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Register a new userOn Turbulent Pressure Confinement of Ultra-Compact HII Regions
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It has been proposed recently that the small size and long lifetime of ultra-compact HII regions (UCHIIs) could be due to pressure confinement if the thermal pressure of the ambient gas is higher than previous estimates. We point out that confinement by thermal pressure alone implies emission measures in excess of observed values. We show that turbulent pressure, inferred from observed non-thermal velocities, is sufficient to confine UC HIIs and explain their longevity. We predict an anti-correlation between the size of UCHIIs and the velocity dispersion of the ambient neutral gas, and show that it is consistent with existing observations.
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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 radio and infrared observations of 4 hyper-compact HII regions and 4 ultra-compact HII regions in the southern Galactic plane. These objects were selected from a blind survey for UCHII regions using data from two new radio surveys of the southern sky; the Australia Telescope 20 GHz survey (AT20G) and the 2nd epoch Molonglo Galactic Plane Survey (MGPS-2) at 843 MHz. To our knowledge, this is the first blind radio survey for hyper- and ultra-compact HII regions. We have followed up these sources with the Australia Telescope Compact Array to obtain H70-alpha recombination line measurements, higher resolution images at 20 GHz and flux density measurements at 30, 40 and 95 GHz. From this we have determined sizes and recombination line temperatures as well as modeling the spectral energy distributions to determine emission measures. We have classified the sources as hyper-compact or ultra-compact on the basis of their physical parameters, in comparison with benchmark parameters from the literature. Several of these bright, compact sources are potential calibrators for the Low Frequency Instrument (30-70 GHz) and the 100-GHz channel of the High Frequency Instrument of the Planck satellite mission. They may also be useful as calibrators for the Australia Telescope Compact Array, which lacks good non-variable primary flux calibrators at higher frequencies and in the Galactic plane region. Our spectral energy distributions allow the flux densities within the Planck bands to be determined, although our high frequency observations show that several sources have excess emission at 95 GHz (3 mm) that can not be explained by current models.
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.
We present numerical radiation-hydrodynamic simulations of cometary HII regions for a number of champagne flow and bowshock models. For the champagne flow models we study smooth density distributions with both steep and shallow gradients. We also consider cases where the ionizing star has a strong stellar wind, and cases in which the star additionally has a proper motion within the ambient density gradient. We present simulated emission-measure maps and long-slit spectra of our results. Our numerical models are not tailored to any particular object but comparison with observations from the literature shows that, in particular, the models combining density gradients and stellar winds are able to account for both the morphology and radial velocity behavior of several observed cometary HII regions, such as the well-studied object G29.96-0.02.
The ionized core in the Sgr B2 Main star-forming region was imaged using the Submillimeter Array archival data observed for the H26$alpha$ line and continuum emission at 0.86 millimeter with an angular resolution 0.3arcsec. Eight hyper-compact H26$alpha$ emission sources were detected with a typical size in the range of 1.6--20$times10^2$ AU and electron density of 0.3--3$times10^7$ cm$^{-3}$, corresponding to the emission measure 0.4--8.4$times10^{10}$ cm$^{-6}$ pc. The H26$alpha$ line fluxes from the eight hyper-compact HII sources imply that the ionization for each of the sources must be powered by a Lyman continuum flux from an O star or a cluster of B stars. The most luminous H26$alpha$ source among the eight detected requires an O6 star that appears to be embedded in the ultra-compact HII region F3. In addition, $sim$ 23 compact continuum emission sources were also detected within the central 5arcsec$times$3arcsec,($sim0.2$ pc) region. In the assumption of a power-law distribution for the dust temperature, with the observed brightness temperature of the dust emission we determined the physical properties of the submillimeter emission sources showing that the molecular densities are in the range of 1--10$times10^8$ cm$^{-3}$, surface densities between 13 to 150 $g$ cm$^{-2}$, and total gas masses in the range from 5 to $gtrsim$ 200 $M_odot$ which are 1 or 2 orders of magnitude greater than the corresponding values of the Bonnor-Ebert mass. With a mean free-fall time scale of 2$times10^3$ y, each of the massive protostellar cores are undergoing gravitational collapse to form new massive stars in the Sgr B2 Main core.
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