© ESO, 2011

1. Introduction

The composition of cometary nuclei is of strong interest in understanding their origin. Having spent most of their time in a very cold environment, these objects should not have evolved much since their formation. Thus, their composition provides clues to the composition in the outer regions of the Solar Nebula where they formed. The last two decades have proven the efficiency of microwave spectroscopy in investigating the chemical composition of cometary atmospheres. About 20 different cometary molecules have now been identified at radio wavelengths (Bockelée-Morvan et al. 2004a).

In this paper, we extend our investigations of the composition of cometary atmospheres from radio observations (Bockelée-Morvan et al. 2004a; Biver et al. 2002a, 2006a, 2007b) to three comets observed in 2003 and 2007. These observations were designed to measure the molecular abundances of comets approaching close to the Sun to investigate how the strength of the solar heating of both the comet nucleus and of its environment affects the coma composition. Previous observations have suggested that the relative production rates of several molecules vary with heliocentric distance (Biver et al. 2006a). The passages of comets C/2002 X5 (Kudo-Fujikawa), C/2002 V1 (NEAT), and C/2006 P1 (McNaught) provided us with the opportunity to measure molecular abundances at heliocentric distances (r_h) between 0.1 and 0.25 AU, about one order of magnitude smaller than usual. This study is complementary to the long-term monitoring of comet C/1995 O1 (Hale-Bopp) (Biver et al. 2002b), which provided information on the outgassing of a comet between 0.9 and 14 AU.

Opportunities to plan observations of comets passing within 0.2 AU from the Sun are rare. For example, comet C/1998 J1 (SOHO) was discovered too late to establish an accurate ephemeris around perihelion time. In addition, Sun-grazing comets often do not survive and even disintegrate before reaching perihelion, making observing plans extremely difficult. The last opportunity was comet C/1975 V1 (West). The observations reported here are unique, and required to develop specific observing strategies and analyses.

The organization of the paper is as follows. In Sect. 2, we present the observations of comets C/2002 X5 (Kudo-Fujikawa), C/2002 V1 (NEAT), and C/2006 P1 (McNaught) performed with the 30-m telescope of the Institut de Radioastronomie Millimétrique (IRAM) and the Nançay radio telescope. In Sects. 3 and 4, the analysis of these observations is presented. A summary follows in Sect. 5.

2. Observations

Owing to the late discovery or assessment of their activity, the three comets were observed as targets of opportunity using the IRAM 30-m and Nançay radio telescopes. This was possible because these telescopes do not have tight solar elongation constraints. The small solar elongation (< 10° at r_h  <  0.2 AU) resulted in limited observing support from other observatories.

2.1. C/2002 X5 (Kudo-Fujikawa)

Comet C/2002 X5 (Kudo-Fujikawa) was discovered visually at m_v = 9 on 13, 14 December 2002 by two Japanese amateur astronomers, T. Kudo and S. Fujikawa (Nakano 2002). At about 1.2 AU from the Earth and the Sun at that time, it was then a moderately active comet. It passed perihelion on 29 January 2003 at a perihelion distance q = 0.190 AU from the Sun.

One of the main goals of the observations of comet C/2002 X5 at the IRAM 30-m was to measure the evolution of the HNC/HCN production rate ratio as it approached the Sun. A first observing slot was scheduled on 4–5 January 2003, at r_h = 0.8 AU, but the weather prevented any observation. Observations performed on 13 January (r_h = 0.55 AU) only partly succeeded because of technical problems. Most data were acquired on 26.5 January, 2 days before perihelion, at r_h = 0.21 AU. At that time, the comet was only observable visually by the C3 coronagraph aboard the SOlar Heliospheric Observatory (SOHO) – the solar elongation was 5.5° (Bout et al. 2003; Povich et al. 2003). The pointing of the comet was a real challenge, since the ephemeris uncertainty was expected to be on the order of the beam size (10–20″). Since at r_h = 0.2 AU molecular lifetimes are typically shorter than an hour, hence photodissociation scale lengths are smaller than the beam size, accurate pointing was required. From coarse mapping, we found the comet about 10″ south of its predicted position (Fig. 1, Table 1). The last observing run at IRAM took place on 12 March 2003 (r_h = 1.2 AU, Fig. 3), when the comet was receding from the Sun and about 100 times less productive. A log of the observations and measured line areas are given in Table 2. Sample spectra are shown in Figs. 2–4.

To monitor the water production rate, observations of OH at 18-cm using the Nançay radio telescope were scheduled on a daily basis from 1 January to 10 April 2003. The OH lines were only detected when the comet was between 1 and 0.4 AU from the Sun, inbound and outbound.

2.2. C/2002 V1 (NEAT)

Comet C/2002 V1 (NEAT) was discovered on 6 November 2002 by the Near Earth Asteroid Tracking (NEAT) program telescope on the Haleakala summit of Maui, Hawaii (Pravdo 2002). At that time, it was a relatively faint object (m_v = 17). Comet C/2002 V1 (NEAT) brightened rapidly after its discovery, becoming a potentially interesting target for observations at perihelion on 18 February 2003 at q = 0.099 AU. The orbital period was estimated to be 9000 years (Nakano note NK965¹), suggesting that it has survived a close passage to the Sun at its previous perihelion and might be expected to do so again.

The observations at IRAM were undertaken on 16 and 17 February 2003 (r_h = 0.13–0.11 AU), i.e., just one day before perihelion. As for comet C/2002 X5 (Kudo-Fujikawa), the observations were challenging because of the lack of supporting optical astrometry, the comet being 8–6${\mathrm{◦}}^{}$ away from the Sun and only seen by the SOHO coronagraph when it became as bright as magnitudes −1 to −2. A coarse map of the HCN J = 3−2 line at IRAM revealed the comet to be at about 5.5″ (half a beam, Fig. 5, Table 1) from its expected position. Observational data are given in Table 3.

OH 18-cm observations were obtained nearly every day from 31 December 2002 to 25 March 2003. The comet was only detected at r_h > 0.4 AU from the Sun, though the water production rate (Q_H2O) likely exceeded 2 × 10³⁰ molec. s^-1 at perihelion (Sect. 3).

