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Open Access
Issue
A&A
Volume 689, September 2024
Article Number A77
Number of page(s) 7
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202450634
Published online 04 September 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1 Introduction

Comets formed from dust and gas in our proto-planetary disc about 4.6 billion years ago. After their formation, they were redistributed in their current reservoirs as a consequence of strong gravitational interactions with the forming giant planets (Gomes et al. 2005; Brasser & Morbidelli 2013; Walsh & Morbidelli 2011). There, the nuclei have remained frozen to this day, retaining most of the chemical and mineralogical properties associated with their formation site. A proper understanding of the composition of comets can thus offer a unique window into the chemistry and physics of the early Solar System (Eistrup et al. 2019; Altwegg et al. 2019; Mumma & Charnley 2011; Lippi et al. 2024).

Since 1995, spectroscopic surveys of comets in the mid-infrared (≈2.5 to 5 µm) have allowed the study of the emission lines produced by solar-pumped fluorescence of several gas species sublimating directly from the nucleus (i.e. primary species, e.g. H2O, NH3, HCN, CO, CH4, C2H2, C2H6, CH3OH, and H2CO). Currently about 35 comets have been studied thoroughly using this technique, revealing a rich chemical complexity and diversity among the comet population (Biver et al. 2022; Lippi et al. 2021; Bockelée-Morvan & Biver 2017; Dello Russo et al. 2016). Nevertheless, the statistic is still insufficient to identify classes and sub-classes based on comets’ chemistry and to create a taxonomy for these objects.

Comet C/2023 H2 (Lemmon) (hereafter C/2023 H2) is a long period comet discovered on April 23, 20231, that reached perihelion on October 29, 2023, at 0.89 au from the Sun. After perihelion C/2023 H2 brightened far above the expectations up to Mυ ≈ 6, showing also a weak ion tail (see Fig. 1). Thanks to its successive close encounter with our planet (minimum geocentric distance 0.19 au, on November 10, 2023) observations of C/2023 H2 provided the rare opportunity to measure the composition of an object that would be otherwise too faint to be observed in standard circumstances.

In this paper, we present the chemical composition of C/2023 H2 as retrieved with the CRyogenic InfraRed Echelle Spectrograph (CRIRES+) at the ESO Very Large Telescope (VLT) (Kaeufl et al. 2004; Dorn et al. 2014), under the Discretionary Director Time program 2112.C-5015. We further complement the infrared data with those acquired from close-in-time optical observations using the Isaac Newton Telescope’s Intermediate Dispersion Spectrograph (INT-IDS). Finally, we discuss the overall results in the context of the potential existence of a sub-class of CO-enriched comets.

2 Observations and data reduction

We targeted comet C/2023 H2 with CRIRES+ during four nights between November 24 to 27, 2023 (see observation log in Table 1). To investigate the overall composition of the comet, we selected three settings: L3377 for the study of H2O, HCN, C2H2, NH3, and C2H6; L3302 for H2CO, CH3OH, H2O, and CH4; and M4318 for H2O and CO. The slit width was fixed to 0.4″ and oriented along the extended Sun-comet radius vector direction.

We processed all data sets in a systematic way using NASA-Goddard custom and semi-automated tools that include the latest procedures for flat-fielding and the removal of high dark current pixels and cosmic ray hits, along with spatial and spectral straightening with milli-pixel precision (cf., Bonev 2005; DiSanti et al. 2006; Villanueva et al. 2013). Spectral calibration and compensation for telluric absorption was achieved by comparing the data with highly precise atmospheric radiance and transmittance models obtained with the Planetary Spectrum Generator (PSG, Villanueva et al. 2018, 2022). Flux calibration was obtained using a suitable standard star observed closely in time with the comet and processed with the same algorithms. A selection of the reduced spectra obtained for C/2023 H2 are shown in Fig. 2, revealing a clear contrast in signal to noise between hyper-volatile and non-hyper-volatile species, as well as a very low dust continuum.

We extracted the spectra over 21 pixels centred on the nucleus (the central pixel and ± 10 pixels, equivalent to approximately ±250 km). Nucleus-centred production rates QNC and rotational temperatures Tгot were obtained by fitting well-established quantum mechanical fluorescence models (see e.g. DiSanti et al. 2006; Radeva et al. 2011; Villanueva et al. 2012a,b; Lippi et al. 2013) to the extracted spectra using two different and complementary fit methodologies: a χ2 minimisation process based on the Levenberg–Marquardt non-linear fitting algorithm (Villanueva et al. 2011) and a correlation analysis (Bonev 2005); in all the calculations, we assumed a spherically symmetric uniform outflow v=0.8 Rh−0.5$\v = 0.8R_{\rm{h}}^{ - 0.5}$ km s−1, where Rh is the heliocentric distance in au.

