EDP Sciences
Free Access
Issue
A&A
Volume 509, January 2010
Article Number A80
Number of page(s) 9
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/200912477
Published online 22 January 2010
A&A 509, A80 (2010)

High-dispersion infrared spectroscopic observations of comet 8P/Tuttle with VLT/CRIRES[*]

H. Kobayashi1 - D. Bockelée-Morvan2 - H. Kawakita1 - N. Dello Russo3 - E. Jehin4 - J. Manfroid4 - A. Smette5 - D. Hutsemékers4 - J. Stüwe6 - M. Weiler7 - C. Arpigny4 - N. Biver2 - A. Cochran8 - J. Crovisier2 - P. Magain4 - H. Sana5 - R. Schulz9 - R. J. Vervack3 - H. Weaver3 - J.-M. Zucconi10,[*]

1 - Department of Physics, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto, Japan
2 - LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
3 - Space Department, The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA
4 - Institut d'Astrophysique et de Géophysique, Sart-Tilman, 4000 Liège, Belgium
5 - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago 19, Chile
6 - Leiden Observatory, 2300 RA Leiden, The Netherlands
7 - GEPI, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
8 - Department of Astronomy and McDonald Observatory, University of Texas at Austin, Austin, USA
9 - ESA/RSSD, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
10 - Observatoire de Besançon, 25010 Besançon Cedex, France

Received 13 May 2009 / Accepted 13 October 2009

Abstract
We report on the composition of the Halley-family comet (HFC) 8P/Tuttle investigated with high-dispersion near-infrared spectroscopic observations. The observations were carried out at the ESO VLT (Very Large Telescope) with the CRIRES instrument as part of a multi-wavelength observation campaign of 8P/Tuttle performed in late January and early February 2008. Radar observations suggested that 8P/Tuttle is a contact binary, and it was proposed that these components might be heterogeneous in chemistry. We determined mixing ratios of organic volatiles with respect to H2O and found that mixing ratios were consistent with previous near infrared spectroscopic observations obtained in late December 2007 and in late January 2008. It has been suggested that because 8P/Tuttle is a contact binary, it might be chemically heterogeneous. However, we find no evidence for chemical heterogeneity within the nucleus of 8P/Tuttle. We also compared the mixing ratios of organic molecules in 8P/Tuttle with those of both other HFCs and long period comets (LPCs) and found that HCN, C2H2, and C2H6 are depleted whereas CH4 and CH3OH have normal abundances. This may indicate that 8P/Tuttle was formed in a different region of the early solar nebula than other HFCs and LPCs. We estimated the conversion efficiency from C2H2 to C2H6 by hydrogen addition reactions on cold grains by employing the C2H6/(C2H6+C2H2) ratio. The C2H6/(C2H6+C2H2) ratio in 8P/Tuttle is consistent with the ratios found in other HFCs and LPCs within the error bars. We also discuss the source of C2 and CN based on our observations and conclude that the abundances of C2H2 and C2H6 are insufficient to explain the C2 abundances in comet 8P/Tuttle and that the abundance of HCN is insufficient to explain the CN abundances in the comet, so at least one additional parent is needed for each species, as pointed out in previous study.

Key words: comets: individual: 8P/Tuttle - planets and satellites: formation - Kuiper belt: general

1 Introduction

Comets are remnants of planetesimals formed in the early solar nebula. Their nuclei consist of volatile ices and dust grains and are thought to preserve primordial information such as temperature, degree of ionization, and chemistry. Comet composition may also provide clues about the dynamical evolution of planetesimals in the early solar system. Comets are dynamically classified by their Tisserand invariants with respect to Jupiter ($T_{\rm J}$): ecliptic comets ( $T_{\rm J} > 2$, here we call them Jupiter family comets: JFCs) and nearly isotropic comets (NICs, $T_{\rm J} < 2$) (Levison 1996). The NICs are further divided into two sub-classes: long period comets (LPCs, semi-major axis (a) > 40 AU) and Halley-family comets (HFCs, a < 40 AU) (Levison 1996). The dynamical origin of LPCs is thought to be the Oort cloud whereas JFCs are thought to originate in the classical Kuiper Belt and/or the scattered disk. The dynamical origin of HFCs is unclear and still under debate. They were injected either into the inner part of the Oort cloud or into the outer part of the scattered disk (Duncan, 2008).

HFCs have long been studied by spectrophotometry at optical wavelengths (e.g., A'Hearn et al. 1995). Optical spectroscopic observations allow us to observe the daughter species produced by photo-dissociation of parent (or grand-parent, etc.) species in the coma, e.g., H2O, HCN, and C2H2. Strong vibrational and/or rotational transitions of parent species are observable in the near infrared and radio regions of the spectrum (Bockelée-Morvan et al. 2004). As an example, Biver et al. (2002) and Crovisier et al. (2008) reported the mixing ratios of parent molecules in four HFCs observed at radio wavelengths. In the near infrared region, DiSanti & Mumma (2008) summarized the mixing ratios of organic volatiles for two HFCs (153P/Ikeya-Zhang and 1P/Halley). Because of the small sample size, the chemistry of HFCs as a group is still not well-characterized.

