1,996 research outputs found
Frequency Space Correlation Between REITs and Capital Market Indices
Several studies have examined real estate investment trust (REIT) co-movement with stocks or bonds using traditional time domain based methods, such as linear regression or correlation. Results of these studies have produced inconsistent statistical model parameters. The erratic behavior of the models may have resulted from the different time periods in the studies, the REITs included in a study or the market indices. Another factor contributing to the variation of the models comes from the compression of cyclical information over a study?s time period by time domain based techniques. Cross-spectral analysis provides a frequency space method of examining the coherency (i.e., frequency space correlation) between two time series across all frequencies. This article contains an examination of the coherency between REITs and stock market indices and REITs and U.S. Treasury debt indices for the period 1989-95. Results of the coherency spectra show significant co-movement between REITs and stock market indices, while debt instruments show very few frequencies with significant coherency. Furthermore, phase spectra provide evidence of contemporaneous movement between REITs and stock indices at all frequencies.
Cool White Dwarfs Revisited -- New Spectroscopy and Photometry
In this paper we present new and improved data on 38 cool white dwarfs
identified by Oppenheimer et al. 2001 (OHDHS) as candidate dark halo objects.
Using the high-res spectra obtained with LRIS, we measure radial velocities for
13 WDs that show an H alpha line. We show that the knowledge of RVs decreases
the UV-plane velocities by only 6%. The radial velocity sample has a W-velocity
dispersion of sig_W = 59 km/s--in between the values associated with the thick
disk and the stellar halo. We also see indications for the presence of two
populations by analyzing the velocities in the UV plane. In addition, we
present CCD photometry for half of the sample, and with it recalibrate the
photographic photometry of the remaining WDs. Using the new photometry in
standard bands, and by applying the appropriate color-magnitude relations for H
and He atmospheres, we obtain new distance estimates. New distances of the WDs
that were not originally selected as halo candidates yield 13 new candidates.
On average, new distances produce velocities in the UV plane that are larger by
10%, with already fast objects gaining more. Using the new data, while applying
the same UV-velocity cut (94 km/s) as in OHDHS, we find a density of cool WDs
of 1.7e-4 pc^-3, confirming the value of OHDHS. In addition, we derive the
density as a function of the UV-velocity cutoff. The density (corrected for
losses due to higher UV cuts) starts to flatten out at 150 km/s (0.4e-4 pc^-3),
and is minimized (thus minimizing a possible non-halo contamination) at 190
km/s (0.3e-4 pc^-3). These densities are in a rough agreement with the
estimates for the stellar halo WDs, corresponding to a factor of 1.9 and 1.4
higher values.Comment: Accepted to ApJ. New version contains some additional data. Results
unchange
A galaxy as the source of a Civ absorption system close to the epoch of reionization
We find a bright (L_{UV}=2.5 L*_{z=6}) Lyman alpha emitter at redshift
z=5.719 at a projected distance of 79 physical kpc from a strong triply ionized
carbon (Civ) absorption system at redshift z=5.7238 previously reported in the
spectrum of the z_{em} = 6.309 QSO SDSS J1030+0524. This is the highest
redshift galaxy-absorber pair detected to-date, supporting the idea that
galaxy-wide outflows were already in place at the end of the epoch of
reionization. The proximity of this object makes it the most likely source of
metals, consistent with models of outflows at lower redshift where significant
observational evidence relates metal absorption systems with galaxies hosting
outflows.
In a typical outflow scenario, a wind of 200 km/s, active since the universe
was only 0.6 Gyr old (z ~8.4), could eject metals out to 79 kpc at z=5.719.
Although the origin of metals in the intergalactic medium (IGM) is still under
debate, our results are consistent with predictions from cosmological
simulations which reproduce the evolution of the cosmic density of Civ, from z
~ 6 to the present day based on outflow-driven enrichment of the IGM.
We also report two more Lyman alpha emitters in this field, at z=5.973\pm
0.002 and z=5.676\pm 0.002 respectively, the former confirming the original
identification by Stiavelli et al. Our results suggest that the colour cut
typically used to identify i-dropouts (i_{775}-z_{850}>1.3) misses a
non-negligible fraction of blue galaxies with faint UV continuum at z \geq 5.7.Comment: Accepted for publication in MNRAS, 9 pages, 3 figures, 1 tabl
Observations of Ultracool White Dwarfs
We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object
Another Majorana Idea: Real and Imaginary in the Weinberg Theory
The Majorana discernment of neutrality is applied to the solutions of
Weinberg equations in the representation of the Poincar\`e
group.Comment: ReVTeX file, 6pp., no figure
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
Neutron stars with isovector scalar correlations
Neutron stars with the isovector scalar -field are studied in the
framework of the relativistic mean field () approach in a pure nucleon
plus lepton scheme. The -field leads to a larger repulsion in dense
neutron-rich matter and to a definite splitting of proton and neutron effective
masses. Both features are influencing the stability conditions of the neutron
stars. Two parametrizations for the effective nonlinear Lagrangian density are
used to calculate the nuclear equation of state () and the neutron star
properties, and compared to correlated Dirac-Brueckner results. We conclude
that in order to reproduce reasonable nuclear structure and neutron star
properties within a approach a density dependence of the coupling
constants is required.Comment: 11 pages, 5 figures, revtex4 styl
Impossibility of Unlimited Gravitational Collapse
It is shown that the gravitational field, as a physical field developing in
the Minkowsky space, does not lead to unlimited gravitational collapse of
massive bodies and, hence, excludes a possibility of the formation of the
``black holes''.Comment: 6 page
Holographic Magnetic Star
A warm fermionic AdS star under a homogeneous magnetic field is explored. We
obtain the relativistic Landau levels by using Dirac equation and use the
Tolman-Oppenheimer-Volkoff (TOV) equation to study the physical profiles of the
star. Bulk properties such as sound speed, adiabatic index, and entropy density
within the star are calculated analytically and numerically. Bulk temperature
increases the mass limit of the AdS star but external magnetic field has the
opposite effect. The results are partially interpreted in terms of the
pre-thermalization process of the gauge matter at the AdS boundary after the
mass injection. The entropy density is found to demonstrate similar temperature
dependence as the magnetic black brane in the AdS in certain limits regardless
of the different nature of the bulk and Hawking temperatures. Total entropy of
the AdS star is also found to be an increasing function of the bulk temperature
and a decreasing function of the magnetic field, similar behaviour to the mass
limit. Since both total entropy and mass limit are global quantities, they
could provide some hints to the value of entropy and energy of the dual gauge
matter before and during the thermalization.Comment: 39 pages, 14 figures, 1 table, comments and references added, to
appear in JHE
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