  We have studied the relationship between the high- and low-ionization [O IV]
lambda 25.89 micron, [Ne III] lambda 15.56 micron and [Ne II] lambda 12.81
micron emission lines with the aim of constraining the active galactic nuclei
(AGN) and star formation contributions for a sample of 103 Seyfert galaxies. We
used the [O IV] and [Ne II] emission as tracers for the AGN power and star
formation to investigate the ionization state of the emission-line gas. We find
that Seyfert 2 galaxies have, on average, lower [O IV]/[Ne II] ratios than
those of Seyfert 1 galaxies. This result suggests two possible scenarios: 1)
Seyfert 2 galaxies have intrinsically weaker AGN, or 2) Seyfert 2 galaxies have
relatively higher star formation rates than Seyfert 1 galaxies. We estimate the
fraction of [Ne II] directly associated with the AGN and find that Seyfert 2
galaxies have a larger contribution from star formation, by a factor of ~1.5 on
average, than what is found in Seyfert 1 galaxies. Using the stellar component
of [Ne II] as a tracer of the current star formation we found similar star
formation rates in Seyfert 1 and Seyfert 2 galaxies. We examined the mid- and
far-infrared continua and find that [Ne II] is well correlated with the
continuum luminosity at 60 micron and that both [Ne III] and [O IV] are better
correlated with the 25 micron luminosities than with the continuum at longer
wavelengths, suggesting that the mid-infrared continuum luminosity is dominated
by the AGN, while the far-infrared luminosity is dominated by star formation.
Overall, these results test the unified model of AGN, and suggest that the
differences between Seyfert galaxies cannot be solely due to viewing angle
dependence.
