共查询到20条相似文献，搜索用时 15 毫秒
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J. P. Phillips Luis Cuesta S. N. Kemp 《Monthly notices of the Royal Astronomical Society》2005,357(2):548-558
We have undertaken visual spectroscopy of the highly evolved planetary nebulae (PNe) A8, A13, A62, A72, A78 and A83 over a wavelength range 4330 < λ < 6830 Å . This permits us to specify relative line intensities in various sectors of the nebular shells, and to investigate the variation of emission as a function of radius. We determine that the spectrum of the central star of A78 has varied appreciably over a period of 25 yr. There is now evidence for strong P Cygni absorption in the λ4589 and λ5412 transitions of He ii , implying terminal velocities of the order of V ∞ ≅ 3.83 × 103 km s−1 . We also note that the emission-line profiles of the sources can be used to investigate their intrinsic emission structures. We find that most PNe show appreciable levels of emission throughout their volumes; only one source (A13) possesses a thin-shell structure. Such results are in conformity with evolutionary theory, and probably reflect the consequences of adiabatic cooling in highly evolved outflows. 相似文献
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A. Ali O.M. Shalabiea M.S. El-Nawawy T.J. Millar 《Monthly notices of the Royal Astronomical Society》2001,325(2):881-885
We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (104 –105 cm−3 ) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC3 N and SiC2 are in good agreement with observations, while those of CN, HCO+ , CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH2 , CH2 + , HCl, OH and H2 O are anticipated to be highly abundant in these objects. 相似文献
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S. Kimeswenger F. Kerber & R. Weinberger 《Monthly notices of the Royal Astronomical Society》1998,296(3):614-618
We present ISO observations of A58 and A78. Both objects are suspected to have undergone late He flashes ('born-again' nebulae). With ISOCAM we have been able to obtain data of much higher spatial resolution over the wavelength range 4.5–18 μm than has been possible in the past. In order to extract full information from our data we have developed a method to eliminate residual variations in the dark field. The results for A58 and A78 are compared with literature data and with A30 – the most prominent 'born-again' planetary nebula (PN). 相似文献
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S. Hyung L. H. Aller W. A. Feibelman W. B. Lee A. de Koter 《Monthly notices of the Royal Astronomical Society》2000,318(1):77-91
With the Hamilton echelle spectrograph at the Lick Observatory, emission-rich spectral lines of the planetary nebula NGC 6543 were secured in the wavelength range from 3550 to 10 100 Å. We chose two bright regions, ∼8 arcsec east and ∼13 arcsec north of the central star, the physical conditions and chemical abundances of which may differ as a result of the different physical characteristics involving the mass ejection of different epochs. By combining Hamilton echelle observations with archive UV data secured with the International Ultraviolet Explorer ( IUE ), we obtain improved diagnostics and chemical compositions for the two observed regions. The diagnostic diagram gives the average value of T e =8000∼8300 K, and the electron number density near N e ∼5000 cm−3 for most ions, while some low-excitation lines indicate much higher temperatures, i.e. T e ∼10 000 K. With the construction of a photoionization model, we try to fit the observed spectra in a self-consistent way: thus, for most elements, we employ the same chemical abundances in the nebular shell; and we adopt an improved Sobolev approximation model atmosphere for the hydrogen-deficient Wolf–Rayet type central star. Within the observational errors, the chemical abundances do not seem to show any positional variation except for helium. The chemical abundances of NGC 6543 appear to be the same as in average planetary nebulae. The progenitor star may have been an object of one solar mass, most of the heavier elements of which were less plentiful than in the Sun. 相似文献
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Joseph L. Hora Lynne K. Deutsch William F. Hoffmann Giovanni G. Fazio 《Astrophysics and Space Science》1995,224(1-2):361-364
We have obtained near diffraction-limited images of three bipolar PPN at UKIRT in October, 1993: AFGL 915 (the Red Rectangle), AFGL 618, and AFGL 2688 (the Egg Nebula). Images were taken at unidentified infrared (UIR) emission feature wavelengths and at several continuum wavelengths in the 10 and 20µm atmospheric windows. In all three PPN the emission is dominated by a central point source with fainter emission extending for several arcsec. In AFGL 2688, the mid-IR emission is extended in the same direction as the main optical lobes. In AFGL 915, the UIR feature emission is spatially separated from the central source. The spikes that have been observed at 2µm and give the nebula its rectangular appearance are also visible at 10µm. 相似文献
