THE DISCOVERY OF THE MOST METAL-RICH WHITE DWARF: COMPOSITION OF A TIDALLY DISRUPTED EXTRASOLAR DWARF PLANET

Abstract

Cool white dwarf stars are usually found to have an outer atmosphere that is\npractically pure in hydrogen or helium. However, a small fraction have traces\nof heavy elements that must originate from the accretion of extrinsic material,\nmost probably circumstellar matter. Upon examining thousands of Sloan Digital\nSky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS\nJ073842.56+183509.6 shows the most severe metal pollution ever seen in the\noutermost layers of such stars. We present here a quantitative analysis of this\nexciting star by combining high S/N follow-up spectroscopic and photometric\nobservations with model atmospheres and evolutionary models. We determine the\nglobal structural properties of our target star, as well as the abundances of\nthe most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca,\nand Fe. The relative abundances of these elements imply that the source of the\naccreted material has a composition similar to that of Bulk Earth. We also\nreport the signature of a circumstellar disk revealed through a large infrared\nexcess in JHK photometry. Combined with our inferred estimate of the mass of\nthe accreted material, this strongly suggests that we are witnessing the\nremains of a tidally disrupted extrasolar body that was as large as Ceres.\n