The enigmatic B[e]-star Henize 2-90: the non-spherical mass loss history from an analysis of forbidden lines

Abstract

We study the optical spectrum of the exciting B[e] star Hen 2-90 based on new high-resolution observations that cover the innermost 2 of this object whose total extent is more than 3. Our investigation is split in two parts, (i) a qualitative study of the presence of the numerous emission lines and classification of their line profiles, which indicate a circumstellar environment of high complexity, (ii) and a quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [OII], [SII], [SIII], [ArIII], [CIII], [ClIII], and [NII]. We find correlation between the different ionization states of the elements and the velocities derived from the line profiles: the highly ionized atoms have the highest outflow velocity, while the neutral lines have the lowest. The recent HST image of Hen 2-90 (Sahai et al. 2002, ApJ, 573, L123) reveals a bipolar, highly ionized region, a neutral disk-like structure, and an intermediate region of moderate ionization. This HST image covers about the same innermost regions as our observations. When combining the velocity information with the HST image of Hen 2-90, it seems that a non-spherical stellar wind model is a good option to explain the ionization and spatial distribution of the circumstellar material. Such a wind might expand into the cavity formed during the AGB phase of the star, which is still visible as a large nebula, seen e.g. on Ho plates. We modelled the forbidden lines under the assumption of a non-spherically symmetric wind that can be split into a polar, a disk forming, and an intermediate wind, based on the HST image. We find that in order to fit the observed line luminosities, the mass flux, surface temperature, and terminal wind velocities need to be latitude dependent, which might be explained in terms of a rapidly rotating central star. A rotation speed of 75-80% of the critical velocity was derived from the terminal velocities extracted from the observed line wings considering the inclination of the system as suggested from the HST image. The total mass loss rate of the star was determined to be on the order of 3 x 10 -5 M○. yr -1 . The combination of this wind scenario and the underabundance of C, O, and N in comparison to the solar abundance of S. Ar, and Cl might be explained in terms of a rapidly rotating post-AGB star.