Dust depletion of Ca and Ti in QSO absorption line systems
  To explore the role of titanium- and calcium-dust depletion in gas in and
around galaxies we systematically study Ti/Ca abundance ratios in intervening
absorption-line systems at low and high redshift. We investigate
high-resolution optical spectra obtained by the UVES instrument at the Very
Large Telescope and spectroscopically analyze 34 absorption-line systems at
z<=0.5 to measure column densities (or limits) for CaII and TiII. We complement
our UVES data set with previously published absorption-line data on Ti/Ca for
redshifts up to z~3.8. Our absorber sample contains 110 absorbers (DLAs,
sub-DLAs & LLSs). We compare our Ti/Ca findings with results from the Milky Way
and the Magellanic Clouds and discuss the properties of Ti/Ca absorbers in the
general context of quasar absorption-line systems.Our analysis indicates that
there are two distinct populations of absorbers with either high or low Ti/Ca
ratios with a separation at [Ti/Ca}]~1. While the calcium dust depletion in
most of the absorbers appears to be severe, the titanium depletions are mild in
systems with high Ti/Ca ratios. The derived trend indicates that absorbers with
high Ti/Ca ratios have dust-to-gas ratios that are substantially lower than in
the Milky Way. We characterize the overall nature of the absorbers by
correlating Ti/Ca with other observables (e.g., metallicity, velocity-component
structure) and by modeling the ionization properties of singly-ionized Ca and
Ti in different environments. We conclude that CaII and TiII bearing
absorption-line systems trace predominantly neutral gas in the disks and inner
halo regions of galaxies, where the abundance of Ca and Ti reflects the local
metal and dust content of the gas. Our study suggests that the Ti/Ca ratio
represents a useful measure for the gas-to-dust ratio and overall metallicity
in intervening absorption-line systems.
