Detection of silicates in the 51 Ophiuchi system

We have detected a prominent 10-micron silicate emission feature in 51 Oph, a B9.5 Ve star with circumstellar emission. 51 Oph shows the 'Vega phenomenon', probable disk emission from a main-sequence star. We obtained narrow-band spectrophotometry of the inner 5-arcsec-diameter region. The emission feature is more than 25 times stronger than the photospheric flux at 10 microns. We estimate that the temperatures of grains within 180 AU of the star range between 400 and 1000 K. The total flux density at the peak of the emission feature is 1.4 times the underlying dust continuum, implying particle sizes smaller than 8 microns. The shape of the silicate feature in 51 Oph resembles those in Beta Pictoris and in cometary spectra, but the grains around 51 Oph are warmer and probably larger than those around Beta Pic.