Characterizing cometary dust grains with polarimetry

Light scattering experiments have been carried out at visible light for two kinds of dust samples: narrow size distributions consisting of low-absorbing particles with sizes in the micron domain, plus millimeter-sized single grains. The polarimetric analysis of the scattered light has revealed as a very precise tool for size characterization of the samples of particles in the resonance regime, while it let us distinguish the compact/fluffy structure feature of particles much larger than the wavelength.

First, four narrow size distributions of forsterite particles, spanning a broad range of scattering sizes in the resonance regime, were measured. Forsterite is a Mg-rich olivine representative of low-absorbing cosmic dust particles that has been identified in several comets, e.g. 1P/Halley, Hale–Bopp, 9P/Tempel and 81P/Wild2 (see [1] and references therein). The maximum of the degree of linear polarization (DLP) decreases with the size of the particles. That maximum occurs around 90º in all cases, but the angle ranges from 75º to 140º, increasing with the grain size [2]. Furthermore, dust particles with size parameters from ∼ 6 to ∼ 20 seem to be responsible for the negative polarization branch (NBP). This is in agreement with the idea that the NBP might be caused by coherent back-scattering by wavelength-scaled features of the grains (see .e.g. [3]).

On the other hand, millimeter-sized particles were identified to reproduce the flux curve of Fomalhaut protoplanetary disk [4]. Light scattering was then
measured for the same kind of grains in an attempt to reproduce the comet 67P/Churyumov-Gerasimenko phase function obtained in situ by the OSIRIS
camera system onboard the ESA Rosetta mission. In that case it was found that large porous particles with organic inclusions can reproduce both, ground-based observations of the DLP and the OSIRIS phase functions [5].

 

References

1. Frattin, E., Muñoz, O., Moreno, F., Nava, J., Escobar-Cerezo, J., Gomez Martin, J.C., Guirado, D., Cellino, A., Coll, P., Raulin, F., Bertini, I., Cremonese, G., Lazzarin, M., Naletto, G., Forgia, F.L.: Experimental phase function and degree of linear polarization of cometary dust analogues. Monthly Notices of the Royal Astronomical Society 484, 2198–2211 (2019)

2. Muñoz, O., Frattin, E., Jardiel, T., Gómez-Martín, J.C., Moreno, F., Ramos, J.L., Guirado, D., Peiteado, M., Caballero, A.C., Milli, J., Ménard, F.: Retrieving dust grain sizes from photopolarimetry: An experimental approach. The Astrophysical Journal Supplement Series 256(17 (12pp)) (2021)

3. Muinonena, K., Penttilä, A., Videen, G.: Polarimetry of Stars and Planetary Systems, chap. Multiple Scattering of Light in Particulate Planetary Media. Cambridge University Press, Cambridge (2015)

4. Muñoz, O., Moreno, F., Vargas-Martín, F., Guirado, D., Escobar-Cerezo, J., Min, M., Hovenier, J.W.: Experimental phase functions of millimeter-sized cosmic dust grains. The Astrophysical Journal 846(85 (8pp)) (2017)

5. Muñoz, O., Moreno, F., Gómez-Martín, J.C., Vargas-Martín, F., Guirado, D., Ramos, J.L., Bustamante, I., Bertini, I., Frattin, E., Markannen, J., Tubiana, C., Fulle, M., Güttler, C., Sierks, H., Rotundi, A., Corte, V.D., Ivanovski, S., Zakharov, V.V., Bockelée-Morvan, D., Blum, J., Merouane, S., Levasseur-Regourd, A.C., Kolokolova, L., Jardiel, T., Caballero, A.C.: Experimental phase function and degree of linear polarization curves of millimetersized cosmic dust analogs. The Astrophysical Journal Supplement Series 247(19 (13pp)) (2020)