831 research outputs found
High-Field Low-Frequency Spin Dynamics
The theory of exchange symmetry of spin ordered states is extended to the
case of high magnetic field. Low frequency spin dynamics equation for
quasi-goldstone mode is derived for two cases of collinear and noncollinear
antiferromagnets.Comment: 2 page
Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity
The mass-loss rates of hot, massive, luminous stars are considered a decisive
parameter in shaping the evolutionary tracks of such stars and influencing the
interstellar medium on galactic scales. The small-scale structures (clumps)
omnipresent in such winds may reduce empirical estimates of mass-loss rates by
an evolutionarily significant factor of >=3. So far, there has been no direct
observational evidence that wind-clumping may persist at the same level in
environments with a low ambient metallicity, where the wind-driving opacity is
reduced. Here we report the results of time-resolved spectroscopy of three
presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud,
where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale
emission peaks moving outwards in the accelerating parts of the stellar
winds.The general properties of the moving features, such as their velocity
dispersions,emissivities and average accelerations, closely match the
corresponding characteristics of small-scale inhomogeneities in the winds of
Galactic Wolf-Rayet stars.Comment: 9 pages, 3 figures; accepted by ApJ Letter
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
Is the CPT-norm always positive?
We give an explicit example of an exactly solvable PT-symmetric Hamiltonian
with the unbroken PT symmetry which has one eigenfunction with the zero
PT-norm. The set of its eigenfunctions is not complete in corresponding Hilbert
space and it is non-diagonalizable. In the case of a regular Sturm-Liouville
problem any diagonalizable PT-symmetric Hamiltonian with the unbroken PT
symmetry has a complete set of positive CPT-normalazable eigenfunctions. For
non-diagonalizable
Hamiltonians a complete set of CPT-normalazable functions is possible but the
functions belonging to the root subspace corresponding to multiple zeros of the
characteristic determinant are not eigenfunctions of the Hamiltonian anymore
Octupolar order in the multiple spin exchange model on a triangular lattice
We show how a gapless spin liquid with hidden octupolar order arises in an
applied magnetic field, in a model applicable to thin films of 3He with
competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions.
Evidence is also presented for nematic -- i.e., quadrupolar -- correlations
bordering on ferromagnetism in the absence of a magnetic field.Comment: 4 pages, 5 figure
Transfer matrix solution of the Wako-Sait\^o-Mu\~noz-Eaton model augmented by arbitrary short range interactions
The Wako-Sait{\^o}-Mu\~noz-Eaton (WSME) model, initially introduced in the
theory of protein folding, has also been used in modeling the RNA folding and
some epitaxial phenomena. The advantage of this model is that it admits exact
solution in the general inhomogeneous case (Bruscolini and Pelizzola, 2002)
which facilitates the study of realistic systems. However, a shortcoming of the
model is that it accounts only for interactions within continuous stretches of
native bonds or atomic chains while neglecting interstretch (interchain)
interactions. But due to the biopolymer (atomic chain) flexibility, the
monomers (atoms) separated by several non-native bonds along the sequence can
become closely spaced. This produces their strong interaction. The inclusion of
non-WSME interactions into the model makes the model more realistic and
improves its performance. In this study we add arbitrary interactions of finite
range and solve the new model by means of the transfer matrix technique. We can
therefore exactly account for the interactions which in proteomics are
classified as medium- and moderately long-range ones.Comment: 15 pages, 2 figure
A 10-hour period revealed in optical spectra of the highly variable WN8 Wolf-Rayet star WR 123
Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8
stars in general and WR123 in particular? A dedicated spectroscopic campaign
targets the ten-hour period previously found in the high-precision photometric
data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a
series of high signal-to-noise, mid-resolution spectra from several sites in
the {\lambda}{\lambda} 4000 - 6940 A^{\circ} domain. We also followed the star
with occasional broadband (Johnson V) photometry. The acquired spectroscopy
allowed a detailed study of spectral variability on timescales from \sim 5
minutes to months. Results. We find that all observed spectral lines of a given
chemical element tend to show similar variations and that there is a good
correlation between the lines of different elements, without any significant
time delays, save the strong absorption components of the Hei lines, which tend
to vary differently from the emission parts. We find a single sustained
periodicity, P \sim 9.8 h, which is likely related to the relatively stable
pulsations found in MOST photometry obtained one year later. In addition,
seemingly stochastic, large-amplitude variations are also seen in all spectral
lines on timescales of several hours to several days.Comment: 6 pages, 4 figures, 2 tables, data available on-line, accepted in A&A
Research Note
The massive eclipsing LMC Wolf-Rayet binary BAT99-129. 1 Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful of extra-galactic eclipsing
Wolf-Rayet binaries known. We present blue, medium-resolution, phase-dependent
NTT-EMMI spectra of this system that allow us to separate the spectra of the
two components of the binary and to obtain a reliable orbital solution for both
stars. We assign an O5V spectral type to the companion, and WN3(h)a to the
Wolf-Rayet component. We discuss the spectroscopic characteristics of the
system: luminosity ratio, radii, rotation velocities. We find a possible
oversynchronous rotation velocity for the O star. Surprisingly, the extracted
Wolf-Rayet spectrum clearly shows the presence of blueshifted absorption lines,
similar to what has been found in all single hot WN stars in the SMC and some
in the LMC. We also discuss the presence of such intrinsic lines in the context
of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binary evolution and GRB
progenitors. Altogether, BAT99~129 is the extragalactic counterpart of the
well-known Galactic WR binary V444 Cygni.Comment: 14 pages, 9 figures, accepted by A&A for publicatio
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