Observations of the Hydrogen Cyanide in Comet 46P/Wirtanen at a 3.4 mm Wavelength

Comets are generally believed to have originated beyond the snow line in the protoplanetary disk (Disanti et al. 2009). Subsequent gravitational scattering processes during the orbital migration of the growing giant planets would lead to the large-scale distribution of these icy planetesimals (Bockelee-Morvan et al. 2004b). Before being injected into the inner solar system within Jupiter's orbit, these icy primitive bodies have not experienced significant solar heating and hence surface reprocessing. This means that multiwavelength spectroscopic observations of the molecular abundances in the expanding cometary comas (Jones et al. 2006; Tseng et al. 2007; Wang et al. 2017) could provide us with important clues on the physicochemical condition and temperature of the solar nebula where they formed.

It has been known for a long time that the CN radical must be the daughter product of one or more parent molecules (Ip 1989; Biver et al. 1996). One of them is hydrogen cyanide HCN which was first detected in the Oort-cloud comet C/1973E1 Kohoutek at 3.4 mm wavelength from the J = 1–0 rotational transition by Huebner et al. (1974) in December 1973. Since then, the radio emission of HCN has been detected in many comets (Ekelund et al. 1981; Irvine et al. 1984; Despois et al. 1986; Schloerb et al. 1986, 1987; Bockelee-Morvan et al. 1987; Winnberg et al. 1987; Biver et al. 1999, 2000, 2002a, 2002b, 2006, 2007, 2011, 2012; Disanti et al. 2009; Drahus et al. 2010, 2012; Wirstrom & Lerner 2016).

The Jupiter family comet, 46P/Wirtanen (hereafter 46P), with an orbital period of 5.5 yr was discovered on 1948 January 15 at Lick Observatory by Carl Alvar Wirtanen. The physical properties of 46P such as its effective radius of about 0.56–0.7 km (Boehnhardt et al. 2002), rotation period of 6.0–7.6 hr (Meech et al. 1997; Lamy et al. 1998), and water production rate of 7.24 × 10²⁷–1.7 × 10²⁸ molecules s⁻¹ (Groussin 2003; Kobayashi & Kawakita 2010) had been determined relatively precisely because it was a candidate target of the Rosetta mission of ESA before comet 67P/Churuyumov–Gerasimenko was selected instead. In its perihelion passage at a heliocentric distance of 1.055 au in 2018 December, 46P's geocentric distance was only 0.077 au on 2018 December 16 thus making it the tenth closet comet encounter with the Earth in modern time. This favorable observational condition provided an excellent opportunity to study its coma in detail. An observational network (the East-Asian 46P/Wirtanen Observation Campaign) was organized to coordinate observations by different facilities in China and other regions. In this paper, we report the observational results of the 3.4 mm transition of HCN (1–0) radio emission obtained using the Purple Mountain Observatory (PMO) 13.7 m radio telescope. The organization of the paper is as follows. The observational equipment and observational method are described in Section 2. The data analysis and results are given in Section 3. The discussion in Section 4 is followed by a conclusion in Section 5.

The observations of the HCN(1–0) emission in 46P were performed for about 8 hr each day from 2018 December 14 to 15, using the 13.7 m aperture ratio telescope of PMO in Delingha, China, which is located at longitude 97° 43'46'' E, 37° 22'40'' N and 3169.21 m above the sea level. We searched for hydrogen cyanide in its HCN(J = 1–0), F = 1–1, 2–1, and 0–1 lines at 88630.4157, 88631.8473, and 88633.9360 MHz, respectively. At that time, 46P was close to its perigee at a geocentric distance of Δ = 0.077 au and a heliocentric distance of r_h = 1.056 au.

