Emission lines of FeI and NiI are commonly found in the coma of Solar System comets, even at large heliocentric distances. These atoms are most likely released from the surface of the comet’s nucleus or from a short-lived parent. The presence of these lines in cometary spectra is unexpected because the surface blackbody equilibrium temperature is too low to allow the sublimation of refractory minerals containing these metals. These lines were also found in the interstellar comet 2I/Borisov, which has a NiI/FeI abundance ratio similar to that observed in Solar System comets. On average, this ratio is one order of magnitude higher than the solar Ni/Fe abundance ratio. Here, we report observations of the interstellar comet 3I/ATLAS, which were carried out with the ESO Very Large Telescope equipped with the UVES and X-shooter spectrographs. Spectra were obtained at heliocentric distances ranging from 3.14 to 1.85 au. Nil was detected at all epochs. FeI was only detected at heliocentric distances smaller than 2.64 au. We estimated the Nil and FeI production rates by comparing the observed line intensities with those produced by a dedicated fluorescence model. Comet 3I first exhibited extreme and unusual NiI/FeI abundance ratios during the initial stages of its activity. However, as its heliocentric distance decreased, this ratio became indistinguishable from those observed in Solar System comets and in comet 2I∕Borisov. Comet 3I was found to be C2-depleted, with a NiI/FeI abundance ratio finally consistent with other C2-depleted comets. Nevertheless, comet 3I remains exceptional due to its high, total production rate of NiI and FeI, which is at least one order of magnitude larger than that of other comets. We interpreted these observations assuming that the NiI and FeI atoms were released through the sublimation of Ni(CO)4 and Fe(CO)5 carbonyls. This scenario provides a straightforward explanation for the asymmetric release of NiI and FeI atoms in the cometary coma and how it depends on the heliocentric distance. It also supports the presence of carbonyls in the cometary material.

We present high-resolution UVES+VLT observations of neutral nickel and iron atoms in the coma of the interstellar comet 3I/ATLAS taken after perihelion. Metal emission was strong shortly after perihelion and persisted at large heliocentric distances. At r_h ∼ 2 au the total metal production rate was found to be at least an order of magnitude larger than that of typical solar-system comets. Post-perihelion production rates exhibit pronounced asymmetry compared to the pre-perihelion behavior: production rates are higher after perihelion and decline more gradually with r_h, the difference being stronger for FeI. The NiI/FeI abundance ratio, initially anomalously large before perihelion, evolved toward values comparable to solar-system comets near 2 au, and shows a weaker r_h dependence after perihelion. To interpret these results, we revisited and extended the carbonyl hypothesis in which FeI and NiI are produced by the rapid photodissociation of Fe(CO)_5 and Ni(CO)_4 vaporized from the nucleus. Fits that include direct sublimation of carbonyls reproduce the observed rates and the high NiI/FeI line ratio, which is determined by the higher volatility of Ni(CO)_4. Desorption of carbonyls from sublimating CO_2 and H_2O ices is found to be negligible. The temperature profiles needed to reproduce the observations were found to be shallower than the equilibrium T ∝ r_h^-1/2 relation, suggesting that the sublimation could occur below the surface of the nucleus. Fits using temperature profiles from thermal models require sublimation from depths of several cm, especially post-perihelion. An additional transient heat source (T ≃ 100-140 K), possibly linked to the amorphous-crystalline ice transition, is proposed to explain the early NiI excess before perihelion.

