# Constraints on interstellar dust models from extinction and spectro-polarimetry

## TL;DR

This study models interstellar dust to explain observed extinction and polarisation spectra, revealing that aligned silicate grains of 70-200nm dominate polarisation, with a complex eight-parameter model fitting the data.

## Contribution

It introduces a detailed dust model with eight free parameters that accurately reproduces interstellar extinction and polarisation spectra, constrained by cosmic abundance limits.

## Key findings

- Aligned silicates dominate polarisation spectra.
- Best-fit grain size lower limit is 70-200nm.
- A complex eight-parameter dust model fits observations well.

## Abstract

We present polarisation spectra of seven stars in the lines-of-sight towards the Sco OB1 association. Our spectra were obtained within the framework of the Large Interstellar Polarization Survey carried out with the FORS instrument of the ESO VLT. We have modelled the wavelength-dependence of extinction and linear polarisation with a dust model for the diffuse interstellar medium which consists of a mixture of particles with size ranging from the molecular domain of 0.5 nm up to 350 nm. We have included stochastically heated small dust grains with radii between 0.5 and 6 nm made of graphite and silicate, as well as polycyclic aromatic hydrocarbon molecules (PAHs), and we have assumed that larger particles are prolate spheroids made of amorphous carbon and silicate. Overall, a dust model with eight free parameters best reproduces the observations. Reducing the number of free parameters leads to results that are inconsistent with cosmic abundance constraints. We found that aligned silicates are the dominant contributor to the observed polarisation, and that the polarisation spectra are best-fit by a lower limit of the equivolume sphere radius of aligned grains of 70 - 200nm.