The dense and asymmetric central star wind of the young PN He 2-138

Abstract

We present optical European Southern Observatory time series and UV archival (Far Ultraviolet Spectroscopic Explorer (FUSE), Hubble Space Telescope, International Ultraviolet Explorer) spectroscopy of the H-rich central star of He 2-138. Our study targets the central star wind in a very young planetary nebula, and explores physical conditions that may provide clues to the nature of the preceding post-asymptotic giant branch superwind phases of the star. We provide evidence for a dense, slowly accelerating outflow that is variable on time-scales of hours. Line-synthesis modelling (Sobolev with exact integration and CMFGEN) of low- and high-ionization UV and optical lines is interpreted in terms of an asymmetric, two-component outflow, where high-speed high-ionization gas forms mostly in the polar region. Slower, low-ionization material is then confined primarily to a cooler equatorial component of the outflow. A dichotomy is also evident at photospheric levels. We also document temporal changes in the weak photospheric lines of He 2-138, with tentative evidence for a 0.36-day modulation in blue-to-red migrating features in the absorption lines. These structures may betray `wave leakage' of prograde non-radial pulsations of the central star. These multiwaveband results on the aspherical outflow of He 2-138 are discussed in the context of current interest in understanding the origin of axi- and point-symmetric planetary nebulae.