The long-period Galactic Cepheid RS?Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry

摘要

Context. The southern long-period Cepheid RS?Pup is surrounded by a large circumstellar dusty nebula reflecting the light from the central star. Due to the changing luminosity of the central source, light echoes propagate into the nebula. This remarkable phenomenon was the subject of Paper I. The origin and physical properties of the nebula are however uncertain: it may have been created through mass loss from the star itself, or it could be the remnant of a pre-existing interstellar cloud. Aims. Our goal is to determine the three-dimensional structure of the light-scattering nebula, and estimate its mass. This will bring us new clues on the origin of the nebula. Knowing the geometrical shape of the nebula will also allow us to retrieve the distance of RS?Pup in an unambiguous manner using a model of its light echoes (in a forthcoming work). Methods. The scattering angle of the Cepheid light in the circumstellar nebula can be recovered from its degree of linear polarization. We thus observed the nebula surrounding RS?Pup using the polarimetric imaging mode of the VLT/FORS instrument, and obtained a map of the degree and position angle of linear polarization. Results. From our FORS observations, we derive a three-dimensional map of the distribution of the dust around RS?Pup, whose overall geometry is an irregular and geometrically thin layer. The nebula does not present a well-defined central symmetry. Using a simple scattering model, we derive a total dust mass of Mdust?=?2.9?±?0.9 M⊙ for the light-scattering dust within 1.8′ of the Cepheid. This translates into a total mass of Mgas?+?dust?=?290?±?120 M⊙, assuming a dust-to-gas ratio of Mdust/Mgas?=?1.0?±?0.3%. Conclusions. The high mass of the dusty nebula excludes that it was created by mass-loss from the star. However, the thinness of the dust distribution is an indication that the Cepheid participated to the shaping of the nebula, e.g. through its radiation pressure or stellar wind. RS?Pup therefore appears as a regular long-period Cepheid located in an exceptionally dense interstellar environment.