© ESO, 2014

1. Introduction

Comets are among the best-preserved specimens of the primitive solar nebula because the nucleus composition did not evolve much since their formation 4.6 billion years ago. Studying the composition of comet nuclei is thus essential to understand the formation and evolution of material within our solar system. Oxygen is one of the most abundant elements in comets since most of the cometary ices consist of H₂O, CO₂ , and CO molecules. All these molecules generated by the sublimation of cometary ices can produce oxygen atoms in the coma by photodissociation. Oxygen atoms in comets are analyzed through the three forbidden oxygen lines located at 5577.339 Å for the green line, and at 6300.304 Å and 6363.776 Å for the red doublet. These emission lines are a result of the electronic transition of the oxygen atoms from the ¹S state to the ¹D state (green line) and from the ¹D to the ground state (red doublet). In previous papers (Cochran 1984; Cochran & Cochran 2001; Cochran 2008; Capria et al. 2010, and references therein), oxygen atoms were found to come mainly from the photodissociation of water molecules at a heliocentric distance of about 1 au. Decock et al. (2013) analyzed about 50 spectra belonging to 12 comets observed at r~1 au and compared the theoretical ratio of the green-to-red-doublet emission intensity (G/R ratio; see ¹S/¹D in Table 1) with the observed ratios. This work confirmed that H₂O is the main parent source of atomic oxygen in the cometary coma within a radial distance of 8000 km from the nucleus. The observation of these emission lines at larger heliocentric distances (>2.5 au) has shown a higher G/R ratio caused by significant CO₂ contribution to the production of oxygen atoms (Decock et al. 2013). A similar higher G/R ratio value on comets has also been observed by McKay et al. (2012, 2013). In the present paper, we study the three forbidden oxygen lines in the inner coma as closely as possible to the nucleus to determine the parent species that produce oxygen atoms at different nucleocentric distances. This analysis was made for four comets of different types (New, Jupiter family, Halley type) observed with the ESO Very Large Telescope with a good spatial resolution and high signal-to-noise ratio. We also study the effect of the collisional quenching of O(¹S) and O(¹D) with the ambient cometary species (mainly water) on the G/R ratio profile close to the nucleus (≲1000 km).

2. Observations

We carried out the observations on four comets, C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and 103P/Hartley 2 (hereafter C/2002 T7, 73P-C, 8P and 103P, respectively), using the high-resolution UVES spectrograph at ESO (VLT). The observations of comet C/2002 T7 have been made in May 2004 shortly after its perihelion (April 2004). In 1995, comet 73P/Schwassmann-Wachmann 3 was very active, and the nucleus split into several fragments (Böhnhardt et al. 1995). The oxygen emission lines have been observed on the brightest fragment, which is 73P-C. Spectra of comet 8P were obtained on three different nights during January and February 2008. The Jupiter family comet 103P/Hartley 2 was studied in great detail by the EPOXI NASA mission and many telescopes from September to November 2010 (A’Hearn et al. 2011; Meech et al. 2011). Our observations were scheduled at the Paranal Observatory during the EPOXI flyby period.

All the data correspond to spectroscopic observations made with the high-resolution UVES spectrograph mounted on the UT2 of the VLT (Manfroid et al. 2009; Jehin et al. 2009; Decock et al. 2013, and references therein). The peculiarity of these observations is that these comets were observed close to Earth, from 0.61 au for C/2002 T7 down to 0.15 au for 73P-C. These small geocentric distances provide a high spatial resolution (from 400 to 100 km per arc second) and permit us to analyze the coma species very closely to the nucleus. The complete sample consists of five spectra for C/2002 T7, eight for 103P, three for 8P, and one for 73P-C. The 0.4′′ ×12′′ slit is usually centered on the nucleus. For C/2002 T7 and 103P, we also obtained spectra at offset positions, enabling us to study the [OI] lines at large distances from the nucleus. All the observing details and the offset values are given in Table 2.

