Interstellar polarization and grain alignment: the role of iron and silicon

Вощинников, Н. В.; Henning, Th.; Prokopjeva, M. S.; Das, H. K.

doi:10.1051/0004-6361/201117763

Cited by 36 publications

(54 citation statements)

References 62 publications

(51 reference statements)

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“…The abundance of O is uncertain within a factor of two. Variations in absolute abundance estimates are noticed for elements such as Si, Mg, and Fe, for which one assumes that they are completely condensed (for a recent review on dust abundances see Voshchinnikov et al 2012). By averaging over all stars of the Voshchinnikov & Henning (2010) sample the total Si abundance is 25 ± 3 ppm.…”

Section: Element Abundancesmentioning

confidence: 99%

“…The total emission is shown as black line (middle). The observed linear polarisation normalised to the maximum polarisation as given by Voshchinnikov et al (2012) is shown as dashed line and 1σ variation as hatched area. The normalised linear polarisation of silicates is shown for prolates as a black line, and oblates as dotted magenta line (bottom).…”

Section: Dust In the Solar Neighbourhoodmentioning

confidence: 99%

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Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

Self Cite

107

123

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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.

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Section: Element Abundancesmentioning

confidence: 99%

Section: Dust In the Solar Neighbourhoodmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

Self Cite

107

123

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“…The linear function (1) well describes the general dependence of K on λ max derived for several dark clouds, but the observational data for some lines of sight can essentially differ from this dependence Voshchinnikov, 2012). A qualitative explanation of the relation between the width of the IS polarization curve and the position of its maximum is connected to the growth of dust grains in the accretion and coagulation processes which lead to narrowing of the particle size distribution (see, e.g., Whittet et al, 1992).…”

Section: Introductionmentioning

confidence: 89%

Systematic variations of interstellar linear polarization and growth of dust grains

et al. 2013

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Abstract. A quantitative interpretation of the observed relation between the interstellar linear polarization curve parameters K and λ max characterizing the width and the wavelength of a polarization maximum, respectively, is given. The observational data available for 57 stars located in the dark clouds in Taurus, Chamaeleon, around the stars ρ Oph and R CrA are considered. The spheroidal particle model of interstellar dust grains earlier applied to simultaneously interpret the interstellar extinction and polarization curves in a wide spectral range is utilized. The observed trend K ≈ 1.7λ max is shown to be most likely related to a growth of dust grains due to coagulation rather than mantle accretion. The relation of the parameters K and λ max with an average size of silicate dust grains is discussed.

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“…Recent observations suggest that these latter conditions are not met in many regions of the ISM. For example, no correlations are observed between the amounts of solid iron in dust (as inferred from depletion measurements) and the level of observed polarization [13], as might be expected if superparamagnetic inclusions are required for grain alignment. Additionally, polarized lines of CO ice have been observed in molecular clouds [14], in regions where column densities are large enough that one expects near equilibrium between gas and dust temperatures, thereby severely limiting the ability of grains to reach suprathermal rotation rates [6].…”

Section: Alignment Mechanismsmentioning

confidence: 96%

Dust Grain Alignment in the Interstellar Medium

Vaillancourt¹,

Andersson²,

Lazarían³

2014

Proceedings of the Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments — PoS(LCDU2013)

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The first observations of interstellar polarization at visible wavelengths over 60 years ago were quickly attributed to the net alignment of irregular dust grains with local magnetic fields. This mechanism provides a method to measure the topology and strength of the magnetic field and to probe the physical characteristics of the dust (e.g., material, size, and shape). However, to do so with confidence, the physics and variability of the alignment mechanism(s) must be quantitatively understood. The description of the physical alignment mechanism has a long history with key contributions spanning decades; the last 15 years have seen major advances in both the theoretical and observational understanding of the problem. For example, it is now clear that the canonical process of paramagnetic relaxation, in which grain rotational components perpendicular to the magnetic field are damped out, is inadequate to align grains on the necessary timescales (compared to damping via collisions) for typical interstellar medium conditions. However, the modern theory of radiative alignment has been more successful; in this theory grains are aligned with respect to the magnetic field via photon-grain interactions that impart the necessary torques to the rotation axes of grains. Here we highlight key observational tests of these alignment mechanisms, especially those involving spectropolarimetry of both dust extinction at near-optical wavelengths and dust emission at far-infrared through millimeter wavelengths. Observations in both these regimes can place limits on such grain aspects as their size and temperature. To date, most observations of the polarized emission have been in the densest regions of the interstellar medium where interpretation in terms of grain alignment models is complicated by regions containing embedded stars and a wide range of temperatures. Additionally, direct comparison of the optical extinction polarization (A V 10 magnitudes) with dust emission polarization (A V 25 magnitudes) has not been possible. Future observations with increased sensitivity and spectral coverage in the far-infrared may reach the lower extinction levels necessary to allow more definitive tests of grain alignment models.

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Interstellar polarization and grain alignment: the role of iron and silicon

Cited by 36 publications

References 62 publications

Dust in the diffuse interstellar medium

Dust in the diffuse interstellar medium

Systematic variations of interstellar linear polarization and growth of dust grains

Dust Grain Alignment in the Interstellar Medium

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