The cooled superconductor-insulator-superconductor (SIS) receiver working at 3 mm with a frequency coverage of a 85–115 GHz band was employed. The receiver noise is 60 K in this range (Shan et al. 2012). The system temperature was around 120–150 K taking into account the dome noise. The typical atmospheric absorption τ was about 0.10–0.15 associated with weather conditions and telescope elevation during the observation period. The data were collected using a fast Fourier transform spectrometer (FFTS) of 16384 spectral channels with a bandwidth of 1 GHz, and a spectral resolution Δν of ∼61.04 kHz corresponding to a velocity width of ∼0.21 km s⁻¹ per channel. The half power beamwidth (HPBW) reflects the resolution power of a telescope, which can be calculated using HPBW = $\tfrac{k\lambda }{D}$, where λ is the working wavelength, D is the aperture size, and k depends on the illuminating function. At a local oscillating frequency of 88.6 GHz, the HPBW is ∼622. The pointing rms were estimated to be better than 5'' throughout the observations. The main beam efficiency ${\eta }_{{}_{B}}$ of ∼58.8% at this frequency was calculated from the measured antenna temperature ${T}_{A}^{* }$ and the main beam temperature T_MB (Wilson & Rohlfs 2013) according to ${\eta }_{{}_{B}}\,=\,\tfrac{{T}_{A}^{* }}{{T}_{\mathrm{MB}}}$. The effective aperture factor of antenna η was ∼38.9%.⁹ The conversion factor of ∼48.13 Jy K⁻¹, which is estimated from Equation (8.17) of Wilson & Rohlfs (2013), from antenna temperatures (${T}_{A}^{* }$) to flux density (S) lead to the following formula $S\,=\,\tfrac{2k}{\eta \cdot A}\cdot {T}_{A}^{* }$, where S is the system equivalent flux density of the observed source, jansky (Jy); k is Boltzmann constant, erg K⁻¹; η is the effective aperture factor of the antenna; A is the antenna area, cm²; and ${T}_{A}^{* }$ is the antenna temperature of the signal from the observed source in the sky.

The observational trajectory parameters and the Sun–Comet–Earth geometry are provided by the Minor Planet Ephemeris Service of MPC 111773 on 2018 December 14 and 15, as shown in Table 1. The rest frequency of the observed 88631.8473 MHz HCN line was obtained from the molecular database at "Splatalogue,"¹⁰ which is a compilation of the Jet Propulsion Laboratory catalog¹¹ (Pickett et al. 1998) and other catalogs. The size of the visible coma diameter of 46P was estimated to be 75'' field angle¹² during our observational period, thus the size of the cometary atmosphere was about 100,000 km, a value typical for a comet at 1 au from the Sun; this comet was considered to be a diffuse source. The observing strategy was performed in the position-switching observation mode, namely, switching observations between the cometary nucleus (on-source) for 1 minute and the adjacent blank sky (off-source) for 1 minute. The off positions were offset 3° in the antenna azimuth direction away from the targeted positions, to avoid the contamination by the background emission, while no emission was found in each off position. The galactic source S140 was observed several times in individual sessions in our observing periods, to provide calibration of the antenna pointing, the receiver system, and the accuracy of absolute flux density.

Table 1.  Observational Parameters and Sun–Earth–Comet Geometry

	J2000 Coordinates		Heliocentric Distance	Topocentric Distance	Topocentric Radio Velocity
46P/Wirtanen	R.A. (α)	Decl. (δ)	rh	Δ	$\dot{{\rm{\Delta }}}$
Date (UT)	h m s	° '''	(au)	(au)	($\mathrm{km}\,{{\rm{s}}}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)
2018 Dec 14/11:25–20:00
11:25	03 43 17.9	+14 31 53	1.056	0.078	−1.977
15:00	03 44 25.9	+15 05 09	1.056	0.078	−1.606
20:00	03 46 01.5	+15 51 15	1.056	0.078	−1.059
2018 Dec 15/11:20–20:15
11:20	03 51 35.3	+18 14 09	1.056	0.078	−1.191
15:00	03 52 49.9	+18 48 44	1.056	0.078	−0.819
20:15	03 54 37.4	+19 37 37	1.056	0.078	−0.254

Download table as:  ASCIITypeset image

The spectral data were reduced and analyzed with the GILDAS/CLASS software package.¹³ We processed and analyzed the spectral line data of HCN(1–0) spectrum. Linear baseline subtractions were performed for all spectra. The satellite line of HCN(1–0) F = 1–1 at 88630.4157 MHz was marginal detection and the main line F = 2–1 at 88631.8473 MHz hyperfine component was clearly detected during the observational session. The satellite line HCN(1–0) F = 0–1 at 88633.936 MHz was not detected. Their emission profiles relative to the reference frame of the cometary nucleus are shown in Figure 1.