Our scientific goal in the present work is to determine the relative production rates (mixing ratios) of organic volatiles in the HFC 8P/Tuttle by using high-dispersion spectroscopic observations in the near infrared spectral region. This project is part of a multi-wavelength study of 8P/Tuttle performed at the VLT (see Sect. 2). In the present paper, we will focus on observations taken with the CRIRES spectrometer. In Sect. 3, we will derive rotatinal temperatures and mixing ratios of organic volatiles in comet 8P/Tuttle. Bonev et al. (2008) and Boehnhardt et al. (2008) also reported mixing ratios of organic volatiles in comet 8P/Tuttle from near infrared high-dispersion spectroscopic observations. We compare thses results with ours in Sect. 4.

Table 1:   Overview of the CRIRES observations.

2 Observations and data reduction

Comet 8P/Tuttle was observed at multiple wavelengths using the ESO Very Large Telescope (VLT) located on Cerro Paranal in Chile. We observed this comet using UVES (Unit Telescope (UT) 2) on January 16, 28 and February 4, 2008, and FORS1 (UT2) and CRIRES (UT1), on January 28 and February 4, 2008. The V-band magnitudes of 8P/Tuttle were 7-8 in this period. Here we focus on the near-infrared high dispersion spectroscopic observations with CRIRES. The CRIRES (CRyogenic high-resolution InfraRed Echelle Spectrograph, Käufl et al. 2004) can achieve high resolving power ($\lambda $/ $\Delta \lambda \sim 100~000$ with the slit width of 0.2 $^{\prime\prime}$, i.e., $\Delta\lambda \sim 0.03$ nm at 3000 nm). CRIRES records a single echelle order on a mosaic of four InSb Aladdin III arrays with 27 $\mu$m pixels. The mechanical gaps between 2 arrays correspond to about 280 pixels. The telescope was nodded between two different positions (A and B, separated by 15 $^{\prime\prime}$) in a dithering sequence of ABBA (position A (image ``A1'') $\rightarrow$ position B (image ``B1'') $\rightarrow$ position B (image ``B2'') $\rightarrow$ position A (image ``A2'')) with jittering (small random offset). We used two different wavelength settings on Jan. 28 (a setting #1 for H2O, and a setting #2 for HCN and C2H2) and 3 settings on Feb. 4 (a setting #$1^\prime$ for H2O, a setting #3 for CH4 and C2H6, and a setting #4 for CH3OH). The wavelength coverage of each setting is given in Table 1. The slit widths were 0.2 $^{\prime\prime}$ and 1.0 $^{\prime\prime}$ for the comet and a flux standard star (30 Mon; spectral type: A0V), respectively. The slit length was 31 $^{\prime\prime}$ for both cases.

Data were processed using the Image Reduction and Analysis Facility (IRAF) software package distributed by NOAO. For each sequence of ABBA, we calculated (A1 - B1 - B2 + A2) to remove sky background emission. Wavelength calibration and re-sampling of spectra were performed by comparing measured sky background emission lines and modeled atmospheric radiance spectra. Since we also employed ``jittering'' along the slit (random small offsets at both A and B positions to avoid severe defects on the arrays), we spatially registered the spectra before combining them into a single two-dimensional spectrum (Fig. 1). We extracted a one-dimensional (1-D) spectrum from an aperture (0.2 $^{\prime\prime}$ $\times$ $ \sim$ 0.8 $^{\prime\prime}$) centered on the nucleus.

The 1-D spectrum was flux calibrated as follows. The spectra of the standard star were compared with the synthesized stellar spectra (combined with the telluric absorption at a given airmass by the LBLRTM code, Clough et al. 1995) to evaluate the efficiency of both the telescope and the CRIRES instrument. The flux loss of the standard star at the slit (1.0 $^{\prime\prime}$ slit width) was estimated from the brightness spatial profile of the star along the slit. Since the spatial profile has a concentrated core and a wing part, we modeled the spatial profile as a linear combination of two 2-D Gaussian functions (one for the core and another for the wing part). The throughput of the slit was estimated based on this modeled profile. After that, we calibrated the cometary spectra based on the efficiencies determined for the standard star. Therefore, the telluric absorption was not corrected for the comet at this stage and this would be corrected for each emission line later. The difference in airmass between the comet and the standard star was taken into account by using the LBLRTM code.