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Frédéric Zagury 《Journal of Astrophysics and Astronomy》2005,26(4):395-420
This paper presents an analysis of a series of spectra in the Red Rectangle nebula. Only the reddest part of the spectra can
safely be attributed to light from the nebula, and indicates Rayleigh scattering by the gas, in conformity with the large
angles of scattering involved and the proximity of the star. In the blue, light from HD 44179, refracted or scattered in the
atmosphere, dominates the spectra. This paper questions the reliability of ground-based broad-band spectra of extended objects
in the blue. 相似文献
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X.-W. Liu M. J. Barlow Y. Zhang R. J. Bastin P. J. Storey 《Monthly notices of the Royal Astronomical Society》2006,368(4):1959-1970
We present high-quality optical spectroscopic observations of the planetary nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination lines (ORLs) from heavy-element ions. Analysis of the H i and He i recombination spectrum yields an electron temperature of ∼900 K, a factor of 10 lower than given by the collisionally excited [O iii ] forbidden lines. The ionic abundances of heavy elements relative to hydrogen derived from ORLs are about a factor of 70 higher than those deduced from collisionally excited lines (CELs) from the same ions, the largest abundance discrepancy factor (adf) ever measured for a PN. By comparing the observed O ii λ4089/λ4649 ORL ratio to theoretical value as a function of electron temperature, we show that the O ii ORLs arise from ionized regions with an electron temperature of only ∼630 K. The current observations thus provide the strongest evidence that the nebula contains another previously unknown component of cold, high-metallicity gas, which is too cool to excite any significant optical or ultraviolet CELs and is thus invisible via such lines. The existence of such a plasma component in PNe provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other. 相似文献
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D. J. Harman M. Bryce M. P. Redman A. J. Holloway 《Monthly notices of the Royal Astronomical Society》2003,342(3):823-829
The little studied PN, Sa 2-21 has been observed using the Manchester echelle spectrometer at the Anglo-Australian telescope. Narrow band, long-slit spectra were obtained at six positions over two perpendicular position angles in both the [N ii ]λ6584 Å and [O iii ]λ5007 Å emission lines. An [O iii ] halo has been detected for the first time. A morphological modelling program was used to help determine the geometry, structure and kinematics of this ellipsoidal PN. It is proposed that the structure includes a pair of mid-latitude rings of [N ii ] emission, not previously seen in elliptical PNe. Radial spokes of [O iii ] emission have been detected in the main nebular shell indicating the presence of dynamical instabilities. 相似文献
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L. Zaninetti 《Monthly notices of the Royal Astronomical Society》2009,395(2):667-691
Planetary nebulae are imaged using three different physical processes. The first process is the expansion of the shell, which can be modelled by the canonical laws of motion in the spherical case and by momentum conservation when gradients of density are present in the interstellar medium. The second process is the diffusion of particles that radiate from the advancing layer. Three-dimensional diffusion from a sphere and one-dimensional diffusion with drift are analysed. The third process is the composition of an image through an integral operation along the line of sight. The framework developed is applied to A39, the Ring nebula and the etched hourglass nebula MyCn 18. 相似文献
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R. Vázquez R. L. Kingsburgh & J. A. López 《Monthly notices of the Royal Astronomical Society》1998,296(3):564-568
Flux-calibrated low-resolution spectra covering the optical wavelength range from 3400 to 7500 Å have been obtained over the central region and the surroundings of the extraordinary planetary nebula (PN) KjPn 8 (PNG 112.5-00.1). The spectrum from the core is of low excitation with T e (N II ) = 8000 K and n e (S II ) = 550 cm−3 . KjPn 8 is found to be a Type I PN according to the original classification scheme of Peimbert & Torres-Peimbert, with enriched He/H and N/O ratios with respect to mean values for PN. Increased O/H, Ne/H and Ar/H ratios over those of average PN reflect the possible metal-rich environment from which the progenitor star formed, and also are similar to those found in the extreme Type I PN He 2-111. The N/H ratio is found to be only moderately high compared to the average PN and consequently, the large O abundance pulls the N/O ratio towards the lower limit of the criterion for Type I planetary nebulae (PNe) in this case. In addition, the spectra of some knots and faint regions in the KjPn 8 surroundings are presented, which show only a few spectral lines. Low electron densities ranging from 100 to 300 cm−3 have been derived in these outer regions. 相似文献
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