28,766 research outputs found
Cosmological parameter inference with Bayesian statistics
Bayesian statistics and Markov Chain Monte Carlo (MCMC) algorithms have found
their place in the field of Cosmology. They have become important mathematical
and numerical tools, especially in parameter estimation and model comparison.
In this paper, we review some fundamental concepts to understand Bayesian
statistics and then introduce MCMC algorithms and samplers that allow us to
perform the parameter inference procedure. We also introduce a general
description of the standard cosmological model, known as the CDM
model, along with several alternatives, and current datasets coming from
astrophysical and cosmological observations. Finally, with the tools acquired,
we use an MCMC algorithm implemented in python to test several cosmological
models and find out the combination of parameters that best describes the
Universe.Comment: 30 pages, 17 figures, 5 tables; accepted for publication in Universe;
references adde
Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes - diffuse interstellar bands connection
We present high-resolution (R~15000) VLT/UVES optical spectra of two
planetary nebulae (PNe; Tc 1 and M 1-20) where C60 (and C70) fullerenes have
already been found. These spectra are of high-quality (S/N > 300) for PN Tc 1,
which permits us to search for the expected electronic transitions of neutral
C60 and diffuse interstellar bands (DIBs). Surprisingly, we report the
non-detection of the most intense optical bands of C60 in Tc 1, although this
could be explained by the low C60 column density estimated from the C60
infrared bands if the C60 emission peaks far away from the central star. The
strongest and most common DIBs in both fullerene PNe are normal for their
reddening. Interestingly, the very broad 4428 A DIB and the weaker 6309 A DIB
are found to be unusually intense in Tc 1. We also report the detection of a
new broad (FWHM~5 A) unidentified band at ~6525 A. We propose that the 4428 A
DIB (probably also the 6309 A DIB and the new 6525 A band) may be related to
the presence of larger fullerenes (e.g., C80, C240, C320, and C540) and
buckyonions (multishell fullerenes such as C60@C240 and C60@C240@C540) in the
circumstellar envelope of Tc 1.Comment: Accepted for publication in Astronomy & Astrophysics Letters (6
pages, 4 figures, and 1 Table
A search for hydrogenated fullerenes in fullerene-containing planetary nebulae
Detections of C60 and C70 fullerenes in planetary nebulae (PNe) of the
Magellanic Clouds and of our own Galaxy have raised the idea that other forms
of carbon such as hydrogenated fullerenes (fulleranes like C60H36 and C60H18),
buckyonions, and carbon nanotubes, may be widespread in the Universe. Here we
present VLT/ISAAC spectra (R ~600) in the 2.9-4.1 microns spectral region for
the Galactic PNe Tc 1 and M 1-20, which have been used to search for
fullerene-based molecules in their fullerene-rich circumstellar environments.
We report the non-detection of the most intense infrared bands of several
fulleranes around ~3.4-3.6 microns in both PNe. We conclude that if fulleranes
are present in the fullerene-containing circumstellar environments of these
PNe, then they seem to be by far less abundant than C60 and C70. Our
non-detections together with the (tentative) fulleranes detection in the
proto-PN IRAS 01005+7910 suggest that fulleranes may be formed in the short
transition phase between AGB stars and PNe but they are quickly destroyed by
the UV radiation field from the central star.Comment: Accepted for publication in Astronomy & Astrophysics (7 pages, 3
figures, and 3 Tables
Trade Shoks and Aggregate Fluctuations in an Oil-Exporting Economy
In this paper we analyze the role of trade shocks in shaping aggregate fluctuations in Venezuela from 1950 to 1995. To this end a stochastic general equilibrium model of a small open economy whose main productive activity rests in the exports of a single basic product is specified. Shocks to the terms of trade which are directly associated to oil price changes are modelled as a foreign transfer. We find that this approach gives predictions that are consistent with the time series properties of Venezuela when i) the income efect of consumption more than compensates the substitution effect that generates the oil transfer and, ii) there is imperfect capital mobility. In particular, our model specification captures the observed patterns of the main aggregates after the oil resource boom of 1974.Trade shocks, Aggregate fuctuations, Emerging economies.
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