10,699 research outputs found
Near-Infrared, Adaptive Optics Observations of the T Tauri Multiple-Star System
With high-angular-resolution, near-infrared observations of the young stellar
object T Tauri at the end of 2002, we show that, contrary to previous reports,
none of the three infrared components of T Tau coincide with the compact radio
source that has apparently been ejected recently from the system (Loinard,
Rodriguez, and Rodriguez 2003). The compact radio source and one of the three
infrared objects, T Tau Sb, have distinct paths that depart from orbital or
uniform motion between 1997 and 2000, perhaps indicating that their interaction
led to the ejection of the radio source. The path that T Tau Sb took between
1997 and 2003 may indicate that this star is still bound to the presumably more
massive southern component, T Tau Sa. The radio source is absent from our
near-infrared images and must therefore be fainter than K = 10.2 (if located
within 100 mas of T Tau Sb, as the radio data would imply), still consistent
with an identity as a low-mass star or substellar object.Comment: 11 pages, 3 figures, submitted to ApJ
Bogoliubov angle and visualization of particle-hole mixture in superconductors
Superconducting excitations --Bogoliubov quasiparticles -- are the quantum
mechanical mixture of negatively charged electron (-e) and positively charged
hole (+e). Depending on the applied voltage bias in STM one can sample the
particle and hole content of such a superconducting excitation. Recent Scanning
Tunneling Microscope (STM) experiments offer a unique insight into the inner
workings of the superconducting state of superconductors. We propose a new
observable quantity for STM studies that is the manifestation of the
particle-hole dualism of the quasiparticles. We call it a {\em Bogoliubov
angle}. This angle measures the relative weight of particle and hole amplitude
in the superconducting (Bogoliubov) quasiparticle. We argue that this quantity
can be measured locally by comparing the ratio of tunneling currents at
positive and negative biases. This Bogoliubov angle allows one to measure
directly the energy and position dependent particle-hole admixture and
therefore visualize robustness of superconducting state locally. It may also
allow one to measure the particle-hole admixture of excitations in normal state
above critical temperature and thus may be used to measure superconducting
correlations in pseudogap state.Comment: 16 pages, latex file, 9 eps figure
Winds from accretion disks driven by the radiation and magnetocentrifugal force
We study the 2-D, time-dependent hydrodynamics of radiation-driven winds from
luminous accretion disks threaded by a strong, large-scale, ordered magnetic
field. The radiation force is due to spectral lines and is calculated using a
generalized multidimensional formulation of the Sobolev approximation. The
effects of the magnetic field are approximated by adding a force that emulates
a magnetocentrifugal force. Our approach allows us to calculate disk winds when
the magnetic field controls the flow geometry, forces the flow to corotate with
the disk, or both. In particular, we calculate models where the lines of the
poloidal component of the field are straight and inclined to the disk at a
fixed angle. Our numerical calculations show that flows which corotate with the
disk have a larger mass loss rate than their counterparts which conserve
specific angular momentum. The difference in the mass loss rate between these
two types of winds can be several orders of magnitude for low disk luminosities
but vanishes for high disk luminosities. Winds which corotate with the disk
have much higher velocities than angular momentum conserving winds. Fixing the
wind geometry stabilizes winds which are unsteady when the geometry is derived
self-consistently. The inclination angle between the poloidal velocity and the
normal to the disk midplane is important. Non-zero inclination angles allow the
magnetocentrifugal force to increase the mass loss rate for low luminosities,
and increase the wind velocity for all luminosities. Our calculations also show
that the radiation force can launch winds from magnetized disks. The line force
can be essential in producing MHD winds from disks where the thermal energy is
too low to launch winds or where the field lines make an angle of < 30^o with
respect to the normal to the disk.Comment: LaTeX, 11 pages, 6 color postscript or PJEG files, to appear in Ap
The Extraordinary Mid-infrared Spectrum of the Blue Compact Dwarf Galaxy SBS0335-052
SBS0335-052 is a blue compact dwarf galaxy (BCD) with one of the lowest known
metallicities, ZZ_{\sun}/41, making it a local example of how
primordial starburst galaxies and their precursors might appear. A spectrum
obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope
clearly shows silicate absorption features, emission lines of [SIV] and
[NeIII], and puts strong upper limits on the PAH emission features. The
observed low resolution spectrum (R~90) extends from 5.3 to 35microns and peaks
at ~28microns. The spectrum is compared to IRS observations of the prototypical
starburst nucleus NGC7714. SBS0335-052 is quite unlike normal starburst
galaxies, which show strong PAH bands, low ionization emission lines, and a
continuum peak near 80microns. The continuum difference for m
implies a substantial reduction in the mass of cold dust. If the spectrum of
this very low metallicity galaxy is representative of star forming galaxies at
higher redshifts, it may be difficult to distinguish them from AGNs which also
show relatively featureless flat spectra in the mid-IR.Comment: Accepted in ApJ Sup. Spitzer Special Issue, 4 pages, 2 figure
Site-Dilution in quasi one-dimensional antiferromagnet Sr2(Cu1-xPdx)O3: reduction of Neel Temperature and spatial distribution of ordered moment sizes
We investigate the Neel temperature of Sr2CuO3 as a function of the site
dilution at the Cu (S=1/2) sites with Pd (S=0), utilizing the muon spin
relaxation (muSR) technique. The Neel temperature, which is Tn=5.4K for the
undoped system, becomes significantly reduced for less than one percent of
doping Pd, giving a support for the previous proposal for the good
one-dimensionality. The Pd concentration dependence of the Neel temperature is
compared with a recent theoretical study (S. Eggert, I. Affleck and M.D.P.
