10,275 research outputs found
Patsey Doolin\u27s Chivaree
https://digitalcommons.library.umaine.edu/mmb-me/1203/thumbnail.jp
CAFE: Calar Alto Fiber-fed Echelle spectrograph
We present here CAFE, the Calar Alto Fiber-fed Echelle spectrograph, a new
instrument built at the Centro Astronomico Hispano Alem\'an (CAHA). CAFE is a
single fiber, high-resolution (70000) spectrograph, covering the
wavelength range between 3650-9800\AA. It was built on the basis of the common
design for Echelle spectrographs. Its main aim is to measure radial velocities
of stellar objects up to 13-14 mag with a precision as good as a few
tens of . To achieve this goal the design was simplified at maximum,
removing all possible movable components, the central wavelength is fixed, so
the wavelentgth coverage; no filter wheel, one slit and so on, with a
particular care taken in the thermal and mechanical stability. The instrument
is fully operational and publically accessible at the 2.2m telescope of the
Calar Alto Observatory.
In this article we describe (i) the design, summarizing its manufacturing
phase; (ii) characterize the main properties of the instrument; (iii) describe
the reduction pipeline; and (iv) show the results from the first light and
commissioning runs. The preliminar results indicate that the instrument fulfill
the specifications and it can achieve the foreseen goals. In particular, they
show that the instrument is more efficient than anticipated, reaching a
20 for a stellar object as faint as 14.5 mag in 2700s
integration time. The instrument is a wonderful machine for exoplanetary
research (by studying large samples of possible systems cotaining massive
planets), galactic dynamics (high precise radial velocities in moving groups or
stellar associations) or astrochemistry.Comment: 12 pages, 23 figures; Acepted for publishing in A&A, 201
Vector magnetic hysteresis of hard superconductors
Critical state problems which incorporate more than one component for the
magnetization vector of hard superconductors are investigated. The theory is
based on the minimization of a cost functional
which weighs the changes of the magnetic field vector within the sample. We
show that Bean's simplest prescription of choosing the correct sign for the
critical current density in one dimensional problems is just a particular
case of finding the components of the vector . is
determined by minimizing under the constraint , with a bounded set. Upon the selection of
different sets we discuss existing crossed field measurements and
predict new observable features. It is shown that a complex behavior in the
magnetization curves may be controlled by a single external parameter, i.e.:
the maximum value of the applied magnetic field .Comment: 10 pages, 9 figures, accepted in Phys. Rev.
Autonomous clustering using rough set theory
This paper proposes a clustering technique that minimises the need for subjective
human intervention and is based on elements of rough set theory. The proposed algorithm is
unified in its approach to clustering and makes use of both local and global data properties to
obtain clustering solutions. It handles single-type and mixed attribute data sets with ease and
results from three data sets of single and mixed attribute types are used to illustrate the
technique and establish its efficiency
Astrometric search for a planet around VB 10
We observed VB 10 in August and September 2009 using the FORS2 camera of the
VLT with the aim of measuring its astrometric motion and of probing the
presence of the announced planet VB 10b. We used the published STEPS
astrometric positions of VB 10 over a time-span of 9 years, which allowed us to
compare the expected motion of VB 10 due to parallax and proper motion with the
observed motion and to compute precise deviations. The achieved single-epoch
precisions of our observations are about 0.1 mas and the data showed no
significant residual trend, while the presence of the planet should have
induced an apparent proper motion larger than 10 mas/yr. Subtraction of the
predicted orbital motion from the observed data produces a large trend in
position residuals of VB 10. We estimated the probability that this trend is
caused by random noise. Taking all the uncertainties into account and using
Monte-Carlo resampling of the data, we are able to reject the existence of VB
10b with the announced mass of 6.4 M_J with the false alarm probability of only
0.0005. A 3.2 M_J planet is also rejected with a false alarm probability of
0.023.Comment: 6 pages, 6 figures, 2 tables, accepted for publication in A&
Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method
The detection of planets around very low-mass stars with the radial velocity
method is hampered by the fact that these stars are very faint at optical
wavelengths where the most high-precision spectrometers operate. We investigate
the precision that can be achieved in radial velocity measurements of low mass
stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the
precision achievable in the optical. For early-M stars, radial velocity
measurements in the nIR offer no or only marginal advantage in comparison to
optical measurements. Although they emit more flux in the nIR, the richness of
spectral features in the optical outweighs the flux difference. We find that
nIR measurement can be as precise than optical measurements in stars of
spectral type ~M4, and from there the nIR gains in precision towards cooler
objects. We studied potential calibration strategies in the nIR finding that a
stable spectrograph with a ThAr calibration can offer enough wavelength
stability for m/s precision. Furthermore, we simulate the wavelength-dependent
influence of activity (cool spots) on radial velocity measurements from optical
to nIR wavelengths. Our spot simulations reveal that the radial velocity jitter
does not decrease as dramatically towards longer wavelengths as often thought.
