C/2013 R1 (LOVEJOY) AT IR WAVELENGTHS AND THE VARIABILITY OF CO ABUNDANCES AMONG OORT CLOUD COMETS

Gravitational collapse of dense cores in molecular clouds triggers the genesis of young stellar objects. During formation of the star, the circumstellar disk and envelope contain gas, ice, and dust that are the basic ingredients of planet formation (van Dishoeck & Blake 1998; van Dishoeck & Hogerheijde 1999; Evans et al. 2009; Bergin 2011). The gas clears within the first few million years, and dynamical processes transform the accretion disk through a "transitional" disk phase, ultimately leaving major planets and an extended debris disk composed of leftovers from the planetary formation process. Our Sun and planetary system experienced a similar process, and comets, asteroids, and trans-Neptunian objects are the surviving leftovers. Thus, studying the chemical composition of these bodies helps us interpret their origins, and active comets afford a means of accomplishing this through remote sensing.

We can now build comet taxonomies based on cosmogonic parameters (such as chemical composition, isotopic fractionation, and spin temperatures) of primary (typically referred to as "parent") volatiles released from the nucleus (e.g., Bockelée-Morvan et al. 2004; Mumma & Charnley 2011), of product species (e.g., A'Hearn et al. 1995; Fink 2009; Cochran et al. 2012), along with dust signatures such as crystallinity and mineralogy (e.g., Wooden et al. 1997; Zolensky et al. 2006; Ootsubo et al. 2007). Developing such taxonomies is a critical step toward testing the physicochemical conditions that prevailed during the formation of our solar system, some 4.6 billion years ago.

Indeed, the composition of cometary nuclei is key to understanding the formation and evolution of matter in the early solar system. Once formed and expelled to their current reservoirs—the Kuiper Belt (the ecliptic comets, or ECs, come from the scattered Kuiper disk) and the Oort Cloud (OC; source of the nearly isotropic comets, or NICs)—cometary nuclei remain relatively preserved from external alteration, although post-processing (Stern 2003) and additional erosional and evolutionary effects may provide possible sources of chemical modification.

IR spectroscopy is a powerful technique for building a chemical taxonomy of comets through detections of primary volatiles (i.e., native species stored originally as ices in the nucleus), such as H₂O, CO, CO₂, and other less abundant species. Recently, the Akari infrared survey of 18 comets (both ECs and NICs) emphasized the importance of CO₂, by demonstrating its high abundance (second only to H₂O) in many comets (Ootsubo et al. 2012). Only a few high-CO₂ comets were identified earlier (e.g., 103P/Hartley 2; Weaver et al. 1994; Crovisier et al. 1999).

Direct measurements of CO₂ in the cometary coma are only possible from space (a limitation of ground-based observations due to strong corresponding CO₂ absorption in the terrestrial atmosphere), yet ground-based IR spectroscopy can gather unique insights from a large inventory of several other (hyper) volatiles, including CO. Comets rich in CO have been hypothesized since the early 20th century (e.g., Fowler 1910, 1912; Pluvinel & Baldet 1911), based on strong emission from the comet-tail bands of ionized carbon monoxide (CO⁺). However, detailed knowledge of the ionization and dissociation efficiencies of its possible precursors (e.g., CO, CO₂, H₂CO, etc.) is required in order to assess their contributions to the observed CO⁺. Lacking such details, quantitative measurements of CO (whether native or product) are highly uncertain based on the observed CO⁺ emissions.

Direct observations of neutral CO in multiple wavelength regimes (radio, infrared, and ultraviolet) have established a wide range for measured CO abundances (relative to H₂O), ranging from a few tenths of a percent to ∼30% (Bockelée-Morvan et al. 2004; Mumma & Charnley 2011; Feldman et al. 2004, and references therein). But the largest complexity when interpreting measurements of CO stems from its possible parent species (i.e., extended sources). For instance, CO is a principal product of CO₂ dissociation, so comets rich in CO₂ should also reveal a significant production rate for CO—this product CO is extended and its detection is strongly dependent on the instrumental field-of-view (FOV). Native and extended sources of CO are discussed by Bockelée-Morvan et al. (2004), Cottin & Fray (2008), and Mumma & Charnley (2011).

