Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) | Astronomy & Astrophysics (A&A)

© ESO, 2016

1. Introduction

Comets are the most pristine remnants of the formation of the solar system 4.6 billion years ago. Investigating the composition of cometary ices provides clues to the physical conditions and chemical processes at play in the primitive solar nebula. Comets may also have played a role in the delivery of water and organic material to the early Earth (see Hartogh et al. 2011,and references therein). The latest simulations of the early solar system evolution (Brasser & Morbidelli 2013; O’Brien et al. 2014), suggest a more complex scenario. On the one hand, ice-rich bodies formed beyond Jupiter may have been implanted in the outer asteroid belt and participated in the supply of water to the Earth or, on the other hand, current comets coming from either the Oort Cloud or the scattered disk of the Kuiper belt may have formed in the same trans-Neptunian region sampling the same diversity of formation conditions. Understanding the diversity in composition and isotopic ratios of the comet material is thus essential in order to assess such scenarios (Altwegg & Bockelée-Morvan 2003; Bockelée-Morvan et al. 2015).

The recent years have seen significant improvement in the sensitivity and spectral coverage of millimeter receivers. The EMIR receivers (Carter et al. 2012) at the Institut de radioastronomie millimétrique (IRAM) are equipped with a fast Fourier transform spectrometer that offers a wide frequency coverage at a high spectral resolution (0.2 MHz). The combination enables sensitive spectral surveys of cometary atmospheres and simultaneous observations of several molecules, including isotopologues in brighter comets. We report here observations of isotopic ratios with the IRAM 30 m radio telescope in two very active (maximum water outgassing rate close to 10³⁰ molec s^-1) Oort cloud comets, C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy), carried out in 2013 and 2015. They are both dynamically old Oort cloud comets: an original orbital period of 9800 years and an orbit inclination of 83° for comet Lemmon, and a period of 11 000 years and an inclination of 80° for comet Lovejoy.

The analysis of the observation in terms of molecular abundances and detection of rare and new molecular species has been reported (Biver et al. 2014, 2015). In the present article we concentrate on the measurement of several isotopic ratios. We report the detection of HDO in both comets. The compounds H₂¹⁶O and H₂¹⁸O were detected in comet Lovejoy with the Odin submillimeter space observatory, which helped to accurately determine the D/H and ¹⁶O/¹⁸O ratio in water in this comet.

2. Observations

2.1. IRAM Observations

Comet C/2014 Q2 (Lovejoy) was observed with the IRAM 30 m radio telescope during two periods: on 13.8, 15.8, and 16.8 January 2015 (geocentric distance Δ = 0.496−0.528 AU) and on 23.7, 24.7, 25.7, and 26.7 January 2015 (Δ = 0.624−0.675 AU) under very good weather conditions. The heliocentric distances were 1.31 and 1.29 AU, respectively. Perihelion was on 30.07 January UT, at 1.290 AU from the Sun. The 13–25 and 26 January observations were conducted with the EMIR 1 mm and 3 mm receivers, respectively. The main backend used is a Fourier transform spectrometer, which covers a frequency range of 2 × 8 GHz (two sidebands separated by 8 GHz, each in two linear polarizations) in a single setup, with a high spectral resolution of 200 kHz. The spectral resolution (0.3−0.2 km s^-1 when converted into Doppler velocity) allows the velocity profiles of the narrow cometary lines (~ 2 km s^-1) to be resolved. We also used the high resolution VESPA autocorrelator set to 40 kHz sampling on dedicated lines of interest (e.g., CH₃OH (5_{+ 2}−4_{+ 1}E) and H¹⁵CN (3−2)), but VESPA units can only cover lines within a 0.5 GHz window centered at 6.25 GHz in the IF band (upper and lower side band). Using four different tunings we covered the 210−218, 225−233, and 240−272 GHz frequency ranges and 85−93 and 101−109 GHz.

