CHAPTER 4 ANALYSES
4.2. GAS PRODUCTION RATES
Molecules which are evaporated directly from cometary nuclei are called parent molecules. Here we will apply the Haser model for parent molecular species (Haser 1957) to calculate gas production rates for cometary molecules we detected. Even though CS is known to be a daughter molecule of CS2, due to the very short lifetime of CS2, (~500 sec at 1 AU), we will still treat CS as a parent molecule and apply the Haser model for parent molecules to calculate the CS production rate. Assuming that the parent molecules were sublimated from the nucleus directly outward to the coma, we have the averaged production rate, Q:
40
4 n2
Q= πr E, (4.5)
where E is the evaporation rate (cm-2 s-1) and rn is the radius of cometary nucleus. If the gas expands at a constant speed, Vexp, then the number density, n, is a function of the distance to the nuclear surface, namely, rn. The spherical shell can be presented as:
2
As these mo oving outward, their number densities are also decayed . (4.6)
lecules are m
exponentially to their original abundances and form a new generation of daughter molecules. Thus, Equation 4.6 becomes: center to the edge of the coma. Here the scale length is defined as
4.8) where τ is the molecule lifetime s
photodissociation rate at a distance 1 AU from the quiet Sun (listed in Table 4.2), and
observed comets 73P and Boattini close to the Earth (~0.08 and ~0.21 AU, respectively), the beam sizes (< 103 and < 104 km) of the
les
le length of parent molecules
L = Vexp τ rh2, ( which i equal to the reciprocal of the molecular
rh is the heliocentric distance (in AU).
However, in our case, in which we
te copes are much smaller than the observed molecular scale lengths (typically ~105 km). Thus, the average column density of a particular molecule can be approximated in the following form (Bockelée-Morvan 2005):
exp
(4.9)
where d is the primary beam diameter in linea
densities, we can simply apply Equation (4.9) to obtain Q. All of the calculated results are summarized in
us molecules in a comet or chemical compositions N Q
=V d ,
r scale. With the derived total column
Table 4.2.
Water is known to be the most abundant molecule in comets. When comparing chemical abundances of vario
41
42
between different comets, we usually normalize the gas production rate of a certain molecule measured with water production rate first. To increase the accuracy and minimize possible uncertainties, we normally assume the values of water production rate measured at the time closest to our observations. Thus, for Comet 73P-C, the water production rates adopted for calculations are 1.06 × 1028 sec-1 and 1.26 × 1028 sec-1 (Dello Russo et al. 2007) for May 5, 7 and 9, accordingly, and 1.65 × 1028 sec-1 and 1.43 × 1028 sec-1 (Schleicher et al. 2011) for May 8 and 10, respectively. For comets 73P-B and Boattini, water production rates of 1.9 × 1028 sec-1 (Kobayashi et al.
2007) and 1.22 × 1028 sec-1 (Villanueva et al. 2011), respectively, were employed. The gas production rates relative to water (in percentage) in 73P and Boattini are listed in the last column of Table 4.2.
Table 4.1 Molecular and physical parameters of the detected cometary molecular species.
Molecules Dipole Moments a Rotational Constants a Photodissociation Ratec Lifetimef μa A B C
(Debye) (MHz) (sec-1) (sec)
CS 1.957 - 24495.562 - 1.0 × 10-5 d 1.00 × 105
HCN 2.984 - 44315.975 - 1.3 × 10-5 e 7.69 × 104
a Taken from the JPL molecular database.
c At 1 AU from the quiet Sun. The reciprocal of the molecular photodissociation rate is the lifetime of the molecule.
d Biver et al. (1999).
e Jackson et al. (1982).
f The duration in which the number of certain molecular species is reduced to a factor of e-1 of its original quantity after being released from cometary nucleus.
