Some QSOs show an abrupt increase in polarization at rest wavelengths below about 750 Angstrom. Blaes & Agol interpreted this in terms of stellar atmosphere effects in an accretion disk around a supermassive black hole. We have computed the locally emitted continuum energy distribution and polarization from a QSO disk and the resulting observed polarized spectrum including the general, relativistic transfer function. Relativity adds an additional blueshift to the wavelength of the polarization rise, relative to the rest wavelength of the Lyman edge (912 Angstrom), such that the model does not agree with the observations. A good fit results from a model in which the polarization jumps sharply at the Lyman edge in the rest frame of the orbiting gas. Relativistic effects give an abrupt, blueshifted rise in polarization, in agreement with the observations. This model offers a potential method for constraining the inclination of the disk and the angular momentum of the black hole.

We describe a non-LTE analysis of seven subdwarf O stars and four central stars of planetary nebulae. The analysis yields metal abundances (or upper limits to these) of C, N, O, Mg, Al, and Si for most of these stars. Also effective temperature, surface gravity and helium abundance were determined for those programme stars for which no reliable estimates of these parameters were available. The derived abundance patterns prove to be disparate. We also compare the sdO stars in our sample with other sdO analyses (18 object in total) and find that this stellar class is not homogeneous with respect to metal abundances. A tentative subclassification according to helium content and luminosity-to-mass ratio indicates that the compact, high-gravity sdO stars have similar metal abundances with a strong signature of hydrogen burning in the CNO cycle. They only differ in the amounts of helium and carbon present in the photosphere, probably caused by varying efficiencies of mixing or surface erosion. Luminous sdOs, on the other hand, show markedly different metallicities, suggesting different origins.The star CPD-31 degrees 1701 is identified as a peculiar object. As compared with other O subdwarfs with nearly identical parameters, its He II line profiles appear shallow and rounded, lacking a well defined core. They can also not be fitted by theoretical profiles. No explanation for this discrepancy could be found yet.Our analysis yields microturbulence velocities in the range 0-10 km/sec for all stars with a sufficient number of metal lines. No indication was found for higher values like the 20 km/sec reported by Hubeny et al. (1991) for the sdO star BD+75 degrees 325.

High-resolution CCD spectra obtained with the Palomar 60 inch (1.5 m) echelle spectrograph and subsequently confirmed and extended using the CTIO 4 m echelle spectrograph have revealed that the hot star CD -41-degrees 13967 is the young central star of a previously unknown low-excitation planetary nebula. Presented in this paper is a detailed analysis of our CCD echelle spectra of CD -41-degrees 13967, which are well fitted by non-LTE model atmosphere line profiles corresponding to T(eff) = 27,000 K, log (g) = 3.1, and a He abundance 3 times solar. These best-fit model parameters imply a mass of 0.63 M. and a post-asymptotic giant branch (AGB) age of 1250 yr for the central star, based on a comparison with central star of planetary nebula (CSPN) evolutionary tracks in the distance-independent T(eff)-log (g) plane. Using recent UBVRI CCD photometry corrected for extinction along with the Eddington flux emerging from the best-fit non-LTE model atmosphere, we derive a spectroscopic distance estimate of 3.5 kpc to CD -41-degrees 13967. This together with the Galactic coordinates (l(II) = 359.0; b(II) = -33.5) strongly suggests that CD -41-degrees 13967 belongs to the Galactic halo population, a suggestion supported by the results of our stellar abundance analyses for silicon, [Si/H] = -0.20 +/- 0.10, and oxygen, [O/H] = -0.43 +/- 0.20, as well as by the mean of our stellar radial velocity measurements (* = -91 +/- 2 km s-1; compare also with our result, PN = -97 +/- 2 km s-1).The surrounding planetary nebula, He 3-1863, shows an angular extent greater than 6" (FWHM) on the E/W slit of our CTIO spectrograms. The excitation class of the nebula is zero, with only H-alpha, H-beta, H-gamma, H-delta, [N II] 5755, 6548, 6584 angstrom, [O I] 6300, 6364 angstrom, [O II] 7320, 7330 angstrom, and [S II] 6717, 6731 angstrom lines in evidence; lacking is any [O III] 4363, 4959, 5007 angstrom emission. This is consistent with the low T(eff) determined for the central star. Subsequent narrow-band H-alpha CCD imaging reveals that He 3-1863 is a centrally concentrated elliptical nebula with a total E/W extent greater-than-or-equal-to 17" and N/S extent greater-than-or-equal-to 13" on the sky.