The half power beam width of the IRAM 30 m telescope in the 1 mm band ranges from 9.1′′ to 11.6′′, which corresponds to 3300 to 5400 km at the comet distance (22−28′′ at 3 mm corresponding to 11 000−14 000 km on 26.7 Jan.). Given the expansion velocity of the coma (~ 0.8 km s^-1) and heliocentric distance, this means that we are not very sensitive to the photo-dissociation rate of the molecules, provided that the molecule lifetime at 1 AU is larger than 4000 s. The pointing accuracy (1′′−2′′) was regularly checked on reference pointing sources and also on the comet with coarse maps. The time variation of the activity of the comet could be monitored with the multiple strong CH₃OH lines present in all setups. We did not see any variations significantly larger than ± 20% during the observations (Q_CH3OH = 1.2 ± 0.2 × 10²⁸ molec s^-1, 13−16 January, and 1.4 ± 0.2 × 10²⁸ molec s^-1, 23−25 January), which justified the averaging of several days of observations.

Comet C/2012 F6 (Lemmon) was observed in March and April 2013 (as already presented in Biver et al. 2014). Perihelion was on 24.51 March 2013 UT at 0.731 AU from the Sun. We mostly used the same tunings as for comet Lovejoy with the EMIR 1mm receiver combined with the FTS and VESPA spectrometers. The best weather conditions were found on 14.5 and 18.5 March, but with a comet at low elevation (15−20°), and on 6.5 April 2013. We covered the 240−272 GHz frequency range and obtained noisier data on the 85−93, 166−170, and 210−242 GHz bands with the EMIR 3 mm, 2 mm, and 1 mm receivers, respectively. A summary of the observational setups is given in Table A.1.

In this paper we focus only on the measurement of isotopic ratios, and spectra of isotopologues are presented in Figs. 3−8. The chemical inventory of these comets based on these observations with the IRAM telescope has been presented in Biver et al. (2014, 2015) and will be further detailed in a future paper.

2.2. Odin observations of comet C/2014 Q2 (Lovejoy)

Because comet C/2014 Q2 (Lovejoy) clearly became a very active comet, observations with the Odin 1.1 m submillimeter satellite (Frisk et al. 2003) were triggered on a late notice. Observations took place between 30.5 January and 3.4 February 2015. These observations were scheduled to map the emission of H₂¹⁶O at 556.9 GHz, follow its temporal evolution, and detect the H₂¹⁸O line at 547.7 GHz. The half power beamwidth is close to 2.1′ at these frequencies and the main beam efficiency is 0.89. Odin uses three spectrometers: a 1 GHz bandwidth acousto-optical spectrometer (AOS with 0.6 MHz sampling) and two autocorrelators (AC1 and AC2) used in their highest resolution mode (~0.15 MHz with 100 MHz bandwidth). For the first setup, two receivers tuned to the H₂¹⁶O line were used in parallel, coupled to the three spectrometers: AOS and AC2 on receiver 555-B2 and AC1 on 549-A1. For the second setup, the AOS was connected to the 572-B1 receiver tuned near the ammonia line at 572.5 GHz, but not frequency locked, while AC2 was connected to the receiver 555-B2 and AC1 to 549-A1, both tuned to the H₂¹⁸O line.

As Odin is in a helio-synchronous polar Earth orbit at 550 km altitude, the comet was only observable during ~55 min of each 96 min orbit. The Earth was in the field of view during the remaining time, and the atmospheric lines were used to calibrate the frequency scale. Some of the observations failed to track the comet correctly, but half were successful and provided good results. Spectra of the nucleus-centered H₂¹⁶O and H₂¹⁸O lines are shown in Fig. 3 and a map of the H₂¹⁶O line is shown in Fig. 1.

3. Modeling of the lines and model parameters

3.1. Velocity and outgassing pattern

The mean gas expansion velocity was derived from the shape of the lines with the highest signal-to-noise ratio. We determined a value of 1.1 to 1.0 km s^-1 for comet Lemmon and 0.8 km s^-1 for comet Lovejoy (Table A.1). As discussed hereafter, comet Lovejoy was observed with the larger beam of Odin; a value of 0.85 km s^-1 provided a better fit to the width of the water lines. This is expected, as acceleration of the gas expansion velocity in the coma is predicted and observed in the outer part of the comae of active comets.

