Abstract

The 2009 apparition of comet 107P was observed with a variety of instruments from six observatories. The main results of this investigation are: (1) 107P/Wilson-Harrington was found to be active not only in 1949 but also in 1979, 1992, 2005 and 2009. The activity is very weak and produces only a slight brightness increase above the nucleus. (A_SEC=V_NUC(1,1,0)-m(1,1)<1 mag). Since the amount of solar energy received by the object at perihelion has been monotonically increasing since 1928, we conclude that the comet has been active at all apparitions ever after. The probability that the activity in 1949 or in 2009 was due to a surface impact is very small. (2) The rotational period has been determined. We find P_ROT=6.093±0.002 h. The amplitude of the rotational light curve (peak to valley) is A_ROT=0.20±0.01 mag in V. From this data the minimum ratio of semi-axis is (a/b)_MIN=1.20±0.02. The rotational light curve is identical to the shape of a saw tooth. The shape of the object must be peculiar and has sharp edges. (3) This is the first time that the phase curve of a comet has been determined in three colors, B, V and R. We find B(1,1,α)=16.88(±0.06)+0.041(±0.001)α, V(1,1,α)=16.31(±0.04)+0.043(±0.001)α, and R(1,1,α)=15.90(±0.04)+0.039(±0.001)α. (4) From the phase curves color indices can be deduced when the comet is not active. We find V-B=0.57±0.07 and V-R=0.41±0.06. (5) Assuming a geometric albedo p_V=0.04 and the absolute magnitude from the phase plot, we find an effective diámeter free from rotational modulation D_EFFE=3.67±0.06 km. (6) The secular light curves are presented. The comet is slightly active above the nucleus line (A_SEC(1,LAG)=0.36±0.10 mag), but did not exhibited a coma. Activity started +26±1 d after perihelion. Using the definition of photometric age, we find T-AGE=4700 (+6000, -1700) cy (comet years), an exceedingly old object, a methuselah comet. (7) In the 2009 apparition no tail and no coma were detected using more sensitive detectors than on 1949. This is good evidence to conclude that this object is very near extinction and belongs to the graveyard of comets. In a Remaining Revolutions vs. Water-Budget Age plot, we have identified the region of the graveyard, and 107P is a member of this group. (8) Using the amplitude of the secular light curve (A_SEC) vs. Diameter (D_EFFE) diagram (Fig. 15), we show that 107P is the most evolved object in the current data base of comets. In the classification scheme of Ferrín (PSS, 58, 365-391, 2010), 107P is a methuselah comet (WB-AGE=7800>100 comet years), medium size (1.4<D_EFFE=3.67<6 km) nucleus, fast rotating (P_ROT=6.093 h<7 h), belonging to the graveyard of comets (the region with A_SEC<1 mag in Fig. 15 or 1.000 cy<WB-AGE inFig. 18), that must be very near its dormant phase, were not for the fact that this comet is temporarily being rejuvenated due to a trend of apparitions with decreasing perihelion distances. (9) Since the general flow of the sample is down and to the left in the A_SEC vs. D_EFFE diagram, and since this flow has been going on for centuries, and since dwarf comets evolve much more rapidly than large comets, the existence of a significant population of dormant and extinct comets located in the lower left region of the diagram is predicted. This is the graveyard. Three comets have been identified as members of the graveyard, 107P, 133P and D/1891W1 Blanpain.