The cometary continuum component (reflected sunlight) was modeled as a product of the smoothed continuum and the synthesized atmospheric transmittance calculated by the LBLRTM code. The continuum component was removed by using this modeled spectrum. The calibrated (but not corrected for the telluric absorption) cometary emission spectra are shown in Figs. 2 and 3, and the detected emission lines are listed in Table 2. Additional details of our observing procedure and data analysis can be obtained from Kobayashi et al. (2007) and Kawakita & Kobayashi (2009).

\begin{figure}
\par\includegraphics[width=9cm]{12477fg1.eps}
\end{figure} Figure 1:

Examples of two-dimensional spectra of comet 8P/Tuttle. Spectra shown in a)- d) were taken simultaneously by the four CRIRES arrays on January 28, 2008 using setting #1 (Table 1).

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\begin{figure}
\par\includegraphics[width=9cm]{12477fg2.eps}
\end{figure} Figure 2:

Spectra of comet 8P/Tuttle. Each panel corresponds to each grating setting on each date. Part of spectra indicated by a)- g) are also shown in Fig. 3 for more detail. Solid lines and dashed lines are zero levels and $\pm $$1\sigma $ error levels, respectively in all panels. The field of view of the aperture is $0.2\hbox {$^{\prime \prime }$ }\times \sim 0.8\hbox {$^{\prime \prime }$ }$.

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\begin{figure}
\par\includegraphics[width=9cm]{12477fg3.eps}
\end{figure} Figure 3:

Selected spectra of comet 8P/Tuttle. a) Spectrum of H2O and OH observed on January 28, 2008 (setting #1, see Table 1). b) and c) Spectra of HCN and C2H2 observed on January 28, 2008 (setting #2). d) Spectrum of H2O observed on February 4, 2008 (setting #1'). e)- g) Spectrum with CH4 $\nu _{3}$ R0 line, the $\nu _{2}$ band of CH3OH and the $\nu _{7}$ band of C2H6 observed on February 4, 2008 (setting #3). Solid lines and dashed lines are zero levels and $\pm $$1\sigma $ error levels, respectively in all panels. The field of view of the aperture is $0.2\hbox {$^{\prime \prime }$ }\times \sim 0.8\hbox {$^{\prime \prime }$ }$.

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3 Results

The gas production rates (Q, s-1) were calculated from the observed spectra using modeled fluorescence efficiencies (g-factors) for the appropriate rotational excitation temperature ( $T_{\rm rot}$). Except for CH3OH and C2H6, we used the g-factors derived from the fluorescence excitation models described in Kobayashi & Kawakita (2009).

Ro-vibrational lines from fundamental vibrational bands were observed for HCN, C2H2, CH4, C2H6 and CH3OH. These molecules are pumped from the ground vibrational state to the upper vibrational state by the solar radiation field and then relax down to the ground vibrational state. The population distribution among the rotational levels in the ground vibrational state is described by the Boltzmann distribution at a given rotational temperature $T_{\rm rot}$.

Lines from the fundamental bands of H2O cannot be detected by ground-based observations because of extinction from associated telluric lines. Instead, ``hot-bands'' are routinely observed from ground-based observatories (Dello Russo et al. 2005,2004). The fluorescence excitation model of water hot-bands is based on Dello Russo et al. (2005,2004); H2O molecules are pumped from the ground vibrational state to the upper vibrational states by the solar radiation field and then cascade to the ground vibrational state through the intermediate vibrational states. The population distribution in the ground vibrational state is described by the Boltzmann distribution as for HCN, C2H2, CH4, C2H6, and CH3OH.

The detection of several lines of H2O and HCN allows us to determine the rotational temperature $T_{\rm rot}$ of H2O on Jan. 28 and Feb. 4 and of HCN on Jan. 28. We used the method outlined by Dello Russo et al. (2004). We plotted the ratios of line flux to the corresponding line g-factor (F/g) vs. the upper state rotational energy (E'-E' (lowest)) (Fig. 4). At the appropriate $T_{\rm rot}$, the slope of the line for F/g vs. (E'-E' (lowest)) should be equal to zero because at this temperature F/g should be independent of (E'-E' (lowest)).