43
44
Table 4.2 Molecular column densities and gas production rates in comet 73P-B, 73P-C and comet Boattini. Molecules Transition
b The linear scale of the primary beam of KP12M at the comet.
c Scale length of molecules when comets are at 1 AU from the Sun. Values listed were derived using Equation (4.8).
d Column density; Trot = 71 K derived from 73P-C is also adopted for 73P-B, and Trot = 78 K (Villanueva et al. 2011) is assumed for Boattini.
e Gas production rate.
h The water production rates adopted for calculations are: a)for 73P-C, 1.06 × 1028 sec-1 onMay 5 and 7, and 1.26 × 1028 sec-1 (Dello Russo et al. 2007) on May 9, and 1.65 × 1028 sec-1 and 1.43 × 1028 sec-1 (Schleicher et al. 2011) for May 8 and 10, respectively. b) for 73P-B and Boattini, water production rates of 1.98 × 1028 sec -1 (Kobayashi et al. 2007) and 1.16 × 1028 sec-1 (Lippi et al. 2010), on 2006 May 12 and 2008 May 30, respectively, were applied.
Chapter 5
Discussions and Summary
5.1. HCN in Comet 73P/Schwassmann-Wachmann 3
Our pre-perihelion observations of comets 73P-B and 73P-C covering a heliocentric distance from 0.974 to 1.210 AU in early May, 2006 had HCN emission detected.
HCN has been widely seen in JFCs and has been found in more than twenty comets previously regardless which orbital family they belong to. In this study, we report an HCN production rate of 1.92 1.581.08×1025 sec-1 in 73P-Bon 2006 May 13.76.
+−
Listed in Table 5.1 are the published HCN production rates of fragment B from other studies taken on various dates during the 2006 apparition of Comet 73P. We found in general, together with our data, the HCN production rate of 73P-B did show a clear trend of decrease during 2006 May 09–15 when the comet was moving from a distance 1.033 AU from the Sun to 1.000 AU. The reducing HCN gas production while the comet was getting closer to the Sun appears to be counterintuitive. On the other hand, during 2006 May 8–9, 73P-B was reported to experience a sudden gas outburst (Lara et al.2006b, Bertini 2009). However, this outburst event did not last long and the cease of the outburst was noticeable at different wavelengths across most of the EM spectrum, and was also reflected by the sudden plunge of HCN production rates from 6.60×1025 sec-1 on May 10 to 3.23×1025 sec-1 the next day (Table 5.1). Thus it becomes apparent that the decreasing HCN gas production from 2006 May 09 to 16 was largely due to the momentary event of outburst occurred on May 8–9.
45
If we now consider the HCN gas production rate with respect to water, we find the Q(HCN)/Q(H2O) ratio also decreased during the time right after the outburst event from May 10 to 13, but starting from May 14 the ratio soon rose back up. The gas production rates relative to water, Q(X)/QH2O, are often called mixing ratios or, sometimes, simply abundances in cometary study. The nontrivial inconsistency noticed between HCN production rate and HCN mixing ratio suggests HCN molecules may not be well mixed with water ice over the surface layers on the cometary nucleus of 73P-B.
46
For Comet 73P-C, the HCN production rate was relatively more stable during pre-perihelion if compared to 73P-B (Table 5.2). The HCN gas production rate of 73P-C was increased by a factor of ~2 from the early April, 2006, to the early May when the heliocentric distance, rh, of 73P-C shortened from 1.27 AU to 1.04 AU. In 2006 May, 73P-C was about 1.0 AU from the Sun basically and the HCN production rate had been rather steady around 2×1025 sec-1 during the period from 2006 May 05 to May 20, except a couple of irregularities occurred with a factor of 2 surge up to
~4×1025 sec-1 around 2006 May 09. The apparent rising of HCN production rate seen near 2006 May 8 and 9 seems real (Table 5.2).
If we consider our own results only (Table 5.2), we would see a stable value,
~0.10%, of the measured HCN production rate relative to water, i.e., the abundance or mixing ratio, in 73P-C between 2006 May 5.56 and 8.65. However, it is not apparent in the results obtained by Paganini et al. (2010) who also used the SMT to observe 73P but assumed constant water production rates of 1.9×1028 sec-1 and 1.14×1028 sec-1 for comets 73P-B and 73P-C, respectively. In contrast, we adopted the day-to-day water production rates collected from the literature, as shown in Table 5.2, for HCN mixing ratio computation. For the KP12M observations of 73P-C on 2006 May 9.66 and 10.65, the HCN mixing ratios were found to be 0.11% and 0.15%, respectively, which are, again, relative stable during our observation. Our HCN measurements are consistent with the study done by Bockelée-Morvan et al. (2004) who concluded that HCN abundance is nearly constant around ~0.1%, in most comets – including JFCs.
