3,436 research outputs found
The yellow hypergiants HR 8752 and rho Cassiopeiae near the evolutionary border of instability
High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752
and rho Cassiopeiae indicate high effective temperatures placing both stars
near the T_eff border of the ``yellow evolutionary void''. At present, the
temperature of HR 8752 is higher than ever. For this star we found
Teff=7900+-200 K, whereas rho Cassiopeiae has Teff=7300+-200 K. Both, HR 8752
and rho Cassiopeiae have developed strong stellar winds with Vinf ~ 120 km/s
and Vinf ~ 100 km/s, respectively. For HR 8752 we estimate an upper limit for
the spherically symmetric mass-loss of 6.7X10^{-6}M_solar/yr. Over the past
decades two yellow hypergiants appear to have approached an evolutionary phase,
which has never been observed before. We present the first spectroscopic
evidence of the blueward motion of a cool super/hypergiant on the HR diagram.Comment: 13 pages including 3 figures. Accepted for publication in ApJ Letter
Ecological agriculture in South-India : an agro-economic comparison and study of transition
This report describes two research programmes carried out on ecological agriculture in South-India. Experiences of twelve farmers in transition towards ecological agriculture are described and analysed. The comparative performance of seven farmer pairs, consisting of one ecological and one conventional reference farm, is analysed in relation to agronomic and economic performanc
Pulsational instability of yellow hypergiants
Instability of population I (X=0.7, Y=0.02) massive stars against radial
oscillations during the post-main sequence gravitational contraction of the
helium core is investigated. Initial stellar masses are in the range from
65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar
oscillations we assumed that energy transfer in the envelope of the pulsating
star is due to radiative heat conduction and convection. The convective heat
transfer was treated in the framework of the theory of time-dependent turbulent
convection. During evolutionary expansion of outer layers after hydrogen
exhaustion in the stellar core the star is shown to be unstable against radial
oscillations while its effective temperature is Teff > 6700K for
Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability
is due to the \kappa-mechanism in helium ionization zones and at lower
effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of
evolution does not exceed 200 day. Radial oscillations of the hypergiant resume
during evolutionary contraction of outer layers when the effective temperature
is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot.
Initially radial oscillations are due to instability of the first overtone and
transition to fundamental mode pulsations takes place at higher effective
temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for
Mzams=90M_\odot). The upper limit of the period of radial oscillations of
evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow
hypergiants are stable against radial stellar pulsations during the major part
of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter
Inclusive Quasi-Elastic Charged-Current Neutrino-Nucleus Reactions
The Quasi-Elastic (QE) contribution of the nuclear inclusive electron
scattering model developed in Nucl. Phys. A627 (1997) 543 is extended to the
study of electroweak Charged Current (CC) induced nuclear reactions, at
intermediate energies of interest for future neutrino oscillation experiments.
The model accounts for, among other nuclear effects, long range nuclear (RPA)
correlations, Final State Interaction (FSI) and Coulomb corrections.
Predictions for the inclusive muon capture in C and the reaction
C near threshold are also given. RPA correlations are
shown to play a crucial role and their inclusion leads to one of the best
existing simultaneous description of both processes, with accuracies of the
order of 10-15% per cent for the muon capture rate and even better for the LSND
measurement.Comment: 31 pages and 14 figures, accepted for publication as a regular
article in Physical Review
Crossing the `Yellow Void' -- Spatially Resolved Spectroscopy of the Post- Red Supergiant IRC+10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define the empirical
upper luminosity boundary in the HR diagram. During their post--RSG evolution,
these massive stars enter a temperature range (6000-9000 K) of increased
dynamical instability, high mass loss, and increasing opacity, a
semi--forbidden region, that de Jager and his collaborators have called the
`yellow void'. We report HST/STIS spatially resolved spectroscopy of IRC +10420
and its reflection nebula with some surprising results. Long slit spectroscopy
of the reflected spectrum allows us to effectively view the star from different
directions. Measurements of the double--peaked Halpha emission profile show a
uniform outflow of gas in a nearly spherical distribution, contrary to previous
models with an equatorial disk or bipolar outflow. Based on the temperature and
mass loss rate estimates that are usually quoted for this object, the wind is
optically thick to the continuum at some and possibly all wavelengths.
