20 research outputs found
The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN I: Catalog of Molecular Clouds
The second survey of the molecular clouds in 12CO (J = 1-0) was carried out
in the Large Magellanic Cloud by NANTEN. The sensitivity of this survey is
twice as high as that of the previous NANTEN survey, leading to a detection of
molecular clouds with M_CO > 2 x 10^4 M_sun. We identified 272 molecular
clouds, 230 of which are detected at three or more observed positions. We
derived the physical properties, such as size, line width, virial mass, of the
164 GMCs which have an extent more than the beam size of NANTEN in both the
major and minor axes. The CO luminosity and virial mass of the clouds show a
good correlation of M_VIR propto L_CO^{1.1 +- 0.1} with a Spearman rank
correlation of 0.8 suggesting that the clouds are in nearly virial equilibrium.
Assuming the clouds are in virial equilibrium, we derived an X_CO-factor to be
~ 7 x 10^20 cm^-2 (K km s^-1)^-1. The mass spectrum of the clouds is fitted
well by a power law of N_cloud(>M_CO) proportional to M_CO^{-0.75 +- 0.06}
above the completeness limit of 5 x 10^4 M_sun. The slope of the mass spectrum
becomes steeper if we fit only the massive clouds; e.g., N_cloud (>M_CO) is
proportional to M_CO^{-1.2 +- 0.2} for M_CO > 3 x 10^5 M_sun.Comment: 54 pages in total, 18 figures (21 files) and 4 tables, to appear in
Astrophysical Journal Supplement Series. A full color version with higher
resolution figures is available at
http://www.a.phys.nagoya-u.ac.jp/~kawamura/research/NANTEN_LMC_1_preprint_highres.pd
The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN. II. Star Formation
We studied star formation activities in the molecular clouds in the Large
Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds
obtained by NANTEN to compare the cloud distribution with signatures of massive
star formation including stellar clusters, and optical and radio HII regions.
We find that the molecular clouds are classified into three types according to
the activities of massive star formation; Type I shows no signature of massive
star formation, Type II is associated with relatively small HII region(s) and
Type III with both HII region(s) and young stellar cluster(s). The radio
continuum sources were used to confirm that Type I GMCs do not host optically
hidden HII regions. These signatures of massive star formation show a good
spatial correlation with the molecular clouds in a sense they are located
within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC
Types, we favor that the Types indicate an evolutionary sequence; i.e., the
youngest phase is Type I, followed by Type II and the last phase is Type III,
where the most active star formation takes place leading to cloud dispersal.
The number of the three types of GMCs should be proportional to the time scale
of each evolutionary stage if a steady state of massive star and cluster
formation is a good approximation. By adopting the time scale of the youngest
stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II
and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a
lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5
x 10^4 Msun.Comment: accepted to the Astrophysical Journal Supplement Series. 20 figures
and 4 tables. Higher resolution color PDF is found at
http://www.a.phys.nagoya-u.ac.jp/~kawamura/research/NANTEN_LMC_2_preprint.pdf
(47 pages,32MB
Dense Clumps in Giant Molecular Clouds in the Large Magellanic Cloud: Density and Temperature Derived from CO() Observations
In order to precisely determine temperature and density of molecular gas in
the Large Magellanic Cloud, we made observations of optically thin
CO() transition by using the ASTE 10m telescope toward 9 peaks
where CO() clumps were previously detected with the same
telescope. The molecular clumps include those in giant molecular cloud (GMC)
Types I (with no signs of massive star formation), II (with HII regions only),
and III (with HII regions and young star clusters). We detected
CO() emission toward all the peaks and found that their
intensities are 3 -- 12 times lower than those of CO(). We
determined the intensity ratios of CO() to CO(),
, and CO() to CO(),
, at 45\arcsec resolution. These ratios were used for
radiative transfer calculations in order to estimate temperature and density of
the clumps. The parameters of these clumps range kinetic temperature
= 15 -- 200 K, and molecular hydrogen gas density
= 8 -- 7 cm. We confirmed
that the higher density clumps show higher kinetic temperature and that the
lower density clumps lower kinetic temperature at a better accuracy than in the
previous work. The kinetic temperature and density increase generally from a
Type I GMC to a Type III GMC. We interpret that this difference reflects an
evolutionary trend of star formation in molecular clumps. The
and kinetic temperature of the clumps are well correlated
with H flux, suggesting that the heating of molecular gas
= -- cm can be explained by stellar FUV
photons.Comment: 39 pages, 7 figures, 4 tables. Accepted for publication in The
Astronomical Journa
Warm and Dense Molecular Gas in the N159 Region: 12CO J=4-3 and 13CO J=3-2 Observations with NANTEN2 and ASTE
New 12CO J=4-3 and 13CO J=3-2 observations of the N159 region in the Large
Magellanic Cloud have been made. The 12CO J=4-3 distribution is separated into
three clumps. These new measurements toward the three clumps are used in
coupled calculations of molecular rotational excitation and line radiation
transfer, along with other transitions of the 12CO as well as the isotope
transitions of 13CO. The temperatures and densities are determined to be
~70-80K and ~3x10^3 cm-3 in N159W and N159E and ~30K and ~1.6x10^3 cm-3 in
N159S. These results are compared with the star formation activity. The N159E
clump is associated with embedded cluster(s) as observed at 24 micron and the
derived high temperature is explained as due to the heating by these sources.