Table 5.3 gives the observed abundances of CS and HCN relative to water of some bright comets in different dynamical groups. Comet 73P broke up further into several more pieces of fragments in its 2006 return and revealed fresh materials once buried deep within the nuclear mantle or beneath the crust. It appears that the HCN mixing ratios of both Fragments B and C of Comet 73P are pretty much consistent with each other. For 73P-B, the HCN mixing ratio is 0.10 with respect to water (H2O
= 100), and for the largest fragment C of 73P, an averaged abundance of 0.11 is derived. The similarity in chemical abundance between these two fragments has also been confirmed by IR observations; Dello Russo (2007) suggested high degree of homogeneity exists among 73P-B and 73P-C. With no detection of CS in 73P and with HCN the only molecule detected securely with similar abundances in 73P Fragments B and C, we conclude that 73P-B and C are of similar chemical composition and have gone through similar chemical evolution since their breakup.
47
5.2. CS in Comet C/2007 W1 (Boattini)
CS is believed to be the daughter molecule of carbon disulfide (CS2). However, the photodissociation lifetime of CS2 is very short (~500 sec at rh = 1 AU). Therefore, when estimating CS production rates, carbon monosulfide may be treated as a parent molecule instead of a daughter molecule without introducing a large error. Meanwhile, CS can still be used as an indicator of the physical environment of the cometary inner coma due to the very short scale length (≦500 km) of CS2. CS is the only molecular species detected securely in Comet C/2007 W1 (Boattini) from our millimeter radio observations.
Lippi et al. (2010) reported detection of some parent molecules, such as HCN, C2H2 and CH4, by infrared observations during 2008 May 11–30 and the first week of June, 2008. In this study, we reported that the CS production rate is of 4.30×1025 sec-1 on 2008 June 14, which corresponds to a mixing ratio of 0.35 (with H2O = 100) in Boattini. To date, there has been no other report of CS detection in Comet C/2007 W1 (Boattini) for further comparison. As CS is only detectable in the ultraviolet and radio spectra, results from our radio observations of CS are thus important in filling out the overall picture of cometary studies in the future.
5.3. Comparisons to Other Comets
HCN is one of the most commonly seen molecules in comets, in situ, and has been detected in over two dozen comets. Table 5.3 (see also Figure 5.1) summarizes the comets in which HCN has been detected, sorted by their orbital categorization, i.e., HTCs, JFCs and OCCs. Even though HCN has a consistent mixing ratio of ~0.1%
(Biver et al, 2002) between these cometary groups, there are still some differences.
Typically, lower HCN abundances are found in HTCs and JFCs compared to OCCs.
For instance, Comet 8P/Tuttle is an HTC and shows a low mixing ratio of 0.07% only, while Comet 6P/d’Arrest, a JFC, has an even lower abundance of 0.03% relative to water. The rest of the short-period comets in Table 5.3, including our 73P measurements, have a ratio around 0.1%, except for Comet 9P/Tempel 1 (hereafter 9P).
Table 5.1. HCN production rates of 73P-B measured on various dates during the 2006 apparition.
a References: [1] Villanueva et al. (2006); [2] Russo et al. (2007);[3] Kobayashi et al. (2007); [4] Paganini et al. (2010).
b Assumed values.
48
Molecule Transition
Table 5.2. HCN production rates of 73P-C measured on various dates during the 2006 apparition.
a References: [1] Villanueva et al. (2006);[2] Paganini et al. (2010); [3] Russo et al. (2007).
49
b Assumed values.
+ Heliocentric distance.
* Spectral observations in infrared.
# Total column density of HCN molecule.
50
When 73P fragmented, it exposed fresh material from the nucleus interior, which represents the (little evolved) original environment in which 73P was formed. Our 73P-B and 73P-C measurements indicate an HCN mixing ratio similar to the average of typical JFCs. The HCN abundances of OCCs are often higher than 0.1%, such as 0.27% for Comet C/1995 O1 (Hale–Bopp), 0.24% for C/2006 P1 (McNaught), and 0.41% for C/2001 Q4 (NEAT). HCN abundances of 73P-B and C are similar to those of other JFCs listed in Table 5.3 (see also Figure 5.1) which implies JFCs were formed in regions of similar distances from the Sun.
