Shock emission in the bipolar post-AGB star IRAS 16594-4656

Abstract

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 $A_V=7.5\pm0.4$ mag and derived a distance of $2.2\pm0.4$ 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 μ 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 ~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 ~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 ~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.