Astron. Astrophys. 343, L87-L90 (1999)

3. Discussion

The SW spectrum of 103P/Hartley 2 is characterised by typical vibrational bands of volatile species present in the coma. The band positions and intensity are listed in Table 1. Here following we concentrate on their attribution and interpretation.

Table 1. Bands observed in 103P/Hartley 2 with PHT-S.
Notes:
g: emission rate, assuming resonant fluorescence excited by the Sun at 1 AU
Q: production rate computed by assuming a molecule distribution with expansion velocity of 0.8 km s^-1
(1) The upper limit is calculated for 3

The band observed in our spectrum at 2.7 µm, with a shoulder around 2.8 µm, is mostly attributable to the transition of water molecules. The shoulder at 2.8 µm is also presumably due to hot bands of water (Bockelèe-Morvan and Crovisier 1989). The presence of the H₂O molecule in comet 1P/Halley was determined by detecting the IR band at 2.66 µm in observations from the Kuiper Airborne Observatory (Mumma et al., 1986) and by in situ infrared spectroscopy (Moroz et al., 1987) and gas mass spectroscopy, by the NMS instrument onboard Giotto (Krankowsky et al., 1986). By neglecting the emission excess in the shoulder present in our spectrum, that anyway represents only less than 10% of the entire band flux, we have derived the water production rate, Q(H₂O), from the integrated band intensity. We have considered an expansion velocity of 0.8 km s^-1 (resulting from an hydro-dynamic model, Combi and Smyth, 1988). The computed Q(H₂O) = (3.1 0.2) 10²⁸ molec. s^-1 value is higher than that, (1.24 0.2) 10²⁸ molec. s^-1, derived by Crovisier et al. (1999a) from SWS observations of P/Hartley 2 performed on 31 Dec. 1997. As the two observations are close in time, the difference could be explained with a sudden change of H₂O emission. The values reported above are smaller than Q(H₂O) 6.3 10²⁸ molec. s^-1, based on UV observations performed with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope, during the previous passage of the comet, in September 1991, when the comet was at heliocentric distance 1 AU (Weaver et al., 1994). This result was derived from the brightness analysis of several bands of the hydroxyl radical, which is believed to be the primary daughter product of the photo-dissociation of H₂O. Actually, the shoulder at 2.8 µm, present in our spectrum, could be due to the rather weak OH (1-0) resonance.

Our SW spectrum presents a well defined band at 4.26 µm, due to carbon dioxide. CO₂ cannot be directly observed in the IR from ground, due to telluric absorption. It was first identified in comet 1P/Halley by in situ gas mass spectrometry (Krankowsky et al., 1986) and infrared spectroscopy (Combes et al., 1988). Weaver et al. (1994) detected in P/Hartley 2 several bands of the Cameron system of CO, almost certainly due to photo-dissociative emission of CO₂. By assuming an expansion velocity of 0.8 km s^-1, we have derived, directly from our integrated band flux, the production rate Q(CO₂) = (2.5 0.3) 10²⁷ molec. s^-1. This value is quite consistent with Q(CO₂) = (2.6 10²⁷) molec. s^-1, inferred by Weaver et al. (1994) at almost the same heliocentric distance. The relative abundance CO₂/H₂O derived from our observations is 8%, well compatible, within errors, with the 10%, obtained by Crovisier et al. (1999a) from SWS (for H₂O) and CAM-CVF (for CO₂) spectra. These direct determinations are a bit higher than that inferred by Weaver et al. (1994), likely due to the decrease in the water production rate, and reveal that P/Hartley 2 has a relative CO₂ abundance higher than 1P/Halley and several other comets (Feldman et al., 1997).

The attribution of the band falling at 3.69 µm is rather puzzling and various interpretations could be proposed. A band of the deuterated water HDO, due to the OD stretch, falls at 3.68 µm and could match the observed band, within the spectral resolution limits of the ISOPHOT-S spectrum (in this spectral region, the ISOPHOT-S is 0.04 µm). HDO has been detected in comet C/1996 B2 (Hyakutake), through its rotational transition at 464.925 GHz (Bockelèe-Morvan et al., 1998), and in comet Hale-Bopp (Meier et al. 1998) with a similar abundance. Unfortunately, we cannot be sure of the attribution of the 3.69 µm band in our spectrum. Actually, a more intense band (OH stretch) would be expected at 2.7 µm, but it could not be resolved from the more intense water band at 2.66 µm and a band, expected at 7.13 µm, is not evident in our LW spectrum. Moreover, if we derive the production rate of HDO by assuming that the 3.69 µm is entirely due to this molecule, the resulting D/H(H₂O) ratio is too high with respect to the measured values for comets 1P/Halley (Eberhardt et al., 1995) and Hyakutake (Bockelèe-Morvan et al., 1998). Finally, contrarily to our observed spectrum, the band width should spread over more pixels, even for a cold (5-10 K) rotational temperature (Crovisier, private communication ). Alternatively, the 3.69 µm band could be attributed to the methyladyne radical (CH), whose (1-0) band falls at 3.66 µm and is, however, expected to be broad.

Carbon monoxide is probably a major volatile cometary component, but its abundance is highly variable from comet to comet. The (1-0) band at 4.67 µm of CO is not observed in our SW spectrum. Our derived upper limit for Q(CO) (see Table 1) is in agreement with the 9 10²⁶ molec. s^-1 measured by Weaver et al. (1994).

The spectral region from 3.1 to 3.6 µm displays some weak and wide features, not very much evident above the noise level, that could be interpreted with the presence of C-H bearing molecules, already observed in several comets (e.g. Moroz et al., 1987; Brooke et al., 1990; Brooke et al., 1991). Recently, methanol and/or ethanol have been proposed among the potential contributors to these bands (Disanti et al., 1995; Mumma et al., 1996). No attempt of specific attribution is possible for our very faint structures.

Finally, the feature at 4.87 µm, which is as bright as the 3.69 µm feature, corresponds to a notoriously bad pixel of the instrument (it has been already seen in other ISOPHOT-S observation), so we consider it as an instrumental effect.

The LW spectrum of P/Hartley 2 presents a featureless profile that can be fitted by a black-body at colour temperature T_c = 285 K (Fig. 2). This result is similar to the T_c = 295 K value, determined by Crovisier et al. (1999a) from CAM spectra. The slight difference could be due to the different spectral ranges used to derive T_c and/or to the fact that the real spectrum departs from a pure black-body spectrum. For comparison, T_c = 309 K is derived by scaling to the P/Hartley 2 heliocentric distance the power law obtained from pre- and post-perihelion observations of 1P/Halley (Tokunaga et al. 1988). The equilibrium blackbody temperature would be 272 K. We conclude that grains in P/Hartley 2 are hot, as expected for very small particles, but not as much as typical of P/Halley-like dust. After subtraction of the blackbody profile, our residual spectrum does not show any 10 µm silicate emission band, in contrast with the finding by Crovisier et al. (1999a), who detected a broad 10 µm band, with evidence of a sharp feature at 11.3 µm, attributable to amorphous and crystalline silicate grains respectively. We notice that the silicate emission in their spectrum is about 20% of the continuum in intensity and, thus, it is too faint to be detected above 3 in our spectrum, but should be (faintly) observable above 1. We note however that the 10 µm silicate emission in the same comet was not seen by Fomenkova et al. (1998) on 24 January, while Lynch et al. (1998) reported it on 9 February.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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