718 research outputs found
Infrared emission from ultracompact H II regions
Models of circumstellar dust shells around ultracompact (UC) H II regions were constructed that accurately fit the observed IR flux distributions. The models assume spherically symmetric dust shells illuminated by stars whose bolometric luminosity is inferred from the integrated FIR flux densities. Assuming ionization by a single zero age main sequence (ZAMS) star, the relations of Panagia were used to infer the stellar radius and effective temperature for a given luminosity. The grain mixture in the dust shell consists of bare graphite and silicate grains with the optical properties of Draine and Lee and the size distribution of Mathis et al. The computer code of Wolfire et al was used to solve the radiative transfer equations through a spherical dust shell. The model provides monochromatic luminosities, dust temperatures, and opacities through the shell. Aside from the stellar and dust properties, the only other input parameters to the model are the distance to the shell, the form of its density distribution, and its outer radius. Predictions of the model are compared with observations of a typical UC H II region and the run of dust temperature with radius and the optical depth with frequency are discussed
Relative Evolutionary Time Scale of Hot Molecular Cores with Respect to Ultra Compact HII Regions
Using the Owens Valley and Nobeyama Radio Observatory interferometers, we
carried out an unbiased search for hot molecular cores and ultracompact UC HII
regions toward the high-mass star forming region G19.61--0.23. In addition, we
performed 1.2 mm imaging with SIMBA, and retrieved 3.5 and 2 cm images from the
VLA archive data base. The newly obtained 3 mm image brings information on a
cluster of high-mass (proto)stars located in the innermost and densest part of
the parsec scale clump detected in the 1.2 mm continuum. We identify a total of
10 high-mass young stellar objects: one hot core (HC) and 9 UC HII regions,
whose physical parameters are obtained from model fits to their continuum
spectra. The ratio between the current and expected final radii of the UC \HII
regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars
formed simultaneously. Under the opposite assumption -- namely that star
formation occurred randomly -- we estimate that HC lifetime is less than
1/3 of that of UCHII regions on the basis of the source number ratio
between them.Comment: 13 pages, 2 figs, including a color fi
The environment of the infrared dust bubble N65: a mutiwavelength study
AIMS: We investigate the environment of the infrared dust bubble N65 and
search for evidence of triggered star formation in its surroundings. METHODS:
We performed a multiwavelength study of the region around N65 with data taken
from large-scale surveys: Two Micron All Sky Survey, GLIMPSE, MIPSGAL, SCUBA,
and GRS. We analyzed the distribution of the molecular gas and dust in the
environment of N65 and performed infrared photometry and spectral analysis of
point sources to search for young stellar objects and identify the ionizing
star candidates. RESULTS: We found a molecular cloud that appears to be
fragmented into smaller clumps along the N65 PDR. This indicates that the
so-called collect and collapse process may be occurring. Several young stellar
objects are distributed among the molecular clumps. They may represent a second
generation of stars whose formation was triggered by the bubble expanding into
the molecular gas. We dentified O-type stars inside N65, which are the most
reliable ionizing star candidates.Comment: 15 pages, 10 figures, accepted for publication in A&A. Figures
degraded to reduce file siz
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
The molecular clump towards the eastern border of SNR G18.8+0.3
The eastern border of the SNR G18.8+0.3, close to an HII regions complex, is
a very interesting region to study the molecular gas that it is probably in
contact with the SNR shock front. We observed the aforementioned region using
the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO
J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of 22". To
complement these observations, we analyzed IR, submillimeter and radio
continuum archival data. In this work, we clearly show that the radio continuum
"protrusion" that was early thought to belong to the SNR is an HII regions
complex deeply embedded in a molecular clump. The new molecular observations
reveal that this dense clump, belonging to an extended molecular cloud that
surrounds the SNR southeast border, is not physically in contact with SNR
G18.8+0.3, suggesting that the SNR shock front have not yet reached it or maybe
they are located at different distances. We found some young stellar objects
embedded in the molecular clump, suggesting that their formation should be
approximately coeval with the SN explosion.Comment: Accepted for publication in A&A (Sept. 7, 2012
The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles
The Milky Way Project citizen science initiative recently increased the
number of known infrared bubbles in the inner Galactic plane by an order of
magnitude compared to previous studies. We present a detailed statistical
analysis of this dataset with the Red MSX Source catalog of massive young
