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Register a new userHigh Resolution Mid-Infrared Imaging of Radio Ultra-Compact HII Regions
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We present data from mid-infrared Keck Telescope imaging of 18 radio-selected ultra-compact HII region candidates at diffraction-limited resolution. The goal of these observations is to determine the sizes, luminosities, and morphologies of the mid-infrared emitting dust surrounding the stellar sources. All 18 sources were imaged at 11.7um and at 17.65um, and 10 of them were imaged also at 24.5um. All the sources were resolved. We have generated dust temperature and optical depth maps and combine them with radial velocity measurements and radio data (1.4 and 5 GHz) to constrain the properties of these star-forming regions. Half of our objects are excited by B-stars, and all our objects have derived types that are later than an O6 star. We find a significant correlation between infrared and radio flux densities, and a weaker one between infrared diameters and the central source ionizing photon rates. This latter correlation suggests that the more compact sources result from later spectral types rather than young age. Our new data may suggest a revision to infrared color selection criteria of ultra-compact HII regions at resolutions <1. These 18 sources are part of a sample of 687 sources dominated by ultra-compact HII regions selected by matching radio and infrared maps of the first Galactic quadrant by Giveon and coworkers. The new mid-infrared images constitute a significant improvement in resolving sub-structure at these wavelengths. If applied to all of this sample our analysis will improve our understanding of embedded star-formation in the Galaxy.
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We present a study of the mid-infrared properties and dust content of a sample of 27 HII ``blobs, a rare class of compact HII regions in the Magellanic Clouds. A unique feature of this sample is that even though these HII regions are of high and low excitation they have nearly the same physical sizes ~1.5-3 pc. We base our analysis on archival 3-8 microns infrared imagery obtained with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. We find that despite their youth, sub-solar metallicity and varied degrees of excitation, the mid-infrared colors of these regions are similar to those of typical HII regions. Higher excitation ``blobs (HEBs) display stronger 8 micron emission and redder colors than their low-excitation counterparts (LEBs).
We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young, embedded star birth regions, which are likely to be clusters of ultracompact HII regions. All 35 of the sources presented in this article have positive radio spectral indices alpha>0 suggesting an optically thick thermal bremsstrahlung emission arising in the HII region surrounding hot stars. Energy requirements indicate a range of a several to >500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF, this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly half of the sources in our sample, there is no obvious optical counterpart, giving further support for their deeply embedded nature. Their luminosities and radio spectral energy distributions are consistent with HII regions having electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We suggest that the less luminous of these sources are extragalactic ultracompact HII region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, while the brightest will be counterparts to 30 Doradus. These objects constitute the lower mass range of extragalactic ``ultradense HII regions which we argue are the youngest stages of massive star cluster formation yet observed. This sample is beginning to fill in the continuum of objects between small associations of ultracompact HII regions and the massive extragalactic clusters that may evolve into globular clusters.
We present results of our diffraction-limited mid-infrared imaging of the massive star-forming region W3(OH) with SpectroCam-10 on the 5-m Hale telescope at wavelengths of 8.8, 11.7, and 17.9 micron. The thermal emission from heated dust grains associated with the ultracompact HII region W3(OH) is resolved and has a spatial extent of ~2 arcsec in the N band. We did not detect the hot core source W3(H_2O) which implies the presence of at least 12 mag of extinction at 11.7 micron towards this source. These results together with other data were used to constrain the properties of W3(OH) and W3(H_2O) and their envelopes by modelling the thermal dust emission.
G339.88-1.26 is considered to be a good candidate for a massive star with a circumstellar disk. This has been supported by the observations of linearly distributed methanol maser spots believed to delineate this disk, and mid-infrared observations that have discovered a source at this location that is elongated at the same position angle as the methanol maser distribution. We used the mid-infrared imager/spectrometer OSCIR at Keck to make high-resolution images of G339.88-1.26. We resolve the mid-infrared emission into 3 sources within 1.5 arcsec of the location of the masers. We determine that the methanol masers are most likely not located in a circumstellar disk. Furthermore we find that the observed radio continuum emission most likely comes from two sources in close proximity to each other. One source is an unobscured massive star with an extended HII region that is responsible for the peak in the radio continuum emission. A second source is embedded and centered on the elongation in the radio continuum emission that is believed to be tracing an outflow in this region.
We investigate the diffuse absolute calibration of the InfraRed Array Camera on the Spitzer Space Telescope at 8.0microns using a sample of 43 HII regions with a wide range of morphologies near GLON=312deg. For each region we carefully measure sky-subtracted,point-source- subtracted, areally-integrated IRAC 8.0-micron fluxes and compare these with Midcourse Space eXperiment (MSX) 8.3-micron images at two different spatial resolutions, and with radio continuum maps. We determine an accurate median ratio of IRAC 8.0-micron/MSX8.3-micron fluxes, of 1.55+/-0.15. From robust spectral energy distributions of these regions we conclude that the present 8.0-micron diffuse calibration of the SST is 36% too high compared with the MSX validated calibration, perhaps due to scattered light inside the camera. This is an independent confirmation of the result derived for the diffuse calibration of IRAC by the Spitzer Science Center (SSC). From regression analyses we find that 843-MHz radio fluxes of HII regions and mid-infrared (MIR) fluxes are linearly related for MSX at 8.3-microns and Spitzer at 8.0 microns, confirming the earlier MSX result by Cohen & Green. The median ratio of MIR/843-MHz diffuse continuum fluxes is 600 times smaller in nonthermal than thermal regions, making it a sharp discriminant. The ratios are largely independent of morphology up to a size of ~24 arcsec. We provide homogeneous radio and MIR morphologies for all sources. MIR morphology is not uniquely related to radio structure. Compact regions may have MIR filaments and/or diffuse haloes, perhaps infrared counter- parts to weakly ionized radio haloes found around compact HII regions. We offer two IRAC colour-colour plots as quantitative diagnostics of diffuse HII regions.
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