Unveiling Vela – time variability of Na I D lines in the direction of the Vela supernova remnant

Abstract

High-resolution spectral profiles of Na I D lines from the interstellar medium towards 64 stars in the direction of the Vela supernova remnant are presented. This survey conducted mostly between 2011 and 12 complements an earlier survey of the same stars by Cha & Sembach done in the 1993–96 period. The interval of 15–18 yr provides a base line to search for changes in the interstellar profiles. Dramatic disappearance of strong absorption components at low radial velocity is seen towards three stars – HD 63578, HD 68217, HD 76161 – over 15–18 yr; HD 68217 and HD 76161 are associated with the Vela SNR but HD 63578 is likely associated with the wind bubble of γ 2 Velorum. The vanishing of these cold neutral clouds in the short time of 15–18 yr needs some explanation. Other changes are seen in high-velocity Na D components.