8,841 research outputs found
Quantum discord in spin-cluster materials
The total quantum correlation (discord) in Heisenberg dimers is expressed via
the spin-spin correlation function, internal energy, specific heat or magnetic
susceptibility. This allows one to indirectly measure the discord through
neutron scattering, as well as calorimetric or magnetometric experiments. Using
the available experimental data, we found the discord for a number of binuclear
Heisenberg substances with both antiferro- and ferromagnetic interactions. For
the dimerized antiferromagnet copper nitrate Cu(NO_3)_2*2.5H_2O, the three
independent experimental methods named above lead to a discord of approximately
0.2-0.3 bit/dimer at a temperature of 4 K. We also determined the temperature
behavior of discord for hydrated and anhydrous copper acetates, as well as for
the ferromagnetic binuclear copper acetate complex [Cu_2L(OAc)]*6H_2O, where L
is a ligand.Comment: 7 pages, 6 figure
The shapes of Milky Way satellites: looking for signatures of tidal stirring
We study the shapes of Milky Way satellites in the context of the tidal
stirring scenario for the formation of dwarf spheroidal galaxies. The standard
procedures used to measure shapes involve smoothing and binning of data and
thus may not be sufficient to detect structural properties like bars, which are
usually subtle in low surface brightness systems. Taking advantage of the fact
that in nearby dwarfs photometry of individual stars is available we introduce
discrete measures of shape based on the two-dimensional inertia tensor and the
Fourier bar mode. We apply these measures of shape first to a variety of
simulated dwarf galaxies formed via tidal stirring of disks embedded in dark
matter halos and orbiting the Milky Way. In addition to strong mass loss and
randomization of stellar orbits, the disks undergo morphological transformation
that typically involves the formation of a triaxial bar after the first
pericenter passage. These tidally induced bars persist for a few Gyr before
being shortened towards a more spherical shape if the tidal force is strong
enough. We test this prediction by measuring in a similar way the shape of
nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in
Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of
eleven studied dwarfs show elongated stellar distributions in the outer parts
that may signify transition to tidal tails. We thus find the shapes of Milky
Way satellites to be consistent with the predictions of the tidal stirring
model.Comment: 14 pages, 11 figures, accepted for publication in Ap
Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Bootes
We report the results of a spectroscopic study of the Bootes (Boo) dwarf
spheroidal (dSph) galaxy carried out with the WIYN telescope and the Hydra
multifiber spectrograph. Radial velocities have been measured for 58 Boo
candidate stars selected to have magnitudes and colors consistent with its red
and asymptotic giant branches. Within the 13' half-light radius, seven members
of Boo yield a systemic velocity of V_r=95.6+-3.4 km/s and a velocity
dispersion of 6.6+-2.3 km/s. This implies a mass on the order of 1 x 10^7
M_sun, similar to the inferred masses of other Galactic dSphs. Adopting a total
Boo luminosity of L=1.8 x 10^4 L_sun to 8.6 x 10^4 L_sun implies M/L ~ 610 to
130, making Boo, the most distorted known Milky Way dwarf galaxy, potentially
also the darkest. From the spectra of Boo member stars we estimate its
metallicity to be [Fe/H] ~ -2.5, which would make it the most metal poor dSph
known to date.Comment: Accepted for publication in ApJ Letter
Kinematics in Kapteyn's Selected Area 76: Orbital Motions Within the Highly Substructured Anticenter Stream
We have measured the mean three-dimensional kinematics of stars in Kapteyn's
Selected Area (SA) 76 (l=209.3, b=26.4 degrees) that were selected to be
Anticenter Stream (ACS) members on the basis of their radial velocities, proper
motions, and location in the color-magnitude diagram. From a total of 31 stars
ascertained to be ACS members primarily from its main sequence turnoff, a mean
ACS radial velocity (derived from spectra obtained with the Hydra multi-object
spectrograph on the WIYN 3.5m telescope) of V_helio = 97.0 +/- 2.8 km/s was
determined, with an intrinsic velocity dispersion sigma_0 = 12.8 \pm 2.1 km/s.
