Photoionization modelling of circumstellar nebulae using irregular grains

TL;DR

This study models circumstellar nebulae using irregular grain optical properties, revealing significant differences from spherical models, especially for larger grains, impacting dust mass estimates.

Contribution

It introduces irregular grain optical properties into photoionization models, showing their effects on nebula spectra and dust mass calculations.

Findings

Infrared luminosities can be up to 60% higher with irregular grains for large sizes.

Differences between spherical and irregular grains increase with grain size.

Traditional spherical grain assumptions may overestimate dust mass.

Abstract

We study the effects of using the optical properties of irregular hexahedral grains in photoionization models of circumstellar nebulae around evolved stars. Dust opacities for the irregular grains were obtained from the scattering properties available in the TAMUdust2020 database and these were implemented in the spectral synthesis code cloudy. A sample of photoionization models that use opacities from both spherical and irregular hexahedral grains across a standard MRN size distribution (0.005 to 0.25 um) was produced. We consider the optical properties of graphite, amorphous carbon and silicate dust grains and find that differences between the model nebula continua calculated using spherical and irregular dust grains increase with the grain size, especially for graphite. In particular, we find that the luminosities at the infrared peak for the hexahedral grain models can be up to 60% higher than those from the equivalent spherical grain models for the largest grains. This result suggests that traditional spherical grain assumptions may lead to an overestimate of the dust mass in photoionized nebulae.