925 research outputs found
A multi-method approach to radial-velocity measurement for single-object spectra
The derivation of radial velocities from large numbers of spectra that
typically result from survey work, requires automation. However, except for the
classical cases of slowly rotating late-type spectra, existing methods of
measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to
the idiosyncrasies of individual spectra. The radial velocity spectrometer
(RVS) on the Gaia mission, which will start operating very soon, prompted a new
attempt at creating a measurement pipeline to handle a wide variety of spectral
types.
The present paper describes the theoretical background on which this software
is based. However, apart from the assumption that only synthetic templates are
used, we do not rely on any of the characteristics of this instrument, so our
results should be relevant for most telescope-detector combinations.
We propose an approach based on the simultaneous use of several alternative
measurement methods, each having its own merits and drawbacks, and conveying
the spectral information in a different way, leading to different values for
the measurement. A comparison or a combination of the various results either
leads to a "best estimate" or indicates to the user that the observed spectrum
is problematic and should be analysed manually.
We selected three methods and analysed the relationships and differences
between them from a unified point of view; with each method an appropriate
estimator for the individual random error is chosen. We also develop a
procedure for tackling the problem of template mismatch in a systematic way.
Furthermore, we propose several tests for studying and comparing the
performance of the various methods as a function of the atmospheric parameters
of the observed objects. Finally, we describe a procedure for obtaining a
knowledge-based combination of the various Doppler-shift measurements.Comment: 16 pages, 4 figure
Self-gravitating clouds of generalized Chaplygin and modified anti-Chaplygin Gases
The Chaplygin gas has been proposed as a possible dark energy, dark matter
candidate. As a working fluid in a Friedmann-Robertson-Walker universe, it
exhibits early behavior reminiscent of dark matter, but at later times is more
akin to a cosmological constant. In any such universe, however, one can expect
local perturbations to form. Here we obtain the general equations for a
self-gravitating relativistic Chaplygin gas. We solve these equations and
obtain the mass-radius relationship for such structures, showing that only in
the phantom regime is the mass-radius relationship large enough to be a serious
candidate for highly compact massive objects at the galaxy core. In addition,
we study the cosmology of a modified anti-Chaplygin gas. A self-gravitating
cloud of this matter is an exact solution to Einstein's equations.Comment: 16 page
Dowe really need an accurate calibration pattern?
The metrology of calibration patterns is often a real problem in computer vision to obtain a reliable estimation of the intrinsic
and extrinsic parameters that model a video camera device . In order to take into account these errors a self-calibration method
is described, that enables us to compute in the same time the traditional calibration parameters and the 3D geometry of the
calibration pattern using a multi-images calibration algorithm . Experimental results shows that the proposed algorithm leads to
reliable calibration results and proves that calibration errors no longer depend on the accuracy of calibration point measurement,
but on the accuracy of calibration point detection in the image plane .Cet article soulĂšve le problĂšme de l'influence des erreurs introduites par la mĂ©trologie d'une mire d'Ă©talonnage sur la dĂ©termination des paramĂštres intrinsĂšques d'un capteur vidĂ©o. Afin de s'affranchir de ces erreurs, une approche d'autocalibrage est dĂ©crite permettant conjointement d'estimer les paramĂštres internes du capteur vidĂ©o et la gĂ©omĂ©trie de la mire, au sein d'un formalisme d'Ă©talonnage multi-images. Par la problĂ©matique qu'il soulĂšve, cet article cotoie les approches de Shape from Motion et d'Auto-calibrage des capteurs, en gardant cependant la spĂ©cificitĂ© d'ĂȘtre dĂ©crit selon un formalisme photogrammĂ©trique et donc d'intĂ©grer implicitement les aberrations gĂ©omĂ©triques des objectifs. Nous nous sommes attachĂ©s Ă insister sur l'aspect descriptif de la mĂ©thode pour permettre aux lecteurs une vision globale de la mise en oeuvre du procĂ©dĂ©
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
CH 3 GHz Observations of the Galactic Center
A 3 3 map of the Galactic Center was made at 9\arcmin resolution
and 10\arcmin spacing in the CH , J=1/2, F=1-1 transition at
3335 MHz. The CH emission shows a velocity extent that is nearly that of the
CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335
MHz CH transition primarily traces low-density molecular gas and our
observations indicate that the mass of this component within 30 pc of
the Galactic Center is 9 10 M. The CO-H
conversion factor obtained for the low-density gas in the mapped region is
greater than that thought to apply to the dense molecular gas at the Galactic
Center. In addition to tracing the low-density molecular gas at the Galactic
Center, the CH spectra show evidence of emission from molecular clouds along
the line of sight both in the foreground and background. The scale height of
these clouds ranges from 27 - 109 pc, consistent with previous work based on
observations of molecular clouds in the inner Galaxy.Comment: 29 pages, 12 figure
CH 3 GHz Observations of Molecular Clouds Along the Galactic Plane
Spectra in the CH , J=1/2, F=1-1 transition at 3335 MHz were
obtained in three 5-point crosses centered on the Galactic plane at
50\arcdeg, 100\arcdeg, and 110\arcdeg. The lines of sight traverse both
Giant Molecular Clouds (GMCs) and local, smaller entities. This transition is a
good tracer of low-density molecular gas and the line profiles are very similar
to CO(1-0) data at nearly the same resolution. In addition, the CH 3335 MHz
line can be used to calibrate the CO-H conversion factor (X) in
low-density molecular gas. Although this technique underestimates X
in GMCs, our results are within a factor of two of X values
calibrated for GMCs by other techniques. The similarity of CH and CO line
profiles, and that of X values derived from CH and more traditional
techniques, implies that most of the molecular gas along the observed lines of
sight is at relatively low densities ( 10 cm).Comment: 26 pages, 12 figures, submitted to the AJ, revised after referee
repor
Multi--Pressure Polytropes as Models for the Structure and Stability of Molecular Clouds. I. Theory
Molecular clouds are supported by thermal pressure, magnetic pressure, and
turbulent pressure. Each of these can be modeled with a polytropic equation of
state, so that overall the total pressure is the sum of the individual
components. We model the turbulent pressure as being due to a superposition of
Alfven waves. The theory of polytropes is generalized to allow for the flow of
entropy in response to a perturbation, as expected for the entropy associated
with wave pressure. The equation of state of molecular clouds is "soft", so
that the properties of the clouds are generally governed by the conditions at
the surface. In general, the polytropes are not isentropic, and this permits
large density and pressure drops to occur between the center and the edge of
the polytropes, as is observed.Comment: Submitted to ApJ with 10 figure
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