Comets are considered the most well-preserved fossils of the early solar system. Analyzing their volatile inventories allows us to quantify the extent of chemical and thermal evolution within the protoplanetary disk, offering key clues into the origins of planetary materials (Mumma M. J. & Charnley S. B., 2011, Lippi M., et al., 2024). The recent discovery of interstellar objects (ISOs, Fitzsimmons A., et al, 2018, 2019) has opened a new window, offering a direct baseline to compare our Solar System’s building blocks with those of other planetary systems. Investigating ISOs in the context of our solar system can ultimately reveal whether the chemical foundations of planets are universal or unique to their birth environments.In this contribution, we present high-resolution infrared (2–5 µm) data of the third interstellar visitor 3I/ATLAS, collected using CRIRES+ at ESO-VLT telescope (PI of the observing program C. Opitom). We sampled the object’s activity both before and after perihelion, and across heliocentric distances ranging from 1.8 to 2.7 au. In Figure 1, we show two selcted spectra. We report detections of CO, CH3OH, CH4, C2H6, and HCN, and significant upper limits for H2O, NH3, H2CO, and C2H2. Furthermore, we examine the spectral data for emission lines that may be attributed to isotopic species (e.g., HDO and CH3D), as well as HCl and C2H4.Finally, we compare the volatile composition of 3I/ATLAS to the existing cometary database (Dello Russo N., et al, 2016, Lippi M., et al, 2021) to place this interstellar object within the broader context of planet formation and chemical inheritance.Figure 1: Detection of CO and tentative detection of HDO for comet 3I/ATLAS in December 2025.References: Mumma M. J., Charnley S. B., Annual Review of Astronomy and Astrophysics, 2011, 49, 471-524; Lippi M., Podio L., Codella C., Faggi S., De Simone M., Villanueva G. L., Mumma M. J., Ceccarelli C., The Astrophysical Journal, 2024, 970; Fitzsimmons A., Snodgrass C., Rozitis B., Yang B., Hyland M., Seccull T., Bannister M. T., Fraser W. C., Jedicke R., Lacerda P., Nature Astronomy, 2018, 2, 133-137; Fitzsimmons A., Hainaut O., Meech K. J., Jehin E., Moulane Y., Opitom C., Yang B., Keane J. V., Kleyna J. T., Micheli M., Snodgrass C., The Astrophysical Journal, 2019, 885, L9; Dello Russo N., Kawakita H., Vervack R. J., Weaver H. A., Icarus, 2016, 278, 301-332; Lippi M., Villanueva G. L., Mumma M. J., Faggi S., The Astronomical Journal, 2021, 162, 74

We report initial observations aimed at the characterization of a third interstellar object. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity of e ∼ 6.1, perihelion of q ∼ 1.36 au, inclination of ∼175°, and hyperbolic velocity of V _∞ ∼ 58 km s ^−1 . We report deep stacked images obtained using the Canada–France–Hawaii Telescope and the Very Large Telescope that resolve a compact coma. Using images obtained from several smaller ground-based telescopes, we find minimal light-curve variation for the object over a ∼4 day time span. The visible/near-infrared spectral slope of the object is 17.1% ± 0.2%/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids). Moreover, 3I/ATLAS will be observable through early 2025 September, then unobservable by Earth-based observatories near perihelion due to low solar elongation. It will be observable again from the ground in late 2025 November. Although this limitation unfortunately prohibits detailed Earth-based observations at perihelion when the activity of 3I/ATLAS is likely to peak, spacecraft at Mars could be used to make valuable observations at this time.

We report Very Large Telescope spectroscopy of the interstellar comet 3I/Asteroid Terrestrial-impact Last Alert System (C/2025 N1), from r h ≃ 4.4 to 2.85 au, using X-Shooter (300–550 nm, R ≃ 3000) and the Ultraviolet and Visual Echelle Spectrograph (optical, R ≃ 35–80 k). The coma is dust-dominated, with a fairly constant red optical continuum slope (∼21%–22%/1000 Å). We report the detection of CN emission and also detect numerous Ni i lines, while Fe i remains undetected, potentially implying efficiently released gas-phase Ni. At r h ≃ 3.14 au, we derive 3 σ limits of Q (OH) < 1.48 × 10 26 s −1 but find no indications for [O i ], C 2 , C 3 , or NH 2 . From our latest X-Shooter measurements, conducted on 2025 August 21 ( r h = 2.85 au), we measure production rates of log Q ( CN ) = 24.81 ± 0.01 molecules s −1 and log Q (Ni) = 23.30 ± 0.07 atoms s −1 and characterize their evolution as the comet approaches perihelion. We observe a steep heliocentric distance scaling for the production rates Q ( Ni ) ∝ r h − 7.7 ± 1.0 and Q ( CN ) ∝ r h − 6.7 ± 0.2 , and we predict an Ni–CO (2) correlation if the Ni i emission is driven by the carbonyl formation channel. Energetic considerations of activation barriers show that this behavior is inconsistent with the direct sublimation of canonical metal/sulfide phases and instead favors low-activation-energy release from dust—e.g., photon-stimulated desorption or mild thermolysis of metalated organics or Ni-rich nanophases, possibly including Ni–carbonyl-like complexes. These hypotheses will be testable with future coordinated ground-based and space-based monitoring, as 3I becomes more active during its continued passage through the solar system.

