Zanstra Temperatures Research Articles

Nebular Abundances and Central Star Parameters for eight PN in the Magellanic Clouds

We have obtained ultraviolet and optical spectrophotometry for two PN in the SMC (N87, L302) and for six in the LMC (N28, N66, N97, N141, N201, N203). The data were obtained with the IUE (eight nebulae), the 3.9 m AAT (six nebulae) and the 1.9 m SAAO reflector (two nebulae). Nebular temperatures, densities and abundances are presented. The nebular continua were calculated and subtracted from the observed continua, allowing the central star energy distributions and hydrogen and helium Zanstra temperatures to be derived. Our results for the effective temperatures and absolute magnitudes and luminosities of the central stars are presented, including data on two Wolf-Rayet central stars (WC5 and WC8) and one Of central star.

Ultra-Violet Spectrophotometry of Some Hotter Central Stars

A spectrophotometric survey has been made for about 20 central stars of planetary nebulae, with emphasis mainly on hot stars. We use low-resolution IUE spectra, observed by ourselves or obtained from Data Center, together with, in some cases, results from optical observations. Data have been extracted and merged, regions of saturation eliminated, ITF errors corrected and nebular continua subtracted. Careful assessments have been made of reddening constants, and of data used to calculate Zanstra temperatures.

Luminosities and masses for three central stars of planetary nebulae in the Magellanic Clouds from ultraviolet spectroscopy with the IUE

Physical parameters of the central stars of the planetary nebulae (PN) P40, N2, and N5 are determined from UV observations by the IUE spacecraft and emission line-data. The observations took place on May 23 and 26, 1981. Derived properties, such as the H-beta, color excess, nebular hydrogen emission, helium emission, and stellar continuum emission, the stellar radius, surface gravity, Zanstra temperature, luminosity, absolute bolometric magnitude, apparent visual magnitude, mass, and Zanstra parameter for the three PN central stars are presented. All the masses were over one solar mass, suggesting a specific PN nuclei mass. The finding was corroborated by all luminosities exceeding the Eddington luminosity for a 0.6 solar mass star, which has previously been calculated for the white dwarf core of a PN.

Ultraviolet spectra of planetary nebulae - VI. NGC 7662

A detailed study of NGC 7662 is based on UV results obtained from 15 IUE spectra and on observations of other workers at optical, IR and radio wavelengths. Improved techniques are used to extract IUE data for an extended source. Relative fluxes in the different apertures which have been used are obtained using the brightness contours of Coleman, Reay & Worswick. There is close agreement between the reddening deduced from the ratios He II (λ 1640)/(λ 4686) and (radio)/(Hβ) and the nebular continuum emission observed with the IUE large slots agrees closely with that predicted using absolute radio and Hβ fluxes. The fluxes in nebular emission lines observed with the small slots are smaller than expected from brightness distributions; it is concluded that, for an extended source, the small slots have aperture transmission factors of 0.85 for SWP and 0.46 for LWR. The central star is fainter than has been previously supposed (by more than two magnitudes). The blackbody He II Zanstra temperature of 113 000 K is consistent with the UV colour temperature. Previous work on colour temperatures of central stars is discussed critically. Two models are discussed. For Model I the star is assumed to radiate like a blackbody and absorption by internal dust is neglected. This gives results which are not in agreement with observations for the flux in the C III λ 2297 di-electronic recombination line, and for the thermal infrared (TIR) emission measured by Moseley. For Model II the stellar flux is interpolated from LTE model atmosphere results of Hummer & Mihalas. The He II Zanstra temperature is 120 000 K. The C3+ abundance is such as to give a C III λ 2297 flux in agreement with observations but an emission in the C IV λ 1549 resonance lines greater than that observed by a factor of 3. The C IV emission is assumed to be attenuated by dust. The required dust optical depth is τD = 0.1. The dust is assumed to consist of large graphite grains. The calculated TIR flux agrees with that observed. Model II gives fluxes for lines of H, He, C, N, 0, Ne, Mg, Si and S with an agreement with UV, optical and IR observations which is generally satisfactory. Abundances are deduced. The use of the model atmosphere flux gives a marked improvement for the C3+/C2+ and N3+/N2+ ratios. Model II is considered to be much more realistic than Model I. The O abundance is less than half of the solar value, C/O three times solar, N/O is 40 per cent higher than solar and Ne/O is solar, as is S/H. The gasphase Mg abundance is less than solar by a factor of at least 50, and that of Si by a factor of 7.

Nebular observations and stellar coronae

We examine the ways in which nebular observations of He II lambda4686 can be used to infer the existence of coronae or hot winds of early-type stars. The O VI lambdalambda3811, 3834 doublet is seen in the spectra of some central stars of planetary nebulae, and several authors have considered this to be evidence of a coronal wind. Observations of NGC 6751 reinforce this view by showing that radiative equilibrium model atmospheres cannot sufficiently photoexcite the O VI transitions without producing too much nebular emission at lambda4686. The hot gas in the coronal model produces a significant amount of radiation at energies above 54 eV; therefore, the He II Zanstra temperatures can be affected. Discrepancies between the He II Zanstra temperatures and effective temperatures derived from other methods may indicate coronal emission from planetary nebula central stars.Observations of diffuse nebulae about Population I early-type stars may also prove useful in detecting hot winds. The Lamers and Morton model of the wind of the O4f star xi Pup predicts a small amount of He/sup +/ ionizing radiation. If NGC 7635 is associated with the Of star BD +60/sup 0/2522, then the observation of nebular emission at lambda4686 may indicate coronal emission.

Atmospheres of Central Stars

This review begins with a brief summary of atmospheric models that are of possible relevance to the central stars of planetary nebulae, and then discusses the extent to which these models accord with the observations of both nebulae and central stars. Particular attention is given to the significance of the very high Zanstra temperature implied by the nebular He II λ4686 Å line, and to the discrepancy between the Zanstra He II temperature and the considerably lower temperatures suggested by the appearance of the visual stellar spectrum for some of these objects.