Salut Pierre !



Merci pour l’informatin.



Bye

Michel





De : Observateur@... <mailto:[mailto:Observateur@...]>
[mailto:Observateur@...]

De la part de Pierre Paquette
Envoyé : 30 juin 2009 18:46
À : Observateur@...
Objet : [Observateur] NGC 2818

Bonsoir,

Je suggère d'afficher ce message avec une police («font») de largeur
fixe comme Monospace. Et je m'excuse pour l'anglais.

Selon «Star Clusters», de Brent A. Archinal et Steven J. Hynes
(Willmann-Bell, 2003):

Name Type h min s deg min con Tr.type diam mag nb* mag*br magsur distpc
sa u2k ref
NGC 2818 clpn 09 16 10 –36 37.1 Pyx III1m n 8.0 8.2 298 11.32 12.7 3221
20 170 4
The NGC designation applies to the cluster and planetary nebula.
Aliases: Cr 206; Lund 497; Mel 96; OCl 743; Raab 82; Dunlop 564; h 3154;
vdB-Ha 59; C0914–364

NGC 2818 = C0914–364: The first observation of this object was made by
James Dunlop [1828] from Paramatta, in New South Wales, Australia, and
was cataloged by him as Δ564. He noted the cluster first with a smaller
nebulosity involved. John Herschel also observed this field, listing it
as h 3154 and describing it as a planetary nebula in the field of an
open cluster. In the NGC description it appears that the 2818
designation is applied specifically to the planetary nebula. However, to
complicate matters, its presence in the cluster is mentioned, but the
cluster is not given any separate designation. Arguably, from the
historical records, the designation NGC 2818 could be said to apply to
the cluster/planetary nebula combination, but this may cause confusion.
In an attempt to resolve the matter, Sulentic and Tifft [1973] split the
two objects in the RNGC, calling the open cluster NGC 2818 and the
planetary nebula NGC 2818A. However, there are no historical grounds for
this, and we feel that this designation should be avoided. A more
satisfactory solution might be to identify the planetary nebula as NGC
2818, and identifying the cluster by one of its alternative
designations, Melotte 96 or Collinder 206.

Of course, the presence of a planetary nebula within an open cluster
might just be a projection effect, as it is in the more well-known case
of NGC 2438 and the cluster NGC 2437 (= M 46). Evidence of a genuine
physical association in the case of NGC 2818/2817A, though, was first
provided by Tifft, et al., [1972] who noted that the radial velocity of
the nebula was similar to that of two stars in the cluster. Further
observational evidence of the association was also presented by Dufour
[1984] based on an analysis of its physical properties and chemical
composition. This occurrence is very important in the study of planetary
nebulae, which are notoriously difficult to investigate because of the
problems of measuring their distance. In this instance the distance,
size and progenitor mass can be determined to an unusual degree of
accuracy. The cluster itself is not otherwise particularly exceptional,
being an intermediate-age Population I grouping at a distance of some
3.5kpc.

Brian Skiff [private communication 1995] gives a position for the
cluster of approximately (2000.0) 09h 16m 10s –36° 36.1', a value which
is used here. He also gives the position of the planetary nebula as
(2000.0) 09h 16m 01.48s –36° 37' 36.5" and notes it is GSC 7164–3813.

Le catalogue de Lynga (rendu obsolète par le New catalog of optically
visible open clusters and candidates (V2.8) de Dias W.S., Alessi B.S.,
Moitinho A., Lepine J.R.D., dans Astronomy & Astrophysics vol. 389, no.
871, paru en 2002) est disponible sur mon site et contient 1151 entrées:
<http://cielprofond.info/Lynga.zip> http://cielprofond.info/Lynga.zip

Bonnes observations!

Pierre Paquette



[Les parties de ce message comportant autre chose que du texte seul ont été
supprimées]