611 research outputs found
Comparison of the histological and immunohistochemical features of the thymus in young- and elderly-onset myasthenia gravis without thymoma
ArticleJOURNAL OF CLINICAL NEUROSCIENCE. 14(2): 110-115 (2007)journal articl
Arm & Interarm Star Formation in Spiral Galaxies
We investigate the relationship between spiral arms and star formation in the
grand-design spirals NGC 5194 and NGC 628 and in the flocculent spiral NGC
6946. Filtered maps of near-IR (3.6 micron) emission allow us to identify "arm
regions" that should correspond to regions of stellar mass density
enhancements. The two grand-design spirals show a clear two-armed structure,
while NGC 6946 is more complex. We examine these arm and interarm regions,
looking at maps that trace recent star formation - far-ultraviolet (GALEX NGS)
and 24 micron emission (Spitzer, SINGS) - and cold gas - CO (Heracles) and HI
(Things). We find the star formation tracers and CO more concentrated in the
spiral arms than the stellar 3.6 micron flux. If we define the spiral arms as
the 25% highest pixels in the filtered 3.6 micron images, we find that the
majority (60%) of star formation tracers occurs in the interarm regions; this
result persists qualitatively even when considering the potential impact of
finite data resolution and diffuse interarm 24 micron emission. Even with a
generous definition of the arms (45% highest pixels), interarm regions still
contribute at least 30% to the integrated star formation rate tracers. We look
for evidence that spiral arms trigger star or cloud formation using the ratios
of star formation rate (SFR, traced by a combination of FUV and 24 micron
emission) to H_2 (traced by CO) and H_2 to HI. Any enhancement of SFR / M(H_2)
in the arm region is very small (less than 10%) and the grand design spirals
show no enhancement compared to the flocculent target. Arm regions do show a
weak enhancement in H_2/HI compared to the interarm regions, but at a fixed gas
surface density there is little clear enhancement in the H_2/HI ratio in the
arm regions. Thus, it seems that spiral arms may only act to concentrate the
gas to higher densities in the arms.Comment: 11 pages, 9 Figures, accepted by Ap
The properties and polarization of the H2O and CH3OH maser environment of NGC7538-IRS1
NGC7538 is a complex massive star-forming region. The region is composed of
several radio continuum sources, one of which is IRS1, a high-mass protostar,
from which a 0.3 pc molecular bipolar outflow was detected. Several maser
species have been detected around IRS1. The CH3OH masers have been suggested to
trace a Keplerian-disk, while the H2O masers are almost aligned to the outflow.
More recent results suggested that the region hosts a torus and potentially a
disk, but with a different inclination than the Keplerian-disk that is supposed
to be traced by the CH3OH masers. Tracing the magnetic field close to
protostars is fundamental for determining the orientation of the disk/torus.
Recent studies showed that during the protostellar phase of high-mass star
formation the magnetic field is oriented along the outflows and around or on
the surfaces of the disk/torus. The observations of polarized maser emissions
at milliarcsecond resolution can make a crucial contribution to understanding
the orientation of the magnetic field and, consequently, the orientation of the
disk/torus in NGC7538-IRS1. The NRAO Very Long Baseline Array was used to
measure the linear polarization and the Zeeman-splitting of the 22GHz H2O
masers toward NGC7538-IRS1. The European VLBI Network and the MERLIN telescopes
were used to measure the linear polarization and the Zeeman-splitting of the
6.7GHz CH3OH masers toward the same region. We detected 17 H2O masers and 49
CH3OH masers at high angular resolution. We detected linear polarization
emission toward two H2O masers and toward twenty CH3OH masers. The CH3OH
masers, most of which only show a core structure, seem to trace rotating and
potentially infalling gas in the inner part of a torus. Significant
Zeeman-splitting was measured in three CH3OH masers. [...] We also propose a
new description of the structure of the NGC7538-IRS1 maser region.Comment: 13 pages, 9 figures, 4 Tables, accepted by Astronomy & Astrophysic
Probing the Protostellar Envelope around L1157: the Dust and Gas Connection
We present observations of the Class 0 protostar L1157-mm using the Combined
Array for Research in Millimeter-wave Astronomy (CARMA) in 3 mm dust continuum
and N2H+ line emission. In the N2H+ line, we detect a large-scale envelope
extended over a linear size of ~20,000AU flattened in the direction
perpendicular to the outflow. This N2H+ feature coincides with the outer
envelope seen in the 8 micron extinction by Looney et al. Meanwhile, the dust
continuum traces the compact, nearly spherical structure of the inner envelope,
where N2H+ becomes depleted. This highly flattened N2H+ envelope also shows
dynamical signatures consistent with gravitational infall in the inner region,
but a slow, solid-body rotation at large scales. This flattened structure is
not a rotationally supported circumstellar disk; instead, it resembles a
prestellar core both morphologically and kinematically, representing the early
phase of a Class 0 system. In this paper, we construct a simple model to
interpret both the dust continuum and N2H+ emission and suggest a possible
dynamical scenario for the overall properties of the envelope.Comment: Accepted for publication by the ApJ, 34 pages, 10 figures and 2
table
Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius
We report high-resolution (0.07 arcsec) near-infrared polarized intensity
images of the circumstellar disk around the star 2MASS J16042165-2130284
obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our
-band data, which clearly exhibits a resolved, face-on disk with a large
inner hole for the first time at infrared wavelengths. We detect the
centrosymmetric polarization pattern in the circumstellar material as has been
observed in other disks. Elliptical fitting gives the semimajor axis, semiminor
axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14
, respectively. The disk is asymmetric, with one dip located at P.A.s
of . Our observed disk size agrees well with a previous study
of dust and CO emission at submillimeter wavelength with Submillimeter Array.
Hence, the near-infrared light is interpreted as scattered light reflected from
the inner edge of the disk. Our observations also detect an elongated arc (50
AU) extending over the disk inner hole. It emanates at the inner edge of the
western side of the disk, extending inward first, then curving to the
northeast. We discuss the possibility that the inner hole, the dip, and the arc
that we have observed may be related to the existence of unseen bodies within
the disk.Comment: 21 pages, 3 figures, published 2012 November 7 by ApJL, typo
correcte
Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae
We report high-resolution 1.6 \micron polarized intensity () images of
the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of
22 AU () up to 554 AU (3.85), which have been obtained by the
high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed
complicated and asymmetrical structures in the inner part (140 AU) of
the disk, while confirming the previously reported outer ( 200 AU)
spiral structure. We have imaged a double ring structure at 40 and
100 AU and a ring-like gap between the two. We found a significant
discrepancy of inclination angles between two rings, which may indicate that
the disk of AB Aur is warped. Furthermore, we found seven dips (the typical
size is 45 AU or less) within two rings as well as three prominent
peaks at 40 AU. The observed structures, including a bumpy double ring, a
ring-like gap, and a warped disk in the innermost regions, provide essential
information for understanding the formation mechanism of recently detected
wide-orbit ( 20 AU) planets.Comment: 12 pages, 3 figure
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