Using the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains

Draine, B. T.; Hensley, Brandon S.

doi:10.48550/arxiv.2101.07277

Cited by 2 publications

(3 citation statements)

References 49 publications

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“…For a moderate filling factor of 0.1, our simulations reach polarisation degrees of 1.9% in the innermost disk regions, which is in agreement with several observations. An only moderately porous dust phase is also in agreement with recent studies concerning the interstellar medium (Hirashita et al 2021;Draine & Hensley 2021), and the rotational disruption of dust grains in protoplanetary disks (Tatsuuma & Kataoka 2021). Furthermore, the polarisation of inclined disks follows the same trends concerning the filling factor as mentioned above in the face-on case.…”

Section: Discussionsupporting

confidence: 90%

“…The dust model was chosen to consist only of spherical grains. For the chemical composition, we used the refractive indices of astronomical silicate (astrosil) and the two main orientations of graphite (parallel and perpendicular, Draine & Malhotra 1993) from Draine (2003). To mimic porous dust grains in our study, we considered spherical grains with different filling factors Φ and explored their effect on the scattering polarisation.…”

Section: Dust Modelmentioning

confidence: 99%

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Self-scattering on large, porous grains in protoplanetary disks with dust settling

Brunngräber

Wolf

2021

A&A

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Context. Observations of protoplanetary disks in the sub-millimetre wavelength range suggest that polarisation is caused by scattering of thermal re-emission radiation. Most of the dust models that are used to explain these observations have major drawbacks: they either use much smaller grain sizes than expected from dust evolution models, or result in polarisation degrees that are lower than observed. Aims. We investigate the effect of dust grain porosity on the observable polarisation due to scattering at sub-millimetre wavelengths arising from grain size distributions up to millimetre sizes, as they are expected to be present close to the midplane of protoplanetary disks. Methods. Using the effective medium theory, we calculated the optical properties of porous dust and used them to predict the behaviour of the observable polarisation degree due to scattering. Subsequently, Monte Carlo radiative transfer simulations for protoplanetary disks with porous dust grains were performed to analyse the additional effect of the optical depth structure, and thus the effect of multiple scattering events and inhomogeneous temperature distributions on the net observable polarisation degree. Results. We find that porous dust grains with moderate filling factors of about 10% increase the degree of polarisation compared to compact grains. For higher grain porosities, that is, grains with a filling factor of 1% or lower, the extinction opacity decreases, as does the optical depth of a disk with constant mass. Consequently, the unpolarised direct radiation dominates the scattered flux, and the degree of polarisation drops rapidly. Even though the simulated polarisation degree is higher than in the case of compact grains, it is still below the typical observed values for face-on disks. However, the polarisation degree can be increased when crucial model assumptions derived from disk and dust evolution theories, for instance, dust settling and millimetre-sized dust grains, are dropped. In the case of inclined disks, however, our reference model is able to achieve polarisation degrees of about 1%, and using higher disk masses, even higher than this.

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Section: Discussionsupporting

confidence: 90%

Section: Dust Modelmentioning

confidence: 99%

Self-scattering on large, porous grains in protoplanetary disks with dust settling

Brunngräber

Wolf

2021

A&A

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“…For a moderate filling factor of 0.1, our simulations reach polarisation degrees of 1.9 % in the innermost disk regions, which is in agreement with several observations. An only moderately porous dust phase is also in agreement with recent studies concerning the interstellar medium (Hirashita et al 2021;Draine & Hensley 2021), and the rotational disruption of dust grains in protoplanetary disks (Tatsuuma & Kataoka 2021). Furthermore, the polarisation of inclined disks follows the same trends concerning the filling factor as mentioned above in the face-on case.…”

Section: Discussionsupporting

confidence: 90%

Self-scattering in protoplanetary disks with dust settling

Brunngräber

Wolf²

2020

A&A

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Scattering of re-emitted flux is considered to be at least partially responsible for the observed polarisation in the (sub-)millimetre wavelength range of several protoplanetary disks. Although the degree of polarisation produced by scattering is highly dependent on the dust model, early studies investigating this mechanism relied on the assumption of single grain sizes and simple density distribution of the dust. However, in the dense inner regions where this mechanism is usually most efficient, the existence of dust grains with sizes ranging from nanometres to millimetres has been confirmed. Additionally, the presence of gas forces larger grains to migrate vertically towards the disk midplane, introducing a dust segregation in the vertical direction. Using polarisation radiative transfer simulations, we analyse the dependence of the resulting scattered light polarisation at 350 μm, 850 μm, 1.3 mm, and 2 mm on various parameters describing protoplanetary disks, including the effect of dust grain settling. We find that the different disk parameters change the degree of polarisation mostly by affecting the anisotropy of the radiation field, the optical depth, or both. It is therefore very challenging to deduce certain disk parameter values directly from polarisation measurements alone. However, assuming a high dust albedo, it is possible to trace the transition from optically thick to optically thin disk regions. The degree of polarisation in most of the considered disk configurations is lower than what is found observationally, implying the necessity to revisit models that describe the dust properties and disk structure.

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Using the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains

Cited by 2 publications

References 49 publications

Self-scattering on large, porous grains in protoplanetary disks with dust settling

Self-scattering on large, porous grains in protoplanetary disks with dust settling

Self-scattering in protoplanetary disks with dust settling

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