7,376 research outputs found
Molar volume of solid isotopic helium mixtures
Solid isotopic helium mixtures have been studied by path-integral Monte Carlo
simulations in the isothermal-isobaric ensemble. This method allowed us to
study the molar volume as a function of temperature, pressure, and isotopic
composition. At 25 K and 0.2 GPa, the relative difference between molar volumes
of isotopically-pure crystals of 3He and 4He is found to be about 3%. This
difference decreases under pressure, and for 12 GPa it is smaller than 1%. For
isotopically-mixed crystals, a linear relation between lattice parameters and
concentrations of helium isotopes is found, in agreement with Vegard's law. The
virtual crystal approximation, valid for isotopic mixtures of heavier atoms,
does not give reliable results for solid solutions of helium isotopes.Comment: 7 pages, 5 figure
Light Higgs bosons from a strongly interacting Higgs sector
The mass and the decay width of a Higgs boson in the minimal standard model
are evaluated by a variational method in the limit of strong self-coupling
interaction. The non-perturbative technique provides an interpolation scheme
between strong-coupling regime and weak-coupling limit where the standard
perturbative results are recovered. In the strong-coupling limit the physical
mass and the decay width of the Higgs boson are found to be very small as a
consequence of mass renormalization. Thus it is argued that the eventual
detection of a light Higgs boson would not rule out the existence of a strongly
interacting Higgs sector.Comment: 2 figure
Rare-gas solids under pressure: A path-integral Monte Carlo simulation
Rare-gas solids (Ne, Ar, Kr, and Xe) under hydrostatic pressure up to 30 kbar
have been studied by path-integral Monte Carlo simulations in the
isothermal-isobaric ensemble. Results of these simulations have been compared
with available experimental data and with those obtained from a quasiharmonic
approximation (QHA). This comparison allows us to quantify the overall
anharmonicity of the lattice vibrations and its influence on several structural
and thermodynamic properties of rare-gas solids. The vibrational energy
increases with pressure, but this increase is slower than that of the elastic
energy, which dominates at high pressures. In the PIMC simulations, the
vibrational kinetic energy is found to be larger than the corresponding
potential energy, and the relative difference between both energies decreases
as the applied pressure is raised. The accuracy of the QHA increases for rising
pressure.Comment: 9 pages, 6 figure
Near-infrared K-band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples
We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert 2
galaxies in the CfA and 12 micron samples. The strength of the CO absorption
features at 2.3-2.4 micron produced by stars is measured in terms of a
spectroscopic CO index. A clear anti-correlation between the observed CO index
and the nuclear K-L color is present, suggesting that a featureless hot dust
continuum heated by an AGN contributes significantly to the observed K-band
fluxes in the nuclei of Seyfert 2 galaxies. After correction for this AGN
contribution, we estimate nuclear stellar K-band luminosities for all sources,
and CO indices for sources with modestly large observed CO indices. The
corrected CO indices for 10 (=40%) Seyfert 2 nuclei are found to be as high as
those observed in star-forming or elliptical (=spheroidal) galaxies. We combine
the K-band data with measurements of the L-band 3.3 micron polycyclic aromatic
hydrocarbon (PAH) emission feature, another powerful indicator for
star-formation, and find that the 3.3 micron PAH to K-band stellar luminosity
ratios are substantially smaller than those of starburst galaxies. Our results
suggest that the 3.3 micron PAH emission originates in the putative nuclear
starbursts in the dusty tori surrounding the AGNs, because of its high surface
brightness, whereas the K-band CO absorption features detected at the nuclei
are dominated by old bulge (=spheroid) stars, and thus may not be a powerful
indicator for the nuclear starbursts. We see no clear difference in the
strength of the CO absorption and PAH emission features between the CfA and 12
micron Seyfert 2s.Comment: 28 pages, 6 figures, accepted for publication in ApJ (10 October
2004, v614 issue
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
Multi-wavelength Observations of the Giant X-ray Flare Galaxy NGC 5905: signatures of tidal disruption
NGC 5905 is one of the few galaxies with no prior evidence for an AGN in
which an X-ray flare, due to the tidal disruption of a star by the massive
black hole in the center of the galaxy, was detected by the RASS in 1990-91.
Here we present analysis of late-time follow-up observations of NGC 5905 using
Chandra, Spitzer VLA 3 GHz and 8 GHz archival data and GMRT 1.28 GHz radio
observations. The X-ray image shows no compact source that could be associated
with an AGN. Instead, the emission is extended -- likely due to nuclear star
formation and the total measured X-ray luminosity is comparable to the X-ray
luminosity determined from the 2002 Chandra observations. Diffuse X-ray
emission was detected close to the circum-nuclear star forming ring. The
Spitzer 2006 mid-infrared spectrum also shows strong evidence of nuclear star
formation but no clear AGN signatures. The semi-analytical models of Tommasin
et. al. 2010 together with the measured [OIV]/[NeII] line ratio suggest that at
most only 5.6% of the total IR Flux at 19 m is being contributed by the
AGN. The GMRT 1.28 GHz observations reveal a nuclear source. In the much higher
resolution VLA 3 GHz map, the emission has a double lobed structure of size
2.7'' due to the circumnuclear star forming ring. The GMRT 1.28 GHz peak
emission coincides with the center of the circumnuclear ring. We did not detect
any emission in the VLA 8 GHz (1996) archival data. The 3 upper limits
for the radio afterglow of the TDE at 1.28 GHz, 3 GHz and 8 GHz are 0.17 mJy,
0.09 mJy and 0.09 mJy, respectively. Our studies thus show that (i) NGC 5905
has a declining X-ray flux consistent with a TDE, (ii) the IR flux is dominated
by nuclear star formation, (iii) the nuclear radio emission observed from the
galaxy is due to circumnuclear star formation, (iv) no compact radio emission
associated with a radio afterglow from the TDE is detected.Comment: 12 pages, 8 figures, accepted to be published in Astrophysics and
Space Scienc
3He-rich SEP Events Observed by STEREO-A
Using the SIT (Suprathermal Ion Telescope) instrument on STEREO-A we have
examined the abundance of the rare isotope 3He during the rising activity phase
of solar cycle 24 between January 2010 and December 2011. We have identified
six solar energetic particle (SEP) events with enormous abundance enhancements
of 3He (3He/4He >1). The events were short lasting, typically ~0.5-1 day and
most of them occurred in association with high-speed solar wind streams and
corotating interaction regions. With one exception the events were not
associated with ~100 keV solar electron intensity increases. The events showed
also enhanced NeS/O and Fe/O ratios. The solar images indicate that the events
were generally associated with the active regions located near a coronal hole.Comment: accepted for publication in AIP Conference Proceedings for
'Thirteenth International Solar Wind Conference
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