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The evolution of Pre-Planetary Nebulae as traced by molecular gas (H₂)

Background

Kwok (2000) suggests a Planetary Nebula (PN) might be defined as "ionized circumstellar shells showing some degree of symmetry surrounding a hot, compact star evolving between the Asymptotic Giant Branch (AGB) and the White Dwarf (WD) phases''. There is no agreement on the mechanisms that shape PNs. To understand these mechanisms better we study their progenitors, a short-lived (10² → 10⁵ yrs) population of objects that occupy the AGB to PN transition stage of evolution to post-AGB stars or Proto-Planetary Nebulae (PPN). These are low- and intermediate- mass stars (0.8M_☉ → 8M_☉) whose AGB phase has terminated due to the depletion of their envelope through mass loss. During this phase of evolution the central star's temperature increases while remaining at a constant luminosity. The temperature of the central star eventually becomes hot enough to ionise the cast-off AGB envelope. It is during the early post-AGB/late AGB stage that the shaping process is believed to take place. There are no conclusive theories as to how the initially spherical circumstellar envelope becomes distorted into a multitude of axisymmetric structures. Hubble Space Telescope (HST) images of post-AGB stars indicate that many display a bipolar structure (Balick & Frank 2002). Stars in the post-AGB phase possess fast winds which can shock excite the molecular hydrogen (H₂), and its emission can be used to investigate these processes. This project will use emission from H2 in the infrared to identify and characterise regions of shock excitation around post-AGB stars, thus providing insights into the shaping process occurring at the end of the AGB.

The detection of H₂ line emission depends on evolutionary phase (Garcia-Hernandez et al. 2002). Many post-AGB stars displaying H₂ tend to be of a spectral type earlier than A-type, but there are instances of detecting H₂ in objects of a later spectral classification (e.g., CRL 2688 [F-type]). We intend to use the H₂ lines as a tracer for the interaction of the fast winds with the slower AGB outflow. We will use a sample of post-AGB stars at several evolutionary/spectral phases (M → B type) to this end. In this way we will encompass the range of cool early type post-AGB stars, to objects at the point of becoming ionised PNe.

Ongoing Work

Several studies of these objects have been carried out using a variety of different instruments. We intend to present, for the first time, VLT/SINFONI Integral Field Spectroscopy (IFS) data of these post-AGB objects. In contrast to the previous studies, using long-slit spectroscopy and/or imaging, the IFS data provides spectral information over the full extent of the objects, simultaneously. This advantage will not only allow us to perform a much more in-depth analysis, but also provide us with a unique IFS dataset of post-AGB stars. We intend to investigate the excitation mechanisms at work in post-AGB stars and to model the shocks in order to quantify our results. Line-ratio diagnostics coupled with detailed magneto-hydrodynamic shock models (Smith 1990), will determine properties of these stellar outflows such as, magnetic field strength, gas densities, and shock velocities, which are otherwise difficult to measure.