共查询到20条相似文献，搜索用时 296 毫秒
1.
Using a numerically accurate radiation-gas dynamical method we investigate the winds scenario for aspherical planetary nebulae
(PNe). Our model includes the interaction of two winds: as low high mass-loss rate wind (a `super wind'); and a fast wind;
low mass-loss rate wind. Our model also includes the evolution of the UV spectrum of the PNe centeral star. As stated in the
section3 of Paper I (Ganbari and Khesali, 2001), we consider a three dimensional density distribution ρ(r,θφ for the super wind, in this way we enter the effects of cooling and heating mechanisms in our model. Taking into account
the above assumptions, we introduce the code (DIS3D) and numerically we study the dynamical and ionization properties of the
planetary nebula NGC3132. We show that it is possible by simulations to reproduce the shape of PNe in three dimensions, and
calculating the physical quantities throughout the entire nebula.
This revised version was published online in July 2006 with corrections to the Cover Date. 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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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. Casassus P. F. Roche M. J. Barlow 《Monthly notices of the Royal Astronomical Society》2000,314(4):657-671
We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Si ix ] 3.934 μm, are interpreted using photoionization models (including cloudy ), which allow us to re-estimate the temperature of the central star to be about 250 000 K. All of the detected lines are consistent with this value, except for [Al v ] and [Al vi ]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154 eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22 km s−1 FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Ne v ] 3426 Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1 arcsec and 10 km s−1 scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be −34.8±1 km s−1 and −17.8±3 km s−1 . We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle. 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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Geraint Harker Shaun Cole John Helly Carlos Frenk Adrian Jenkins 《Monthly notices of the Royal Astronomical Society》2006,367(3):1039-1049
We study the environmental dependence of the formation epoch of dark matter haloes in the Millennium Simulation: a ten billion particle N -body simulation of standard Lambda cold dark matter cosmology. A sensitive test of this dependence – the marked correlation function – reveals highly significant evidence that haloes of a given mass form earlier in denser regions. We define a marked cross-correlation function, which helps quantify how this effect depends upon the choice of the halo population used to define the environment. The mean halo formation redshift as a function of the local overdensity in dark matter is also well determined, and we see an especially clear dependence for galaxy-sized haloes. This contradicts one of the basic predictions of the excursion set model of structure formation, even though we see that this theory predicts other features of the distribution of halo formation epochs rather well. It also invalidates an assumption usually employed in the popular halo, or halo occupation distribution, models of galaxy clustering, namely that the distribution of halo properties is a function of halo mass but not of halo environment. 相似文献
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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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K. V. K. Iyengar 《Journal of Astrophysics and Astronomy》1985,6(4):227-231
As dust emission in the far infrared (FIR) is a characteristic property of planetary nebulae we searched the Infrared Astronomical
Satellite (IRAS) point-source catalogue for confirmatory evidence on the two new possible planetary nebulae S 68 and 248 -
5 identified by Fesen, Gull & Heckathorn (1983) and the high-excitation planetary nebula 76 + 36 detected by Sanduleak (1983).
We identify the nebulae 248 - 5 and 76 + 36 with IRAS sources 07404 - 3240 and 17125 + 4919, respectively and have determined
their dust temperature, total FIR emission and optical depth. We also set a lower limit ranging in value from 1.2 × 10-6 to 3.7 × 10-5 forM
dust
/M
bd
of the nebula 248 - 5 depending on whether its grain material is silicate or graphite. S 68 could not be identified with
an IRAS source. 相似文献
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J.P. Phillips 《Astronomische Nachrichten》2003,324(3):191-201
We have investigated the variation of planetary nebula number densities as a function of nebular radius, taking account of uncertainties arising from interstellar extinction. We find that the trend is composed of two components: one (a “spike” component) located at radii R < 0.035 pc, and the other (a “plateau” component) extending to larger radii. The plateau component appears to follow a Gaussian fall‐off law with scale radius R0 = 0.28 pc. It is shown that this latter trend is not consistent with the assumption that larger shells are optically thin and density bounded. Rather, it seems likely thatmany of the larger sources have appreciable Lyman continuum optical depths and are ionization bounded. The deduced variation in N(R) then suggests that the velocities of the ionization fronts increase with radius. The nature of the spike component is less easy to fathom, and this may arise as a result of sharply lower ionization front velocities at radii R < 0.035 pc, or through contraction of the shells following a down‐turn in central star luminosities. 相似文献
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D. R. Gonçalves A. Mampaso R. L. M. Corradi C. Quireza 《Monthly notices of the Royal Astronomical Society》2009,398(4):2166-2176
We obtained optical long-slit spectra of four planetary nebulae (PNe) with low-ionization pair of knots, namely He 1-1, IC 2149, KjPn 8 and NGC 7662.
These data allow us to derive the physical parameters and excitation of the pairs of knots, and those of higher ionization inner components of the nebulae, separately.
Our results are as follows. (1) The electron temperatures of the knots are within the range 9500–14 500 K, similar to the temperatures of the higher ionization rims/shells. (2) Typical knots' densities are 500–2000 cm−3 . (3) Empirical densities of the inner rims/shells are higher than those of the pairs of knots, by up to a factor of 10. Theoretical predictions, at variance with the empirical results, suggest that knots should be denser than the inner regions, by at least a factor of 10. (4) Empirical and theoretical density contrasts can be reconciled if we assume that at least 90 per cent of the knots' gas is neutral (likely composed of dust and molecules). (5) By using the new Raga et al. shock modelling and diagnostic diagrams appropriated for spatially resolved PNe, we suggest that high-velocity shocked knots travelling in the photoionized outer regions of PNe can explain the emission of the pairs of knots analysed in this paper. 相似文献
These data allow us to derive the physical parameters and excitation of the pairs of knots, and those of higher ionization inner components of the nebulae, separately.
Our results are as follows. (1) The electron temperatures of the knots are within the range 9500–14 500 K, similar to the temperatures of the higher ionization rims/shells. (2) Typical knots' densities are 500–2000 cm