2.3. C/2006 P1 (McNaught)

C/2006 P1 was discovered by Robert McNaught at m_v = 17.3 on 7 August 2006 (McNaught 2006) as it was at 3.1 AU from the Sun. The geometry was very unfavorable for observing this comet as it approached the Sun. The comet was basically lost in the glare of the Sun in November and December 2006 from r_h = 1.5 to 0.5 AU. At r_h > 0.5 AU, its intrinsic brightness was similar to that of comet C/1996 B2 (Hyakutake). It brightened rapidly in early January 2007 to peak at m_v  ~  –5 and became visible to the naked eye in broad daylight (Green 2007). It passed perihelion on 12 January 2007 at q = 0.17 AU. Comet C/2006 P1 (McNaught) was the brightest and most productive comet since C/1965 S1 (Ikeya-Seki). The absence of an ion tail led Fulle et al. (2007) to argue that, because of a very high outgassing rate, the diamagnetic cavity was so large that ions were photodissociated before reaching the region where they could interact with the solar wind. A week after perihelion, C/2006 P1 displayed a fantastic dust tail with many striae curving around 1/3 of sky at a mean distance of 20° from the Sun.

The IRAM observations were performed on 15, 16, and 17 January 2007 (r_h = 0.21–0.25 AU). The comet ephemeris was expected to be possibly wrong by up to 1′ (corresponding to the 3-σ uncertainty from the JPL’s HORIZONS ephemeris, Giorgini et al. 1996). An on-the-fly map of the HCN J(3–2) line on 15.6 January UT with the IRAM 30-m HERA array of receivers showed that the comet was 30″ north of its predicted position (Fig. 7). Orbit updates later confirmed the observed offset (Table 1). Most observations consisted of five-point integrations at 0 and 6″  offsets in RA and Dec to pinpoint the maximum emission (e.g., Fig. 9). On 17.5 January, the observations were affected by strong anomalous refraction due to the low elevation of the comet (21–17°). The effect was estimated to be equivalent to a mean pointing offset of up to 12″, implying a loss of 90% of the signal. Observational data are summarized in Table 4.

OH 18-cm observations were performed daily between 10 and 20 January. The comet was detected intermittently. OH 18-cm lines had never been detected that close to the Sun (0.17 AU) in a comet before.

3. Analysis of Nançay data – H₂O production rates

The characteristics of the Nançay radio telescope and the OH observations of comets may be found in Crovisier et al. (2002). The OH 18-cm lines usually observed in comets are maser lines pumped by UV solar radiation (Despois et al. 1981). However, for high water-production rates, the maser emission is quenched by collisions in a large part of the coma, and most of the signal may originate from thermal emission. The radius of the region where the maser emission is quenched is estimated to be $\mathrm{50}\hspace{0.17em}\mathrm{000}{\mathit{r}}_{\mathrm{h}}\sqrt{{\mathit{Q}}_{\mathrm{29}}}$ km, where Q₂₉ is the water production rate in units of 10²⁹ molec. s^-1 (Gérard et al. 1998). For production rates above 6 × 10²⁹ molec. s^-1 and the heliocentric distances where the comets were observed (r_h < 0.3 AU), most OH radicals dissociate within the quenching zone. Hence, the signal is dominated by OH thermal emission. The OH production rates or upper limits are given in Table 5 and sample spectra are provided in Fig. 11.

Table 5 also includes water-production rate measurements from other instruments for comet C/2002 X5. The H₂O line at 557 GHz was observed in this comet with Odin during March 2003 (Biver et al. 2007a). Observations were conducted with the SOHO coronagraph spectrometer on the perihelion date: from H Lyman α measurements, the peak outgassing rate of water is estimated to  ~3 × 10³⁰ molec. s^-1 (Combi et al. 2008). According to Povich et al. (2003) and Bout et al. (2003), this comet displayed a strongly variable activity around perihelion with a  ~ two day period. During the perihelion period, the comet was not or only very marginally detected at Nançay. Averaging the data obtained during the five days around perihelion, we obtain a possible 4-σ detection (subject to baseline uncertainties) suggesting a production rate of  ~6 × 10³⁰ molec. s^-1 (Table 5). This is higher than the SOHO estimate, but still within the same order of magnitude. Power laws fitted to Nançay and Odin data (Fig. 18) yield values in the same range 1.5−5.4 × 10³⁰ molec. s^-1 at 0.214 AU from the Sun. A mean value of 3.5 × 10³⁰ molec. s^-1 is adopted.

For C/2002 V1 (NEAT), the pre-perihelion observations can be fitted by a power law ($\mathrm{0.33}\hspace{0.17em}\mathrm{\pm }\hspace{0.17em}\mathrm{0.03}\mathrm{\times }{\mathrm{10}}^{\mathrm{29}}\mathrm{\times }{\mathit{r}}_{\mathrm{h}}^{\mathrm{-}\mathrm{1.95}\mathrm{\pm }\mathrm{0.23}}$), which extrapolates to Q_H2O = 1.6 × 10³⁰ and 2.5 × 10³⁰ molec. s^-1 for 16 and 17 January 2003, respectively, consistent with the upper limit determined for these days. Extrapolation of SOHO-SWAN measurements yield values that are four times higher but indicative of abundances relative to water that are abnormaly low for all molecules. We assume that Q_H2O = 2.0 and 2.5 × 10³⁰ molec. s^-1, respectively, these values being more compatible with the day-to-day variations observed in millimeter spectra of other molecules.

As for C/2006 P1 (McNaught), the water production rates for 13 and 19 January deduced from the Nançay data agree with the estimated HDO production rate from the marginally detected line at 225.9 GHz (Fig. 8) and the HDO/H₂O = 6 × 10^-4 ratio measured in comets. The total production rate varied from  ≈ 40 × 10³⁰ down to  ≈ 5 × 10³⁰ molec. s^-1 at that time (Sect. 3).

The estimated visual magnitudes of these comets at perihelion were m₁ ≈  + 3, m₁ ≈ −1.5, and m₁ ≈  −5 for C/2002 X5, C/2002 V1, and C/2006 P1, respectively. Correcting for the  −2 mag surge in brightness of C/2006 P1 caused by forward scattering (Marcus 2007), and using the correlation law between heliocentric magnitude and Q_H2O of Jorda et al. (2008), this would imply water production rates of 1, 11, and 26 × 10³⁰ molec. s^-1  respectively. The comparison to measured production rates in Table 5 suggests that C/2002 V1 was a more dusty comet than the two others since, unlike C/2002 X5 and C/2006 P1, the actual outgassing rate being much lower than the value (11 × 10³⁰ molec. s^-1 ) inferred from visual magnitudes.