Comet 9P was the target of the Deep Impact (DI) mission and was hit by a 364-kg artificial impactor on 2005 July 4 (A’Hearn et al. 2005a, 2005b). Comet 9P revealed its inner layers and the chemical differences between the nucleus layers at the impact. Many observatories monitored the DI event before and after the impact.
Comet 9P had the relatively consistent ratios of HCN/H2O pre- and post-impact, from 0.18% up to 0.21% (Table 5.3), and similarly consistent ratios of CH3OH/H2O.
However, NIR measurements carried out by either DI spacecraft or ground-based observatories indicated an increase in abundances of organic molecules (i.e., CH-X,
~3.4–3.5 μm) and enriched C2H6, post-impact. Furthermore, C2H6/H2O mixing ratios that are somewhat similar to the values in OCCs (Mumma et al. 2005) indicate that cometary chemical compositions are not characteristics of orbital classes. These studies also suggested heterogeneous layers in 9P.
In the case of 73P, the outburst event exposed the inner parts of its nucleus and provided a good opportunity to view the possible similarities to 9P. Both 9P and 73P are JFCs, so one may have expected there to be not much difference between their cometary nuclei. However, even though the HCN/H2O ratios of 73P-B and 73P-C are similar to pre- and post-impact 9P, many studies suggested a homogeneous nucleus for 73P. Infrared observations performed by Dello Russo et al. (2007) toward 73P during its 2006 perihelion showed that 73P-B and 73P-C are remarkably similar, by determining their day-to-day volatile molecular emissions. Thus, our conclusion of the homogeneity of these two fragments 73P-B and 73P-C is supported.
CS was first detected in comet C/1975 V1 (West) in the ultraviolet wavelength (Smith et al. 1976). However, it was not until 1996 that Biver et al. (1999) detected CS in Comet C/1996 B2 (Hyakutake). For comets with shorter periods, i.e., HTCs and JFCs, abundances of water and other molecules are often lower than in OCCs. Most
HTCs and JFCs in Table 5.3 showed no detection of CS, but CS is more often seen in OCCs. We derived a mixing ratio for CS/H2O of 0.35%, which, compared to other comets (Figure 5.2), is a bit high, for instance, Comet C/1995 O1 (Hale–Bopp) and C/2006 M4 (SWAN) showed abundances relative to water of 0.40% and 0.25%, respectively. The rather high abundance of CS obtained could be intrinsic in nature.
However, we cannot rule out the possibility that it may also be largely due to the parent-molecule approximation we adopted.
5.4. Summary
Study of chemical compositions of comets is fundamental to our understanding of the origin and evolution of comets in our Solar System. We thus observed Comet 73P/Schwassmann-Wachmann 3, a Jupiter-family comet which passed the Earth at a distance of mere 0.08 AU in May 2006, and Comet C/2007 W1 (Boattini), an Oort-Cloud comet with an orbital period of about 63,000 years which was at perihelion in 2008 June at a distance of 0.21 AU from Earth when closest. These two comets offered us rare opportunities to investigate cometary chemical compositions in detail.
By using the Submillimeter Telescope (SMT) and the Kitt Peak 12m telescope (KP12M) of Arizona Radio Observatory, we thus observed these two comets during their perihelion and detected CS and HCN emission. We found an HCN production rate of 1.92 ×1025 sec-1 toward 73P-B on 2006 May 13, and of 1.40+−1. –4.26+− × 1025 sec-1 toward 73P-C on 2006 May 5 and between May 7 and 10; yet there was no detection of HCN in the Oort-Cloud comet Boattini. In comparison, the CS 3-2 line was only visible in Boattini (with a production rate of 4.30 ×1025 sec-1) but not in 73P/S-W 3. The c-C3H2 52,3-43,2 line was detected tentatively toward 73P/S-W 3-C on 2006 May 06 but was fading away after the first 18-minute integration; if confirmed, this will be the first detection of cyclopropenylidene in any comet. Searches for H2CO and its isotope HDCO were also attempted without success.
