The effect of temperature mixing on the observable (T,beta)-relation of interstellar dust clouds

Detailed studies of the shape of dust emission spectra are possible thanks to the current instruments capable of observations in several sub-millimetre bands (e.g., Herschel and Planck). However, some controversy remains even on the basic effects resulting from the mixing of temperatures along the line-of-sight. Studies have suggested either a positive or a negative correlation between the colour temperature T_C and the observed spectral index beta_Obs. Our aim is to show that both cases are possible and to determine the factors leading to either behaviour. We start by studying the sum of two or three modified black bodies of different temperature. With radiative transfer modelling, we examine the probability distributions of the dust mass as a function of the physical dust temperature. With these results as a guideline, we examine the (T_C, beta_Obs) relations for different sets of clouds. Even in the case of modified blackbodies at temperatures T_0 and T_0+ Delta T_0, the correlation between T_C and beta_Obs can be either positive or negative. If one compares models where Delta T_0 is varied, the correlation is negative. If the models differ in their mean temperature T_0 rather than in Delta T_0, the correlation remains positive. Radiative transfer models show that externally heated clouds have different mean temperatures but the widths of their temperature distributions are rather similar. Thus, the correlation between T_C and beta_Obs is expected to be positive. The same result applies to clouds illuminated by external radiation fields of different intensity. For internally heated clouds a negative correlation is the more likely alternative. If the signal-to-noise ratio is high, the observed negative correlation could be explained by the temperature dependence of the dust optical properties but that intrinsic dependence could be even steeper than the observed one.Comment: Accepted to A&

Recent Results of Solid-State Spectroscopy

Solid state spectroscopy continues to be an important source of information on the mineralogical composition and physical properties of dust grains both in space and on planetary surfaces. With only a few exceptions, artificially produced or natural terrestrial analog materials, rather than 'real' cosmic dust grains, are the subject of solid state astrophysics. The Jena laboratory has provided a large number of data sets characterizing the UV, optical and infrared properties of such cosmic dust analogs. The present paper highlights recent developments and results achieved in this context, focussing on 'non-standard conditions' such as very low temperatures, very high temperatures and very long wavelengths.Comment: 15 pages, 10 figures. Contribution to an IAU Conference "The Molecular Universe" held in Toledo in June 201

Dust and star formation properties of a complete sample of local galaxies drawn from the Planck Early Release Compact Source Catalogue

We combine Planck High Frequency Instrument data at 857, 545, 353 and 217 GHz with data from Wide-field Infrared Survey Explorer (WISE), Spitzer, IRAS and Herschel to investigate the properties of a well-defined, flux-limited sample of local star-forming galaxies. A 545 GHz flux density limit was chosen so that the sample is 80 per cent complete at this frequency, and the resulting sample contains a total of 234 local, star-forming galaxies. We investigate the dust emission and star formation properties of the sample via various models and calculate the local dust mass function. Although single-component-modified blackbodies fit the dust emission longward of 80 \u3bcm very well, with a median \u3b2 = 1.83, the known degeneracy between dust temperature and \u3b2 also means that the spectral energy distributions are very well described by a dust component with dust emissivity index fixed at \u3b2 = 2 and temperature in the range 10-25 K. Although a second, warmer dust component is required to fit shorter wavelength data, and contributes approximately a third of the total infrared emission, its mass is negligible. No evidence is found for a very cold (6-10 K) dust component. The temperature of the cold dust component is strongly influenced by the ratio of the star formation rate to the total dust mass. This implies, contrary to what is often assumed, that a significant fraction of even the emission from \u2dc20 K dust is powered by ongoing star formation, whether or not the dust itself is associated with star-forming clouds or `cirrus'. There is statistical evidence of a free-free contribution to the 217 GHz flux densities of 7220 per cent. We find a median dust-to-stellar mass ratio of 0.0046; and that this ratio is anticorrelated with galaxy mass. There is good correlation between dust mass and atomic gas mass (median Md/MHI = 0.022), suggesting that galaxies that have more dust (higher values of Md/M*) have more interstellar medium in general. Our derived dust mass function implies a mean dust mass density of the local Universe (for dust within galaxies), of 7.0 \ub1 1.4 7 105 M 99 Mpc-3, significantly greater than that found in the most recent estimate using Herschel data. \ua9 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies

Aims. The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample. Methods. This study is performed by exploiting the multi-wavelength DustPedia database, from ultraviolet (UV) to sub-millimeter bands, in addition to molecular (12CO) and atomic (Hi) gas maps and metallicity abundance information available in the literature. We fitted the surface-brightness profiles of the tracers of dust and stars, the mass surface-density profiles of dust, stars, molecular gas, and total gas, and the SFR surface-density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (αCO) per galaxy by using dust- and gas-mass profiles. Results. Although each galaxy has its own peculiar behavior, we identified a common trend of the exponential scale-lengths versus wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec2 radius decrease from UV to 70 μm, from 0.4 to 0.2, and then increase back up to ~0.3 at 500 microns. The main result is that, on average, the dust-mass surface-density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 μm surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 μm scale-length tend to increase from earlier to later types, the scale-length at 70 μm tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 μm that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types. Our αCO determinations are in the range (0.3−9) M⊙ pc-2 (K km s-1)-1, almost invariant by using a fixed dust-to-gas ratio mass (DGR) or a DGR depending on metallicity gradient

Infrared and Raman Spectroscopy of Methylcyanodiacetylene (CH3C5N)

International audienceA spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH3C5N). Gas-phase, cryogenic matrix-isolated, and pure solid-phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas-phase IR absorption bands were revisited and extended. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei