BIMA and VLA Observations of Comet Hale-Bopp at 22-115 GHz

We observed comet Hale-Bopp with the Berkeley-Illinois-Maryland Association (BIMA) array at frequencies between 85 and 115 GHz and with the VLA at 43 and 22.5 GHz during 1997 March and April. The continuum radiation of the comet was "easily" detected in the 3 mm wavelength band, where a peak flux density of 12.8 ± 0.65 mJy beam-1 was obtained for all the data averaged together (scaled to a geocentric distance of 1.322 AU). The beam size for this observation was 119 × 88. The total flux density was 47 ± 4 mJy. We believe that only about 2 mJy can be attributed to thermal emission from the nucleus itself; the remaining emission is thermal emission from large (roughly millimeter-sized or greater ) grains in the comet's coma. Roughly 25% of the emission comes from an unresolved source at the center (i.e., an area with a radius of 4500 km), which is surrounded by a much larger area of low-level emission, visible in the BIMA images up to distances of at least ~25,000 km. Most of this emission can be reconciled with thermal emission from dust flowing away from the nucleus, although there is also clear evidence of distinct emission features ~25,000 km to the southeast and southwest of the nucleus, suggesting former outbursts and/or dust jets. The spectral index of the millimeter emission is consistent with a thermal blackbody spectrum, while a comparison with data at submillimeter wavelengths suggests a steepening in spectral index at shorter wavelengths. The total mass of the dust in the coma is at least a few times 1011 kg, and the dust production rate is of the order ~106 kg s-1.

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