1.58
Compared to other known comets, 73P-B and 73P-C showed an HCN abundance typical to JFCs, 0.10% and 0.11%, respectively. With no detection of CS in 73P, and with HCN the only molecule detected securely in both 73P-B and C with similar abundances, we conclude that these two cometary fragments are of similar chemical composition and have gone through similar chemical evolution since their breakup; in
51
52
addition, it may imply most JFCs were formed in regions at an approximately equal distance from the Sun. Comet C/2007 W1 (Boattini) was observed to have a large quantity of CS molecules presented but with very few HCN molecules. Furthermore, the observed CS mixing ratio of Boattini (0.35%) relative to water was found to be closer to Comet Hale–Bopp (0.40%); our finding made Boattini a very special comet among all comets observed to date, though we cannot rule out the possibility that it may also be largely due to the parent-molecule approximation we adopted.
Table 5.3. The abundances (or mixing ratios) of CS and HCN molecules relative to water (H2O = 100) of comets in different dynamical groups.
Comets Classification CS HCN observations References b
1P/Halley
21P/Giacobini-Zinner 0.08 0.09 IR, Radio [6]
22P/Kopff < 0.15 0.13 Radio [7]
73P/Schwassmann-Wachmann 3-B … 0.10 +0.080.05
− Radio this work
a The mean value obtained by averaging over dates and transitions 3–2 and 1–0.
b [1] Despois et al. (1986); [2] Bonev et al. (2008); [3] Crovisier et al. (2005) [4] Dello Russo et al. (2009); [5] Mumma et al. (2005); [6] Biver et al. (2002); [7] Biver et al. (1997); [8] Mumma et al. (2011) [9] Lewis et al. (2001) [10] Magee-Sauer et al. (1999); [11] Magee-Sauer et al. (2002);
[12] Biver et al. (1999); [13] Bockelée-Morvan et al. (2001); [14] Mumma et al. (2001); [15] Remijan et al. (2006); [16] Chuang et al. (2006); [17]
Benov et al. (2009); [18] Disanti et al. (2009); [19] Dello Russo et al. (2009); [20] Lippi et al. (2010).
53
Figure 5.1. Ratios of HCN production rates relative to water (H2O = 100), namely HCN abundances or mixing ratios, of comets in different dynamical groups (HTCs, JFCs and OCCs). Dash lines denote upper limits and red arrows indicate results from our work.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
55
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
C/2007 W1 (Boattini) C/2006 M4 (SWAN) C/2002 T7 (LINEAR) C/2001 Q4 (NEAT) C/1999 S4 (LINEAR) C/1996 B2 (Hyakutake) C/1995 O1 (Hale–Bopp) 22P/Kopff 21P/Giacobini-Zinner 9P/Tempel 1 (before Deep Impact) 9P/Tempel 1 (after Deep Impact)
OCCs JFCs
Figure 5.2. Ratios of CS production rates relative to water (H2O = 100), namely CS abundances or mixing ratios, of comets in different dynamical groups (HTCs, JFCs and OCCs). Dash lines denote upper limits and red arrows indicate results from our work.
56
Bibliography
A'Hearn, M. F., Millis, R. L., Schleicher, D.G., Osip, D. J., Birch, P. V., 1995, Icarus, 118, 223–
270
Balsiger, H., Altwegg, K., and Geiss, J. 1995, J. Geophys. Res. 100, 5827–5834
Bertini, I., Lara, L. M., Vincent, J. B., Boehnhardt, H., Küpper, M., Rodrigo, R., 2009, Astro.
Astrophys 496, 235–247
Biver, N., Bockelée-Morvan, D., Colom, P., Crovisier, J., Davies, J. K., Dent, W. R., Despois, D., Gérard, E., Lellouch, E., Rauer, H., Moreno, R., Paubert, G., 1997, Science 275, 1915–1918 Biver, N., Bockelée-Morvan, D., Crovisier, J., Davies, J. K., Matthews, H. E., Wink, J. E., Rauer, H., Colom, P., Dent, W. R. F., Despois, D., Moreno, R., Paubert, G., Jewitt, D., and Senayn, M.
