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 H 2 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 H 2 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 H 2 line emission at 15 radius from the star. This H 2 line emission in NGC 2316 is consistent with UV-pumped fluorescence in an H 2 production zone. The correlation between the H 2 and PAH emissions in the H 2 shell suggest efficient formation of H 2 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 A v ∼ 4.5 mag. Their emission characteristics and variations add new constraints to the PAH composition and excitation modes.

Ring-like features around young B stars

“…Therefore, in the source I05358-IRS1 the emission from the first ring is composed of a combination of PAH and H 2 emission, but with an excess PAH emission closer to the central star. This is similar to the case of NGC 2316 for which Velusamy & Langer (2008) detected a H 2 ring and intense PAH emission within the H 2 ring around the young B3 star.…”

“…1 of Bally 2008;Beuther et al 2004). The Spitzer Space Telescope (SST) IRAC bands are dominated by emission from very small grains (VSG) (Wood et al 2008), polycyclic aromatic hydrocarbons (PAH) (Povich et al 2007;Velusamy & Langer 2008), and several H 2 lines arising in warm molecular gas and shocked gas (Neufeld & Yuan 2009). Thus, with the aid of Spitzer IRAC images and Hi-Res processing , we discovered well-defined ring features around I20293-IRS1 and I05358-IRS1.…”

Planckearly results. XX. New light on anomalous microwave emission from spinning dust grains

“…For the present preliminary model, we represent this interface with two effective media at low (200 cm −3 ) and high (2 × 10 4 cm −3 ) densities irradiated by strong and weak radiation fields. In the more excited ρ Ophiuchi region we are able to match the AME by requiring: (a) an attenuated radiation field in the dense gas; and (b) smaller and less abundant PAHs in stronger radiation fields, in line with recent work suggesting that PAHs are photoevaporated from larger carbonaceous grains (Berné et al 2007;Compiègne et al 2008;Velusamy & Langer 2008). The PAH abundances we assume for both regions are within the range found by Habart et al (2003).…”

The role of PAHs in the physics of the interstellar medium

“…PAHs are attractive candidates for the formation of H 2 because they represent two thirds of the total grain surface, e.g., Habart et al 2004) and carry chemically bonded H atoms. Observations indicate an expected trend between H 2 and PAH integrated emissions (Habart et al 2004) but the corresponding emissivity profiles are not always correlated, (Habart et al 2003;Berné et al 2009;Velusamy & Langer 2008). These results may be related to the radiative transfer of UV photons across cloud borders with different density profiles.…”

“…PAH coagulation is suggested by observations of molecular clouds (Boulanger et al 1994;Flagey et al 2009). Conversely, PAHs may be photoevaporated off larger grains when fresh molecular material is exposed to stellar UV radiation thus producing a limb brightening effect (Boulanger et al 1990;Bernard et al 1993;Rapacioli et al 2005;Berné et al 2007;Compiègne et al 2008;Velusamy & Langer 2008;Rapacioli et al this volume).…”