4. Analysis of IRAM data

Molecular production rates were derived using models of molecular excitation and radiation transfer (Biver et al. 1999, 2000, 2006a). The excitation model of CH₃OH was updated. The computation of the partition function considers now the first torsional state, which is populated significantly in the hot atmospheres of the comets studied in this paper. For these productive comets observed at small r_h, a significant fraction of the observed molecules are photodissociated before leaving the collision-dominated region. Thus, the determination of the gas kinetic temperature (which controls the rotational level populations in the collision zone) and strong constraints on molecular lifetimes were essential. As shown below, line shapes and brightness distributions obtained from coarse maps provide information on the gas outflow velocity and molecular lifetimes.

4.1. Gas temperature

Table 6 summarizes rotational temperatures deduced from relative line intensities. Using our excitation models, we then constrained the gas temperature (Table 6) following the methods outlined in, e.g., Biver et al. (1999). Several data are indicative of relatively high temperatures, which are difficult to measure because the rotational population is spread over many levels making individual lines weaker. Hence, the uncertainties in derived values are high. We note that the values derived for C/2002 X5 from CH₃OH lines do not reflect the marginal detection of some lines. The inconsistency between the different measurements, especially from CH₃OH and HCN lines in comet C/2006 P1, is possibly related to differences in collision cross-sections and temperature variations in the coma.

Gas temperature laws as a function r_h were obtained for comets C/1995 O1 (Hale-Bopp) (Biver et al. 2002b), C/1996 B2 (Hyakutake) (Biver et al. 1999), and 153P/Ikeya-Zhang (Biver et al. 2006a). The laws ($\mathrm{100}{\mathit{r}}_{\mathrm{h}}^{-1.1}$ for Hale-Bopp and $\mathrm{60}{\mathit{r}}_{\mathrm{h}}^{-0.9}$ for other comets on average), extrapolate to T = 200–1000 K for r_h = 0.25–0.11 AU. Photolytic heating increases with decreasing r_h and the increasing production rate of water. Hence, we expect higher temperatures for the most productive comet C/2006 P1. Combi & Smyth (1988) predicted a maximum temperature on the order of 500–900 K for comet Kohoutek at 0.25–0.14 AU, but temperatures are generally below 300 K at distances from the nucleus where molecules are not photodissociated. Given also that the HCN J(3–2) v₂ = 1 line at 265852.709 MHz is not detected, we estimate that temperatures are lower than 300 K below r_h = 0.25 AU.

For C/2002 X5, we adopted the gas kinetic temperature values of 50 K, 180 K, and 40 K for mid-January, end of January, and mid-March all in 2003, respectively. For C/2002 V1 and C/2006 P1, we used 150 K and 300 K, respectively. We later discuss (in Sect. 5.1) the influence of the assumed temperature on the inferred production rates.

4.2. Screening of photolysis by water molecules

Given high water-production rates (>10³⁰–10³¹ molec. s^-1 ), self-shielding against photodissociation by solar UV radiation is significant for these comets. As a consequence, molecules can have longer lifetimes on the night side, and reduced H₂O photolysis may limit gas acceleration. We studied the screening of photolysis using simplified assumptions: isotropic outgassing at a constant expansion velocity and an infinite lifetime for the screening molecules (i.e., we assumed that their scale length is larger than the size of the optically thick region). According to Nee & Lee (1985), when considering Lee & Suto (1986) and Lee (1984), the OH photoabsorption cross-section does not differ much from the H₂O one: it peaks close to the Lyman α wavelength and is slightly stronger (1–2 times). On the other hand, the cometary hydrogen may not absorb significantly the Solar Lyman α spectral line. The comet Lyman α line is too narrow (velocity dispersion of 20 km s^-1) to absorb significantly the solar emission (≈ 180 km s^-1  line width). Consequently, if the sum of the scalelengths of H₂O and OH is larger than the distances from the nucleus considered hereafter, the infinite lifetime assumption should not underestimate the screening effect.

In cylindrical coordinates, with the vertical z-axis pointing towards the Sun, a point in the coma has coordinates (θ, ρ = rsin(φ), z_r = rcos(φ)), where r is the distance to the nucleus and φ the co-latitude angle. The photodissociation rate of a molecule M, characterized by its photodissociation absorption cross-section σ_M(λ), in a radiation field F(λ) (in photons m^-2 s^-1 nm^-1) is given by ${\mathit{\beta }}_{\mathrm{M}}\mathrm{=}{\mathrm{\int }}_{\mathit{\lambda }}{\mathit{\sigma }}_{\mathrm{M}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\mathit{F}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\mathrm{d}\mathit{\lambda }\mathit{.}$(1)The problem is symmetric around the z-axis. At (ρ, z_r), the solar flux at wavelength λ will be attenuated to $\mathit{F}\mathrm{\left(}\mathit{\lambda ,\rho ,\phi }\mathrm{\right)}\mathrm{=}{\mathit{F}}_{\mathrm{0}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\exp \mathrm{[}\mathrm{-}\mathit{\tau }\mathrm{(}\mathit{\lambda ,\rho ,\phi }\mathrm{\left)}\mathrm{\right]}$(2)because of the optical thickness along the comet-sun axis. We consider water as the major molecule responsible for the opacity, which is connected to its absorption cross-section σ_H2O(λ) by $\mathit{\tau }\mathrm{\left(}\mathit{\lambda ,\rho ,\phi }\mathrm{\right)}\mathrm{=}{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}{\mathrm{\int }}_{{\mathit{z}}_{\mathit{r}}}^{\mathrm{\infty }}{\mathit{n}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}\mathit{\rho ,z}\mathrm{\right)}\mathrm{d}\mathit{z}\mathit{.}$(3)The photons absorbed are mostly those responsible for photodissociation and not fluorescence. Photodissociation of molecules takes place for λ < 200 nm and a significant solar UV field (λ > 80 nm). Using the assumptions for the density, we can integrate the density along the z-axis to obtain $\begin{array}{ccc}{\mathrm{\int }}_{{\mathit{z}}_{\mathit{r}}}^{\mathrm{\infty }}{\mathit{n}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}\mathit{\rho ,z}\mathrm{\right)}\mathrm{d}\mathit{z}& \mathrm{=}& \int \begin{array}{}\mathrm{\infty }\\ {\mathit{z}}_{\mathit{r}}\mathrm{=}\mathit{\rho }\mathit{/}\tan \mathrm{\left(}\mathit{\phi }\mathrm{\right)}\end{array}\frac{{\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }\mathrm{(}{\mathit{\rho }}^{\mathrm{2}}\mathrm{+}{\mathit{z}}^{\mathrm{2}}\mathrm{)}}\mathrm{d}\mathit{z}\\ & \mathrm{=}& \frac{{\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }\mathit{r}}\frac{\mathit{\phi }}{\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\mathrm{·}\end{array}$Hence at any point in the coma, we estimate the opacity $\mathit{\tau }\mathrm{\left(}\mathit{\lambda ,r,\phi }\mathrm{\right)}\mathrm{=}\mathrm{⟨}\mathit{\tau }\mathrm{\left(}\mathit{\lambda ,r}\mathrm{\right)}\mathrm{⟩}\frac{\mathrm{4}\mathit{\phi }}{{\mathit{\pi }}^{\mathrm{2}}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\mathrm{=}\mathit{C}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\frac{\mathit{\phi }}{\mathit{r}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\mathit{,}$(4)where the average value of the opacity over 4π steradians at the distance r from the nucleus is $\mathrm{⟨}\mathit{\tau }\mathrm{\left(}\mathit{\lambda ,r}\mathrm{\right)}\mathrm{⟩}\mathrm{=}\mathit{C}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\frac{{\mathit{\pi }}^{\mathrm{2}}}{\mathrm{4}}\frac{\mathrm{1}}{\mathit{r}}$(5)with $\mathit{C}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\mathrm{=}\frac{{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}{\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }}\mathrm{·}$(6)The surface corresponding to an opacity τ(λ,r,φ) = 1.0 is defined by ${\mathit{r}}_{\mathrm{thick}}\mathrm{\left(}\mathit{\phi }\mathrm{\right)}\mathrm{=}\mathit{C}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\frac{\mathit{\phi }}{\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\mathrm{=}\frac{{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}{\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }}\frac{\mathit{\phi }}{\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\mathrm{·}$(7)According to Eq. (7), the size of the optically thick region tends to infinity in the anti-sunward direction (φ = π). It has a finite length determined by the apparent size of the Sun (2.7° at r_h = 0.2 AU). This region is plotted in Fig. 12 for the three comets, considering only absorption of Lyman α photons by water molecules with a cross-section σ_H2O(Ly   α) = 15 × 10^-22 m² (Lewis et al. 1983).

