54 research outputs found
Evolution of HII regions in hierarchically structured molecular clouds
We present observations of the H91 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
We present 247-GHz molecular line observations of methanol (CHOH) toward
sixteen massive star-forming regions, using the APEX telescope with an angular
resolution of . 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 104168 K and column densities
from to cm. Using the average methanol
line parameters, we estimate virial masses, which fall in the range from 145 to
720 M 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,
at 247.228 GHz, is predicted to show class II maser
emission, similar in intensity to previously reported 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
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 10 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
Arecibo Observations of Formaldehyde in L1551
We report observations of the formaldehyde (H2CO) 6 cm (4.8 GHz) line toward L1551. The observations were conducted with the Arecibo Telescope (beam FWHP ~ 1') to verify the tentative detection of H2CO thermal emission reported by Duncan and collaborators in 1987. The H2CO emission lines were expected to be present with a signal-to-noise ratio of 10 in our spectra. However, we did not detect H2CO emission; i.e., our data rule out their tentative detection. The absence of H2CO emission is also confirmed by the fact that the H2CO line profiles at the two positions of expected emission are well fitted by a single absorption component (accounting for the hyperfine structure of the line) in one of the positions and by a single absorption line plus a red-wing absorption component in the second position. The Orion BN/KL region remains the only H2CO 6 cm thermal emitter known. Our observations also demonstrate that the H2CO 6 cm absorption line traces not only the quiescent molecular cloud but also the kinematics associated with the star formation process in L1551-IRS 5
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