The jitter strongly depends on the details of the spots, i.e., on spot
temperature and the spectral appearance of the spot. Forthcoming nIR
spectrographs will allow the search for planets with a particular advantage in
mid- and late-M stars. Activity will remain an issue, but simultaneous
observations at optical and nIR wavelengths can provide strong constraints on
spot properties in active stars.Comment: accepted by ApJ, v2 accepted revision with new precision
calculations, abstract abride
Determination of the critical current density in the d-wave superconductor YBCO under applied magnetic fields by nodal tunneling
We have studied nodal tunneling into YBa2Cu3O7-x (YBCO) films under magnetic
fields. The films' orientation was such that the CuO2 planes were perpendicular
to the surface with the a and b axis at 450 form the normal. The magnetic field
was applied parallel to the surface and perpendicular to the CuO2 planes. The
Zero Bias Conductance Peak (ZBCP) characteristic of nodal tunneling splits
under the effect of surface currents produced by the applied fields. Measuring
this splitting under different field conditions, zero field cooled and field
cooled, reveals that these currents have different origins. By comparing the
field cooled ZBCP splitting to that taken in decreasing fields we deduce a
value of the Bean critical current superfluid velocity, and calculate a Bean
critical current density of up to 3*10^7 A/cm2 at low temperatures. This
tunneling method for the determination of critical currents under magnetic
fields has serious advantages over the conventional one, as it avoids having to
make high current contacts to the sample.Comment: 8 pages, 2 figure
Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}
Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to
J. Mag. Mag. Ma
18 Sco: a solar twin rich in refractory and neutron-capture elements. Implications for chemical tagging
We study with unprecedented detail the chemical composition and stellar
parameters of the solar twin 18 Sco in a strictly differential sense relative
to the Sun. Our study is mainly based on high resolution (R ~ 110 000) high S/N
(800-1000) VLT UVES spectra, which allow us to achieve a precision of about
0.005 dex in differential abundances. The effective temperature and surface
gravity of 18 Sco are Teff = 5823+/-6 K and log g = 4.45+/-0.02 dex, i.e., 18
Sco is 46+/-6 K hotter than the Sun and log g is 0.01+/-0.02 dex higher. Its
metallicity is [Fe/H] = 0.054+/-0.005 dex and its microturbulence velocity is
+0.02+/-0.01 km/s higher than solar. Our precise stellar parameters and
differential isochrone analysis show that 18 Sco has a mass of 1.04+/-0.02M_Sun
and that it is ~1.6 Gyr younger than the Sun. We use precise HARPS radial
velocities to search for planets, but none were detected. The chemical
abundance pattern of 18 Sco displays a clear trend with condensation
temperature, showing thus higher abundances of refractories in 18 Sco than in
the Sun. Intriguingly, there are enhancements in the neutron-capture elements
relative to the Sun. Despite the small element-to-element abundance differences
among nearby n-capture elements (~0.02 dex), we successfully reproduce the
r-process pattern in the solar system. This is independent evidence for the
universality of the r-process. Our results have important implications for
chemical tagging in our Galaxy and nucleosynthesis in general.Comment: ApJ, in pres
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