Astron. Astrophys. 353, 1101-1114 (2000)

4. Discussion of cometary molecular abundances

4.1. Sulphur species

The SO and SO₂ abundances measured in comet Hale-Bopp are 0.3 and 0.2 % relative to water, respectively (Table 4). These values are more than one order of magnitude larger than the upper limits derived from UV observations in other comets (Kim & A'Hearn 1991). In comet P/Halley, the upper limits obtained for the SO and SO₂ mixing ratios relative to water were of 4 10^-4 and 2 10^-5, respectively; in comet C/1983 H1 (IRAS-Araki-Alcock), still lower upper limits of 1.5 10^-4 and 8 10^-7 were obtained. As discussed by Kim et al. (1999), the SO abundances based on the UV upper limits were underestimated due to an erroneous radiative lifetime of the A electronic state. The SO and SO₂ abundances measured in comet Hale-Bopp can be also compared to the stringent upper limits obtained in comets C/1990 K1 (Levy) and Hyakutake (Crovisier et al. 1993; Lis et al. 1997). For comet Levy, there are no conflicting results within a factor of 2. We have computed the 3 upper limit on the SO production rate in comet Hyakutake on March 30, 1996 from the observation of the 6₅ - 5₄ SO line at 251.826 GHz made by Lis et al. (1997). The inferred value is 2.2 10²⁶ mol. s^-1 when SO is assumed to be produced by SO₂, using the photodissociation rates given in Table 2. The upper limit given in Lis et al. (1997) assumes a parent molecule distribution and the SO photodissociation rate of Summers & Strobel (1996). This translates into Q(SO)/Q(HCN) 80%, and Q(SO)/Q(H₂O) 0.1%, which might suggest that the SO abundance relative to water varies from comet to comet.

Our measurements of SO and SO₂ production rates give [SO]/[SO₂] 1.6 in March, averaging the results of the two SO₂ lines, and 1.4 on April 5-8. Taken at their face values, these ratios might suggest that SO₂ is not the sole parent of SO. We have investigated to which extent the derived production rates are sensitive to the assumed photorates. Using a photodissociation rate four times higher for SO (i.e., (SO) = 6.2 10^-4 s^-1 from Huebner et al. (1992)) and the SO₂ photodissociation rate given in Table 2, we infer SO production rates and abundances in average 3 times higher, which exacerbates the difference between SO and SO₂ abundances. Similarly, we have investigated the sensitivity of the SO and SO₂ production rate to the assumed SO₂ lifetime: using the photodissociation rate (SO₂) = 1.4 10^-4 s^-1 proposed by Kumar (1982), the inferred SO₂ production rates are 40% lower and the SO production rates are in average 15% times higher. Therefore, no agreement can be found between the SO and SO₂ abundances, when considering the various published photorates. Table 3 illustrates the influence of SO and SO₂ assumed photorates on the derived production rates. A better modelling of the excitation and density distributions of SO and SO₂ are required to go further in this discussion of the origin of SO. As discussed in Sect. 3, the assumption that we made on the rotational population distribution of SO₂ likely underestimate the SO₂ production rates.

Table 3. Influence of the SO and SO₂ photodissociation rates on the production rates of  SO (219.9 GHz line on March 13) and  SO₂ (221.9 GHz line on March 18, 20-21). The rotational temperatures of SO and SO₂ are taken equal to 120 K

Table 4. Molecular abundances in comet Hale-Bopp at 1 AU from the Sun. Abundances with respect to H₂O are computed assuming [HCN]/[H₂O] = 0.25% (see text).
Notes:
a) Adapted from Crovisier et al. (1997) (see text).
b) Weaver et al. (1999).
c) Bird et al. (1999).
d) Woodney et al. (1999); L. Woodney, personal communication .