A rotational temperature of 50 ± 3 K was retrieved from the spectra of CO and C2H6, both showing many strong emission lines, and was assumed to be the same for all the other species; in fact, it was not possible to measure a reliable rotational temperature from emission lines of water or other species due to the low signal-to-noise. To obtain the final total production rates QT, the nucleus-centred production rates QNC were then multiplied by a common growth factor of (1.65 ± 0.07) that takes into account slit losses and aperture effects, and that was derived from the spatial analysis of ethane and CO lines (for details on the growth factor formalism, see, e.g. Villanueva et al. 2011; Faggi et al. 2019 or Appendix B2 in Dello Russo et al. 1998). Since the growth factor is the same for all the species, relative abundances with respect to water were calculated using nucleus-centred production rates (i.e. QNC/QNC(H2O)%). The results are reported in Table 2 and discussed in the next section.

In addition to the infrared data set, six long-slit spectra of C/2023 H2 were obtained on November 16, 2023, using the INT-IDS, each with an exposure time of 180s. The blue-sensitive EEV10 detector with the R400B grating yield a spectral coverage of 300-650 nm, with a resolution of 0.14 nm. A slit width of 2″ was used, with an orientation of 28 degrees measured from the north towards the east, that is, perpendicular to the horizon, in order to avoid losing flux because of atmospheric refraction. The data sets were bias-subtracted and flat-fielded, and atmospheric emissions and cosmic rays were removed. The flux was integrated along the slit within a 10 000 km distance from the nucleus and the extracted spectra were extinction-corrected and flux-calibrated using standard star observations. The dust contribution was removed by adjusting a slope-modified spectrum of a solar analogue to the continuum. The final average spectrum is shown in Fig. 3.

Line fluxes from the different species, that is, OH, CN, C2, C3, NH, and CH, were converted to molecular production rates using a standard Haser model (υp = υd = 1 km s−1) (Haser 1957). We used parent and daughter scale lengths from Randall et al. (1992) for C2, C3, CN, and NH; from Cochran & Schleicher (1993) for OH; and from Cochran & Cochran (1990) for CH. We used fluorescence factors from Cochran & Barker (1985) for CH and from the Lowell Minor Planet Services2for all other molecules. The calculated production rates are listed in Table 3.

Table 1

CRIRES+ observation log for C/2023 H2.

thumbnail Fig. 1

Image of C/2023 H2, showing a bright coma and faint ion tail. The image was obtained by G. Masi on November 8, 2023 (see the Virtual Telescope Project at www.virtualtelescope.eu).
thumbnail Fig. 2

Selection of spectra of C/2023 H2 obtained with CRIRES+. The top panel shows the identification of many lines of CO in the M3418 setting (all the detectors, order 12). In the middle plots, we show from left to right emission lines of CH4 (setting L3302, detectors 1 and 2, order 17) and C2H6 (setting L3377, detector 1, order 17); other species such as methanol and OH are barely visible. In the bottom panel, we show detection of water and OH, in setting L3302, detectors 1 and 2, and order 19. For each panel, the modelled spectra is shown in red; single models are shifted from the observed spectra and colour-coded (CO: orchid, H2O: green, C2H6: dark magenta, CH4: dark cyan, CH3OH: chocolate, OH (prompt emission): olive). Below the models, we plotted in grey the residual (after subtraction of the modelled emissions) together with the expected stochastic noise envelope (±1σ) in yellow.

3 Results and discussion

3.1 Mixing ratios in C/2023 H2 and comparison with other comets

We retrieved a water production rate of (59 ± 7) × 1026 mol/s by combining the flux of 15 lines detected over the entire spectral range covered by the three settings. This value is unusually low compared to that measured in other Oort Cloud comets at similar heliocentric distances (see for example Lippi et al. 2021). The retrieved production rates and mixing ratios with respect to water for the other molecular species reveal a volatile-rich chemistry very similar to that of other comets with a high content of CO, namely C/2009 P1 (Garrad), C/1999 T1 (McNaught-Hartley), and C/2013 R1 (Lovejoy), as shown in Fig. 4 (see for example DiSanti et al. 2014; Lippi et al. 2021; Paganini et al. 2014, respectively). Compared to other comets observed within 1.5 au from the Sun, C/2023 H2 does indeed display the highest mixing ratio with respect to water of CO (16.3 ± 0.6%), as well as high abundances of C2H6, CH4, HCN, and CH3 OH (1.00 ± 0.06%, 1.22 ± 0.02%, 0.26 ± 0.08%, and 2.9 ± 0.3%, respectively); 3σ upper limits for C2H2, H2CO, and NH3 are also compatible with an enriched chemistry.