The mean absolute proper motions of these 31 ACS members are mu_alpha cos
(delta) = -1.20 +/- 0.34 mas/yr and mu_delta = -0.78 \pm 0.36 mas/yr. At a
distance to the ACS of 10 \pm 3 kpc, these measured kinematical quantities
produce an orbit that deviates by ~30 degrees from the well-defined swath of
stellar overdensity constituting the Anticenter Stream in the western portion
of the Sloan Digital Sky Survey footprint. We explore possible explanations for
this, and suggest that our data in SA 76 are measuring the motion of a
kinematically cold sub-stream among the ACS debris that was likely a fragment
of the same infalling structure that created the larger ACS system. The ACS is
clearly separated spatially from the majority of claimed Monoceros ring
detections in this region of the sky; however, with the data in hand, we are
unable to either confirm or rule out an association between the ACS and the
poorly-understood Monoceros structure.Comment: Accepted to ApJ. 48 pages, 20 figures, preprint forma
Migrações na fase pelágica do Cherne, Polyprion americanus (Schneider, 1801), evidenciadas por marcação e recaptura
Vários chernes juvenis, Polyprion americanus (Schneider. 1801) foram capturados Ă superfĂcie da água e marcados em vários locais prĂłximos das ilhas do Grupo Central dos Açores. Um dos exemplares foi recapturado 3 meses depois ter sido marcado, a 217 km do local onde tinha sido libertado e a uma profundidade de 254 m. A recaptura deste espĂ©cime indica que o cherne passa para uma vida demersal quando atinge um comprimento total de cerca de 50 cm.ABSTRACT: Pelagic juvenile wreckfish, Polyprion americanus (Schneider, 1801) were tagged in the surface waters around the Central Group of the Azores. One wreckfish was recaptured three months after tagging, and 217 km from its release point. It had settled to the bottom in 254 m of water. The recapture of this specimen suggests that wreckfish take up a demersal life at a total length of about 50 cm
A Megacam Survey of Outer Halo Satellites. IV. Two foreground populations possibly associated with the Monoceros substructure in the direction of NGC2419 and Koposov2
The origin of the Galactic halo stellar structure known as the Monoceros ring
is still under debate. In this work, we study that halo substructure using deep
CFHT wide-field photometry obtained for the globular clusters NGC2419 and
Koposov2, where the presence of Monoceros becomes significant because of their
coincident projected position. Using Sloan Digital Sky Survey photometry and
spectroscopy in the area surrounding these globulars and beyond, where the same
Monoceros population is detected, we conclude that a second feature, not likely
to be associated with Milky Way disk stars along the line-of-sight, is present
as foreground population. Our analysis suggests that the Monoceros ring might
be composed of an old stellar population of age t ~ 9Gyr and a new component ~
4Gyr younger at the same heliocentric distance. Alternatively, this detection
might be associated with a second wrap of Monoceros in that direction of the
sky and also indicate a metallicity spread in the ring. The detection of such a
low-density feature in other sections of this halo substructure will shed light
on its nature.Comment: 10 pages, 10 figures, accepted for publication in Ap
Nonlinear response of single-molecule nanomagnets: equilibrium and dynamical
We present an experimental study of the {\em nonlinear} susceptibility of
Mn single-molecule magnets. We investigate both their
thermal-equilibrium and dynamical nonlinear responses. The equilibrium results
show the sensitivity of the nonlinear susceptibility to the magnetic
anisotropy, which is nearly absent in the linear response for axes distributed
at random. The nonlinear dynamic response of Mn was recently found to be
very large and displaying peaks reversed with respect to classical
superparamagnets [F. Luis {\em et al.}, Phys. Rev. Lett. {\bf 92}, 107201
(2004)]. Here we corroborate the proposed explanation -- strong field
dependence of the relaxation rate due to the detuning of tunnel energy levels.
This is done by studying the orientational dependence of the nonlinear
susceptibility, which permits to isolate the quantum detuning contribution.
Besides, from the analysis of the longitudinal and transverse contributions we
estimate a bound for the decoherence time due to the coupling to the phonon
bath.Comment: 13 pages, 8 figures, resubmitted to Phys. Rev. B with minor change
Gpr126/Adgrg6 has Schwann cell autonomous and nonautonomous functions in peripheral nerve injury and repair
Schwann cells (SCs) are essential for proper peripheral nerve development and repair, although the mechanisms regulating these processes are incompletely understood. We previously showed that the adhesion G protein-coupled receptor Gpr126/Adgrg6 is essential for SC development and myelination. Interestingly, the expression of Gpr126 is maintained in adult SCs, suggestive of a function in the mature nerve. We therefore investigated the role of Gpr126 in nerve repair by studying an inducible SC-specific Gpr126 knock-out mouse model. Here, we show that remyelination is severely delayed after nerve-crush injury. Moreover, we also observe noncell-autonomous defects in macrophage recruitment and axon regeneration in injured nerves following loss of Gpr126 in SCs. This work demonstrates that Gpr126 has critical SC-autonomous and SC-nonautonomous functions in remyelination and peripheral nerve repair. SIGNIFICANCE STATEMENT Lack of robust remyelination represents one of the major barriers to recovery of neurological functions in disease or following injury in many disorders of the nervous system. Here we show that the adhesion class G protein-coupled receptor (GPCR) Gpr126/Adgrg6 is required for remyelination, macrophage recruitment, and axon regeneration following nerve injury. At least 30% of all approved drugs target GPCRs; thus, Gpr126 represents an attractive potential target to stimulate repair in myelin disease or following nerve injury
Proteasome inhibitor-based therapy for antibody-mediated rejection
The development of donor-specific anti-human leukocyte antigen antibodies (DSAs) following renal transplantation significantly reduces long-term renal graft function and survival. The traditional therapies for antibody-mediated rejection (AMR) have provided inconsistent results and transient effects that may be due to a failure to deplete mature antibody-producing plasma cells. Proteasome inhibition (PI) is a novel AMR therapy that deletes plasma cells. Initial reports of PI-based AMR treatment in refractory rejection demonstrated the ability of bortezomib to deplete plasma cells producing DSA, reduce DSA levels, provide histological improvement or resolution, and improve renal allograft function. These results have subsequently been confirmed in a multicenter collaborative study. PI has also been shown to provide effective primary AMR therapy in case reports. Recent studies have demonstrated that PI therapy results in differential responses in early and late post-transplant AMR. Additional randomized studies are evaluating the role of PI in transplant induction, acute AMR, and chronic rejection in renal transplantation. An important theoretical advantage of PI-based regimens is derived from several potential strategies for achievement of synergy
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