The SWAN all-sky camera on the SOHO spacecraft observed the hydrogen Lyman-alpha coma of Comet Hale-Bopp (1995 O1) from January through May 1997. The observations covered heliocentric distances from 1.75 AU before perihelion to 1.29 AU after, showing the hydrogen coma to vary in brightness, shape, and orientation with the comet's orbital position. The water production rate of the comet was determined from an analysis of these images with a model that accounts for the detailed physics and chemistry. The model incorporates coma dynamics (expansion velocity and kinetic temperatures), which have been shown to be consistent with a variety of other observations. The distribution of brightness in the modeled images reproduces the absolute magnitude and distribution of brightness seen in the observations. We find that the maximum water production rate occurred about two weeks after perihelion, and that the magnitude and variation with time are consistent with other published observations which were analyzed in a consistent manner.
Shown are the results of a one-dimensional spherical hybrid kinetic/dusty-gas hydrodynamic calculations from Figure 1 in the paper by Combi et al. (1999b). Above is the variation of the radial outflow velocity with distance from the nucleus for different heliocentric distances of Comet Hale-Bopp. Below are similar curves for the gas kinetic temperature.
Contours of brightness in kilorayleighs for the SWAN data (solid lines) are compared with those from a model (dashed lines). The model calculation (see Combi & Smyth 1988b) is able to reproduce both the shape and overall radial gradient of the coma. The observed shape is produced by a combination of H atom velocity distribution from photodissociation and partial thermalization, radiation pressure acceleration from solar fluorescent scattering, and the relative orbital motions of the nucleus and H atoms.
In (a) is a comparison of the effective speed distribution of H atoms exiting the inner coma with (thick line) and without (thin line) the full physics of partial collisional thermalization from the model for comet Hale-Bopp at perihelion on April 1, 1997. In (b) is a similar comparison except from the model for January 1, 1997 when the comet was 1.75 AU from the sun. Note that for January 1 only a minor modification of the speed distribution results from collisions, whereas the effect is quite important near perihelion.
The water production rates of Hale-Bopp determined from the SWAN observations of H Ly-alpha are compared with those determined from IRTF observations of H2O by Dello Russo et al. (1999) and various observations of OH by Colom et al. (1999) from Nancay, Weaver et al. (1999) with HST and IUE, and Schleicher et al. (1997, 1999) from ground-based photometry at Lowell Observatory.
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