4,891 research outputs found
Near-Infrared Properties of Metal-poor Globular Clusters in the Galactic Bulge Direction
Aims. J, H, and K' images obtained from the near-infrared imager CFHTIR on
the Canada-France-Hawaii Telescope are used to derive the morphological
parameters of the red giant branch (RGB) in the near-infrared color-magnitude
diagrams for 12 metal-poor globular clusters in the Galactic bulge direction.
Using the compiled data set of the RGB parameters for the observed 12 clusters,
in addition to the previously studied 5 clusters, we discuss the properties of
the RGB morphology for the clusters and compare them with the calibration
relations for the metal-rich bulge clusters and the metal-poor halo clusters.
Methods. The photometric RGB shape indices such as colors at fixed magnitudes
of MK = MH = (-5.5, -5, -4, and -3), magnitudes at fixed colors of (J - K)o =
(J - H)o = 0.7, and the RGB slope are measured from the fiducial normal points
defined in the near- infrared color-magnitude diagrams for each cluster. The
magnitudes of RGB bump and tip are also estimated from the differential and
cumulative luminosity functions of the selected RGB stars. The derived RGB
parameters have been used to examine the overall behaviors of the RGB
morphology as a function of cluster metallicity. Results. The correlations
between the near-infrared photometric RGB shape indices and the cluster
metallicity for the programme clusters compare favorably with the previous
observational calibration relations for metal-rich clusters in the Galactic
bulge and the metal-poor halo clusters. The observed near-infrared magnitudes
of the RGB bump and tip for the investigated clusters are also in accordance
with the previous calibration relations for the Galactic bulge clusters.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2
Thermodynamic experiments as well as Raman scattering have been used to study
the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl
and Br. While the phase transition observed in the Cl system at T_o=18.2 K is
consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br
system has several unusual features. We propose an explanation in terms of
weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.Comment: 4 pages, 4 figure
Mean Escape Time in a System with Stochastic Volatility
We study the mean escape time in a market model with stochastic volatility.
The process followed by the volatility is the Cox Ingersoll and Ross process
which is widely used to model stock price fluctuations. The market model can be
considered as a generalization of the Heston model, where the geometric
Brownian motion is replaced by a random walk in the presence of a cubic
nonlinearity. We investigate the statistical properties of the escape time of
the returns, from a given interval, as a function of the three parameters of
the model. We find that the noise can have a stabilizing effect on the system,
as long as the global noise is not too high with respect to the effective
potential barrier experienced by a fictitious Brownian particle. We compare the
probability density function of the return escape times of the model with those
obtained from real market data. We find that they fit very well.Comment: 9 pages, 9 figures, to be published in Phys. Rev.
Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae
We present infrared (IR) and optical echelle spectra of the Classical T Tauri
star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis
to fit atomic and molecular absorption lines and determine key stellar
parameters: Teff = 4126 \pm 24 K, log g = 4.84 \pm 0.16, [M/H] = -0.10 \pm
0.12, vsini = 5.8 \pm 0.6 km/s. The IR spectrum is used to look for Zeeman
broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I
lines near 2.2 microns and find the average value of the magnetic field over
the entire surface is 2.61 \pm 0.23 kG. In addition, several nearby
magnetically insensitive CO lines show no excess broadening above that produced
by stellar rotation and instrumental broadening, reinforcing the magnetic
interpretation for the width of the Ti I lines. We carry out extensive tests to
quantify systematic errors in our analysis technique which may result from
inaccurate knowledge of the effective temperature or gravity, finding that
reasonable errors in these quantities produce a 10% uncertainty in the mean
field measurement.Comment: The tar file includes one Tex file and four .eps figures. The paper
is accepted and tentatively scheduled for the ApJ 1 December 2005, v634, 2
issue. ApJ manuscript submission # 6310
Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains
By means of thermal expansion and specific heat measurements on the
high-pressure phase of (VO)PO, the effects of two energy scales of
the weakly dimerized antiferromagnetic = 1/2 Heisenberg chain are explored.
