147 research outputs found
Deep Near-Infrared Imaging af an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernovae Triggered Star Formation
While conducting a near-infrared (NIR) survey of ``Digel Clouds'', which are
thought to be located in the extreme outer Galaxy (EOG), Kobayashi & Tokunaga
found star formation activity in ``Cloud 2'', a giant molecular cloud at the
Galactic radius of ~ 20 kpc. Additional infrared imaging showed two embedded
young clusters at the densest regions of the molecular cloud. Because the
molecular cloud is located in the vicinity of a supernova remnant (SNR) HI
shell, GSH 138-01-94, it was suggested that the star formation activity in
Cloud 2 was triggered by this expanding HI shell. We obtained deep J (1.25 um),
H (1.65 um) and K (2.2 um) images of one of the embedded clusters in Cloud 2
with high spatial resolution (FWHM ~0".3) and high sensitivity (K ~ 20 mag, 10
sigma). We identified 52 cluster members. The estimated stellar density (~ 10
pc^{-2}) suggests that the cluster is a T-association. This is the deepest NIR
imaging of an embedded cluster in the EOG. The observed K-band luminosity
function (KLF) suggests that the underlying initial mass function (IMF) of the
cluster down to the detection limit of ~ 0.1 M_sun is not significantly
different from the typical IMFs in the field and in the near-by star clusters.
The overall characteristics of this cluster appears to be similar to those of
other embedded clusters in the far outer Galaxy. The estimated age of the
cluster from the KLF, which is less than 1 Myr, is consistent with the view
that the star formation was triggered by the HI shell whose age was estimated
at 4.3 Myr (Stil & Irwin). The 3-dimensional geometry of SNR shell, molecular
cloud and the embedded cluster, which is inferred from our data, as well as the
cluster age strongly suggest that the star formation in Cloud 2 was triggered
by the SNR shell.Comment: 19pages, 8 figures, 1 table, accepted to ApJ. Full paper (pdf) with
high resolution figures available at
http://www.ioa.s.u-tokyo.ac.jp/~ck_yasui/papers/Cloud2N_1.pd
Dissociative recombination and electron-impact de-excitation in CH photon emission under ITER divertor-relevant plasma conditions
For understanding carbon erosion and redeposition in nuclear fusion devices,
it is important to understand the transport and chemical break-up of
hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH
A^2\Delta - X^2\Pi Ger\"o band around 430 nm. The CH A-level can be excited
either by electron-impact or by dissociative recombination (D.R.) of
hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity
transport code ERO. A series of methane injection experiments was performed in
the high-density, low-temperature linear plasma generator Pilot-PSI, and
simulated emission intensity profiles were benchmarked against these
experiments. It was confirmed that excitation by D.R. dominates at T_e < 1.5
eV. The results indicate that the fraction of D.R. events that lead to a CH
radical in the A-level and consequent photon emission is at least 10%.
Additionally, quenching of the excited CH radicals by electron impact
de-excitation was included in the modeling. This quenching is shown to be
significant: depending on the electron density, it reduces the effective CH
emission by a factor of 1.4 at n_e=1.3*10^20 m^-3, to 2.8 at n_e=9.3*10^20
m^-3. Its inclusion significantly improved agreement between experiment and
modeling
The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter
We use radio-wave scattering data to constrain the distribution of ionized
gas in the outer Galaxy. Like previous models, our model for the H II disk
includes parameters for the radial scale length and scale height of the H II,
but we allow the H II disk to warp and flare. Our model also includes the
Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7
pulsars. Scattering in the Perseus arm is no more than 60% of the level
contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz
scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring
disk with a 1 kpc scale height, though this may reflect the non-uniform and
coarse coverage provided by the available data. The lack of a warp indicates
that VLBI observations near 1 GHz with an orbiting station having baseline
lengths of a few Earth diameters will not be affected by interstellar
scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is
15--20 kpc, but the data cannot distinguish between a gradual decrease in the
electron density and a truncated distribution. We favor a truncated one,
because we associate the scattering with massive star formation, which is also
truncated near 20 kpc. The distribution of electron density turbulence
decreases more rapidly with Galactocentric distance than does the hydrogen
distribution. Alternate ionizing and turbulent agents---the intergalactic
ionizing flux and satellite galaxies passing through the disk---do not
contribute significantly to scattering. We cannot exclude the possibility that
a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that
for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript
files, accepted for publication in Ap
The W51 Giant Molecular Cloud
We present 45"-47" angular resolution maps at 50" sampling of the 12CO and
13CO J=1-0 emission toward a 1.39 deg x 1.33 deg region in the W51 HII region
complex. These data permit the spatial and kinematic separation of several
spectral features observed along the line of sight to W51, and establish the
presence of a massive (1.2 x 10^6 Mo), large (83 pc x 114 pc) giant molecular
cloud (GMC), defined as the W51 GMC, centered at (l,b,V) = (49.5 deg, -0.2 deg,
61 km/s). A second massive (1.9 x 10^5 Mo), elongated (136 pc x 22 pc)
molecular cloud is found at velocities of about 68 km/s along the southern edge
of the W51 GMC. Of the five radio continuum sources that classically define the
W51 region, the brightest source at lambda 6cm (G49.5-0.4) is spatially and
kinematically coincident with the W51 GMC and three (G48.9-0.3, G49.1-0.4, and
G49.2-0.4) are associated with the 68 km/s cloud. Published absorption line
spectra indicate that the fifth prominent continuum source (G49.4-0.3) is
located behind the W51 molecular cloud. The W51 GMC is among the upper 1% of
clouds in the Galactic disk by size and the upper 5-10% by mass. While the W51
GMC is larger and more massive than any nearby molecular cloud, the average H2
column density is not unusual given its size and the mean H2 volume density is
comparable to that in nearby clouds. The W51 GMC is also similar to other
clouds in that most of the molecular mass is contained in a diffuse envelope
