Multiwavelength study of the nebula associated with the Galactic LBV candidate HD 168625

We present high-resolution HST imaging of the nebula associated with the Galactic luminous blue variable candidate HD 168625, together with ISO imaging and Anglo-Australian Telescope echelle spectroscopy. The overall nebular morphology is elliptical, with the major axis at P. A. similar or equal to 120degrees. The dimensions of the nebula are 12 x 16.7 at Halpha and 15.5 x 23.5 at 4 mum. In the HST Halpha image the nebula is resolved into a complex structure of laments and arcs of different brightness. The asymmetry is lost in the HST continuum image, where the nebula appears more diffuse and richer in laments and clumps with the shape of cometary tails. At 11.3 mum the nebular emission peaks in two diametrically opposite lobes located on the nebula boundaries and along its major axis. A very faint loop is also visible at optical wavelengths north and south of the shell. We suggest that the nebula is an ellipsoid with projected sizes of 14 and 9 (0.19 pc x 0.12 pc) along the right ascension and declination directions, respectively. This ellipsoid is expanding at 19 km s(-1) and is dynamically as old as similar or equal to4800 yr; it probably interacts with the stellar wind and the loop so that polycyclic aromatic hydrocarbon emission is detected from its caps, i.e., the lobes seen in the ISO images. The chemistry of the loop suggests that it is composed of unprocessed material, probably from the local interstellar medium swept by the stellar wind.