Prior to 2013, only six comets within 2.5 AU of the Sun (where both H₂O and CO are active) were identified as being enriched in native CO. These comets are C/1995 O1 (Hale-Bopp; DiSanti et al. 1999, 2001; Bockelée-Morvan et al. 2010), C/1996 B2 (Hyakutake; Mumma et al. 1996; Biver et al. 1999; DiSanti et al. 2003), C/1999 T1 (McNaught-Hartley; Mumma et al. 2003; Biver et al. 2006), C/2001 Q4 (NEAT; Lupu et al. 2007), C/2008 Q3 (Garradd; Ootsubo et al. 2012), and C/2009 P1 (Garradd; e.g., Paganini et al. 2012b; Feaga et al. 2013).

During late 2013, we confirmed a relatively high abundance of CO in C/2013 R1 (Lovejoy; hereafter C/2013 R1), supporting the existence of a probably sparse (yet growing) fraction of CO-rich comets, especially if we consider the scarcity of CO in most comets observed by the Akari survey. Here, we present our recent IR observations of comet C/2013 R1 and discuss the results found during our six-night observing campaign.

Comet C/2013 R1 was discovered by Terry Lovejoy on UT 2013 September 7, at R_h = 1.98 AU and geocentric distance (Δ) 1.93 AU, with an apparent magnitude of ∼15. Its large eccentricity (e = 0.998), original semi-major axis (a = 339.3 AU), Tisserand parameter (T_j = 0.500), and inclination to the ecliptic (i = 6404) classify it as a nearly isotropic (long-period) comet originating from the OC (Nakano 2013).

The significant activity and interesting coma features observed by amateur astronomers prompted our IR observations in 2013 mid-October, using the Near InfraRed SPECtrometer (NIRSPEC) at the 10 m W. M. Keck Observatory atop Mauna Kea, Hawaii. We observed comet C/2013 R1 on six dates spanning R_h = 1.35–1.16 AU pre-perihelion (perihelion was at 0.81 AU on 2013 December 22). Spectra were acquired in the usual four-step sequence (ABBA) with an integration time of 30 s (or 60 s) per step, nodding the telescope along the (0432 × 24'') slit by 12'' between A- and B-beam positions (see DiSanti et al. 2001; Bonev 2005; Villanueva et al. 2011a; Paganini et al. 2012a for details).

We measured abundances for nine gaseous species in C/2013 R1 (H₂O, OH*, CO, CH₄, HCN, C₂H₆, CH₃OH, NH₃, and NH₂) and obtained upper limits for two others (C₂H₂ and H₂CO). A list of temperatures, production rates, abundances, and other pertinent information is given in Table 1; weighted mean mixing ratios (i.e., abundances relative to H₂O) are given in table note "d." We display selected emissions in Figures 1 and 2.

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Table 1. Retrieved Molecular Parameters for Primary and Product Volatiles in Comet C/2013 R1 (Lovejoy)^a

