arXiv subjects

Ant Jones

Publications and source records attributed to Ant Jones.

4 recordsLinked to original sources

arXiv2026

Spheroidal core-mantle particle absorption, scattering, and polarisation in the long-wavelength limit

The numerical calculation of optical properties (extinction, absorption, scattering and polarisation efficiencies) is often time-consuming for non-spherical and inhomogeneous particles. Where possible analytical methods are therefore to be preferred. We provide an analytical tool to derive the optical properties of mantled spheroidal particles, of arbitrary axis ratio, in the long wavelength limit (a << lambda), where the mantle form may be confocal, co-axial or of constant depth with respect to the particle core. We have developed an analytical approach to spheroidal core/mantle particle optical property calculations. The analytical method compares well with DDSCAT numerical calculations and, under limited circumstances, with those made using the Bruggemann effective medium theory (EMT).The analytical method presented here provides a useful tool to explore the optical and polarisation properties of core/mantle spheroidal particles at long wavelengths (lambda >~ 8mu) and is simpler and faster to implement than corresponding numerical methods. We caution against the use of EMT methods in approximating the optical properties of core/mantle particles.

astro-ph.GA↗

arXiv2025

The nature and evolution of a-C(:H) nanoparticle substructures and speculations on the origin of the 3-4$\mu$m emission bands

The nature and evolution of hydrocarbonaceous grains within interstellar and circumstellar media is still far from resolved, perhaps owing to the rather complex nature of their seemingly simple binary atomic compositions. This work explores the fine details of amorphous hydrocarbon nanoparticle, a-C(:H), composition and the evolution of the inherent sub-structures under extreme conditions, focusing on the characteristic CH$_n$ bands in the 3-4 micron wavelength region. Particular attention is paid to the role of dehydrogenation and its effects on the sp^3 and sp^2 hybridisations, leading to an extensive conjugated domain functionalisation of the contiguous structural network within a-C(:H) nanoparticles. Qualitatively this approach is able to explain the origin and evolution, including the appearance and disappearance, of emission bands observed in the 3-4 micron wavelength regime without a significant aromatic moiety content within the structures. A diatomic a-C(:H) phase is likely at the heart of the observed dust evolution in the interstellar medium, and circumstellar and photodissociation regions, as observed at short wavelengths. It appears that we have some way to go in fully understanding these complex materials. Much laboratory work will be required in order to elucidate their chemical and structural evolution at nanoparticle sizes under extreme conditions.

astro-ph.GA↗

arXiv2015

Interstellar dust modelling: Interfacing laboratory, theoretical and observational studies (The THEMIS model)

The construction of viable and physically-realistic interstellar dust models is only possible if the constraints imposed by laboratory data on interstellar dust analogue materials are respected and used within a meaningful theoretical framework. These physical dust models can then be directly compared to observations without the need for any tuning to fit the observations. Such models will generally fail to achieve the excellent fits to observations that empirical models are able to achieve. However, the physically-realistic approach will necessarily lead to a deeper insight and a fuller understanding of the nature and evolution of interstellar dust. The THEMIS modelling approach, based on (hydrogenated) amorphous carbons and amorphous silicates with metallic Fe and/or FeS nano-inclusions appears to be a promising move in this direction.

astro-ph.GA↗

arXiv2014

The physical and compositional properties of dust: what do we really know?

Many things in current interstellar dust studies are taken as well understood givens by much of the community. For example, it is widely held that interstellar dust is made up of only three components, i.e., astronomical silicates, graphite and polycyclic aromatic hydrocarbons, and that our understanding of these is now complete and sufficient enough to interpret astronomical observations of dust in galaxies. To zeroth order this is a reasonable approximation. However, while these three pillars of dust modelling have been useful in advancing our understanding over the last few decades, it is now apparent that they are insufficient to explain the observed evolution of the dust properties from one region to another. Thus, it is time to abandon the three pillars approach and to seek more physically-realistic interstellar dust analogues. The analy- sis of the pre-solar grains extracted from meteorites, interplanetary dust particles and from the Stardust mission, and the interpretation of x-ray scattering and absorption observations, supports the view that our current view of the interstellar dust composition(s) is indeed too naive. The aim of this review is to point out where our current views are rather secure and, perhaps more importantly, where they are far from secure and we must re-think our ideas. To this aim ten as- pects of interstellar dust will be scrutinised and re-evaluated in terms of their validity within the current observational, experimental, modelling and theoretical constraints. It is concluded from this analysis that we really do need to re-assess many of the fundamental assumptions relating to what we think we really do know about interstellar dust. In particular, it is clear that unravelling the nature dust evolution in the interstellar medium is perhaps the key to significantly advancing our current understanding of interstellar dust. ...

astro-ph.GA↗