The low energy scale, given by the spin gap , is found to manifest
itself in a pronounced thermal expansion anomaly. A quantitative analysis,
employing T-DMRG calculations, shows that this feature originates from changes
in the magnetic entropy with respect to , . This term, inaccessible by specific heat, is visible only in the
weak-dimerization limit where it reflects peculiarities of the excitation
spectrum and its sensitivity to variations in .Comment: 4 pages, 4 figures now identical with finally published versio
Spectropolarimetry of the Classical T Tauri Star TW Hydrae
We present high resolution (R ~ 60,000) circular spectropolarimetry of the
classical T Tauri star TW Hydrae. We analyze 12 photospheric absorption lines
and measure the net longitudinal magnetic field for 6 consecutive nights. While
no net polarization is detected the first five nights, a significant
photospheric field of Bz = 149 \pm 33 G is found on the sixth night. To rule
out spurious instrumental polarization, we apply the same analysis technique to
several non-magnetic telluric lines, detecting no significant polarization. We
further demonstrate the reality of this field detection by showing that the
splitting between right and left polarized components in these 12 photospheric
lines shows a linear trend with Lande g-factor times wavelength squared, as
predicted by the Zeeman effect. However, this longitudinal field detection is
still much lower than that which would result if a pure dipole magnetic
geometry is responsible for the mean magnetic field strength of 2.6 kG
previously reported for TW Hya. We also detect strong circular polarization in
the He I 5876 and the Ca II 8498 emission lines, indicating a strong field in
the line formation region of these features. The polarization of the Ca II line
is substantially weaker than that of the He I line, which we interpret as due
to a larger contribution to the Ca II line from chromospheric emission in which
the polarization signals cancel. However, the presence of polarization in the
Ca II line indicates that accretion shocks on Classical T Tauri stars do
produce narrow emission features in the infrared triplet lines of Calcium.Comment: One tar file. The paper has 22 pages, 5 figures. Accepted by AJ on
Sep 10, 200
Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood
The chemical composition of Earth's atmosphere has undergone substantial
evolution over the course of its history. It is possible, even likely, that
terrestrial planets in other planetary systems have undergone similar changes;
consequently, the age distribution of nearby stars is an important
consideration in designing surveys for Earth-analogues. Valenti & Fischer
(2005) provide age and metallicity estimates for 1039 FGK dwarfs in the Solar
Neighbourhood. Using the Hipparcos catalogue as a reference to calibrate
potential biases, we have extracted volume-limited samples of nearby stars from
the Valenti-Fischer dataset. Unlike other recent investigations, our analysis
shows clear evidence for an age-metallicity relation in the local disk, albeit
with substantial dispersion at any epoch. The mean metallicity increases from
-0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day.
Supplementing the Valenti-Fischer measurements with literature data to give a
complete volume-limited sample, the age distribution of nearby FGK dwarfs is
broadly consistent with a uniform star-formation rate over the history of the
Galactic disk. In striking contrast, most stars known to have planetary
companions are younger than 5 Gyrs; however, stars with planetary companions
within 0.4 AU have a significantly flatter age distribution, indicating that
those systems are stable on timescales of many Gyrs. Several of the older,
lower metallicity host stars have enhanced [alpha/Fe] ratios, implying
membership of the thick disk. If the frequency of terrestrial planets is also
correlated with stellar metallicity, then the median age of such planetary
system is likely to be ~3 Gyrs. We discuss the implications of this hypothesis
in designing searches for Earth analogues among the nearby stars.Comment: Accepted for publication in Ap
Spin-Charge Separation in Two Dimensions - A Numerical Study
The question of spin-charge separation in two-dimensional lattices has been
addressed by numerical simulations of the motion of one hole in a half-filled
band. The calculations have been performed on finite clusters with Hubbard and
t-J models. By comparing the time evolution of spin and charge polarisation
currents in one and two dimensions, evidence in favor of spin-charge separation
in two dimensions is presented. In contrast with this, spin-charge separation
is absent in a highly doped, metallic, system.Comment: RevTeX 3.0, 10 Pages, 6 PostScript Figures (on request
First Detection of the White-Dwarf Cooling Sequence of the Galactic Bulge
We present Hubble Space Telescope data of the low-reddening Sagittarius
window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar
Planet Search field (3'x3'), together with three more Advanced Camera for
Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and
F814W filters, approximately every two weeks for two years, with the principal
aim of detecting a hidden population of isolated black holes and neutron stars
through astrometric microlensing. Proper motions were measured with an accuracy
of ~0.1 mas/yr (~4 km/s) at F606W~25.5 mag, and better than ~0.5 mas/yr (20
km/s) at F606W~28 mag, in both axes. Proper-motion measurements allowed us to
separate disk and bulge stars and obtain a clean bulge color-magnitude diagram.
We then identified for the first time a white dwarf (WD) cooling sequence in
the Galactic bulge, together with a dozen candidate extreme horizontal branch
stars. The comparison between theory and observations shows that a substantial
fraction of the WDs (30%) are systematically redder than the cooling tracks for
CO-core H-rich and He-rich envelope WDs. This evidence would suggest the
presence of a significant number of low-mass WDs and WD - main sequence
binaries in the bulge. This hypothesis is further supported by the finding of
two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables,
and a few candidate cataclysmic variables in the same field.Comment: 9 pages, 5 figures, accepted for publication on Ap
A bioeconomic analysis of the potential of seaweed Hypnea pseudomusciformis farming to different targeted markets.
Simulations were performed to evaluate the economic potential of farming the seaweed Hypnea pseudomusciformis in two production scales for the carrageenan, human food, and glycolic extract markets in Brazil
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