Species	νb	Lines	Trot	Global Qc	Abundanced
	(cm−1)		(K)	(1026 s−1)	(% Relative to H2O)
2013 October 22
eRh = 1.35 AU; Δ = 0.84 AU; Δ-dot = −40 km s−1; P.A. = 2808; α = 463. Time on source, KL1: 24 minutes, KL2: 28 minutes
H2O	3428.04	18	49$^{+11}_{-8}$	185.4 ± 19.2	100
CH4	3041.70	5	54$^{+7}_{-6}$	1.54 ± 0.14	0.68 ± 0.12
C2H6	2983.46	8	55$^{+7}_{-5}$	1.38 ± 0.17	0.72 ± 0.09
CH3OH	2850.53	17	46$^{+6}_{-4}$	5.40 ± 0.49	2.75 ± 0.38
HCN	3312.15	6	(50)f	0.59 ± 0.12	0.32 ± 0.07
NH3	3295.41	1	(50)f	<3.44	<1.86
NH2	3301.86	2	(50)f	<0.35	<0.19
C2H2	3295.96	6	(50)f	<0.92	<0.50
H2CO	2780.97	12	(50)f	<0.49	<0.26
2013 October 24
eRh = 1.34 AU; Δ = 0.79 AU; Δ-dot = −39 km s−1; P.A. = 2814; α = 475. Time on source, M-wide: 14 minutes
H2O	2073.75	4	(50)f	153.2 ± 25.2	100
CO	2143.12	6	47 ± 3	15.15 ± 1.85	9.89 ± 2.03
2013 October 25
eRh = 1.33 AU; Δ = 0.77 AU; Δ-dot = −39 km s−1; P.A. = 2817; α = 481. Time on source, KL2: 28 minutes
H2O	3442.05	12	55 ± 10	231.8 ± 33.1	100
CH4	3041.66	3	63$^{+1}_{-5}$	2.87 ± 0.17	1.24 ± 0.19
C2H6	2904.94	9	(50)f	2.17 ± 0.35	0.94 ± 0.20
HCN	3313.65	10	54$^{+8}_{-7}$	0.68 ± 0.06	0.29 ± 0.05
NH3	3315.17	2	(50)f	<2.54	<1.09
NH2	3301.86	2	(50)f	<0.22	<0.10
C2H2	3299.44	6	(50)f	<0.15	<0.07
H2CO	2779.99	12	(50)f	<0.34	<0.15
2013 October 27
eRh = 1.30 AU; Δ = 0.72 AU; Δ-dot = −38 km s−1; P.A. = 2822; α = 493. Time on source, KL2: 20 minutes
H2O	3446.17	9	62 ± 15	155.6 ± 24.9	100
CH4	3041.66	3	63$^{+7}_{-6}$	1.81 ± 0.25	1.16 ± 0.25
C2H6	2908.57	4	(60)f	1.74 ± 0.37	1.12 ± 0.30
HCN	3305.63	9	(60)f	0.36 ± 0.10	0.23 ± 0.07
NH3	3306.30	2	(60)f	<0.36	<0.23
NH2	3304.94	3	(60)f	<0.29	<0.19
C2H2	3296.59	7	(60)f	<0.22	<0.14
H2CO	2777.33	15	(60)f	<0.30	<0.19
2013 October 29
eRh = 1.28 AU; Δ = 0.68 AU; Δ-dot = −37 km s−1; P.A. = 2826; α = 506. Time on source, KL2: 24 minutes
H2O	3440.70	13	59$^{+3}_{-6}$	145.0 ± 40.9	100
CH4	3048.09	5	63$^{+4}_{-3}$	1.81 ± 0.21	1.25 ± 0.38
C2H6	2908.10	7	(60)f	1.08 ± 0.25	0.75 ± 0.27
HCN	3307.94	10	56 ± 5	0.47 ± 0.06	0.32 ± 0.10
NH3	3306.25	2	(60)f	<0.36	<0.25
NH2	3302.60	4	(60)f	<0.33	<0.22
C2H2	3298.44	4	(60)f	<0.17	<0.12
H2CO	2779.17	13	(60)f	<0.28	<0.19
2013 November 7
eRh = 1.16 AU; Δ = 0.51 AU; Δ-dot = −29 km s−1; P.A. = 2852; α = 581. Time on source, KL2: 20 minutes
H2O	3450.15	9	70 ± 2	367.9 ± 25.8	100
CH4	3048.09	5	61 ± 4	3.35 ± 0.25	0.91 ± 0.09
C2H6	2907.58	10	72$^{+12}_{-10}$	2.18 ± 0.25	0.59 ± 0.08
HCN	3312.74	12	70$^{+11}_{-9}$	0.87 ± 0.07	0.24 ± 0.03
NH3	3295.36	1	(70)f	0.36 ± 0.05	0.10 ± 0.02
NH2	3304.99	3	(70)f	0.44 ± 0.26	0.12 ± 0.07
C2H2	3305.13	6	(70)f	<0.16	<0.12
H2CO	2779.77	8	(70)f	<0.23	<0.06

