Near-infrared echelle spectroscopy of protoplanetary nebulae: probing the fast wind in H2

Abstract

Echelle spectroscopy of H22.122μm, [Feii]1.644μm and Brγ line emission from a very young planetary nebula (PN), IRAS21282+5050, and from four protoplanetary nebulae (PPN), IRAS 19343+2926 (M1−92), IRAS17150−3224 (AFGL 6815), IRAS17423−1755 (Hen 3−1475) and IRAS17441−2411, is presented. H2 line emission is detected in discrete shock fronts in the lobes of each nebula, regardless of source spectral type [although non-detections in IRAS 09371+1212 (Frosty Leo) support claims that late spectral types do not produce bright H2 line emission]. In IRAS17150−3224, we also uncover possible signs of rotation, as would be expected if the H2 features were excited in a magnetocentrifugal disc wind. [Feii]1.644μm emission was detected in only one source, M1−92 (notably, the source with the brightest H2 features). Again, the emission is predominantly excited in high-velocity shocks in the bipolar lobes of the PPN. The H2 and [Feii] observations of M1−92, and the complex H2 profiles in IRAS21282+5050, are explained using the shock models of Smith and collaborators. We show that bow shock models are generally able to account for the observed line profiles, peak velocities, the double-peaked profiles in IRAS21282+5050, and the spatial distribution of H2 and [Feii] in M 1−92. J-type bow models are adequate in each case, i.e. a strongly magnetized wind is not required. Finally, Brγ is detected in each of the five targets; in absorption in the G-type PPN, though in emission in the O- and B-type sources. Brγ emission is detected predominantly towards the near-infrared (near-IR) continuum peak in each PPN, with only very weak emission detected in the extended lobes of the O- and B-type sources. In Brγ, low peak velocities, though very broad profile widths, are measured in each target, regardless of the nebula inclination angle. The emission must therefore derive from ionized regions in a fast wind very close to the central star (rather than from shocks in the bipolar lobes), or, in the late-type sources, from absorption in an equatorial torus.