The interstellar comet 3I/ATLAS is only the third interstellar object to be discovered. Pre-perihelion measurements provide a unique opportunity to study its activity and composition, which may alter as it is heated in the coming months. We provide an initial baseline from optical spectroscopic observations obtained only two days after discovery, using the MUSE instrument on the VLT on 2025 July 3, while 3I was at 4.47 au from the Sun and 3.46 au from the Earth. These observations confirm the cometary nature of 3I, and reveal a red coma with a spectral slope of (18 +/- 4) %/ 1000 angstrom in the 5000-9000 A range, redder than most Solar System comets but similar to the surface colour of some Solar System Trans-Neptunian Objects or Centaurs. We searched for but did not detect gas emission from C-2, NH2, CN, and [OI], which is consistent with volatile non-detections for Solar System comets at this heliocentric distance. At present, the coma appears entirely dusty. Future observations of 3I as it comes closer to the Sun will provide an invaluable opportunity to witness the evolution of its activity, study its composition, test predictions of interstellar object population models, and compare 3I to Solar System comets.

2I/Borisov is the first active interstellar comet observed in the Solar Sytem, allowing for the first time to sample the composition of a planetary building block from an extrasolar system. We report on the monitoring of 2I with the FORS low resolution spectrograph of the ESO VLT at Paranal during four months, from November 19, 2019 to March 20, 2020. We collected a dozen spectra at 8 different epochs allowing to follow the evolution of the comet activity and composition around perihelion. We also observed with the same instrumental setup an Oort Cloud comet, C/2019 U6 (Lemmon), at about the same heliocentric and geocentric distance than 2I/Borisov at perihelion (rh=Delta=2 au) and with similar AfRho value and Q(CN) in order to use it as a reference for the Solar System. The usual species are detected in the optical spectrum of 2I (CN, C3, C2, and NH2) and their production rates and abundance ratios are computed. The dust production rate and colors are also derived, compared to C/2019 U6 and other comets of the Solar System, and their evolutions are followed with the heliocentric distance.

Long Period Comets (LPCs) have orbital periods longer than 200 years, perturbed from their resting place in the Oort cloud. Such gravitational influences may send these icy bodies on a path towards the center of the Solar system in highly elliptical orbits. In this work, we present the activity and composition evolution of several LPCs observed with both TRAPPIST telescopes (TS and TN) during the period of 2019-2020. These comets include: C/2017 T2 (PANSTARRS), C/2018 Y1 (Iwamoto), C/2018 W2 (Africano), and disintegrated comet C/2019 Y4 (ATLAS). We monitored the OH, NH, CN, C2 and C3 production rates evolution and their chemical mixing ratios with respect to their distances to the Sun as well as the dust production rate proxy (A(0)fp) during the journey of these comets into the inner Solar system.C/2017 T2 (PANSTARRS) is a very bright comet which was discovered on October 2, 2017 when it was 9.20 au from the Sun. We started observing this comet with TS at the beginning of August 2019 when it was at 3.70 au. The comet made the closest approach to the Earth on December 28, 2019 at a distance of 1.52 au and it passed the perihelion on May 4, 2020 at 1.61 au. The water production rate of the comet reached a maximum of (4,27±0,12)1028 molecules/s and its dust production rate (A(0)fp(RC)) also reached the peak of 5110±25 cm on January 26, 2020, when the comet was at 2.08 au from the Sun (-100 days pre-perihelion). At the time of writing, we still monitoring the activity of the comet with TN at heliocentric distance of 1.70 au. Our observations show that C/2017 T2 is a normal LPC.C/2018 Y1 (Iwamoto) is a nearly parabolic comet with a retrograde orbit discovered on December 18, 2018 by Japanese amateur astronomer Masayuki Iwamoto. We monitored the activity and composition of Iwamoto with both TN and TS telescopes from January to March 2019. The comet reached its maximum activity on January 29, 2019 when it was at 1.29 au from the Sun (-8 days pre-perihelion) with Q(H2O)=(1,68±0,05)1028 molecules/s and A(0)fp(RC)= 92±5 cm. These measurements show that it was a dust-poor comet compared to the typical LPCs.C/2018 W2 (Africano) was discovered on November 27, 2018 at Mount Lemmon Survey with a visual magnitude of 20. The comet reached its perihelion on September 6, 2019 when it was at 1.45 au from the Sun. We monitored the comet from July 2019 (rh=1.71 au) to January 2020 (rh=2.18 au) with both TN and TS telescopes. The comet reached its maximum activity on September 21, 15 days post-perihelion (rh=1.47 au) with Q(H2O)=(0,40±0,03)1028 molecules/s.C/2019 Y4 (ATLAS) is a comet with a nearly parabolic orbit discovered on December 18, 2019 by the ATLAS survey. We started to follow its activity and composition with broad- and narrow-band filters with the TN telescope on February 22, 2019 when it was at 1.32 au from the Sun until May 3, 2020 when the comet was at a heliocentric distance of 0.90 au inbound. The comet activity reached a maximum on March 22 (rh=1.65 au) 70 days before perihelion. At that time, the water-production rate reached (1,53±0,04)1028 molecules/s and the A(0)fp reached (1096±14) cm in the red filter. After that, the comet began to fade and disintegrated into several fragments.

