Small-scale structure in the interstellar medium: Time-varying interstellar absorption towards κ velorum

Abstract

Ultra-high spectral resolution observations of time-varying interstellar absorption towards κ Vel are reported, using the Ultra-High Resolution Facility on theAnglo-Australian Telescope. Detections of interstellar Ca I, Ca II, KI, NaI and CH are obtained, whilst an upper limit on the column density is reported for C2. The results show continued increases in column densities of KI and Ca I since observations ̃4 yr earlier, as the transverse motion of the star carried it ̃10 au perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrowcomponents (Aand B) are identified for all species except CH. The column density N(K I) is found to have increased by 82+10-9 per cent between 1994 and 2006, whilst N(Ca I) is found to have increased by 32 ± 5 per cent over the shorter period of 2002-2006. The line widths are used to constrain the kinetic temperature to Tk,A > 67182+18-17 K and Tk,B < 114+15-14 K. Electron densities are determined from the Ca I/Ca II ratio, which in turn place lower limits on the total number density of nA >̃ 7 × 103 cm-3 and nB >̃ 2 × 104 cm-3. Calcium depletions are estimated from the Ca I/K I ratio. Comparison with the chemical models of Bell et al. confirms the high number density, with n = 5 × 104 cm-3 for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of 40 per cent is placed on their variation over ̃9 yr. The DIBs are unusually weak for the measured E(B - V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the λ5780 to λ5797 DIBs is amongst the highest known, which may indicate that the carrier of λ5797 is more sensitive to ultraviolet radiation than to local density. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.