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تسجيل مستخدم جديدOn the nitrogen abundance of FLIERs: the outer knots of the planetary nebula NGC 7009
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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 7009s 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.
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We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009. These data allow us to discuss the physical, excitation and chemical properties of all the morphological components of the nebula, including its remarkable sy
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The spatial structure of the emission lines and continuum over the 50 arcsecond extent of the nearby, O-rich, planetary nebula NGC 7009 (Saturn Nebula) have been observed with the MUSE integral field spectrograph on the ESO Very Large Telescope. Scie
nce Verification data, in <0.6 arcsecond seeing, have been reduced and analysed as images over the wavelength range 4750-9350A. Emission line maps over the bright shells are presented, from neutral to the highest ionization available (He II and [Mn V]). For collisionally excited lines (CELs), maps of electron temperature (T_e from [N II] and [S III]) and electron density (N_e from [S II] and [Cl III]) are available and for optical recombination lines (ORLs) temperature (from the Paschen jump and ratio of He I lines) and density (from high Paschen lines). These estimates are compared: for the first time, maps of the differences in CEL and ORL T_es have been derived, and correspondingly a map of t^2 between a CEL and ORL temperature, showing considerable detail. Total abundances of He and O were formed, the latter using three ionization correction factors. However the map of He/H is not flat, departing by ~2% from a constant value, with remnants corresponding to ionization structures. Ionization correction factor methods are compared for O abundance, but none delivers a flat map. An integrated spectrum over an area of 2340 square arcseconds was also formed and compared to 1D photoionization models. The spatial variation of a range of nebular parameters illustrates the complexity of the ionized media in NGC 7009. These MUSE data are very rich with detections of many lines over areas of hundreds of square arcseconds and follow-on studies are indicated. (Abridged)
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