We report on the regular observation with broad band and cometary narrow band filters of the first interstellar active comet, 2I/Borisov, with both TRAPPIST-South and -North telescopes (TS and TN) [1]. We followed 2I activity since its discovery on September 11, 2019 (rh=2.8 au inbound) until the beginning of March 2020 when it was 2.9 au outbound and TS operations had to stop because of Covid-19 and la Silla Observatory closure. The comet activity reached a maximum on November 29, 2019 (rh=2.01au), 10 days before perihelion with an apparent magnitude of 16.50±0.04 measured within an aperture radius of 5 arcsec in R filter, an A(0)fp(R) dust proxy = 135±7 cm and a Q(CN)=(4.5±0.7) 1024 molecules/s (using a Haser model and vp=vd=1km/s [2]). Unlike the first interstellar object, 1I/Oumuamua, discovered in 2017 [3,4], 2I was showing an extended coma surrounding its nucleus and a short tail. We first detected CN in 2I with TN on October 18 when the comet was 2.65 au from the Sun while we never detected C2. We were able to follow the CN activity of 2I for more than 3 months.Figure 1: Light curve (R mag) and A(0)fp dust proxy of 2I/Borisov as a function of days to perihelion. The A(0)fp values are computed at 10 000 km from the nucleus and normalized to phase angle of 0°.References:[1] Jehin, E., Gillon, M., Queloz, D., et al. 2011, The Messenger, 145, 2[2] Haser, 1957, Bulletin de l'Académie Royale de Belgique, vol. 43, p. 740-750[3] Meech, K. J., Weryk, R., Micheli, M., et al. 2017, Nature, 552, 378[4] Micheli, M., Farnocchia, D., Meech, K. J., et al. 2018, Nature, 559, 223

