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We detect HCN J=5-4 emission from the ultraluminous quasar APM08279+5255 atz=3.911 using the IRAM Plateau de Bure interferometer. This object is stronglygravitationally lensed, yet still thought to be one of the most intrinsicallyluminous objects in the Universe. The new data imply a line luminosityL'_HCN(J=5-4) = 4.0+/-0.5 x 10^(10) K km/s pc^2. The ~440 km/s full width halfmaximum of the HCN J=5-4 line matches that of the previously observed high-J COlines in this object and suggests that the emission from both species emergesfrom the same region: a warm, dense circumnuclear disk. Simple radiativetransfer models suggest an enhanced abundance of HCN relative to CO in thenuclear region of APM08279+5255, perhaps due to increased ionization, orpossibly the selective depletion of oxygen. The ratio of far-infraredluminosity to HCN luminosity is at the high end of the range found for nearbystar forming galaxies, but comparable to that observed in the few high redshiftobjects detected in the HCN J=1-0 line. This is the first clear detection ofhigh-J HCN emission redshifted into the 3-millimeter atmospheric window.Comment: Accepted for publication in ApJ

HCN J = 5-4 Emission in APM 08279+5255 at z = 3.91