DATA REDUCTION

The 1.3-, 2-, and 3-mm observations were performed using the SMT and KP12M dual-polarization SIS mixers operating in single-sideband mode. System temperatures ranged from 100–500 K with typical image sideband rejections greater than 20 dB. For all measurements, the chopper-wheel method (Penzias and Burrus 1973) was introduced, which is a way to measure ambient temperature toward the sky using a chopper or a vane absorber as the calibration signal.

The temperature scale achieved by the SMT is the observed source antenna temperature, TA*, which is corrected for atmospheric attenuation, radiative loss, rearward scattering and spillover. For column density calculation, in which we need to know the source radiation temperature (TR) or the main-beam brightness temperature (Tmb), one can use the formula TR =TA*/ ηb, where ηb is the main beam efficiency; according to the SMT website, the average value of ηb is 0.74 over the 1-mm band (Figure 2.3).

The KP12M intensity scale is the observed source antenna temperature, TR*, which is again corrected for atmospheric attenuation, radiative loss, rearward and forward scattering, and spillover. Compared to the TA* used at the SMT, TR* includes in it forward scattering losses. To derive the radiation temperature, TR* was converted using the formula TR =TR*/ ηc, where ηc is the corrected main beam efficiency, which is available in the KP12M user manual (Table 2.8).

Data reduction was carried out using the Continuum and Line Analysis Single-dish Software (CLASS) package, which was developed at IRAM. For a given molecular spectral line, we first added all scans that were collected, and then removed continuum baselines. Compared to rms noises, spectral features with intensities greater than 3σ were identified as firm detections, while no-detections correspond to those lower than 3σ.

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Table 2.8. KP12M telescope corrected main beam efficiencies.

Frequency (GHz)

Beam width

(arcsec) ηc a

70 90 0.98

90 70 0.95

115 55 0.85

145 43 0.80

a Corrected main beam efficiency (percentage of power in the main diffraction beam relative to the outlying error beam).

Figure 2.3. The SMT main beam efficiencies by measuring Mars and Saturn were found in fall 2007–spring 2008 across 1 mm, and the average main beam efficiency is 0.74. (Taken from ARO website: http://aro.as.arizona.edu/smt_docs/smt_beam_eff.htm.) This value is in agreement with the Ruze formula, which is related to the gain of an antenna and the rms of the random surface errors, and is applied to calculate telescope main beam efficiencies here.

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Chapter 3 Results

3.1. Carbon Monosulfide

Carbon monosulfide (CS) was first detected in the comet 1976 VI (West) in rocket ultraviolet spectrum. (Smith et al. 1980). Since then, thanks to the work of the IUE satellite, CS at (0,0), (1,0), and (0,1) bands have been detected in many other comets (Weaver et al. 1981). Observations toward Comet 1979 X (Bradfield) suggested that CS was not released from the nucleus itself, but rather from the dissociation of other bigger parent molecules, such as carbon sulfide (CS2) (Jackson et al. 1982). However, the very short lifetime of CS2 makes the detection of this molecule less likely.

Therefore, like other radio astronomers, our observations toward 73P-B, 73P-C, and C/2007 W1 (Boattini) focused on the CS lines.

CS J = 2–1 and J = 3–2 transitions were observed on 2006 May 14, toward 73P-B and 73P-C, respectively (Tables 3.1 & 3.2). CS J = 2–1 was searched for toward 73P-B earlier in the day of May 14, but we did not detect CS at this transition.

We then tuned to a higher frequency for the J = 3–2 transition and changed our target to the larger component, 73P-C. As 73P-C is the main component of Comet 73P after its fragmentation in 1995, we expected a higher possibility of detection of CS;

however, we did not find any toward 73-P.

CS J = 3–2 was also searched for on 2008 June 14 in the long-period comet C/2007 W1 (Boattini), and CS was clearly detected this time (Figure 3.1; Table 3.3).

CS has also been detected in several other comets, such as Comet Hale–Bopp (Biver et al. 1999), Comet Hyakutake (Biver et al. 1997) and Comet Lee (Biver et al. 2000).

Our detection of CS in Comet Boattini can be used to compare chemical compositions and abundances in OCCs, JFCs, and in other orbital groups.