In Table 4, we further compare C/2023 H2 to additional CO-rich comets considering the QCO/QHCN abundance ratio, following Wierzchos & Womack (2018) and Cordiner et al. (2020). We define as CO-enriched comets those with a CO/H2O ratio greater than 7.7%, that is, than the 75th percentile resulting from the box plot statistic in Lippi et al. (2021); additionally we sorted comets by heliocentric distances and we distinguished between those observed within and beyond 2.5 au from the Sun, as the latter tends to show significantly higher CO/HCN ratios on average. It is possible to notice that the CO/HCN ratio in C/2023 H2 is well in agreement with the majority of CO-enriched comets observed within 2.5 au from the Sun. There are no similarities instead with comets observed at larger heliocentric distances, and especially with C/2016 R2 (PANSTARRS), which showed the highest CO abundance ever recorded in a comet, and with the interstellar object 2I/Borisov which is also particularly rich in CO (e.g. McKay et al. 2019; Cordiner et al. 2020, respectively).

In the optical range, the spectrum is dominated by bright gas emissions in comparison to a faint dust contribution. We report a clear detection of OH, CN, C2, C3, CH, NH, and NH2.

Additionally some peaks are visible at 378 nm, at 400 nm in between the lines of the C3 complex, as well as a doublet at 425 nm and 427 nm. These lines are consistent with some of the strongest emission features from CO+ ions in this region, corresponding to the (4−0), (3−0), and (2−0) transitions, respectively (Magnani & A’Hearn 1986). Although we derived lower production rates than Jehin et al. (2023), which could indicate variations in activity or anisotropic outgassing, we measured similar mixing ratios between OH, C2, and CN. While we calculated a significantly lower C3 production rate, we partially explain this difference by contamination between different molecules in this spectral region and differences in methods, in particular in dust-correction procedures. The low OH production rate and presence of CO+ ions, which are rarely detected in optical spectra, are in agreement with the volatile-rich composition observed in the infrared.

According to the composition-based classification of comets established by A’Hearn et al. (1995), our optical measurements place C/2023 H2 into the ‘typical’ composition group, as opposed to the carbon-chain depleted one. Our abundance ratios for C2, C3, CN, and NH with respect to OH are within the expected range for the typical class, but they are above the average ratios within this class (which are 0.32% for CN/OH, 0.36% for C2/OH, 0.026% for C3/OH, and 0.43% for NH/OH), indicating a composition rich in organics. For comparison, optical observations of C/2013 R1 (Lovejoy) (hereafter C/2013 R1) made at a heliocentric distance of 1.09 au by Opitom et al. (2015) show that C/2013 R1 has a CN/OH ratio of about (0.35 ± 0.02%), in agreement with the one we measured in C/2023 H2 (0.423 ± 0.073%) and with the HCN/H2O ratio for C/2023 H2 inferred from the infrared observations (0.26 ± 0.08 %). The C2/OH ratios are similar in both comets (0.497 ± 0.121% and 0.523 ± 0.094% for C/2013 R1 and C/2023 H2, respectively). Opitom et al. (2015) suggest that extended sources of C2 in the coma of C/2013 R1 are contributing to an increased C2 measurement, in which case this is also compatible with C/2023 H2 being richer in parent molecules of C2 with respect to water.

If we consider the A(0)fρ value of (39 ± 1) reported in Jehin et al. (2023), we estimate a dust-to-gas ratio of about −25.93 (based on log[A(0)fρ/Q(OH)], see A’Hearn et al. 1995), which would classify this comet as being dust-poor. This would be consistent with the low dust signal that we observed both at infrared and optical wavelengths. At the same time, we measured a water production rate that is one order of magnitude lower than other long period comets observed at heliocentric distances of about 1 au, suggesting that this comet may also have a small nucleus. Unfortunately, we could not find any estimates of the nucleus size of C/2023 H2 in the literature, and therefore we are unable to confirm this hypothesis.

