511 research outputs found
An improved version of the Implicit Integral Method to solving radiative transfer problems
Radiative transfer (RT) problems in which the source function includes a
scattering-like integral are typical two-points boundary problems. Their
solution via differential equations implies to make hypotheses on the solution
itself, namely the specific intensity I(tau;n) of the radiation field. On the
contrary, integral methods require to make hypotheses on the source function
S(tau). It looks of course more reasonable to make hypotheses on the latter
because one can expect that the run of S(tau) with depth be smoother than that
of I(tau;n).
In previous works we assumed a piece-wise parabolic approximation for the
source function, which warrants the continuity of S(tau) and its first
derivative at each depth point. Here we impose the continuity of the second
derivative S"(tau). In other words, we adopt a cubic spline representation to
the source function, which highly stabilize the numerical processes.Comment: Accepted for publication in Astrophysics (2012, N.1
Structural properties of disk galaxies I. The intrinsic ellipticity of bulges
(Abridged) A variety of formation scenarios was proposed to explain the
diversity of properties observed in bulges. Studying their intrinsic shape can
help in constraining the dominant mechanism at the epochs of their assembly.
The structural parameters of a magnitude-limited sample of 148 unbarred S0--Sb
galaxies were derived in order to study the correlations between bulges and
disks as well as the probability distribution function (PDF) of the intrinsic
equatorial ellipticity of bulges. It is presented a new fitting algorithm
(GASP2D) to perform the two-dimensional photometric decomposition of galaxy
surface-brightness distribution. This was assumed to be the sum of the
contribution of a bulge and disk component characterized by elliptical and
concentric isophotes with constant (but possibly different) ellipticity and
position angles. Bulge and disk parameters of the sample galaxies were derived
from the J-band images which were available in the Two Micron All Sky Survey.
The PDF of the equatorial ellipticity of the bulges was derived from the
distribution of the observed ellipticities of bulges and misalignments between
bulges and disks. Strong correlations between the bulge and disk parameters
were found. About 80% of bulges in unbarred lenticular and
early-to-intermediate spiral galaxies are not oblate but triaxial ellipsoids.
Their mean axial ratio in the equatorial plane is = 0.85. There is not
significant dependence of their PDF on morphology, light concentration, and
luminosity. The interplay between bulge and disk parameters favors scenarios in
which bulges assembled from mergers and/or grew over long times through disk
secular evolution. But all these mechanisms have to be tested against the
derived distribution of bulge intrinsic ellipticities.Comment: 24 pages, 13 figures, accepted for publication in A&A, corrected
proof
Inversion of stellar statistics equation for the Galactic Bulge
A method based on Lucy (1974, AJ 79, 745) iterative algorithm is developed to
invert the equation of stellar statistics for the Galactic bulge and is then
applied to the K-band star counts from the Two-Micron Galactic Survey in a
number of off-plane regions (10 deg.>|b|>2 deg., |l|<15 deg.).
The top end of the K-band luminosity function is derived and the morphology
of the stellar density function is fitted to triaxial ellipsoids, assuming a
non-variable luminosity function within the bulge. The results, which have
already been outlined by Lopez-Corredoira et al.(1997, MNRAS 292, L15), are
shown in this paper with a full explanation of the steps of the inversion: the
luminosity function shows a sharp decrease brighter than M_K=-8.0 mag when
compared with the disc population; the bulge fits triaxial ellipsoids with the
major axis in the Galactic plane at an angle with the line of sight to the
Galactic centre of 12 deg. in the first quadrant; the axial ratios are
1:0.54:0.33, and the distance of the Sun from the centre of the triaxial
ellipsoid is 7860 pc. The major-minor axial ratio of the ellipsoids is found
not to be constant. However, the interpretation of this is controversial. An
eccentricity of the true density-ellipsoid gradient and a population gradient
are two possible explanations.