3.2. Hydrogen Cyanide and its

N Isotopomer

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HCN has been widely observed in many OCCs; in fact, hydrogen cyanide is the best studied cometary molecule. HCN is an abundant cometary parent molecule; more line was clearly detected In both fragments. In 73-B, a slightly blue-shifted (0.1 km s^-1) line profile was revealed (Figure 3.2), which could be due to small fragmentation events that were still active in the 73-B component (Vincent et al. 2009). Toward 73P-C, the line intensity illustrated a trend of weakness from observations on May 9 and 10 to May 13 (Figure 3.3). During these three dates, 73P was moving away from the Sun while closer to the Earth. Since heating from the Sun governs cometary activities dominantly, the increasing heliocentric distance may be a plausible cause of the diminishing line intensity observed.

Because of the nitrogen atom in the molecule, the HCN J = 1–0 ―line‖ actually includes three transitions of hyperfine structure with F = 0–1, F = 2–1 and F = 1–1 which are discernible at high enough spectral resolution (Figure 3.2 and 3.3). In an optically thin case, the expected line intensity ratio between these three hyperfine components of F = 0–1, 2–1 and 1–1 would be about 1:5:3 accordingly in the local thermodynamic equilibrium (LTE) condition; however, due to the weakness of the HCN emission and high noise levels of the observed spectra, except the strongest hyperfine transition at F = 2–1, the remaining two hyperfine components at F = 1–1 and 0–1 were not visible in the HCN spectrum taken toward 73P-B. Likewise, the line intensity ratio of the hyperfine components of HCN toward 73P-C was not apparent either, even though the F = 1–1 transition was revealed marginally on May 9.7 and was securely detected on May 10.7. The HCN emission appeared to be fading during the time of our observations and disappeared completely on May 13.7.

On the other hand, the HCN J = 3–2 line at 265.8 GHz was clearly detected toward 73P-C sometime earlier on 2006 May 5, 7 and 8 (Figure 3.4). There was no obvious temporal variation in the observed HCN line intensity for the duration in early May; the primary beam of the SMT was of 28˝or ~1,800 km in linear size at the time, with 1˝ corresponding to a linear scale of ~65 km when the comet was ~0.09 AU away. The nearly constant HCN 3–2 intensity indicates that our beam is smaller than the gas outflow shell of HCN and we were looking into the inner coma of the comet.

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HCN is thought to be a parent molecule and is released from comet nucleus directly into its expanding atmosphere (Bockelée-Morvan et al. 1984). The double-peak line profile of HCN (visible in Figure 3.4) may imply an expanding inner coma with a shell-like structure. The stronger red-shifted line component of the HCN spectral profile, when compared to the blue-shifted one, seen on 2006 May 5.56 may suggest that some additional HCN molecules could have been evaporated directly from the red-shifted dust tail.

The detection of HCN in the J = 3–2 transition in 73P-C inspired us to observe its isotopomer, HC¹⁵N. We would be able to derive the ¹⁴N to ¹⁵N isotope ratio if we had HC¹⁵N detected. The cometary ¹⁴N/¹⁵N ratio can be a useful tool to understand the physical conditions in the early Solar System (Hutsemekérs et al. 2009); a ¹⁴N/¹⁵N ratio of about 330 was derived in the long-period comet Hale-Bopp (Jewitt et al. 1997;

Ziurys et al. 1999). However, we did not detect HC¹⁵N in both 73P-B and 73P-C on 2006 May 13, and only upper limits were obtained (Tables 3.1 & 3.2).

The HCN J = 1–0 transition was also observed toward Comet C/2007 W1 (Boattini) with the KP12M on 2008 June 14 (Table 3.3); however, we did not detect HCN in Boattini during this period of time. The strong HCN J = 3–2 line detected in 73P encouraged us to observe this higher transition of HCN in Boattini. Unfortunately, due to an instrumental failure of the SMT, we were not able to observe the HCN 3–2 line in Comet Boattini. Observations of the HC¹⁵N isotope of HCN were also attempted yet were unsuccessful. The non-detection of HCN 1–0 emission in Boattini is kind of unusual as HCN has been found in abundance in most comets. The non-detection of the three hyperfine components of HCN 1–0 transition may be mainly due to the bad weather experienced. During the HCN observation, the only available planet then, Uranus, was not bright enough in a poor weather condition, therefore good focus and pointing were not attainable. Another factor causing the HCN non-detection may be instrumental. Channel-1 receiver was very unstable and jumpy during the run so channel-1 data are not useable through out the run which reduced our integration time to half equivalently hence made HCN data noiser.

