604 research outputs found
Decoherence in trapped ions due to polarization of the residual background gas
We investigate the mechanism of damping and heating of trapped ions
associated with the polarization of the residual background gas induced by the
oscillating ions themselves. Reasoning by analogy with the physics of surface
electrons in liquid helium, we demonstrate that the decay of Rabi oscillations
observed in experiments on 9Be+ can be attributed to the polarization phenomena
investigated here. The measured sensitivity of the damping of Rabi oscillations
with respect to the vibrational quantum number of a trapped ion is also
predicted in our polarization model.Comment: 26 pdf pages with 5 figures, http://www.df.ufscar.br/~quantum
The R136 star cluster dissected with Hubble Space Telescope/STIS. II. Physical properties of the most massive stars in R136
We present an optical analysis of 55 members of R136, the central cluster in the Tarantula Nebula of the Large Magellanic Cloud. Our sample was observed with STIS aboard the Hubble Space Telescope, is complete down to about 40 Mâ, and includes 7 very massive stars with masses over 100 Mâ. We performed a spectroscopic analysis to derive their physical properties. Using evolutionary models we find that the initial mass function (IMF) of massive stars in R136 is suggestive of being top-heavy with a power-law exponent Îł â 2 ± 0.3, but steeper exponents cannot be excluded. The age of R136 lies between 1 and 2 Myr with a median age of around 1.6 Myr. Stars more luminous than logâL/Lâ = 6.3 are helium enriched and their evolution is dominated by mass loss, but rotational mixing or some other form of mixing could be still required to explain the helium composition at the surface. Stars more massive than 40 Mâ have larger spectroscopic than evolutionary masses. The slope of the wind-luminosity relation assuming unclumped stellar winds is 2.41 ± 0.13 which is steeper than usually obtained (âŒ1.8). The ionising (logâQ0â[ph/s] = 51.4) and mechanical (logâLSWâ[erg/s] = 39.1) output of R136 is dominated by the most massive stars (>100âMâ). R136 contributes around a quarter of the ionising flux and around a fifth of the mechanical feedback to the overall budget of the Tarantula Nebula. For a census of massive stars of the Tarantula Nebula region we combined our results with the VLT-FLAMES Tarantula Survey plus other spectroscopic studies. We observe a lack of evolved Wolf-Rayet stars and luminous blue and red supergiants
cGMP stimulation of cystic fibrosis transmembrane conductance regulator Cl- channels co-expressed with cGMP-dependent protein kinase type II but not type Ibeta
In order to investigate the involvement of cGMP-dependent protein kinase
(cGK) type II in cGMP-provoked intestinal Cl- secretion, cGMP-dependent
activation and phosphorylation of cystic fibrosis transmembrane
conductance regulator (CFTR) Cl- channels was analyzed after expression of
cGK II or cGK Ibeta in intact cells. An intestinal cell line which stably
expresses CFTR (IEC-CF7) but contains no detectable endogenous cGK II was
infected with a recombinant adenoviral vector containing the cGK II coding
region (Ad-cGK II) resulting in co-expression of active cGK II. In these
cells, CFTR was activated by membrane-permeant analogs of cGMP or by the
cGMP-elevating hormone atrial natriuretic peptide as measured by 125I-
efflux assays and whole-cell patch clamp analysis. In contrast, infection
with recombinant adenoviruses expressing cGK Ibeta or luciferase did not
convey cGMP sensitivity to CFTR in IEC-CF7 cells. Concordant with the
activation of CFTR by only cGK II, infection with Ad-cGK II but not Ad-cGK
Ibeta enabled cGMP analogs to increase CFTR phosphorylation in intact
cells. These and other data provide evidence that endogenous cGK II is a
key mediator of cGMP-provoked activation of CFTR in cells where both
proteins are co-localized, e. g. intestinal epithelial cells. Furthermore,
they demonstrate that neither the soluble cGK Ibeta nor cAMP-dependent
protein kinase are able to substitute for cGK II in this cGMP-regulated
function
Experimental evidence for 56Ni-core breaking from the low-spin structure of the N=Z nucleus 58Cu
Low-spin states in the odd-odd N=Z nucleus 58Cu were investigated with the
58Ni(p,n gamma)58Cu fusion evaporation reaction at the FN-tandem accelerator in
Cologne. Seventeen low spin states below 3.6 MeV and 17 new transitions were
observed. Ten multipole mixing ratios and 17 gamma-branching ratios were
determined for the first time. New detailed spectroscopic information on the
2+,2 state, the Isobaric Analogue State (IAS) of the 2+,1,T=1 state of 58Ni,
makes 58Cu the heaviest odd-odd N=Z nucleus with known B(E2;2+,T=1 --> 0+,T=1)
value. The 4^+ state at 2.751 MeV, observed here for the first time, is
identified as the IAS of the 4+,1,T=1 state in 58Ni. The new data are compared
to full pf-shell model calculations with the novel GXPF1 residual interaction
and to calculations within a pf5/2 configurational space with a residual
surface delta interaction. The role of the 56Ni core excitations for the
low-spin structure in 58Cu is discussed.Comment: 15 pages, 7 figures, submitted to Phys. Rev.
