60 research outputs found
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 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
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
Representation Theorem and Green’s Function : Finite Domain Representation of Elastic Field in terms of Green’s Function in Infinite Medium
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
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
The Distance to the Galactic Center Derived From Infrared Photometry of Bulge Red Clump Stars
On the basis of the near infrared observations of bulge red clump stars near
the Galactic center, we have determined the galactocentric distance to be R_0 =
7.52 +- 0.10 (stat) +- 0.35 (sys) kpc. We observed the red clump stars at |l| <
1.0 deg and 0.7 deg < |b| < 1.0 deg with the IRSF 1.4 m telescope and the
SIRIUS camera in the H and Ks bands. After extinction and population
corrections, we obtained (m - M)_0 = 14.38 +- 0.03 (stat) +- 0.10 (sys). The
statistical error is dominated by the uncertainty of the intrinsic local red
clump stars' luminosity. The systematic error is estimated to be +- 0.10
including uncertainties in extinction and population correction, zero-point of
photometry, and the fitting of the luminosity function of the red clump stars.
Our result, R_0 = 7.52 kpc, is in excellent agreement with the distance
determined geometrically with the star orbiting the massive black hole in the
Galactic center. The recent result based on the spatial distribution of
globular clusters is also consistent with our result. In addition, our study
exhibits that the distance determination to the Galactic center with the red
clump stars, even if the error of the population correction is taken into
account, can achieve an uncertainty of about 5%, which is almost the same level
as that in recent geometrical determinations.Comment: 14 pages, 4 figures, accepted by Ap
Interstellar Extinction Law in the J, H, and Ks Bands toward the Galactic Center
We have determined the ratios of total to selective extinction in the
near-infrared bands (J, H, Ks) toward the Galactic center from the observations
of the region |l| < 2.0deg and 0.5deg < |b| < 1.0deg with the IRSF telescope
and the SIRIUS camera. Using the positions of red clump stars in
color-magnitude diagrams as a tracer of the extinction and reddening, we
determine the average of the ratios of total to selective extinction to be
A(Ks)/E(H-Ks) = 1.44+-0.01, A(Ks)/E(J-Ks) = 0.494+-0.006, and A(H)/E(J-H) =
1.42+-0.02, which are significantly smaller than those obtained in previous
studies. From these ratios, we estimate that A(J) : A(H) : A(Ks) = 1 :
0.573+-0.009 : 0.331+-0.004 and E(J-H)/E(H-Ks) = 1.72+-0.04, and we find that
the power law A(lambda) \propto lambda^{-1.99+-0.02} is a good approximation
over these wavelengths. Moreover, we find a small variation in A(Ks)/E(H-Ks)
across our survey. This suggests that the infrared extinction law changes from
one line of sight to another, and the so-called ``universality'' does not
necessarily hold in the infrared wavelengths.Comment: 18 pages, 9 figures, Accepted for publication in the Ap
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