3.3. Formaldehyde and Deuterated Formaldehyde

Formaldehyde, H2CO, and its deuterated isotopomer HDCOwere searched for in both 73P and Boattini without success. We looked for formaldehyde emission at different H2CO transitions in 73P-C on 2006 May 5, 6, 9 and 14. Cometary formaldehyde was

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first identified in Comet Halley and its spatial distribution implies the presence of an extended source of H2CO, e.g. native polymerized H2CO or POM (polyoxymethylene) in the coma (Bockelée-Morvan et al. 2004). Although H2CO has often been seen in many comets, we did not detect H2CO in 73P-C (Table 3.2). The non-detection could be due to instrumental limitations or simply because 73P-C was too small and too faint and contained very little H2CO. Therefore it was no surprise that we did not detect the deuterated H2CO isotope, HDCO, at the same 21,1–11,0 transition at 134,284 MHz in 73P-C either on 2006 May 14 (Table 3.2).

The H2CO 21,1–11,0 transition at 150,498 MHz was also searched for in comet C/2007 W1 (Boattini) during 2008 June 17, 19, 20 and 22. Despite our long-hour integration, no H2CO emission was obvious at this transition (Table 3.3). We also observed HDCO emission in Boattini at the same 21,1–11,0 transitionon 2008 June 19 and 22 (Table 3.3). On June 19, a slightly blue-shifted (Figure 3.5), likely spectral feature of HDCO at a cometocentric velocity of -1.2 km s^-1 was marginally detected but faded away over time. As mentioned previously, comet C/2007 W1 (Boattini) was rather active still which could explain the marginal detection of HDCO; on June 22, there was no more HDCO line profile visible.

3.4. Deuterated Water and Hydrogen Cyanide

In addition to HDCO (the deuterated formaldehyde, see Section 3.3), deuterated hydrogen cyanide (DCN or deuterium cyanide) was also searched for in 73P-C without success on 2006 May 6 in an attempt to derive the cometary D/H ratio of JFCs. Deuterium has been detected in only a few comets, and D/H ratios have only been derived in several others — Comet 1P/Halley (Balsiger et al. 1995), Comet C/1995 O1 Hale–Bopp (Meier et al. 1998), and Comet C/2006 B2 Hyakutake (Bockelee-Morvan et al. 1998) — and none of them is a Jupiter-family member. In fact, even isotope ratios in JFCs are poorly known due to their weak activities. We expect that, however, more advanced instruments (like the ALMA array), which have higher sensitivities, will be much more efficient and effective for observing isotopes in JFCs.

Deuterated water, HDO, was also searched for in 73P-C on 2006 May 8 and 10.

The 1.3-mm 21,1–21,2 and 2-mm 42,2–42,3 transitions of HDO were observed with the SMT and KP12M, respectively. On 2006 May 8, observations of the 21,1–21,2

25

transition of HDO was carried out toward 73P-C with no luck; however, a 3-σ upper limit of line intensity was determined. On 2006 May 10.5, more observations of the HDO 42,2–42,3 transition at 143,727 MHz toward 73P-C resulted in, unfortunately, a 2-σ blue-shifted spectral feature (Figure 3.6). Obviously deeper observations with much longer integration time should have been contributed for a firm detection.

3.5. Cyclopropenylidene (c-C

H

)

With its ring structure and many rotational transitions in radio, cyclopropenylidene (c-C3H2) has been observed already in many star-forming regions in our galaxy, such as in the TMC-1 dark cloud (Matthews and Irvine, 1985). However, c-C3H2 has never been detected in any comets to date.

Complex molecules with large structure are not often seen in comets, and the existence of complex organic molecules in comets may provide new evidence for an entirely different scenario for the chemical evolution of comets which is important for astrobiology study. As discussed in Section 3.3, for instance, the spatial distribution of H2CO disclosed so far indicates formaldehyde may not be a parent molecule, but rather a daughter molecule coming from an extended source of H2CO in the coma which may be either (native) polymerized H2CO, POM (polyoxymethylene), or even POM-like polymers (Cottini et al. 2004). Likewise it seems likely that cometary c-C3H2 would be produced through the photolytic decomposition of hydrocarbon polymers or aromatic compounds, as this has been suggested as a possible origin for c-C3H2 in UV-irradiated interstellar sources (e.g. Pety et al. 2005). Therefore, cometary c-C3H2 can be a good tracer for studying its precursor hydrocarbon polymers or aromatic compounds in comets.

