A surprisingly large number of short-period comets have been observed with significant activity (a coma and even a well-developed dust tail) at heliocentric distances greater than 3 au, where the water sublimation rate is low and thus the sublimation of other volatiles, such as for example CO, could drive the presence of a coma. As CO is not the main ice in the comet nucleus, the dust release from the nucleus surface as a result of CO drag is expected to be very different from that caused by water.
In order to investigate the complexity of the transition between bare nucleus and the presence of a well-developed coma, to compare activity levels and to obtain information concerning evolutionary differences resulting from distinct dynamical histories, we started a long-term programme of CCD imaging of distant short-period comets. In this paper we present the results of the first observing run, performed at the 3.5-m Telescopio Nazionale Galileo on La Palma on 2004 December 17. Five comets were imaged in the R band (three numbered short-period comets and the two fragments of an unnumbered short-period comet): 36P/Whipple, 111P/Helin–Roman–Crockett, 159P/LONEOS, and P/2004 V5 (LINEAR–Hill) A and B. The heliocentric distance of targets was 3.47 ≤Rh≤ 4.43 au. The targets presented several levels of activity, ranging from a stellar appearance to a well-developed coma and tail. The stellar appearance of 111P/allowed a range to be derived for the nuclear radius rnucleus from 0.46 to 1.39 km (assuming a ‘classical’ albedo value of 0.04), depending on the presence of an unresolved coma. For the active comets, we measured dust production levels in terms of the quantity Afρ, which was in the range 11 ≤Afρ≤ 224 cm.
The distant activity of the target comets is analysed in relation to the target dynamical history (in terms of the perihelion heliocentric distance). A possible preliminary conclusion can be obtained, namely that the hypothesis of distant activity ‘induced’ by higher temperature owing to perihelion lowering cannot be univocally invoked for short-period comets.