Integral field spectroscopy of H2 and CO emission in IRAS 18276−1431: evidence for ongoing post-AGB mass-loss




We present K-band integral field spectroscopy of the bipolar post-asymptotic giant branch (post-AGB) object IRAS 18276−1431 (OH 17.7−2.0) using SINFONI on the VLT. This allows us to image both the continuum and molecular features in this object from 1.95 to 2.45 μm with a spatial resolution down to 70 mas and a spectral resolution of ∼5000. We detect a range of H2 rovibrational emission lines which are consistent with shock excitation in regions of dense (∼107 cm−3) gas with shock velocities in the range of 25–30 km s−1. The distribution of H2 emission in the bipolar lobes suggests that a fast wind is impinging on material in the cavity walls and tips. H2 emission is also seen along a line of sight close to the obscured star as well as in the equatorial region to either side of the stellar position which has the appearance of a ring with radius 0.3 arcsec. This latter feature may be radially cospatial with the boundary between the AGB and post-AGB winds. The first overtone 12CO bandheads are observed longward of 2.29 μm with the v = 2–0 bandhead prominently in emission. The CO emission has the same spatial distribution as the K-band continuum and therefore originates from an unresolved central source close to the star. We interpret this as evidence for ongoing mass-loss in this object. This conclusion is further supported by a rising K-band continuum indicating the presence of warm dust close to the star, possibly down to the condensation radius. The redshifted scattered peak of the CO bandhead is used to estimate a dust velocity along the bipolar axis of 95 km s−1 for the collimated wind. This places a lower limit of ∼125 yr on the age of the bipolar cavities, meaning that the collimated fast wind turned on very soon after the cessation of AGB mass-loss.