We present new infrared observations of the emission/reflection nebula IC 405 obtained with the Spitzer Space Telescope .
Infrared images in the four IRAC bands ( 3.6 , 4.5 , 5.8 , and 8.0 \mu m ) and two MIPS bands ( 24 and 70 \mu m ) are complemented by IRS spectroscopy ( 5 – 30 \mu m ) of two nebular filaments .
The IRAC ( 8.0 \mu m ) and MIPS imaging shows evidence of a bow shock associated with the runaway O9.5V star , HD 34078 , created by the interaction between the star and nebular material .
The ratio of emission at 24 to 70 \mu m is higher in the immediate vicinity of HD 34078 than in the outer filaments , providing evidence for elevated dust temperatures ( T _ { d } \gtrsim 90 K ) in the shock region .
The nebular imaging reveals that the morphology is band dependent , with varying contributions from aromatic emission features , H _ { 2 } , and dust emission .
Nebular spectroscopy is used to quantify these contributions , showing several aromatic emission bands between 6 – 14 \mu m , the S ( 5 ) , S ( 3 ) , S ( 2 ) , and S ( 1 ) pure rotational emission lines of H _ { 2 } , and atomic fine structure lines of Ne , S , and Ar .
The low-dispersion spectra provide constraints on the ionization state of the large molecules responsible for the aromatic infrared features .
H _ { 2 } rotational temperatures of the two bright nebular filaments are determined from the observed line strengths .
An average T ( H _ { 2 } ) \sim 400 K is inferred , with evidence for additional non-uniform excitation by UV photons in the intense radiation field of HD 34078 .
The photoexcitation hypothesis is supported by direct measurement of the far-UV H _ { 2 } fluorescence spectrum , obtained with FUSE .