Nebular Shell Research Articles

The ionization structure of planetary nebulae. VII. New observations of the Ring Nebula

New optical spectrophotometric observations of emission-line intensities have been made in eight positions in the Ring Nebula corresponding to those observed previously with the Ultraviolet Explorer satellite; the total coverage is therefore 1400 to 7200 A. The intensities are in generally good agreement with those found previously in corresponding positions. The Oand Balmer continuum electron temperatures agree well on the average and, like the Nelectron temperatures, decrease with increasing distance from the central star. As found previously for the Ring Nebula and for other planetaries in this series, the lambda 4267 C 2 line intensity near the central star implies a Cabundance that is higher than that determined from the lambda 1906, 1909 C 3 lines. The discrepancy again decreases with increasing distance from the central star and vanishes from the outermost positions, again suggesting that the excitation mechanism from the lambda 4267 line is not understood. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give results that are consistent and in appropriate agreement with abundances calculated using UV lines. The rather high abundances of O, N, and C, and, to some extent N, indicate that some mixing of CNO processed material into the nebular shell may have occurred in the Ring Nebula.

New evolutionary relationships for Magellanic Cloud planetary nebulae

view Abstract Citations (24) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Evolutionary Relationships for Magellanic Cloud Planetary Nebulae Dopita, Michael A. ; Meatheringham, Stephen J. ; Wood, Peter R. ; Webster, B. Louise ; Morgan, David H. ; Ford, Holland C. Abstract Two new evolutionary correlations have been discovered to apply to the population of planetary nebulae (PN) in the Magellanic Clouds. First, the age of the nebular shell is found to follow a relationship, which is shown to be consistent with a model in which the total energy of the ionized and swept-up gas drives the expansion down the density gradient in the precursor AGB wind. Second, a tight correlation is found between the expansion velocity and a combination of the excitation class and the Hβ flux. This appears to be determined by the mass of the planetary nebula nuclear star. These correlations provide strong observational support for the idea that the PN shells are ejected at low velocity during the asymptotic giant branch phase of evolution, and that they are continually accelerated during their nebular lifetimes. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/184870 Bibcode: 1987ApJ...315L.107D Keywords: Ionized Gases; Magellanic Clouds; Planetary Nebulae; Stellar Evolution; Stellar Mass; White Dwarf Stars; Asymptotic Giant Branch Stars; H Beta Line; Hertzsprung-Russell Diagram; High Resolution; Line Of Sight; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; NEBULAE: PLANETARY; STARS: EVOLUTION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (2) NED (2)

Mass loss by stars at the stage of the asymptotic giant branch

For a given initial stellar mass function, star formation function, and initial chemical composition, distributions have been constructed for stars of the asymptotic giant branch by luminosity, and for white dwarfs by mass, by calculating the approximate evolution of a large number of stars. Variants are calculated with different assumptions about the mass loss in the asymptotic branch. Theory can be reconciled with observation only if it is assumed that at this stage there is also a still large mass loss in addition to the stellar wind and the ejection of a planetary nebula shell. This provides the explanation for the absence in the Magellanic clouds of carbon stars with M /sub bol/ 1.0M /sub ./. The degenerate carbon-oxygen nuclei of stars evolving along the asymptotic giant branch cannot attain the Chandrasekhar limit on account of the great mass loss by the stars. The luminosity of stars of the asymptotic giant branch in the globular clusters of the Magellanic Clouds is a good indicator of the age of the clusters.

On the Mass and Luminosity of V348 Sgr

In this short note, an attempt has been made to estimate mass and luminosity of the unique object, V348 Sgr, which appears to be a “Rosetta Stone” for our understanding of late stages of stellar evolution, as i)its photosphere is virtually hydrogen-free and carbon-rich as with an extreme helium star,ii)it fades irregularly as does a R CrB-star andiii)it is surrounded by a nebular shell similar to that surrounding Planetary Nebulae.We know a great deal more about the above-mentioned objects than about V348 Sgr and this knowledge can be used to obtain estimates on mass and luminosity of the latter.

