Infrared Spectroscopy of Ultracompact HiiRegions

Abstract

We present intermediate-resolution (j/*j B 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 km. The 9.7 km silicate feature is seen in absorption, and the 12.8 km [Ne II] Ðne structure line is seen in emission toward most of the observed nebulae. The strengths of both features vary enormously from nebula to nebula, suggesting large variations in the column densities of both Ne II and silicates toward these objects. Near-IR features attributed to polycyclic aromatic hydrocarbons (PAHs) are detected in six of the sources.Spherically symmetric dust shell models were calculated to obtain the best Ðts to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 km. The models are used to infer properties of the dust cocoon such as the distribution of density and temperature with radius, shell thickness, outer shell radius, and dust abundances. Our results are consistent with previous models that predict large dust cocoons with central cavities, sharp temperature gradients, and approximately constant density in the outer regions.

High‐Resolution Far‐Infrared Observations and Models of the Star Formation Core of G34.3+0.2C

“…Campbell et al (2000) made radiative transfer models of the emission due to C and A in which a small amount of dust is located in or just outside the UC H ii regions. Very deep silicate absorption is present at these sources, as had also been observed by Faison et al (1998). This feature could not be due to self-absorption in the models of Campbell et al (2000), and they attributed it to an external absorbing cloud with a ' 3:5 at 20.6 m. Hatchell et al (2001) have recently concluded that the lack of MIR emission at the UC H ii component B is due to extremely strong extinction of the source.…”

“…Models that are based on IRAS data but are not constrained by data on the source sizes typically have large central cavities with no dust, surrounded by one or more shells of uniform dust density, ¼ 0 (Chini, Krügel, & Kreysa 1986;Churchwell, Wolfire, & Wood 1990;Faison et al 1998). Models of high-and intermediate-mass deeply embedded young stellar objects that are based on SEDs covering MIR to millimeter wavelengths and are also constrained by KAO high-resolution scans in the FIR have a range in from 0 to 3, (Campbell et at.…”

Far and mid infrared observations of two ultracompact $\ion{H}{ii}$ regions and one compact CO clump

“…Also, the flux densities of Faison et al (1998) are lower than those of IRAS LRS as well as from ISO observations. This is because of the small aperture (9 ) of Faison et al (1998). The SED from the best fit radiation transfer model is shown in Fig.…”

“…An attempt was made to fit the SED as well as the radial profile of the intensities. of IRAS 20178+4046 using the flux densities from balloonborne, IRAS and ISO observations, given in Tables 2 and 3, IRAS LRS data from Volks & Cohen (1989) and ground based observations of Faison et al (1998). This SED is shown in Fig.…”

“…No maser has been found to be associated with IRAS 20178+4046. Between 8 and 22 µm an IRAS low resolution spectrometer (LRS) spectrum is available for IRAS 20178+4046 (Volk & Cohen 1989) and groundbased intermediate resolution spectroscopic observations have been made between 3 to 13 µm (Faison et al 1998). In the latter, PAH features 1 at 3.3 µm, 8.7 µm and 11.3 µm, the [Ne II] line at 12.8 µm and silicate feature at 9.7 µm have been detected.…”