44 research outputs found

    Evolution of HII regions in hierarchically structured molecular clouds

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    We present observations of the H91α\alpha recombination line emission towards a sample of nine HII regions associated with 6.7-GHz methanol masers, and report arcsecond-scale emission around compact cores. We derive physical parameters for our sources, and find that although simple hydrostatic models of region evolution reproduce the observed region sizes, they significantly underestimate emission measures. We argue that these findings are consistent with young source ages in our sample, and can be explained by existence of density gradients in the ionised gas.Comment: 11 pages, 6 figures; accepted for publication in MNRA

    APEX Millimeter Observations of Methanol Emission Toward High-Mass Star-Forming Cores

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    We present 247-GHz molecular line observations of methanol (CH3_3OH) toward sixteen massive star-forming regions, using the APEX telescope with an angular resolution of 25′′25''. The sample covers a range of evolutionary states, including warm molecular cores, hot molecular cores, and ultracompact HII regions. The hot cores, all of which include UC HII regions, show rich molecular line spectra, although the strength of different species and transitions varies from source to source. In contrast, the warm cores do not show significant molecular line emission. Multiple methanol transitions are detected toward nine of the hot cores; eight of these had enough transitions to use the rotation diagram method to estimate rotational temperatures and column densities. The temperatures lie in the range 104−-168 K and column densities from 3×10163\times10^{16} to 7×10187\times10^{18} cm−2^{-2}. Using the average methanol line parameters, we estimate virial masses, which fall in the range from 145 to 720 M⊙_\odot and proved to be significantly higher than the measured gas masses. We discuss possible scenarios to explain the chemical differences between hot cores and warm molecular cores. One of the observed methanol lines, 42−51A+4_{2}-5_{1}A^{+} at 247.228 GHz, is predicted to show class II maser emission, similar in intensity to previously reported J0−J−1EJ_0-J_{-1}E masers at 157 GHz. We did not find any clear evidence for maser emission among the observed sources; however, a weak maser in this line may exist in G345.01+1.79.Comment: Accepted for publication in The Astronomical Journa

    Searching for compact radio sources associated to UCHII regions

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    Ultra-Compact (UC)HII regions represent a very early stage of massive star formation whose structure and evolution are not yet fully understood. Interferometric observations in recent years show that some UCHII regions have associated compact sources of uncertain nature. Based on this, we carried out VLA 1.3 cm observations in the A configuration of selected UCHII regions in order to report additional cases of compact sources embedded in UCHII regions. From the observations, we find 13 compact sources associated to 9 UCHII regions. Although we cannot establish an unambiguous nature for the newly detected sources, we assess some of their observational properties. According to the results, we can distinguish between two types of compact sources. One type corresponds to sources that probably are deeply embedded in the dense ionized gas of the UCHII region. These sources are being photo-evaporated by the exciting star of the region and will last for 104−105^4-10^5 yr. They may play a crucial role in the evolution of the UCHII region as the photo-evaporated material could replenish the expanding plasma and might provide a solution to the so-called lifetime problem for these regions. The second type of compact sources is not associated with the densest ionized gas of the region. A few of these sources appear resolved and may be photo-evaporating objects such as those of the first type but with significantly lower mass depletion rates. The rest of sources of this second type appear unresolved and their properties are varied. We speculate on the similarity between the sources of the second type and those of the Orion population of radio sources.Comment: 33 pages, 6 figures, 4 tables. Accepted for publication in Ap

    A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations

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    We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6 cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6 cm continuum emission toward five sources, and observations of the H2CO masers in IRAS 18566+0408 and NGC 7538. In the case of NGC 7538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGC 7538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy

    First Detection of an H2CO 6 cm Maser Flare: A Burst in IRAS 18566+0408

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    We report the discovery of a short-duration (less than 3 months) outburst of the H2CO 6 cm maser in IRAS 18566+0408 (G37.55+0.20). During the flare, the peak flux density of the maser increased by a factor of 4; after less than a month, it decayed to the preflare value. This is the first detection of a short, burstlike variability of an H2CO 6 cm maser. The maser shows an asymmetric line profile that is consistent with the superposition of two Gaussian components. We did not detect a change in the velocity or the line width of the Gaussian components during the flare. If the two Gaussian components trace two separate maser regions, then very likely an event outside the maser gas triggered simultaneous flares at two different locations