3. Data reduction

The extraction of the 1D spectra and the reduction procedure are described in Sect. 3 of Decock et al. (2013). To study the [OI] lines as closely as possible to the comet nucleus, we spatially extracted the spectra line by line. We divided the slit of each spectrum into different pixel zones corresponding to different distances to the nucleus. As shown in Fig. 1, various subslit zones are represented by different symbols and are also used in Figs. 3 to 6. We obtained 14 spectra for comets C/2002 T7, 73P-C, and 103P along the 64 pixel slit, while for comet 8P, five spectra were extracted for the 52 pixels slit length. For the spectra taken at large offset positions, we used the average value over the whole slit to increase the signal-to-noise ratio, because in this case the change of distance from the nucleus is small within the slit.

3.1. C₂ emission line subtraction

Many C₂ lines are located in the vicinity of the green line, and we have identified two of them at 5577.331 Å and 5577.401 Å that are blended with the oxygen line. The C₂ contamination becomes stronger at large distances from the nucleus because the [OI] flux decreases faster than C₂. This blend can affect both the flux and the width of the green line. To remove this contamination, we used another C₂ line close to the oxygen line (the P₃(25) line of the (1,2) Swan band at 5577.541 Å) and created a synthetic C₂ spectrum by fitting both the intensity and the profile of the lines. This synthetic spectrum was built using a Boltzmann distribution of the respective populations of the rotational levels with a temperature of 4000 K. Because of the very close energy levels involved (same rotational number value for both the upper and lower vibronic states, see Rousselot et al. 2012, for more details of the Swan band structure) the exact rotational temperature value has very little influence on the final C₂ fit. The spectrum considered in our work is the spectrum observed after subtracting the C₂ synthetic spectrum (see Fig. 2). This step was not necessary for 73P-C because there is no C₂ feature detected around the green line. This comet is a C-chain depleted comet (Schleicher et al. 2006) and thus very poor in C₂.

4. Results and discussion

We used the software IRAF to measure the intensities and widths of the three [OI] lines by making a Gaussian fit.

4.1. G/R ratio

When collisional quenching is neglected, the intensity I of an emission line, expressed in Rayleighs¹ (R), is given theoretically by Festou & Feldman (1981), (1)where τ_p is the photodissociative lifetime of the parent species in seconds, α is the yield of photodissociation, β corresponds to the branching ratio for the transition, and N is the column density of the parent species in cm^-2. The measured green and red-doublet emission intensities and the corresponding G/R ratios for each spectrum of our observations are listed in Table 3. The G/R ratio is displayed for the four comets with respect to the nucleus distance in Figs. 3−6. It is given in Table 4 with the errors on the y-axis estimated from the rms of the N spectra available for a given comet. Errors on the x-axis correspond to the spatial area (in km) covered by the considered subslit. The atmospheric seeing limits the spatial resolution and defines the smallest nucleocentric distance that can be resolved. This smallest nucleocentric distance is provided in the last column of Table 2 and is also included in the x-axis errors. In Figs. 3−6 the G/R ratio decreases monotonically with the projected distance from the nucleus. This is due to strong collisional quenching of the excited oxygen atoms by H₂O (Bhardwaj & Raghuram 2012). Since the lifetime of O(¹D) atoms is ~100 times longer than the lifetime of O(¹S), the O(¹D) is destroyed more frequently by quenching (Raghuram & Bhardwaj 2014), which implies an increase of the G/R ratio close to the nucleus. This is what we see in our observations. Without accounting for collisional quenching of O(¹S) and O(¹D), the model-calculated G/R ratio profiles are plotted with dash-dotted curves in Figs. 3 to 6. The G/R ratio is almost flat throughout the projected coma of all these comets and is significantly loweer than the observed values very close to the nucleus. This calculation shows how important collisional quenching is in determining the G/R ratio in comets. The destruction rate profiles of O(¹S) and O(¹D) by H₂O in the innermost coma depend on the size of the collisional coma of the comet, which is proportional to the water production rate. The higher the H₂O production rate, the more extended the collisional coma and the stronger the quenching by H₂O molecules. That is why comet C/2002 T7 (LINEAR), with a higher Q_H2O ~ 5 × 10²⁹ s^-1, reaches the lowest G/R ratio at larger distances (~1000 km) than the other comets (~500 km).

