973 research outputs found
The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB
We have investigated the dynamical interaction of low- and-intermediate mass
stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first
paper, we examine the structures generated by the stellar winds during the
Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind
history predicted by stellar evolution. The influence of the external ISM is
also taken into account. We find that the wind variations associated with the
thermal pulses lead to the formation of transient shells with an average
lifetime of 20,000 yr, and consequently do not remain recorded in the density
or velocity structure of the gas. The formation of shells that survive at the
end of the AGB occurs via two main processes: shocks between the shells formed
by two consecutive enhancements of the mass-loss or via continuous accumulation
of the material ejected by the star in the interaction region with the ISM. Our
models show that the mass of the circumstellar envelope increases appreciably
due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar
model). We also point out the importance of the ISM on the deceleration and
compression of the external shells. According to our simulations, large regions
(up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars
are expected. These large regions of gas are formed from the mass-loss
experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
The Evolution of the Kinematics of Nebular Shells in Planetary Nebulae in the Milky Way Bulge
We study the line widths in the [\ion{O}{3}]5007 and H lines
for two groups of planetary nebulae in the Milky Way bulge based upon
spectroscopy obtained at the Observatorio Astron\'omico Nacional in the Sierra
San Pedro M\'artir (OAN-SPM) using the Manchester Echelle Spectrograph. The
first sample includes objects early in their evolution, having high H
luminosities, but [\ion{O}{3}]. The second
sample comprises objects late in their evolution, with \ion{He}{2} . These planetary nebulae represent evolutionary
phases preceeding and following those of the objects studied by Richer et al.
(2008). Our sample of planetary nebulae with weak [\ion{O}{3}]5007 has
a line width distribution similar to that of the expansion velocities of the
envelopes of AGB stars, and shifted to systematically lower values as compared
to the less evolved objects studied by Richer et al. (2008). The sample with
strong \ion{He}{2} has a line width distribution
indistinguishable from that of the more evolved objects from Richer et al.
(2008), but a distribution in angular size that is systematically larger and so
they are clearly more evolved. These data and those of Richer et al. (2008)
form a homogeneous sample from a single Galactic population of planetary
nebulae, from the earliest evolutionary stages until the cessation of nuclear
burning in the central star. They confirm the long-standing predictions of
hydrodynamical models of planetary nebulae, where the kinematics of the nebular
shell are driven by the evolution of the central star.Comment: accepted for publication in the Astrophysical Journa
Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets
We investigate the effect of magnetic fields on the propagation dynamics and
morphology of overdense, radiatively cooling, supermagnetosonic jets, with the
help of fully three-dimensional SPMHD simulations. Evaluated for a set of
parameters which are mainly suitable for protostellar jets (with density ratios
between the jet and the ambient medium 3-10, and ambient Mach number ~ 24),
these simulations are also compared with baseline non-magnetic and adiabatic
calculations. We find that, after amplification by compression and
re-orientation in nonparallel shocks at the working surface, the magnetic field
that is carried backward with the shocked gas into the cocoon improves the jet
collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude,
approximately equally spaced internal shocks (which are absent in the HD
systems) are produced by MHD K-H reflection pinch modes. The longitudinal field
geometry also excites non-axisymmetric helical modes which cause some beam
wiggling. The strength and amount of these modes are, however, reduced (by ~
twice) in the presence of radiative cooling relative to the adiabatic cases.
Besides, a large density ratio between the jet and the ambient medium also
reduces, in general, the number of the internal shocks. As a consequence, the
weakness of the induced internal shocks makes it doubtful that the magnetic
pinches could produce by themselves the bright knots observed in the overdense,
radiatively cooling protostellar jets.Comment: To appear in ApJ; 36 pages + 16 (gif) figures. PostScript files of
figures are available at http://www.iagusp.usp.br/preprints/preprint.htm
Evolution of Planetary Nebulae I. An improved synthetic model
We present a new synthetic model to follow the evolution of a planetary
nebula (PN) and its central star, starting from the onset of AGB phase up to
the white dwarf cooling sequence. The model suitably combines various
analytical prescriptions to account for different (but inter-related) aspects
of planetary nebulae, such as: the dynamical evolution of the primary shell and
surrounding ejecta, the photoionisation of H and He by the central star, the
nebular emission of a few relevant optical lines (e.g. Hbeta; HeII4686;
[OIII]5007). Predictions of the synthetic model are tested by comparison with
both findings of hydrodynamical calculations, and observations of Galactic PNe.
The sensitiveness of the results to the models parameters (e.g. transition
time, mass of the central star, H-/He-burning tracks, etc.) is also discussed.
We briefly illustrate the systematic differences that are expected in the
luminosities and lifetimes of PNe with either H- or He-burning central stars,
which result in different ``detection probabilities'' across the H-R diagram,
in both Hbeta and [OIII]5007 lines. Adopting reasonable values of the model
parameters, we are able to reproduce, in a satisfactory way, many general
properties of PNe, like the ionised mass--nebular radius relationship, the
trends of a few main nebular line ratios, and the observed ranges of nebular
shell thicknesses, electron densities, and expansion velocities. The models
naturally predict also the possible transitions from optically-thick to
optically-thin configurations (and vice versa). In this context, the origin of
the Zanstra discrepancy is also analysed. (abridged)Comment: 28 pages, 23 postscript figures, to appear in A&
Conduction and cooling flows
Chandra and XMM-Newton observations have confirmed the presence of large
temperature gradients within the cores of many relaxed clusters of galaxies.
