Mass Loss Rates in Central Stars of Planetary Nebulae
Central stars of planetary nebulae (PNCS) frequently exhibit fast winds (cf. Cerruti-Sola and Perinotto, 1985; CP. and Grewing, this volume). They may be important for the structure of the whole nebula as well as for the evolution of the central star. Their speed is typically two orders of magnitude higher than that of the classical optically visible nebula, which in turn expands a few times faster than the most external winds detected in few cases in the radio domain. I review here the status
... ew here the status of art in the determination of the mass loss rates (M) associated with these fast winds. I restrict myself to the 'observational' determinations. Only at the end I will say something about the predictions of multi-scattering line radiation wind driven theory in connection with one best studied object: N6C 6543. This allows one to conclude that this theory may be the right explanation also for these winds. INTRODUCTION The mass loss rates associated with a stellar wind might be obtained by using: i) Ρ Cygni profiles of lines of heavy ions, ii)P Cygni-like profiles of hydrogen or helium lines, iii) free-free radiation in the infrared or iv) free-free radiation in the radio domain. Use of iii) and iv) has not yet successfully performed in PNCS because of the faintness of the radiation to be measured and of difficulties of taking into account the nebular contamination. Use of ii) has been done so far only in one or two objects. Thus the available M in PNCS almost all come from i). Since various strong lines of heavy ions fall in the UV range observed by the IUE satellite (λΐ200-3200 A), the observational basis for the existing values of M in PNCS rests almost exclusively in IUE data. These consist of low (6 A) and high (0.15 A) spectral resolution spectra, taken with a large aperture (oval «10"x20") or a small aperture (circular, 3" in diameter). The low resolution spectra are adequate to reveal Ρ Cygni-type profiles, but fail if the profile is essentially in emission, as in Wolf-Rayet stars, or is dominated by a blue-shifted absorption (cf. CP). They are also useful for measuring the edge velocity associated with the wind and for first approximation determination of M. The high resolution spectra are very appropriate to study in detail the whole profile, but they can be used only in objects bright enough to give a measurable stellar continuum. The lines in which Ρ Cygni profiles have been seen in PNCS are the resonance lines NV λ 1238.82, 1242.82; CIV 1548.20, 1550.77; SilV 1393.73, 1402.73, the subordinate lines OIV 1338.60, 1342.98, 1343.51; OV 1371.29; NIV 1718.55 and the recombination line He Π 1640.5 A. METHODS TO INTERPRET THE Ρ CYGNI PROFILES OF HEAVY IONS LINES The methods developed to interpret these lines are: i) the escape probability method (EP), ii) the first moment of flux distribution method (FM), iii) the comoving frame method (CF), and iv) the SEI method (Sobolev plus exact integration). 293 S. Torres-Peimbert (ed.), Planetary Nebulae, 293-300. ©1989 by the IAU. available at https://www.cambridge.org/core/terms. https://doi.