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We measure the parsec–scale relationship between integrated CO intensity (ICO) and visual extinction (AV) in 24 local molecular clouds using maps of CO emission and dust optical depth from Planck. This relationship informs our understanding of CO emission across environments, but clean Milky Way measurements remain scarce. We find uniform ICO for a given AV, with the results bracketed by previous studies of the Pipe and Perseus clouds. Our measured ICO−AV relation broadly agrees with the standard Galactic CO-to-H2 conversion factor, the relation found for the Magellanic clouds at coarser resolution, and numerical simulations by Glover & Clark (2016). This supports the idea that CO emission primarily depends on shielding, which protects molecules from dissociating radiation. Evidence for CO saturation at high AV and a threshold for CO emission at low AV varies remains uncertain due to insufficient resolution and ambiguities in background subtraction. Resolution of order 0.1 pc may be required to measure these features. We use this ICO−AV relation to predict how the CO-to-H2 conversion factor (XCO) would change if the Solar Neighbourhood clouds had different dust-to-gas ratio (metallicity). The calculations highlight the need for improved observations of the CO emission threshold and H i shielding layer depth. They are also sensitive to the shape of the column density distribution. Because local clouds collectively show a self-similar distribution, we predict a shallow metallicity dependence for XCO down to a few tenths of solar metallicity. However, our calculations also imply dramatic variations in cloud-to-cloud XCO at subsolar metallicity. [ABSTRACT FROM AUTHOR]

The 'torus' obscurer of active galactic nuclei (AGN) is poorly understood in terms of its density, sub-structure and physical mechanisms. Large X-ray surveys provide model boundary constraints, for both Compton-thin and Compton-thick levels of obscuration, as obscured fractions are mean covering factors fcov. However, a major remaining uncertainty is host-galaxy obscuration. In Paper I, we discovered a relation of NH∝M*1/3 for the obscuration of galaxy-scale gas. Here, we apply this observational relation to the AGN population, and find that galaxy-scale gas is responsible for a luminosity-independent fraction of Compton-thin AGN, but does not produce Compton-thick columns. With the host-galaxy obscuration understood, we present a model of the remaining nuclear obscurer, which is consistent with a range of observations. Our radiation-lifted torus model consists of a Compton-thick component (fcov ~35?per?cent) and a Compton-thin component (fcov ~40?per?cent), which depends on both black hole mass and luminosity. This provides a useful summary of observational constraints for torus modellers who attempt to reproduce this behaviour. It can also be employed as a sub-grid recipe in cosmological simulations that do not resolve the torus. We also investigate host-galaxy X-ray obscuration inside cosmological, hydrodynamic simulations (Evolution and Assembly of Galaxies and their Environment; Illustris). The obscuration from ray-traced galaxy gas can agree with observations, but is highly sensitive to the chosen feedback assumptions. [ABSTRACT FROM AUTHOR]

Using near-infrared United Kingdom Infrared Deep Sky Survey (UKIDSS) Galactic Plane Survey data, we make extinction measurements to individual stars along the same line of sight as molecular clouds. Using an existing 3D extinction map of the inner Galaxy, that provides line of sight specific extinction–distance relationships, we convert the measured extinction of molecular clouds to a corresponding distance. These distances are derived independently from kinematic methods, typically used to derive distances to molecular clouds, and as such they have no near/far ambiguity. The near/far distance ambiguity has been resolved for 27 clouds, and distances have been derived to 20 clouds. The results are found to be in good agreement with kinematic measurements to molecular clouds where the ambiguity has already been resolved, using H i self-absorption techniques. [ABSTRACT FROM AUTHOR]

The origin of grain size distribution in the interstellar medium is one of the most fundamental problems in the interstellar physics. In the Milky Way, smaller grains are more abundant in number, but their origins are not necessarily specified and quantified. One of the most efficient drivers of small grain production is interstellar turbulence, in which dust grains can acquire relative velocities large enough to be shattered. Applying the framework of shattering developed in previous papers, we show that small grains reach the abundance level observed in the Milky Way in (i.e. within the grain lifetime) by shattering in warm neutral medium. We also show that if part of grains experiences additional shattering in warm ionized medium, carbonaceous grains with are redistributed into smaller sizes. This could explain the relative enhancement of very small carbonaceous grains with Å. Our theory also explains the ubiquitous association between large grains and very small grains naturally. Some tests for our theory are proposed in terms of the metallicity dependence. [ABSTRACT FROM AUTHOR]