We observed multiple transitions of c-C3H2 on 2006 May 6, 7 and 9 toward 73P-C. The c-C3H2 line of the 52,3–43,2 transition was tentatively detected on May 6.47 in an 18-minute observation (Figure 3.7). Shown in Figure 3.6 are the c-C3H2 spectra taken at 18-minute interval. c-C3H2 was clearly detected in the 1^st 18-minute interval;

and noticeably the c-C3H2 intensity dropped with time, and the c-C3H2 emission was no longer visible after 36 minutes and could not be further detected in the following days. Evidently the intensity of c-C3H2 was a function of time and dropped as the time went by. Since the linear scale of our beam size was only ~1,700 km in diameter at the

26

time of the c-C3H2 detection, a gas outburst at ~1 km s^-1 would escape from our beam within ~20 minutes. Therefore, it is very likely that we happened to catch the c-C3H2

gas outburst activity in 73P-C in time.

3.6. Methanol (CH

OH)

Methanol, CH3OH, was first reported in observations of Comet 1P/Halley, in its low-resolution spectra (Knacke et al. 1986). Further observations of other comets have also identified spectral features at 3.52 µm as CH3OH line (Hoban et al. 1991).

By using the IRAM 30-m telescope, Bockelée-Morvan et al. (1991, 1994) detected several CH3OH rotational lines toward Comets C/1989 X1 (Austin) and C/1990 K1 (Levy) in the radio spectrum as well.

Cometary methanol was searched for at a favorable 50,5–5-1,5 transition at 157,179 MHz with an upper energy level (Eu) of 48 K toward 73P-C on 2006 May 10.

No CH3OH detection was found. We then checked the wideband receivers MAC of the KP12M and AOS of the SMT for possible chance interlopers of methanol while observing other molecular lines. After careful checking the likely favored CH3OH transitions with their rest frequencies by chance also included in these wideband spectral windows, we found no methanol lines at all in either fragments of 73P or Boattini. It is obvious now that both comets 73P (with a diameter of the nucleus less than 0.5 km) and Boattini were too small thus too faint to be good targets for radio observations even though they were extremely close to the Earth (within 0.2 AU).

27

Figure 3.1. The CS J = 3–2 transition observed with the KP12M telescope toward the comet C/2007 W1 (Boattini), on 2008 June 14. The rms noise level is of 31 mK in the T_R* scale at a spectral resolution of 250 kHz ch^-1. CS was detected only in Comet Boattini, but not in both components B or C of the comet 73P.

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Figure 3.2. The HCN spectrum of the J = 1–0 transition observed toward the comet 73P-B on 2006 May 13 with the KP12M telescope, a slightly blue-shifted (0.1 km s^-1) line profile was revealed. The spectral resolution is at 100 kHz ch^-1 and the rms noise level is of 33 mK. The three transitions of the hyperfine structure of HCN due to nuclear spin of nitrogen atom are denoted by the fiducial marks, from the left to right, at F = 0–1, 2–1 and 1–1.

29

Figure 3.3. HCN J = 1–0 spectra observed toward Comet 73P-C on 2006 May 09, 10 and 13 (top to bottom). The three transitions of the hyperfine structure of HCN due to nuclear spin of nitrogen atom are denoted by the fiducial marks, from the left to right, at F = 0–1, 2–1 and 1–1 in each panel. The spectral resolution is 100 kHz ch^-1 for all spectra, with noise levels in the T_R* scale at 17, 14 and 19 mK in the top, middle and the bottom panels, respectively.

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Figure 3.4. HCN J = 3–2 spectra observed toward 73P-C on 2006 May 5, 7 and 8. The double-peak HCN line profiles are apparent which suggest an expanding shell-like structure of HCN emission. The spectral resolution is of 93.6 kHz ch^-1 (smoothed) for all spectra, and the rms noise levels in TA* scales are 59, 67 and 61 mK of the spectra in the top, middle and bottom panels, respectively.

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Figure 3.5. HDCO 21,1–11,0 spectrum of comet C/2007 W1 (Boattini) taken on 2008 June 19.69. A marginal 2-σ detection of a slightly blue-shifted spectral feature at V = -1.2 km s^-1 is visible. The spectral resolution is of 100 kHz ch^-1 and the rms noise is at 25 mK in T^*R scale.

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Figure 3.6. HDO 42,2–42,3 spectrum of 73P-C taken on 2006 May 10.5. A 2-σ marginal detection of a slightly blue-shifted spectral feature at V = -1.3 km s^-1 is visible. The spectral resolution is of 100 kHz ch^-1 and the rms noise is at 11 mK in T^*R scale.