Zoom In Zoom Out Reset image size

Figure 1 shows the integrated spectra of the HCN at F = 1–1 and F = 2–1, respectively. The emission line profiles were processed by the Gaussian fit approach. From these fittings, the integrated line intensity $\int {T}_{\mathrm{MB}}d\upsilon$, the peak intensity at T_MB, the center velocity relative to the reference frame of the cometary nucleus v, and the FWHM line width ΔV can be derived. Their values are summarized in Table 2. The effective integration time was 3.2 hr and 3.4 hr on December 14 and December 15, as shown as blue line and red line in Figure 1, respectively. The baseline rms noise level of T_MB of both emission features at F = 1–1 and F = 2–1, respectively, were about 7.09 mK and 6.42 mK. The total effective integration time of the two-day average for the HCN spectra was about 6.6 hr, corresponding to the baseline rms noise level of T_MB of 4.57 mK. Therefore, the signal-to-noise ratio of the average F = 2–1 component at 88631.8473 MHz was 7σ. The velocity of the HCN spectral line at the F = 2–1 main hyperfine component in coma, with their line center relative to the cometary nucleus of 46P, was approximately −0.32 ± 0.06 km s⁻¹.

Table 2.  Parameters of Gaussian Fits to the Radio HCN Spectral Feature Detected in 46P/Wirtanen

		HCN (J = 1–0) Gaussian Fits				Production Rate
46P/Wirtanen	Rest Freq.	v	ΔV	$\int {T}_{\mathrm{MB}}{dv}$	TMB	σrms	Q(T = 75 K)
Date (UT)	(MHz)	($\mathrm{km}\,{{\rm{s}}}^{-1}$)	($\mathrm{km}\,{{\rm{s}}}^{-1}$)	($K\,\mathrm{km}\,{{\rm{s}}}^{-1}$)	(K)	(K)	(molec s−1)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
Dec 14/11:25–20:00
	88630.4157	4.40(0.40)	1.94(0.67)	0.0247(0.0090)	1.20 × 10−2	7.09 × 10−3
	88631.8473	−0.37(0.11)	1.05(0.16)	0.0355(0.0060)	3.19 × 10−2	7.09 × 10−3	(7.09 ± 1.61) × 1024
	88633.9360	⋯	⋯	⋯	⋯	⋯	⋯
Dec 15/11:20–20:15
	88630.4157	4.00(0.25)	2.62(0.44)	0.0396(0.0070)	1.42 × 10−2	6.42 × 10−3
	88631.8473	−0.27(0.06)	0.91(0.12)	0.0319(0.0040)	3.28 × 10−2	6.42 × 10−3	(5.52 ± 1.01) × 1024
	88633.9360	⋯	⋯	⋯	⋯	⋯	⋯
Dec 14–15
Average	88630.4157	4.26(0.21)	2.19(0.40)	0.0311(0.0040)	1.33 × 10−2	4.57 × 10−3
	88631.8473	−0.32(0.06)	1.00(0.10)	0.0339(0.0060)	3.18 × 10−2	4.57 × 10−3	(6.45 ± 1.31) × 1024
	88633.9360	⋯	⋯	⋯	⋯	⋯	⋯

Note. Column (1): molecule name; column (2): rest frequency of the transition; columns (3)–(6): the line center velocity v, the FWHM line width ΔV, the integrated intensity $\int {T}_{\mathrm{MB}}{dv}$, and the amplitude T_MB from the Gaussian fits for the detected HCN line, where the error is given in parenthesis. Column (7): after subtracting the Gaussian fitting profile, the 1σ noise of the base residuals in observed spectra (T_MB scale). Column (8): production rate. The transition for which the 88633.9360 MHZ lines were not detected are marked with "⋯."

Download table as:  ASCIITypeset image

4.1. The HCN Production Rates

To convert the detected line area $\int {T}_{{mB}}{dv}$ of the transition intensities into molecular production rates, some assumptions must be made on the excitation mechanisms of the transitions in cometary atmospheres. It has been shown that, for the molecules investigated in the present work, the prevailing excitation mechanisms are thermal excitation by collisions in the inner coma, and solar infrared excitation, e.g., the pumping of the fundamental bands of vibration by fluorescent effects, in the outer coma (Crovisier 1985, 1987; Bockelee-Morvan et al. 1987). The gas emissions vary from thermal equilibrium to fluorescence equilibrium. The dynamical evolution of the relative rotational population distribution changed as the molecules expanded from the surface of the nucleus to the inner parts, then the outer regions of the coma. At r ≥ 10,000 km from the cometary nucleus, collision may be completely ignored and the population distribution is dominated by the competition between radiative excitation by the local radiation field and spontaneous de-excitation (Crovisier 1985). The balance between infrared excitation and spontaneous decay tends to put the molecular rotational population distributions into fluorescent equilibrium. The field of view of our telescope beam radius corresponds to the spatial scale r = 3527 km when r_h = 1.056 au and Δ = 0.078 au for 46P. So the collision of the molecules should be important in our observed results.

