115 research outputs found
A VLA Study of Ultracompact and Hypercompact H II Regions from 0.7 to 3.6 cm
We report multi-frequency Very Large Array observations of three massive star
formation regions (MSFRs) containing radio continuum components that were
identified as broad radio recombination line (RRL) sources and hypercompact
(HC) H II region candidates in our previous H92alpha and H76alpha study:
G10.96+0.01 (component W), G28.20-0.04 (N), and G34.26+0.15 (B). An additional
HC H II region candidate, G45.07+0.13, known to have broad H66alpha and
H76alpha lines, small size, high electron density and emission measure, was
also included. We observed with high spatial resolution (0.9" to 2.3") the
H53alpha, H66alpha, H76alpha, and H92alpha RRLs and the radio continuum at the
corresponding wavelengths (0.7 to 3.6 cm). The motivation for these
observations was to obtain RRLs over a range of principal quantum states to
look for signatures of pressure broadening and macroscopic velocity structure.
We find that pressure broadening contributes significantly to the line widths,
but it is not the sole cause of the broad lines. We compare radio continuum and
dust emission distributions and find a good correspondence. We also discuss
maser emission and multi-wavelength observations reported in the literature for
these MSFRs.Comment: Accepted for publication in ApJ; 55 pages, 10 tables, 12 figure
Radio Imaging of the NGC 2024 FIR 5/6 Region: a Hypercompact H II Region Candidate in Orion
The NGC 2024 FIR 5/6 region was observed in the 6.9 mm continuum with an
angular resolution of about 1.5 arcsec. The 6.9 mm continuum map shows four
compact sources, FIR 5w, 5e, 6c, and 6n, as well as an extended structure of
the ionization front associated with the optical nebulosity. FIR 6c has a
source size of about 0.4 arcsec or 150 AU. The spectral energy distribution
(SED) of FIR 6c is peculiar: rising steeply around 6.9 mm and flat around 1 mm.
The possibility of a hypercompact H II region is explored. If the millimeter
flux of FIR 6c comes from hot ionized gas heated by a single object at the
center, the central object may be a B1 star of about 5800 solar luminosities
and about 13 solar masses. The 6.9 mm continuum of FIR 6n may be a mixture of
free-free emission and dust continuum emission. Archival data show that both
FIR 6n and 6c exhibit water maser activity, suggesting the existence of shocked
gas around them. The 6.9 mm continuum emission from FIR 5w has a size of about
1.8 arcsec or 760 AU. The SEDs suggest that the 6.9 mm emission of FIR 5w and
5e comes from dust, and the masses of the dense molecular gas are about 0.6 and
0.5 solar masses, respectively.Comment: To appear in the Astrophysical Journa
An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408
We report observations of 6cm, 3.6cm, 1.3cm, and 7mm radio continuum,
conducted with the Very Large Array towards IRAS18566+0408, one of the few
sources known to harbor H2CO 6cm maser emission. Our observations reveal that
the emission is dominated by an ionized jet at cm wavelengths. Spitzer/IRAC
images from GLIMPSE support this interpretation, given the presence of 4.5um
excess emission at approximately the same orientation as the cm continuum. The
7mm emission is dominated by thermal dust from a flattened structure almost
perpendicular to the ionized jet, thus, the 7mm emission appears to trace a
torus associated with a young massive stellar object. The H2CO 6cm maser is
coincident with the center of the torus-like structure. Our observations rule
out radiative pumping via radio continuum as the excitation mechanism for the
H2CO 6cm maser in IRAS18566+0408.Comment: 20 pages, 4 figures, ApJ (in press
Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge
The Magellanic Bridge is the nearest low-metallicity, tidally stripped
environment, offering a unique high-resolution view of physical conditions in
merging and forming galaxies. In this paper we present analysis of candidate
massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star
formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary
optical and near-IR photometry. While we definitely find YSOs in the Bridge,
the most massive are , found in the Large
Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears
decreasing, as the most massive YSOs are less massive than those formed in the
past. To investigate environmental effects on MSF, we have compared properties
of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge
are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical
counterparts, compared to only 56% of LMC sources with the same range of mass,
circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes
are evidently more porous or clumpy in the Bridge's low-metallicity
environment. Second, we have used whole samples of YSOs in the LMC and the
Bridge to estimate the probability of finding YSOs at a given \hi\ column
density, N(HI). We found that the LMC has higher probability than
the Bridge for N(HI) cm, but the trend reverses at
lower N(HI). Investigating whether this lower efficiency relative to HI is due
to less efficient molecular cloud formation, or less efficient cloud collapse,
or both, will require sensitive molecular gas observations.Comment: 41 pages, 20 figures, 6 tables; accepted for publication in ApJ;
several figures are in low resolution due to the size limit here and a high
resolution version can be downloaded via
http://www.astro.virginia.edu/~cc5ye/ms_bridge20140215.pd
Relations Between Molecular Cloud Structure Sizes and Line Widths in the Large Magellanic Cloud
We present a comparative study of the size-line width relation for
substructures within six molecular clouds in the Large Magellanic Cloud (LMC)