3.2. Temperature and collisional excitation

For all molecules we used the latest spectroscopic data available in the JPL (Pickett et al. 1998) and CDMS (Müller et al. 2005) databases, both for line identification and computation of line strengths. We employed the code previously used for other comets (Biver et al. 2007; Hartogh et al. 2011; Bockelée-Morvan et al. 2012) to compute the excitation of the rotational levels of the molecules and the line intensities. We take into account the pumping of the rotational levels by the fluorescence of the infrared vibrational bands, which is not expected to play a major role at the cometocentric distances ≤5000 km sampled in the case of comet Lovejoy. In our model molecules slowly evolve from the local thermal equilibrium (T = 100−110 and 73 K, for comets Lemmon and Lovejoy, respectively; Table A.1) maintained by collisions with neutrals and electrons close to the nucleus to spontaneous decay of the rotational population in the less dense parts of the coma (Biver et al. 2000, 2006). Within the accuracy of the detections, line intensity ratios (or rotational temperatures) for methanol, and also for most other species, are in agreement with the model (e.g., T_rot(CH₃CHO, Lovejoy) = 67 ± 15 K for 68 K predicted). The density is described with the Haser model, using lifetimes from Crovisier (1994), Bockelée-Morvan et al. (2000), and Crovisier et al. (2004a,b). Line intensities are computed with a radiative transfer code that takes opacity effects into account. Except for H₂¹⁶O and HCN, lines are optically thin, meaning that line intensities are proportional to the production rates.

4. Water production rate and time evolution

4.1. Odin maps and excitation of water in the coma of comet Lovejoy

Several maps (e.g., Fig. 1) of the water emission were performed for comet Lovejoy with Odin in order to precisely derive the water production rate. The extension of the emission of optically thick submillimeter water lines in cometary comae is in large part governed by the excitation mechanism of the water rotational levels. It depends both on the temperature profile and collision rates with electrons and neutrals in the inner part of the coma (Biver et al. 2007; Hartogh et al. 2011; Bockelée-Morvan et al. 2012). However, in the outer part (beyond 150′′) the line intensity mostly depends on the water production rate (and assumed photo-dissociation lifetime), and very little on the assumed temperature profile. Dividing the gas temperature by a factor of 2 only increases the derived production rate by 10%. We used the same excitation parameters for the H₂¹⁶O observations with Odin, as those derived for the molecular lines observed with the IRAM 30 m instrument: a constant temperature of 73 K throughout the coma and an electron density scaling factor x_ne = 0.5 (Zakharov et al. 2007; Biver et al. 2007). The expansion velocity inferred from the H₂¹⁶O and H₂¹⁸O line widths is v_exp = 0.85 km s^-1, slightly larger than for the lines observed with the smaller beam of the IRAM 30 m telescope. The fit to the radial profile of the line intensity from the map (Fig. 1) is satisfactory and corresponds to Q_H2O = 8.7 ± 0.7 × 10²⁹ molec s^-1and the reduced χν2=2.4 when we include the 5% uncertainty due to the 10′′ (Frisk et al. 2003) pointing uncertainty. Taking into account an increase in the velocity with distance from the nucleus does not significantly change the goodness of the fit and the derived production rate.

4.2. Periodic variation in H162O and H182O production rates

Since H₂¹⁶O and H₂¹⁸O measurements with Odin were not simultaneous, but interlaced, in order to derive an accurate H₂¹⁶O /H₂¹⁸O production rate ratio it is necessary to determine if significant short-time variation of the activity is present. Inspection of production rates does suggest that time variation is present in the data. Since the observations of H₂¹⁸O constituted a series of ten successive Odin orbits, we did a time averaging of ≈ 2.5 h, corresponding to two consecutive orbits of observations, yielding a sufficient signal-to-noise ratio (5−10) to look for time variations. We fitted a sinusoidal time variation to the individual measurements of the production rates of H₂¹⁶O and H182O, independently. We only selected the H₂¹⁶O measurements with pointing offsets less than ~100′′. The results are provided in Table 1. For both molecules, the sinusoidal fit is better than assuming no time variation (χν2 reduced by a factor ~2.5), and the inferred period is similar (T_p = 0.922−0.940 day). A simulation of periodic variations of the production rates anticipated a longer delay between peak outgassing and peak signal for H₂¹⁶O than H₂¹⁸O (0.25 versus 0.14 day), which we took into account. We found different reference times T₀ for H₂¹⁶O and H182O, but these are strongly correlated to the T_p value. Formally, the T_p-independent 1σ uncertainty δT₀ is on the order of 0.4 days. We also tried to fit the data with the same time dependence for H₂¹⁶O and H182O, using a fixed mean period of T_p = 0.935 days (weighted average) and a corresponding reference time 31.742 Jan. 2015 UT. The goodness of the fits is similar (Table 1). A larger damping of the apparent production rate amplitude was expected for H₂¹⁶O due to optical depth effects. From the observations the effect is not so obvious: Δ Q/Q₀ is only slightly smaller for H₂¹⁶O than for H₂¹⁸O (0.19 versus 0.24). We did not investigate the possible variation of the gas temperature with the production rate, but this could explain a variation of the line intensity with time that was larger than anticipated.

