91 research outputs found
DQ white-dwarf stars with low C abundance: Possible progenitors
The present paper focuses on the evolution of hydrogen-deficient white dwarfs
with the aim of exploring the consequences of different initial envelope
structures on the carbon abundances expected in helium-rich,
carbon-contaminated DQ white dwarfs. In particular, the evolutionary link
between the DQs with low detected carbon abundances and the PG1159, extreme
horizontal branch, and helium-rich R Coronae Borealis (RCrB) stars is explored.
We present full evolutionary calculations that take a self-consistent treatment
of element diffusion into account as well as expectations for the outer layer
chemical stratification of progenitor stars upon entering the white dwarf
regime. We find that PG1159 stars cannot be related to any DQ white dwarfs with
low C abundances. Instead, we suggest that the latter could constitute the
progeny of the giant, helium-rich RCrB stars.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Time variation of the fine structure constant in the early universe and the Bekenstein model
We calculate bounds on the variation of the fine structure constant at the
time of primordial nucleosynthesis and at the time of neutral hydrogen
formation. We use these bounds and other bounds from the late universe to test
Bekenstein model. We modify the Kawano code, CAMB and CosmoMC in order to
include the possible variation of the fine structure constant. We use
observational primordial abundances of \De, \He and \Li, recent data from
the Cosmic Microwave Background and the 2dFGRS power spectrum, to obtain bounds
on the variation of . We calculate a piecewise solution to the scalar
field equation of Bekenstein model in two different regimes; i) matter and
radiation, ii) matter and cosmological constant. We match both solutions with
appropriate boundary conditions. We perform a statistical analysis using the
bounds obtained from the early universe and other bounds from the late universe
to constrain the free parameters of the model. Results are consistent with no
variation of in the early universe. Limits on are
inconsistent with the scale length of the theory being larger than Planck
scale. In order to fit all observational and experimental data, the assumption
implied in Bekenstein's model has to be relaxed.Comment: 13 pages, 8 figures,version accepted to be published in Astronomy and
Astrophysic
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The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological implications of the large-scale two-point correlation function
We obtain constraints on cosmological parameters from the spherically
averaged redshift-space correlation function of the CMASS Data Release 9 (DR9)
sample of the Baryonic Oscillation Spectroscopic Survey (BOSS). We combine this
information with additional data from recent CMB, SN and BAO measurements. Our
results show no significant evidence of deviations from the standard
flat-Lambda CDM model, whose basic parameters can be specified by Omega_m =
0.285 +- 0.009, 100 Omega_b = 4.59 +- 0.09, n_s = 0.96 +- 0.009, H_0 = 69.4 +-
0.8 km/s/Mpc and sigma_8 = 0.80 +- 0.02. The CMB+CMASS combination sets tight
constraints on the curvature of the Universe, with Omega_k = -0.0043 +- 0.0049,
and the tensor-to-scalar amplitude ratio, for which we find r < 0.16 at the 95
per cent confidence level (CL). These data show a clear signature of a
deviation from scale-invariance also in the presence of tensor modes, with n_s
<1 at the 99.7 per cent CL. We derive constraints on the fraction of massive
neutrinos of f_nu < 0.049 (95 per cent CL), implying a limit of sum m_nu < 0.51
eV. We find no signature of a deviation from a cosmological constant from the
combination of all datasets, with a constraint of w_DE = -1.033 +- 0.073 when
this parameter is assumed time-independent, and no evidence of a departure from
this value when it is allowed to evolve as w_DE(a) = w_0 + w_a (1 - a). The
achieved accuracy on our cosmological constraints is a clear demonstration of
the constraining power of current cosmological observations.Comment: 26 pages, 15 figures. Minor changes to match version accepted by
MNRA
Dimensionless cosmology
Although it is well known that any consideration of the variations of
fundamental constants should be restricted to their dimensionless combinations,
the literature on variations of the gravitational constant is entirely
dimensionful. To illustrate applications of this to cosmology, we explicitly
give a dimensionless version of the parameters of the standard cosmological
model, and describe the physics of Big Bang Neucleosynthesis and recombination
in a dimensionless manner. The issue that appears to have been missed in many
studies is that in cosmology the strength of gravity is bound up in the
cosmological equations, and the epoch at which we live is a crucial part of the
model. We argue that it is useful to consider the hypothetical situation of
communicating with another civilization (with entirely different units),
comparing only dimensionless constants, in order to decide if we live in a
Universe governed by precisely the same physical laws. In this thought
experiment, we would also have to compare epochs, which can be defined by
giving the value of any {\it one} of the evolving cosmological parameters. By
setting things up carefully in this way one can avoid inconsistent results when
considering variable constants, caused by effectively fixing more than one
parameter today. We show examples of this effect by considering microwave
background anisotropies, being careful to maintain dimensionlessness
throughout. We present Fisher matrix calculations to estimate how well the fine
structure constants for electromagnetism and gravity can be determined with
future microwave background experiments. We highlight how one can be misled by
simply adding to the usual cosmological parameter set
The formation and evolution of hydrogen-deficient post-AGB white dwarfs: the emerging chemical profile and the expectations for the PG1159-DB-DQ evolutionary connection
This paper is designed to explore the formation and evolution of
hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete
evolution of an initially 2.7 M_sun star from the zero-age main sequence
through the thermally pulsing and mass-loss phases to the white dwarf stage.
