共查询到20条相似文献，搜索用时 31 毫秒
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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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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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D. H. Morgan Q. A. Parker D. Russeil 《Monthly notices of the Royal Astronomical Society》2001,322(4):877-884
The central stars of two of the new planetary nebulae found during scans of the AAO/UKST H α Survey of the Milky Way have been found to exhibit Wolf–Rayet (WR) emission features. One (PMR 1) is an early-type star of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC] star which, depending on the classification scheme used, is either intermediate in class between [WC9] and [WC10] or the sole member of the [WC10] class. Both stars exhibit unusual spectral features which may be attributed to enhanced nitrogen in their atmospheres and could be indicative of unusual stellar evolution. 相似文献
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Orsola De Marco Paul A. Crowther 《Monthly notices of the Royal Astronomical Society》1999,306(4):931-942
We present a detailed analysis of the planetary nebula M4–18 (G146.7+07.6) and its WC10-type Wolf–Rayet (WR) central star, based on high‐quality optical spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging ( HST /WFPC2). From a non-LTE model atmosphere analysis of the stellar spectrum, we derive T eff =31 kK, v ∞ =160 km s−1 and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to those of He 2–113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He 2–113, we obtain a distance of 6.8 kpc to M4–18 [ E ( B−V )=0.55 mag from nebular and stellar techniques]. This implies that the planetary nebula of M4–18 has a dynamical age of ∼3100 yr, in contrast to ≥270 yr for He 2–113. This is supported by the much higher electron density of the latter. These observations may be reconciled with evolutionary predictions only if [WC]-type stars exhibit a range in stellar masses.
Photoionization modelling of M4–18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain. 相似文献
Photoionization modelling of M4–18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain. 相似文献
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Noam Soker 《Monthly notices of the Royal Astronomical Society》1998,299(2):562-566
We study the formation of radially aligned condensations and tails through the compression of material inside ionization shadows at early ionization phases of planetary nebulae. A dense clump, formed before ionization starts, forms an ionization shadow behind it. The surroundings, which are ionized before the shadow, have a higher temperature, and as a result compress the material in the shadow, forming a compressed tail. If the compressed tail crosses a dense shell, a dense condensation (clump) is formed there. At later stages this condensation is ionized and observed as a bright knot, radially aligned with the inner clump. We find that for the shadow to be effective, the clump should be already present as the ionization by the central star starts, and its density enhancement should be by a factor of ≳ 5. We propose this mechanism as an explanation for the radially aligned condensations recently found in the planetary nebula IC 4593. 相似文献
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There are about 50 galactic planetary nebulae know to have [WR] type nuclei. We have compared their nebular properties with those of the other planetary nebulae in the Galaxy. We have found that the nebular morphological types are similarly distributed in the two groups. Bipolar nebulae constitute only 20% of the total in each group. The distribution of the nebular electron densities and abundance ratios N/O, He/H and C/O are the same in the two groups. The only marked difference is that nebular expansion velocities are larger in the group of planetary nebulae with [WR] central stars. We argue that the WR phenomenon does not preferentially occur in more massive central stars of planetary nebulae, contrary to what has been suggested in some former studies. We demonstrate that, for most of the observed [WR] type objects, the WR phenomenon cannot be triggered by a late helium shell flash event.The results of our investigation are published inAstronomy & Astrophysics
303, 893 (1995) and in the proceedings of the 2nd International Colloquium on Hydrogen-deficient Stars, C.S. Jeffery & U. Heber (eds), Astronomical Society of the Pacific Conference Series, Vol. 96, p. 209 (1996). 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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Accurate optical coordinates of 734 PNe, measured on the charts of the Digitized Palomar Sky Survey, are presented. As a result of the discussion about the external accuracy the constants –0.8″ in RA and +0.8″ in DEC should be added to the coordinates measured by us. They were used but rounded off already in CGPN(2000). The list and measurements of new 31 candidates of central stars are given which might be interesting for stellar evolution. 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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Intricate filamentary structure and multiple shell-like appearance are very common phenomena in Planetary Nebulae.In addition, recent observations also indicate that the individual filaments present in these objects can have larger velocities than the adjacent smooth background (Pascoli, 1992 PASP 104, 350 and paper quoted therein).We have hypothesized that non linear hydrodynamical processes existing within the nebular gas are, possibly, responsible for these structures. As a matter of fact, it is argued that such a characteristic morphology, reinterpreted as a intermingled network of solitary waves or solitons, can be spontaneously generated in Planetary Nebulae as soon as one assumes that the nebular gas is permeated by a weak magnetic field whose strength is about 10–5 to 10–4 gauss.Main results of this work and further comments will be subsequently published in Ap&SS. 相似文献
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D. H. Morgan Q. A. Parker Martin Cohen 《Monthly notices of the Royal Astronomical Society》2003,346(3):719-730
We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s−1 , suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula. 相似文献