Dust in the diffuse interstellar medium

Abstract

We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light. The model is set up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5-3 greater absorption cross section in the far-infrared than spherical grains of the same volume do. Mass estimates derived from submillimetre observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates, and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified so that absolute element abundances are not direct input parameters. The model is compared to the average properties of the Milky Way, which seem to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars, with one of them located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributors to the polarisation.

The global dust modelling framework THEMIS

“…Basing their work upon the MRN dust model Draine & Lee (1984) semi-empirically tuned graphite and silicate optical properties to fit observations, resulting in "astronomical" graphite and silicate. Polycyclic aromatic hydrocarbon (PAH) molecules were later added into the mix (Desert et al 1990;Siebenmorgen & Kruegel 1992;Dwek et al 1997;, 2002Siebenmorgen et al 2014) but the physical basis for these models, even including the later improvements, is fundamentally little different from the MRN model. It should be noted that in all of these models the different grain populations, for example, graphite and olivine (MRN), astronomical graphite and silicate (Draine & Lee 1984) or astronomical graphite, silicate and PAH (Siebenmorgen & Kruegel 1992;Dwek et al 1997;, 2002Siebenmorgen et al 2014) are considered to reside in distinct and separate dust populations.…”

“…Polycyclic aromatic hydrocarbon (PAH) molecules were later added into the mix (Desert et al 1990;Siebenmorgen & Kruegel 1992;Dwek et al 1997;, 2002Siebenmorgen et al 2014) but the physical basis for these models, even including the later improvements, is fundamentally little different from the MRN model. It should be noted that in all of these models the different grain populations, for example, graphite and olivine (MRN), astronomical graphite and silicate (Draine & Lee 1984) or astronomical graphite, silicate and PAH (Siebenmorgen & Kruegel 1992;Dwek et al 1997;, 2002Siebenmorgen et al 2014) are considered to reside in distinct and separate dust populations. Some later dust models followed this same basic approach but abandoned astronomical graphite in favour of physically more-realistic amorphous carbons (e.g., Zubko et al 2004;Compiègne et al 2011;Galliano et al 2011).…”

Planck intermediate results

“…These data were used to define the dust models of Draine & Li (2007), Compiègne et al (2011) and Siebenmorgen et al (2014), and the analytical fit proposed by Finkbeiner et al (1999), which has been widely used by the CMB community to extrapolate the IRAS all-sky survey to microwave frequencies. Today the Planck data allow us to characterize the dust emission at millimetre wavelengths directly from observations.…”

Planckintermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353 GHz with interstellar polarization in the visible

“…Such complex regions are deliberately not considered here. We limit our analysis to lines of sight through the diffuse ISM, for which more homogeneous properties might be expected, and for which the most comprehensive observational constraints on dust models are already available and exploited (e.g., Draine & Li 2007;Compiegne et al 2011;Jones et al 2013;Siebenmorgen et al 2014).…”

“…The most recent are further constrained by fitting the (pre-Planck) spectral energy distribution of dust emission in the infrared and submillimetre (Draine & Fraisse 2009;Siebenmorgen et al 2014). Efforts have also been made to predict the polarized thermal emission quantitatively (Martin 2007;Draine & Fraisse 2009;Draine & Hensley 2013).…”