3. Results and discussion
The first gaseous species was firmly detected at r = 2.35 AU on Sept. 10, 1996. Fig. 1 shows the co-added spectra of the observing runs where gaseous emissions were detected. The spectrum was still featureless in July and attempts to detect any gaseous emission with the 1.5 Danish telescope in August 1996 also led to negative results (N. Thomas, priv. com.). All spectra obtained during the observing run in September clearly exhibit the CN (0-0) band at 387.5 nm. The C3 band at 405.0 nm is also visible in the co-added spectra. Due to the low spectral resolution in September (Table 1) the solar Ca doublet around 395 nm is not resolved. It shows as a broad dip in the spectrum. As a consequence about 50% of the CN and most of the C3 band depicted in the September spectrum of Fig. 1 are actually due to the underlying continuum. Thus, C3 emission is only marginally present above the detection limit. C2 and NH2 were first detected in mid-October. Four NH2 bands (7-0, 8-0, 9-0, 10-0) were clearly detected in high-resolution spectra in the red out to 7300 Å (not shown here). The presence of the 6-0 band is probable. As in comet Hale-Bopp at large r (Rauer et al., 1997), we observe, though to a lesser degree, an "odd-even anomaly", the even bands of NH2 being appreciably stronger than the odd bands. Because of the limited space we cannot discuss these observations here and confine ourselves to quoting the NH2 production rate determined from them. Fig. 2 demonstrates the evolution of the production rates as a function of heliocentric distance for all gaseous species and the dust (in Af ).
Between 2.3 AU and 1.8 AU the gas production rates are slowly increasing with decreasing solar distance. A steeper increase in the production rate curves of all gaseous species and the dust is obvious between 1.8 AU and 1.6 AU. The inclusion of photometric data obtained around 1.1 AU (Farnham and Schleicher, 1997 and priv. com.) shows that the production rate of C2 continues to increase at a higher level than the CN production. Consequently, the C2/CN ratio rises from 0.25 around 2 AU to 1.1 around 1.1 AU.
The steep increase in production rates between 1.8 AU and 1.6 AU and the increase in the C2/CN ratio was also confirmed with the Haser model. Note that the simple connection of the available data points in Fig. 1 results in a smooth increase in production rates between 1.6 AU and 1.1. AU. This does not necessarily reflect the real behaviour of the comet. However, due to the lack of observations it is impossible to say how the production rates in this region increase with decreasing r. A strong increase of the C2 production rate as compared to CN between 2.0 AU and 1.5 AU was already reported for comet West (A'Hearn et al., 1977). (Our re-calculation with the model parameters used in this paper confirm this result.) A subsequent study of 14 comets indicated an apparent depletion of C2 relative to CN for both, periodic and dynamically new comets (A'Hearn & Millis, 1980) at distances larger than 2 AU. Unfortunately, the data were not sensitive to the location of a possible division within the interval from 1.5 AU to 2.0 AU since only one comet, 81P/Wild 2, was observed in that region (around 1.80 AU and 1.50 AU). Newburn and Spinrad (1989) conclude from a later study that the C2/CN production rate ratio changes continuously with r in the 5 comets for which they have measurements at different distances.
Unfortunately, they have no data beyond 2 AU, rather large gaps in the coverage of the relevant region or they mixed pre- and postperihelion data in order to increase the number of data points. A relatively sharp change of gas and/or dust production and of the C2/CN ratio is therefore very unlikely to be detected in these data. It is equally unlikely to find it in the photometric study of 85 comets by A'Hearn et al. (1995), because here the step size of r was binned such that the region relevant to this effect lies within one binning interval (1.58 AU - 2.00 AU) with the result that a possibly present abrupt change is averaged out. Nevertheless, A'Hearn et al. (1995) still see the change in the C2/CN ratio described by Newburn and Spinrad (1989), but the size of the effect is much smaller.
A comparison of the activity curves depicted in Fig. 2 with the lightcurve of comet 46P/Wirtanen obtained in broad-band R reveals that also the R brightness of the comet much steepens between 1.8 AU and 1.6 AU (Boehnhardt et al., 1997; Meech et al., 1997). Furthermore, comet 46P/Wirtanen was previously reported to have displayed a similar rapid rise of brightness during its past two apparitions (Morris, 1994). These events were, however, manifested by only one data point before the increase and the subsequent measurements continued only at around 1.2 AU. The concomitant increase in the production rates of gaseous trace species, the dust production, the R brightness and in the C2/CN ratio between 1.8 AU and 1.6 AU indicates a distinct change of the outgassing conditions during this part of the orbit. As the dust is dragged out of the nucleus by sublimating volatiles, the observed increase of dust production by a significant factor probably requires a parallel increase in production of a more abundant gaseous species and may be related to water becoming the main driver of activity in 46P/Wirtanen. Unfortunately, no data exists on the water production rate for this part of the orbit. The only measurement close to it was obtained at 2.47 AU (Aug. 25.9, 1996) revealing a 5.5 detection of OH corresponding to a water production rate of (2.6 0.5) s-1 (Stern et al., 1997). This value is consistent with the extrapolated CN production rate assuming the typical CN/OH abundance ratio of 0.003.
The fact that the C2/CN production rate ratio in comet 46P/Wirtanen strongly depends on its heliocentric distance has implications to the practical application of the new taxonomy of comets introduced by A'Hearn et al. (1995). Here two taxonomic groups were introduced that distinguish comets with typical abundance ratios from comets that are carbon-chain depleted . The criterion for this taxonomy is the C2/CN production rate ratio with a comet being depleted if C2/CN 0.66. When applying this taxonomy to comet 46P/Wirtanen it would fall into the category depleted beyond 1.6 AU and be designated typical at smaller r . Thus, one has to be careful when designating a comet depleted if it was observed exclusively at larger distances from the sun. It is well known that the production rates of CN and C2 vary with r according to different laws in many comets. A'Hearn et al. (1995) assume power laws to represent the approximate r -dependences, i.e. Q(CN) and Q(C2) vary as and with ( often negative) as obtainable from their Table V. In such cases the production rate curves (straight lines in double logarithmic representation) would cross at a certain r and the comet will necessarily become depleted beyond this distance. For positive reverse changes may occur. Reviewing the data by A'Hearn et al. (1995) with respect to the distribution of observations along the orbit it turns out that of the 29 comets designated depleted , 22 were observed exclusively beyond 1.7 AU. For one of these comets, 81P/Wild 2, the C2/CN ratio is derived near 2.33 AU and the approximate r-dependence of the production rates is given in their Table V resulting in -3. Consequently, comet 81P/Wild 2 should be expected to show typical abundance ratios below 1.7 AU if the above argumentation is valid. This was indeed the case for its 1978 passage, when it was measured near 1.8 AU and 1.5 AU (A'Hearn & Millis, 1980). Comet 67P/Churyumov-Gerasimenko on the other hand is depleted at 1.4 AU with a positive and should therefore change to typical beyond 2.3 AU.
We conclude that the C2/CN ratio is strongly dependent on the heliocentric distance for some comets and that the criterion for carbon-chain depletion must take this into account. As long as this is not accurately done the existence of an entire population of carbon-chain depleted comets that might come from the Kuiper belt as proposed by A'Hearn et al. (1995) is in question. New observations are therefore needed to re-examine the outgassing behaviour of comets along their orbits. Our new knowledge of 46P/Wirtanen not only helps to optimize the ROSETTA Mission in particular in view to science operations, but emphasizes the need for this kind of long-term rendez-vous missions with a comet.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998