We determined the best $T_{\rm rot}$ for H2O by the $\chi^2$-fitting method. Errors ($\pm $$1\sigma $) in $T_{\rm rot}$ and the ortho-to-para ratio (OPR) are estimated based on the reduced $\chi^2$. The $T_{\rm rot}$ of H2O is determined to be $70 \pm 15$ K and 65 +15-13 K for Jan. 28 and Feb. 4, respectively. The OPR for H2O was also determined but it was poorly constrained. The best-fit values are 2.35 +0.77-0.58 and 2.56 +1.50-0.74 on Jan. 28 and Feb. 4, respectively. These values are consistent with the high-temperature limit (OPR = 3) within their uncertainties. Therefore, we assume OPR = 3 in the following part. For HCN, we determined $T_{\rm rot} = 54 \pm 9$ K on Jan. 28 (Fig. 4). The $\pm $$1\sigma $ uncertainty for the $T_{\rm rot}$ is estimated from the error in the slope. The rotational temperatures for H2O and HCN on Jan. 28 are consistent within their uncertainties. The spatial profiles of H2O and HCN are also shown in Fig. 4 and these profiles are consistent with each other. These facts suggest that the rotational excitation of these molecules was controlled by intermolecular collisions in the inner coma, so these molecules were thermalized. Therefore, we assumed a $T_{\rm rot}$ of 70 K on both Jan. 28 and Feb. 4 for molecules where rotational temperatures could not be directly determined (C2H2, CH4, C2H6 and CH3OH).

For C2H6, g-factors of detected lines were provided by Dello Russo et al. (2001) at $T_{\rm rot} = 70$ K. We used those g-factors to determine the mixing ratio of C2H6. Usually, CH3OH production rates from high-resolution infrared measurements are determined from the flux of the $\nu _{3}$ Q-branch. However, the $\nu _{3}$ Q-branch was observed when the comet was at high airmass ($ \sim$2.5), so flux-calibration for this setting was more uncertain. Therefore, a CH3OH production rate was determined from the flux of $\nu _{2}$ lines detected near 3000 cm-1. These same lines were detected in comet C/1999 H1 (Lee) along with the $\nu _{3}$ Q-branch of CH3OH, and fluxes for these lines were reported in Dello Russo et al. (2006). The gas rotational temperatures derived from C/1999 H1 (Lee) were between 70 and 80 K (Mumma et al. 2001a; Dello Russo et al. 2005), similar to the derived gas rotational temperatures reported here for 8P/Tuttle. Therefore, assuming the relative intensities of the $\nu _{2}$ CH3OH lines and the $\nu _{3}$ Q-branch are the same in both comets, effective g-factors could be determined for the $\nu _{2}$ CH3OH lines using the modeled g-factor for the $\nu _{3}$ Q-branch. We used the CH3OH $\nu _{3}$ Q-branch g-factor at 70 K used by Bonev et al. (2008) and Boehnhardt et al. (2008) to enable a direct comparison with their derived CH3OH production rate (DiSanti, private comm.).

\begin{figure}
\par\includegraphics[width=9cm]{12477fg4.eps}
\end{figure} Figure 4:

Rotational temperature analysis of H2O (on January 28 and February 4, 2008) and HCN (on January 28, 2008), and spatial profiles of H2O and HCN ( lower-right panel). For the rotatinal temperature analysis, we plot F/g vs. E'-E'(lowest) (upper state energy), where F is the line flux and g is the line g-factor. If the rotational temperature ( $T_{\rm rot})$ used to compute the g-factors is correct, the line fitted to the F/g values should be flat. The inferred $T_{\rm rot}$ of H2O is $70 \pm 15$ K and 65 +15-13 K on January 28 and February 4, respectively. $T_{\rm rot}$ of HCN is determined as $54 \pm 9$ K. The solid lines in the plots are the averaged F/g values (because F/g is normalized by the averaged F/g, the averaged F/g is unity). The errors-bars of $T_{\rm rot}$ and dashed lines in the figures correspond to $\pm $$1\sigma $ errors. For spatial profiles, the solid blue line corresnoinds to $\rho ^{-1}$ profile ($\rho $: nucleocentric distance), and the error-bars correspond to $\pm $$1\sigma $ errors for each data point.

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Gas production rates are determined based on the isotropic expanding coma model (e.g., Kobayashi et al. 2007). We assumed the expansion velocity of gas to be $0.8 \times r^{-0.5}$ km s-1 (r is the heliocentric distance of the comet in AU). Photodissociation rates of the molecules are taken from Huebner et al. (1992). In order to determine the absolute production rates, the ``Q-curve'' method (correction for the slit-loss of the comet) was used (DiSanti & Mumma 2008). We assumed that the ``growth factor'' (a ratio of the global Q relative to the nucleus-centered Q) is the same for all species observed at the same time. For example, we determined the growth factor of C2H6 from its emission lines and we applied it to CH4 and CH3OH (these emission lines were observed simultaneously) since the emission lines of both CH4 and CH3OH are too weak to determine the growth-factor based on their signals only. We determined the growth-factors of H2O (January 28: 2.22; February 4: 2.55) and C2H6 (3.54) and we applied them to other molecules in each setting. On February 4, the growth-factor of C2H6 is 40% higher than that of H2O. This difference might be caused by the difference of seeing for these settings (seeing is much better on setting #1' than setting #3).

