4,112 research outputs found
Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex
We carry out three-dimensional MHD simulations of star formation in
turbulent, magnetized clouds, including ambipolar diffusion and feedback from
protostellar outflows. The calculations focus on relatively diffuse clouds
threaded by a strong magnetic field capable of resisting severe tangling by
turbulent motions and retarding global gravitational contraction in the
cross-field direction. They are motivated by observations of the Taurus
molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an
ordered large-scale magnetic field, as well as elongated condensations that are
generally perpendicular to the large-scale field. We find that stars form in
earnest in such clouds when enough material has settled gravitationally along
the field lines that the mass-to-flux ratios of the condensations approach the
critical value. Only a small fraction (of order 1% or less) of the nearly
magnetically-critical, condensed material is turned into stars per local
free-fall time, however. The slow star formation takes place in condensations
that are moderately supersonic; it is regulated primarily by magnetic fields,
rather than turbulence. The quiescent condensations are surrounded by diffuse
halos that are much more turbulent, as observed in the Taurus complex. Strong
support for magnetic regulation of star formation in this complex comes from
the extremely slow conversion of the already condensed, relatively quiescent
CO gas into stars, at a rate two orders of magnitude below the maximum,
free-fall value. We analyze the properties of dense cores, including their mass
spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap
Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation
The efficiency of star formation, defined as the ratio of the stellar to
total (gas and stellar) mass, is observed to vary from a few percent in regions
of dispersed star formation to about a third in cluster-forming cores. This
difference may reflect the relative importance of magnetic fields and
turbulence in controlling star formation. We investigate the interplay between
supersonic turbulence and magnetic fields using numerical simulations, in a
sheet-like geometry. We demonstrate that star formation with an efficiency of a
few percent can occur over several gravitational collapse times in moderately
magnetically subcritical clouds that are supersonically turbulent. The
turbulence accelerates star formation by reducing the time for dense core
formation. The dense cores produced are predominantly quiescent, with subsonic
internal motions. These cores tend to be moderately supercritical. They have
lifetimes long compared with their local gravitational collapse time. Some of
the cores collapse to form stars, while others disperse away without star
formation. In turbulent clouds that are marginally magnetically supercritical,
the star formation efficiency is higher, but can still be consistent with the
values inferred for nearby embedded clusters. If not regulated by magnetic
fields at all, star formation in a multi-Jeans mass cloud endowed with a strong
initial turbulence proceeds rapidly, with the majority of cloud mass converted
into stars in a gravitational collapse time. The efficiency is formally higher
than the values inferred for nearby cluster-forming cores, indicating that
magnetic fields are dynamically important even for cluster formation.Comment: submitted to Ap
Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores
We use magnetic collapse models to place some constraints on the formation
and angular momentum evolution of circumstellar disks which are embedded in
magnetized cloud cores. Previous models have shown that the early evolution of
a magnetized cloud core is governed by ambipolar diffusion and magnetic
braking, and that the core takes the form of a nonequilibrium flattened
envelope which ultimately collapses dynamically to form a protostar. In this
paper, we focus on the inner centrifugally-supported disk, which is formed only
after a central protostar exists, and grows by dynamical accretion from the
flattened envelope. We estimate a centrifugal radius for the collapse of mass
shells within a rotating, magnetized cloud core. The centrifugal radius of the
inner disk is related to its mass through the two important parameters
characterizing the background medium: the background rotation rate \Omb and
the background magnetic field strength \Bref. We also revisit the issue of
how rapidly mass is deposited onto the disk (the mass accretion rate) and use
several recent models to comment upon the likely outcome in magnetized cores.
Our model predicts that a significant centrifugal disk (much larger than a
stellar radius) will be present in the very early (Class 0) stage of
protostellar evolution. Additionally, we derive an upper limit for the disk
radius as it evolves due to internal torques, under the assumption that the
star-disk system conserves its mass and angular momentum even while most of the
mass is transferred to a central star.Comment: 23 pages, 1 figure, aastex, to appear in the Astrophysical Journal
(10 Dec 1998
Possible association of CUX1 gene polymorphisms with antidepressant response in major depressive disorder
Association between response to antidepressant treatment and genetic polymorphisms was examined in two independent Japanese samples of patients with major depressive disorder (MDD). Genome-wide approach using the Illumina Human CNV370-quad Bead Chip was utilized in the analysis of the 92 MDD patients in the first sample. In all, 11 non-intergenic single-nucleotide polymorphisms with uncorrected allelic P-value <0.0001 were selected for the subsequent association analyses in the second sample of 136 MDD patients. Difference in allele distribution between responders and nonresponders were found in the second-stage sample for rs365836 and rs201522 of the CUX1 gene (P=0.005 and 0.004, respectively). The allelic P-values for rs365836 and rs201522 in both samples combined were 0.0000023 and 0.0000040, respectively. Our results provide the first evidence that polymorphisms of the CUX1 gene may be associated with response to antidepressant treatment in Japanese patients with MDD.ArticlePHARMACOGENOMICS JOURNAL. 13(4):354-358 (2013)journal articl
The photometric properties of a vast stellar substructure in the outskirts of M33
We have surveyed sq.degrees surrounding M33 with CFHT MegaCam in the
g and i filters, as part of the Pan-Andromeda Archaeological Survey. Our
observations are deep enough to resolve the top 4mags of the red giant branch
population in this galaxy. We have previously shown that the disk of M33 is
surrounded by a large, irregular, low-surface brightness substructure. Here, we
quantify the stellar populations and structure of this feature using the PAndAS
data. We show that the stellar populations of this feature are consistent with
an old population with dex and an interquartile range in
metallicity of dex. We construct a surface brightness map of M33 that
traces this feature to mags\,arcsec. At these low surface
brightness levels, the structure extends to projected radii of kpc from
the center of M33 in both the north-west and south-east quadrants of the
galaxy. Overall, the structure has an "S-shaped" appearance that broadly aligns
with the orientation of the HI disk warp. We calculate a lower limit to the
integrated luminosity of the structure of mags, comparable to a
bright dwarf galaxy such as Fornax or AndII and slightly less than $1\$ of the
total luminosity of M33. Further, we show that there is tentative evidence for
a distortion in the distribution of young stars near the edge of the HI disk
that occurs at similar azimuth to the warp in HI. The data also hint at a
low-level, extended stellar component at larger radius that may be a M33 halo
component. We revisit studies of M33 and its stellar populations in light of
these new results, and we discuss possible formation scenarios for the vast
stellar structure. Our favored model is that of the tidal disruption of M33 in
its orbit around M31.Comment: Accepted for publication in ApJ. 17 figures. ApJ preprint forma
Parker Instability in a Self-Gravitating Magnetized Gas Disk: I. Linear Stability Analysis
To be a formation mechanism of such large-scale structures as giant molecular
clouds (GMCs) and HI superclouds, the classical Parker instability driven by
external gravity has to overcome three major obstacles: The convective motion
accompanying the instability generates thin sheets than large condensations.