To retrieve the global outgassing production rate Q from the volume integration within the instrument beam of the line area $\int {T}_{{mB}}{dv}$, a model involving kinematics, chemistry, and excitation of the molecules, along with light propagation properties, must be used. The excitation mode used to interpret our results can be found in detail in Drahus et al. (2010). It assumed a stationary molecular production rate, spherical symmetry, uniform expansion with velocity, an optically thin coma, and the LTE condition. Under these assumptions the production rate Q, according to formula (2) of Drahus et al. (2010), depends linearly on the line area $\int {T}_{{mB}}{dv}$, $Q=\displaystyle \frac{2}{\sqrt{\pi \mathrm{ln}2}}\displaystyle \frac{k}{h}\tfrac{b}{D}\tfrac{{\rm{\Delta }}}{I(T)}\tfrac{{v}_{\exp }}{\nu }({e}^{\tfrac{h\nu }{{kT}}}-1)\int {T}_{{mB}}{dv}$, where h = $6.63\times {10}^{-34}\,\mathrm{Js}$ is the Planck constant; b = 1.22 is a dimensionless factor, intrinsic to every dish for the PMO; D = 13.7 m in the dish diameter; Δ is the geocentric distance; and I(T) is the integrated line intensity (at temperature T) as defined in the JPL spectral line catalog (Pickett et al. 1998). Note that in order to calculate Q in molecules per second, $\int {T}_{{mB}}{dv}$ must be provided in the commonly used unit of Km s⁻¹. Here, we adopted T = 75 K, I(75 K) = 0.050913 nm² MHz, referring to the analysis of comet 73P-C/Schwassmann-Wachmann 3 (Drahus et al. 2010). The observed linewidths of the F = 2–1 hyperfine components were 1.05 ± 0.16 km s⁻¹ for the spectrum on December 14 and 0.91±0.12 km s⁻¹ on December 15. In other words, it is 1.00 ± 0.10 km s⁻¹ on December 14–15. Since these linewidths cannot be thermal broadening, they must be attributed to the gas velocity field, corresponding to twice the gas expansion velocity v_exp. From this consideration, the expansion velocity v_exp ≈ 0.5 ± 0.05 km s⁻¹ on December 14–15.

The HCN production rates of 46P were estimated as follows: Q_{(75 K)} = (7.09 ± 1.61) × 10²⁴ molec s⁻¹ on December 14, Q_{(75 K)} = (5.52 ± 1.01) × 10²⁴ molec s⁻¹ on December 15, and Q_{(75 K)} = (6.45 ± 1.31) × 10²⁴ molec s⁻¹ on December 14–15. From a comparative study of 24 comets observed at millimeter and submillimeter wavelengths, Q_[HCN]/${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$, was found to vary from 0.08% to 0.25%, with a mean value around 0.13% (Biver et al. 2002b; Bockelee-Morvan et al. 2004a, 2004b). For comparison, D. Schleicher measured the peak water production rate to be ${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$ = 7.24 × 10²⁷ molec s⁻¹ on 2018 December 16.¹⁴ We have therefore Q_[HCN]/${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$ = (0.09 ± 0.01)% of 46P. This suggests that our derived HCN abundance is consistent with the statistical results from previous apparitions (Biver et al. 2002b). As can be seen from Table 3, comparing to other comets, the HCN abundance Q_[HCN]/${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$ of 46P is approximately equal to that of the Jupiter family comets/Halley family comets and is slightly smaller than that of the long-period comets.