1999, Astro. J. 118, 1850–1872
Biver, N., Bockelée-Morvan, D., Crovisier, J., Henry, F., Davies, J. K. Matthews, H., Colom, P., Gérard, E., Lis, D. C., Phillips, T. G., Rantakyrö, F., Haikala, L., Weaver, H.A. 2000, Astro. J. 120, 1554–1570
Biver, N., Bockelée-Morvan, D., Crovisier, J., Colom, P., Henry, F., Lellouch, E., Winnberg, A., Johansson, L. E. B., Gunnarsson, M., Rickman, H., Rantakyrö, F, Davies, J. K., Dent, W. R. F., Paubert G., Moreno, R., Wink, J., Despois, D., Benford, D., Gardner, M., Lis, D.C., Mehringer, D., Phillips, T.G., Rauer, H., 2002, Earth Moon and Planets, 90, 5–14
Biver, N., Bockelée-Morvan, D., Boissier, J., Crovisier, J., Colom, P., Lecacheux, A., Moreno, R., Paubert, G., Lis, D. C., Sumner, M., Frisk, U., Hjalmarson, Å., Olberg, M., Winnberg, A., Florén, H. G., Sandqvist, A., Kwok, S., 2007, Icarus 187, 253–271
Boehnhardt, H., Käufl, H. U. 1995, Comet 73P/Schwassmann–Wachmann 3, IAU Circ. 6274 Bonev, B. P., Mumma, M. J., Radeva, Y. L., DiSanti, M. A., Gibb, E. L., Villanueva, G. L. 2008, ApJ 680, L61–L64
Bonev, B. P., Mumma, M. J., Gibb, E. L., DiSanti, M. A., Villanueva, G. L., Magee- Sauer, K., Ellis, R. S. 2009, ApJ 699, 1563–1572
Bockelée-Morvan, D., Crovisier, J., Colom, P., Despois, D., Perault, M., Irvine, W. M., Schloerb, F. P., Swade, D. 1984, Astro. Astrophys. 141, 411–418
Bockelée-Morvan, D., Colom, P., Crovisier, J., Despois, D., Paubert, G. 1991, Nature, 350, 318–320
Bockelée-Morvan, D., Crovisier, J., Baudry, A., Despois, D. 1994, Astro. Astrophys. 287,
57
647–665
Bockelée-Morvan, D., Gautier, D., Lis, D. C., Young, K., Keene, Y. J., Phillips, T., Owen, T., Crovisier, J., Goldsmith, P. F., Bergin, E. A., Despois, D., Wootten, A. 1998, Icarus 133, 147–
162
Bockelée-Morvan, D., Crovisier, J., Mumma, M.J., Weaver, H.A. 2004, in Comets II, 391–423, (Tucson: Univ. Ariz. Press.)
Capria, M.T., Coradini, A., De Sanctis, M.C., and Orosei, R. 2000, Astro. Astrophys., 357, 359–366
Crovisier, J., 2006, In Astrobiology: Future Perspectives, Astrophysics and Space Science Library. 305, 179–203
Crovisier, J., Biver, N., Bockelée-Morvan, D., Boissier, J., Colom, P., Lis, D.C.,2009, Earth Moon and Planets. 105, 267–272
Crovisier, J., Bockelée-Morvan, D., Biver, N., Colom, P., Despois, D., Lis, D.C. 2004, Astro.
Astrophys. 418, L35–L38
Daniel, W. E. 2006, Comet 73P/Schwassmann–Wachmann, IAU Circ. 8708
Dello Russo, N., Vervack Jr, R. J., Weaver, H. A., Biver, N., Bockelée-Morvan, D., Crovisier, J., Lisse, C. M. 2007, Nature 448, 172–175
Dello Russo, N., Vervack Jr, R. J., Weaver, H. A., Lisse, C. M. 2009, Icarus 200, 271–279
Dello Russo, N., Vervack Jr, R. J., Weaver, H. A., Kawakita, H., Kobayashi, H. Biver, N., Bockelée-Morvan, D,. Crovisier, J. 2009, ApJ 703, 187–197
Despois, D., Crovisier, J., Bockelée-Morvan, D., Schraml, J., Forveille, T., Gérard, E. 1986, Astro.
