scholarly journals Helium Rich Subdwarf O Stars and Central Stars of Planetary Nebulae

1985 ◽  
Vol 87 ◽  
pp. 322-343
Author(s):  
R.H. Méndez ◽  
C.H. Miguel ◽  
U. Heber ◽  
R.P. Kudritzki
Keyword(s):  
Absorption Line ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
Evolutionary Status ◽  
Line Formation ◽  
Line Profiles ◽  
Plane Parallel ◽  
Computational Work ◽  
Work First ◽  
Central Stars

In this review we will discuss the hottest subluminous H-deficient stars, namely those with Teff > 30000 K. In the absence of reliable distance determinations for hot subluminous stars, the best way to discuss their properties and evolutionary status is to find their positions on the log g - log Teff diagram. In the last few years, after extensive computational work, first in Kiel and more recently also in Munich, it has become possible to obtain log g and Teff, together with the surface He abundance, directly by fitting the observed H and He absorption line profiles with theoretical profiles obtained from non-LTE model atmospheres and associated line formation codes. The non-LTE models are plane-parallel, in hydrostatic and radiative equilibrium, and the atmosphere is assumed to consist of H and He only. A recent paper by Groth et al. (1985) gives most of the references on the application of this non-LTE model atmosphere approach to the study of all kinds of hot subluminous stars.

scholarly journals Spectroscopic Distances to Central Stars of Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 168-168 ◽  
Author(s):  
R. H. Méndez ◽  
R. P. Kudritzki ◽  
A. Herrero ◽  
D. Husfeld ◽  
H. G. Groth
Keyword(s):  
Absorption Line ◽  
Planetary Nebulae ◽  
Interstellar Extinction ◽  
Model Atmosphere ◽  
Line Profiles ◽  
Available Information ◽  
Central Stars

We present spectroscopic distances for 22 central stars of planetary nebulae. These distances have been determined using information provided by our non-LTE model atmosphere analyses of the stellar H and He absorption line profiles. In this way, no assumptions about nebular properties are necessary.Our spectroscopic distances turn out to be larger than many other frequently cited values. We show that our distances are not in contradiction with the available information about the interstellar extinction, and we describe additional evidence supporting them.

scholarly journals Unified Model Atmosphere Studies of Central Stars of Planetary Nebulae

1993 ◽  
Vol 155 ◽  
pp. 82-82
Author(s):  
R. Gabler ◽  
A. Gabler ◽  
R. H. Méndez ◽  
R. P. Kudritzki
Keyword(s):  
High Resolution ◽  
Absorption Line ◽  
Spectroscopic Analysis ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
Unified Model ◽  
Parallel Model ◽  
Line Profiles ◽  
Atmospheric Parameters ◽  
Central Stars

A first step in the accurate quantitative spectroscopic analysis of central stars of PN has been based on fitting the results of NLTE, hydrostatic, plane-parallel model atmosphere calculations to the observed H and He absorption-line profiles in high-resolution spectra of bright central stars (Méndez et al. 1988, A&A 190, 113 and subsequent papers). Such analyses have provided very useful determinations of the basic atmospheric parameters: Teff, log g and He abundance.

scholarly journals Non-LTE Model Atmosphere Analysis of Central Stars

1983 ◽  
Vol 103 ◽  
pp. 343-357 ◽  
Author(s):  
R.H. Méndez ◽  
R. P. Kudritzki ◽  
K. P. Simon
Keyword(s):  
Absorption Line ◽  
Model Atmosphere ◽  
Helium Abundance ◽  
Line Profiles ◽  
Atmospheric Parameters ◽  
Central Stars

This review will be concentrated on the determination of the main atmospheric parameters (Teff, log g, helium abundance) of PN nuclei, and of other subluminous objects, by fitting the observed absorption line profiles with theoretical profiles obtained from non-LTE model atmosphere calculations.

scholarly journals Echelle Spectra of a Large Sample of Planetary Nebula Nuclei

1989 ◽  
Vol 131 ◽  
pp. 316-316
Author(s):  
James K. McCarthy
Keyword(s):  
High Resolution ◽  
Absorption Line ◽  
Planetary Nebula ◽  
Planetary Nebulae ◽  
The Sun ◽  
Line Profiles ◽  
Large Sample ◽  
Central Stars

We have undertaken at Palomar Observatory to obtain high resolution spectra of a large sample of planetary nebula nuclei (PNN) in order to systematically investigate their spectral morphologies and then to derive temperatures and surface gravities by comparing absorption line profiles to model atmospheres. We have taken as our sample all those central stars of planetary nebulae within 1.3 kpc of the sun according to the distance determinations of Daub (Ap, J., 260, 612, 1982); of the 94 objects in this unbiased sample, 64 are in the sky visible from Palomar and 33 have central stars bright enough to be observed at a resolution of 5 000 with an “echellette” spectrograph on the 5-m Hale telescope, leaving 7 PNN (11% of the northern sample of 64 PNN) which are too faint to be observed at present.

scholarly journals Luminosities of Central Stars of PN in the Galactic Bulge

1993 ◽  
Vol 155 ◽  
pp. 81-81
Author(s):  
R. H. Méndez
Keyword(s):  
Absorption Line ◽  
Galactic Center ◽  
Interstellar Extinction ◽  
Model Atmosphere ◽  
Line Profiles ◽  
Probable Explanation ◽  
Lyman Continuum ◽  
Most Probable Explanation ◽  
Central Stars ◽  
Balmer Decrement