The OCS abundance in comet Hale-Bopp inferred from our radio observations ( 0.4 % relative to water) is in very good agreement with IR determinations (Dello Russo et al. 1998). OCS was marginally identified in comet Hyakutake on March 19, 1996 at the NRAO 12-m telescope by Woodney et al. (1997), who evaluated a production rate of 1.4 10²⁶ s^-1 and an abundance relative to water of 0.1%. Using our excitation model and the parameters used in Biver et al. (1999b) for interpreting other radio molecular data of comet Hyakutake, we reevaluate the OCS production rate to 3.5 10²⁶ s^-1, which translates into a abundance relative to water on the order of 0.2 % using the water production rate of 2 10²⁹ s^-1 derived by Gérard et al. (1998). This abundance is twice lower than the value that we infer in comet Hale-Bopp. As there are remaining uncertainties in the water production rate of comet Hyakutake in March 1996 (lower values were published for March 19 by, e.g., Millis & Schleicher 1996; see also Woodney et al. 1997), this difference might not be significant. Using the HCN production rate measured on March 20.5 by Lis et al. (1997), the inferred [OCS]/[HCN] ratio is 1, which is not significantly different from the value found in comet Hale-Bopp (Table 4).

Table 4 shows that sulphur in the gas phase of comet Hale-Bopp, and presumably in its nuclear ices, is mostly in the form of the hydrogenated species H₂S. The oxygenated species SO₂ and OCS, although less abundant, are however significant contributors to the sulphur inventory. Taking into account the uncertainties in the abundance determinations of SO₂ discussed previously, it is safe to conclude that the [SO₂]/[H₂S] ratio is at least 0.1. CS₂, believed to be the parent of the CS radical (Jackson et al. 1982), is also a significant component. On the other hand, the H₂CS molecule, another new sulphur species detected in comet Hale-Bopp, has a very small abundance (0.02%) relative to water (Woodney et al. 1999). The total elemental sulphur abundance with respect to oxygen in the gas phase of comet Hale-Bopp is [S]/[O] = 0.02, which is the Solar System ratio. Previous estimations based on the study of atomic sulphur in cometary comae suggested a depletion by a factor of 2 or more (Meier & A'Hearn 1997). A large fraction of sulphur is also stored in grains, with [S]/[Mg] marginally above the solar ratio (Schulze et al. 1997).

4.2. N-bearing and CHO-bearing species

The abundances of the newly detected N-bearing and CHO-bearing species in comet Hale-Bopp are in agreement with the upper limits obtained in other comets (Crovisier et al. 1993; Lis et al. 1997). The HNCO abundance measured in comet Hyakutake (Lis et al. 1997) is only marginally lower than that inferred in comet Hale-Bopp.

HC₃N and NH₂CHO, as well as HNCO, HNC and CH₃CN, have minor abundances with respect to the main species NH₃ (Table 4) and thus do not modify previous conclusions (Wyckoff et al. 1991) concerning the apparent strong depletion of nitrogen in comets, both in the gas ([N]/[O] depletion 15 in comet Hale-Bopp) and refractory ([N]/[Mg] depletion of 3 in P/Halley; Schulze et al. 1997) phases. Newly detected CHO-species have very small abundances (in the range 0.01-0.1% relative to water) and do not contribute significantly either to the carbon or to the oxygen contents in comets, which seem essentially locked into H₂O, CO, CO₂, CH₃OH and CHON grains with Solar System elemental abundances (Schulze et al. 1997).

4.3. The volatile composition of comet Hale-Bopp

Table 4 summarizes molecular abundances in comet Hale-Bopp inferred at 1 AU from the Sun, near perihelion. For completeness, abundances of CO₂, CH₄, C₂H₂ and C₂H₆, derived from IR observations (Crovisier et al. 1997; Weaver et al. 1999) and of NH₃ and H₂CS, observed at radio wavelengths (Bird et al. 1999, 1997; Woodney et al. 1999; L. Woodney, personal communication ), are also given. Other abundances are from this study and from Paper I. Note that CO₂ has been observed using the Infrared Space Observatory (ISO) only at 2.8 AU, due to solar elongation constraints (Crovisier et al. 1997). The abundance relative to water given in Table 4 assumes that the [CO₂]/[CO] ratio did not change with heliocentric distance.

