METAL: The Metal Evolution, Transport, and Abundance in the Large
  Magellanic Cloud Hubble program. I. Overview and Initial Results
  Metal Evolution, Transport, and Abundance in the LMC (METAL) is a large Cycle
24 program on the Hubble Space Telescope aimed at measuring dust extinction
properties and interstellar depletions in the Large Magellanic Cloud (LMC) at
half-solar metallicity. The 101-orbit program is comprised of COS and STIS
spectroscopy toward 33 LMC massive stars between 1150 A and 3180 A, and
parallel WFC3 imaging in 7 NUV-NIR filters. The fraction of silicon in the
gas-phase (depletion) obtained from the spectroscopy decreases with increasing
hydrogen column density. Depletion patterns for silicon differ between the
Milky Way, LMC, and SMC, with the silicon depletion level offsetting almost
exactly the metallicity differences, leading to constant gas-phase abundances
in those galaxies for a given hydrogen column density. The silicon depletion
correlates linearly with the absolute-to-selective extinction, R$_V$,
indicating a link between gas depletion and dust grain size. Extinction maps
are derived from the resolved stellar photometry in the parallel imaging, which
can be compared to FIR images from Herschel and Spitzer to estimate the
emissivity of dust at LMC metallicity. The full METAL sample of depletions, UV
extinction curves, and extinction maps will inform the abundance, size,
composition, and optical properties of dust grains in the LMC, comprehensively
improve our understanding of dust properties, and the accuracy with which
dust-based gas masses, star formation rates and histories in nearby and
high-redshift galaxies are estimated. This overview paper describes the goals,
design, data reduction, and initial results of the METAL survey.
