CHAPTER 5 DISCUSSIONS AND SUMMARY
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
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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).
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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.
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