Photometry and direct imaging of comet P/Faye 1991 XXI

Abstract

Broad-band B and V photometry over a period of 74 days including the perihelion passage, and CCD direct-imaging (V and I) is presented for the coma of short-period comet Faye 1991 XXI. The V lightcurve, corrected for the projected diaphragm diameter varying with geocentric distance, and for the cometary dust phase function, reveals a brightness “outburst” at perihelion, corresponding to an increase of the water production rate by about 10–15% during 2.0 days. This “outburst” cannot be explained by an increase of the gas sublimation rate due to a change of temperature with heliocentric distance. Rather, it must be attributed to the activation of a “dormant” active area on a very slowly rotating nucleus, or to a change in the physical structure of the emission region, like an “explosion” of volatile ices, or an exothermal phase transition in the nucleus material. From the total visual brightness of the comet a water production rate of roughly 2 × 10 28 s −1 can be estimated for the time of perihelion at 1.6 AU. The “apparent” post-perihelion brightness decrease is extraordinarily steep with n = 13.2 (or larger), which is difficult to explain by a simple dependence of the sublimation from heliocentric distance. The correction of the lightcurve for the cometary dust phase function of the coma, i.e. reduction to zero phase angle, leads to a “more normal” brightness decrease for Jupiter family comets of n = 6−7. Radial surface-brightness profiles show a ϱ −1 dependence for the inner coma indicating an almost isotropic dust emission of the near-nucleus region. A cut-off of the ϱ −1 profile in the sunward sector can be interpreted as a dust reflection envelope at about 5 × 10 3 km in front of the nucleus. The dust expansion velocities near perihelion are of the order of 200 m s −1 for the smallest grains visible in the coma. Moreover, apart from the sun-tail elongation, a slight deviation from the isotropy of the coma can be recognized perpendicular to the sun-comet radius vector, and may be interpreted in terms of solar radiation pressure acting on cometary dust grains, or anisotropic outgassing of the nucleus.