We next consider HCN photodissociation, but similar equations can be established for other molecules. The effective HCN photodissociation rate at a point (r, φ) in the coma can then be derived from Eq. (1) ${\mathit{\beta }}_{\mathrm{HCN}}\mathrm{\left(}\mathit{r,\phi }\mathrm{\right)}\mathrm{=}{\mathrm{\int }}_{\mathit{\lambda }}{\mathit{\sigma }}_{\mathrm{HCN}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}{\mathit{F}}_{\mathrm{0}}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\exp \left(\mathrm{-}\frac{\mathit{C}\mathrm{\left(}\mathit{\lambda }\mathrm{\right)}\mathit{\phi }}{\mathit{r}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\right)\mathrm{d}\mathit{\lambda ,}$(8)where σ_HCN(λ) is the photodissociation cross-section for HCN. The integration can be divided over several wavelength intervals, corresponding to the different absorption bands of HCN and H₂O $\begin{array}{ccc}{\mathit{\beta }}_{\mathrm{HCN}}\mathrm{\left(}\mathit{r,\phi }\mathrm{\right)}& \mathrm{=}& \sum _{{\mathit{\lambda }}_{\mathit{i}}}{\mathit{\beta }}_{\mathrm{HCN}\mathit{,}{\mathit{\lambda }}_{\mathrm{i}}}\mathrm{\left(}\mathit{r,\phi }\mathrm{\right)}\\ & \mathrm{=}& {\mathit{\beta }}_{\mathrm{0}\mathit{,}\mathrm{HCN}}\mathrm{\times }\sum _{{\mathit{\lambda }}_{\mathit{i}}}{\mathit{x}}_{\mathit{i}}\exp \left(\mathrm{-}\mathit{C}\mathrm{\left(}{\mathit{\lambda }}_{\mathit{i}}\mathrm{\right)}\frac{\mathit{\phi }}{\mathit{r}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\right)\mathit{,}\\ & & \end{array}$where x_i corresponds to the fraction of the photodissociation rate due to radiation around the wavelength λ_i. For HCN, 88% of the contribution comes from solar Lyman α, i.e., x_Ly   α = 0.88 (Bockelée-Morvan & Crovisier 1985). To ease computations, we make the approximation $\sum _{{\mathit{\lambda }}_{\mathit{i}}}{\mathit{x}}_{\mathit{i}}\exp \left(\mathrm{-}\frac{\mathit{C}\mathrm{\left(}{\mathit{\lambda }}_{\mathit{i}}\mathrm{\right)}\mathit{\phi }}{\mathit{r}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\right)\mathrm{\approx }\exp \left(\mathrm{-}\sum _{{\mathit{\lambda }}_{\mathit{i}}}\frac{{\mathit{x}}_{\mathit{i}}\mathit{C}\mathrm{\left(}{\mathit{\lambda }}_{\mathit{i}}\mathrm{\right)}\mathit{\phi }}{\mathit{r}\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}\right)\mathrm{·}$(9)This rough assumption is valid for small opacities (r > r_thick) and if the water absorption cross-sections are the same at all wavelengths λ_i. Otherwise, it will slightly overestimate the screening effect in the opaque region. We then define an effective water absorption cross-section for HCN (likewise for the other molecules) $\begin{array}{ccc}\mathrm{⟨}{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{HCN}}\mathrm{⟩}\mathrm{=}\sum _{{\mathit{\lambda }}_{\mathit{i}}}{\mathit{x}}_{\mathit{i}}{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{\left(}{\mathit{\lambda }}_{\mathit{i}}\mathrm{\right)}\mathrm{=}\mathrm{13.8}\mathrm{\times }{\mathrm{10}}^{-22} {\mathrm{m}}^{\mathrm{2}}\mathit{,}& & \end{array}$(10)where most of the contribution comes from Lyman α (σ_H2O(Ly   α) = 15 × 10^-22 m², Lee & Suto 1986; Lewis et al. 1983). This approximation was validated by ourselves for HCN: we estimated numerically that using this mean value for the screening cross-section instead of summing over various wavelength intervals yields only a  ≈ 2% excess error in the estimate of the increase in the number of molecules due to screening.

