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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 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]

Abstract: With AKARI, we have systematically performed near- to far-infrared (NIR–FIR) observations of the interstellar medium (ISM) in our Galaxy and nearby galaxies as one of AKARI mission programs called ISMGN (ISM in our Galaxy and Nearby galaxies). Our scientific objective is to increase our knowledge on the properties of the ISM exposed to an extensive range of environments, the processing, evolution, and destruction of interstellar dust, and their connection with physical conditions of interstellar gas and star-forming activity. Our Galaxy contains ideal laboratories for probing the life cycle of the ISM, while nearby external galaxies provide a much wider range of physical conditions. We present the contents of the AKARI observations relevant to our ISM studies, together with some results demonstrative of the AKARI uniqueness obtained during the AKARI cold mission phase (Phases 1 and 2). For the ISM in our Galaxy and the LMC, we have performed spectroscopic observations of several regions intensively in the NIR to FIR, including detailed studies of 14 Galactic and 21 LMC SNRs. We have observed about 60 nearby galaxies, for many of which NIR to FIR images in the 10 photometric bands centered at the wavelengths of 3, 4, 7, 11, 24, 15, 65, 90, 140, and as well as 2–14 low-resolution spectra are obtained. Our data, especially the 11 and imaging, the FIR 4-band imaging, and the NIR spectral data will be complementary to the Spitzer data of nearby galaxies such as those from the SINGS legacy program. We further refer to our on-going observations in the AKARI post-helium mission phase (Phase 3). [Copyright &y& Elsevier]

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]