Water production rate of comet C/1997 H2 (SOHO) near perihelion

Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: mancuso@oato.inaf.it

Received: 18 March 2015
Accepted: 18 May 2015

Abstract

Comet C/1997 H2 is a non-Kreutz comet that was observed near perihelion at 0.137 AU on 1997 May 2 with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) satellite. The observed H i Lyman α radiance profile at 1215.67 Å, which was obtained during the crossing of the cometary coma through the UVCS slit positioned at 8.48 R_⊙, was least-squares fitted to a Haser model with hydrogen atom velocities of ~20 km s^-1 and ~8 km s^-1 (from photodissociation of H₂O and OH, respectively). In order to obtain a better agreement between the observed H i Lyα radiance profile and the model, an additional population of low-velocity (~4 km s^-1) hydrogen atoms was required. This low-velocity component is attributed to thermalization of a substantial fraction (≈30%) of fast hydrogen atoms within the cometary collision zone. The hydrogen production rate of comet C/1997 H2 for the crossing near perihelion corresponds to a water production rate of Q_H2O = 6.64 × 10²⁸ molecules per second.