Table 2

Averaged molecular abundances measured in C/2023 H2 using CRIRES+.

thumbnail Fig. 3

Optical spectrum of C/2023 H2 acquired on November 16, 2023, using the INT-IDS. Vertical lines indicate the detected emission features of OH (dashed purple), NH (dashed grey), CN (solid green), C3 (dashed yellow), CH (dashed blue), C2 (solid red), NH2 (dashed grey), and [OI] (dotted purple). Tentatively detected emissions of CO+ are indicated by dotted pink lines.
Table 3

Molecular abundances measured in C/2023 H2 using the INT-IDS on November 16, 2023, while the comet was at Rh = 0.95 au and ∆ = 0.29 au.

thumbnail Fig. 4

Comparison of C/2023 H2 with other comets. In the upper panel, C/2023 H2 is compared with C/2009P1, C/1999T1, and C/2013 R1, which show the closest relative abundance proportions. In the bottom plot, for each molecular species, we compare C/2023 H2 (marked with a magenta star) with the box plot statistic from other infrared results relative to comets observed within 2 au from the Sun (data are from Lippi et al. 2021 and references therein). For each box the middle line corresponds to the median, the box limits to the 25th and 75th percentiles, and the whiskers to the 5th and 95th percentiles. C/2009P1, C/1999T1, and C/2013 R1 are also shown with a grey square, diamond, and triangle, respectively.
Table 4

QCO/QHCN abundance ratios measured in CO-rich comets.

3.2 Two hypotheses to interpret the high abundance of hyper volatiles in C/2023 H2

A first scenario that could explain the C/2023 H2 composition points to an original chemistry, with the formation of this comet in a region of our proto-planetary disc where CO and other organic species were particularly enriched. To explain the existence of such comets, Price et al. (2021) modelled the CO/H2O abundance ratios in the mid-plane of a proto-planetary disc, managing to reproduce a population of CO-rich objects in the outer region, and suggesting that these comets may be more common than previously known. The capture of comets from a close CO-rich forming disc in the Sun’s birth cluster is also a possibility (Levison et al. 2010). A sub-class of comets enriched in CO is indeed emerging from spectroscopic measurements as shown in Table 4, although it is probably still underrepre-sented since infrared observations typically favour bright comets characterised by high water (and/or OH) production rates. In this regard, also C/2023 H2 was not initially considered a suitable infrared target, but was selected only when its magnitude increased more than expected.

Alternatively, water sublimation in this comet may have been mitigated. In fact, C/2023 H2 is a long-period comet with an orbital period of about 3730 years3 that has already experienced a few perihelion passages. This may have resulted in the formation of a thick dust mantle on its nucleus over time. If the activity around perihelion was not enough to remove the outer layers of the nucleus, this may have left this dust mantle and/or only water-poor pebbles exposed (Ciarniello et al. 2022), limiting the sublimation of water but not affecting hyper-volatile species in the same way (Marboeuf & Schmitt 2014).

In situ observations of comets 67P/Churyumov-Gerasimenko (e.g. Altwegg et al. 2019; Fornasier et al. 2023; Läuter et al. 2020; Hässig et al. 2015) and 103P/Hartley 2 (e.g. A’Hearn et al. 2011) have indeed shown that cometary nucleus surfaces are quite heterogeneous, and that strong seasonal variations in the outgassing may be present. This is reflected in ground-based measurements of the gas in the coma, which can similarly show spatial and temporal variability, making it more difficult to correlate the observed molecular abundances to the ice composition inside the nucleus (Altwegg et al. 2019; Läuter et al. 2019). Indeed, if water sublimation in C/2023 H2 was inefficient due to the presence of a surface thick dust mantel, the production rate we obtained from the gas in the coma might not be representative of the H2O abundance inside the nucleus. Hyper-volatile species, on the other hand, might not be impacted by this dust layer in the same way, and thus the resulting relative abundances reported in Table 2 may be biased with respect to the bulk composition. This highlights the need to establish alternative and complementary references for determining the composition of comets, which should be immune from the large fluctuation of water (e.g. C2H6 as proposed by Bonev et al. 2021).

