53 research outputs found
Distortion of Magnetic Fields in a Starless Core III: Polarization--Extinction Relationship in FeSt 1-457
The relationship between dust polarization and extinction was determined for
the cold dense starless molecular cloud core FeSt 1-457 based on the background
star polarimetry of dichroic extinction at near-infrared wavelengths. Owing to
the known (three-dimensional) magnetic field structure, the observed
polarizations from the core were corrected by considering (a) the subtraction
of the ambient polarization component, (b) the depolarization effect of
inclined distorted magnetic fields, and (c) the magnetic inclination angle of
the core. After these corrections, a linear relationship between polarization
and extinction was obtained for the core in the range up to
mag. The initial polarization vs. extinction diagram changed dramatically after
the corrections of (a) to (c), with the correlation coefficient being refined
from 0.71 to 0.79. These corrections should affect the theoretical
interpretation of the observational data. The slope of the finally obtained
polarization--extinction relationship is
, which is close to the statistically estimated upper
limit of the interstellar polarization efficiency (Jones 1989). This
consistency suggests that the upper limit of interstellar polarization
efficiency might be determined by the observational viewing angle toward
polarized astronomical objects.Comment: Accepted to the Astrophysical Journal (ApJ
N2H+ Observations of Molecular Cloud Cores in Taurus
N2H+ observations of molecular cloud cores in Taurus with the Nobeyama 45 m
radio telescope are reported. We compare ``cores with young stars'' with
``cores without young stars''. The differences in core radius, linewidth, and
core mass are small. Linewidth is dominated by thermal motions in both cases.
N2H+ maps show that the intensity distribution does not differ much between
cores without stars and those with stars. This is in contrast to the result
previously obtained in H13CO+ toward Taurus molecular cloud cores. Larger
degree of depletion of H13CO+ in starless cores will be one possible
explanation for this difference. We studied the physical state of molecular
cloud cores in terms of ``critical pressure'' for the surface (external)
pressure. There is no systematic difference between starless cores and cores
with stars in this analysis. Both are not far from the critical state for
pressure equilibrium. We suggest that molecular cloud cores in which thermal
support is dominated evolve toward star formation by keeping close to the
critical state. This result is in contrast with that obtained in the
intermediate-mass star forming region OMC-2/3, where molecular cloud cores
evolve by decreasing the critical pressure appreciably. We investigate the
radial distribution of the integrated intensity. Cores with stars are found to
have shallow (-1.8 to -1.6) power-law density profiles.Comment: 19 pages, 5 figure
Sequential Star Formation in the filamentary structures of Planck Galactic cold clump G181.84+0.31
We present a multi-wavelength study of the Planck cold clump G181.84+0.31,
which is located at the northern end of the extended filamentary structure
S242. We have extracted 9 compact dense cores from the SCUBA-2 850 um map, and
we have identified 18 young stellar objects (YSOs, 4 Class I and 14 Class II)
based on their Spitzer, Wide-field Infrared Survey Explorer (WISE) and
Two-Micron All-Sky Survey (2MASS) near- and mid-infrared colours. The dense
cores and YSOs are mainly distributed along the filamentary structures of
G181.84 and are well traced by HCO(1-0) and NH(1-0)
spectral-line emission. We find signatures of sequential star formation
activities in G181.84: dense cores and YSOs located in the northern and
southern sub-structures are younger than those in the central region. We also
detect global velocity gradients of about 0.80.05 km spc and
1.00.05 km spc along the northern and southern
sub-structures, respectively, and local velocity gradients of 1.20.1 km
spc in the central substructure. These results may be due to the
fact that the global collapse of the extended filamentary structure S242 is
driven by an edge effect, for which the filament edges collapse first and then
further trigger star formation activities inward. We identify three
substructures in G181.84 and estimate their critical masses per unit length,
which are 10115 M pc, 568 M
pc and 284 M pc, respectively. These values are
all lower than the observed values ( 200 M pc),
suggesting that these sub-structures are gravitationally unstable.Comment: 20 pages, 17 figures, article, accepte
Discovery of X rays from Class 0 protostar candidates in OMC-3
We have observed the Orion Molecular Clouds 2 and 3 (OMC-2 and OMC-3) with
the Chandra X-ray Observatory (CXO). The northern part of OMC-3 is found to be
particularly rich in new X-ray features; four hard X-ray sources are located in
and along the filament of cloud cores. Two sources coincide positionally with
the sub- dust condensations of MMS 2 and 3 or an outflow radio source
VLA 1, which are in a very early phase of star formation. The X-ray spectra of
these sources show an absorption column of (1-3) x 10^23 H cm-2. Assuming a
moderate temperature plasma, the X-ray luminosity in the 0.5-10 keV band is
estimated to be ~10^30 erg s^-1 at a distance of 450 pc. From the large
absorption, positional coincidence and moderate luminosity, we infer that the
hard X-rays are coming from very young stellar objects embedded in the
molecular cloud cores. We found another hard X-ray source near the edge of the
dust filament. The extremely high absorption of 3 x 10^23 H cm^-2 indicates
that the source must be surrounded by dense gas, suggesting that it is either a
YSO in an early accretion phase or a Type II AGN (e.g. a Seyfert 2), although
no counterpart is found at any other wavelength. In contrast to the hard X-ray
sources, soft X-ray sources are found spread around the dust filaments, most of
which are identified with IR sources in the T Tauri phase.Comment: 9 pages, To be appeared in ApJ v554 n2 Jun 20, 2001 issue, related
press release is available at http://science.psu.edu/alert/Tsuboi11-2000.htm,
Figure 1 and figure 2 with the best resolution is available at
ftp.astro.psu.edu/pub/tsuboi/OMC/010205