Notes. ^aUncertainties represent 1σ, and upper limits represent 3σ. The reported error in production rate includes the line-by-line scatter in measured column densities, along with photon noise, systematic uncertainty in the removal of the cometary continuum, and (minor) uncertainty in rotational temperature. ^bMean wavenumber of all emission lines (used for this reduction) from a particular species. ^cGlobal production rate, after applying a measured growth factor to compensate slit losses. ^dWe also estimated the weighted mean relative abundance (%) for each species: CO (9.89 ± 2.03), CH₃OH (2.75 ± 0.38), CH₄ (0.91± 0.06), C₂H₆ (0.69 ± 0.06), HCN (0.26 ± 0.02), NH₃ (0.10 ± 0.02), NH₂ (0.12 ± 0.07), C₂H₂ (<0.07), and H₂CO (<0.06). ^eR_h: heliocentric distance; Δ: geocentric distance; Δ-dot: geocentric velocity; P.A.: position angle of the extended Sun–comet vector; α: solar phase (Sun–comet–Earth) angle. These values represent the mid-point of data acquisition. KL1, KL2, M-wide refer to standard NIRSPEC settings. ^fAssumed value based on retrieval of contemporary species.

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3.1. Rotational Temperatures

Near-IR spectroscopy is sensitive to molecular ro-vibrational transitions, and the ability to detect multiple emission lines grants an accurate determination of the molecular abundances in the coma. Sampling of each trace gas simultaneously with water (the most abundant ice in cometary nuclei) reveals its relative abundance ratio directly, and removes most sources of systematic error that are often introduced when measured separately. To accurately determine the production rate for each molecular species we need to establish its rotational temperature (T_rot) with high precision. This is essential for characterizing correct rotational level populations. Once T_rot is known, accurate production rates are determined using fluorescence models we developed for each species: H₂O (Villanueva et al. 2012a), OH* (Bonev et al. 2006), C₂H₆ (Villanueva et al. 2011b; Radeva et al. 2011), CO, C₂H₂, and CH₄ (Paganini et al. 2013; Villanueva et al. 2011a; Gibb et al. 2003), NH₃, HCN (Villanueva et al. 2013; Lippi et al. 2013), NH₂ (Kawakita & Mumma 2011), H₂CO (DiSanti et al. 2006), and CH₃OH (Villanueva et al. 2012b; DiSanti et al. 2013).

From the observed molecular emission lines, we retrieved rotational temperatures (T_rot) for six molecules (H₂O, CO, CH₄, HCN, C₂H₆, and CH₃OH). For species whose T_rot could not be determined, e.g., due to insufficient lines, restricted energy range, and/or poor signal-to-noise ratio, we adopted a value based on the results of other simultaneously observed molecules in the coma. In support of this approach, all derived rotational temperatures were consistent (within the 1σ–2σ level) on each date, suggesting a common temperature among species in the inner coma, as we generally find for a given comet. Figure 3(A) shows a gradual (and expected) increase of rotational temperature (from ∼50 K to ∼70 K) with decreasing R_h. The best-fit temperature variation followed a power law in R_h of −2.0 (T_rot = T_1AU R_h^{−2.0 ± 0.2}, with T_1AU = (93 ± 3) K representing the fitted value extrapolated to R_h = 1 AU; see Figure 3). Although our measurements sample a relatively small range of R_h, this power law is consistent with "typical" values for other comets (e.g., see Biver et al. 1997, 1999).

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3.2. Production Rates

3.3. Relative Abundances

3.4. Spatial Profiles

We centered the slit on the comet and aligned it along the Sun–comet direction (Figure 4). From October 22 to November 7, the phase angle (i.e., the observer–comet–Sun angle) ranged between 46° and 58°. A viewing geometry with phase angle of 90° and the slit oriented along the Sun–comet direction would be "optimal," since all pixels at positive distances along the slit from the comet's center would sample the sunward hemisphere and all negative distances the anti-sunward hemisphere. Keeping this difference in mind, we will refer to the positive distances in Figure 4 as the projected "sunward" position, and negative distances as "anti-sunward" position.