Sect. 5 presents a comparison between the volatile composition of comets and that inferred in several regions of the interstellar medium. We assume that the molecular abundances measured in the coma of comet Hale-Bopp are representative of abundances in cometary ices. This is only partly true for CO and OCS due to the additional contribution ( 50%) of a distributed source (DiSanti et al. 1999; Dello Russo et al. 1998). The abundance of H₂CO in cometary ices is also a debated question. In situ measurements in comet P/Halley showed that the density distribution of the H₂CO molecules was not compatible with a parent molecule distribution, suggesting the presence of an extended source (Meier et al. 1993). This was confirmed by radio observations in, e.g., comets Hyakutake (Biver et al. 1999b; Lis et al. 1997) and Hale-Bopp (Bockelée-Morvan et al. 1998a; Wink et al. 1999). Therefore, the H₂CO abundance given in Table 4, which was computed assuming production from an extended source (Paper I), does not reflect the abundance inside the nucleus, which could be much smaller.

Since gas densities were high in the inner coma of comet Hale-Bopp near perihelion ( 10⁹ cm^-3 at 1000 km from the nucleus), one can wonder whether the minor species found in this comet could not be the product of chemical reactions in the coma. This question is debated for HNC. The / production rate ratio in comet Hale-Bopp increased as the comet approached the Sun (Biver et al. 1997; Paper I; Irvine et al. 1998). This has been interpreted by the production of a major part of HNC by ion-molecule reactions in the coma or by isomerisation of HCN from impacts with high energy H atoms (Irvine et al. 1998; Rodgers and Charnley 1998). But present coma chemistry models are unable to reproduce the ratio / of 6% measured in comet Hyakutake (Irvine et al. 1996; Rodgers and Charnley 1998), a fortiori the value of 12% inferred in the moderately active comet C/1999 H1 (Lee) (Biver et al. 1999c). Therefore, the presence of a significant amount of HNC in cometary nuclei cannot be excluded. The importance of chemical reactions in the coma for producing the other minor species discussed in this paper can quantitatively only be addressed with a detailed model. The main routes for producing neutral molecules in cometary atmospheres are via ion-molecule reactions followed by electron dissociative recombination or possibly ion-neutral charge-exchange reaction (Huebner et al. 1991). However, only a small amount of new molecules are expected to form in this way. The first reason is that the ionic content of the inner coma is very low (fractional ionization less than 10^-4 within 10 000 km from the nucleus). Photoionization, which is the dominant ionization process here, is indeed a relatively slow process (photoionization rates 3 10^-7 s^-1). Second, the formation of new neutral molecules requires at least two subsequent reactions, and the step involving ion-neutral and recombination reactions is only efficient in the innermost collisional region where ions and neutral densities are the highest and the electrons are thermalysed at the temperature of the neutrals. Finally, as electron recombination is often a dissociative reaction, the formation of the observed neutral species requires first the formation of still more complex ions.

In relating coma to nuclear abundances, further complications might arise from chemical fractionation effects occurring during sublimation. This should affect predominantly the abundances of the most volatile species (Espinasse et al. 1991).

Finally there is this issue: how typical is comet Hale-Bopp? The abundance ratios of the trace species C₂ and CN classify this comet as "typical" in the compositional taxonomy of A'Hearn et al. (1995) (Schleicher et al. 1998). This comet is among those with the highest observed content of the highly volatile species CO and H₂S (Biver 1997), which might indicate only moderate fractionation effects. Its CH₃OH abundance is in the low range of measured values in comets (1-8 %) (e.g., Bockelée-Morvan et al. 1995). When comparison is possible, the abundances of the other parent molecules are similar to those found in other long-period comets (Crovisier 1993), so that, to first approximation, the composition of comet Hale-Bopp can be taken as representative of the volatile composition of such comets. Such an approximation is all the more justified for the comparison between comets and interstellar sources presented in the next section, as molecular abundances differing by several orders of magnitude are considered.

© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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