4 Conclusions

We reported on production rates and relative abundances for nine different primary species in C/2023 H2 as measured by CRIRES+ between November 24 and 27, 2023, and five secondary species retrieved using the IDS-INT on November 16, 2023. Our results can be summarised as follows:

  • In comet C/2023 H2, the hyper-volatile species CO, C2H6, and CH4 display high abundances and probably play an important role in the activity of this comet; on the contrary, H2O shows a rather low production rate compared to other comets observed at similar heliocentric distances;

  • Close-in-time INT-IDS measurements are in line with the infrared results, especially considering the low production rate for OH, the low dust signal, and the presence of CO+ lines, rarely observed in optical spectra;

  • Overall, C/2023 H2 looks normal to enriched in organics, and shows many similarities with three other comets, namely C/2009 P1 (Garrad), C/1999 T1 (McNaught-Hartley), and C/2013 R1 (Lovejoy) and more in general with CO-enriched comets observed within 2.5 au from the Sun.

Our results suggest that C/2023 H2 may belong to a CO-enriched class of comets. This class may be not too rare, but it is probably still understudied since it suffers from observational biases. However, we cannot completely exclude that the unusual composition of this comet is the result of physical processes and heterogeneity of the ices in the nucleus and the gas in the coma. This highlights the need of additional observations especially tailored to understand whether this sub-class of comets is real, and to better comprehend the nature of the processes occurring on the surface of the nucleus and in the coma of active comets.

Acknowledgements

The presented results are based on observations under the DDT program 2112.C-5015, at the European Southern Observatory (ESO), Cerro Paranal. M. Lippi acknowledges funding from the “NextGen- erationEU” program, in the context of the Italian “Piano Nazionale di Ripresa e Resilienza (PNRR)”, project code SOE_0000188. S. Faggi, G. L. Villanueva and M. J. Mumma acknowledge support from the NASA’s Center for Astrobiology (GCA), Goddard’s Fundamental Laboratory Research (FLaRe) and the Sellers Exoplanet Environments Collaboration (SEEC).

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3

Source: Small-Body Database at https://ssd.jpl.nasa.gov/tools/

All Tables

Table 1

CRIRES+ observation log for C/2023 H2.

In the text
Table 2

Averaged molecular abundances measured in C/2023 H2 using CRIRES+.

In the text
Table 3

Molecular abundances measured in C/2023 H2 using the INT-IDS on November 16, 2023, while the comet was at Rh = 0.95 au and ∆ = 0.29 au.

In the text
Table 4

QCO/QHCN abundance ratios measured in CO-rich comets.

In the text

All Figures

thumbnail Fig. 1

Image of C/2023 H2, showing a bright coma and faint ion tail. The image was obtained by G. Masi on November 8, 2023 (see the Virtual Telescope Project at www.virtualtelescope.eu).
In the text
thumbnail Fig. 2

Selection of spectra of C/2023 H2 obtained with CRIRES+. The top panel shows the identification of many lines of CO in the M3418 setting (all the detectors, order 12). In the middle plots, we show from left to right emission lines of CH4 (setting L3302, detectors 1 and 2, order 17) and C2H6 (setting L3377, detector 1, order 17); other species such as methanol and OH are barely visible. In the bottom panel, we show detection of water and OH, in setting L3302, detectors 1 and 2, and order 19. For each panel, the modelled spectra is shown in red; single models are shifted from the observed spectra and colour-coded (CO: orchid, H2O: green, C2H6: dark magenta, CH4: dark cyan, CH3OH: chocolate, OH (prompt emission): olive). Below the models, we plotted in grey the residual (after subtraction of the modelled emissions) together with the expected stochastic noise envelope (±1σ) in yellow.
In the text
thumbnail Fig. 3

Optical spectrum of C/2023 H2 acquired on November 16, 2023, using the INT-IDS. Vertical lines indicate the detected emission features of OH (dashed purple), NH (dashed grey), CN (solid green), C3 (dashed yellow), CH (dashed blue), C2 (solid red), NH2 (dashed grey), and [OI] (dotted purple). Tentatively detected emissions of CO+ are indicated by dotted pink lines.
In the text
thumbnail Fig. 4

Comparison of C/2023 H2 with other comets. In the upper panel, C/2023 H2 is compared with C/2009P1, C/1999T1, and C/2013 R1, which show the closest relative abundance proportions. In the bottom plot, for each molecular species, we compare C/2023 H2 (marked with a magenta star) with the box plot statistic from other infrared results relative to comets observed within 2 au from the Sun (data are from Lippi et al. 2021 and references therein). For each box the middle line corresponds to the median, the box limits to the 25th and 75th percentiles, and the whiskers to the 5th and 95th percentiles. C/2009P1, C/1999T1, and C/2013 R1 are also shown with a grey square, diamond, and triangle, respectively.
In the text

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Astronomy & Astrophysics (A&A)

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