A Cometary Bow Shock and Mid-infrared Emission Variations Revealed in Spitzer

We present new infrared observations of the emission/reflection nebula IC 405 obtained with the Spitzer Space T elescope. Infrared images in the four IRAC bands (3.6, 4.5, 5.8, and 8.0 μm) and two MIPS bands (24 and 70 μm) are complemented by IRS spectroscopy (5 – 30 μm) of two nebular filaments. The IRAC (8.0 μm) 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 μm is higher in the immediate vicinity of HD 34078 than in the outer filaments, providing evidence for elevated dust temperatures (Td & 90 K) in the shock region. The nebular imaging reveals that the morphology is band dependent, with varying contributions from aromatic emission features, H2, and dust emission. Nebular spectroscopy is used to quantify these contributions, showing several aromatic emission bands between 6 – 14 μm, the S(5), S(3), S(2), and S(1) pure rotational emission lines of H2, 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. H2 rotational temperatures of the two bright nebular filaments are determined from the observed line strengths. An average T(H2) ∼ 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 H2 fluorescence spectrum, obtained with FUSE. Subject headings: dust — ISM: molecules — ISM: individual (IC 405) — reflection nebulae — Infrared: ISM