view Abstract Citations (51) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolutionary versus Dynamical Time Scales for the Evolution of the Central Stars of Planetary Nebulae McCarthy, J. K. ; Mould, J. R. ; Mendez, R. H. ; Kudritzki, R. P. ; Husfeld, D. ; Herrero, A. ; Groth, H. G. Abstract The disagreement between evolutionary and dynamical time scales for the evolution of the central stars of planetary nebulae (CSPN) through the H-R diagram is investigated. Three possible reasons for the disagreement are examined: (1) the nebulae could have experienced a phase of rapid photoionization of material ejected previously while the stars were still on the AGB; (2) the central stars could have undergone a late helium shell flash and returned to the AGB; and (3) the AGB-CSPN evolutionary transition times could have been increased by small additional amounts of residual envelope material remaining after the superwind mass-loss phase. Mechanism (3) is favored here because it can account for evolutionary ages both less than and greater than the corresponding dynamical ages. Publication: The Astrophysical Journal Pub Date: March 1990 DOI: 10.1086/168458 Bibcode: 1990ApJ...351..230M Keywords: Asymptotic Giant Branch Stars; Planetary Nebulae; Stellar Evolution; Absorption Spectra; Hertzsprung-Russell Diagram; Photoionization; Stellar Envelopes; Stellar Mass; Stellar Winds; Astrophysics; NEBULAE: PLANETARY; STARS: EVOLUTION full text sources ADS | data products SIMBAD (23)

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.

Results of a non-LTE fine analysis based mainly on high-resolution CASPEC spectra for three extremely helium-rich sdO stars are discussed in order to explain hydrogen deficiency in single stars. High temperature (Teff = 70,000 to 75,000 K) and a position in the log Teff - log g diagram were found close to the Eddington limit. Various abundance estimates are derived for hydrogen (upper limits only), carbon, nitrogen, and magnesium. Hydrogen is reduced to less than 10 percent by number in LSE 153 and LSE 263, and to less than 5 percent in LSE 259. The hydrogen deficiency is accompanied by nitrogen- and carbon-enrichment in LSE 153 and LSE 259 only. In LSE 263, carbon is depleted by about 1 dex. Stellar masses obtained by assuming that a core mass-luminosity relation holds for these stars, were found to be in the range 0.6-0.9 solar mass, yielding luminosities log L/L:solar = 3.7-4.5. Two of the program stars (LSE 153 and 259) appear to be possible successors of the R CrB and helium B stars, whereas the third star (LSE 263) displays a much lower carbon content in its photosphere making it an exceptional case among the known hydrogen deficient stars.

Les spectres de 4 des etoiles les plus brillantes dans l'amas ouvert NGC 346 dans le Petit Nuage de Magellan sont analyses en utilisant des modeles d'atmosphere non-ETL et des calculs de formation de raies. Les parametres atmospheriques sont determines et fournissent, en combinaison avec le module de distance du Petit Nuage de Magellan, les luminosites et les masses. L'influence du recouvrement par le vent sur la determination des parametres stellaires est discutee dans le cas d'une metallicite faible du Petit Nuage de Magellan. L'ionisation de la region HII geante N66 dans laquelle sont plongees les etoiles de l'amas est discutee

First reported at the IAU Colloquium No. 53 on White Dwarfs (McGraw et al . 1979), PG 1159-035 (GW Vir) is the prototype of a new class of very hot, pulsating, pre-white dwarf stars. It shows complicated, nonradial pulsation modes which have been studied exhaustively, both observationally and theoretically. The effective temperature has been crudely estimated as 100,000 K with log g ~ 7 (Wesemael, Green and Liebert 1985, hereafter WGL).

In the upper left corner of the HR-diagram various stars have been found that are clearly not members of the hydrogen main sequence. The majority of them lie to the left and below the main sequence, indicating that they are highly evolved stars close to the extinction of their thermonuclear power source and hence to rapid cooling towards the white dwarf domain. The implication is that a “short” time ago they were red giants and as such experienced phenomena like mixing, dredge-up and heavy mass loss. Therefore, one expects the hot evolved stars to display the consequences of the afore mentioned processes that are up to now not satisfactorily understood.

The massive stars of the Magellanic Clouds are of considerable current interest with regard to questions of initial mass function, star formation mechanisms, stellar evolution with mass loss and the chemical evolution of galaxies. The effective temperatures, surface gravities and helium abundances of 6 main sequence O-type stars, obtained by fitting non-LTE model atmospheres to high quality spectra, are presented here; these are the first results from a long-term program to determine accurately the parameters and chemical abundances of massive stars in the Magellanic Clouds. The program stars were selected to be main sequence objects, according to the classification of Conti et al. (1985, in prep.), with He II λ 4686 Å in absorption, and to have minimal reddening and nebular emission. Spectra were obtained in 1984 December with the Cassegrain echelle spectrograph (CASPEC) and a CCD detector at the ESO 3.6 m telescope. A preliminary analysis of these spectra has been carried out by fitting the equivalent widths of He I λ 4471 Å and the profiles of Hγ and the Pickering lines (for details of the technique, see Kudritzki, 1980). The resulting values of effective temperature and gravity are given in Table I, along with the identification, spectral type and mv, of each star.

Nine helium rich sdO stars are found to show no trace of hydrogen on high resolution visual spectra. Effective temperatures derived from UV fluxes range from 42500 K to 80000 K. A dichotomy with respect to the C/N ratio is found which is reminiscent of the OBN and OBC stars near the main sequence. It is estimated that about 20% of the sdO stars are extremely helium rich. This fraction compares nicely with those of the (helium rich) DB white dwarfs (20%) and the helium rich central stars of planetary nebulae (less than 35%).