78 research outputs found
Does the momentum flux generated by gravitational contraction drive AGB mass-loss?
Gravitational contraction always generates a radially directed momentum flux.
A particularly simple example occurs in the electron-degenerate cores of AGB
stars, which contract steadily under the addition of helium ashes from shell
hydrogen burning. The resulting momentum flux is quantified here. And since the
cores of AGB stars lack efficient momentum cancellation mechanisms, they can
maintain equilibrium by exporting their excess momentum flux to the stellar
envelope, which disposes of much of it in a low velocity wind. Gravitational
contraction easily accounts for the momentum flux in the solar wind, as well as
the flux required to lift mass into the dust formation zone of every AGB star,
whereon radiation pressure continues its ejection as a low velocity wind. This
mechanism explains the dependence of the AGB mass-loss rate on core mass; its
generalization to objects with angular momentum and/or strong magnetic fields
suggests a novel explanation of why most planetary nebulae and proto planetary
nebulae exhibit axial symmetry.
Quasistatic contraction is inherently biased to the generation of the maximum
possible momentum flux. Its formalism is therefore readily adapted to providing
an upper limit to the momentum flux needed to sustain mass loss when this
begins from a semi-continuous rather than impulsive process.Comment: 35 pages, including 1 fig and 2 tables, to appear in Astrophysical
Journal -- ps documen
The Infrared Continuum Spectrum of VY CMa
We combine spectra of VY CMa obtained with the short- and long-wavelength
spectrometers, SWS and LWS, on the Infrared Space Observatory to provide a
first detailed continuum spectrum of this highly luminous star. The
circumstellar dust cloud through which the star is observed is partially
self-absorbing, which makes for complex computational modeling. We review
previous work and comment on the range of uncertainties about the physical
traits and mineralogical composition of the modeled disk. We show that these
uncertainties significantly affect the modeling of the outflow and the
estimated mass loss. In particular, we demonstrate that a variety of quite
diverse models can produce good fits to the observed spectrum. If the outflow
is steady, and the radiative repulsion on the dust cloud dominates the star's
gravitational attraction, we show that the total dust mass-loss rate is yr, assuming that the star is at a distance of
1.5 kpc. Several indications, however, suggest that the outflow from the star
may be spasmodic. We discuss this and other problems facing the construction of
a physically coherent model of the dust cloud and a realistic mass-loss
analysis
HI in circumstellar environments
We present new results of a spectroscopic survey of circumstellar HI in the
direction of evolved stars made with the Nancay Radiotelescope. The HI line at
21 cm has been detected in the circumstellar shells of a variety of evolved
stars: AGB stars, oxygen-rich and carbon-rich, Semi-Regular and Miras, and
Planetary Nebulae. The emissions are generally spatially resolved, i.e. larger
than 4', indicating shell sizes of the order of 1 pc which opens the
possibility to trace the history of mass loss over the past ~ 10^4-10^5 years.
The line-profiles are sometimes composite. The individual components have
generally a quasi-Gaussian shape; in particular they seldom show the
double-horn profile that would be expected from the spatially resolved
optically thin emission of a uniformly expanding shell. This probably implies
that the expansion velocity decreases outwards in the external shells (0.1-1
pc) of these evolved stars. The HI line-profiles do not necessarily match those
of the CO rotational lines. Furthermore, the centroid velocities do not always
agree with those measured in the CO lines and/or the stellar radial velocities.
The HI emissions may also be shifted in position with respect to the central
stars. Without excluding the possibility of asymmetric mass ejection, we
suggest that these two effects could also be related to a non-isotropic
interaction with the local interstellar medium. HI was detected in emission
towards several sources (rho Per, alpha Her, delta^2 Lyr, U CMi) that otherwise
have not been detected in any radio lines. Conversely it was not detected in
the two oxygen-rich stars with substantial mass-loss rate, NML Tau and WX Psc,
possibly because these sources are young with hydrogen in molecular form,
and/or because the temperature of the circumstellar HI gas is very low (< 5 K).Comment: Accepted for publication in The Astronomical Journa
The Effective Temperature Scale of Galactic Red Supergiants: Cool, But Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCS
stellar atmosphere models to determine the effective temperatures of 74
Galactic red supergiants. From these we find a new effective temperature scale
that is significantly warmer than those in the literature. We show that this
temperature scale, along with the newly derived bolometric corrections, gives
much better agreement between our red supergiants and stellar evolutionary
tracks. This agreement provides an independent verification of our new
temperature scale. The combination of effective temperature and bolometric
luminosities allows us to calculate stellar radii; the coolest and most
luminous stars have radii of roughly 1500 solar radii (7 AU), in excellent
accordance with the largest stellar radii predicted from current evolutionary
theory. We find that similar results are obtained for the effective
temperatures and bolometric luminosities using only the de-reddened V-K colors,
providing a powerful demonstration of the self-consistency of the MARCS models.Comment: 32 pages, 16 figures; Accepted by the Astrophysical Journa
Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project
In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum-dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory
Improving policy coherence for food security and nutrition in South Africa: a qualitative policy analysis
Like most other low and middle-income countries, South Africa must address a rising burden of diet-related chronic disease in a
situation of persistent food insecurity and undernutrition. Supply-side policy interventions are a critical component of action to
address the double burden of malnutrition. However, the food supply is governed by a number of different policy sectors, and
policy incoherence can occur between government action to promote a healthy food supply and objectives for economic
liberalization. We analysed the coherence of food supply policy content with respect to nutrition and food security in South
Africa, and conducted 14 in-depth interviews with 22 public and private sector actors to identify opportunities to improve policy
coherence across sectors governing the food supply. Drawing on Sabatier’s conceptualization of actors as influential in shaping
policy outcomes, we identified three coalitions of actors related to food security and nutrition in South Africa: the dominant
Economic Growth coalition, the Food Security coalition, and the Health coalition. Understanding the frames, beliefs and
resources held by these coalitions offers insights into the policy tensions faced by the Government of South Africa with respect
to the food supply. The analysis indicates that the current reconsideration of economic policy agendas favouring liberalization in
SouthAfrica, including the termination of most bilateral investment treaties, may present an opportunity for increased recognition
of food security and nutrition priorities in food supply policy making. Opportunities to strengthen policy coherence across the
food supply for food security and nutrition include: specific changes to economic policy relating to the food supply that achieve
both food security/nutrition and economic objectives; creating links between producers and consumers, through markets and
fiscal incentives that make healthy / fresh foods more accessible and affordable; increasing formal avenues for engagement by
Civil Society in nutrition and food security policy making; and including consideration of the nutritional quality of the food
supply in policy objectives across sectors, to create a framework for policy coherence across sectors relating to the food supply
La fabrication de composants nucléaires et son impact sur l'environnement à Chalon-sur-Saône
Entrée en service en octobre 1975, l'usine Framatome de Chalon-sur-Saône a une capacité de réalisation de huit chaudières nucléaires par an. Les auteurs décrivent la façon dont cette grande unité industrielle a été intégrée à l'environnement de la Saône-et-Loire, sur le plan de son implantation et de son impact extérieur. Ils présentent également l'environnement interne de l'usine
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