After solving the differential balance equation for the HCN density at the distance r from the nucleus in the coma, we find that ${\mathit{n}}_{\mathrm{HCN}}\mathrm{\left(}\mathit{r,\phi }\mathrm{\right)}\mathrm{=}\frac{{\mathit{Q}}_{\mathrm{HCN}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }{\mathit{r}}^{\mathrm{2}}}\exp \left(\mathrm{-}\frac{\mathit{r}{\mathit{\beta }}_{\mathrm{0}}}{{\mathit{v}}_{\exp }}\mathit{G}\left(\frac{\mathit{r}}{{\mathit{C}}_{\mathrm{HCN}}\frac{\mathit{\phi }}{\sin \mathrm{\left(}\mathit{\phi }\mathrm{\right)}}}\right)\right)$(11)with ${\mathit{C}}_{\mathrm{HCN}}\mathrm{=}\frac{\mathrm{⟨}{\mathit{\sigma }}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{HCN}}\mathrm{⟩}{\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}}{\mathrm{4}\mathit{\pi }{\mathit{v}}_{\exp }}\mathrm{·}$(12)The function $\mathit{G}\mathrm{\left(}\mathit{x}\mathrm{\right)}\mathrm{=}\frac{\mathrm{1}}{\mathit{x}}{{}^{\mathrm{\int }}}_{\mathrm{0}}^{\mathit{x}}\exp \mathrm{(}\mathrm{-}\mathrm{1}\mathit{/}\mathit{t}\mathrm{)}\mathrm{d}\mathit{t}$ is equal to the exponential integral G(x) = E₂(1/x), which can easily be computed by numerical integration. We note that G(∞) = 1.0, so that Eq. (11) gives the classical Haser formula for negligible opacities.

We compared the production rates determined using the density from Eq. (11) to those obtained with the Haser formula. The largest effect is for comet C/2006 P1 with a  ≈ 60% decrease of the HCN production rate, and  ≈ 40% decrease for CH₃OH, CH₃CN, or HDO. We did not develop a full 3-D model to take into account phase angles different from 0°  or 180°. But the comparison between the case of a phase angle of 180°, where we can simply use Eq. (11), and replacing Eq. (4) by the averaged value in Eq. (5), only yields a 3% difference.

Table 7 provides characteristic scalelengths for the three comets and for comparison C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) observed in 1996–1997. In all cases, the optically thick region (at the Lyman α wavelength) is within the water coma (L_H2O >   ⟨ r_thick(Ly   α) ⟩ ) and if one takes into account OH, the assumption of an infinite lifetime for the screening molecules is valid since the coma encompassing H₂O and OH is definitely larger than the optically thick region and the HCN coma. One can also note that for comets C/1996 B2 and C/1995 O1, L_HCN ≫   ⟨ r_thick(Ly   α) ⟩ , so that the screening effect is negligible.

4.3. Line shapes and gas expansion velocity

We used the line shapes to estimate the gas expansion velocity following e.g., Biver et al. (1999). Table 9 lists the velocities (VHM) corresponding to the half maximum intensity on both negative and positive sides of the lines. For each comet, we used the most reliably detected lines. To infer the gas velocity from the measured VHM, the processes contributing to line broadening should be considered. For the HCN(3–2) line, we took into account its hyperfine structure (broadening of about 0.03 km s^-1 and 0.06 km s^-1, on the negative and positive sides, respectively). Thermal broadening affects the widths of the lines, and depends on the actual gas temperature. When gas expansion velocities are in the range 1–2 km s^-1, and temperatures are 100–300 K, thermal dispersion essentially smooths the line shape and widens it by less than 0.05 km s^-1. Optically thick lines, such as HCN(3–2) in C/2006 P1, are broadened by an additional 0.05 km s^-1 when the expansion velocity is constant in the coma.

At heliocentric distances r_h < 0.25 AU, the photodissociation lifetimes of HCN, HNC, CH₃OH, CS, HC₃N, and H₂CO are shorter than 1 h. In most cases, the beam size (5000 to 14 000 km at the comet for C/2002 X5, C/2002 V1, and C/2006 P1 near perihelion) is larger than the molecular scale-length. Therefore, velocity measurements pertain to nucleus distances fixed by the molecular lifetimes and the line widths are mostly representative of the expansion velocity in the coma at a distance equal to the molecular scale-length. Figures 4–10 show that the molecular lines have different widths. The measured “VHMs” velocities are plotted versus the molecular scale-lengths in Figs. 13–16. In the following sections, we constrain the evolution of the gas expansion velocity with distance to nucleus by fitting the measured VHMs, taking into account all sources of line broadening.

4.3.1. Outgassing pattern and anisotropy

Lines of C/2002 X5 detected around perihelion, and those of C/2006 P1, show an asymmetric shape, while C/2002 V1 lines are relatively symmetric.

For C/2002 X5, we likely observed a jet or outburst in progress on the night side. All the lines are strongly red-shifted (+0.25 km s^-1 on average, Table 2) suggesting gas outflow in a preferential direction, independent of acceleration effects. The phase angle was small (26°, Table 1), so the observed asymmetry corresponds to preferential outgassing from the night side. Anisotropy in screening of photodissociation (Sect. 4.2) also contributes to the asymmetry of the lines because lifetimes are longer on the night side. Considering the lines the less affected by screening (HC₃N, CH₃OH), we found that the outgassing rate is 1.35 ± 0.1 times higher on the night side. The mean gas velocity is smaller (0.9 versus 1.2 km s^-1) on the dayside of the nucleus. This is most likely owing to shorter molecular lifetimes on the dayside, the measurements sampling thereby molecules closer to the nucleus (Sect. 4.2).

For C/2006 P1, the phase angle was large when the comet was observed (120–140°, Fig. 12). The line velocity shifts are slightly negative (−0.05 km s^-1, Table 4), which could indicate some excess emission from the night side. However, evidence of asymmetric gas acceleration is present. The mean gas velocities inferred from long-lived molecules (HCN, CH₃OH) are 1.7 and 1.3 km s^-1 on the rear (≈ Sun facing) and front side of the nucleus, respectively, but  ≈ 1.2 km s^-1 on both sides for species with shorter lifetimes (CS and H₂CO) (Table 9). The less significant gas acceleration on the night side is likely due to a reduced photolytic heating caused by the screening of water photolysis. The small line blue-shifts suggest that the outgassing rate is barely larger (by a factor of 1.1 ± 0.1) in the day side, after taking into account screening effects (Sect. 4.2).

In the case of C/2002 V1, the phase angle was close to 90°  (Table 1), so that the line shapes are unaffected by day/night asymmetries.