GGD30 has been suggested to be either a small reflection nebulosity or a Herbig–Haro (HH) object formed in the outflow from a nearby obscured star. Observations to date have not been able to distinguish between these two scenarios. In addition, there are conflicting proposals for the location of the exciting source for GGD30. To resolve these questions, we have carried out optical spectroscopy and near-infrared ( J, K and 3.6-μm) imaging of GGD30. Taken together, these observations reveal that the bright optical knot in GGD30 must be a HH object, excited by the outflow from an optically obscured pre-main-sequence (PMS) star located ∼3 arcsec to the southwest. Based on mid-infrared fluxes from the Mid-course Space Experiment ( MSX) satellite, we estimate the luminosity of this PMS star to be which suggests it is an intermediate-mass object rather than low-mass as previously proposed. The optical spectroscopy indicates projected velocities of associated with the HH object. The fact that these velocities are blueshifted and relatively high compared to the velocities typical of HH flows suggests that the outflow from the PMS star must be almost aligned with the line of sight. There is an additional low-velocity component but its origin is not clear. [ABSTRACT FROM AUTHOR]

The butterfly-shaped planetary nebula, NGC 6302, shows a unique, dense equatorial dark lane, which is presumably a dusty disc, obscuring an unobserved, very hot central star. We trace the structure of this disc usingHubble Space TelescopeHα and[Nii] images, Very Large Telescope L-and M-band images at 0.4-arcsec resolution, including Brα and polycyclic aromatic hydrocarbon (PAH) images, and a James Clerk Maxwell Telescope 450-μm image. Extinction maps are derived from these images. Within the disc, the extinction isand. The 450-μm map shows a north–south elongated central core, tracing the massive dust disc, and extended emission from dust in the bipolar flows. A fit to the spectral energy distribution yields the disc dust mass of. The innermost region shows an ionized shell. The orientation of the polar axis shows a marked change between shell, disc and inner and outer outflow. The structures are well described by the warped-disc model of. PAH images are presented: PAH emission is found in the shell but avoids the disc. An infrared source is found close to the expected location of the central star. [ABSTRACT FROM AUTHOR]

We investigate the attenuation law seen through an interstellar medium (ISM) with clumpy spatial distributions of stars and dust. The clumpiness of the dust distribution is introduced by a multiphase ISM model. We solve a set of radiative transfer equations with multiple anisotropic scatterings through the clumpy ISM in a one-dimensional plane-parallel geometry by using the mega-grain approximation, in which dusty clumps are regarded as very large particles (i.e. mega-grains). The clumpiness of the stellar distribution is introduced by the youngest stars embedded in the clumps. We assume a smooth spatial distribution for older stars. The youngest stars are surrounded by denser dusty gas and suffer stronger attenuation than diffuse older stars (i.e. age-selective attenuation). The apparent attenuation law is a composite of the attenuation laws for the clumpy younger stars and for the diffuse older stars with a luminosity weight. In general, the stellar population dominating the luminosity changes from older stars to younger stars as the wavelength decreases. This makes the attenuation law steep; the composite attenuation rapidly increases from small attenuation for older stars at a long wavelength to large attenuation for younger stars at a short wavelength. The resultant attenuation law of normal disc galaxies is expected to be much steeper than that of starburst galaxies observed by Calzetti et al. Finally, the Calzetti attenuation law is regarded as a special case with a large density in our framework. [ABSTRACT FROM AUTHOR]

