Ultraviolet and visible variability of the coma of Comet Levy (1990c)

Abstract

Short-term variability of the coma Levy (1990c) was detected and monitored with the International Ultraviolet Explorer (IUE) satellite observatory during August and September 1990 including 24 hr of continuous observation on 18 September. The visible lightcurve obtained on this date with the IUE Fine Error Sensor (FES) shows two distinct maxima separated by 17.0 ± 0.1 hr. However, this period cannot properly match in phase the FES data obtained during 8-hr shifts on 11 and 13 September, and suggests a decrease in apparent period of ∼1.3% per day. A similar decrease is derived from a comparison with the period derived from ground-based data taken in late August. The variation in the ultraviolet emissions of OH, CS, and CO 2 +, as well as that of the ultraviolet continuum (λ = 2960 A ̊ ) , was also determined from 18 consecutive long-wavelength IUE spectra taken on 18 September. From these data it is possible to model a production rate source function based on a nucleus whose activity exhibits a hemispherical asymmetry. Moreover, the ratio of gas to dust production rates is found to show a similar asymmetry. It is also found that the dust outflow velocity is about one-fourth the gas velocity. A sharp change in CO 2 + brightness suggests that a subsurface region of volatiles, including CO 2, may be partly responsible for the observed variability, although the origin of the decrease in period remains unexplained. CO was also observed, with a production rate 0.04 ± 0.01 that of H 2O, but the long exposure time required for this measurement precluded any determination of CO variability. IUE observations made postperihelion on 9–10 January 1991 showed no variability.