We also analyzed the theoretical estimates of production rates for O(¹S) and O(¹D) atoms, which depend on the water production rate (Raghuram & Bhardwaj 2013, 2014). For comets 73P-C, 8P, and 103P with Q_H2O ~ 10²⁸ s^-1, O(¹S) comes from both CO₂ and H₂O, while O(¹D) is only produced by the photodissociation of H₂O. However, for C/2002 T7 (LINEAR) the O(¹S) atoms are formed by the photodissociation of CO₂ close to the nucleus (<50 km) and, beyond, by both CO₂ and H₂O. Therefore, the O(¹S)/O(¹D) values in Table 1 should report a higher G/R ratio below 50 km for this comet. Unfortunately, the atmospheric seeing does not permit us to resolve the coma at such small distances to the nucleus. Beyond 10³ km, the main contribution to both O(¹S) and O(¹D) is the photodissociation of H₂O molecules. As shown in Figs. 3−6, at these distances the G/R ratio reaches a constant value of ~0.05, which is very similar to the O(¹S)/O(¹D) value for the pure water case (cf. Table 1).

4.1.1. Estimation of CO₂ relative abundances

We used the coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) to estimate the CO₂ relative abundance (i.e., the CO₂/H₂O abundance ratio) in these comets by comparing the observed slit-centered G/R data points. The model accounts for the main production and loss processes of O(¹S) and O(¹D) atoms in the inner cometary coma. It also includes the collisional quenching of O(¹S) (4 × 10^-10 molecule^-1 cm³ s^-1, Stuhl & Welge 1969) and O(¹D) (2.1 × 10^-10 molecule^-1 cm³ s^-1, Atkinson et al. 2004; Takahashi et al. 2005) by H₂O in the inner coma. More details about the model calculations are given in Bhardwaj & Raghuram (2012) and Raghuram & Bhardwaj (2013, 2014). We took the atmospheric seeing effect into account, which is especially strong very close to the nucleus. For this, we convolved the spatial profile provided by the model with a Gaussian of FWHM equal to the seeing value. The seeing values were obtained from the Paranal seeing monitor. The H₂O production rates and the seeing values used in the model as well as the corresponding heliocentric distances are given in Tables 2 and 5. The CO relative abundances for comets C/2002 T7 and 73P-C are assumed to be equal to 5% in the model. However, earlier works (Bhardwaj & Haider 2002; Raghuram & Bhardwaj 2013, 2014) have shown that the role of CO in the determining G/R ratio is insignificant. The derived CO₂ relative abundances obtained by fitting the observed G/R profiles with the model are provided in the last column of Table 6.

The comparison between the observed and computed G/R profiles of comet C/2002 T7 shown in Fig. 3 indicates that the relative abundance of CO₂ in this comet is lower than 2%. The computed G/R profile for comet 73P-C with 5% CO₂ relative abundance is presented in Fig. 4 together with the observations. The model calculations for comet 8P suggest that for 8P/Tuttle, the CO₂ relative abundance probably is between 0 and 5%. In comet 103P (shown in Fig. 6), the required CO₂ relative abundances necessary to reproduce the observations are about 15 to 20%, which agrees with the CO₂ measurements obtained by EPOXI observations (A’Hearn et al. 2011). This is the only comet with such a high CO₂ abundance in our sample. The observed data close to the nucleus are very well fitted with the model calculations for all these comets, which indicates that the O(¹S) and O(¹D) collisional quenching rates measured by Stuhl & Welge (1969) and Takahashi et al. (2005) lie in a plausible range.