Evolutionary Calculations for Planetary Nebula Nuclei with Continuing Mass Loss and Realistic Starting Conditions

The mechanism by which planetary nebula (PN) shells are ejected is still subject to considerable uncertainty. It is generally assumed that the precursors of these objects are low mass (M< 5 M⊙) asymptotic giant branch (AGB) stars, and that the nucleus of a planetary nebula (NPN) is undergoing a final gravitational contraction to the white dwarf state. The shell consists of some or all of the remaining unburnt (though not necessarily uncontaminated), hydrogen-rich material out of which the star was originally formed.

The H-R Diagram of NGC 1962-65-66-70

The complex NGC 1962–65–66–70 is one of the many giant filamentary shell nebulae in the LMC. The nebulosity, also known as N144 was studied originally in H-alpha by Henize (1956). It has an angular size of 698″ x 620″ and surrounds an association of very young blue stars. The cluster has been studied first photographically in V and B by Westerlund (1961).

Near-infrared spectroscopy and monochromatic isophotometry of NGC 6302

A near-infrared spectrum on NGC6302 shows evidence for anomalously enhanced HeI 2'S-2'P, and the presence of H2S(1)v=1→0 emission. The S(1) line is found to be confined to the inner regions of NGC6302, and we explore two processes whereby this might arise: shock interaction between a stellar wind and neutral inclusions, and formation of shocks behind ionization fronts. The strength of the Bγ line suggests the presence of relatively high central emission measures, and indicates densities of order Ne~104 cm-3 or higher. Contour maps in the lines [NII] λ6584 and [OIII] λ4959/5007 are also presented and these reveal the possible presence of low-excitation knots within the main nebular shell.

Physical Processes in Nebular Shells and the Interpretation of Nebular Spectra

Computed models are now recognized as useful tools for interpretation of the spectra of planetary nebulae. However, even the most detailed models need geometrical parameters such as filling factors which are poorly determined by observations. Some effects may be seen more clearly by modeling the stratification than by just using total fluxes. A simple model for NGC 6720 is presented which reproduces the behavior of (Ne III) λ3869 observed by Hawley and Miller (1977), clearly showing the effects of charge transfer. The behavior of C II λ4267 remains puzzling. Finally, we comment on the interaction of high velocity stellar winds with nebular shells. Non-equilibrium particle distributions at the contact between the shocked stellar wind and the nebula may result in the rapid cooling of the shocked gas.

On the O III/O II Problem in Medium and High Excitation Planetary Nebulae

Numerical models have been constructed for twelve ionization bounded, medium to high excitation planetary nebulae. In most objects the excitation sensitive line ratio (O III) λ 500.9 nm / (O II) λ 372.7 nm is predicted to be too low as compared to observations. A similar systematic discrepancy is observed for [S III] λ 953.2 nm / (S II) λ 672.0 nm. We investigated the following effects on the ionization structure of the nebulae: 0++ + HO → 0+ + H+ charge exchange reaction, energy distribution of ionizing radiation and density distribution of gas in the nebular shell. The results show that density distribution is the most important factor determining the 0 III/O II and S III/S II line intensity ratios. While a factor of ten decrease in the charge exchange coefficient is required to explain the systematic discrepancy, a reduction of nebular radius by a few percent - truncated nebula (quasi density bounded model, but nebula still optically thick to Lyman photons) - suffices to produce the correct 0 III/O II ratio. Also, a density gradient of n ∼ r−1 to r−2 yields much better agreement with observations. Realistic variations in stellar spectrum hardly affects the 0 III/O II line intensity ratio.

Optical and UV Nebular Spectra of NGC 40

The planetary nebula NGC 40 has a WC8 central star. From analysis of IUE spectra of the star, Benvenuti, Perinotto and Willis (1981, IAU Symposium 99) obtain the remarkable abundance ratio C/He ~ 0.2 and probably no hydrogen. The question arises as to whether the nebular shell has abnormal abundances.

