Spectroscopic studies of four planetary nebulae with emission-line nuclei

Spectroscopic observations of four planetary nebulae (PNe) with emission-line central stars of different spectral types are presented: Cn 1-5, Pe 1-1, NGC 5873, and M1-19. The interstellar extinction, physical conditions (n _e , T _e ), and abundances of several elements (He, N, O, Ne, S, Ar, Cl) have been determined for all nebulae. The nebula Cn 1–5 with fairly high abundances of helium and nitrogen is shown to belong to type I PNe. Possible variability of the intensities of low-excitation emission lines in NGC 5873 has been found; it can be related to variations of the stellar wind from the central star. The measured α-element abundance ratios (S/O, Ne/O, Ar/O, Cl/O) are in good agreement with those typical of HII regions.     

Chemical Evolution of the System of Galactic Planetary Nebulae

The problem of the chemical evolution of the system of Galactic planetary nebulae, starting with the early stage of development of the Galaxy, is investigated. The radial and vertical gradients of C, N, O, Ne, Ar, Cl, and S abundances are determined for different ages of the precursor stars of the nebulae. A statistically significant age dependence of the gradients is derived.     

Chemical abundances in LMC and SMC planetary nebulae

Abundances of He, N, O, and Ne are calculated for 29 faint planetary nebulae in the LMC and SMC from spectrophotometry obtained by Boroson and Liebert. When the results are combined with abundances from Aller and Czyzak's galactic planetary sample, the following conclusions are drawn: (1) O and Ne in planetaries remains generally unchanged throughout the evolution of the progenitor; (2) O and Ne abundances are indicators of progenitor metallicity; (3) Significant amounts of N are produced during evolution of intermediate mass stars, and (4) H production in intermediate mass stars over time can explain the current interstellar abundance of N in the LMC but not in the SMC.     

Metal-poor planetary nebulae: Effects of winds

Some planetary nebulae in the galactic thick disk display extremely low abundances of heavy elements such as O, Ne, S, and Ar, compared with normal or type II nebulae. Their central stars are generally relatively cool and underluminous, indicating that the progenitor stars had very low masses. It is suggested that strong stellar winds have had an important role in the formation of these objects, which is supported by the large mass loss rates now observed.     

Spectroscopy of planetary nebulae in the region of Canis Major

We present the results of a pilot project of spectroscopic observations for planetary nebulae (PNe) and PN candidates in Canis Major, a sky region where the remnant of a disrupted dwarf galaxy cannibalized by the Milky Way may be located. The spectra of seven objects were taken while testing the SALT spectrograph (South African Astronomical Observatory). All elemental abundances have been obtained by the T _e method, where the electron temperature is calculated directly using the measured weak auroral [OIII] ?? 4363 ? and/or [NII] ?? 5755 ? lines. We have measured the intensities of all the detected emission lines and determined the abundances of oxygen and several other elements (N, Ne, S, Cl, C, and He) in all PNe. The radial velocity for one PN has been measured for the first time and the velocities for all of the remaining PNe have been measured with a considerably better accuracy than that of the previously published ones. The elemental abundances for three PNe have been calculated for the first time and the accuracies of determining the abundances for three others have been improved. The measured heavy-element abundance ratios (S/O, Ne/O, Cl/O) are in good agreement with their typical values for HII regions. Among the PNe studied, ESO 428-05 is the first and so far the most likely candidate for belonging to the remnants of a possible dwarf galaxy disrupted by the tidal interaction with the Milky Way.     

The infrared [WC] stars

A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for10⁴ yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M_⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars.     

Planetary nebula chemical abundances: Z-distribution and connection with the central star masses

Dependencies of galactic planetary nebula chemical abundances and their central star masses on the distance from the galactic plane are discussed.Z-dependencies of He/H, N/H, N/O and Ar/H and dependencies of He/H, N/H, N/O, Ne/H and Ar/H on central star mass are found. Three galactic planetary nebula distance scale samples are used and it is shown that the distance scale system (where distances of each planetary nebula mass class are determined with the separate scale) is the most reliable. The correlations obtained for the Magellanic Cloud planetary nebulae are used for comparison.     

Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution

Planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the Hubble Space Telescope Data Archive and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, and S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extragalactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe-specifically, that the genesis of PNe structure should relate strongly to the population type, and by inference the mass, of the progenitor star and less strongly on whether the central star is a member of a close binary system.     

Chemical enrichment by massive stars

Heavy element abundances derived from high-quality ground-based and Hubble Space Telescope (HST) spectroscopic observations of low-metallicity blue compact galaxies (BCGs) with oxygen abundances 12+log O/H between 7.1 and 8.3 are discussed. None of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z_⊙/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M≥10 M_⊙) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only ±0.03 dex and ±0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs, and hence against production of these elements by intermediate-mass (3 M_⊙≤M≤9 M_⊙) stars at very low metallicities, as commonly thought.In higher metallicity BCGs (7.6<12+log O/H<8.2), the Ne/O, Si/O, S/O, Ar/O and Fe/O abundance ratios retain the same constant value they had at lower metallicities. By contrast, there is an increase of the C/O and N/O ratios along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. BCGs show the same O/Fe overabundance with respect to the Sun (∼0.4 dex) as galactic halo stars, suggesting the same chemical enrichment history.     

On the evolution of central stars of planetary nebulae

The evolution of nuclei of planetary nebulae has been calculated from the end of the ejection stage that produces the nebulae to the white, dwarf stage. The structure of the central star is in agreement with the general picture of Finzi (1973) about the mass ejection from the progenitors of planetary nebulae. It has been found that in order to obtain evolutionary track consistent with the Harman-Seaton track (O'Dell, 1968) one has to assume that the masses of the nuclei stars are less than 0.7M . The calculated evolutionary time scale of the central stars of planetary nebulae is 2×10⁴ yr. This time scale is negatively correlated with the stellar mass: the heavier the stellar mass, the shorter the evolutionary time scale.     

The masses of sdB and sdO stars

Mass is a fundamental parameter, but the masses are not well known for most hot subdwarfs. We propose a method of determining the masses of hot subdwarfs. Using this method, we studied the masses of hot subdwarfs from the ESO supernova Ia progenitor survey and the Hamburg quasar survey. The study shows that most of the subdwarf B stars have masses between 0.42 and 0.54 M _⊙, whilst most sdO stars are in the range 0.40～0.55 M _⊙. Comparing our study to the theoretical mass distributions of Han et al. (Mon. Not. R. Astron. Soc. 341:669, 2003), we found that sdO stars with mass less than ～0.5 M _⊙ may evolve from sdB stars, whilst most high-mass (>0.5 M _⊙) sdO stars result from mergers directly.     

Integrated fluxes for emission lines in the ultraviolet spectra of several planetary nebulae

The IUE satellite observatory has been used to obtain absolutely-calibrated emission line fluxes for diagnostic lines of multiply-ionized C, N, O, Si, Ne, and Ar which occur in the ultraviolet spectral region of planetary nebulae. These data, when combined with data from the blue, visual, and near infrared, will provide improved estimates of ionic concentrations, plasma temperatures and densities, and elemental abundances.     

A mechanism for the production of planetary nebulae

C¹² stars in the range 1.04–1.55M are evolved to simulate the core evolution of the possible precursors of planetary nebulae. The nuclear shell burning in stars above 1.2M advances to within about 0.2M of the surface, where the intense radiation interacts with the surface matter and causes mass loss. Comparison between our theoretical results and observations suggests that this may be a mechanism by which planetary nebulae are formed.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Planetary nebulae in the Magellanic Clouds (II)

Using the classification scheme for planetary nebulae in the Magellanic Clouds using four criteria proposed in Paper I, all nebulae are divided into three classes on the basis of the mass of their central stars. The features of individual chemical abundances in the Magellanic Cloud planetary nebulae and the way in which these differ from the galactic planetary nebulae are investigated separately for each class of nebulae. The role of CN and ON cycling in intermediate mass star evolution is discussed.     

The evolution of 1.40M θ pure He star

The evolution of 1.40M pure He star is calculated from the stage of the ignition in the center up to the very advanced stage of evolution where mass ejection by the very luminous He shell could occur. It is found that C¹² does not ignite by a modest margin. Subsequent evolution and relation to the central stars of planetary nebulae is discussed.     