4.1.2. 103P/Hartley 2

Two data points very close to the nucleus (observed on 2010/11/11, 8:57) appear to be relatively high in Fig. 6, while the other points farther away from the nucleus have G/R values similar to those of the other spectra. First, we considered a possible contamination by a cosmic rays, but an analysis of the three [OI] lines in the 2D spectra shows no cosmic-ray event (see Fig. 7). Figure 7 also confirms that there was no problem during the acquisition of this spectrum by comparing it with the spectrum taken the day before. This suggests that the CO₂/H₂O abundance sometimes strongly varies close to the nucleus. These variations might be generated by icy particles that evaporate very fast in the first ten kilometers and/or by the rotation of the nucleus, because it has been shown that the CO₂ comes from a localized region (the small lobe) of the nucleus (A’Hearn et al. 2011).

4.2. Line widths

Using relation (9) given in Decock et al. (2013), (2)the intrinsic line widths (FWHM(λ)) corrected for the instrumental broadening are evaluated from the measured FWHM_observed (Å). The results are listed in Table 3, the errors on the velocity widths are provided in Table 4. Figures 8–10 plot the intrinsic width of the three forbidden oxygen lines as a function of the nucleocentric distance. Our observations clearly show that the green line is wider than the red lines (~2.5 km s^-1 versus ~1.5 km s^-1) even after the subtracting the C₂ contamination. This was first reported by Cochran (2008) and confirmed by Decock et al. (2013) for a large sample of comets. We emphasize that 73P-C presents, like all comets, a wider green line, but is very poor in C₂. Therefore, the greater width of the 5577 Å line cannot be explained by the C₂ blends alone. A blend with another species cannot be excluded, but is unlikely. The width of the green line decreases when the distance to the nucleus increases (Fig. 8). This trend is more obvious for comets C/2002 T7 and 103P, for which we have spectra taken at large distances from the nucleus. The width of the green line at large distance also becomes similar to the width of the red lines. From extrapolating the linear regression for the width of the green line of 103P and C/2002 T7 with distance, we expect that the width of the green line will become equal to width of the red line at ~5000 km for 103P and at ~35 000 km for C/2002 T7. More observations at larger distances from the nucleus are necessary to confirm this. We cannot explain this behavior by the collisional quenching because this is not observed for the red lines. Indeed, as already mentioned by Raghuram & Bhardwaj (2014), the O(¹D) atoms are more quenched than the O(¹S) atoms because of their longer lifetimes. Therefore, the collisional quenching should make the red line wider than the green line, which is not observed. The greater width of the green line close to the nucleus might arise because the O(¹S) atoms are mainly produced by CO₂ very close to the nucleus. Indeed, the solar flux responsible for the production of O(¹S) from CO₂ comes from the 955−1165 Å region (Bhardwaj & Raghuram 2012), which is more energetic than the Lyman-α photons, which are considered to be the main source of oxygen-atom production from H₂O dissociation. These energetic photons can penetrate the thick coma more deeply than the Lyman-α photons. Since the line width is related to the value of the excess energy after the photodissociation, an excitation by higher energetic photons produces greater widths, as is observed for the green line close to the nucleus. The excess energy of different parent molecules has been evaluated by Raghuram & Bhardwaj (2014). They found that the excess energy from CO₂ is higher than that from H₂O. When the O(¹S) atoms are observed far away from the nucleus, they come from both CO₂ and H₂O, but with Lyman-α photons as the main excitation source, followed by a stronger contribution from the water molecules, which results in a decreasing width of the green line.

Bisikalo et al. (2014) investigated the greater width of the green line and provided a different explanation. They developed a Monte Carlo model to study the [OI] line widths and suggested that the observed line profile also depends on the thermalization by elastic collisions. However, the thermalization process leads to a broadening of the red lines, not of the green line, which disagrees with our observations.