4.3.2. Expansion velocity acceleration

Since there is evidence of gas acceleration in the coma, we used a variable expansion velocity in the coma v_exp(r), which is plotted in Figs. 13–16. This function reproduces the general shape of gas-dynamic simulations (Combi & Smyth 1988). The parameter v_exp(r) ² is adjusted to match the observed line widths (Table 9) when computing line shapes. These velocity plots provide the two free parameters used to compute v_exp(r) and lead to the following comments:

• C/2002 X5 atr_h = 0.21 AU (Fig. 13): positive and negative sides VHMs were both fitted using the same function. The computed line shapes for HC₃N(28–27), CH₃OH(5₀ − 5_-1 + 6₀ − 6_-1 + 7₀ − 7_-1)E, and HCN(3–2), taking into account screening and a night/day outgassing rate ratio of 1.35, have exactly the measured width within the errorbars. Doppler shifts of the lines (+0.095, +0.172, +0.243 km s^-1, predicted, respectively) are also in good agreement.

• C/2002 V1 (Fig. 14): asymmetry in the lines (due to day/night outgassing asymmetry and screening effect) is neither seen nor expected because of the phase angle. We have thus averaged the VHM measurements on both sides of the lines. However, the two days of observations require two different velocity profiles, with stronger acceleration on the second day closer to the Sun. For this comet, we did not observe short-lived species (e.g., HC₃N, H₂CO, or H₂S), so the velocity profile is poorly constrained close to the nucleus. The CS line widths suggest that the CS lifetime is significantly shorter than the HCN lifetime.

• C/2006 P1: data were divided into four subsets: positive and negative velocity sides of the lines on the 16th (Fig. 15) and 17th of January 2007 (Fig. 16). The species with the smaller scale length (H₂CO) was not observed on 17 Jan. but we assumed a similar acceleration in the coma for both days, scaled to the HCN lines VHMs. A stronger acceleration is present for molecules moving in an opposite direction to the observer (v > 0), which is consistent with stronger acceleration towards the dayside (Fig. 12: phase angle of 120−140°   ≪ 90°).

4.4. Constraints on CS and HNC lifetimes

We assume that CS is the photodissociation product of CS₂. It forms very close to the nucleus since CS₂ photodissociates into CS in less than 30 s at r_h < 0.25 AU. We assume that there is no additional excitation effect to those not modeled in Biver et al. (1999) that would strongly affect its rotational population mimicking photodissociation. Hence, the width of CS lines (clearly narrower than HCN lines; Figs. 4, 6, 10, and Table 9) can be converted into a scale-length estimate using line-shape modeling with v_exp(r). The scale-length of CS is determined by the gas expansion velocity, its photodissociation rate, and the screening effects. Since the photodissociation process of CS is unknown, we investigated β_0,CS = 1–3 × 10^-5 s^-1 for the photodissociation rate at 1 AU and σ_H2O → CS = 0−12 × 10^-22 m² (i.e. values smaller than or comparable to HCN for the screening cross-section). Table 8 summarizes the findings. There is no unique solution, but grouping measurements comet by comet, the closer matches are found for (β_0,CS ≥ 1.7 × 10^-5 s^-1, σ_H2O → CS ≥ 6 × 10^-22 m²), (β_0,CS = 2.9 ± 0.7 × 10^-5 s^-1, for σ_H2O → CS = 6 × 10^-22 m²), and (β_0,CS = 4.3 ± 1.2 × 10^-5 s^-1, σ_H2O → CS ≥ 6 × 10^-22 m²) for C/2002 X5, C/2002 V1, and C/2006 P1, respectively. The screening of photodissociation is necessary to achieve closer agreement between the night- and dayside measurements. In addition, the detection of CS at only 0.1 AU from the Sun in C/2002 V1 requires a lifetime (β_0,CS ≪ 4 × 10^-5 s^-1) long enough to get realistic abundances (CS/H₂O  ≪  1%, this ratio being  < 0.2% at 1 AU in all comets where it has been measured). Values β_0,CS = 2.5 ± 0.5 × 10^-5 s^-1 and σ_H2O → CS = 6 × 10^-22 m² are consistent with all measurements and are used to determine the production rates.

For HNC, we assume similar spectroscopic properties (e.g. UV absorption spectrum) to those of HCN. Slight differences in line shapes may underline differences. In C/2002 X5 and C/2006 P1, the HNC(3−2) line is slightly narrower than HCN(3−2) suggesting stronger photodissociation, weaker screening, and/or other destruction processes in the coma. But this effect is not observed in C/2002 V1. A possible interpretation is that HNC is partly created in the coma by chemical reactions, as suggested by Rodgers & Charnley (1998). This would partly compensate for the shortening of its scale-length. A parent scale-length of  ≈ 1300 km at r_h = 0.2 AU, β_0,HNC ≈ 1.3 × β_0,HCN, and σ_{H2O → HNC} ≈ 0.9 × σ_{H2O → HCN} would provide the closest agreements between model and data, but the constraints are not strong enough to infer a definite conclusion.

5. Production rates and abundances

The production rates were computed using models that incorporate collisions with neutrals and electrons, and radiative pumping by the solar radiation (Biver et al. 1999, 2000, 2006a). We did not consider infra-red pumping by the large and warm dust coma. Assuming a dust-to-gas ratio of 1.0 (which might be underestimated for the dustier comets C/2006 P1 and C/2002 V1), and the simplified approach by Crovisier & Encrenaz (1983), we estimate that for HCN or CH₃OH in comet C/2006 P1 (McNaught), the vibrational pumping by the dust infrared radiation is stronger than that by the solar radiation field within 2000–7000 km of the nucleus. However, collisions with water still dominate in a region twice as large and control the excitation of the rotational levels.

We used the radial profiles of the gas expansion velocity determined in the previous section. In some cases, we considered different velocity profiles to interpret the positive and negative sides of the lines, but similar production rates were obtained as long as the velocity profiles provided a good fit to the line widths. To compute rotational level populations and collision rates with neutrals, we used the gas temperatures given in Sect. 4.1. Inferred production rates are given in Table 10. As discussed in the next section, they are possibly wrong by a factor 1.5, because of uncertainties in the model parameters.

5.1. Uncertainties due to model parameters

The photodissociative lifetimes were taken from Crovisier (1994), except for CS and HNC, which we constrained from the present observations (Sect. 4.4). We took into account solar activity when computing the photodissociation rates (Crovisier 1989, 1994; Bockelée-Morvan & Crovisier 1985). The uncertainty introduced by our simplifying assumptions in the modeling of photolysis screening is not very large (<15%, not considering water production rate uncertainties). Derived HCN production rates are a factor between 1.4 and 2.5 smaller than when this effect is omitted.