AGB star intershell abundances inferred from UV spectra of extremely hot post-AGB stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. Almost all of the chemical trace elements in these hot stars can only be identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope played a crucial role for this research.     

Self-consistent photoionization models of planetary nebulae luminescence: stellar ionizing continuum,nebular abundances,and evolution of the envelopes

The modern self-consistent photoionization model of planetary nebula luminescence is described. All of the processes which play an important role in the ionization and thermal equilibrium of the nebular gas are taken into consideration. The diffuse ionizing radiation is taken into account completely. The construction of the model is carried out for the radial distribution of gas density in the nebular envelope which is consistent with isophotal map of the nebula. The application of the model is illustrated on the example of the planetary nebulae BD+30°3639 and NGC 7293. It is shown that the continuum of the central star at 912 Å does not correspond to the blackbody spectrum but agrees with the spectrum of corresponding non-LTE model atmosphere. The radial distributions of electron density, electron temperature, and other parameters in the nebular envelopes are found.The evolution of the radial distribution of gas density in the planetary nebulae envelopes is investigated. Approximative analytical expression which describe both such distribution and its change with time is adjusted. It is shown that the nebular envelope is formed as a result of quiet evolution of the slow stellar wind of star-precursor, and the formation of the envelope begins from the decrease of star-precursor's mass loss rate. Obtained radial distributions of gas density in the envelopes of young nebulae rule out the idea that the planetary nebula is formed as a result of a rapid ejection of clear-cut envelope. So, there is no necessity for the superwind which is used for this purpose in theoretical calculations.A new method of the determination of planetary nebulae abundances is proposed. Unobserved ionization stages are taken into account with aid of the correlations between relative abundances of various ions which had been obtained from the grid of the photoionization models of planetary nebulae luminescence. Simple approximative expressions for the determination of He/H, C/H, N/H, O/H, Ne/H, Mg/H, Si/H, S/H, and Ar/H are found. The chemical composition of 130 Galactic planetary nebulae is revised. A comparative analysis of the abundances in the Galactic disk, bulge, and halo nebulae is carried out.     

Stellar adiabatic mass loss model and applications

Roche-lobe overflow and common envelope evolution are very important in binary evolution, which is believed to be the main evolutionary channel to hot subdwarf stars. The details of these processes are difficult to model, but adiabatic expansion provides an excellent approximation to the structure of a donor star undergoing dynamical time scale mass transfer. We can use this model to study the responses of stars of various masses and evolutionary stages as potential donor stars, with the urgent goal of obtaining more accurate stability criteria for dynamical mass transfer in binary population synthesis studies. As examples, we describe here several models with the initial masses equal to 1 M _⊙ and 10 M _⊙, and identify potential limitations to the use of our results for giant-branch stars.     

Probability distribution of terrestrial planets in habitable zones around host stars

With more and more exoplanets being detected, it is paid closer attention to whether there are lives outside solar system. We try to obtain habitable zones and the probability distribution of terrestrial planets in habitable zones around host stars. Using Eggleton’s code, we calculate the evolution of stars with masses less than 4.00 M _⊙. We also use the fitting formulae of stellar luminosity and radius, the boundary flux of habitable zones, the distribution of semimajor axis and mass of planets and the initial mass function of stars. We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 M _⊙, and calculate the habitable zones of host stars, affected by stellar effective temperature. We achieve the probability distribution of terrestrial planets in habitable zones around host stars. We also calculate that the number of terrestrial planets in habitable zones of host stars is 45.5 billion, and the number of terrestrial planets in habitable zones around K type stars is the most, in the Milky Way.     

Evolution of central stars of planetary nebulae towards the crystallizing white dwarf stage

The evolution of the central stars of planetary nebulae, interpreted as hot white dwarfs with liquefying cores, towards the cold white dwarf stage is discussed and theoretical (non-computational) evolutionary tracks are built for such central stars as they cool towards the crystallizing region. The conclusions seem to hint a picture in which crystalline white dwarfs can be looked at as final stages of the central stars of planetary nebulae.