Ultraviolet shell formation at V1016 Cygni

A recent 330 minute high-resolution International Ultraviolet Explorer (IUE) spectrogram of V1016 Cygni shows splitting of the 1550 A component of the C IV resonance doublet, suggesting that a nebular shell is in the process of formation. An expansion velocity of 15 km/s is indicated by the IUE data and corresponds to the low range encountered with normal planetary nebulae, but IUE data of V1016 Cyg, taken approximately one year earlier, do not show split C IV lines. The recent IUE observations are consistent with VLA contour maps at 1.3 cm made by Newell and Hjellming (1981) and by Hjellming and Bignell (1982) that morphologically show an extended shell-type object. Thus, V1016 Cyg may have undergone an event which forms a planetary nebula.

A large shell nebula in NGC 55

view Abstract Citations (24) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A large shell nebula in NGC 55 Graham, J. A. ; Lawrie, D. G. Abstract The optical detection of a large shell nebula, at least several hundred parsecs across, in NGC 55 is reported. The observation was made in the light of the forbidden O III 5007-A emission line using an image-tube camera together with a narrow-band interference filter. The observations confirm the gaseous nature of the nebula and its association with the galaxy. The observed nebulous arc is roughly centered on a bright stellar knot which appears to be an unusually active region of star formation. The shell is similar in form to the neutral-hydrogen supershells observed by Heiles (1979) in the Galaxy. Publication: The Astrophysical Journal Pub Date: February 1982 DOI: 10.1086/183739 Bibcode: 1982ApJ...253L..73G Keywords: Interstellar Matter; Nebulae; Forbidden Bands; Hydrogen; Oxygen Ions; Planetary Nebulae; Stellar Winds; Supernovae; Astronomy full text sources ADS | data products NED (1)

The [Sii] electron density distribution over the planetary nebula NGC 7009

Electron densities have been measured from [Sii] 6716/6731 A line ratios for a grid of points over the surface of the planetary nebula NGC 7009 using a photon counting detector. The radial dependence of the electron density has been modelled, and the relationship provides possible evidence that the planetary nebula shell is driven by a strong stellar wind.

The Evidence for Planetary Progenitor Rotation and for the Long Term Acceleration of the Ejected Nebular Shell

Abstract:Several theories seek to explain the peculiar shapes of planetary nebulae. Those of Louise, Kirkpatrick, and Phillips and Reay rely on progenitor rotation. The velocity-radius relation for the shells of well observed planetaries do not extrapolate back through the origin, but rather fall short, suggesting that the shell acquires its velocity over a significant period of time. Kirkpatrick’s theory relies heavily on long term acceleration of the nebular shell, and other theoretical studies support the idea of acceleration of the nebular shell up to the time it becomes optically thin to the ionizing radiation from the central star.

The evidence for planetary progenitor rotation and for the long term acceleration of the ejected nebular shell

Several theories seek to explain the peculiar shapes of planetary nebulae. Those of Louise, Kirkpatrick, and Phillips and Reay rely on progenitor rotation. The velocity-radius relation for the shells of well observed planetaries do not extrapolate back through the origin, but rather fall short, suggesting that the shell acquires its velocity over a significant period of time. Kirkpatrick's theory relies heavily on long term acceleration of the nebular shell, and other theoretical studies support the idea of acceleration of the nebular shell up to the time it becomes optically thin to the ionizing radiation from the central star.

THE SHELL NEBULAE IN THE MAGELLANIC CLOUDS: A SURVEY IN [S II], HALPHA , AND [O III

ABSTRACT The Curtis Schmidt telescope was used to survey both the Large and Small Magellanic Clouds in the lines of forbidden O III at 5007 A, H-alpha + forbidden N II, and forbidden S II at 6716 and 6731 A, with the purpose of studying the ionization and excitation conditions of the shell nebulae in the Clouds. In the LMC, a number of shell nebulae have images that appear disk-like in forbidden O III; often the disk is concentrated toward what presumably are central stars. This is uncharacteristic of SNRs, and these objects are most likely H II regions. A few of the nebulae have significant forbidden S II strengths, sharp edges, and no obvious possibilities for exciting stars; these are very good candidates for possible identification as SNRs. The SMC is generally lacking in the regions strong in forbidden O III and S II. However, S131 has exceptionally strong S II.

