89 research outputs found
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
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
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
Distortion of Magnetic Fields in a Starless Core: Near-Infrared Polarimetry of FeSt 1-457
Magnetic fields are believed to play an important role in controlling the
stability and contraction of dense condensations of gas and dust leading to the
formation of stars and planetary systems. In the present study, the magnetic
field of FeSt 1-457, a cold starless molecular cloud core, was mapped on the
basis of the polarized near-infrared light from 185 background stars after
being dichroically absorbed by dust aligned with the magnetic field in the
core. A distinct "hourglass-shaped" magnetic field was identified in the region
of the core, which was interpreted as the first evidence of a magnetic field
structure distorted by mass condensation in a starless core. The steep
curvature of the magnetic field lines obtained in the present study indicates
that the distortion was mainly created during the formation phase of the dense
core. The derived mass-to-magnetic flux ratio indicates that the core is in a
magnetically supercritical state. However, the stability of the core can be
considered to be in a nearly critical state if the additional contributions
from the thermal and turbulent support are included. Further diffusion of the
magnetic field and/or turbulent dissipation would cause the onset of dynamical
collapse of the core. The geometrical relationship between the direction of the
magnetic field lines and the elongation of the core was found to be in good
agreement with the theoretical predictions for the formation of Sun-like stars
under the influence of a magnetic field.Comment: Accepted to the Astrophysical Journal (ApJ
A lack of classical Cepheids in the inner part of the Galactic disk
Recent large-scale infrared surveys have been revealing stellar populations
in the inner Galaxy seen through strong interstellar extinction in the disk. In
particular, classical Cepheids with their period-luminosity and period-age
relations are useful tracers of Galactic structure and evolution. Interesting
groups of Cepheids reported recently include four Cepheids in the Nuclear
Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga
et al. and those spread across the inner part of the disk reported by Dekany
and collaborators. We here report our discovery of nearly thirty classical
Cepheids towards the bulge region, some of which are common with Dekany et al.,
and discuss the large impact of the reddening correction on distance estimates
for these objects. Assuming that the four Cepheids in the NSD are located at
the distance of the Galactic Centre and that the near-infrared extinction law,
i.e. wavelength dependency of the interstellar extinction, is not
systematically different between the NSD and other bulge lines-of-sight, most
of the other Cepheids presented here are located significantly further than the
Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region
of the Galactic disk except the NSD. Recent radio observations show a similar
distribution of star-forming regions.Comment: 8 pages, 4 figures, accepted for publication in MNRA
Cepheids and other short-period variables near the Galactic Centre
We report the result of our near-infrared survey of short-period variable
stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre.
Forty-five variables are discovered and we classify the variables based on
their light curve shapes and other evidence. In addition to 3 classical
Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing
binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one
Cepheid-like variable whose nature is uncertain. Eclipsing binaries are
separated into the foreground objects and those significantly obscured by
interstellar extinction. One of the reddened binaries contains an O-type
supergiant and its light curve indicates an eccentric orbit. We discuss the
nature and distribution of type II Cepheids as well as the distance to the
Galactic Centre based on these Cepheids and other distance indicators. The
estimates of R0(GC) we obtained based on photometric data agree with previous
results obtained with kinematics of objects around the GC. Furthermore, our
result gives a support to the reddening law obtained by Nishiyama and
collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would
result in a rather different distance estimate.Comment: 13 pages, 10 figures, 7tables, accepted for publication in MNRA
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
A Chain of Dark Clouds in Projection Against the Galactic Center
In the J, H, and Ks bands survey of the the Galactic Center region over an
area of 2deg x 5deg, we have found many dark clouds, among which a
distinguished chain of dark clouds can be identified with a quiescent CO cloud.
The distances of the clouds is estimated to be 3.2-4.2 kpc, corresponding to
the Norma arm by our new method to determine distance to dark clouds using the
cumulative number of stars against J-Ks colors. Adopting these estimated
distances, the size is about 70 pc in length and the total mass of the cloud is
6x10^4 M_solar. Three compact HII regions harbor in the cloud, indicating that
star forming activities are going on at the cores of the quiescent CO cloud on
the spiral arm.Comment: 13 pages, 7 figures, accepted for publication in PAS
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
Variable Stars in the Magellanic Clouds: Results from OGLE and SIRIUS
We have performed a cross-identification between OGLE-II data and
single-epoch SIRIUS JHK survey data in the LMC and SMC. After eliminating
obvious spurious variables, we determined the pulsation periods for 9,681 and
2,927 variables in the LMC and SMC, respectively. Based on these homogeneous
data, we studied the pulsation properties and metallicity effects on period-K
magnitude (PK) relations by comparing the variable stars in the LMC and SMC.
The sample analyzed here is much larger, and we found the following new
features: (1) variable red giants in the SMC form parallel sequences on the PK
plane, just like those found by Wood (2000) in the LMC; (2) both of the
sequences A and B of Wood (2000) have discontinuities, and they occur at the
K-band luminosity of the TRGB; (3) the sequence B of Wood (2000) separates into
three independent sequences B+- and C'; (4) comparison between the theoretical
pulsation models (Wood et al. 1996) and observational data suggests that the
variable red giants on sequences C and newly discovered C' are pulsating in the
fundamental and first overtone mode, respectively; (5) the theory can not
explain the pulsation mode of sequences A+- and B+-, and they are unlikely to
be the sequences for the first and second overtone pulsators, as was previously
suggested; (6) the zero points of PK relations of Cepheids in the metal
deficient SMC are fainter than those of LMC ones by ~0.1 mag but those of SMC
Miras are brighter than those of LMC ones by ~0.13 mag, which are probably due
to metallicity effects.Comment: 9 pages, 10 figures, accepted for publication in MNRAS. High
resolution version is available at:
http://www.ioa.s.u-tokyo.ac.jp/~yita/scr/astro/papers/RefereedPaper/yitaMD250
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