18,161 research outputs found
Sub-arcsecond Morphology of Planetary Nebulae
Planetary nebulae (PNe) can be roughly categorized into several broad
morphological classes. The high quality images of PNe acquired in recent years,
however, have revealed a wealth of fine structures that preclude simplistic
models for their formation. Here we present narrow-band, sub-arcsecond images
of a sample of relatively large PNe that illustrate the complexity and variety
of small-scale structures. This is especially true for bipolar PNe, for which
the images reveal multi-polar ejections and, in some cases, suggest turbulent
gas motions. Our images also reveal the presence or signs of jet-like outflows
in several objects in which this kind of component has not been previously
reported.Comment: 7 pages, 7 figures, Accepted for publication in PAS
On the geometric genus of reducible surfaces and degenerations of surfaces to unions of planes
In this paper we study some properties of degenerations of surfaces whose
general fibre is a smooth projective surface and whose central fibre is a
reduced, connected surface , , which is assumed to be
a union of smooth projective surfaces, in particular of planes. Our original
motivation has been a series of papers of G. Zappa which appeared in the
1940-50's regarding degenerations of scrolls to unions of planes.
Here, we present a first set of results on the subject; other aspects are
still work in progress and will appear later.
We first study the geometry and the combinatorics of a surface like ,
considered as a reduced, connected surface on its own; then we focus on the
case in which X is the central fibre of a degeneration of relative dimension
two over the complex unit disk. In this case, we deduce some of the intrinsic
and extrinsic invariants of the general fibre from the ones of its central
fibre.
In the particular case of a central fibre of a semistable degeneration,
i.e. has only global normal crossing singularities and the total space of
the degeneration is smooth, some of the above invariants can be also computed
by topological methods (i.e., the Clemens-Schmid exact sequence). Our results
are more general, not only because the computations are independent on the fact
that is the central fibre of a degeneration, but also because the
degeneration is not semistable in general.Comment: latex2e, 26 pages, 11 figure
Special scrolls whose base curve has general moduli
In this paper we study the Hilbert scheme of smooth, linearly normal, special
scrolls under suitable assumptions on degree, genus and speciality.Comment: Latex2e, shorter versio
Earth-based lunar atmosphere investigation Final report
Instrumentation and results of earth-based spectrometric identification of lunar atmospheric constituents in visible regio
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