We then combined both QH216O and QH218O to derive our best estimate of the water production rate as a function of time during the period 30 January to 3 February 2015 (last line of Table 1). Figure 2 shows the time evolution of the production rates measured for the two water isotopologues and best fit.

4.3. Reference water production rates

The reference water production rates were primarily derived from contemporaneous observations of the OH radical maser lines at 18 cm with the Nançay radio telescope. During the 12.8−16.8 January 2015 observing period of comet Lovejoy, both observations centered on the comet and at 3.0′ offset position were used to better constrain the quenching of the maser inversion (Gerard et al. 1998) yielding an average water production rate Q_H2O = 5.0 ± 0.2 × 10²⁹ molec s^-1 (Biver et al. 2015). For other periods and for comet Lemmon, the observations of the OH radical could not be used to derive an accurate water production rate owing to significant uncertainties in the quenching of the maser inversion. We either used the water production rates reported elsewhere (Combi et al. 2014; Paganini et al. 2014) or made an interpolation from the Odin observations of comet Lovejoy at perihelion. Based on Nançay and Odin observations, we adopt a mean Q_H2O = 5, 5.5, 6, and 7 × 10²⁹ molec s^-1 for the 13−16 January, 13–25 January, 23−25 January, and 26 January–3 February 2015 time intervals, respectively, with a ± 20% time variation possible within each time interval. Observations done on a short time interval might be affected by a periodic time variation, but this was tracked from the monitoring of the methanol production lines, which are present in all IRAM observations settings (Table A.1).

5. Isotopic ratios

Table A.2 provides the list of molecular lines (main species and isotopologues) observed at IRAM in comets C/2012 F6 (Lemmon) in 2013 and in C/2014 Q2 (Lovejoy) in 2015 and considered in this study. Line intensities and production rates (or upper limits) derived using the models presented in Sect. 3 are given. The corresponding isotopic ratios or 3σ limits are presented in Table 2.

5.1. ¹⁶O/¹⁸O ratio in comet Lovejoy

The analysis of Odin observations in Sect. 4.2 (Table 1) enabled us to derive an accurate H₂¹⁶O /H₂¹⁸O production rate ratio. From the sinusoidal fits and derived mean Q₀ production rates, we obtain:

• Q₀(H₂¹⁶O )/Q₀(H₂¹⁸O ) = 493 ± 29 for the independent fits;

• Q₀(H₂¹⁶O )/Q₀(H₂¹⁸O ) = 499 ± 24 for the phased fits.

The second approach, which corresponds to a H₂¹⁶O /H₂¹⁸O ratio that does not vary with time, yields ¹⁶O/¹⁸O = 499 ± 24, which is exactly the terrestrial value (498.7).

5.2. D/H ratio

Thanks to the wide frequency coverage of IRAM observations, lines of deuterated species of known cometary molecules were observed. None were detected except the HDO (2₁₁−2₁₂) line at 241.562 GHz (Fig. 3). This is not surprising as water is over a hundred times more abundant than HCN, H₂S, or H₂CO. Nevertheless, the upper limits on the D/H ratio in H₂S or H₂CO in comet Lovejoy are the lowest obtained so far in a comet.

5.2.1. D/H ratio in water in comet C/2012 F6 (Lemmon)

We observed the HDO (2₁₁−2₁₂) line at 241.561 GHz in comet C/2012 F6 (Lemmon) in March-April 2013. The detection of HDO (Fig. 4) is also marginal (4σ) and the retrieved intensity was possibly affected by poorer baselines on 8 April when we could not use the telescope wobbler to cancel sky emission. The inferred average HDO production rate for the period 14 March to 8 April 2013 is Q_HDO = 13 ± 3 × 10²⁶ molec s^-1, yielding D/H = 6.5 ± 1.6 × 10^-4. This is four times the Earth value (VSMOW D/H = 1.56 × 10^-4), the highest value found in a comet, but compatible with the Earth value at the 3σ level.

