Near-infrared observations of the proto-planetary nebula AFGL 618

We present near-infrared images and spectroscopy of the bipolar proto-planetary nebula AFGL 618. Emission from molecular hydrogen is detected in the 0.90-1.34 μm spectrum. From molecular hydrogen line ratios, we derive a visual extinction of A_V = 3.4^+4.0_-2.0 mag to the H₂-emtting region. Models of collisionally excited and fluorescent H₂ emission are compared with the data: the near-infrared H₂ spectrum is dominated by emission from collisionally excited molecules at T_ex ≈ 2000 K, but a component of fluorescent emission is also present. The low level of fluorescent emission might indicate a clumpy structure, or one which partly shields the emitting region from the stellar continuum. We show that this type of near-infrared spectrum is an excellent tool for discriminating relatively low levels of fluorescent H₂ emission from a strong collisionally excited component. Images in the H- and K-bandpasses have been analyzed using models which assume single scattering of photons from the central object. The models indicate that the bipolar axis is inclined to the plane of the sky by i ∼ 45°. An unresolved, compact source is seen in both images and is reproduced in the models. It is found that the emission in the H- and K-bands is consistent with single scattering of photons by dust with a distribution such that the dust density decreases with increasing stellar latitude and is nearly zero at the poles. A total visual attenuation at the stellar equator of A_V ∼ 87 mag is required to reproduce the images.