The main source of error in the computation of production rates is the uncertainty in the gas kinetic temperature (except for C/2002 X5 at r_h > 0.5 AU, where the temperature is well constrained). We assumed constant coma temperatures of 180, 150, and 300 K for near perihelion data of comets C/2002 X5, C/2002 V1, and C/2006 P1, respectively (Sect. 4.1). An increase (respectively decrease) in these values by 50% affects the production rate determinations in the following way:

• Q_HCN and Q_HNC are increased by +33 to +40% (C/2006 P1) (respectively, decreased by 31% to 39%);

• the same trend is observed for Q_CS and Q_CH3CN:  ± 45% for a  ± 50% variation in the gas temperature;

• Q_CH3OH is still more sensitive to the assumed temperature, because of excitation of the torsional band at high T: a  ± 50% change in temperature causes changes of +51/–39%, +72/–53%, and +102/–64% in the production rates for C/2002 V1, C/2002 X5, and C/2006 P1, respectively;

• Q_HC3N is minimal for T ≈ 200 K. The retrieved Q_HC3N is 50% higher when either increasing or decreasing the nominal temperature (180 K) by 50%.

Therefore, most abundances relative to HCN are not significantly sensitive to the assumed T. Two exceptions are the CH₃OH/HCN (+20 to +40% for ΔT =  + 50%) and HC₃N/HCN (+100% for ΔT = +50%) ratios.

5.2. Short- and long-term variations in the production rates

The most significant time variations are observed for C/2002 X5. Rapid variations are observed around perihelion (Fig. 17). Strong variations in the light curve and water production rate are also reported by Bout et al. (2003) and Povich et al. (2003). On 26.5 January 2003, the production rates of HCN, CS and HNC increased by  ≈ 70% over a time interval of  ≈ 0.11 days (Table 10, Fig. 17).

Over the 2–3 days of observations of comets C/2002 V1 and C/2006 P1, production rates also varied. For C/2002 V1, an heliocentric dependence Q_CS$\mathrm{\propto }{\mathit{r}}_{\mathrm{h}}^{-0.8}$ to ${\mathit{r}}_{\mathrm{h}}^{-1.5}$ is observed, while for C/2006 P1 ${\mathit{Q}}_{\mathrm{HCN}}\mathrm{\propto }{\mathit{r}}_{\mathrm{h}}^{-4.5}$. For C/2006 P1, the variation in the HCN production rate is steeper than the variation in the H₂O production rate ($\mathrm{2.4}\mathrm{\times }{\mathrm{10}}^{\mathrm{29}}{\mathit{r}}_{\mathrm{h}}^{-3}$ molec. s^-1 ) deduced from the OH observations (Table 5). This inconsistency could be due to rotation-induced time variations and the flux loss cause by anomalous refraction on 17 January being unable to be quantified.

The long-term (over 2 months) evolution of the production rates of C/2002 X5 is shown in Fig. 18. The HCN, HNC, and CH₃OH production rates vary according to $\mathit{Q}\mathrm{\propto }{\mathit{r}}_{\mathrm{h}}^{\mathrm{-}\mathrm{3.5}\mathrm{\pm }\mathrm{0.5}}$, pre and post-perihelion, as for H₂O post-perihelion ($\mathrm{(}\mathrm{2.6}\mathrm{\pm }\mathrm{0.2}\mathrm{)}\mathrm{\times }{\mathrm{10}}^{\mathrm{28}}{\mathit{r}}_{\mathrm{h}}^{\mathrm{-}\mathrm{3.5}\mathrm{\pm }\mathrm{0.2}}$ molec. s^-1 ).

5.3. Molecular abundances and discussion

Table 11 summarizes the molecular abundances relative to water and HCN in the three comets.

The water production rates used as reference for the short heliocentric distances (r_h < 0.3 AU) are uncertain, as discussed in Sect. 3. For C/2002 X5, according to the short-term variations seen for all molecules, we assumed ${\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{=}\mathrm{35}\mathrm{+}\mathrm{10}\mathrm{\times }\sin {}^{\mathrm{(}}\mathrm{2}\mathit{\pi }{\frac{\mathit{t}\mathrm{\left[}\mathit{d}\mathrm{\right]}\mathrm{-}\mathrm{26.54}}{\mathrm{0.2}}}^{\mathrm{)}}\mathrm{\times }{\mathrm{10}}^{\mathrm{29}}$ molec. s^-1. For C/2006 P1, we used ${\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{=}\mathrm{2.4}\mathrm{\times }{\mathrm{10}}^{\mathrm{29}}{\mathit{r}}_{\mathrm{h}}^{-3}$ molec. s^-1, and for C/2002 V1 ${\mathit{Q}}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}\mathrm{=}\mathrm{2.7}\mathrm{\times }{\mathrm{10}}^{\mathrm{29}}{\mathit{r}}_{\mathrm{h}}^{-1}$ molec. s^-1, these values being possibly in error by  ± 50%. Resulting abundances relative to water are roughly in agreement with abundances measured in other comets (Biver et al. 2002a). The CH₃OH/H₂O ratio seems however quite low in comet C/2002 V1 and especially in C/2006 P1. The HDO/H₂O ratio in comet C/2006 P1, is not well constrained, both OH and HDO being only marginally detected, but production rates are compatible with the ratio measured in other comets (Bockelée-Morvan et al. 1998; Meier et al. 1998; Jehin et al. 2009,  ≈ 6 × 10^-4).

We now turn our discussion to abundances relative to HCN, which are more reliable since HCN was always observed simultaneously. The main observed features are:

• A decrease in the CH₃OH/HCN ratio at low r_h$\mathrm{\propto }{\mathit{r}}_{\mathrm{h}}^{\mathrm{0.5}}$ is observed for C/2002 X5, confirming the trend observed in comet C/1996 B2 (Hyakutake) (Biver et al. 1999). Using this ${\mathit{r}}_{\mathrm{h}}^{\mathrm{0.5}}$ variation, we infer CH₃OH/HCN ratios at 1 AU of 18 and 10 for C/2002 V1 and C/2006 P1, respectively, which are within the range of abundances measured in comets, though C/2006 P1 belongs to the methanol-poor category (Biver et al. 2002a).