The mixing ratios of parent volatiles in 8P/Tuttle are determined as production rate ratios relative to H2O (i.e., Q(X)/Q(H2O)). For HCN and C2H2, we derived the mixing ratios by comparing directly their emission lines with H2O lines (taken simultaneously, see Table 2). The growth factors are canceled in these cases. In other cases (CH4, C2H6, and CH3OH), the molecular emission lines were not observed with H2O simultaneously and both the growth factors of H2O and each molecular species are used to derive the mixing ratios. The uncertainties in production rates and mixing ratios are dominated by systematic uncertainty (e.g., accuracy of the fluorescence model for each molecule, accuracy of model calculation of the atmospheric transmittance etc.) and not generally by the stochastic noise mainly caused by sky-background emission. We employed the standard deviation of the F/g values as the systematic uncertainty. Production rates and mixing ratios for all detected molecules are listed in Table 3.

Table 3:   Production rates and mixing ratios of comet 8P/Tuttle.

4 Discussion

Radar observations obtained by Harmon et al. (2008) suggest that the nucleus of 8P/Tuttle is a ``contact binary'' with a rotational period of 11.4 h. Because of this, Bonev et al. (2008) hypothesized that the components of 8P/Tuttle might be chemically heterogeneous. Comparing mixing ratios obtained from our observations to those obtained from other high-resolution infrared observations carried out in December 2007 and January 2008 (Boehnhardt et al. 2008; Bonev et al. 2008) suggests no evidence for temporal variation of the coma composition in 8P/Tuttle (Table 4 and Fig. 5) because these mixing ratios are consistent within three $\sigma$ error-bars. This is confirmed by a $\chi^2$ test of hypothsis which, even at the 0.01 significance level, does not allow us to reject the hypothesis of constant mixing ratios. We note that the emission lines used to determine the mixing ratio of CH3OH in our analysis and in others (Boehnhardt et al. 2008; Bonev et al. 2008) were different i.e., the $\nu _{2}$ lines in ours as listed in Table 2 and the $\nu _{3}$ Q-branch in others. This implies that the error of g-factor we used may be larger because we converted the g-factor of the $\nu _{3}$ Q-branch to $\nu _{2}$ lines based on the observations of comet Lee for CH3OH. Since we did not consider the error of the g-factor of the $\nu _{3}$Q-branch and the error for the conversion from the $\nu _{3}$ Q-branch to the $\nu _{2}$ lines, the error of CH3OH mixing ratio might be larger.

Table 4:   Mixing ratios in comet 8P/Tuttle.

\begin{figure}
\par\includegraphics[width=9cm]{12477fg5.eps}
\end{figure} Figure 5:

Comparison of mixing ratios in comet 8P/Tuttle measured at different dates (this work, Bonev et al. 2008; and Boehnhardt et al. 2008, see Table 4). All error-bars are $\pm $$1\sigma $ error levels.

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\begin{figure}
\par\includegraphics[width=9cm]{12477fg6.eps}
\end{figure} Figure 6:

Comparison between mixing ratios measured in 8P/Tuttle (this work) and in other Halley Family comets and 6 Long Period comets (DiSanti & Mumma 2008, see Table 5). The black solid lines are the range of 6 Long Period comets. For the C2H2, C2H6, and CH3OH of C/1999 S4 (LINEAR) is derived the upper limit (these molecules are shown as the ranges with the dashed lines below solid lines). Although all mixing ratios are within the range for 6 LPCs, HCN and C2H2 are at the lower end of each range. In contrast, CH3OH is at the higher end of the range. All error bars are $\pm $$1\sigma $ error levels. Note that there are no reported error bars for some molecular species in the case of comet 1P/Halley (HCN, C2H2, and C2H6). For the HCN in comet 1P/Halley we show the range reported in some reports (see the references in Table 5).

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The absolute production rate of water in late January is almost the same between our work on January 28 UT and Boehnhardt et al. (2008) on January 27 UT. Given the rotation period measured by Harmon et al. (2008) and Lamy et al. (2008), our observations of January 28 and February 4 were separated by $\approx$14.65 rotations, i.e., they were done at rotation phases separated by one third of the nucleus rotation period. The measurements of Boehnhardt et al. (2008) are at phases in between our measurements. Given the uncertainties in the rotation period of 8P/Tuttle, the rotation phase of the December observations (Bonev et al. 2008) cannot be properly scaled with respect to our measurements. If the nucleus had two components of significantly different chemical composition, as suggested by Bonev et al. (2008), variations of the coma composition should have been observed. Based on these near-IR measurements, there is no evidence for chemical heterogeneity within the nucleus of 8P/Tuttle.