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We obtained spatial profiles for the continuum and for six parent volatiles (H₂O, CO, CH₄, HCN, C₂H₆, and CH₃OH)—some of them on multiple dates. On October 22, ethane displayed sunward enhancements (to projected nucleocentric distance ∼2000 km), while H₂O, HCN, and CH₃OH showed symmetric profiles about the nucleus (extensions of CH₃OH were measured only to ∼± 1000 km). The continuum, on the other hand, was enhanced in the anti-sunward direction. On October 24, our observations revealed asymmetric profiles for CO and the continuum, both enhanced toward the sunward side. On October 25 (when we observed an apparent increase in production rates), the spatial profiles of HCN and H₂O were rather symmetric, while methane and the continuum favored release toward the sunward side. The behavior of methane showed similar enhancement (i.e., sunward release) on October 29 (as did HCN), while H₂O showed flux excess in the anti-sunward direction (as did the continuum). On November 7, H₂O, HCN, and CH₄ showed slight enhancements in the sunward hemisphere, while the continuum was symmetric.

In summary, the complexity in our observed spatial profiles does not show clear delineation of outgassing sources as, for instance, would arise from polar and apolar ice phases—as was observed in comets C/2007 W1 (Boattini), 103P/Hartley2, and C/2009 P1 (Garradd; Villanueva et al. 2011a; Mumma et al. 2011; Paganini et al. 2012b). The evolving patterns in the profiles measured for C/2013 R1, however, could be related to nucleus rotation or to alternative physical effects like sequential activation of multiple outgassing jets (cf. CBET 3710). Additional observations at other wavelengths could test this possibility.

Thanks to a combination of prompt observing strategies, technical advances, and a rigorous survey, we have quantified CO, H₂O, and other volatile species in 17 OC comets since 1996, of which four were rich in native CO. Indeed, in comet C/2013 R1, the measured CO abundance (∼10%, relative to H₂O) was enhanced, classifying it as a CO-rich comet—the fifth highest among the 17 comets from the ground-based IR survey.

But how common are these CO-rich comets? Are they outliers of the typical population of comets? Three other surveys (all space-based) sampled CO and H₂O simultaneously (or nearly so) with the same beam size: the Akari IR survey (H₂O is sampled directly) and UV surveys with Hubble and the International Ultraviolet Explorer (using OH as the proxy for H₂O).

Among comets surveyed in the water-activated zone (R_h < 2.5 AU), Akari found that 6 of 13 comets were CO-poor (3σ upper limits less than 4.3% with respect to H₂O), contrasting strongly with the fact that 11 of 13 showed significant CO₂ abundance (>10% with respect to H₂O; Ootsubo et al. 2012). Of these 13 comets, only 3 were OC comets, and only C/2008 Q3 (at R_h = 1.8 AU) showed high abundance ratios for both CO and CO₂, (∼24% and ∼30%, respectively). Considering the large Akari aperture for the C/2008 Q3 observations (diameter of ∼27,000 km), some fraction of the CO observed in it could have been produced by dissociation of precursors such as CO₂, H₂CO, or by polymers such as polyoxymethylene (Eberhardt 1999; Fray et al. 2006). In the entire survey (at all heliocentric distances), CO was detected in only 3 of 18 comets sampled: C/2008 Q3 (Garradd), C/2006 W3 (Christensen) at 3.13 and 3.61 AU, and 29P/Schwassmann–Wachmann 1 at 6.18 AU. The lack of CO detections in most comets by the Akari survey is intriguing, although the sensitivity for CO (∼2 × 10²⁶ molecules s⁻¹, 3σ detection limit) could have restricted further detections.

Figure 5 displays the CO/H₂O mixing ratios based on the ground-based IR survey (17 OC comets; filled circles), with the addition of (6 OC) comets for which there were no ground-based IR results (open circles). We excluded ECs to avoid any biases, such as could be produced by (possible) CO depletion after multiple perihelion passages.

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While the native CO abundances among comets show a range from ∼0.3% to values of about 30% (Figure 5), OC comets having high native CO abundances (>8%) seem to represent a small fraction of the sampled population. Considering a "typical" (median) abundance of ∼4% for CO (relative to H₂O), we arbitrarily define comets with abundances larger than twice the median CO abundance (i.e., >8%) to represent "CO-rich" comets (shaded green in Figure 5). By this standard, the only comets rich in native CO are C/2001 Q4 (NEAT), C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 T1 (McNaught-Hartley), C/2009 P1 (Garradd; that also displayed larger CO values post-perihelion (e.g., Bodewits et al. 2014)), C/2008 Q3 (Garradd), and now C/2013 R1 (Lovejoy).