that is not currently forming massive stars. We speculate that much of the
massive star formation activity in this region has resulted from a collision
between the 68 km/s cloud and the W51 GMC.Comment: Accepted for publication by the Astronomical Journal. 21 pages, plus
7 figures and 1 tabl
Star Formation in the Most Distant Molecular Cloud in the Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of the Galaxy's Formation
We report the discovery of active star formation in Digel's Cloud 2, which is
one of the most distant giant molecular clouds known in the extreme outer
Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite
different environment from that in the solar neighborhood, including lower
metallicity, much lower gas density, and small or no perturbation from spiral
arms. With new wide-field near-infrared (NIR) imaging that covers the entire
Cloud 2, we discovered two young embedded star clusters located in the two
dense cores of the cloud. Using our NIR and 12CO data as well as HI, radio
continuum, and IRAS data in the archives, we discuss the detailed star
formation processes in this unique environment. We show clear evidences of a
sequential star formation triggered by the nearby huge supernova remnant, GSH
138-01-94. The two embedded clusters show a distinct morphology difference: the
one in the northern molecular cloud core is a loose association with
isolated-mode star formation, while the other in the southern molecular cloud
core is a dense cluster with cluster-mode star formation. We propose that high
compression by the combination of the SNR shell and an adjacent shell caused
the dense cluster formation in the southern core. Along with the low
metallicity range of the EOG, we suggest that EOG could be an excellent
laboratory for the study of star formation processes, such as those triggered
by supernovae, that occured during an early epoch of the Galaxy's formation. In
particular, the study of the EOG may shed light on the origin and role of the
thick disk, whose metallicity range matches with that of the EOG well.Comment: Accepted by The Astrophysical Journal (18 pages, 9 figures; a version
w/full-resolution color figures is available at
http://www.ioa.s.u-tokyo.ac.jp/~naoto/papers/apj.cl2_quirc/ms2p_final.pdf
Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters
As a first step for studying star formation in the extreme outer Galaxy
(EOG), we obtained deep near-infrared images of two embedded clusters at the
northern and southern CO peaks of Cloud 2, which is one of the most distant
star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With
high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the
IRCS imager at the Subaru telescope, we detected cluster members with a mass
detection limit of < 0.1 M_{sun}, which is well into the substellar regime.
These high quality data enables a comparison of EOG to those in the solar
neighborhood on the same basis for the first time. Before interpreting the
photometric result, we have first constructed the NIR color-color diagram
(dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the
Mauna Kea Observatory filter system and also for the low metallicity
environment since the metallicity in EOG is much lower than those in the solar
neighborhood. The estimated stellar density suggests that an ``isolated type''
star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation
is ongoing in Cloud 2-S. Despite the difference of the star formation mode,
other characteristics of the two clusters are found to be almost identical: (1)
K-band luminosity function (KLF) of the two clusters are quite similar, as is
the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the
estimated star formation efficiencies (SFEs) for both clusters are typical
compared to those of embedded clusters in the solar neighborhood (~ 10 %). The
similarity of two independent clusters with a large separation (~ 25 pc)
strongly suggest that their star formation activities were triggered by the
same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap
Molecular Hydrogen Outflows in W51
We present the results of a deep search for the molecular hydrogen shock
fronts associated with young stellar outflows in the giant molecular cloud and
massive star forming region W51. A total of 14 outflows were identified, and a
few of these were studied in detail with high-resolution imaging and
spectroscopy.Comment: 21 pages + 14 figure
Examination and Evaluation of the Use of Screen Heaters for the Measurement of the High Temperature Pyrolysis Kinetics of Polyethene and Polypropene
The HII Region KR 140: Spontaneous Formation of a High Mass Star
We have used a multiwavelength data set from the Canadian Galactic Plane
Survey (CGPS) to study the Galactic HII region KR 140, both on the scale of the
nebula itself and in the context of the star forming activity in the nearby
W3/W4/W5 complex of molecular clouds and HII regions. From both radio and
infrared data we have found a covering factor of about 0.5 for KR 140 and we
interpret the nebula as a bowl-shaped region viewed close to face on.
Extinction measurements place the region on the near side of its parent
molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735,
which is less than a few million years old. CO data show that VES 735 has
disrupted much of the original molecular cloud for which the estimated mass and
density are about 5000 and 100 cm, respectively. KR 140 is
isolated from the nearest star forming activity, in W3. Our data suggest that
KR 140 is an example of spontaneous (i.e., non-triggered) formation of,
unusually, a high mass star.Comment: 46 pages; includes 15 figures; accepted by the Ap
A large atomic hydrogen shell in the outer Galaxy: SNR or stellar wind bubble?
We report the detection of a ring like HI structure toward l=90.0, b=2.8 with
a velocity of v_LSR=-99 km/s. This velocity implies a distance of d=13 kpc,
corresponding to a Galactocentric radius of R_gal=15 kpc. The l-v_LSR diagram
implies an expansion velocity of v_exp ~ 15 km/s for the shell. The structure
has an oblate, irregular shell-like appearance which surrounds weak infrared
emission as seen in the 60 micrometer IRAS data. At a distance of 13 kpc the
size of the object is about 110 x 220 pc and placed 500 pc above the Galactic
plane with a mass of 1e5 solar mass. An expanding shell with such a high mass
and diameter cannot be explained by a single supernova explosion or by a single
stellar wind bubble. We interpret the structure as a relic of a distant stellar
activity region powered by the joint action of strong stellar winds from early
type stars and supernova explosions.Comment: Accepted for publication by The Astrophysical Journal, 5 Pages, 4
Figure
- …