In this paper we study the near-infrared emission spectrum of IRAS 16594 - 4656 , a bipolar post-AGB star with spectral type B7 and no observed ionization .
Using optical and near-infrared photometry we determined the total extinction towards this object to be A _ { V } = 7.5 \pm 0.4 mag and derived a distance of 2.2 \pm 0.4 kpc , assuming a luminosity of 10 ^ { 4 } L _ { \odot } .
The near-infrared spectrum shows strong H _ { 2 } emission lines and some typical metastable shock excited lines such as [ Fe ii ] 1.257 & 1.644 \mu m. We determined the rotational and vibrational excitation temperatures , as well as the ortho-to-para ratio of the molecular hydrogen .
Based on these we argue that the H _ { 2 } emission is mainly collisionally excited .
Line ratios indicate that the H _ { 2 } emission originates in a \sim 25 km s ^ { -1 } C-type shock .
On the other hand , the metastable lines , and especially the [ Fe ii ] emission lines , indicate the presence of a \sim 75 km s ^ { -1 } J-type shock .
Hence we postulate that the H _ { 2 } emission originates where the stellar wind ( with an observed terminal velocity of \sim 126 km s ^ { -1 } ) is funneled through an equatorial density enhancement , impinging almost tangentially upon the circumstellar material .
The [ Fe ii ] emission either occurs along the walls of the bipolar lobes where the transverse shock velocity would be higher , or could originate much closer to the central star in shocks in the post-AGB wind itself , or possibly even an accretion disk .
Further high resolution near-infrared spectra are currently being obtained to confirm the proposed geometry and kinematics .