Small-scale structure in the interstellar medium: time-varying
  interstellar absorption towards {\kappa} Velorum
  Ultra-high spectral resolution observations of time-varying interstellar
absorption towards {\kappa} Vel are reported, using the Ultra-High Resolution
Facility on the Anglo-Australian Telescope. Detections of interstellar Ca I, Ca
II, K I, Na I and CH are obtained, whilst an upper limit on the column density
is reported for C_2. The results show continued increases in column densities
of K I 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 narrow components (A & B) are
identified for all species except CH. The column density N(K I) is found to
have increased by 82 +10-9 % between 1994 and 2006, whilst N(Ca I) is found to
have increased by 32 +- 5 % over the shorter period of 2002-2006. The line
widths are used to constrain the kinetic temperature to T_k,A < 671 +18-17 K
and T_k,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 n_A >
7 * 10^3 cm^-3 and n_B > 2 * 10^4 cm^-3. Calcium depletions are estimated from
the Ca I / K I ratio. Comparison with the chemical models of Bell et al. (2005)
confirms the high number density, with n = 5 * 10^4 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 % is placed on
their variation over ~ 9 years. 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 {\lambda}5780 to {\lambda}5797 DIBs is amongst the
highest known, which may indicate that the carrier of {\lambda}5797 is more
sensitive to UV radiation than to local density.
