2008A&A...480..775V

The spectrum of IRAS 16594-4656 shows shock-excited H₂ emission and collisionally excited emission lines such as [OI], [CI], and [FeII]. The goal is to determine the location of the H₂ and [FeII] shock emission, to determine the shock velocities, and to constrain the physical properties in the shock. High resolution spectra of the H₂ 1-0 S(1), H₂ 2-1 S(1), [FeII], and Paβ emission lines were obtained with the near infrared spectrograph Phoenix on Gemini South. The position-velocity diagrams of H₂ 1-0 S(1), H₂ 2-1 S(1), and [FeII] are presented. The H₂ and [FeII] emission is spatially extended. The collisionally excited [OI] and [CI] optical emission lines have a similar double-peaked profile compared to the extracted H₂ profile and appear to be produced in the same shock. They all indicate an expansion velocity of ∼8km/s and the presence of a neutral, very high-density region with n_e about 3x10⁶ to 5x10⁷cm^–3. However, the [Feii] emission is single-peaked. It has a Gaussian FWHM of 30km/s and a total width of 62km/s at 1% of the peak. The Paβ profile is even wider with a Gaussian FWHM of 48km/s and a total width of 75km/s at 1% of the peak. The H₂ emission is excited in a slow 5 to 20km/s shock into dense material at the edge of the lobes, caused by the interaction of the AGB ejecta and the post-AGB wind. The 3D representation of the H₂ data shows a hollow structure with less H₂ emission in the equatorial region. The [FeII] emission is not present in the lobes, but originates close to the central star in fast shocks in the post-AGB wind or in a disk. The Paβ emission also appears to originate close to the star.