5.2.2. D/H ratio in comet C/2014 Q2 (Lovejoy)

The tracking of the comet activity via monitoring of the CH₃OH production rate and the precise determination of the water outgassing rate using OH, H₂¹⁶O and H₂¹⁸O make us confident that we have a good H₂O production rate reference in order to derive the D/H ratio. We report here the detection of HDO with the IRAM 30 m telescope in this comet. The HDO (2₁₁−2₁₂) line was observed on 13, 15, 16, 23, and 24 January 2015. The average yields a 4σ detection (Fig. 3) corresponding to Q_HDO = 15.3 ± 4.1 × 10²⁵ molec s^-1, hence a D/H ratio of 1.4 ± 0.4 × 10^-4. Although the detection is marginal, in the worse case taking a 5σ upper limit would yield D/H< 1.86 × 10^-4, suggesting that comet Lovejoy has a D/H ratio for water similar to that of the Earth (VSMOW D/H = 1.56 × 10^-4), or possibly even lower. Previous measurements of the D/H ratio in Oort cloud comets yielded values larger than VSMOW (see Sect. 6.1).

5.3. ¹⁴N/¹⁵N ratio in HCN

One of the primary goals of the observing program of comet Lovejoy was to measure the ¹⁴N/¹⁵N ratio in HCN. On January 13, 15, and 16 we first observed the main isotopologue transition HCN (3−2) at 265.886 GHz, then carried out longer integrations using the setup covering the H¹³CN (3−2) line at 259.012 GHz and the HC¹⁵N (3−2) line at 258.157 GHz (Fig. 5).

The HCN reference production rate was computed taking opacity effects into account. The average value for the 13−16 January period is Q_HCN = 4.87 ± 0.02 × 10 × 10²⁶ molec s^-1, using pointing at the nucleus and at 5−8′′ offsets – which both yield exactly the same value. The HCN production rate based on the hyperfine component HCN (J = 3−2,F = 2−2) with a total statistical weight of 3.7% is marginally higher (+14%, 2σ), possibly because of contamination by the wings of the main HCN hyperfine components.

In order to minimize biases due to time variation of the outgassing and other observing uncertainties (calibration, pointing uncertainty, etc.), we rescaled the HCN production rate to the time when H¹³CN (3−2) and HC¹⁵N (3−2) were observed, using the CH₃OH production rate as a reference since several methanol lines were observed in each setup. The correction applied to Q_HCN is +10% for the 13–16 January observations and +27% for 23–25 January. The compound HC¹⁵N (3−2) is clearly detected during both periods (signal-to-noise ratios of 9 and 6). The derived ¹⁴N/¹⁵N ratios are 145 ± 16 and 144 ± 23, for 13–16 January and 23–25 January, respectively.

The compounds HC¹⁵N (3−2) and H¹³CN (3−2) were not clearly detected (Fig. 6) in comet C/2012 F6 (Lemmon), but the 2σ value or lower limit we obtained for the ¹⁴N/¹⁵N ratio in HCN (≥ 106 or 152 ± 72) is fully compatible with values found in other comets.

5.4. ¹²C/¹³C ratio

The H¹³CN (3−2) line at 259.012 GHz is in the same spectral band as HC¹⁵N (3−2). The production rates (assuming the same photo-dissociation lifetime as for HCN) and relative abundances are derived in the same way for both molecules. Owing to its hyperfine structure similar to that of H¹²CN (3−2) (Fig. 5), the H¹³CN (3−2) line only contains 93% of the flux in the (−1.2−1.2 km s^-1) window and this was taken into account.

The derived ¹²C/¹³C ratios in comet Lovejoy and Lemmon are 109 ± 14 and 124 ± 64, respectively. Within the uncertainties, they are compatible with the terrestrial value (89.7) although slightly higher (1.4σ for Lovejoy). This trend was also observed for comet 17P/Holmes (Bockelée-Morvan et al. 2008) and Hale-Bopp (Jewitt et al. 1997). Combining the three ¹²C/¹³C measurements yields a value 2.5σ larger than the Earth value. This interesting result needs to be further investigated in other comets.