We also compared the mixing ratios with other HFCs and LPCs listed in DiSanti & Mumma (2008), and summarized them in Table 5 and Fig. 6. Although the black bars shown in Fig. 6 indicate a wide range (by a factor of $ \sim$10) of mixing ratios, most of this variation is caused by C/1999 S4 (LINEAR) and C/2001 A2 (LINEAR). The former is highly depleted in organic volatiles while the latter is rich in organics (see Table 5). From near-IR measurements of comets, some chemical groups are proposed by Mumma et al. (2009); organics-enriched, -normal, and -depleted group. Comet C/1999 S4 (LINEAR) and C/2001 A2 (LINEAR) represent comets of the organics-depleted and the organics-enriched group, respectively. On the other hand, Crovisier et al. (2009) reported that there is no clear evidence of such grouping by chemistry in radio observations. The chemical taxonomy of comets is still under debate and the lack of samples makes this problem more complex. Therefore, we plotted the mixing ratios of comets without such a grouping, just as a range. In comparison to other HFCs (1P/Halley and 153P/Ikeya-Zhang), 8P/Tuttle is strongly depleted in C2H2 and may also be depleted in HCN and C2H6. On the other hand, in comparison to 6 LPCs, the mixing ratios in comet 8P/Tuttle are within the ranges typically seen, with HCN and C2H2 at the depleted end while CH3OH is in the upper range.

Table 5:   Mixing ratios of comet 8P/Tuttle, other HFCs and LPCs.

\begin{figure}
\par\includegraphics[width=9cm]{12477fg7.eps}
\end{figure} Figure 7:

The C2H6/(C2H2+C2H6) ratio in comets listed in Table 6. The C2H6/(C2H2+C2H6) ratio is considered as the conversion rate from C2H2 to C2H6 by hydrogen-addition reactions on cold grains in the solar nebula (or in the molecular cloud). All comets show similar conversion rates. The solid line shows the mean value of the C2H6/(C2H2+C2H6) ratio and dashed lines show the $\pm $$1\sigma $ error levels of the C2H6/(C2H2+C2H6) ratio. All error-bars are $\pm $$1\sigma $ error levels.

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A similar composition was also reported by Bonev et al. (2008). They suggested that the composition of 8P/Tuttle was unusual and that it might be caused by chemical heterogeneity in the nucleus if the binary components are chemically distinct. However, as we mentioned above, the mixing ratios of species in 8P/Tuttle are not atypical and there is no evidence of heterogeneity in chemistry by comparing the composition on different dates. Does the chemistry of 8P/Tuttle suggest a different formation region than the HFCs and LPCs? HFCs and LPCs are thought to have formed in the region from 5-30 AU in the early solar nebula. JFCs likely formed in a more compact region (16-30 AU) (Morbidelli et al. 2008). While 8P/Tuttle may have formed in a different region of the early solar nebula from other HFCs and LPCs, based on the derived volatile mixing ratios there is no conclusive evidence.

Bonev et al. (2008) reported that C2H6 might be formed from C2H2 by hydrogen atom addition reactions, and that this conversion may have occurred with higher efficiency in 8P/Tuttle than in typical OC comets. Our observations show no conclusive evidence for this hypothesis. From our detection of C2H2 we determined that the mixing ratio of C2H2 to H2O is consistent with the upper-limit reported in Bonev et al. (2008). In order to test this hypothesis, we use the C2H6/(C2H6+C2H2) ratio to estimate the conversion rate from C2H6 to C2H2. We compared the ratio obtained in 8P/Tuttle with other HFCs and LPCs listed in Table 5. Table 6 and Fig. 7 show the C2H6/(C2H6+C2H2) ratios in these different comets. The conversion efficiency from C2H2 to C2H6 in 8P/Tuttle is consistent with the values obtained in other HFCs and LPCs within the limitations of sample size (within the 99% confidence limit (hereafter, c.l.), see Fig. 7).

