The Nebula around HD 168625: Morphology, Dynamics, and Physical Properties

Abstract

We present a new set of medium-resolution spectra and high-resolution coronographic images of the nebula around the Galactic luminous blue variable (LBV) candidate HD 168625. The observations were carried out at the WHT and INT (La Palma) in 1994 June and 1995 October, and at the ESO/New Technology Telescope (La Silla) in 1995 May. We find that, between 1995 May and 1995 October, HD 168625 has dimmed by 0.3 mag, and its temperature has diminished from 15,000 to 12,000 K.With the coronograph, we clearly resolve the structure in the circumstellar nebula. We find that the nebula has a complex morphology, which includes an inner elliptical shell (12'' × 16'' in size), and fainter filaments that form two outer loops in the northern and southern regions, suggesting an overall bipolar structure. The nebula is clearly associated with the star, on the basis of evidence from dynamical and chemical considerations. The nebular dynamics show a shell expanding at 40 km s–1, centered on the star, and the N enrichment detected suggests that the nebula is formed by material ejected by the star. We also find that the nebular parameters are very similar to those found in most nebulae around LBVs: although the mass of the ionized gas of 0.5 M☉ (from the integrated Hα flux) is somewhat small, the temperature (an upper limit of 7000 K) and the electron density (average 1000 cm–3) are fairly typical. Given the position of HD 168625 in the H-R diagram (at the lower end of the LBV strip, in close proximity to HR Carinae), we conclude that, although HD 168625 has not displayed LBV-type variations in the past few decades, it has certainly undergone an LBV-type outburst 103 years ago. In addition, the observed nebular bipolar morphology strengthens the suggestion (Nota et al. 1995) that all LBV-type nebulae are shaped by the same mechanism, involving a wind interacting with a density contrast.