8,430 research outputs found
Anelastic relaxation and La NQR in LaSrCuO around the critical Sr content x=0.02
Anelastic relaxation and La NQR relaxation measurements in
LaSrCuO for Sr content x around 2 and 3 percent, are presented
and discussed in terms of spin and lattice excitations and ordering processes.
It is discussed how the phase diagram of LaSrCuO at the
boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02)
could be more complicate than previous thought, with a transition to a
quasi-long range ordered state at T = 150 K, as indicated by recent neutron
scattering data. On the other hand, the La NQR spectra are compatible
with a transition to a conventional AF phase around T = 50 K, in agreement with
the phase diagram commonly accepted in the literature. In this case the
relaxation data, with a peak of magnetic origin in the relaxation rate around
150 K at 12 MHz and the anelastic counterparts around 80 K in the kHz range,
yield the first evidence in LaSrCuO of freezing involving
simultaneously lattice and spin excitations. This excitation could correspond
to the motion of charged stripes.Comment: 10 pages, 8 figure
A New Feature in the Spectrum of the Superluminous LMC Supergiant HDE 269896
We have found strong selective emission of the N II 5000A complex in the
spectrum of the LMC hypergiant HDE 269896, ON9.7 Ia. Since this object also
has anomalously strong He II 4686 emission for its spectral type, an
unusually wide range of ionization in its extended atmosphere is indicated. The
published model of this spectrum does not reproduce these emission features,
but we show that increased nitrogen and helium abundances, together with small
changes in other model parameters, can do so. The morphological and possible
evolutionary relationships of HDE 269896, as illuminated by the new spectral
features, to other denizens of the OB Zoo are discussed. This object may be in
an immediate pre-WNVL (Very Late WN) state, which is in turn the quiescent
state of at least some Luminous Blue Variables.
More generally, the N II spectrum in HDE 269896 provides a striking
demonstration of the occurrence of two distinctly different kinds of line
behavior in O-type spectra: normal absorption lines that develop P Cygni
profiles at high wind densities, and selective emission lines from the same
ions that do not. Further analysis of these features will advance understanding
of both atomic physics and extreme stellar atmospheres.Comment: 12 pages, 2 tables, 4 figures (quality downgraded due to size
constraints); to appear in PASP January 200
Dimensionality of the spatio-temporal entanglement of PDC photon pairs
In this work the Schmidt number of the two-photon state generated by
parametric-down conversion (PDC) is evaluated in the framework of a fully
spatio-temporal model for PDC. A comparison with the results obtained in either
purely spatial or purely temporal models shows that the degree of entanglement
of the PDC state cannot be trivially reduced to the product of the Schmidt
numbers obtained in models with lower dimensionality, unless the detected
bandwidth is very narrow. This result is a consequence of the non-factorability
of the state in the spatial and temporal degrees of freedoms of twin photons.
In the limit of a broad pump beam, we provide a geometrical interpretation of
the Schmidt number, as the ratio between the volume of the phase matching
region and of a correlation volume.Comment: 17 pages, 10 figures. Submitted to Phys. Rev.
A deep and wide-field view at the IC 2944 / 2948 complex in Centaurus
We employed the ESO MPI wide-field camera and obtained deep images in the VIc
pass-bands in the region of the IC 2944/2948 complex (l ~ 294; b ~ -1), and
complemented them with literature and archival data. We used this material to
derive the photometric, spectroscopic and kinematic properties of the brightest
(V < 16) stars in the region. The VI deep photometry on the other end, helped
us to unravel the lower main sequence of a few, possibly physical, star groups
in the area.
Our analysis confirmed previous suggestions that the extinction toward this
line of sight follows the normal law (Rv = 3.1). We could recognize B-type
stars spread in distance from a few hundred pc to at least 2 kpc. We found two
young groups (age ~ 3 Myr) located respectively at about 2.3 and 3.2 kpc from
the Sun. They are characterized by a significant variable extinction (E(B-V)
ranging from 0.28 to 0.45 mag), and host a significant pre-main sequence
population. We computed the initial mass functions for these groups and
obtained slopes Gamma from -0.94 to -1.02 (e_Gamma = 0.3), in a scale where the
classical Salpeter law is -1.35. We estimated the total mass of both main
stellar groups in ~ 1100 Mo, respectively. Our kinematic analysis
indicated that both groups of stars deviate from the standard rotation curve of
the Milky Way, in line with literature results for this specific Galactic
direction.
Finally, along the same line of sight we identified a third group of
early-type stars located at ~ 8 kpc from the Sun. This group might be located
in the far side of the Sagittarius-Carina spiral arm.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Photometric and spectroscopic variations of the Be star HD 112999
Be objects are stars of B spectral type showing lines of the Balmer series in
emission. The presence of these lines is attributed to the existence of an
extended envelope, disk type, around them. Some stars are observed in both the
Be and normal B-type spectroscopic states and they are known as transient Be
stars. In this paper we show the analysis carried out on a new possible
transient Be star, labelled HD 112999, using spectroscopic optical observations
and photometric data.Comment: 10 pages, 5 figures, accepted for publication in IBV
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