The best fit for the stellar density, for 1300 pc<t<3000 pc, are calculated
for both cases, assuming an ellipsoidal distribution with constant axial
ratios, and when K_z is allowed to vary. From these, the total number of bulge
stars is ~ 3 10^{10} or ~ 4 10^{10}, respectively.Comment: 19 pages, 23 figures, accepted in MNRA
Physical requirements for modeling stellar atmospheres according to the different spectral features observed
n/
Astrophysical inputs on the SUSY dark matter annihilation detectability
If dark matter (DM), which is considered to constitute most of the mass of
galaxies, is made of supersymmetric (SUSY) particles, the centers of galaxies
should emit gamma-rays produced by their self-annihilation. We present accurate
estimates of continuum gamma-ray fluxes due to neutralino annihilation in the
central regions of the Milky Way. We use detailed models of our Galaxy, which
satisfy available observational data, and include some important physical
processes, which were previously neglected. Our models predict that spatially
extended annihilation signal should be detected at high confidence levels by
incoming experiments assuming that neutralinos make up most of the DM in the
Universe and that they annihilate according to current SUSY models.Comment: 4 pages, submitted to Physical Review Letter
Riociguat for the treatment of chronic thromboembolic pulmonary hypertension.
BACKGROUND: Riociguat, a member of a new class of compounds (soluble guanylate cyclase stimulators), has been shown in previous clinical studies to be beneficial in the treatment of chronic thromboembolic pulmonary hypertension. METHODS: In this phase 3, multicenter, randomized, double-blind, placebo-controlled study, we randomly assigned 261 patients with inoperable chronic thromboembolic pulmonary hypertension or persistent or recurrent pulmonary hypertension after pulmonary endarterectomy to receive placebo or riociguat. The primary end point was the change from baseline to the end of week 16 in the distance walked in 6 minutes. Secondary end points included changes from baseline in pulmonary vascular resistance, N-terminal pro-brain natriuretic peptide (NT-proBNP) level, World Health Organization (WHO) functional class, time to clinical worsening, Borg dyspnea score, quality-of-life variables, and safety. RESULTS: By week 16, the 6-minute walk distance had increased by a mean of 39 m in the riociguat group, as compared with a mean decrease of 6 m in the placebo group (least-squares mean difference, 46 m; 95% confidence interval [CI], 25 to 67; P<0.001). Pulmonary vascular resistance decreased by 226 dyn · sec · cm-5in the riociguat group and increased by 23 dyn · sec · cm-5in the placebo group (least-squares mean difference, -246 dyn · sec · cm-5; 95% CI, -303 to -190; P<0.001). Riociguat was also associated with significant improvements in the NT-proBNP level (P<0.001) and WHO functional class (P = 0.003). The most common serious adverse events were right ventricular failure (in 3% of patients in each group) and syncope (in 2% of the riociguat group and in 3% of the placebo group). CONCLUSIONS: Riociguat significantly improved exercise capacity and pulmonary vascular resistance in patients with chronic thromboembolic pulmonary hypertension. (Funded by Bayer HealthCare; CHEST-1 and CHEST-2 ClinicalTrials.gov numbers, NCT00855465 and NCT00910429, respectively.) Copyright © 2013 Massachusetts Medical Society
Impact of pump OSNR on noise figure for fiber-optical parametric amplifiers
Electrical measurements of the noise figure of a fiberoptical parametric amplifier are presented and compared with optical measurements. The transfer of pump noise by four-wave mixing was clearly demonstrated. A numerical model was developed to simulate the transfer of pump noise and validated by these measurements. Using this model, we determine, for practical systems, a minimum required pump optical signal-to-noise ratio of 65 dB
Impact of pump phase modulation on system performance of fibre-optical parametric amplifiers
Bit error rate induced in a parametric amplifier has been measured with a 10 Gbit=s line rate. It is shown experimentally that the phase modulation of the pump distorts the mark level of the channel and may cause system penalty. Different phase modulation schemes have been compared
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