Astrophys. 160, L11–L12
DiSanti, M. A., Mumma, M. J. 2008, Space Science Reviews 138, 127–145
DiSanti, M. A., Villanueva, G. L., Milam, S. N., Zack, L. N., Bonev, B. P., Mumma, M. J., Ziurys, L. M., Anderson, W. M. 2009, Icarus 203, 589–598
Duncan, M., Quinn, T., Tremaine, S. 1987, Astro. J. 94, 1330–1338 Duncan, M., Levison, H., 1997. Science 276, 1670–1672
Duncan, M., Levison, H., Done, L. 2004, in Comets II, 193–204, (Tucson: Univ. Ariz. Press.) Gomes, R., Levison, H. F., Tsiganis, K., Morbidelli, A. 2005, Science 435, L466–L469
Haser, L., 1957, Bull. Acad. R. Sci. Liege 43, 740–750.
Hitomi, Kobayashi., Hideyo, Kawakita., Michael, J., Mumma, M. J., Boncho, P., Bonev,
58
Watanabe, Jun-ichi., Fuse, Tetsuharu., 2007, Astro. J. 668, L75–L78
Hollenbach, D. J., Tielens, A. G. G. M., 1997, Annu. Rev. Astro. Astrophys. 35, 179–215
Howell, E. S., Nolan, M. C., Harmon, J. K., Lovell, A. J., Benner, L. A., Ostro, S. J., Campbell, D.
B., Margot, J., 2007b, Bull American Astro. Soc. 39, 486 Hsieh, H. H., Jewitt, D., 2006, Science 312, 561–563
Hutsemekérs, D., Manfroid, J., Jehin, E., Arpigny, C., 2009, Icarus 204, 346–348 Jackson, W. M., Butterworth, P. S., Ballard, D., 1982, Astro. J. 304, 515–518
Jackson, W. M., Halpern, J. B., Feldman, P. D., Rahe, J., 1982, Astro. Astrophys. 107, 385–389 Jewitt, D., Moro-Martìn, A., Lacerda, P., 2009, In Astrophysics in the Next Decade: The James Webb Space Telescope and Concurrent Facilities 509
Kobayashi, H., Kawakita, H., Mumma, M. J., Bonev, B. P., Watanabe, J., Fuse, T., 2007, ApJ, 668, L75–L78
Levison, H. 1996. Comet Taxonomy, in Completing the Inventory of the Solar System, vol. 107 of Astronomical Society of the Pacific Conference Proceedings, Rettig, T.W., Hahn, J.M., Eds., pp. 173-191
Levison, H.F., Duncan, M. J., 1997, Science 276, 1670–1672
Levison, H. F., Morbidelli, A., VanLaerhoven, C., Gomes, R., Kleomenis, Tsiganis, K.,2008, Icarus 196, 258–273
Lippi, M., Villanueva, G. L., DiSanti, M. A., Bonev, B. P., Mumma, M. J., Boehnhardt, H., 2010, European Planetary Science Congress 5, 496
Matthews, H. E., Irvine, W. M., 1985, Astro. J. 298, L61–L65
Magee-Sauer, K., Mumma, M., DiSanti, M. A., Dello Russo, N., Retting, T. W., 1999, Icarus 142, 498–508
Meier, R., Owen, T. C., Matthews, H. E., Jewitt, D. C., Bockelée-Morvan, D., Biver, N., Crovisier, J., Gautier, D., 1998, Science 279, 842–844
Mumma, M. J., DiSanti, M.A., Dello Russo, N., Magee-Sauer, K., Gibb, E., Novak, R., 2003, Advances in Space Research 31, 2563–2575
Mumma, M. J., Dello Russo, N., Disanti, M. A., Magee-Sauger, K., Novak, R. E., Brittain, S., Retting, T., McLean, I. S., Reuter, D. C., Xu, L. H., 2001, Science 292, 1334–1339
Mumma, M. J., Bonev B. P., Villanueva G. L., Paganini L., DiSanti M. A., Gibb E. L., Keane J. V., Meech K. J., Blake G. A., Ellis R. S., Lippi M., Boehnhardt H., and Magee-Sauer K., 2011, ApJ
Mumma, M. J., Bonev B. P., Villanueva G. L., Paganini L., DiSanti M. A., Gibb E. L., Keane J. V., Meech K. J., Blake G. A., Ellis R. S., Lippi M., Boehnhardt H., and Magee-Sauer K., 2011, ApJ