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

TL;DR

This study analyzes the complex, asymmetric stellar wind of the young planetary nebula He 2-138 using multi-wavelength spectroscopy, revealing a variable, two-component outflow and potential stellar pulsations that inform nebula formation theories.

Contribution

It provides the first detailed evidence of a dense, asymmetric, two-component outflow in He 2-138, linking stellar wind structure to nebula morphology.

Findings

Evidence for a dense, slowly accelerating outflow

Detection of a polar high-ionization component

Possible stellar pulsation with 0.36-day period

Abstract

We present optical ESO time-series and UV archival (FUSE, HST, IUE) 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-AGB super-wind phases of the star. We provide evidence for a dense, slowly accelerating outflow that is variable on time-scales of hours. Line-synthesis modelling (SEI 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 multi-waveband 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.