A Cometary Bow Shock and Mid‐Infrared Emission Variations Revealed inSpitzerObservations of HD 34078 and IC 405

We present new infrared observations of the emission/reflection nebula IC 405obtained with the Spitzer Space Telescope. Infrared images in the four IRACbands (3.6, 4.5, 5.8, and 8.0 um) and two MIPS bands (24 and 70 um) arecomplemented by IRS spectroscopy (5-30 um) of two nebular filaments. The IRAC(8.0 um) and MIPS imaging shows evidence of a bow shock associated with therunaway O9.5V star, HD 34078, created by the interaction between the star andnebular material. The ratio of emission at 24 to 70 um is higher in theimmediate vicinity of HD 34078 than in the outer filaments, providing evidencefor elevated dust temperatures (T_d > 90 K) in the shock region. The nebularimaging reveals that the morphology is band dependent, with varyingcontributions from aromatic emission features, H2, and dust emission. Nebularspectroscopy is used to quantify these contributions, showing several aromaticemission bands between 6-14 um, the S(5), S(3), S(2), and S(1) pure rotationalemission lines of H2, and atomic fine structure lines of Ne, S, and Ar. Thelow-dispersion spectra provide constraints on the ionization state of the largemolecules responsible for the aromatic infrared features. H2 rotationaltemperatures of the two bright nebular filaments are determined from theobserved line strengths. An average T(H2) ~ 400 K is inferred, with evidencefor additional non-uniform excitation by UV photons in the intense radiationfield of HD 34078. The photoexcitation hypothesis is supported by directmeasurement of the far-UV H2 fluorescence spectrum, obtained with FUSE.Comment: ApJ accepted, uses emulateapj, images compressed for astro-p