Fragmented Comet 73P/Schwassmann-Wachmann 3

Comet 73P/Schwassmann-Wachmann 3 has been observed to fragment on several occasions, yet the cause of its fragmentation remains poorly understood. We use previously unpublished archival Hubble Space Telescope data taken in 2006 to study the properties of the primary fragment, 73P-C, in order to constrain the potential fragmentation mechanisms. Currently the literature presents a wide range of measured rotational periods, some of which suggest that the nucleus might have split due to rotational instability. However, we find the most likely value of the rotation period to be 10.38 +/- 0.04 hr (20.76 +/- 0.08 hr if double-peaked), much longer than the critical period for rotational instability for any reasonable nucleus density and shape, even in the absence of tensile strength. We also find strong, cyclic photometric variations of about 0.31 +/- 0.01 mag in the central light from this object, while similar variations with a smaller range are apparent in the surrounding dust coma. These observations are compatible with rotational modulation of the mass-loss rate and with dust having a mean outflow speed of 107 +/- 9 m s(-1). Finally, we also estimate the radius of the nucleus to be 0.4 +/- 0.1 km accounting for dust contamination and assuming a geometric albedo of 0.04.