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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-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 1.95-2.45μm with a spatial resolution down to 70 mas and a spectral resolution of approx. 5000. We detect a range of H2 ro-vibrational emission lines which are consistent with shock excitation in regions of dense (approx. 10^7 cm^-3) gas with shock velocities in the range 25 - 30 km/s. 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 12^CO 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 red-shifted scattered peak of the CO bandhead is used to estimate a dust velocity along the bipolar axis of 95 km/s for the collimated wind. This places a lower limit of approx. 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.