Spectropolarimetric constraints on interstellar grain models
Authors:   , Mills Bradley W. 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] It is now well recognized that the interstellar medium acts as an efficient polarizer of electromagnetic radiation, resulting in the partial plane polarization of starlight by dichroic absorption by aligned, non-spherical dust grains on the line of sight. The discovery of the linear polarization of starlight provided a valuable mechanism for constraining the physical properties of interstellar dust, requiring the need for a grain population which both departs from spherical symmetry and efficiently aligns with the galactic magnetic field. Observations of light from distant stars have shown the degree of polarization to scale with reddening, suggesting that the grains which are responsible for effectively absorbing and scattering electromagnetic radiation are also responsible for the partial plane polarization of the transmitted light. The exact chemical and physical nature of interstellar dust remains a point of contention, being generally recognized to consist of both amorphous silicate and carbonaceous material. Grain models consisting of bare and separated silicate and carbon, silicates covered with a mantle of carbonaceous dust and porous composites of small silicates and carbonaceous particles have been invoked to explain the main observational constraints, most notably being the observed wavelength dependent extinction, polarization and far infrared emission. While each dust model appears to be consistent in explaining such constraints, each makes unique assumptions regarding the physical relationship of the main dust forming components, with no further methods of differentiating between such models being discussed. Spectropolarimetric observations across solid state absorption features have recently provided a means in which to distinguish between dust models. The core-mantle model of interstellar dust postulates a grain morphology consisting of an amorphous silicate core coated by (i.e., being physically associated with) a mantle of water ice, or carbonaceous material, being dependent on the environment in which the grain resides. For spectral features whose carrier resides in the mantle atop an elongated silicate core, a correlation between the polarization profiles of such features is expected with that of the silicate. Two studies are proposed: (i) Spectroscopic observations of dense, star forming regions from 2 to 13 Âµm have been carried out for several young, high luminosity infrared sources, protostars. The 3.1 Âµm feature, attributed to absorption by amorphous H2O ice, and 9.7 Âµm feature, attributed to absorption by amorphous silicates, appear concurrently in all sources with molecular clouds intervening along the line of sight, indicative of both ice and silicates as important grain constitutes in dense phases of the interstellar medium. Freeze out of gas phase elements onto refractory grain cores result in a silicate core-ice mantle grain morphology. Spectropolarimetric observations of the Becklin-Neugebauer (BN) object have shown the 3.1 Âµm and 9.7 Âµm features to be polarized, highly suggestive of an aligned silicate core-ice mantle grain morphology present on the line of sight. Subsequent observations of a source similar in nature to BN, the embedded protostar AFGL 2591, have raised a challenge to this model, with excess polarization detected in the 9.7 Âµm absorption band, combined with a 3.1 Âµm ice band feature being devoid of polarization. A model which physically associates the carrier material of the ice feature with that of an aligned silicate component requires a polarization signature across the 3.1 Âµm feature commensurate with that of the silicate feature. We seek to alleviate this challenge on the coreâ€"mantle model by computing the degree of polarization across both the 3.1 Âµm ice and 9.7 Âµm silicate features for spheroidal silicate core-spherical ice core-mantle grains; on the basis that grains with spherical mantles, being much less elongated in nature, should reduce the degree of polarization across the ice feature. By considering variations in specific grain parameters, including variations in core elongation and mantle thickness, we seek to reduce the degree of 3.1 Âµm polarization while simultaneously reproducing 9.7 Âµm spectropolarimetric observations. Such constraints provide a valuable test of the physical relationship of silicate and ice components in star forming regions. (ii)Spectroscopic observations from 2 to 13 Âµm have been carried out on multi- ple lines of sight which sample the diffuse interstellar medium. Absorption features at 3.4 Âµm, attributed to carbonaceous material, and 9.7 Âµm, attributed to amorphous silicates are indicative of both carbon and silicates materials as important grain constitutes. Spectropolarimetric observations have shown the 9.7 Âµm feature to be polarized, suggesting highly aligned silicate grains present on the line of sight. Subsequent observations of the 3.4 Âµm feature on the same Galactic sightlines have shown a carbonaceous dust feature to be devoid of polarization. A model which physically associates the carrier material of the carbonaceous feature with that of an aligned silicate component requires a polarization signature across the 3.4 Âµm feature commensurate with that of the silicate feature. No such excess rise is currently detected, indicative of a carbonaceous component being devoid of polarization. We seek to alleviate this challenge on the core-mantle model by computing the degree of polarization across both the 3.4 Âµm carbonaceous and 9.7 Âµm silicate features for spheroidal silicate core -'equal-thickness' carbonaceous core-mantle grains; on the basis that grains with equal thickness mantles should reduce the degree of polarization associated with the carbonaceous feature, a result of equal attenuation of light along both the grains semi-major and semi-minor axis. By considering variations in specific grain parameters, including variations in core elongation and mantle thickness, we seek to reduce the degree of 3.4 Âµm polarization while simultaneously reproducing both 9.7 Âµm and optical (0.55Âµm) spectropolarimetric observations. Such constraints provide a valuable test of the physical relationship of silicate and carbonaceous dust components in diffuse regions of the interstellar medium.