounting for the scattering of radiation by dust grains mixed with the gas, as well as the absorption and emission of radiation by both the gas and the dust components. An escape probability method is implemented for the transfer of resonance lines that may be absorbed by the grains, thus contributing to their energy balance. The energetics of the co-existing dust and gas components must also take into account the effects of dust–gas collisions and photoelectric emission from the dust grains, which are dependent on the grain charge. These are included in our code using the average grain potential approximation scheme. A set of rigorous benchmark tests have been carried out for dust-only spherically symmetric geometries and 2D disc configurations. The results of mocassin are found to be in agreement with those obtained by well-established dust-only RT codes that employ various approaches to the solution of the RT problem. A model of the dust and of the photoionized gas components of the planetary nebula NGC 3918 is also presented as a means of testing the correct functioning of the RT procedures in a case where both gas and dust opacities are present. The two components are coupled via the heating of dust grains by the absorption of both UV continuum photons and resonance line photons emitted by the gas. The mocassin results show agreement with those of a 1D dust and gas model of this nebula published previously, showing the reliability of the new code, which can be applied to a variety of astrophysical environments. [ABSTRACT FROM AUTHOR]

Deep optical spectra of the high-excitation planetary nebula NGC 1501 and its W04 central star are presented. A recombination line abundance analysis of the emission-line spectrum of the central star yields He : C : O mass fractions of 0.36 : 0.48 : 0.16, similar to those of PG 1159 stars. A detailed empirical analysis of the nebular collisionally excited line (CEL) and optical recombination line (ORL) spectrums are presented, together with fully three-dimensional photoionization modelling of the nebula. We found very large ORL–CEL abundance discrepancy factors (ADFs) for O2+ (32) and Ne2+ (33). The mean value of∼5100 K for theTe derived from He irecombination lines ratios is 6000 K, lower than the value of 11 100 K implied by the[O iii] line ratio. This result indicates the existence of a second, low-temperature nebular component, which could account for the observed ORL emission. Electron temperature fluctuationscannot account for the high ADFs found from our optical spectra of this nebula.A three-dimensional photoionization model of NGC 1501 was constructed using the photoionization codemocassin, based on our new spectroscopic data and using the three-dimensional electron density distribution determined from long-slit echellograms of the nebula by Ragazzoni et al. The central star ionizing radiation field is approximated by a model atmosphere, calculated using the Tübingen non-local thermodynamic equilibrium model atmosphere package, for abundances typical of the W04 nucleus of NGC 1501 and PG 1159 stars. The nebular emission-line spectrum was best reproduced using a central star model with an effective temperature ofand a luminosity of. The initial models showed higher degrees of ionization of heavy elements than indicated by observations. We investigated the importance of the missing low-temperature dielectronic recombination rates for third-row elements and have estimated upper limits to their rate coefficients.Our single-phase, three-dimensional photoionization model heavily underpredicts the optical recombination line emission. We conclude that the presence of a hydrogen-deficient, metal-rich component is necessary to explain the observed ORL spectrum of this object. The existence of such knots could also provide a softening of the radiation field, via the removal of ionizing photons by absorption in the knots, thereby helping to alleviate the overionization of the heavy elements in our models. [ABSTRACT FROM AUTHOR]