The degree of density enhancement achieved by the instability is too low to
make dense interstellar clouds. The time and the length scales of the
instability are significantly longer and larger than the estimated formation
time and the observed mean separation of the GMCs, respectively. This paper
examines whether a replacement of the driving agent from the external to the
self gravity might remove these obstacles by activating the gravitational
instability in the Galactic ISM disk. The self gravity can suppress the
convective motions, and a cooperative action of the Jeans and the Parker
instabilities can remove all the obstacles confronting the classical version of
the Parker instability. The mass and mean separation of the structures
resulting from the odd-parity undular mode solution are shown to agree better
with the HI superclouds than with the GMCs. We briefly discuss how inclusions
of the external gravity and cosmic rays would modify behaviors of the
odd-parity undular mode solution.Comment: 53 pages, 21 figure
Three Dimensional Structures of Particles Recovered from the Asteroid Itokawa by the Hayabusa Mission and a Role of X-Ray Microtomography in the Preliminary Examination
Particles of regolith on S-type Asteroid 25143 Itokawa were successfully recovered by the Hayabusa mission of JAXA (Japan Aerospace Exploration Agency). Near-infrared spectral study of Itokawa s surface indicates that these particles are materials similar to LL5 or LL6 chondrites. High-resolution images of Itokawa's surface suggest that they may be breccias and some impact products. At least more than 1500 particles were identified as Itokawa origin at curation facility of JAXA. Preliminary analysis with SEM/EDX at the curation facility shows that they are roughly similar to LL chondrites. Although most of them are less than 10 micron in size, some larger particles of about 100 micron or larger were also identified. A part of the sample (probably several tens particles) will be selected by Hayabusa sample curation team, and sequential examination will start from January 2011 by Hayabusa Asteroidal Sample Preliminary Examination Team (HASPET). In mainstream of the analytical flow, each particle will be examined by microtomography, XRD and XRF first as nondestructive analyses, and then the particle will be cut by an ultra-microtome and examined by TEM, SEM, EPMA, SIMS, PEEM/XANES, and TOF-SIMS sequentially. Three-dimensional structures of Itokawa particles will be obtained by microtomography sub-team of HASPET. The results together with XRD and XRF will be used for design of later destructive analyses, such as determination of cutting direction and depth, to obtain as much information as possible from small particles. Scientific results and a role of the microtomography in the preliminary examination will be presented
Spin Pseudo Gap in La2-xSrxCuO4 Studied by Neutron Scattering
Spin excitations of La2-xSrxCuO have been studied using inelastic neutron
scattering techniques in the energy range of 2 meV =< w =< 12 meV and the
temperature range of 8 K =< T =< 150 K. We observed a signature of a spin
pseudo gap in the excitation spectrum above Tc for the slightly overdoped
sample with x = 0.18. On heating, the spin pseudo gap gradually collapses
between T = 80 K and 150 K. For the x = 0.15 and 0.20, although the visibility
of gap-like structure at T ~ Tc is lower compared to the x = 0.18 sample, the
broad bump of kai"(w) appears at w ~ 5 meV,close to the spin-gap energy at base
temperature, suggests the existence of the spin pseudo gap in the normal state.Comment: revtex, 7 pages, 8 eps figures, PRB (2003) in pres
Observation of Tau Neutrino Interactions
The DONUT experiment has analyzed 203 neutrino interactions recorded in
nuclear emulsion targets. A decay search has found evidence of four tau
neutrino interactions with an estimated background of 0.34 events. This number
is consistent with the Standard Model expectation.Comment: 12 pages, 3 figures, PD
Performance and calibration of the CHORUS scintillating fiber tracker and opto-electronics readout system
An essential component of the CERN WA95/CHORUS experiment is a scintillating fiber tracker system for precise track reconstruction of particles. The tracker design, its opto-electronics readout and calibration system are discussed. Performances of the detector are presented
- …