Multi-wavelength study of the nebula associated with the galactic LBV candidate HD 168625

Abstract

We present high resolution HST imaging of the nebula associated with the galactic LBV candidate HD 168625, together with ISO imaging and AAT echelle spectroscopy. The overall nebular morphology is elliptical with the major axis at PA ~ 120 deg. The dimensions of the nebula are 12'' x 16''.7 at Halpha and 15''.5 x 23''.5 at 4 micron. In the HST Halpha image, the nebula is resolved into a complex structure of filaments and arcs of different brightness. The asymmetry is lost in the HST continuum image where the nebula appears more diffuse and richer in filaments and clumps with the shape of cometary tails. At 11.3 micron the nebular emission peaks in two diametrically opposite lobes, placed 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 RA and DEC directions, respectively. This ellipsoid is expanding at 19 km/s and is dynamically as old as ~ 4800 yrs; it probably interacts with the stellar wind and the loop so that PAH 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 un-processed material, probably from the local interstellar medium swept by the stellar wind.