The Metallicity and Dust Content of HVC 287.5+22.5+240: Evidence for a Magellanic Clouds Origin

We estimate the abundances of S and Fe in the high velocity cloud HVC287.5+22.5+240, which has a velocity of +240 km/s with respect to the localstandard of rest and is in the Galactic direction l~287, b~23. The measurementsare based on UV absorption lines of these elements in the Hubble SpaceTelescope spectrum of NGC 3783, a background Seyfert galaxy, as well as new H I21-cm interferometric data taken with the Australia Telescope. We findS/H=0.25+/-0.07 and Fe/H=0.033+/-0.006 solar, with S/Fe=7.6+/-2.2 times thesolar ratio. The S/H value provides an accurate measure of the chemicalenrichment level in the HVC, while the super-solar S/Fe ratio clearly indicatesthe presence of dust, which depletes the gas-phase abundance of Fe. Themetallicity and depletion information obtained here, coupled with the velocityand the position of the HVC in the sky, strongly suggest that the HVCoriginated from the Magellanic Clouds. It is likely (though not necessary) thatthe same process(es) that generated the Magellanic Stream is also responsiblefor HVC 287.5+22.5+240.Comment: AASTEX, 3 postscript figures, AJ, 1998, Jan issu