2,268 research outputs found
On the formation of oxygen-neon white dwarfs in close binary systems
The evolution of a star of initial mass 10 , and metallicity in a Close Binary System (CBS) is followed from its main sequence until
an ONe degenerate remnant forms. Restrictions have been made on the
characteristics of the companion as well as on the initial orbital parameters
in order to avoid the occurrence of reversal mass transfer before carbon is
ignited in the core. The system undergoes three mass loss episodes. The first
and second ones are a consequence of a case B Roche lobe overflow. During the
third mass loss episode stellar winds may play a role comparable to, or even
more important than Roche lobe overflow. In this paper, we extend the
previously existing calculations of stars of intermediate mass belonging to
close binary systems by following carefully the carbon burning phase of the
primary component. We also propose different possible outcomes for our scenario
and discuss the relevance of our findings. In particular, our main result is
that the resulting white dwarf component of mass more likely has
a core composed of oxygen and neon, surrounded by a mantle of carbon-oxygen
rich material. The average abundances of the oxygen-neon rich core are , , and . This result has important consequences for the Accretion
Induced Collapse scenario. The average abundances of the carbon-oxygen rich
mantle are , and . The existence of
this mantle could also play a significant role in our understanding of
cataclysmic variables.Comment: 13 pages, 12 figures, accepted for publication in A&
Learning from medical data streams: an introduction
Clinical practice and research are facing a new challenge created by the rapid growth of health information science and technology, and the complexity and volume of biomedical data. Machine learning from medical data streams is a recent area of research that aims to provide better knowledge extraction and evidence-based clinical decision support in scenarios where data are produced as a continuous flow. This year's edition of AIME, the Conference on Artificial Intelligence in Medicine, enabled the sound discussion of this area of research, mainly by the inclusion of a dedicated workshop. This paper is an introduction to LEMEDS, the Learning from Medical Data Streams workshop, which highlights the contributed papers, the invited talk and expert panel discussion, as well as related papers accepted to the main conference
13CO at the centre of M82
Using the IRAM interferometer, we have observed the nearby starburst galaxy
M82 with a 4.2" resolution (~70 pc) in the 1->0 line of 13CO and in the 2.6-mm
continuum.
The spatial distribution of the 13CO line shows the same gross features as
the 12CO(1->0) map of Shen & Lo (1995), namely two lobes and a compact central
source, though with different relative intensities. The lobes are more
conspicuous and the central source is fainter in 13CO than in 12CO.
The velocity field observed around the nucleus shows a very steep gradient
(140 km/s over 75 pc), which is very probably caused by the stellar bar visible
in the near infrared. The dynamical centre coincides with the IR peak and is
shifted 6" north-east of the compact 13CO source. The two CO lobes appear to be
associated with the ends of the bar and not with a molecular ring, as usually
assumed. They are probably shaped by the strong UV radiation from the central
region. 13CO must be more photodissociated than the self-shielded 12CO
molecules in the central ~250 pc region, which may explain the relative
weakness of the 13CO central source.
A 130 pc-wide bubble of molecular gas has been identified, which happens to
host the most luminous compact radio source in M82. It lies 120 pc west of the
IR peak between the central source and the western lobe and seems characterized
by warmer gas, strong UV radio free-free radiation, and an enhanced cosmic ray
production rate.Comment: Accepted by A&A; 9 pages, 9 ps figures, needs LaTeX 2e A&A macro and
psfig Styl
The complete submillimetre spectrum of NGC 891
Submillimetre maps of NGC 891 have been obtained with the PRONAOS
balloon-borne telescope and with the ISOPHOT instrument on board the ISO
satellite. In this article, we also gather data from IRAS and SCUBA to present
the complete submillimetre spectrum of this nearby edge-on spiral galaxy. We
derive submillimetre emission profiles along the major axis. The modified
blackbody fits, assuming a single dust component, lead to temperatures of 19-24
K toward the centre and 18-20 K toward the edges, with possible variations of
the dust spectral index from 1.4 to 2. The two-component fits lead to a warm
component temperature of 29 K all along the galaxy with a cold component at 16
K. The interstellar medium masses derived by these two methods are quite
different: 4.6 x 10^9 Mo in the case of the one-component model and 12 x 10^9
Mo in the case of the two-component one. This two-component fit indicates that
the cold dust to warm dust ratio is 20 to 40, the highest values being in the
wings of this galaxy. Comparing to dust mass estimates, both estimations of the
ISM mass are consistent with a gas to dust mass ratio of 240, which is close to
the Milky Way value. Our results illustrate the importance of accurate
submillimetre spectra to derive masses of the interstellar medium in galaxies.Comment: 5 pages, 2 figures, accepted May 2003 in the MNRA
Pseudo-Riemannian Jacobi-Videv Manifolds
We exhibit several families of Jacobi-Videv pseudo-Riemannian manifolds which
are not Einstein. We also exhibit Jacobi-Videv algebraic curvature tensors
where the Ricci operator defines an almost complex structure
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