POLYCYCLIC AROMATIC HYDROCARBON AND H2EMISSIONS IN THE ULTRAVIOLET-DOMINATED REGION IN NGC 2316

Abstract

NGC 2316 is a young star cluster embedded in a dense cloud containing a central H II region powered by a B3 star. The ultraviolet (UV) radiation emitted by this young star affects the physical conditions and the chemical composition of the surrounding cloud. We present Spitzer Infrared Spectrograph (IRS) spectral maps of the polycyclic aromatic hydrocarbon (PAH) features and H2 lines in the 9-14 μm (SL1) and 15-20 μm (LL2) bands obtained using multiple slit positions. We show the distribution of 7.7 μm, 8.6 μm, 10-14 μm, 16-18 μm, and 19.0 μm PAH bands and the spatial variations among the bright narrow PAH features at 11.2, 12.0, 12.7, 16.4, 17.4, and 17.7 μm with distance from the ultraviolet (UV) source. We show that the Infrared Array Camera (IRAC) 8 μm emission is primarily due to the 7.7 and 8.6 μm PAH features. The IRAC 8 μm image of the entire cloud containing the cluster and the B3 star shows PAH emission on a large scale, not covered by IRS spectral data. We have also mapped the line intensities of three pure rotational H2 transitions: 0-0 S(1) Ortho 17.0 μm, 0-0 S(2) Para 12.3 μm, and 0-0 S(3) Ortho 9.7 μm. We detect a limb-brightened shell of H2 line emission at 15'' radius from the star. This H2 line emission in NGC 2316 is consistent with UV-pumped fluorescence in an H2 production zone. The correlation between the H2 and PAH emissions in the H2 shell suggest efficient formation of H2 by chemisorption on PAH surfaces. The intensities of all PAH features increase toward the UV source, but their relative contributions to the overall band emissions differ with the distance from the UV source. The detection of PAH emissions closest to the UV source implies the survival of PAHs in the intense, but softer UV radiation field of the B3 star. Using the radial intensities and a phenomenological model for PAH destruction as a function of distance from the UV source, we estimate the relative survival of the emitters (PAH components) in each spectrally resolved PAH emission feature. We identify four distinct PAH emission zones in NGC 2316 with a central UV source embedded in nearly spherical dense cloud with an average Av ~ 4.5 mag. Their emission characteristics and variations add new constraints to the PAH composition and excitation modes.