Planetary Nebulae and Nitrogen Enrichment in the Galaxy

All stars in the mass interval 1-4Mʘprobably evolve through a double shell source phase before losing their entire envelopes through stellar wind and the ejection of planetary nebula shells. The remnant carbon-oxygen core rapidly evolves to a blue nucleus, illuminates the surrounding nebula for a few tens of thousands of years and finally cools off as a white dwarf (Paczynski, 1970, Harm and Schwarzschild 1975). A large fraction of the mass contained in the main-sequence stars of mass 1-4Mʘthus returns to the interstellar medium. A common feature of the double shell source (DSS) evolution of stars is the occurrence of He shell flashes. Evolutionary studies by Schwarzschild and Harm (1967), Iben (1975) and others show that the flash-driven convection zone carries helium burning products towards the hydrogen-rich layers. The consequences of mixing between the outer convective envelope and the intershell region have received a great deal of attention in recent years particularly in connection with the interpretation of carbon stars. If a deep temporary convection zone exists extending from the surface of the star to a point near the helium burning shell and mixing is allowed to take place for sufficiently long time Sackmann, Smith, and Despain (1974) found that the flash-produced12C underwent further ON0 processing enriching the surface of the star in12N. Iben (1976) has also speculated on the possibility of surface enrichment of14N during the DSS evolution of intermediate-mass stars. The subsequent loss of this envelope as a planetary nebula shell can thus cause nitrogen enrichment of the interstellar gas. In the present work we have evaluated the extent of this enrichment and have also derived the gradient of nitrogen abundance in the disc of the Galaxy based on the simple model of galactic evolution due to Talbot and Arnett (1973, hereinafter TA).

Time-dependent effects in the nebular shell of FG Sge.

view Abstract Citations (28) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time-dependent effects in the nebular shell of FG Sagittae. Harrington, J. P. ; Marionni, P. A. Abstract Theoretical models have been constructed of planetary nebulae illuminated by central stars which are evolving on a time scale comparable with the cooling time of the nebular gas. The emission line spectra of the models have been compared with the spectrum of the planetary nebula surrounding FO Sge. The observed spectrum can be reproduced by that of a nebular shell of density = 160 cm -3, illuminated by a central star which has decreased linearly in temperature at constant luminosity from an initial value of 55,000-75,000 K. The nebula appears to have a N/O ratio which is anomalously high even for planetary nebulae. Subject headings: nebulae: abundances - nebulae: planetary - stars: individual Publication: The Astrophysical Journal Pub Date: June 1976 DOI: 10.1086/154401 Bibcode: 1976ApJ...206..458H full text sources ADS | data products SIMBAD (4)

On the white dwarf HZ 43 as an extreme-ultraviolet source

It is confirmed by atmospheric modeling that both the visual continuum and the XUV spectrum (60 to 600 A) of HZ 43 can be fitted self-consistently with the emergent flux from a stellar atmosphere having an effective temperature of about 125,000 K and log g of at least 7. An examination of theoretical white-dwarf cooling sequences and estimates of the stellar luminosity indicate that the parameters of HZ 43 are consistent with a hot white dwarf of 0.6 to 1.2 solar masses. The age is estimated to be as short as 100,000 years, raising the possibility that the star may have ejected a planetary nebula or may be surrounded by an expanding H II region. It is concluded that the detection of a planetary nebula shell or an H II region around HZ 43 may provide an astrophysical test of the direct neutrino-electron interaction.

Differential Deceleration of Nebular Shells and the Displacement of Central Stars

Differential Deceleration of Nebular Shells and the Displacement of Central Stars