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

Abstract

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 H 2 rovibrational emission lines which are consistent with shock excitation in regions of dense (∼10 7 cm −3 ) gas with shock velocities in the range of 25-30 km s −1 . The distribution of H 2 emission in the bipolar lobes suggests that a fast wind is impinging on material in the cavity walls and tips. H 2 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 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.

A fast bipolar H2 outflow from IRAS 16342−3814: an old star reliving its youth

“…This is in contrast to the H2 emission which traces the extended bipolar outflow. The situation is remarkably similar to that of IRAS 18276-1431 discussed by Gledhill et al (2011), where the CO and Na I emission was interpreted as evidence for the presence of hot (> 2000 K) and dense (> 10 10 cm −3 ) gas close to the central star(s).…”

“…Data reduction was accomplished using the European Southern Observatory (ESO) pipeline for SINFONI with further processing and data visualization using the starlink software collection. For further details on our processing of the SINFONI data cubes, see Gledhill et al (2011).…”

Episodes of Molecular Emission in the Optical Spectrum of IRAS 22272+5435

“…Mozurkewich et al (1987) observed variable CO emission/ absorption profiles in the RV Tauri-type star R Scuti and attributed them to shocks in the upper atmosphere. Integral field spectroscopy of the post-AGB object IRAS 18276-1431 supports the idea that CO emissions form close to the star (Gledhill et al 2011).…”

Numerical simulations of wind-driven protoplanetary nebulae – I. near-infrared emission

“…The distribution of H2 emission in pPN suggests that a fast wind is impinging on material in the cavity walls and tips (Gledhill et al 2011). Examples of prolate ellipsoidal nebula are found .…”

“…Hrivnak et al (2008) found the emission to be from the walls of the nebula producing an ellipsoidal velocity structure. Gledhill et al (2011) identified a fast wind impinging on the cavity walls and tips. van de Steene et al (2008) detected shock-excited molecular hydrogen emission as well as collisionally-excited emission lines from [OI], [CI], and [FeII].…”