C/1995 O1 (Hale-Bopp): Gas production curves and their interpretation

The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the fi rst opportunity to follow the activity of a bright comet over a large range of heliocentric distances r(h). Production rates of a number of parent mol ecules and daughter species have been monitored both pre- and postperihelio n. CO was found to be the major driver of the activity far from the Sun, su rpassed by water within 3 AU whose production rate reached 10(31) s(-1) at perihelion. Gas production curves obtained for various species show several behaviours with r(h). Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant t o the study of several other phenomena such as the sublimation from icy gra ins, dust mantling or seasonal effects. For some species, such as H2CO or H NC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predic tions of how such a source may vary as the comet moves along its orbit, app roaching perihelion and receding again. Features in the observed gas produc tion curves of comet Hale-Bopp are generally interpretable in terms of eith er subsurface production (typical example: CO at large r(h)) or free sublim ation (typical example: H2O). Possible implications for the vertical strati fication of the cometary ices are reviewed, and preference is found for a m odel with crystallization of amorphous ice close to the nuclear surface.