Probing the dark matter issue in f(R)-gravity via gravitational lensing
For a general class of analytic f(R)-gravity theories, we discuss the weak
field limit in view of gravitational lensing. Though an additional Yukawa term
in the gravitational potential modifies dynamics with respect to the standard
Newtonian limit of General Relativity, the motion of massless particles results
unaffected thanks to suitable cancellations in the post-Newtonian limit. Thus,
all the lensing observables are equal to the ones known from General
Relativity. Since f(R)-gravity is claimed, among other things, to be a possible
solution to overcome for the need of dark matter in virialized systems, we
discuss the impact of our results on the dynamical and gravitational lensing
analyses. In this framework, dynamics could, in principle, be able to reproduce
the astrophysical observations without recurring to dark matter, but in the
case of gravitational lensing we find that dark matter is an unavoidable
ingredient. Another important implication is that gravitational lensing, in the
post-Newtonian limit, is not able to constrain these extended theories, since
their predictions do not differ from General Relativity.Comment: 7 pages, accepted for publication in EPJ
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
Gas Rich Galaxies and the HI Mass Function
We have developed an automated cross-correlation technique to detect 21cm
emission in sample spectra obtained from the HI Parkes All Sky Survey.
The initial sample selection was the nearest spectra to 2435 low surface
brightness galaxies in the catalogue of Morshidi-Esslinger et al. (1999).
The galaxies were originally selected to have properties similar to Fornax
cluster dE galaxies. As dE galaxies are generally gas poor it is not surprising
that there were only 26 secure detections. All of the detected galaxies have
very high values of . Thus the HI selection of faint
optical sources leads to the detection of predominately gas rich galaxies. The
gas rich galaxies tend to reside on the outskirts of the large scale structure
delineated by optically selected galaxies, but they do appear to be associated
with it. These objects appear to have similar relative dark matter content to
optically selected galaxies. The HI column densities are lower than the
'critical density' necessary for sustainable star formation and they appear,
relatively, rather isolated from companion galaxies. These two factors may
explain their high relative gas content. We have considered the HI mass
function by looking at the distribution of velocities of HI detections in
random spectra on the sky.
The inferred HI mass function is steep though confirmation of this results
awaits a detailed study of the noise characteristics of the HI survey.Comment: MNRAS in pres
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
The sensitivity of a search for sources of TeV neutrinos can be improved by
grouping potential sources together into generic classes in a procedure that is
known as source stacking. In this paper, we define catalogs of Active Galactic
Nuclei (AGN) and use them to perform a source stacking analysis. The grouping
of AGN into classes is done in two steps: first, AGN classes are defined, then,
sources to be stacked are selected assuming that a potential neutrino flux is
linearly correlated with the photon luminosity in a certain energy band (radio,
IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino
production in AGN, this correlation is motivated by hadronic AGN models, as
briefly reviewed in this paper.
The source stacking search for neutrinos from generic AGN classes is
illustrated using the data collected by the AMANDA-II high energy neutrino
detector during the year 2000. No significant excess for any of the suggested
groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic
Cosmological parameters from SDSS and WMAP
We measure cosmological parameters using the three-dimensional power spectrum
P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in
combination with WMAP and other data. Our results are consistent with a
``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt,
tensor modes or massive neutrinos. Adding SDSS information more than halves the
WMAP-only error bars on some parameters, tightening 1 sigma constraints on the
Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter
density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on
neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when
dropping prior assumptions about curvature, neutrinos, tensor modes and the
equation of state. Our results are in substantial agreement with the joint
analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive
consistency check with independent redshift survey data and analysis
techniques. In this paper, we place particular emphasis on clarifying the
physical origin of the constraints, i.e., what we do and do not know when using
different data sets and prior assumptions. For instance, dropping the
assumption that space is perfectly flat, the WMAP-only constraint on the
measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to
t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running
tilt, neutrino mass and equation of state in the list of free parameters, many
constraints are still quite weak, but future cosmological measurements from
SDSS and other sources should allow these to be substantially tightened.Comment: Minor revisions to match accepted PRD version. SDSS data and ppt
figures available at http://www.hep.upenn.edu/~max/sdsspars.htm
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