Consequently the observed variations in apparent spectral type and inferred
temperature are changes in the wind and do not necessarily mean that the
underlying stellar radius and interior structure are evolving on such a short
timescale. To explain the evidence for simultaneous outflow and infall of
material near the star, we propose a `rain' model in which blobs of gas
condense in regions of lowered opacity outside the dense wind. With the
apparent warming of its wind, the recent appearance of strong emission, and a
decline in the mass loss rate, IRC +10420 may be about to shed its opaque wind,
cross the `yellow void', and emerge as a hotter star.Comment: To appear in the Astronomical Journal, August 200
-- coupling in He with the Nijmegen soft-core potentials
The -- coupling in
He is studied with the [ + +
] + [ + + ] + [ + + ] model,
where the particle is assumed as a frozen core. We use the Nijmegen
soft-core potentials, NSC97e and NSC97f, for the valence baryon-baryon part,
and the phenomenological potentials for the parts (=,
, and ). We find that the calculated of He for NSC97e and NSC97f are,
respectively, 0.6 and 0.4 MeV in the full coupled-channel calculation, the
results of which are about half in comparison with the experimental data,
MeV.
Characteristics of the sector in the NSC97 potentials are discussed in
detail.Comment: 18 pages, 4 figure
Drivers of land use change and household determinants of sustainability in smallholder farming systems of Eastern Uganda
Smallholder farming systems in sub-Saharan Africa have undergone changes in land use, productivity and sustainability. Understanding of the drivers that have led to changes in land use in these systems and factors that influence the systems’ sustainability is useful to guide appropriate targeting of intervention strategies for improvement. We studied low input Teso farming systems in eastern Uganda from 1960 to 2001 in a place-based analysis combined with a comparative analysis of similar low input systems in southern Mali. This study showed that policy-institutional factors next to population growth have driven land use changes in the Teso systems, and that nutrient balances of farm households are useful indicators to identify their sustainability. During the period of analysis, the fraction of land under cultivation increased from 46 to 78%, and communal grazing lands nearly completely disappeared. Cropping diversified over time; cassava overtook cotton and millet in importance, and rice emerged as an alternative cash crop. Impacts of political instability, such as the collapse of cotton marketing and land management institutions, of communal labour arrangements and aggravation of cattle rustling were linked to the changes. Crop productivity in the farming systems is poor and nutrient balances differed between farm types. Balances of N, P and K were all positive for larger farms (LF) that had more cattle and derived a larger proportion of their income from off-farm activities, whereas on the medium farms (MF), small farms with cattle (SF1) and without cattle (SF2) balances were mostly negative. Sustainability of the farming system is driven by livestock, crop production, labour and access to off-farm income. Building private public partnerships around market-oriented crops can be an entry point for encouraging investment in use of external nutrient inputs to boost productivity in such African farming systems. However, intervention strategies should recognise the diversity and heterogeneity between farms to ensure efficient use of these external inputs
Numerical simulations of composite supernova remnants for small pulsar wind nebulae
Composite supernova remnants consist of a pulsar wind nebula located inside a
shell-type remnant. The presence of a shell has implications on the evolution
of the nebula, although the converse is generally not true. The purpose of this
paper is two-fold. The first aim is to determine the effect of the pulsar's
initial luminosity and spin-down rate, the supernova ejecta mass, and density
of the interstellar medium on the evolution of a spherically-symmetric,
composite supernova remnant expanding into a homogeneous medium. The second aim
is to investigate the evolution of the magnetic field in the pulsar wind nebula
when the the composite remnant expands into a non-uniform interstellar medium.
The Euler conservation equations for inviscid flow, together with the
magnetohydrodynamic induction law in the kinematic limit, are solved
numerically for a number of scenarios where the ratio of magnetic to particle
energy is . The simulations in the first part of the paper is
solved in a one-dimensional configuration. In the second part of the paper, the
effect of an inhomogeneous medium on the evolution is studied using a
two-dimensional, axis-symmetric configuration
Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares
(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet
emission from coronae of magnetically active stars is entirely (or to a large
part) due to the superposition of flares, using an analytic approach to
determine the amplitude distribution of flares in light curves. The
flare-heating hypothesis is motivated by time series that show continuous
variability suggesting the presence of a large number of superimposed flares
with similar rise and decay time scales. We rigorously relate the amplitude
distribution of stellar flares to the observed histograms of binned counts and
photon waiting times, under the assumption that the flares occur at random and
have similar shapes. Applying these results to EUVE/DS observations of the
flaring star AD Leo, we find that the flare amplitude distribution can be
represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our
analytical results agree with existing Monte Carlo results of Kashyap et al.
(2002) and Guedel et al. (2003). The method is applicable to a wide range of
further stochastically bursting astrophysical sources such as cataclysmic
variables, Gamma Ray Burst substructures, X-ray binaries, and spatially
resolved observations of solar flares.Comment: accepted for publication in Ap
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