A calibration of the nebular intensity of [O III] 5007 (relative to Hβ) in terms of stellar bolometric corrections is presented. This calibration (restricted to low- and medium-excitation PN) is based exclusively on the results of non-LTE model atmosphere analyses of absorption-line profiles in the spectra of bright central stars of PN (Méndez et al. 1988, A&A 190, 113 and subsequent papers). Knowing the bolometric corrections, the distance to the Galactic center, the amount of interstellar extinction and the visual apparent magnitudes (Tylenda et al. 1989, A&AS 77, 39) of central stars of PN in the direction of the Galactic center, luminosities for 20 of these central stars are obtained without any assumptions about nebular properties. The luminosities obtained with this method are substantially higher than those obtained for the same objects by Pottasch and Acker (1989, A&A 221, 123) and Tylenda et al. (1991, A&A 246, 221). The most probable explanation for this discrepancy is that luminosity estimates based on assumptions about nebular properties are too low, because many PN are not completely optically thick in the H Lyman continuum and/or because many ionizing photons in these PN are being absorbed by nebular dust. Which of the two effects is more important cannot be decided without solving the problem of the discrepancy between extinction determinations based on the Balmer decrement versus extinction determinations based on radio and Hβ fluxes (the radio-Hβ extinctions are too low).

Unified Model Atmosphere Studies of Central Stars of Planetary Nebulae

1993 ◽  
pp. 82-82
Author(s):  
R. Gabler ◽  
A. Gabler ◽  
R. H. Méndez ◽  
R. P. Kudritzki
Keyword(s):  
Planetary Nebulae ◽  
Model Atmosphere ◽  
Unified Model ◽  
Central Stars

scholarly journals The Ionization of Planetary Nebulae

1968 ◽  
Vol 34 ◽  
pp. 190-204 ◽  
Author(s):  
Robert E. Williams
Keyword(s):  
Ionizing Radiation ◽  
Emission Line ◽  
Radiation Field ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
Evolutionary Sequence ◽  
Lyman Continuum ◽  
The Mean ◽  
Central Stars

The ionization of the most abundant elements in planetary nebulae has been determined for a number of models of nebulae at different epochs in their expansion. The values used for the temperatures and radii of the central stars and the sizes and densities of the shells have come from Seaton's evolutionary sequence. The ionizing radiation field has been taken from model atmosphere calculations of the central stars by Gebbie and Seaton, and Böhm and Deinzer. Emission-line fluxes have been calculated for the models and compared with observations of planetary nebulae by O'Dell, Osterbrock's group, and Aller and his collaborators. Results indicate that the central stars have strong He+ Lyman continuum excesses, similar to those predicted by Gebbie and Seaton. The mean abundance determinations for the nebulae made by Aller are confirmed, with the exception of nitrogen, which appears to be 3 or 4 times more abundant than his value. It is also seen that the electron temperatures of the nebulae are higher than previous theoretical determinations, providing better agreement with empirically derived values.

scholarly journals Spectrophotometric studies of Planetary Nebulae with [WR] central stars

1997 ◽  
Vol 180 ◽  
pp. 269-269
Author(s):  
M. Peña ◽  
G. Stasińska ◽  
C. Esteban ◽  
R. Kingsburgh ◽  
L. Koesterke ◽  
...  
Keyword(s):  
Ionizing Radiation ◽  
Radiation Field ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
The Other ◽  
Self Consistent ◽  
First Results ◽  
Photoionization Model ◽  
Central Stars

We present the first results of a project on PNe with [WR] nuclei whose aim is twofold. One is to search for possible spatial abundance variations inside the nebula. The other is to check whether, for each object, one can build a self-consistent photoionization model (with the code PHOTO, Stasińska 1990, A&AS, 83, 501) using, as an input, the ionizing radiation field from an expanding model atmosphere reproducing the observed stellar lines of He, C and O (Koesterke et al., these proceedings).

scholarly journals Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae

2003 ◽  
Vol 209 ◽  
pp. 169-176 ◽  
Author(s):  
Klaus Werner ◽  
Jochen L. Deetjen ◽  
Stefan Dreizler ◽  
Thomas Rauch ◽  
Jeff W. Kruk
Keyword(s):  
Temperature Scale ◽  
Model Atmosphere ◽  
Uv Spectra ◽  
Stellar Surface ◽  
Line Profiles ◽  
Considerable Uncertainty ◽  
Metal Abundances ◽  
Effective Temperatures ◽  
Central Stars

The determination of effective temperatures of very hot central stars (Teff > 70 000 K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose.A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff >100000 K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

scholarly journals NLTE analysis of central stars of highly excited Planetary Nebulae

1997 ◽  
Vol 180 ◽  
pp. 132-132
Author(s):  
T. Rauch ◽  
J. Köppen ◽  
R. Napiwotzki ◽  
K. Werner
Keyword(s):  
White Dwarfs ◽  
State Of The Art ◽  
Planetary Nebulae ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Spectral Analyses ◽  
Central Stars

Very hot central stars (CSPN) of highly excited planetary nebulae (PN) display directly the formation of white dwarfs. Only a few of these CSPN have been analyzed so far due to their low brightness and thus, the interpretation of their evolutionary status is hampered by statistical incompleteness. In the last decade many spectral analyses of very hot post-AGB stars by means of state-of-the-art NLTE model atmospheres have been performed (e.g. Rauch et al. 1996; Werner & Rauch 1994; Rauch & Werner 1995) and our picture of post-AGB evolution has been improved.

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