Dust in the diffuse ISM


The variance of magnetic field strengths and structures is a fundamental issue in the structure of the diffuse interstellar medium. Although Zeeman measurements and infrared polarization provide a picture of the dense phase, the translucent medium remains elusive. A survey of approximately 50 lines of sight, using very high resolution UV and optical spectroscopy (resonance transitions for velocity and density information) and optical polarization measurements to decompose the contributors along the lines of sight has been performed. In the majority of cases, the contributors are composite and clouds can be separated in velocity and extinction contributions. A substantial fraction, however, are single clouds (about 15% of the sample). Unsurprisingly, the polarization is more randomized with more line-of-sight contributors, but the exceptional detail also makes this study a warning regarding interpretation of single-direction measurements of dust properties.


The observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light of the diffuse interstellar medium can be explained by a two component dust model. The model (Siebenmorgen et al, 2014, A&A 561, A82) 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 500nm. Dust grains with radii larger than 6nm are spheroids. 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. In this model 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 contributor to the polarisation .
Typical extinction, emission, and polarisation signatures of dust in the solar neighborhod are shown in the Figure below (right). Variations of dust properties along distinguished sight-lines towards stars either including multiple- (open circles) or single aborbing dust cloud (filled symbols) is shown on the left (Siebenmorgen et al, 2018, A&A 611, A5). The number of intersecting clouds is revealed by high resolution spectroscopy of the Doppler components of interstellar lines (CaII , NaI, KI).
Dust in the diffuse ISM

Dust in the solar neighbourhood

The above figure shows:
Top-left: The observed extinction curves in the ISM by Fitzpatrick & Massa (2007), is shown as dashed line and the 1 sigma variation as hatched area. The contribution of the individual dust components to the total extinction of the model are given with scaled cross sections.
Middle of left panels: Emission normalised per H atom when dust is heated by the ISRF. High Galactic latitudes observations from DIRBE (Arendt et al. 1998) and FIRAS (Finkbeiner et al. 1999) with 1 sigma error bars as shown in grey. The model fluxes convolved with the band passes of the observations are shown as filled circles. The emission of the individual dust components is given. The total emission is shown as black line.
Bottom of left panel: The observed linear polarisation normalised to the maximum polarisation as given by Voshchinnikov et al. (2012) is shown as dashed line and 1 sigma variation as hatched area. The normalised linear polarisation of silicates is shown for prolates as a black line, and oblates as magenta dotted line.
Top of middle panels: Dust parameters versus observed extinction and polarisation characteristics. Influence of the exponent of the dust size distribution q on the total to selective extinction Rv,
Top of right panels: The mass ratio of small and large (>6nm) grains, m(vsg) / m(lgr), on the strength of the far UV rise of the extinction curve c4
Bottom of middle: The mean size of aligned silicates versus the wavelength at which the polarisation has its maximum.
Bottom of right panels: The minimum size of aligned silicate grains versus the width of the polarisation curve.
Symbols: Sight-lines to individual stars that intersect multiple dust clouds are shown as open circles and their Pearson correlation coefficient is given in black. Sight-lines to stars where only a single dust cloud is penetrated are shown as red squares together with their Person coefficient and a straight line estimator (MLE) in red. In the course of the archive research of this proposal we identify a few additional single-cloud sight-lines that we mark in orange.

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