Particular attention is given to the chemical abundance changes during the
whole evolution. A time-dependent scheme for the simultaneous treatment of
abundance changes caused by nuclear reactions, diffusive overshooting, salt
fingers and convection is considered. We employed the double-diffusive
mixing-length theory of convection for fluids with composition gradients
(Grossman & Taam 1996). The study can therefore be considered as a test of its
performance in low-mass stars. Also, time-dependent element diffusion for
multicomponent gases is taken into account during the white dwarf evolution.
The evolutionary stages corresponding to the last helium thermal pulse on the
early white-dwarf cooling branch and the following born-again episode are
carefully explored. Relevant aspects for PG1159 stars and DB white dwarf
evolution are studied in the frame of these new evolutionary models that take
into account the history of the white dwarf progenitor. The scope of the
calculations is extended to the domain of the helium-rich, carbon-contaminated
DQ white dwarfs with the aim of exploring the plausibility of the evolutionary
connection PG1159-DB-DQ. In this regard, the implications for the
double-layered chemical structure in pulsating DB white dwarfs is investigated.
Another aspect of the investigation concerned the consequences of mass-loss
episodes during the PG1159 stage for the chemical stratification of the outer
layer of DB and DQ white dwarfs.Comment: 20 pages, 15 figures. Accepted by Astronomy & Astrophysic
The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
The Sloan Digital Sky Survey III (SDSS-III) presents the first spectroscopic
data from the Baryon Oscillation Spectroscopic Survey (BOSS). This ninth data
release (DR9) of the SDSS project includes 535,995 new galaxy spectra (median
z=0.52), 102,100 new quasar spectra (median z=2.32), and 90,897 new stellar
spectra, along with the data presented in previous data releases. These spectra
were obtained with the new BOSS spectrograph and were taken between 2009
December and 2011 July. In addition, the stellar parameters pipeline, which
determines radial velocities, surface temperatures, surface gravities, and
metallicities of stars, has been updated and refined with improvements in
temperature estimates for stars with T_eff<5000 K and in metallicity estimates
for stars with [Fe/H]>-0.5. DR9 includes new stellar parameters for all stars
presented in DR8, including stars from SDSS-I and II, as well as those observed
as part of the SDSS-III Sloan Extension for Galactic Understanding and
Exploration-2 (SEGUE-2).
The astrometry error introduced in the DR8 imaging catalogs has been
corrected in the DR9 data products. The next data release for SDSS-III will be
in Summer 2013, which will present the first data from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) along with another year of
data from BOSS, followed by the final SDSS-III data release in December 2014.Comment: 9 figures; 2 tables. Submitted to ApJS. DR9 is available at
http://www.sdss3.org/dr
The Baryon Oscillation Spectroscopic Survey of SDSS-III
The Baryon Oscillation Spectroscopic Survey (BOSS) is designed to measure the
scale of baryon acoustic oscillations (BAO) in the clustering of matter over a
larger volume than the combined efforts of all previous spectroscopic surveys
of large scale structure. BOSS uses 1.5 million luminous galaxies as faint as
i=19.9 over 10,000 square degrees to measure BAO to redshifts z<0.7.
Observations of neutral hydrogen in the Lyman alpha forest in more than 150,000
quasar spectra (g<22) will constrain BAO over the redshift range 2.15<z<3.5.
Early results from BOSS include the first detection of the large-scale
three-dimensional clustering of the Lyman alpha forest and a strong detection
from the Data Release 9 data set of the BAO in the clustering of massive
galaxies at an effective redshift z = 0.57. We project that BOSS will yield
measurements of the angular diameter distance D_A to an accuracy of 1.0% at
redshifts z=0.3 and z=0.57 and measurements of H(z) to 1.8% and 1.7% at the
same redshifts. Forecasts for Lyman alpha forest constraints predict a
measurement of an overall dilation factor that scales the highly degenerate
D_A(z) and H^{-1}(z) parameters to an accuracy of 1.9% at z~2.5 when the survey
is complete. Here, we provide an overview of the selection of spectroscopic
targets, planning of observations, and analysis of data and data quality of
BOSS.Comment: 49 pages, 16 figures, accepted by A
QUBIC VI: cryogenic half wave plate rotator, design and performances
Inflation Gravity Waves B-Modes polarization detection is the ultimate goal
of modern large angular scale cosmic microwave background (CMB) experiments
around the world. A big effort is undergoing with the deployment of many
ground-based, balloon-borne and satellite experiments using different methods
to separate this faint polarized component from the incoming radiation. One of
the largely used technique is the Stokes Polarimetry that uses a rotating
half-wave plate (HWP) and a linear polarizer to separate and modulate the
polarization components with low residual cross-polarization. This paper
describes the QUBIC Stokes Polarimeter highlighting its design features and its
performances. A common systematic with these devices is the generation of large
spurious signals synchronous with the rotation and proportional to the
emissivity of the optical elements. A key feature of the QUBIC Stokes
Polarimeter is to operate at cryogenic temperature in order to minimize this
unwanted component. Moving efficiently this large optical element at low
temperature constitutes a big engineering challenge in order to reduce friction
power dissipation. Big attention has been given during the designing phase to
minimize the differential thermal contractions between parts. The rotation is
driven by a stepper motor placed outside the cryostat to avoid thermal load
dissipation at cryogenic temperature. The tests and the results presented in
this work show that the QUBIC polarimeter can easily achieve a precision below
0.1{\deg} in positioning simply using the stepper motor precision and the
optical absolute encoder. The rotation induces only few mK of extra power load
on the second cryogenic stage (~ 8 K).Comment: Part of a series of 8 papers on QUBIC to be submitted to a special
issue of JCA
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