The ALMA Survey of 70 ÎŒm Dark High-mass Clumps in Early Stages (ASHES). VI. The Core-scale CO Depletion
Studying the physical and chemical properties of cold and dense molecular clouds is crucial for the understanding of how stars form. Under the typical conditions of infrared dark clouds, CO is removed from the gas phase and trapped onto the surface of dust grains by the so-called depletion process. This suggests that the CO-depletion factor (f D ) can be a useful chemical indicator for identifying cold and dense regions (i.e., prestellar cores). We have used the 1.3 mm continuum and C18O (2-1) data observed at the resolution of âŒ5000 au in the ALMA Survey of 70 ÎŒm Dark High-mass Clumps in Early Stages (ASHES) to construct averaged maps of f D in 12 clumps to characterize the earliest stages of the high-mass star formation process. The average f D determined for 277 of the 294 ASHES cores follows an unexpected increase from the prestellar to the protostellar stage. If we exclude the temperature effect due to the slight variations in the NH3 kinetic temperature among different cores, we explain this result as a dependence primarily on the average gas density, which increases in cores where protostellar conditions prevail. This shows that f D determined in high-mass star-forming regions at the core scale is insufficient to distinguish among prestellar and protostellar conditions for the individual cores and should be complemented by information provided by additional tracers. However, we confirm that the clump-averaged f D values correlate with the luminosity-to-mass ratio of each source, which is known to trace the evolution of the star formation process
Large-scale mapping observations of the CI(3P1-3P0) and CO(J=3-2) lines toward the Orion A molecular cloud
Large scale mapping observations of the 3P1-3P0 fine structure transition of
atomic carbon (CI, 492 GHz) and the J=3-2 transition of CO (346 GHz) toward the
Orion A molecular cloud have been carried out with the Mt. Fuji
submillimeter-wave telescope. The observations cover 9 square degrees, and
include the Orion nebula M42 and the L1641 dark cloud complex. The CI emission
extends over almost the entire region of the Orion A cloud and is surprisingly
similar to that of 13CO(J=1-0).The CO(J=3-2) emission shows a more featureless
and extended distribution than CI.The CI/CO(J=3-2) integrated intensity ratio
shows a spatial gradient running from the north (0.10) to the south (1.2) of
the Orion A cloud, which we interpret as a consequence of the temperature
gradient. On the other hand, the CI/13CO(J=1-0) intensity ratio shows no
systematic gradient. We have found a good correlation between the CI and
13CO(J=1-0) intensities over the Orion A cloud. This result is discussed on the
basis of photodissociation region models.Comment: Text file is 13 pages long, and 3 figure files (pdf format). NRO
Report No. 508 (1999). University of Tokyo, Resceu 41/9
Centimeter Imaging of the R Coronae Australis Region
The R CrA region was observed in the 3.5 and 6.2 cm continuum with high
angular resolutions (0.6--1.7 arcseconds) using the Very Large Array. Archival
data sets were also analyzed for comparison, which provided angular resolutions
up to 0.3 arcseconds. A cluster of young stellar objects was detected, and a
rich array of star forming activities was revealed. IRS 7A showed an enhanced
outflow activity recently. The main peak of IRS 7A positionally coincides with
an X-ray source, which suggests that the X-ray emission is directly related to
the central protostar. The Class 0 source SMA 2 is associated with a double
radio source, B 9a and 9b, and seems to be driving two outflows. The B 9
complex is probably a multiple-protostar system. Both B 9a and 9b are
nonthermal radio sources with negative spectral indices. IRS 7B is a compact
radio source surrounded by an extended structure. The compact source
corresponds to the Class 0/I source SMA 1, and it is also closely associated
with an X-ray source, suggesting that magnetic activities start early in the
protostellar stage of evolution. The extended structure of IRS 7B may be a
bipolar outflow. IRS 5 was resolved into two sources with a separation of 0.9
arcseconds. Both IRS 5a and 5b display radio flares and X-ray emission,
suggesting that energetic magnetic processes are active in both members. The
month-scale active phase of IRS 5b implies that the flare activity must involve
large-scale magnetic fields. During the strong flare event of IRS 5b in 1998,
IRS 5a also showed an enhanced level of radio emission. This concurrent
activity suggests that IRS 5 may be an interacting young binary system, but the
interaction mechanism is unknown. Alternatively, what was seen in the radio
images could be a circumbinary halo.Comment: To appear in the Astrophysical Journa
The ALMA Survey of 70 ÎŒm Dark High-mass Clumps in Early Stages (ASHES). VII. Chemistry of Embedded Dense Cores
We present a study of the chemistry toward 294 dense cores in 12 molecular clumps, using data obtained from the ALMA Survey of 70 ÎŒm dark High-mass clumps in Early Stages. We identified 97 protostellar cores and 197 prestellar core candidates, based on the detection of outflows and molecular transitions of high upper-energy levels (E u /k > 45 K). The detection rate of the N2D+ emission toward the protostellar cores is 38%, which is higher than 9% for the prestellar cores, indicating that N2D+ does not exclusively trace prestellar cores. The detection rates of the DCO+ emission are 35% for the prestellar cores and 49% for the protostellar cores, which are higher than those for N2D+, implying that DCO+ appears more frequently than N2D+ in both prestellar and protostellar cores. Both the N2D+ and DCO+ abundances appear to decrease from the prestellar to the protostellar stage. The DCN, C2D, and 13CS emission lines are rarely seen in the dense cores of early evolutionary phases. The detection rate of the H2CO emission toward dense cores is 52%, three times higher than that for CH3OH (17%). In addition, the H2CO detection rate, abundance, line intensities, and line widths increase with the core evolutionary status, suggesting that the H2CO line emission is sensitive to protostellar activity
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