Radio Observations of Ions in Comet Hale-Bopp

Abstract

We present radio observations of HCO+, H3O+ and CO+ in Hale-Bopp. Radio spectroscopy is a powerful technique for observing molecules in cometary environments and for determining their kinematics. Recently, the exceptionnally bright comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) offered us the opportunity to make unprecedented observations, resulting in the detection of no less than two dozens of new cometary molecular species (Lis et al., 1999; Bockelee-Morvan et al., 2000). Among them, three molecular ions were discovered at radio wavelengths in comet Hale-Bopp: HCO+ (Veal et al., 1997), H3O+ and CO+ (Lis et al., 1999) (this latter was already known from its visible spectrum). We observed these ions with the Caltech Submillimetre Observatory (CSO 10 m), the IRAM 30-m telescope and the IRAM interferometer at Plateau de Bure (PdB 15 m) while comet Hale-Bopp was near perihelion (rh ∼ 1 AU). When cometary ices sublimate, gaseous molecules are ejected and expand with small velocities (∼ 1 km s−1) from the nucleus. But ionized molecules are accelerated by interaction with the interplanetary magnetic field frozen in the solar wind flow. A way to study the acceleration is to measure the velocities at some distances from the nucleus. Such observations have been performed for HCO+ on several days (e.g. we observed the comet with shifts from the nucleus of 1’ (about 100 000 km), 2’, 3’, 4’ and 6’ in the tail direction and 2’ in the sun direction on April 18, 1997, cf Figure 1) and CO+ on April 4, 1997 with the IRAM 30-m. The evolution of the mean velocity of the HCO+ ions with distance to the nucleus is plotted in Figure 1. Models assuming a constant acceleration fit well the observations. The acceleration of HCO+ is between 1 and 5 cm s−2. That of CO+ (whose data are not shown) seems to be smaller (≤ 1 cm s−2). A modelization with a better account of the solar wind interaction should improve the fit. Astrophysics and Space Science 277: 303–304, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.