Here we investigate whether thermal conduction operating over those gradients
can supply sufficient heat to offset radiative cooling. Narayan & Medvedev
(2001) and Gruzinov (2002) have noted, using published results on cluster
temperatures, that conduction within a factor of a few of the Spitzer rate is
sufficient to balance bremsstrahlung cooling. From a detailed study of the
temperature and emission measure profiles of Abell 2199 and Abell 1835, we find
that the heat flux required by conduction is consistent with or below the rate
predicted by Spitzer in the outer regions of the core. Conduction may therefore
explain the lack of observational evidence for large mass cooling rates
inferred from arguments based simply on radiative cooling, provided that
conductivity is suppressed by no more than a factor of three below the full
Spitzer rate. To stem cooling in the cluster centre, however, would necessitate
conductivity values at least a factor of two larger than the Spitzer values,
which we consider implausible. This may provide an explanation for the observed
star formation and optical nebulosities in cluster cores. The solution is
likely to be time dependent. We briefly discuss the possible origin of the
cooler gas and the implications for massive galaxies.Comment: 5 pages, 4 figures, accepted by MNRAS. Minor changes following
referee's comment
Constraining interactions mediated by axion-like particles with ultracold neutrons
We report a new limit on a possible short range spin-dependent interaction
from the precise measurement of the ratio of Larmor precession frequencies of
stored ultracold neutrons and Hg atoms confined in the same volume. The
measurement was performed in a 1 T vertical magnetic holding field
with the apparatus searching for a permanent electric dipole moment of the
neutron at the Paul Scherrer Institute. A possible coupling between freely
precessing polarized neutron spins and unpolarized nucleons of the wall
material can be investigated by searching for a tiny change of the precession
frequencies of neutron and mercury spins. Such a frequency change can be
interpreted as a consequence of a short range spin-dependent interaction that
could possibly be mediated by axions or axion-like particles. The interaction
strength is proportional to the CP violating product of scalar and pseudoscalar
coupling constants . Our result confirms limits from complementary
experiments with spin-polarized nuclei in a model-independent way. Limits from
other neutron experiments are improved by up to two orders of magnitude in the
interaction range of m
Effect of reminders on mitigating participation bias in a case-control study
BACKGROUND: Researchers commonly employ strategies to increase participation in health studies. These include use of incentives and intensive reminders. There is, however, little evidence regarding the quantitative effect that such strategies have on study results. We present an analysis of data from a case-control study of Campylobacter enteritis in England to assess the usefulness of a two-reminder strategy for control recruitment. METHODS: We compared sociodemographic characteristics of participants and non-participants, and calculated odds ratio estimates for a wide range of risk factors by mailing wave. RESULTS: Non-participants were more often male, younger and from more deprived areas. Among participants, early responders were more likely to be female, older and live in less deprived areas, but despite these differences, we found little evidence of a systematic bias in the results when using data from early reponders only. CONCLUSIONS: We conclude that the main benefit of using reminders in our study was the gain in statistical power from a larger sample size
Measurement of the permanent electric dipole moment of the neutron
We present the result of an experiment to measure the electric dipole moment EDM) of the neutron at the Paul Scherrer Institute using Ramsey's method of separated oscillating magnetic fields with ultracold neutrons (UCN). Our measurement stands in the long history of EDM experiments probing physics violating time reversal invariance. The salient features of this experiment
were the use of a Hg-199 co-magnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic field changes. The statistical analysis was performed on blinded datasets by two separate groups while the estimation of systematic effects profited from an
unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is d_{\rm n} = (0.0\pm1.1_{\rm stat}\pm0.2_{\rmsys})\times10^{-26}e\,{\rm cm}
The X-ray luminous cluster underlying the bright radio-quiet quasar H1821+643
We present a Chandra observation of the only low redshift, z=0.299, galaxy
cluster to contain a highly luminous radio-quiet quasar, H1821+643. By
simulating the quasar PSF, we subtract the quasar contribution from the cluster
core and determine the physical properties of the cluster gas down to 3 arcsec
(15 kpc) from the point source. The temperature of the cluster gas decreases
from 9.0\pm0.5 keV down to 1.3\pm0.2 keV in the centre, with a short central
radiative cooling time of 1.0\pm0.1 Gyr, typical of a strong cool-core cluster.
The X-ray morphology in the central 100 kpc shows extended spurs of emission
from the core, a small radio cavity and a weak shock or cold front forming a
semi-circular edge at 15 arcsec radius. The quasar bolometric luminosity was
estimated to be 2 x 10^{47} erg per sec, requiring a mass accretion rate of 40
Msolar per yr, which corresponds to half the Eddington accretion rate. We
explore possible accretion mechanisms for this object and determine that Bondi
accretion, when boosted by Compton cooling of the accretion material, could
provide a significant source of the fuel for this outburst. We consider
H1821+643 in the context of a unified AGN accretion model and, by comparing
H1821+643 with a sample of galaxy clusters, we show that the quasar has not
significantly affected the large-scale cluster gas properties.Comment: 20 pages, 19 figures, accepted by MNRA
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