The N159E clump is likely responsible for a dark lane in a large HII region by
the dust extinction. The N159W clump is associated with embedded clusters
mainly toward the eastern edge of the clump only. These clusters show offsets
of 20"-40" from the 12CO J=4-3 peak and are probably responsible for heating
indicated by the derived high temperature. The N159W clump exhibits no sign of
star formation toward the 12CO J=4-3 peak position and its western region. We
suggest that the N159W peak represents a pre-star-cluster core of ~105M_sol
which deserves further detailed studies. Note that recent star formation took
place between N159W and N159E as indicated by several star clusters and HII
regions, while the natal molecular gas toward the stars have already been
dissipated by the ionization and stellar winds of the OB stars. The N159S clump
shows little sign of star formation as is consistent with the lower temperature
and somewhat lower density. The N159S clump is also a candidate for future star
formation
Sub-millimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J=3-2 and J=1-0 transitions of CO
We have carried out sub-mm 12CO(J=3-2) observations of 6 giant molecular
clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10m sub-mm
telescope at a spatial resolution of 5 pc and very high sensitivity. We have
identified 32 molecular clumps in the GMCs and revealed significant details of
the warm and dense molecular gas with n(H2) 10 cm and
Tkin 60 K. These data are combined with 12CO(J=1-0) and 13CO(J=1-0)
results and compared with LVG calculations. We found that the ratio of
12CO(J=3-2) to 12CO(J=1-0) emission is sensitive to and is well correlated with
the local Halpha flux. We interpret that differences of clump propeties
represent an evolutionary sequence of GMCs in terms of density increase leading
to star formation.Type I and II GMCs (starless GMCs and GMCs with HII regions
only, respectively) are at the young phase of star formation where density does
not yet become high enough to show active star formation and Type III GMCs
(GMCs with HII regions and young star clusters) represents the later phase
where the average density is increased and the GMCs are forming massive stars.
The high kinetic temperature correlated with \Halpha flux suggests that FUV
heating is dominant in the molecular gas of the LMC.Comment: 74 pages, including 41 figures, accepted for publication in ApJ
The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud
We present the properties of an extensive sample of molecular clouds in the
Large Magellanic Cloud (LMC) mapped at 11 pc resolution in the CO(1-0) line. We
identify clouds as regions of connected CO emission, and find that the
distributions of cloud sizes, fluxes and masses are sensitive to the choice of
decomposition parameters. In all cases, however, the luminosity function of CO
clouds is steeper than dN/dL \propto L^{-2}, suggesting that a substantial
fraction of mass is in low-mass clouds. A correlation between size and
linewidth, while apparent for the largest emission structures, breaks down when
those structures are decomposed into smaller structures. We argue that the
correlation between virial mass and CO luminosity is the result of comparing
two covariant quantities, with the correlation appearing tighter on larger
scales where a size-linewidth relation holds. The virial parameter (the ratio
of a cloud's kinetic to self-gravitational energy) shows a wide range of values
and exhibits no clear trends with the CO luminosity or the likelihood of
hosting young stellar object (YSO) candidates, casting further doubt on the
assumption of virialization for molecular clouds in the LMC. Higher CO
luminosity increases the likelihood of a cloud harboring a YSO candidate, and
more luminous YSOs are more likely to be coincident with detectable CO
emission, confirming the close link between giant molecular clouds and massive
star formation.Comment: Accepted by ApJS; 22 pages in emulateapj format; full-resolution
version and data tables available at http://mmwave.astro.illinois.edu/magma