Classifying and modelling spiral structures in hydrodynamic simulations
  of astrophysical discs
  We demonstrate numerical techniques for automatic identification of
individual spiral arms in hydrodynamic simulations of astrophysical discs.
Building on our earlier work, which used tensor classification to identify
regions that were "spiral-like", we can now obtain fits to spirals for
individual arm elements. We show this process can even detect spirals in
relatively flocculent spiral patterns, but the resulting fits to logarithmic
"grand-design" spirals are less robust. Our methods not only permit the
estimation of pitch angles, but also direct measurements of the spiral arm
width and pattern speed. In principle, our techniques will allow the tracking
of material as it passes through an arm. Our demonstration uses smoothed
particle hydrodynamics simulations, but we stress that the method is suitable
for any finite-element hydrodynamics system. We anticipate our techniques will
be essential to studies of star formation in disc galaxies, and attempts to
find the origin of recently observed spiral structure in protostellar discs.