Bonev et al. (2008) also reported that the mixing ratios of HCN and C2H2 are inconsistent with these species being the only native precursors of the CN and C2 radicals observed at optical wavelengths. We computed log10[Q(HCN)/Q(H2O)], log10[Q(C2H2)/Q(H2O)] and log10[Q(C2H2)/Q(HCN)] and compared our results with optical results (see Table 7), including a measurement of the C2/CN ratio at a time coincident with our CRIRES observations (Jehin et al. 2009). Possible precursors of C2 observed in the near infrared are C2H2 and C2H6. Although C2H6 is not a direct parent of C2, it may contribute to its abundance. According to Helbert et al. (2005), C2H6 can photodissociate to C2H5 ($ \sim$33%) and C2H4 ($ \sim$37%). All C2H5 and a fraction of C2H4 ($ \sim$44%) photodissociate to C2H2. Finally, all of C2H2 is photodissociated to C2. Based on this C2 is formed from C2H6 at about 50 % yield ($ \sim$33% (C2H $_{6} \rightarrow$ C2H5  $\rightarrow$ C2H2) + $ \sim$16% (C2H6  $\rightarrow$ C2H $_{4} \rightarrow {\rm C}_{2}$H2)). We note that this is only an approximation as these branching ratios are uncertain and dependent on the wavelength of the irradiating photon (Helbert et al. 2005). Assuming the branching ratios are correct, the mixing ratio log10[Q(C2)/Q(OH)] $ \sim$ log10[(Q(C2H2) + 0.5Q(C2H6))/Q(H2O)] if C2H2 and C2H6 are the primary parents of C2. However, log10[Q(C2)/Q(OH)] is significantly larger than log10[(Q(C2H2) + 0.5Q(C2H6))/Q(H2O)], and this is true even assuming that C2H6 completely photodissociates to C2. Our derived mixing ratio for HCN is also too low to explain the CN abundance in 8P/Tuttle (e.g., log10[Q(CN)/Q(OH)] > log10[Q(HCN)/Q(H2O)]). A direct comparison of the HCN and CN production rates measured simultaneously at the VLT on January 28 yields CN/HCN $ \sim$ 3 (Jehin et al. 2009). To explain the CN and C2 abundances in 8P/Tuttle, at least one additional precursor of both CN and C2 is needed. Our results agree with the conclusion of Bonev et al. (2008).

The discrepancy between gas production rates of parents and their daughter species was reviewed by Fray et al. (2003) for HCN and CN. They compared HCN and CN production rates in eight comets and they found that the CN production rates are significantly higher than those of HCN in four comets while the CN and HCN production rates are similar to each other in other comets. An additional process is required for the CN production other than HCN photolysis in the former case.

Table 6:   C2H6/(C2H6+C2H2) ratios in comets.

Table 7:   HCN/H2O (CN/OH), C2H2/H2O (C2/OH), and C2H2/HCN (C2/CN) ratios in comet 8P/Tuttle.

5 Summary and conclusion

We observed comet 8P/Tuttle on January 28 and February 4 with the ESO VLT. Our observations were carried out in multiple spectral regions (optical and near IR), with this paper focusing on high dispersion spectroscopic observations of near IR data obtained with CRIRES. We determined the rotational temperatures for H2O and HCN, and the production rates and abundance ratios of several parent volatiles in 8P/Tuttle. We compared our results to other high dispersion near IR spectroscopic observations reported by Bonev et al. (2008) and Boehnhardt et al. (2008). Mixing ratios obtained from both studies are consistent with each other and we found no evidence of chemical heterogeneity from these observations. We also compared our results to those for other HFCs and LPCs. 8P/Tuttle is strongly depleted in C2H2 and may also be depleted in HCN and C2H6 relative to other HFCs. On the other hand, relative abundances of volatiles in 8P/Tuttle are not atypical when compared to LPCs, with C2H2 and HCN on the depleted end and CH3OH slightly enhanced. Although it is possible that the formation region of 8P/Tuttle is different from the formation regions of other LPCs and HFCs, we note that diverse chemistry is also seen within the Oort cloud population. We determined conversion efficiencies for hydrogen atom addition reactions through the C2H6/(C2H6+C2H2) ratio in 8P/Tuttle and other comets. We found that C2H6/(C2H6+C2H2) in 8P/Tuttle is consistent with other LPCs and HFCs (99% c.l.) contrary to the high conversion efficiency reported by Bonev et al. (2008). However, we note that it is difficult to discern any trends as the number of comets where C2H6/(C2H6+C2H2) ratios are measured is small. It is generally assumed that CN is formed directly from HCN and that C2 is formed directly from C2H2 and indirectly from C2H6 in cometary comae. However, Fray et al. (2003) reported a discrepancy between CN and HCN production rates in four comets (suggesting an additional process is required for the CN production). In the case of 8P/Tuttle the abundances of HCN, C2H2 and C2H6 are insufficient to explain the CN and C2 abundances. So we concluded that at least one additional parent is needed for each species, as pointed out by Bonev et al. (2008).

Acknowledgements
The authors thank the anonymous referee for helpful comments. H. Kobayashi is JSPS fellow of the Ministry of Education, Science, Sports and Culture (Japan). EJ is Research Associate FNRS (Belgium), JM is Research Director FNRS and DH is Senior Research Associate FNRS.

References

Table 2:   Line identification of detected emissions.

Footnotes

... VLT/CRIRES[*]
Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Prog. 080.C-0615 and 280.C-5053).
...[*]
We regret to note the death of Dr. J. -M. Zucconi in 2009 May.

All Tables

Table 1:   Overview of the CRIRES observations.

Table 3:   Production rates and mixing ratios of comet 8P/Tuttle.

Table 4:   Mixing ratios in comet 8P/Tuttle.

Table 5:   Mixing ratios of comet 8P/Tuttle, other HFCs and LPCs.