Horton, Phys. Rev. Lett. 89, 47202 (2002)) of weakly coupled one-dimensional
antiferromagnetic chains of S=1/2 spins, and a quantitative agreement is found.
The inhomogeneity of the ordered moment sizes is characterized by the muSR time
spectra. We propose a model that the ordered moment size recovers away from the
dopant S=0 sites with a recovery length of \xi = 150-200 sites. The origin of
the finite recovery length \xi for the gapless S=1/2 antiferromagnetic chain is
compared to the estimate based on the effective staggered magnetic field from
the neighboring chains.Comment: 10 pages, 9 figures, submitted to PR
The Birth of High Mass Stars: Accretion and/or Mergers?
The observational consequences of the merger scenario for massive star
formation are explored and contrasted with the gradual accumulation of mass by
accretion. Protostellar mergers may produce high luminosity infrared flares
lasting years to centuries followed by a luminosity decline on the
Kelvin-Helmholtz time-scale of the merger product. Mergers may be surrounded by
thick tori of expanding debris, impulsive wide-angle outflows, and shock
induced maser and radio continuum emission. Collision products are expected to
have fast stellar rotation and a large multiplicity fraction. Close encounters
or mergers will produce circumstellar debris disks with an orientation that
differs form that of a pre-existing disk. The extremely rare merger of two
stars close to the upper-mass end of the IMF may be a possible pathway to
hypernova generated gamma-ray bursters. While accretional growth can lead to
the formation of massive stars in isolation or in loose clusters, mergers can
only occur in high-density cluster environments. It is proposed that the
outflow emerging from the OMC1 core in the Orion molecular cloud was produced
by a protostellar merger that released between to ergs less
than a thousand years ago
Doping-dependent nodal Fermi velocity in Bi-2212 revealed by high-resolution ARPES
The improved resolution of laser-based angle-resolved photoemission
spectroscopy (ARPES) allows reliable access to fine structures in the spectrum.
We present a systematic, doping-dependent study of a recently discovered
low-energy kink in the nodal dispersion of Bi2Sr2CaCu2O8+d (Bi-2212), which
demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212.
The renormalization of the nodal velocity due to this kink becomes stronger
with underdoping, revealing that the nodal Fermi velocity is non-universal, in
contrast to assumed phenomenology. This is used together with laser-ARPES
measurements of the gap velocity, v2, to resolve discrepancies with thermal
conductivity measurements.Comment: Submitted to Phys. Rev. Let
Flow profiling of a surface acoustic wave nanopump
The flow profile in a capillary gap and the pumping efficiency of an acoustic
micropump employing Surface Acoustic Waves is investigated both experimentally
and theoretically. Such ultrasonic surface waves on a piezoelectric substrate
strongly couple to a thin liquid layer and generate an internal streaming
within the fluid. Such acoustic streaming can be used for controlled agitation
during, e.g., microarray hybridization. We use fluorescence correlation
spectroscopy and fluorescence microscopy as complementary tools to investigate
the resulting flow profile. The velocity was found to depend on the applied
power somewhat weaker than linearly and to decrease fast with the distance from
the ultrasound generator on the chip.Comment: 12 pages 20 figure
Autoimmune hyperphosphatemic tumoral calcinosis in a patient with FGF23 autoantibodies
Hyperphosphatemic familial tumoral calcinosis (HFTC)/hyperostosis-hyperphosphatemia syndrome (HHS) is an autosomal recessive disorder of ectopic calcification due to deficiency of or resistance to intact fibroblast growth factor 23 (iFGF23). Inactivating mutations in FGF23, N-acetylgalactosaminyltransferase 3 (GALNT3), or KLOTHO (KL) have been reported as causing HFTC/HHS. We present what we believe is the first identified case of autoimmune hyperphosphatemic tumoral calcinosis in an 8-year-old boy. In addition to the classical clinical and biochemical features of hyperphosphatemic tumoral calcinosis, the patient exhibited markedly elevated intact and C-terminal FGF23 levels, suggestive of FGF23 resistance. However, no mutations in FGF23, KL, or FGF receptor 1 (FGFR1) were identified. He subsequently developed type 1 diabetes mellitus, which raised the possibility of an autoimmune cause for hyperphosphatemic tumoral calcinosis. Luciferase immunoprecipitation systems revealed markedly elevated FGF23 autoantibodies without detectable FGFR1 or Klotho autoantibodies. Using an in vitro FGF23 functional assay, we found that the FGF23 autoantibodies in the patient's plasma blocked downstream signaling via the MAPK/ERK signaling pathway in a dose-dependent manner. Thus, this report describes the first case, to our knowledge, of autoimmune hyperphosphatemic tumoral calcinosis with pathogenic autoantibodies targeting FGF23. Identification of this pathophysiology extends the etiologic spectrum of hyperphosphatemic tumoral calcinosis and suggests that immunomodulatory therapy may be an effective treatment
- âŠ