stellar sources to investigate the association of these bubbles with massive
star formation. We particularly address the question of massive triggered star
formation near infrared bubbles. We find a strong positional correlation of
massive young stellar objects (MYSOs) and H II regions with Milky Way Project
bubbles at separations of < 2 bubble radii. As bubble sizes increase, a
statistically significant overdensity of massive young sources emerges in the
region of the bubble rims, possibly indicating the occurrence of triggered star
formation. Based on numbers of bubble-associated RMS sources we find that
67+/-3% of MYSOs and (ultra)compact H II regions appear associated with a
bubble. We estimate that approximately 22+/-2% of massive young stars may have
formed as a result of feedback from expanding H II regions. Using MYSO-bubble
correlations, we serendipitously recovered the location of the recently
discovered massive cluster Mercer 81, suggesting the potential of such analyses
for discovery of heavily extincted distant clusters.Comment: 16 pages, 17 figures. Accepted for publication in ApJ, comments
welcome. Milky Way Project public data release available at
http://www.milkywayproject.org/dat
Infrared point source variability between the Spitzer and MSX surveys of the Galactic mid-plane
We present a list of 552 sources with suspected variability, based on a
comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which
were carried out nearly a decade apart. We were careful to address issues such
as the difference in resolution and sensitivity between the two surveys, as
well as the differences in the spectral responses of the instruments. We
selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by
more than a factor of two, in order to minimize contamination from sources
where the difference in fluxes at 8 micron is due to a strong 10 micron
silicate feature. We present a subset of 40 sources for which additional
evidence suggests variability, using 2MASS and MIPSGAL data. Based on a
comparison with the variability flags in the IRAS and MSX Point-Source Catalogs
we estimate that at least a quarter of the 552 sources, and at least half of
the 40 sources are truly variable. In addition, we tentatively confirm the
variability of one source using multi-epoch IRAS LRS spectra. We suggest that
most of the sources in our list are likely to be Asymptotic Giant Branch stars.Comment: 47 pages, 12 Figures, 3 Tables, accepted for publication in A
Revealing the environs of the remarkable southern hot core G327.3-0.6
We present a submm study of the massive hot core G327.3-0.6 that constrains
its physical parameters and environment. The APEX telescope was used to image
CO and N2H+ emission, to observe lines from other molecules toward a hot and a
cold molecular core, and to measure the continuum flux density of the hot core.
In the C18O J=3-2 line, two clumps were found, one associated with the HII
region G327.3-0.5 and the other associated with the hot core. An additional
cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+
emission. From the the continuum data, we calculate a mass of 420 Msol and a
size of 0.1 pc for the hot core. A new, more accurate position of the hot core
is reported, which allows the association of the core with a bright
mid-infrared source. The luminosity of the hot core is estimated to be between
5 and 15 10^4 Lsol. This study revealed several different evolutionary stages
of massive star formation in the G327.3-0.6 region.Comment: APEX A&A special issue, accepte
Candidate Rotating Toroids around High-Mass (Proto)Stars
Using the OVRO, Nobeyama, and IRAM mm-arrays, we searched for
``disk''-outflow systems in three high-mass (proto)star forming regions:
G16.59-0.05, G23.01-0.41, and G28.87+0.07. These were selected from a sample of
NH3 cores associated with OH and H2O maser emission and with no or very faint
continuum emission. Our imaging of molecular line (including rotational
transitions of CH3CN and 3mm dust continuum emission revealed that these are
compact, massive, and hot molecular cores (HMCs), that is likely sites of
high-mass star formation prior to the appearance of UCHII regions. All three
sources turn out to be associated with molecular outflows from CO and/or HCO+
J=1--0 line imaging. In addition, velocity gradients of 10 -- 100 km/s per pc
in the innermost densest regions of the G23.01 and G28.87 HMCs are identified
along directions roughly perpendicular to the axes of the corresponding
outflows. All the results suggest that these cores might be rotating about the
outflow axis, although the contribution of rotation to gravitational
equilibrium of the HMCs appears to be negligible. Our analysis indicates that
the 3 HMCs are close to virial equilibrium due to turbulent pressure support.
Comparison with other similar objects where rotating toroids have been
identified so far shows that in our case rotation appears to be much less
prominent; this can be explained by the combined effect of unfavorable
projection, large distance, and limited angular resolution with the current
interferometers.Comment: Accepted by ApJ main journal, the paper with the original quality
figures are available from
http://subarutelescope.org/staff/rsf/publication.htm
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