340 research outputs found
On the nitrogen abundance of FLIERs: the outer knots of the planetary nebula NGC 7009
We have constructed a 3D photoionisation model of a planetary nebula (PN)
similar in structure to NGC 7009 with its outer pair of knots (also known as
FLIERs --fast, low-ionization emission regions). The work is motivated by the
fact that the strong [N II]6583A line emission from FLIERs in many planetary
nebulae has been attributed to a significant local overabundance of nitrogen.
We explore the possibility that the apparent enhanced nitrogen abundance
previously reported in the FLIERs may be due to ionization effects. Our model
is indeed able to reproduce the main spectroscopic and imaging characteristics
of NGC 7009's bright inner rim and its outer pairs of knots, assuming
homogeneous elemental abundances throughout the nebula, for nitrogen as well as
all the other elements included in the model. Because of the fact that the
(N+/N)/(O+/O) ratio predicted by our models are 0.60 for the rim and 0.72 for
the knots, so clearly in disagreement with the N+/N=O+/O assumption of the
ionization correction factors method (icf), the icfs will be underestimated by
the empirical scheme, in both components, rim and knots, but more so in the
knots. This effect is partly responsible for the apparent inhomogeneous N
abundance empirically derived. The differences in the above ratio in these two
components of the nebula may be due to a number of effects including charge
exchange --as pointed out previously by other authors-- and the difference in
the ionization potentials of the relevant species --which makes this ratio
extremely sensitive to the shape of the local radiation field. Because of the
latter, a realistic density distribution is essential to the modelling of a
non-spherical object, if useful information is to be extracted from spatially
resolved observations, as in the case of NGC 7009.Comment: 12 pages including 8 tables and 2 figures. MNRAS in pres
A deep narrowband survey for planetary nebulae at the outskirts of M33
Context: Planetary nebulae (PNe) are excellent tracers of stellar populations
with low surface brightness, and therefore provide a powerful method to detect
and explore the rich system of substructures discovered around the main spiral
galaxies of the Local Group. Aims: We searched the outskirts of the Local Group
spiral galaxy M33 (the Triangulum) for PNe to gain new insights into the
extended stellar substructure on the northern side of the disc and to study the
existence of a faint classical halo. Methods: The search is based on wide field
imaging covering a 4.5 square degree area out to a maximum projected distance
of about 40 kpc from the centre of the galaxy. The PN candidates are detected
by the combination of images obtained in narrowband filters selecting the
[OIII] and H + [NII] nebular lines and in the continuum
g' and r' broadband filters. Results:Inside the bright optical disc of M33,
eight new PN candidates were identified, three of which were spectroscopically
confirmed. No PN candidates were found outside the limits of the disc. Fourteen
additional sources showing [OIII] excess were also discovered. Conclusions:The
absence of bright PN candidates in the area outside the galaxy disc covered by
this survey sets an upper limit to the luminosity of the underlying population
of , suggesting the lack of a massive
classical halo, which is in agreement with the results obtained using the RGB
population.Comment: 13 pages, 18 figure
New candidate planetary nebulae in M81
A 34 x 34 arcmin field centred on the spiral galaxy M81 has been searched for
emission-line objects using the prime focus wide field camera (WFC) of the 2.54
m Isaac Newton Telescope (La Palma, Spain). A total of 171 candidate planetary
nebulae (PNe) are found, 54 of which are in common with the ones detected by
Jacoby et al. (1989). The behaviour of PNe excitation as a function of
galactocentric distance is examined, and no significant variations are found.
The PNe luminosity function is built for the disk and bulge of M81, separately.
A distance modulus of 27.92+-0.23 mag is found for disk PNe, in good agreement
with previous distance measurements for M81 (Jacoby et al. 1989; Huterer et al.
1995).Comment: 7 pages including 2 tables. A&A accepted; also available at
http://www.iac.es/publicaciones/preprints.htm
Para todos tiene la muerte una mirada
Reflexions about the pass of time and old age in human being, the body, the loneliness and the memory, by 14 photografers, 11 men and 3 women, thats confronts face to face to this cuestion. Every artist that look for the beauty and truth in the last chapter of theri lifes, in difents ways, some times in a direct way, and, others, in revindicative one., hilarious or imaginative, but always with grate courage and sense of tenderness and compression
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