113 research outputs found
Chemical Variation in Molecular Cloud Cores in the Orion A Cloud
We have observed molecular cloud cores in the Orion A giant molecular cloud
(GMC) in CCS, HC3N, DNC, and HN13C to study their chemical characteristics. We
have detected CCS in the Orion A GMC for the first time. CCS was detected in
about a third of the observed cores. The cores detected in CCS are not
localized but are widely distributed over the Orion A GMC. The CCS peak
intensity of the core tends to be high in the southern region of the Orion A
GMC. The HC3N peak intensity of the core also tends to be high in the southern
region, while there are HC3N intense cores near Orion KL, which is not seen in
CCS. The core associated with Orion KL shows broad HC3N line profile, and star
formation activity near Orion KL seems to enhance the HC3N emission. The column
density ratio of NH3 to CCS is lower near the middle of the filament, and is
higher toward the northern and southern regions along the Orion A GMC filament.
This ratio is known to trace the chemical evolution in nearby dark cloud cores,
but seems to be affected by core gas temperature in the Orion A GMC: cores with
low NH3 to CCS column density ratios tend to have warmer gas temperature. The
value of the column density ratio of DNC to HN13C is generally similar to that
in dark cloud cores, but becomes lower around Orion KL due to higher gas
temperature.Comment: 26 pages, 18 figures, to be published in Publications of the
Astronomical Society of Japa
Mapping dust column density in dark clouds by using NIR scattered light : Case of the Lupus 3 dark cloud
We present a method of mapping dust column density in dark clouds by using
near-infrared scattered light. Our observations of the Lupus 3 dark cloud
indicate that there is a well defined relation between (1) the H-Ks color of an
individual star behind the cloud, i.e., dust column density, and (2) the
surface brightness of scattered light toward the star in each of the J, H, and
Ks bands. In the relation, the surface brightnesses increase at low H-Ks
colors, then saturate and decrease with increasing H-Ks. Using a simple
one-dimensional radiation transfer model, we derive empirical equations which
plausibly represent the observed relationship between the surface brightness
and the dust column density. By using the empirical equations, we estimate dust
column density of the cloud for any directions toward which even no background
stars are seen. We obtain a dust column density map with a pixel scale of 2.3 x
2.3 arcsec^2 and a large dynamic range up to Av = 50 mag. Compared to the
previous studies by Juvela et al., this study is the first to use color excess
of the background stars for calibration of the empirical relationship and to
apply the empirical relationship beyond the point where surface brightness
starts to decrease with increasing column density
Distortion of Magnetic Fields in a Starless Core V: Near-infrared and Submillimeter Polarization in FeSt 1-457
The relationship between submillimeter (submm) dust emission polarization and
near-infrared (NIR) -band polarization produced by dust dichroic extinction
was studied for the cold starless dense core FeSt 1-457. The distribution of
polarization angles (-rotated for submm) and degrees were found to
be very different between at submm and NIR wavelengths. The mean polarization
angles for FeSt 1-457 at submm and NIR wavelengths are and , respectively. The correlation
between and was found to be linear from outermost regions to
relatively dense line of sight of mag, indicating that NIR
polarization reflects overall polarization (magnetic field) structure of the
core at least in this density range. The flat versus
correlations were confirmed, and the polarization efficiency was found to be
comparable to the observational upper limit (Jones 1989). On the other hand, as
reported by Alves et al., submm polarization degrees show clear linearly
decreasing trend against from mag to the densest center
( mag), appearing as "polarization hole" structure. The power
law index for the versus relationship was obtained to be
, indicating that the alignment for the submm sensitive dust is
lost. These very different polarization distributions at submm and NIR
wavelengths suggest that (1) there is different radiation environment at these
wavelengths or (2) submm-sensitive dust is localized or the combination of
them.Comment: Accepted to the Astrophysical Journal (ApJ
Interstellar Extinction Law toward the Galactic Center II: V, J, H, and Ks Bands
We have determined the ratios of total to selective extinction directly from
observations in the optical V band and near-infrared J band toward the Galactic
center. The OGLE (Optical Gravitational Lensing Experiment) Galactic bulge
fields have been observed with the SIRIUS camera on the IRSF telescope, and we
obtain A(V)/E(V-J)=1.251+-0.014 and A(J)/E(V-J)=0.225+-0.007. From these
ratios, we have derived A(J)/A(V) = 0.188+-0.005; if we combine A(J)/A(V) with
the near-infrared extinction ratios obtained by Nishiyama et al. for more
reddened fields near the Galactic center, we get A(V) : A(J) : A(H) : A(Ks) = 1
: 0.188 : 0.108 : 0.062, which implies steeply declining extinction toward the
longer wavelengths. In particular, it is striking that the Ks band extinction
is \approx 1/16 of the visual extinction A(V) much smaller than one tenth of
A(V) so far employed.Comment: 8 pages, 7 figures, Accepted for publication in Ap
