Velocity discrimination in hydrogen-deficient white dwarfs

Overview

This project investigated unidentified absorption features in the far-ultraviolet (FUV) spectra of hydrogen-deficient white dwarfs, particularly DO white dwarfs and PG1159 stars. The work used spectra obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE), a space telescope that observed in the far-ultraviolet region.

The central problem was that many absorption lines in these stellar spectra had not been securely identified. Some features could originate in the star’s photosphere; others could be produced by material in the interstellar medium (ISM) between the star and Earth. Distinguishing between these origins is an essential first step before assigning an atomic species or transition to an unidentified line.

Research question

How can radial-velocity differences between white dwarfs and the interstellar medium be used to determine whether unidentified FUV absorption lines are photospheric or interstellar in origin?

A related question was whether the remaining unidentified features could be explained by incomplete atomic line lists, including transitions from highly ionised or iron-group species not adequately represented in available data.

Method

The analysis used velocity discrimination. In a simplified case, spectra from two white dwarfs have different separations between their photospheric and ISM radial velocities.

• A line that shifts with the ISM velocity is likely interstellar.
• A line that shifts with the photospheric velocity is likely stellar.
• Comparing the same feature across stars helps distinguish the two possible origins even where a line’s atomic identification is initially unknown.

Known spectral lines were used to establish the characteristic velocities of the photosp