The sensitivity was not sufficient to detect other ¹³C isotopologues. The most sensitive limit was reached for the ¹³CH₃OH (5_{+ 2}−4_{+ 1}E) and ¹²CH₃OH (5_{+ 2}−4_{+ 1}E) lines at 261.113 and 266.838 GHz, respectively. The derived lower limit is ¹²C/¹³C > 61 in comet Lovejoy, compatible with the Earth value.

5.5. ³²S/³⁴S ratio

The wide 8 GHz frequency coverage of the EMIR receiver at IRAM, allows us to observe both C³²S and C³⁴S or H322S and H342S lines in the same FTS spectrum since they are less than 4 GHz apart. As a consequence, the isotopic ratios do not depend on any calibration or pointing effects and, since the lines are mostly optically thin, the abundance ratio is very close to the line intensity ratio. The integration time on the H₂S lines (either around 168 or 216 GHz) was not sufficient to yield a significant constraint on the H322S/H342S ratio. On the other hand, the C³⁴S (5−4) line at 241 016.089 MHz was detected in both comets (Figs. 7 and 8). The accuracy of the ³²S/³⁴S ratio in CS is given by the signal-to-noise ratio of the C³⁴S line, and we found ³²S/³⁴S = 20 ± 5 and 24.7 ± 3.5 in comets Lemmon and Lovejoy, respectively. We do not find any significant departure from the Earth value of ³²S/³⁴S = 22.5. The C³³S line lies between the C³⁴S and C³²S lines. The upper limit for the C³³S (5−4) line at 242.913 GHz in comet Lovejoy yields ³²S/³³S > 50. This is the first reported estimate of ³³S in a comet, but this is only a lower limit compatible with the Earth value (126.7).

6. Discussion

We measured a terrestrial value for the ¹⁶O/¹⁸O ratio in water in comet Lovejoy, as was the case in all comets in which this ratio had been measured before (Balsiger et al. 1995; Biver et al. 2007; Bockelée-Morvan et al. 2012, 2015; Altwegg et al. 2015).

The D/H and ¹⁴N/¹⁵N ratios in solar system bodies are of particular importance as they vary from object to object, with the heavier elements enriched in the Earth and small bodies compared to Jupiter or the protosolar values. These ratios trace the various enrichment and fractionation mechanisms that were at play in the proto-planetary nebula. The other isotopic ratios investigated here do not show significant variations in solar system objects within the accuracy of our measurements. Figure 9 shows the ¹⁴N/¹⁵N versus D/H ratios measured in solar system objects, including our new measurements presented here.

6.1. Earth-like water in comet Lovejoy

We obtained a measurement or at least a stringent upper limit for the D/H ratio in water of two dynamically old Oort cloud comets. We measured the lowest D/H ratio in water in a comet so far: 1.4 ± 0.4 × 10^-4 (< 1.8 × 10^-4; 5σ) in Lovejoy, while the corresponding ratio could be four times higher in comet Lemmon. These two D/H measurements are based on the same line in two comets of similar activity. Comet Lemmon was slightly more active but also two times farther away. We do not expect that the excitation mechanism of HDO in the coma of these comets can yield a factor of four difference in the derived production rates. So the observed diversity in the D/H ratio can be real and would suggest that these two comets, although they come from the same current reservoir, were formed in different places or at different times in the young solar system. In either case, this brings additional proof that comet dynamical origin does not imply a specific formation region in the disk. Previous studies (Hartogh et al. 2011; Lis et al. 2013; Bockelée-Morvan et al. 2012; Altwegg et al. 2015) have shown a growing diversity in the measured values of the D/H ratio in cometary water, and especially variations by more than a factor of 3 among Jupiter family comets, which are thought to originate from the Kuiper belt. Now we find that a similar range of values is present among Oort cloud comets. This is in line with the latest models (Brasser & Morbidelli 2013), which suggest that all comets were formed in the same extended massive original Kuiper Belt beyond the giant planets and were later scattered to the two current reservoirs, the scattered Kuiper disk and the Oort cloud. The diversity in deuterium enrichment would occur because the pre-solar cloud was initially enriched in HDO (higher D/H in water than in H₂: protosolar D/H value in H₂ is 2 × 10^-5, Fig. 9), but the inner warmer parts of the proto-planetary nebula lost more of their initial enrichment in deuterated water.

