CHAPTER 3 RESULTS
3.6. METHANOL (CH 3 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).
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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 TR* 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.
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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 TR* 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 1st 18-minute interval; (b) the of c-C3H2 signal was fading away in the 2nd 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 TR* 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 TR* for KP12M data with the FB1 receiver at 100 kHz ch-1 resolution, or in TA* 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 TR* for KP12M data with the FB1 receiver at 100 kHz ch-1 resolution, or in TA* 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
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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 TR* 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, different transitions of one molecule would be observed simultaneously with the same telescope and receiver. However, during our observations, the HCN J = 1–0 and J = 3–2 transitions were observed in comet 73P-C on 2006 May 5, 7 and 8 with the SMT, and on May 9, 10, and 13 with the KP12M, respectively. Thus, to minimize the extent of possible variations of the physical and chemical environments of the comet caused by its evolution during the perihelion, we have chosen the measurements of HCN J = 1–0 transition conducted on 2006 May 8.65 and the J = 3–2 line on May 9.66 when 73P-C was at a distance 1.032 and 1.026 AU, respectively, from the Sun for the derivation of Trot.
For a given molecule, by assuming the LTE, which is optically thin for all transition lines, and making the excitation temperature of all transitions equal to Trot, the background temperature is much smaller than Trot and all transitions fall in the Rayleigh-Jeans region. We then followed the work of Turner (1991), by using a rotational diagram to derive Trot, as shown below:
3 2 u efficiency (as shown in Tables 3.1, 3.2 and 3.3), S is the line strength, μ is the dipole moment, gI is the reduced nuclear spin weight, gk is the K-level degeneracy, Eu is the
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upper-state energy level of the transition, Qrot is the rotational partition function, k is the Boltzmann’s constant, and N is the total column density.
The rotational partition function, Qrot, can be computed differently according to various molecular structures. For linear species, such as CS and HCN, the partition function can be approximated as (Turner 1991):
rot kTrot
Q =σ hB , (4.2)
where σ is the multiplicity, h is the Planck’s constant and B is the rotational constant.
We note that the left-hand side of Equation 4.1, log (3kW / 8π3νSμ2 gI gk), if plotted on a log–log scale diagram (e.g. Figure 4.1), is a 1st-order linear function of the right-hand side term, (Eu / k), with a negative slope of – [(log e)/Trot]and an intercept of [log (N/Qrot)]. Derivation of the rotational excitation temperature by using Equation 4.1 and plotted on a log–log diagram is called the rotational diagram method.
Figure 4.1. The HCN rotational diagram of Comet 73P-C, based on observations conducted on 2006 May 9 and 10 where goodness of fit was applied to calculate the Trot upper and lower limits. Trot = 71+−7224was derived and was also applied to Comet 73P-B for further calculations.
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By plotting the rotational diagram (Figure 4.1), we measured T for HCN excitation in 73P-C when the comet was at 1.026–1.032 AU from the Sun; the same rotational temperature of 71 K derived from 73P-C is also applied to 73P-B when calculating the total column density of HCN gas in the comet coma. The goodness-of-fit method was applied in order to estimate the error of the Trot derived based on merely two data points. For comet C/2007 W1 (Boattini), we adopted a rotational temperature of Trot = 78 ± 3 K (Villanueva et al. 2011) for further calculations.
72 rot =71+−24
For single-dish observations, as discussed in the previous section, column densities can be derived from the rotational diagram method. With the same argument and assumption, the total column density of an observed molecule can also be obtained from a derivative of Equation 4.1 in a slightly different format by rearranging Equation (4.1). We have
We thus applied the derived Trot = 71− K of 73P-C to all transitions of various molecules observed in both 73P-B and 73P-C. Again, for Boattini, we adopted Trot = 78 ± 3 K for all column density estimations. Table 4.1 lists important molecular and physical parameters of the detected cometary molecules CS and HCN. Table 4.2, meanwhile, lists the column densities in comet 73P and comet Boattini.
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:
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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
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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.
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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.
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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.
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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.
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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
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