Table 3.  Observed HCN Production Rate in Different Comets

	Comet	Date	Heliocentric	Production Rate	Q[HCN]/${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$	Reference
			Distance (au)	(molecules s−1)
	(1)	(2)	(3)	(4)	(5)	(6)
Long	C/1999 S4 Linear	2000 Jul	0.77	<1026	0.1%	Bockelee-Morvan et al. (2001)
Period	C/2006 W3 (Christensen)	2009 Sep	3.20	(1.6 ± 0.1) × 1026	>0.4%	Bockelee-Morvan et al. (2010)
	C/2006 M4 (SWAN)	2006 Nov	1.02	(2.58 ± 0.264) × 1026	∼0.1%	Disanti et al. (2009)
	C/1996 B2 (Hyakutake)	1996 Apr	0.55	(24.9 ± 1.9) × 1026	0.12%	Biver et al. (1999)
	C/1999 H1 (Lee)	1999 Sep	1.35	(0.32 ± 0.01) × 1026	(0.11 ± 0.02)%	Biver et al. (2000)
	C/1999 T1 (McNaught-Hartley)	2001 Jan	1.35	(5.8 ± 0.4) × 1025	(0.08 ± 0.01)%	Biver et al. (2006)
	C/2001 A2 (Linear)	2001 Jul	1.15	(4.9 ± 0.2) × 1025	(0.14 ± 0.01)%	Biver et al. (2006)
	C/2000 WM1 (Linear)	2001 Nov	1.36	(1.6 ± 0.3) × 1025	(0.08 ± 0.01)%	Biver et al. (2006)
	C/2002 V1 (NEAT)	2003 Feb	0.10	(41.1 ± 2.5) × 1026	(0.17 ± 0.01)%	Biver et al. (2011)
	C/2006 P1 (McNaught)	2007 Jan	0.22	(286 ± 28) × 1026	(0.13 ± 0.02)%	Biver et al. (2011)
	C/2013 R1 (Lovejoy)	2013 Dec	0.82	0.7-2.4 × 1026	0.2%	Wirstrom & Lerner (2016)
	C/2014 Q2 (Lovejoy)	2015 Jan	1.3	∼4.5 × 1026	0.09%	Wirstrom & Lerner (2016)
Halley	P/Swift-Tuttle 1992t	1994 Dec	1.01	(2.9 ± 1.0) × 1026	0.10%	Wootten et al. (1994)
Family	P/Halley 1982i	1985 Nov	1.5	7 × 1025	0.09%	Bockelee-Morvan et al. (1987)
	P/Halley 1982i	1985 Nov	0.6	3 × 1027	0.16%	Bockelee-Morvan et al. (1987)
	153P/Ikeya-Zhang	2002 Apr	1.06	(1.7 ± 0.1) × 1026	(0.11 ± 0.01)%	Biver et al. (2006)
Jupiter	19P/Borrelly	1994 Nov	1.37	(3.4 ± 0.5) × 1025	0.11%	Bockelee-Morvan et al. (2004a)
Family	19P/Borrelly	2001 Sep	1.36	(1.9 ± 0.3) × 1025	0.06%	Bockelee-Morvan et al. (2004a)
	73P/SW 3-fragment C	2006 May	1.0	(2.7 ± 0.09) × 1025	∼0.1%	Drahus et al. (2010)
	9P/Tempel 1	2005 May	1.63	(6.8 ± 1.1) × 1024	(0.11 ± 0.01)%	Biver et al. (2007)
	9P/Tempel 1	2005 Jul	1.51	(11.1 ± 1.8) × 1024	(0.12 ± 0.03)%	Biver et al. (2007)
	10P/Tempel 2	2010 Jul	1.42	(1.6 ± 0.4) × 1025	(0.09 ± 0.01)%	Biver et al. (2012)
	46P/Wirtanen	2018 Dec 14	1.05	(7.09 ± 1.61) × 1024	(0.10 ± 0.02)%	This work
	46P/Wirtanen	2018 Dec 15	1.05	(5.52 ± 1.01) × 1024	(0.08 ± 0.01)%	This work
	46P/Wirtanen	2018 Dec 14–15	1.05	(6.45 ± 1.31) × 1024	(0.09 ± 0.01)%	This work

Download table as:  ASCIITypeset image

4.2. Anomalies

We conducted post-perihelion and closest perigee observations of the parent volatile in the comet 46P/Wirtanen on 2018 December 14–15, using the Delingha 13.7 m radio telescope. We have clearly detected the HCN(1–0) molecular lines at 3.4 mm toward 46P. The HCN production rate derived from the excitation model of Drahus et al. (2010) is approximately (6.45 ± 1.31) × 10²⁴ molec s⁻¹, which would imply Q_[HCN]/${Q}_{[{{\rm{H}}}_{2}{\rm{O}}]}$ = (0.09 ± 0.01)%.

The authors would like to thank all the staff of the Delingha Radio Telescope at Purple Mountain Observatory for their assistance in the comet observations. This work was supported by the foresight project funding of center for Astronomical Mega-Science, Chinese Academy of Sciences (grant No. 2019000009), Chinese Academy of Sciences Taiwan Young Talent Programme (grant No. 2018TW2JA0006), Heaven Lake Hundred-Talent Program of Xinjiang Uygur Autonomous Region of China, the National Nature Science Foundation of China (grant No. 11503073), the Chinese Academy of Sciences Foundation of the young scholars of western (grant No. 2015-XBQN-B-02), and the Surface Projects of NSFC (grant No. 11673056).