Herschel imaging and spectroscopy of the nebula around the luminous blue variable star WRAY 15-751 ^⋆,⋆⋆

¹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 août, 17 – Bât. B5c, 4000 Liège, Belgium
e-mail: vamvatira@astro.ulg.ac.be
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium

Received: 7 May 2013
Accepted: 28 June 2013

Abstract

We have obtained far-infrared Herschel-PACS imaging and spectroscopic observations of the nebular environment of the luminous blue variable (LBV) WRAY 15-751. The far-infrared images clearly show that the main, dusty nebula is a shell of radius 0.5 pc and width 0.35 pc extending outside the Hα nebula. Furthermore, these images reveal a second, bigger and fainter dust nebula that is observed for the first time. Both nebulae lie in an empty cavity, very likely the remnant of the O-star wind bubble formed when the star was on the main sequence. The kinematic ages of the nebulae are calculated to be about 2 × 10⁴ and 8 × 10⁴ years, and we estimated that each nebula contains ~0.05 M_⊙ of dust. Modeling of the inner nebula indicates a Fe-rich dust. The far-infrared spectrum of the main nebula revealed forbidden emission lines coming from ionized and neutral gas. Our study shows that the main nebula consists of a shell of ionized gas surrounded by a thin photodissociation region illuminated by an “average” early-B star. We derive the abundance ratios N/O = 1.0 ± 0.4 and C/O = 0.4 ± 0.2, which indicate a mild N/O enrichment. From both the ionized and neutral gas components we estimate that the inner shell contains 1.7 ± 0.6 M_⊙ of gas. Assuming a similar dust-to-gas ratio for the outer nebula, the total mass ejected by WRAY 15-751 amounts to 4 ± 2 M_⊙. The measured abundances, masses and kinematic ages of the nebulae were used to constrain the evolution of the star and the epoch at which the nebulae were ejected. Our results point to an ejection of the nebulae during the red super-giant (RSG) evolutionary phase of an ~40 M_⊙ star. The multiple shells around the star suggest that the mass-loss was not a continuous ejection but rather a series of episodes of extreme mass-loss. Our measurements are compatible with the recent evolutionary tracks computed for an ~40 M_⊙ star with little rotation. They support the O–BSG–RSG–YSG–LBV filiation and the idea that high-luminosity and low-luminosity LBVs follow different evolutionary paths.