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Resumen:
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We present echelle long-slit optical spectra of a sample of objects evolving off the asymptotic giant branch (AGB), most of them in the preplanetary nebula (PPN) phase, obtained with the ESI and MIKE spectrographs at the 10 m Keck II and 6.5 m Magellan-I telescopes, respectively. The total wavelength range covered with ESI (MIKE) is ~3900-10900 Å (~3600-7200 Å). In this paper, we focus our analysis mainly on the H? profiles. Prominent H? emission is detected in half of the objects, most of which show broad H? wings (with total widths of up to ~4000 km s^?1). In the majority of the H?-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles. In ~37% of the objects H? is observed in absorption. In almost all cases, the absorption profile is partially filled with emission, leading to complex, structured profiles that are interpreted as an indication of incipient post-AGB mass loss. The rest of the objects (~13%) are H? nondetections. We investigate correlations between the H? profile and different stellar and envelope parameters. All sources in which H? is seen mainly in absorption have F-G type central stars, whereas sources with intense H? emission span a larger range of spectral types from O to G, with a relative maximum around B, and also including very late C types. Shocks may be an important excitation/ionization agent of the close stellar surroundings for objects with late type central stars. Sources with pure emission or P Cygni H? profiles have larger J ? K color excess than objects with H? mainly in absorption, which suggests the presence of warm dust near the star in the former. The two classes of profile sources also segregate in the IRAS color-color diagram in a way that intense H?-emitters have dust grains with a larger range of temperatures. Spectral classification of the central stars in our sample is presented. For a subsample (13 objects), the stellar luminosity has been derived from the analysis of the O I 7771-7775 Å infrared triplet. The location in the HR diagram of most of these targets, which represent ~30% of the whole sample, is consistent with relatively high final (and, presumably, initial) masses in the range M_f ~ 0.6–0.9 M_? (M_i ~ 3–8 M_?).
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