Table 6:   C2H6/(C2H6+C2H2) ratios in comets.

Table 7:   HCN/H2O (CN/OH), C2H2/H2O (C2/OH), and C2H2/HCN (C2/CN) ratios in comet 8P/Tuttle.

Table 2:   Line identification of detected emissions.

All Figures

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg1.eps}
\end{figure} Figure 1:

Examples of two-dimensional spectra of comet 8P/Tuttle. Spectra shown in a)- d) were taken simultaneously by the four CRIRES arrays on January 28, 2008 using setting #1 (Table 1).

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg2.eps}
\end{figure} Figure 2:

Spectra of comet 8P/Tuttle. Each panel corresponds to each grating setting on each date. Part of spectra indicated by a)- g) are also shown in Fig. 3 for more detail. Solid lines and dashed lines are zero levels and $\pm $$1\sigma $ error levels, respectively in all panels. The field of view of the aperture is $0.2\hbox {$^{\prime \prime }$ }\times \sim 0.8\hbox {$^{\prime \prime }$ }$.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg3.eps}
\end{figure} Figure 3:

Selected spectra of comet 8P/Tuttle. a) Spectrum of H2O and OH observed on January 28, 2008 (setting #1, see Table 1). b) and c) Spectra of HCN and C2H2 observed on January 28, 2008 (setting #2). d) Spectrum of H2O observed on February 4, 2008 (setting #1'). e)- g) Spectrum with CH4 $\nu _{3}$ R0 line, the $\nu _{2}$ band of CH3OH and the $\nu _{7}$ band of C2H6 observed on February 4, 2008 (setting #3). Solid lines and dashed lines are zero levels and $\pm $$1\sigma $ error levels, respectively in all panels. The field of view of the aperture is $0.2\hbox {$^{\prime \prime }$ }\times \sim 0.8\hbox {$^{\prime \prime }$ }$.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg4.eps}
\end{figure} Figure 4:

Rotational temperature analysis of H2O (on January 28 and February 4, 2008) and HCN (on January 28, 2008), and spatial profiles of H2O and HCN ( lower-right panel). For the rotatinal temperature analysis, we plot F/g vs. E'-E'(lowest) (upper state energy), where F is the line flux and g is the line g-factor. If the rotational temperature ( $T_{\rm rot})$ used to compute the g-factors is correct, the line fitted to the F/g values should be flat. The inferred $T_{\rm rot}$ of H2O is $70 \pm 15$ K and 65 +15-13 K on January 28 and February 4, respectively. $T_{\rm rot}$ of HCN is determined as $54 \pm 9$ K. The solid lines in the plots are the averaged F/g values (because F/g is normalized by the averaged F/g, the averaged F/g is unity). The errors-bars of $T_{\rm rot}$ and dashed lines in the figures correspond to $\pm $$1\sigma $ errors. For spatial profiles, the solid blue line corresnoinds to $\rho ^{-1}$ profile ($\rho $: nucleocentric distance), and the error-bars correspond to $\pm $$1\sigma $ errors for each data point.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg5.eps}
\end{figure} Figure 5:

Comparison of mixing ratios in comet 8P/Tuttle measured at different dates (this work, Bonev et al. 2008; and Boehnhardt et al. 2008, see Table 4). All error-bars are $\pm $$1\sigma $ error levels.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg6.eps}
\end{figure} Figure 6:

Comparison between mixing ratios measured in 8P/Tuttle (this work) and in other Halley Family comets and 6 Long Period comets (DiSanti & Mumma 2008, see Table 5). The black solid lines are the range of 6 Long Period comets. For the C2H2, C2H6, and CH3OH of C/1999 S4 (LINEAR) is derived the upper limit (these molecules are shown as the ranges with the dashed lines below solid lines). Although all mixing ratios are within the range for 6 LPCs, HCN and C2H2 are at the lower end of each range. In contrast, CH3OH is at the higher end of the range. All error bars are $\pm $$1\sigma $ error levels. Note that there are no reported error bars for some molecular species in the case of comet 1P/Halley (HCN, C2H2, and C2H6). For the HCN in comet 1P/Halley we show the range reported in some reports (see the references in Table 5).

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm]{12477fg7.eps}
\end{figure} Figure 7:

The C2H6/(C2H2+C2H6) ratio in comets listed in Table 6. The C2H6/(C2H2+C2H6) ratio is considered as the conversion rate from C2H2 to C2H6 by hydrogen-addition reactions on cold grains in the solar nebula (or in the molecular cloud). All comets show similar conversion rates. The solid line shows the mean value of the C2H6/(C2H2+C2H6) ratio and dashed lines show the $\pm $$1\sigma $ error levels of the C2H6/(C2H2+C2H6) ratio. All error-bars are $\pm $$1\sigma $ error levels.

Open with DEXTER
In the text


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