Could there be evidence of chemical post-processing in some comets? A'Hearn et al. (2012) showed that the relative depletion of CO (in ECs and NICs) was not proportional to the reciprocal of the semi-major axis (1/a). But they found that CO₂ was somewhat depleted in comets with q < 1 AU, demonstrating the possible influence of thermal modification—but other hypervolatiles (like CO, CH₄, C₂H₆) should then be depleted, too. The importance of CO is its high volatility (i.e., sensitivity to temperature), so high-CO comets have the potential to place important constraints on the birthplace and processing history of pre-cometary ices.

Can the relatively large CO content challenge and inform our current understanding of their origins? Recent mapping of the solar-nebula analog TW Hydrae with ALMA allowed estimates of the CO frost line (near ∼30 AU; Qi et al. 2013). Certainly, if we consider that the formation of comets occurred in the nebular mid-plane, a large CO abundance in these comets would support formation in cold regions near—or perhaps beyond—heliocentric distances that are consistent with the CO snowline. Earlier consideration of the severely depleted comet C/1999 S4 (and others) suggested formation in the Jupiter–Saturn zone where CO ice is not stable (Mumma et al. 2001a). However, the possible roles of radial mixing, shielding of certain hypervolatiles in water ice mantles, and the existence of diverse structural phases of ice, i.e., polar (H₂O-rich) and apolar (CO-rich, CO₂-rich), could serve as plausible explanations for the wide dispersion of CO content found in comets (e.g., Paganini et al. 2012b, for a discussion). We can only expect to sharpen our conclusions and answer these questions as future observations of CO-rich comets become available and dynamical models deal with the wide range of CO abundances in comets.

Our NIRSPEC observations of comet C/2013 R1 resulted in detections of seven primary volatiles (H₂O, CO, CH₄, HCN, C₂H₆, CH₃OH, and NH₃), upper limits for two others (C₂H₂ and H₂CO), and detections of two product species (OH* and NH₂) at pre-perihelion distances (1.35–1.16 AU) in 2013 October/November. During this period, the measured production rate for water increased from 1.0 to 3.9 × 10²⁸ molecules s⁻¹. We measured (weighted) mean abundances (%, relative to water) for CO (9.89 ± 2.03), CH₃OH (2.75 ± 0.38), CH₄ (0.91 ± 0.06), C₂H₆ (0.69 ± 0.06), HCN (0.26 ± 0.02), NH₃ (0.10 ± 0.02), NH₂ (0.12 ± 0.07), C₂H₂ (<0.07), and H₂CO (<0.06). We obtained consistent rotational temperatures (at the 1σ–2σ level) for six species (H₂O, CO, CH₄, HCN, C₂H₆, and CH₃OH), suggesting a common source of rotational excitation within NIRSPEC's FOV on each date. The OPR for H₂O was 3.01 ± 0.49, corresponding to a spin temperature larger than 29 K (at the 1σ level). Our results demonstrate a rich chemistry that, compared to most OC comets, reveals highly enriched CO, (at most) slightly enriched CH₃OH, C₂H₆, and HCN, and CH₄ consistent with "normal," yet depleted NH₃, C₂H₂, and H₂CO. NH₂ was also depleted, as expected if it is the principal product of NH₃ photolysis.

We have identified a new CO-rich comet. Future observations will test whether these comets stem from a larger population and, if so, assess whether the observed large CO abundance could indeed inform our understanding of the true formative conditions experienced by these icy bodies.

We gratefully acknowledge support by NASA's PAST Program (L.P., M.J.M, M.A.D., G.L.V.) and NAI through its member Teams at GSFC (M.J.M., M.A.D., B.P.B., G.A.B.) and at UH (J.V.K., K.J.M.; No. NNA09DA77A), and NSF (B.P.B., E.L.G.; Award 1211362). The data of October 24 and 25 were collected during part of the NASA Keck time awarded for observations of comet ISON, and are publicly available through the Keck Observatory Archive. We thank the anonymous referee for useful insights on this paper. The authors acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.