A method for determining distances to dark clouds and Bok globules based on broad-band optical and near-infrared photometry is presented. In this method, intrinsic colour indices of stars projected towards the direction of a cloud are computed by dereddening the observed colour indices using various trial values of extinctionand a standard extinction law. The computed intrinsic colour indices for a star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match the standard intrinsic colour indices. Distances (d) to the stars are determined using theand absolute magnitudescorresponding to the spectral types thus obtained. A plot ofagainstdundergoes a sharp rise at a distance corresponding to the distance to the cloud. Using this method, we have determined a distance of 550 pc to the cometary globule CG 12. The distance of 550 pc and a Galactic latitude ofimply that CG 12 is at a height of∼200 pc above the galactic mid-plane. The star formation efficiency in this cloud is found to be relatively high (≳16 per cent, to as large as∼33 per cent). The existence of an H ishell centred atwith CG 12 near its boundary and its tail pointing away from the centre of the shell supports the suggestion of a supernova explosion, near the centre of the H ishell, being responsible for the cometary morphology and the triggering of star formation. Thus CG 12 is a rare example of triggered high-mass star formation at relatively large galactic height. [ABSTRACT FROM AUTHOR]

We present far-infrared (FIR) and submillimetre images of the Crab nebula, taken with the ISOPHOT instrument onboard theInfrared Space Observatory(ISO) and with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The ISOPHOT observations were made in the bands centred at 60, 100 and 170μm, with FWHM of equivalent area Gaussian beams of 44, 47 and 93 arcsec, respectively. The 850-μm SCUBA image was processed using a maximum-entropy method algorithm and has a Gaussian FWHM of 17 arcsec. The 60- and 100-μm images show clear excess of emission, above that expected from an extrapolation of the synchrotron spectrum of the Crab nebula from lower frequencies, as previously seen fromIRASobservations. The superior angular resolution of the ISOPHOT images reveal that about half this excess is attributable to two peaks, separated by≈80 arcsec. We also present spectra taken using the Long-wavelength Spectrometer onboardISO, which show that the FIR excess is not due to line emission. The lower-resolution 170-μm image does not show any excess emission, but is possibly fainter, particularly in the NW, than expected from an extrapolation of the lower-frequency synchrotron emission. These findings are consistent with a picture in which the FIR excess is due to emission from a small amount of warm dust (in the ranges 0.01–0.07 andfor astronomical silicate and graphite, respectively) which radiates predominantly at 60 and 100μm but not at, and which is superimposed on a synchrotron spectrum that gradually steepens towards shorter wavelengths throughout the far and medium infrared spectral range. The dust geometry is consistent with a torus of diameter≈0.8 pc created by the (red supergiant) supernova progenitor prior to its explosion, superimposed upon a broadly distributed component which may be supernova condensates in the filaments. The dust-to-gas ratio in the filaments is comparable to the interstellar value. Therefore, even if the condensates seen at the present epoch could ultimately escape the remnant without being destroyed, the surrounding interstellar medium will not be significantly enriched in dust. Our upper limit ofon the total mass of carbon in the form of graphite is consistent with the inference from the gas-phase carbon abundances that there has been no significant enrichment of the filaments in carbon nucleosynthesis products from the progenitor. To study spectral index variations in the radio synchrotron emission we compared the 850-μm image with a 20-cm Very Large Array (VLA) image. These images are very similar, implying that there is little variation in spectral index across the face of the remnant between these wavelengths. As seen previously, there are some spectral variations near the centre of the remnant. However, given the good agreement between the integrated flux density at 850μm and the extrapolated synchrotron spectrum, together with the different epochs of the 850-μm and 20-cm images, we do not see the need for the second radio synchrotron component from the remnant, which has previously been proposed. [ABSTRACT FROM AUTHOR]

Recent CO measurements of an essentially complete subsample of galaxies from the SCUBA Local Universe Survey (SLUGS) are used to examine their implications for dust and gas masses in this sample. Estimates of dust masses are affected by a contribution to the SCUBA brightness measurements by CO(3–2) emission, and molecular gas masses by the use of a modified value of the CO-to-H2 conversion factor X . The average dust mass is reduced by 25–38 per cent, which has no bearing on earlier conclusions concerning the shape of the dust mass luminosity function derived from the SLUGS. The value of X found from the CO survey, when applied together with the reduction in dust masses, leads to lower estimates for the mean gas-to-dust mass ratios, where the gas includes both H2 and H i . For the CO sample, the mean global ratio is reduced from approximately 430 to about 320–360, but is further reduced to values near 50 when applied to the nuclear regions relevant to the CO observations. We discuss these results and suggest that the differences between the nuclear and outer regions may simply reflect differences in metallicity or the existence of considerable amounts of unobserved cold dust in the outer regions of these galaxies. [ABSTRACT FROM AUTHOR]