mapped with the Atacama Large Millimeter/submillimeter Array (ALMA). Our sample
extends our previous study, which compared a Planck detected cold cloud in the
outskirts of the LMC with the 30 Doradus molecular cloud and found the typical
line width for 1 pc radius structures to be 5 times larger in 30 Doradus. By
observing clouds with intermediate levels of star formation activity, we find
evidence that line width at a given size increases with increasing local and
cloud-scale 8m intensity. At the same time, line width at a given size
appears to independently correlate with measures of mass surface density. Our
results suggest that both virial-like motions due to gravity and local energy
injection by star formation feedback play important roles in determining
intracloud dynamics.Comment: 20 pages, to appear in ApJ. Data presented in this paper can be found
at https://mmwave.astro.illinois.edu/almalmc
Formation of High-Mass stars in an isolated environment in the Large Magellanic Cloud
The aim of this study is to characterize the distribution and basic
properties of the natal gas associated with high-mass young stellar objects
(YSOs) in isolated environments in the Large Magellanic Cloud (LMC). High-mass
stars usually form in Giant Molecular Clouds (GMCs) as part of a young stellar
cluster, but some OB stars are observed far from GMCs. By examining the spatial
coincidence between the high-mass YSOs and 12CO (J = 1-0) emission detected by
NANTEN and Mopra observations, we selected ten high-mass YSOs that are located
away from any of the NANTEN clouds but are detected by the Mopra pointed
observations. The ALMA observations revealed that a compact molecular cloud
whose mass is a few thousand solar masses or smaller is associated with the
high-mass YSOs, which indicates that these compact clouds are the sites of
high-mass star formation. The high-density and high-temperature throughout the
clouds are explained by the severe photodissociation of CO due to the lower
metallicity than in the Galaxy. The star formation efficiency ranges from
several to as high as ~ 40%, indicating efficient star formation in these
environments. The enhanced turbulence may be a cause of the efficient star
formation therein, as judged from the gas velocity information and the
association with the lower density gas.Comment: 31 pages, 14 figures, PASJ accepted for publicatio
Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major
We study a very young star-forming region in the outer Galaxy that is the
most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360
survey. This region, dubbed CMa-l224, is located in the Canis Major OB1
association. CMa-l224 is relatively faint in the mid-infrared, but it shines
brightly at the far-infrared wavelengths as revealed by the Herschel Space
Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 m data from
the Spitzer/GLIMPSE360 survey, combined with the JHK 2MASS and the 70-500
m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection
criteria based on color-color cuts and fitting of the YSO candidates' spectral
energy distributions with YSO 2D radiative transfer models. We identify 293 YSO
candidates and estimate physical parameters for 210 sources well-fit with YSO
models. We select an additional 47 sources with GLIMPSE360-only photometry as
`possible YSO candidates'. The vast majority of these sources are associated
with high H column density regions and are good targets for follow-up
studies. The distribution of YSO candidates at different evolutionary stages
with respect to Herschel filaments supports the idea that stars are formed in
the filaments and become more dispersed with time. Both the supernova-induced
and spontaneous star formation scenarios are plausible in the environmental
context of CMa-l224. However, our results indicate that a spontaneous
gravitational collapse of filaments is a more likely scenario. The methods
developed for CMa-l224 can be used for larger regions in the Galactic plane
where the same set of photometry is available.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 54 pages including appendice
Integral field spectroscopy of massive young stellar objects in the N113 H II region in the Large Magellanic Cloud
The Spitzer Surveying the Agents of Galaxy Evolution (SAGE) survey has allowed the identification and analysis of significant samples of Young Stellar Object (YSO) candidates in the Large Magellanic Cloud (LMC). However, the angular resolution of Spitzer is relatively poor meaning that at the distance of the LMC, it is likely that many of the Spitzer YSO candidates in fact contain multiple components. We present high-resolution K-band integral field spectroscopic observations of the three most prominent massive YSO candidates in the N113 H?II region using Very Large Telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI). We have identified six K-band continuum sources within the three Spitzer sources and we have mapped the morphology and velocity fields of extended line emission around these sources. Br ?, He?I and H2 emission is found at the position of all six K-band sources; we discuss whether the emission is associated with the continuum sources or whether it is ambient emission. H2 emission appears to be mostly ambient emission and no evidence of CO emission arising in the discs of YSOs has been found. We have mapped the centroid velocities of extended Br ? emission and He?I emission and found evidence of two expanding compact H?II regions. One source shows compact and strong H2 emission suggestive of a molecular outflow. The diversity of spectroscopic properties observed is interpreted in the context of a range of evolutionary stages associated with massive star formation
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