FCRAO Millimeter Continuum Observations of Comet Hale-Bopp (C/1995 O1)

M. Senay, B. Rownd, A. Lovell, J. Dickens, C. De Vries, F.P. Schloerb, L. Mayhew, L.M. Yuen, P. Mauskopf (FCRAO)

The number distribution of millimeter-size dust particles in cometary comae is difficult to establish by optical/IR means alone. However, millimeter wave continuum radio observations are sensitive to these large particles that may control the total dust coma mass. The dust/gas mass ratio derived by combining dust and spectral line observations provides very important and basic information about the physical nature of comets.

In late February 1997, FCRAO expanded its intensive comet Hale-Bopp (C/1995 O1) spectral line mapping campaign with a program of millimeter wave bolometry observations of Hale-Bopp's dust coma. The observations were taken with the SuZIE multielement bolometer mounted on the FCRAO 14m radio telescope. This bolometer is capable of taking data simultaneously at wavelengths of 1.1, 1.4, and 2.1mm.

Initial results from late February 1997 indicate that the equivalent blackbody cross section at 1.4mm was (6.1 +/- 0.2) x 10 tex2html_wrap_inline15 km, implying a dust mass of M = 1.7 x 10 tex2html_wrap_inline17 kg in the coma at the time of observation, and a dust mass production rate dM/dt = 3.2 x 10 tex2html_wrap_inline19 kg s tex2html_wrap_inline21 . For log[Q(OH)] = 30.35, this gives a dust/gas mass ratio of approximately 3.5. The slope of the continuum emission spectrum corresponds to an opacity index of tex2html_wrap_inline23 = 1.36 +/- 0.30, intermediate between the values of tex2html_wrap_inline23 for circumstellar dust disks and the ISM. This may be interpreted as a consequence of an evolving dust particle size distribution as the dust grains expanded outwards into the coma of comet Hale-Bopp.