6.2. ¹⁴N/¹⁵N in comets

Measurements of ¹⁴N/¹⁵N in comets are relatively scarce. Although the ¹⁴N/¹⁵N ratio is 5−20 times lower than the H₂¹⁶O /HDO ratio, the nitrogen bearing molecules in cometary comae are less than 1% in abundance relative to water. So the detection of ¹⁵N isotopologues requires very active comets. The most sensitive technique is optical high resolution spectroscopy, which has provided several detections of C¹⁵N (Manfroid et al. 2005, 2009; Hutsemékers et al. 2009) and recently detections of ¹⁵NH₂ (Rousselot et al. 2014; Shinnaka et al. 2015), but this technique only gives access to daughter species. The compounds N₂ and NH₃ are not very abundant (Rubin et al. 2015; Biver et al. 2012) and do not have strong lines observable from the ground, nor do their ¹⁵N isotopologues. Only HCN is easily detectable in the radio and infrared. We report here the third clear (S/N> 10) detection of HC¹⁵N in a comet after its detection in comets Hale-Bopp and 17P/Holmes (Bockelée-Morvan et al. 2008).

We measure the same enrichment in ¹⁵N in HCN in comet Lovejoy as in other comets, in spite of their different dynamical origins (Bockelée-Morvan et al. 2008). The same enrichment was also found in CN in many other comets (Hutsemékers et al. 2009). This is a factor of two enrichment in ¹⁵N in comparison to the Earth atmospheric value (272) and a factor of three in comparison to the proto-planetary value (441, Marty et al. 2011) (Fig. 9). The enrichment is similar to that found in NH₂ (Rousselot et al. 2014; Shinnaka et al. 2015), thought to be mostly produced by the photo-dissociation of NH₃, the main volatile carrier of cometary nitrogen (Biver et al. 2012).

Correspondingly low ¹⁴N/¹⁵N ratios are also detected in carbonaceous interplanetary dust particles (IDPs) (Messenger 2000; Floss et al. 2004). These large enrichments with respect to solar or terrestrial values can be explained by interstellar chemistry theories involving ion-molecule ¹⁵N fractionation at 10 K (e.g., Wirström et al. 2012). Recently, significant nitrogen fractionation has been measured in dark cloud cores with ratios as low as 150 for nitriles (Milam et al. 2015). Thus, the similarity of the ¹⁴N/¹⁵N ratios found in comets and IDPs strengthens a putative link to interstellar chemistry as the origin of isotopically anomalous organic particles in comets.

Nevertheless, our observations confirms that the trend of a twofold enrichment in ¹⁵N compared to Earth observed in C¹⁵N is also present in HCN. A consequence concerning the debate of the origin of CN in cometary coma is that we cannot exclude HCN as the sole parent of CN on the basis of different ¹⁴N/¹⁵N ratios.

7. Conclusions

Chemical diversity is observed in the population of Oort Cloud comets, with abundances varying by up to a factor of ten for several species. The origin of this diversity is unclear and might reflect comet formation at different places or times in the early solar system.

The observations presented here provide further evidence that diversity is present in the enrichment in deuterium in cometary water with respect to the protosolar value (HD/H₂), both in Jupiter family and Oort cloud comets, in line with the latest scenarios of comet origins (Brasser & Morbidelli 2013). On the one hand, we found Earth-like D/H, ¹⁶O/¹⁸O , ¹²C/¹³C , and ³²S/³⁴S ratios in comet C/2014 Q2 (Lovejoy), strengthening the role that some comets may have played in supplying material to the young Earth, especially complex organic molecules (Biver et al. 2015). On the other hand we confirm the trend of finding a systematic twofold enrichment in ¹⁵N in cometary HCN relative to its abundance in Earth N₂, whose origin is puzzling.

We were also able to obtain in comet C/2014 Q2 sensitive limits, some of the best obtained so far with remote observations, on isotopic ratios in other molecules such as D/H in H₂S, H₂CO, or C³²S/C³³S.

Acknowledgments

The IRAM observations were conducted under the target of opportunity proposal D04-14 and regular proposal 128-14 and we gratefully acknowledge the support from the IRAM director for awarding us discretionary time and the IRAM staff for its support and for scheduling the observations on short notice. This research has been supported by the Programme national de planétologie de l’Institut des sciences de l’univers (INSU). S.N.M. acknowledges the NASA Planetary Astronomy program for support.

References

Appendix A: Additional tables

All Tables

All Figures