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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]

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]

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]

Constraining the spatial and thermal structure of the gaseous component of circumstellar disks is crucial for understanding star and planet formation. Models predict that the [Ne II] line at 12.81μm detected in young stellar objects (YSOs) with Spitzer traces disk gas and its response to high-energy radiation, but such [Ne II] emission may also originate in shocks within powerful outflows. To distinguish between these potential origins for mid-infrared [Ne II] emission and to constrain disk models, we observed 32 YSOs using the high-resolution (R ~ 30,000) mid-infrared spectrograph VISIR at the Very Large Telescope. We detected the 12.81μm [Ne II] line in 12 objects, tripling the number of detections of this line in YSOs with high spatial and spectral resolution spectrographs. We obtain the following main results. (1) In Class I objects the [Ne II] emission observed from Spitzer is mainly due to gas at a distance of more than 20-40 AU from the star, where neon is, most likely, ionized by shocks due to protostellar outflows. (2) In transition and pre-transition disks, most of the emission is confined to the inner disk, within 20-40 AU from the central star. (3) Detailed analysis of line profiles indicates that, in transition and pre-transition disks, the line is slightly blueshifted (2-12 km s-1) with respect to the stellar velocity, and the line width is directly correlated with the disk inclination, as expected if the emission is due to a disk wind. (4) Models of EUV/X-ray-irradiated disks reproduce well the observed relation between the line width and the disk inclination, but underestimate the blueshift of the line. [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]

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]

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]

We present nebular phase optical imaging and spectroscopy and near/mid-IR imaging of the Type II SN 2006bc. Observations reveal the central wavelength of the symmetric Hα line profile to be redshifted with respect to the host galaxy Hα emission by day 325. Such a phenomenon has been argued to result from an asymmetric explosion in the iron-peak elements resulting in a larger mass of 56Ni and higher excitation of hydrogen on the far side of the supernova (SN) explosion. We also observe a gradual blueshifting of this Hα peak which is indicative of dust formation in the ejecta. Although showing a normal peak brightness, V ∼ –17.2, for a core-collapse SN, 2006bc fades by ∼6 mag during the first 400 days suggesting either a relatively low 56Ni yield, an increase in extinction due to new dust, or both. A short-duration flattening of the light curve is observed from day 416 to day 541 suggesting an optical light echo. Based on the narrow time window of this echo, we discuss implications on the location and geometry of the reflecting interstellar medium. With our radiative transfer models, we find an upper limit of 2 × 10–3M☼ of dust around SN 2006bc. In the event that all of this dust were formed during the SN explosion, this quantity of dust is still several orders of magnitude lower than that needed to explain the large quantities of dust observed in the early universe. [ABSTRACT FROM AUTHOR]