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