  The photospheres of some white dwarfs are "polluted" by accretion of material
from their surrounding planetary debris. White dwarfs with dust disks are often
heavily polluted and high-resolution spectroscopic observations of these
systems can be used to infer the chemical compositions of extrasolar planetary
material. Here, we report spectroscopic observation and analysis of 19 white
dwarfs with dust disks or candidate disks. The overall abundance pattern very
much resembles that of bulk Earth and we are starting to build a large enough
sample to probe a wide range of planetary compositions. We found evidence for
accretion of Fe-rich material onto two white dwarfs as well as O-rich but
H-poor planetary debris onto one white dwarf. In addition, there is a spread in
Mg/Ca and Si/Ca ratios and it cannot be explained by differential settling or
igneous differentiation. The ratios appear to follow an evaporation sequence.
In this scenario, we can constrain the mass and number of evaporating bodies
surrounding polluted white dwarfs.
