The dust properties of bubble HII regions as seen by Herschel

Abstract

Context: Because of their relatively simple morphology, "bubble" HII regions have been instrumental to our understanding of star formation triggered by HII regions. With the far-infrared (FIR) spectral coverage of the Herschel satellite, we can access the wavelengths where these regions emit the majority of their energy through their dust emission. Aims: We wish to learn about the dust temperature distribution in and surrounding bubble HII regions and to calculate the mass and column density of regions of interest, in order to better understand ongoing star formation. Additionally, we wish to determine whether and how the spectral index of the dust opacity, β, varies with dust temperature. Any such relationship would imply that dust properties vary with environment. Methods: Using aperture photometry and fits to the spectral energy distribution, we determine the average temperature, β-value, and mass for regions of interest within eight bubble HII regions. Additionally, we compute maps of the dust temperature and column density. Results: At Herschel wavelengths (70 μm to 500 μm), the emission associated with HII regions is dominated by the cool dust in their photodissociation regions (PDRs). We find average dust temperatures of 26 K along the PDRs, with little variation between the HII regions in the sample, while local filaments and infrared dark clouds average 19 K and 15 K respectively. Higher temperatures lead to higher values of the Jeans mass, which may affect future star formation. The mass of the material in the PDR, collected through the expansion of the HII region, is between ∼300 M· and ∼10 000 M⊙ for the HII regions studied here. These masses are in rough agreement with the expected masses swept up during the expansion of the HII regions. Approximately 20% of the total FIR emission is from the direction of the bubble central regions. This suggests that we are detecting emission from the "near-side" and "far-side" PDRs along the line of sight and that bubbles are three-dimensional structures. We find only weak support for a relationship between dust temperature andβ, of a form similar to that caused by noise and calibration uncertainties alone. © ESO 2012.