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Register a new userSpitzer--IRAC Imagery and Photometry of Ultracompact HII Regions With Extended Emission
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Eduardo de la Fuente Acosta Ph. D.
Publication date
2008
fields
Physics
and research's language is
English
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We present the results of a morphological study performed to a sample of Ultracompact (UC) HII regions with Extended Emission (EE) using Spitzer--IRAC imagery and 3.6 cm VLA conf. D radio-continuum (RC) maps. Some examples of the comparison between maps and images are presented. Usually there is an IR point source counterpart to the peak(s) of RC emission, at the position of the UC source. We find that the predominant EE morphology is the cometary, and in most cases is coincident with IR emission at 8.0 $mu$m. Preliminary results of Spitzer--IRAC photometry of a sub-sample of 13 UC HII regions with EE based on GLIMPSE legacy data are also presented. Besides, individual IRAC photometry was performed to 19 UC sources within these 13 regions. We show that UC sources lie on specific locus, both in IRAC color-color and AM-product diagnostic diagrams. Counts of young stellar sources are presented for each region, and we conclude that a proportion of ~ 2%, ~10%, and ~88% of sources in the UC HII regions with EE are, in average, Class I, II, and III, respectively.
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We carried out 13CO J=1-0, CS, and C34S J=2-1 and J=3-2 line observations of molecular clouds associated with 16 ultracompact (UC) HII regions with extended envelopes. The molecular clouds are the ones that give birth to rich stellar clusters and/or very massive (O7-O4) stars. Our data show that the clouds are very clumpy and of irregular morphology. They usually have much larger masses, velocity dispersions, and fractions of dense gas than molecular clouds that form early B or late O stars. This is compatible with earlier findings that more massive stars form in more massive cores. 13CO cores are in general associated with compact HII regions regardless of the presence of UC HII regions therein. In contrast, CS cores are preferentially associated with compact HII regions that contain UC HII regions. As with the fact that the compact HII regions containing UC HII regions are more compact than those not associated with UC HII regions, these indicate that the former may be in an earlier evolutionary phase than the latter. The diffuse extended envelopes of HII regions often develop in the direction of decreasing molecular gas density. Based on detailed comparison of molecular line data with radio continuum and recombination line data, the extended ionized envelopes are likely the results of champagne flows in at least 10 sources in our sample. Together these results appear to support a published suggestion that the extended emission around UC HII regions can be naturally understood by combining the champagne flow model with the hierarchical structure of molecular clouds. We discuss the implication of our results for the blister model of HII regions.
The infrared data from the Spitzer Space Telescope has provided an invaluable tool for identifying physical processes in star formation. In this study we calculate the IRAC color space of UV fluorescent molecular hydrogen (H$_2$) and Polycyclic Aromatic Hydrocarbon (PAH) emission in photodissociation regions (PDRs) using the Cloudy code with PAH opacities from Draine & Li 2007. We create a set of color diagnostics that can be applied to study the structure of PDRs and to distinguish between FUV excited and shock excited H$_2$ emission. To test this method we apply these diagnostics to Spitzer IRAC data of NGC 2316. Our analysis of the structure of the PDR is consistent with previous studies of the region. In addition to UV excited emission, we identify shocked gas that may be part of an outflow originating from the cluster.
We have used the Australia Telescope Compact Array (ATCA) to make observations of a sample of eight young ultra-compact HII regions, selected on the basis that they have associated class II methanol maser emission. We have made observations sensitive to both compact and extended structures and find both to be present in most sources. The scale of the extended emission in our sample is in general less than that observed towards samples based on IRAS properties, or large single-dish flux densities. Our observations are consistent with a scenario where extended and compact radio continuum emission coexists within HII regions for a significant period of time. We suggest that these observations are consistent with a model where HII evolution takes place within hierarchically structured molecular clouds. This model is the subject of a companion paper (Shabala et al. 2005) and addresses both the association between compact and extended emission and UCHII region lifetime problem.
Ultracompact and hypercompact HII regions appear when a star with a mass larger than about 15 solar masses starts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbor stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the HII regions formed in the 3D radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the HII regions is a natural outcome of this model. The radio-continuum fluxes of the simulated HII regions, as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (~ 10 %) of observed HII regions should have detectable flux variations (larger than 10 %) on timescales of ~ 10 years, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing HII regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates.
The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e, in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecules radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in LTE and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. We probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC 7538 IRS1 by studying the $^2Pi_{{1/2}}, J = {3/2} - {1/2}$ rotational transition of OH in emission and, toward the last source also the molecules $^2Pi_{3/2}, J = 5/2 - 3/2$ ground-state transition in absorption. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz ($163 mu$m) and 2.51 THz ($119.3 mu$m). We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines.
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