Figures 8 to 10 show that the widths of the three forbidden oxygen lines of C/2002 T7 (LINEAR) are always greater than the widths of the other comets. We explain this by the higher water production rate of this comet. Tseng et al. (2007) have shown that the expansion velocity of the molecules increases when the water production rate increases.

5. Conclusions

Four comets were observed at small geocentric distances with the UVES spectrograph (VLT). Seventeen high-resolution spectra were collected when the comets were very close to Earth (<0.65 au), allowing us to study the [OI] lines as closely as 100 km to the nucleus. Some of them were also recorded with an offset from the nucleus corresponding to a distance of 10 000 km. The 11 centered spectra were spatially extracted and binned per pixel zones to obtain 1D spectra at various distances from the nucleus. Finally, we studied the forbidden oxygen lines in 63 sub-spectra and 6 offset spectra corresponding to different nucleocentric distances. The G/R ratio and the velocity widths were computed and analyzed to better understand the production of oxygen atoms in the coma. The results can be summarized as follows:

• 1.

  In this analysis, C₂ blends were identified and removed from the oxygen green line. This subtraction, made for the first time, is important because this contamination modifies the intensity and the FWHM of the 5577.339 Å line for comets rich in C₂ and more specifically for the analysis made far away from the nucleus². However, the C₂ blends do not explain the greater width of the green line compared with the width of the red lines.

• 2.

  Thanks to the high spatial resolution of the data, we found that the G/R ratio as a function of the nucleocentric distance displays the same profile for all the comets: a rapid increase below 1000 km up to typically 0.2, and a constant value of about 0.05 at larger distances. The G/R value of about 0.1 found in previous studies is thus an average of the values obtained both close to the nucleus and at larger distances. This particular profile is mainly explained by the quenching, which plays an important role in the destruction of O(¹D) atoms in the inner coma (<1000 km). The parent molecules forming oxygen atoms can also contribute to this trend. Indeed, O(¹D) atoms are only formed by H₂O, while O(¹S) are produced by both CO₂ and H₂O. With increasing distance from the nucleus, the relative H₂O contribution becomes stronger and thus the intensity of the green line decreases. In the extended coma, oxygen atoms are only formed by the photodissociation of water molecules. The G/R value of 0.05 agrees well with the pure water case (Table 1).

• 3.

  We fitted our observational [OI] emission lines data with coupled-chemistry-emission model calculations. The comparison between observations and model calculations indicates that the collisional quenching of O(¹S) and O(¹D) by H₂O molecules is significant in the inner coma and cannot be neglected when the G/R ratio is determined. With the measured G/R values close to the nucleus, an estimate of the relative abundance of CO₂ was also derived for all the comets of the sample.

• 4.

  The green line has a much greater width, typically 2.5 km s^-1 , compared with the 1.5 km s^-1 found for the red lines. The greater width of the green line is most likely a result of the involvement of different parent molecules in producing the O(¹S) and O(¹D) atoms (as already suggested by Bhardwaj & Raghuram 2012; Decock et al. 2013): the green oxygen lines are produced by oxygen atoms coming from the photodissociation of both the CO₂ and H₂O molecules, while the red lines are mainly produced by H₂O molecules. While the O(¹S) atoms are mostly provided by CO₂, the contribution of water molecules increases with the nucleocentic distance. This explains the decreasing width of the 5577.339 Å line at distances larger than 500 km.

• 5.

  Measuring the green and red-doublet emission intensities and their line widths is a new way to constrain the CO₂ relative abundances in comets.

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Acknowledgments

A.D. acknowledges the support of the Belgian National Science Foundation F.R.I.A., Fonds pour la formation à la Recherche dans l’Industrie et l’Agriculture. S.R. was supported by ISRO Research associateship during a part of this work. The work of A.B. was supported by ISRO. C. Arpigny is thanked for helpful discussions and important constructive comments. S.R. acknowledges the support of the Science and Technology Facilities Council (STFC) through the Consolidated Grant to Imperial College London.

References

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