76 research outputs found
Molecular clouds towards RCW 49 and Westerlund 2; Evidence for cluster formation triggered by cloud-cloud collision
We have made CO(J=2-1) observations towards the HII region RCW 49 and its
ionizing source, the rich stellar cluster Westerlund 2 (hereafter Wd2), with
the NANTEN2 sub-mm telescope. These observations have revealed that two
molecular clouds in velocity ranges of -11 to +9 km/s and 11 to 21 km/s
respectively, show remarkably good spatial correlations with the Spitzer IRAC
mid-infrared image of RCW 49, as well a velocity structures indicative of
localized expansion around the bright central regions and stellar cluster. This
strongly argues that the two clouds are physically associated with RCW 49. We
obtain a new kinematic distance estimate to RCW 49 and Wd2 of 5.4^{+ 1.1}_{-
1.4} kpc, based on the mean velocity and velocity spread of the associated gas.
We argue that acceleration of the gas by stellar winds from Wd2 is insufficient
to explain the entire observed velocity dispersion of the molecular gas, and
suggest a scenario in which a collision between the two clouds ~4 Myrs ago may
have triggered the formation of the stellar cluster.Comment: A version with higher resolution figures is available from
http://www.a.phys.nagoya-u.ac.jp/~naoko/research/apjl2009/fur09_rev_highreso.pd
Centrally Peaked X-Ray Supernova Remnants in the Small Magellanic Cloud Studied with ASCA and ROSAT
This paper presents ASCA/SIS and ROSAT/HRI results of three supernova
remnants (SNRs) in the Small Magellanic Cloud: 0103-726, 0045-734, and
0057-7226. The ROSAT/HRI images of these SNRs indicate that the most of the
X-ray emissions are concentrated in the center region. Only from 0103-726 are
faint X-rays along the radio shell also detected. The ASCA/SIS spectra of
0103-726 and 0045-734 exhibit strong emission lines from highly ionized metals.
The spectra were well-fitted with non-equilibrium ionization (NEI) plasma
models. The metal abundances are found to be larger than the mean chemical
compositions in the interstellar medium (ISM) of the SMC. Thus, X-rays from
these two SNRs are attributable to the ejecta gas, although the ages estimated
from the ionization timescale are significantly large, ~> 10^4 yr. The chemical
compositions are roughly consistent with the type-II supernova origin of a
progenitor mass ~< 20 M_solar. The SIS spectrum of 0057-7226 was also fitted
with an NEI model of an estimated age ~> 6 x 10^3 yr. Although no constraint on
the metal abundances was obtained, the rather weak emission lines are
consistent with the low metal abundances in the ISM of the SMC. A possible
scenario for the evolution of the morphologies and spectra of SNRs is proposed.Comment: 8 pages, 8 figures, to be published in PASJ. Also available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job/ (with high-resolution
images
Discovery of Molecular Loop 3 in the Galactic Center: Evidence for a Positive-Velocity Magnetically Floated Loop towards
We have discovered a molecular dome-like feature towards and . The large velocity
dispersions of 50--100 km s of this feature are much larger than those
in the Galactic disk and indicate that the feature is located in the Galactic
center, probably within kpc of Sgr A. The distribution has a
projected length of pc and height of pc from the Galactic
disk and shows a large-scale monotonic velocity gradient of km s
per pc. The feature is also associated with HI gas having a
more continuous spatial and velocity distribution than that of CO. We
interpret the feature as a magnetically floated loop similar to loops 1 and 2
and name it "loop 3". Loop 3 is similar to loops 1 and 2 in its height and
length but is different from loops 1 and 2 in that the inner part of loop 3 is
filled with molecular emission. We have identified two foot points at the both
ends of loop 3. HI, CO and CO datasets were used to estimate the
total mass and kinetic energy of loop 3 to be \sim3.0 \times 10^{6} \Mo and
ergs. The huge size, velocity dispersions and energy
are consistent with the magnetic origin the Parker instability as in case of
loops 1 and 2 but is difficult to be explained by multiple stellar explosions.
We argue that loop 3 is in an earlier evolutionary phase than loops 1 and 2
based on the inner-filled morphology and the relative weakness of the foot
points. This discovery indicates that the western part of the nuclear gas disk
of kpc radius is dominated by the three well-developed magnetically
floated loops and suggests that the dynamics of the nuclear gas disk is
strongly affected by the magnetic instabilities.Comment: 30 pages, 10 figures. High resolution figures are available at
http://www.a.phys.nagoya-u.ac.jp/~motosuji/fujishita09_figs
A Detailed Observational Study of Molecular Loops 1 and 2 in the Galactic Center
Fukui et al. (2006) discovered two huge molecular loops in the Galactic
center located in (l, b) ~ (355 deg-359 deg, 0 deg-2 deg) in a large velocity
range of -180-40 km s^-1. Following the discovery, we present detailed
observational properties of the two loops based on NANTEN 12CO(J=1-0) and
13CO(J=1-0) datasets at 10 pc resolution including a complete set of velocity
channel distributions and comparisons with HI and dust emissions as well as
with the other broad molecular features. We find new features on smaller scales
in the loops including helical distributions in the loop tops and vertical
spurs. The loops have counterparts of the HI gas indicating that the loops
include atomic gas. The IRAS far infrared emission is also associated with the
loops and was used to derive an X-factor of 0.7(+/-0.1){\times}10^20 cm^-2 (K
km s^-1)^-1 to convert the 12CO intensity into the total molecular hydrogen
column density. From the 12CO, 13CO, H I and dust datasets we estimated the
total mass of loops 1 and 2 to be ~1.4 {\times} 106 Msun and ~1.9 {\times} 10^6
Msun, respectively, where the H I mass corresponds to ~10-20% of the total mass
and the total kinetic energy of the two loops to be ~10^52 ergs. An analysis of
the kinematics of the loops yields that the loops are rotating at ~47 km s-1
and expanding at ~141 km s^-1 at a radius of 670 pc from the center. Fukui et
al. (2006) presented a model that the loops are created by the magnetic
flotation due to the Parker instability with an estimated magnetic field
strength of ~150 {\mu}G. We present comparisons with the recent numerical
simulations of the magnetized nuclear disk by Machida et al. (2009) and
Takahashi et al. (2009) and show that the theoretical results are in good
agreements with the observations. The helical distributions also suggest that
some magnetic instability plays a role similarly to the solar helical features.Comment: 40 pages, 22 figures, submitted to publication in PAS
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
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