# Unveiling Vela - Time Variability of Interstellar Lines in the Direction of the Vela Supernova Remnant II. Na D and Ca II

## TL;DR

This study investigates time variability in interstellar Na I D and Ca II K lines towards the Vela supernova remnant, revealing significant Na D changes over two decades while Ca II K remains stable, highlighting complex gas dynamics.

## Contribution

It provides new high-resolution SALT observations confirming Na D variability and first evidence of Ca II K stability over similar periods, offering insights into interstellar gas behavior near SNRs.

## Key findings

- Na D absorption weakened significantly from 1993-2012
- Ca II K line remained unchanged between 1993 and 2015
- Contrasting behavior suggests complex gas interactions near SNRs

## Abstract

In a survey conducted between 2011-12 of interstellar Na I D line profiles in the direction of the Vela supernova remnant, a few lines of sight showed dramatic changes in low velocity absorption components with respect to profiles from 1993-1994 reported by Cha & Sembach. Three stars - HD 63578, HD 68217 and HD 76161 showed large decrease in strength over the 1993-2012 interval. HD 68217 and HD 76161 are associated with the Vela SNR whereas HD 63578 is associated with $\gamma^2$ Velorum wind bubble. Here, we present high spectral resolution observations of Ca II K lines obtained with the Southern African Large Telescope (SALT) towards these three stars along with simultaneous observations of Na I D lines. These new spectra confirm that the Na D interstellar absorption weakened drastically between 1993-1994 and 2011-2012 but show for the first time that the Ca II K line is unchanged between 1993-1994 and 2015. This remarkable contrast between the behaviour of Na D and Ca II K line absorption lines is a puzzle concerning gas presumably affected by the outflow from the SNR and the wind from $\gamma^2$ Velorum.