Near-infrared circular polarization survey in star-forming regions : Correlations and trends
We have conducted a systematic near-infrared circular polarization (CP) survey in star-forming regions, covering high-mass, intermediate-mass, and low-mass young stellar objects. All the observations were made using the SIRPOL imaging polarimeter on the Infrared Survey Facility 1.4 m telescope at the South African Astronomical Observatory. We present the polarization properties of 10 sub-regions in 6 star-forming regions. The polarization patterns, extents, and maximum degrees of linear and circular polarizations are used to determine the prevalence and origin of CP in the star-forming regions. Our results show that the CP pattern is quadrupolar in general, the CP regions are extensive, up to 0.65 pc, the CP degrees are high, up to 20%, and the CP degrees decrease systematically from high- to low-mass young stellar objects. The results are consistent with dichroic extinction mechanisms generating the high degrees of CP in star-forming regions.Peer reviewe
Magnetic Field Structure of the HH 1-2 Region: Near-Infrared Polarimetry of Point-Like Sources
The HH 1-2 region in the L1641 molecular cloud was observed in the near-IR J,
H, and Ks bands, and imaging polarimetry was performed. Seventy six point-like
sources were detected in all three bands. The near-IR polarizations of these
sources seem to be caused mostly by the dichroic extinction. Using a
color-color diagram, reddened sources with little infrared excess were selected
to trace the magnetic field structure of the molecular cloud. The mean
polarization position angle of these sources is about 111 deg, which is
interpreted as the projected direction of the magnetic field in the observed
region of the cloud. The distribution of the polarization angle has a
dispersion of about 11 deg, which is smaller than what was measured in previous
studies. This small dispersion gives a rough estimate of the strength of the
magnetic field to be about 130 microG and suggests that the global magnetic
field in this region is quite regular and straight. In contrast, the outflows
driven by young stellar objects in this region seem to have no preferred
orientation. This discrepancy suggests that the magnetic field in the L1641
molecular cloud does not dictate the orientation of the protostars forming
inside.Comment: To appear in the Astrophysical Journa
Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457
Three dimensional (3D) magnetic field information on molecular clouds and
cores is important for revealing their kinematical stability (magnetic support)
against gravity which is fundamental for studying the initial conditions of
star formation. In the present study, the 3D magnetic field structure of the
dense starless core FeSt 1-457 is determined based on the near-infrared
polarimetric observations of the dichroic polarization of background stars and
simple 3D modeling. With an obtained angle of line-of-sight magnetic
inclination axis of and previously
determined plane-of-sky magnetic field strength of
, the total magnetic field strength for FeSt 1-457 is derived to be
. The critical mass of FeSt 1-457, evaluated using
both magnetic and thermal/turbulent support is
, which is identical to the observed core mass, . We thus conclude that the stability of
FeSt 1-457 is in a condition close to the critical state. Without infalling gas
motion and no associated young stars, the core is regarded to be in the
earliest stage of star formation, i.e., the stage just before the onset of
dynamical collapse following the attainment of a supercritical condition. These
properties would make FeSt 1-457 one of the best starless cores for future
studies of the initial conditions of star formation.Comment: Accepted to the Astrophysical Journal (ApJ
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
Near-Infrared Imaging Polarimetry of S106 Cluster-Forming Region with SIRPOL
We present the results of wide-field JHKs polarimetry toward the HII region
S106 using the IRSF (Infrared Survey Facility) telescope. Our polarimetry data
revealed an extended (up to ~ 5') polarized nebula over S106. We confirmed the
position of the illuminating source of most of the nebula as consistent with
S106 IRS4 through an analysis of polarization vectors. The bright portion of
the polarized intensity is consistent with the red wing component of the
molecular gas. Diffuse polarized intensity emission is distributed along the
north--south molecular gas lanes. We found the interaction region between the
radiation from S106 IRS4 and the dense gas. In addition, we also discovered two
small polarization nebulae, SIRN1 and SIRN2, associated with a young stellar
objects (YSO). Aperture polarimetry of point-like sources in this region was
carried out for the first time. The regional magnetic field structures were
derived using point-like source aperture polarimetry, and the magnetic field
structure position angle around the cluster region in S106 was found to be ~
120\arcdeg. The magnetic fields in the cluster region, however, have three
type position angles: ~ 20\arcdeg, ~ 80\arcdeg, and ~ 120\arcdeg. The
present magnetic field structures are consistent with results obtained by
submillimeter continuum observations. We found that the magnetic field
direction in the dense gas region is not consistent with that of the low
density gas region.Comment: 22 pages, 11 figures, accepted for publication in A
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