Observational Constraints on Interstellar Grain Alignment

Andersson, B. G.; Potter, S.

doi:10.1086/519755

Cited by 66 publications

(97 citation statements)

References 83 publications

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“…Hence we can measure the relative grain alignment enhancement in a region by deriving the average relationship between λ max and A V for field stars behind the cloud in question and then probing for localized deviations from this average relationship. We did this in Andersson & Potter (2007, 2010 and found that in Chamaeleon I, but beyond the region around HD 97300 (where its radiation field dominates that of the diffuse radiation), a linear relationship is found:…”

Section: Observations Data Reduction and Analysismentioning

confidence: 96%

“…As discussed in Andersson & Potter (2007), the wavelength of the maximum of the polarization curve (λ max ) is a sensitive probe of the grain alignment and one that is immune to the magnetic field topology along the line of sight. The absolute value of λ max depends on the over-all grain size distributions and likely on the color and intensity of the local diffuse interstellar radiation field (ISRF), but as shown by Andersson & Potter (2007) the slope in the λ max vs. A V relation is universal.…”

Section: Observations Data Reduction and Analysismentioning

confidence: 99%

“…The absolute value of λ max depends on the over-all grain size distributions and likely on the color and intensity of the local diffuse interstellar radiation field (ISRF), but as shown by Andersson & Potter (2007) the slope in the λ max vs. A V relation is universal. Hence we can measure the relative grain alignment enhancement in a region by deriving the average relationship between λ max and A V for field stars behind the cloud in question and then probing for localized deviations from this average relationship.…”

Section: Observations Data Reduction and Analysismentioning

confidence: 99%

“…The FIR data here are from the IRIS reprocessing of the IRAS data (Miville-Deschênes & Lagache 2005). For details see Andersson & Potter (2007). In Fig.…”

Section: Grain Alignmentmentioning

confidence: 99%

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Angle-dependent radiative grain alignment

Andersson

Pintado

Potter

et al. 2011

A&A

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Context. Interstellar grain alignment studies are currently experiencing a renaissance due to the development of a new quantitative theory based on radiative alignment torques (RAT). One of the distinguishing predictions of this theory is a dependence of the grain alignment efficiency on the relative angle (Ψ) between the magnetic field and the anisotropy direction of the radiation field. In an earlier study we found observational evidence for such an effect from observations of the polarization around the star HD 97300 in the Chamaeleon I cloud. However, due to the large uncertainties in the measured visual extinctions, the result was uncertain. Aims. By acquiring explicit spectral classification of the polarization targets, we have sought to perform a more precise reanalysis of the existing polarimetry data. Methods. We have obtained new spectral types for the stars in our for our polarization sample, which we combine with photometric data from the literature to derive accurate visual extinctions for our sample of background field stars. This allows a high accuracy test of the grain alignment efficiency as a function of Ψ. Results. We confirm and improve the measured accuracy of the variability of the grain alignment efficiency with Ψ, seen in the earlier study. We note that the grain temperature (heating) also shows a dependence on Ψ which we interpret as a natural effect of the projection of the grain surface to the illuminating radiation source. This dependence also allows us to derive an estimate of the fraction of aligned grains in the cloud.

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Section: Observations Data Reduction and Analysismentioning

confidence: 96%

Section: Observations Data Reduction and Analysismentioning

confidence: 99%

Section: Observations Data Reduction and Analysismentioning

confidence: 99%

“…The FIR data here are from the IRIS reprocessing of the IRAS data (Miville-Deschênes & Lagache 2005). For details see Andersson & Potter (2007). In Fig.…”

Section: Grain Alignmentmentioning

confidence: 99%

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Angle-dependent radiative grain alignment

Andersson

Pintado

Potter

et al. 2011

A&A

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“…The resulting correlation between A V and λ max has been observed in a number of clouds ( Fig. 1b; [22,23]). …”

Section: Pos(lcdu 2013)003mentioning

confidence: 53%

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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