Observations of Multiphase, High-Velocity, Shocked Gas in the Vela Supernova Remnant

TL;DR

This study analyzes high-resolution ultraviolet spectra to investigate high-velocity shocked gas in the Vela supernova remnant, revealing ionization states, physical conditions, and inhomogeneities in the post-shock environment.

Contribution

It provides detailed diagnostics of the physical conditions and ionization structure of shocked gas in the Vela SNR using archival UV spectra, highlighting the complex, inhomogeneous nature of the interaction.

Findings

Highly ionized species probe gas immediately behind the shock.

Lower ionization species trace downstream cooling regions.

Temporal variations confirm inhomogeneity of the interstellar medium.

Abstract

We present an analysis of high-resolution far-ultraviolet archival spectra obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope of the star HD 75309, which probes high-velocity shocked gas in the Vela supernova remnant (SNR). We examine high-velocity features from intrinsically strong absorption lines of O I, Si II, Si II*, C II, C II*, and Si III. We also detect high-velocity components in the N V doublet and compare these features to observations of high-velocity O VI absorption, available from archival Far Ultraviolet Spectroscopic Explorer data. Kinetic temperatures are derived from the observed fractional abundances of the various ions, while gas densities and thermal pressures are obtained from the relative populations in excited fine-structure levels of C II and Si II. Our results indicate that the highly ionized species at high velocity probe gas in a region immediately behind a shock driven into an interstellar cloud, while the lower ionization species trace material further downstream in the cooling region of the post-shock flow. Low velocity N V and O VI absorption may trace gas in a conductive boundary layer between the unshocked portion of the cloud and the hot X-ray emitting intercloud medium. Temporal variations in high velocity Ca II absorption features observed toward HD 75309 further confirm the highly inhomogeneous nature of the interstellar medium interacting with the Vela SNR.