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Figure 3.7. The c-C₃H₂ spectra of 73P-C taken in three consecutive 18-minute intervals on 2006 May 6.47. (a) A clear detection of c-C₃H₂ during the 1^st 18-minute interval; (b) the of c-C₃H₂ signal was fading away in the 2^nd 18-minute interval, and (c) non-detection of c-C₃H₂ in the last 18-minute interval.

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Table 3.1: Spectral parameters measured of the molecular transitions observed toward comet 73P-B with the KP12M.

Molecules Transition

a The half-power beam width (HPBW) of the primary beam.

b The linear scale of the primary beam of the KP12M at the comet.

c The line intensity shown in T_R^* for KP12M data with the FB1 receiver at 100 kHz ch^-1 resolution; if no detection, adopted a 3-σ upper limit for intensity.

d The correctedmain-beam efficiency.

e The main-beam brightness temperature.

f Gaussian-fitted spectral line width (FWHM) if there is a detection; otherwise, a linewidth of 1.0 km s^-1 is assumed for upper limit calculations.

g The integrated line intensity. Values listed are corrected for the main-beam efficiency.

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Table 3.2: Spectral parameters measured of the molecular transitions observed toward comet 73P-C with the KP12M or SMT.

Molecules Transition

a The half-power beam width (HPBW) of the primary beam.

b The linear scale of the primary beam of the KP12M or the SMT at the comet.

c The line intensity shown in T_R^* for KP12M data with the FB1 receiver at 100 kHz ch^-1 resolution, or in T_A^* for SMT data with the FB2 receiver at 250 kHz ch^-1 resolution, except for the HCN J = 3–2 transition which was measured with the Chirp Transform Spectrometer (CTS) of the SMT at 46.8 kHz ch^-1 resolution; if no detection, adopted a 3-σ upper limit for intensity.

d The correctedmain-beam efficiency.

e The main-beam brightness temperature.

f Gaussian-fitted spectral line width (FWHM) if there is a detection; otherwise, a linewidth of 1.0 km s^-1 is assumed for upper limit calculations.

g The integrated line intensity. Values listed are corrected for the main-beam efficiency.

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Table 3.2 (continued): Spectral parameters measured of the molecular transitions observed toward comet 73P-C with the KP12M or SMT.

a The half-power beam width (HPBW) of the primary beam.

b The linear scale of the primary beam of the KP12M or the SMT at the comet.

c The line intensity shown in T_R^* for KP12M data with the FB1 receiver at 100 kHz ch^-1 resolution, or in T_A^* for SMT data with the FB2 receiver at 250 kHz ch^-1 resolution, except for the HCN J = 3–2 transition which was measured with the Chirp Transform Spectrometer (CTS) of the SMT at 46.8 kHz ch^-1 resolution; if no detection, adopted a 3-σ upper limit for intensity.

d The correctedmain-beam efficiency.

e The main-beam brightness temperature.

f Gaussian-fitted spectral line width (FWHM) if there is a detection; otherwise, a linewidth of 1.0 km s^-1 is assumed for upper limit calculations.

g The integrated line intensity. Values listed are corrected for the main-beam efficiency.

Molecules Transition

37

Table 3.3: Spectral parameters measured of the molecular transitions observed toward comet C/2007 W1 (Boattini) with the KP12M.

a The half-power beam width (HPBW) of the primary beam.

b The linear scale of the primary beam of the KP12M at the comet.

c The line intensity shown in T_R^* for KP12M data with the FB1 receiver at 100 kHz ch^-1 resolution; if no detection, adopted a 3-σ upper limit for intensity.

d The correctedmain-beam efficiency.

e The main-beam brightness temperature.

f Gaussian-fitted spectral line width (FWHM).

g The integrated line intensity. Values listed are corrected for the main-beam efficiency.

Molecules Transition

Chapter 4 Analysis

4.1. Rotational Temperature and Molecular Column Density

Rotational excitation temperature (Trot) is measured from multiple transitions of one molecule, which is an indicator of the physical conditions, such as the (kinetic) temperature and density, of the gas in the inner coma region (Bockelée-Morvan et al.

1994). Thus, Trot can represent kinetic environments in comets and is also an important physical parameter for deriving the column density. In an ideal case,

1994). Thus, Trot can represent kinetic environments in comets and is also an important physical parameter for deriving the column density. In an ideal case,