Echelle long-slit optical spectroscopy of evolved stars
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2008
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University Chicago Press
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Abstract
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 Hα profiles. Prominent Hα emission is detected in half of the objects, most of which show broad Hα wings (with total widths of up to ~4000 km s^−1). In the majority of the Hα-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles. In ~37% of the objects Hα 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 Hα nondetections. We investigate correlations between the Hα profile and different stellar and envelope parameters. All sources in which Hα is seen mainly in absorption have F-G type central stars, whereas sources with intense Hα 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 Hα profiles have larger J − K color excess than objects with Hα 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 Hα-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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© 2008. The American Astronomical Society. All rights reserved. Most of data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We are very grateful to B. F. Madore for kindly providing part of his time at the 6.5 m Magellan-I (Baade) telescope in Las Campanas Observatory to this project. The Magellan telescope is operated by a consortium consisting of the Carnegie Institution of Washington, Harvard University, the Massechusets Institute of Technology, the University of Michigan, and the University of Arizona. We also thank Gregory Walth and Daniel Kelson for processing the MIKE spectra for us. This work has been partially performed at the California Institute of Technology and the Department of Molecular and Infrared Astrophysics of the Instituto de Estructura de la Materia, CSIC, and has been partially supported by the California Institute of Technology optical observatories and by the Spanish MCyT under project AYA 2006-14876 and the Spanish MEC under project PIE 200750I028. R. S. thanks NASA for partially funding this work through an LTSA award (399-20-40-06 ) and ADP award (399-30-00-08); R. S. also received partial support for this work from HST/GO awards (GO-09463.01, 09801.01, and 10185.01) from the Space Telescope Science Institute (operated by the Association of Universities for Research in Astronomy, under NASA contract NAS 05-26555 ). The contribution of R. S. to the research described in this publication was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. A. G. d. P. is partially financed by the MAGPOP EU Marie Curie Research Training Network and by the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA2003-01676 and AYA2006- 02358. We acknowledge the use of data from the UVES Paranal Observatory Project (ESO DDT Program ID 266.D-5655). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and the NASA Astrophysics Data System.