• The CS/HCN ratio varies as $\mathrm{0.35}\mathrm{\times }{\mathit{r}}_{\mathrm{h}}^{-1.0}$. A similar heliocentric dependence (${\mathit{r}}_{\mathrm{h}}^{-0.8}$) was measured in comet C/1996 B2 (Biver et al. 1999). In comet 153P, CS/HCN followed $\mathrm{0.55}{\mathit{r}}_{\mathrm{h}}^{-0.7}$ (Biver et al. 2006a). The puzzling heliocentric variation in the CS/HCN ratio is confirmed down to very low r_h.

• Measuring the HNC/HCN ratio at small r_h was one of the main objectives of our analysis of these observations. HNC was easily detected in the three comets with an abundance ratio HNC/HCN between 0.11 and 0.29 at r_h < 0.26 AU. The measurements in comets C/2002 X5 and C/2002 V1 were included in a previous study of the heliocentric variation of HNC/HCN based on a sample of 11 comets at r_h in the range 0.14–1.5 AU (Lis et al. 2008). The HNC/HCN sample was fitted by a power law in ${\mathit{r}}_{\mathrm{h}}^{-2.3}$, with an indication being found of a possible flattening at r_h  <  0.5 AU. The values inferred for our three comets do not show a trend for the more productive comets being enriched in HNC, as expected for a formation of HNC by chemical reactions (Rodgers & Charnley 1998), but rather the opposite. The HNC/HCN ratio in C/2006 P1 is 0.13, smaller than the value in comet Hale-Bopp at 1 AU (≈ 0.25), this comet being slightly more active than comet Hale-Bopp. Possible origins of HNC in cometary atmospheres are discussed in Rodgers & Charnley (1998) and Lis et al. (2008). Destruction of HNC by reaction processes possibly takes place at low r_h. Small differences in the line shapes between HCN and HNC suggest that HNC might be partly produced in the coma and destroyed further away from the nucleus or be more sensitive to photodissociation than HCN.

Comet C/2006 P1 has compositional similarities with fragments B and C of comet 73P/Schwassmann-Wachmann 3. All are CH₃OH and CO-poor, and the H₂CO/HCN, CH₃CN/HCN ratios are similar (Biver et al. 2006b; Dello Russo et al. 2007b). However, C/2006 P1 is rich in volatile hydrocarbons and in NH₃, while 73P is depleted in these compounds according to Dello Russo et al. (2007a).

Comet C/2002 X5, on the other hand, has a normal CH₃OH abundance. It is unusually rich in HC₃N. The HC₃N/HCN ratio is four times higher than in comet Hale-Bopp (Bockelée-Morvan et al. 2000), and still higher than upper limits found in some comets (Biver et al. 2006a). Interestingly, C/2002 X5 belongs to the class of carbon-rich comets (Povich et al. 2003). Finally, the large abundances of HC₃N, HNC, and CH₃CN relative to HCN in this comet imply that these species are significant contributors to the production of CN radicals in this comet.

5.4. Search for refractory molecules

SiO was searched through its J(6–5) transition at 260518.020 MHz in comets C/2002 X5 and C/2002 V1 around perihelion (Tables 2, 3). It was not detected down to about 0.01% relative to water, assuming thermal equilibrium and a photodissociation rate of 10^-5 s^-1. The same tuning at 260 GHz also allowed us to search for NaCl J(20–19) line in C/2002 V1, while the KCl J(30–29) line was searched for at 230 GHz in C/2002 V1 and C/2006 P1, in both case without success.

6. Conclusion

We have reported on our analysis of millimeter spectroscopic observations of three bright comets (C/2002 X5 (Kudo-Fujikawa), C/2002 V1 (NEAT), and C/2006 P1 (McNaught)) when they were close to the Sun (r_h <  0.25 AU). Our main challenge has been to cope with ephemeris uncertainties by searching for the peak of brightness in HCN emission using coarse mapping. We have obtained the following results:

• The screening effect in photolytic processes, which affects the distribution of the molecules in the coma, was modeled to determine accurate production rates. This effect is significant, especially for C/2006 P1 where it increases by 40 to 60% the number of molecules in the coma.

• The various lines have different widths. Velocity variations in the coma were considered to interpret the line shapes. This provided constraints on the CS and HNC scale-lengths.

• The CS photodissociation rate was estimated to 2.5 ± 0.5 × 10^-5 s^-1 (r_h = 1 AU), which is compatible with other estimates (Boissier et al. 2007).

• The HNC photodissociation lifetime is found to be slightly shorter (by  ≈ 30%) than the lifetime of HCN, though this estimate does not consider destruction paths of HNC by chemical reactions.

• The CS/HCN production rate ratio in cometary atmospheres follows a heliocentric dependence  ≈ ${\mathit{r}}_{\mathrm{h}}^{-0.8}$ down to r_h = 0.1 AU. We rule out artefacts related to uncertainties in CS lifetime. The origin of this variation remains unexplained.

• The CH₃OH/HCN production rate ratio decreases with decreasing r_h according to ${\mathit{r}}_{\mathrm{h}}^{\mathrm{0.5}}$.

• HNC/HCN ratios are high (0.11 to 0.29) in the range r_h = 0.14–0.25 AU, consistent with HNC being a by-product of the thermal degradation of organic grains (Lis et al. 2008). A lower HNC abundance is measured for the more productive comet, possibly indicating destruction of this reactive molecule by chemical processes.

• The H₂CO/HCN abundance ratio measured in C/2006 P1 (2.2 at 0.23 AU) and C/2002 V1 (< 9.5 at 0.12 AU) is in the range of values measured in comets at 1 AU from the Sun (Biver et al. 2002a, 2006a, 1.6–10). This possibly rules out H₂CO production from the thermal degradation of polymers as proposed by Fray et al. (2006).

• The HC₃N abundance in C/2002 X5 is higher, by a factor of four or more, than in any of the other eight comets in which it had previously been measured. HC₃N, CH₃CN, and HNC are all significant contributors to the production of CN radicals in this comet.

• Searches for SiO, NaCl, and KCl were unsuccessful. The upper limit set for SiO is  < 10^-4 relative to H₂O.

Online material

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Acknowledgments

We are grateful to the IRAM staff and to other observers for their assistance during the observations. IRAM is an international institute co-funded by the Centre national de la recherche scientifique (CNRS), the Max Planck Gesellschaft and the Instituto Geográfico Nacional, Spain. This research has been supported by the CNRS and the Programme national de planétologie de l’Institut des sciences de l’univers (INSU).

References

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