Metals have been found in cometary dust by in-situ experiments onboard the Giotto and Rosetta spacecrafts[4,10] as well as in dust particles collected by the Stardust spacecraft[11]. They appear in silicate, sulfide and metal grains. Two sungrazing comets, the Great Comet of 1882[2] and C/1965 S1 (Ikeya-Seki)[9], approached the Sun so close that dust grains have vaporized, revealing lines of several metals in the coma spectrum, in particular FeI and NiI. However, it came as a surprise to find numerous FeI and NiI emission lines in high-resolution spectra of comets observed at large heliocentric distances[7], where the equilibrium temperature T~280r-1∕2K is far too low to allow sublimation of silicates (Tsub≥1200K) and sulfides (Tsub≥600K). In the present contribution, we summarize this discovery and the challenges it raises. Dozens of FeI and NiI emission lines were recently found in the spectral region 3000-4000Å for 17 comets at heliocentric distances between 0.68 and 3.25 au[7]. These comets were observed with the high-resolution spectrograph UVES mounted on the ESO VLT. FeI and NiI lines can also be detected in archival data obtained with other telescopes, as shown in Fig.1 for comet Hyakutake. The spatial extension of the lines is very short indicating that the FeI and NiI atoms originate from the inner coma, close to the nucleus. To compute the FeI and NiI production rates we built a fluorescence model that accounts for the complex absorption structure of the solar spectrum (Swings effect). In Fig.2 we show that the FeI+NiI production rate is correlated with the production rate of major constituents of cometary ices, either H2O or CO. The NiI/FeI abundance ratios are shown in Fig.3. They cluster around NiI/FeI~1 whatever the comet heliocentric distance. The NiI/FeI ratio does not depend on the comet type but there is evidence (indicated by a F-test) that its variance is higher in the Jupiter-family sample than in the Oort cloud sample. This is reminiscent of the wider range of C2/CN abundances observed in Jupiter-family comets[5]. The average abundance ratio is log(NiI/FeI)=-0.07 with a standard deviation of 0.29. This value differs from the ratio log(Ni/Fe)=-1.10±0.23 estimated in the dust of P/Halley[4] and log(NiI/FeI)=-1.11±0.09 in the coma of the sungrazing comet Ikeya-Seki[7]. The latter ratios are similar to the ratio measured in the Sun and the meteorites. Interestingly, the NiI/FeI ratio measured in the interstellar comet 2I/Borisov is in agreement with the ratio found in the solar system sample[8]. These observations raise two questions, still open. How can FeI and NiI atoms be released at such low temperatures? Why is the NiI/FeI abundance ratio one order of magnitude higher than the solar value? The release of FeI and NiI atoms from sulfides is appealing since their sublimation temperature is lower than silicates. Moreover FeNi alloys and sulfides formed in the low temperature range are Ni-rich, such as kamacite and pentlandite[6], so that their sublimation could explain the high NiI/FeI ratios we observed. Superheating is nevertheless required to reach T~600K, which might be possible if the grains are very small, like the metallic nanoparticles found in comet 81P/Wild2[11]. Collisions of cometary dust with high-velocity particles could also produce impact vapor with T~1000K[3]. Alternatively, FeI-NiI atoms could be released from organometallic complexes, yet undetected in the cometary material, such as carbonyls that have much lower sublimation temperatures. In particular Fe(CO)5 and Ni(CO)4 are expected to sublimate at Tsub~100K, intermediate between the sublimation temperature of H2O and CO2[7]. The higher sublimation rate of Ni(CO)4 with respect to Fe(CO)5 would naturally explain the overabundance of nickel. More work is clearly needed to test these hypotheses, both observationnaly and theoretically. The discovery of FeI and NiI atoms in distant comets neverthless indicates that constituents of the nucleus or processes in the coma are still missing. References [1]A’Hearn, M.F., Wellnitz, D.D., Meier, R.: urn:nasa:pds:gbo-kpno:hyakutake_spectra::1.0, NASA Planetary Data System (2015) [2]Copeland, R., Lohse, J.G.: Copernic.Int.J.Astron. 2, 225 (1882) [3]Ip, W.H., Jorda, L.: ApJL 496, L47 (1998) [4]Jessberger, E.K., Christoforidis, A., Kissel, J.: Nature 332, 691 (1988) [5]Levison, H.F.: ASPC 107, 173 (1996) [6]Lewis, J.S.: Physics and chemistry of the solar system, 2dEd., Elsevier (2004) [7]Manfroid, J., Hutsemékers, D., Jehin, E.: Nature 593, 372 (2021) [8]Opitom, C., Jehin, E., Hutsemékers, D., et al.: A&A, submitted (2021) [9]Preston, G.W.: ApJ 147, 718 (1967) [10]Stenzel, O., Hilchenbach, M., Merouane, S., et al.: MNRAS 469, S492 (2017) [11]Zolensky, M.E., Zega, T.J., Yano, H. et al.: Science 314, 1735 (2006) Figure 1: Example of NiI and FeI lines detected in the spectrum of comet C/1996B2 (Hyakutake) obtained on March 26, 1996[1]. Figure 2: Abundance correlations involving CO, H2O and FeI+NiI. Jupiter-family comets are in red. The CO-rich interstellar comet 2I (Borisov) (in blue) appears close to C/2016 R2[8]. Figure 3: NiI/FeI abundance ratios for 18 solar system comets at various heliocentric distances, including C/1996B2 Hyakutake observed using 4 offset slit positions[1]. The solar Ni/Fe ratio is indicated. Jupiter-family comets are in red.

IntroductionComets are pristine relics of the early Solar System, formed from the agglomeration of icy grains and dust particles, offering insights into the evolution of the protosolar nebula (PSN). Despite their usual water-ice richness, the blue comet C/2016 R2 (PanSTARRS) exhibited atypical abundance ratios, with an H2O/CO upper limit of less than 0.32% [1], weak CN lines [2], and an exceptionally high N2/CO ratio of 0.09 [3], suggesting unique formation conditions. Our study revisits historical observations of a historical blue comet, C/1908 R1 (Morehouse), utilizing high-precision scanning technology from the New Astrometric Reduction of Old Observations (NAROO) project [4], combined with numerical integration techniques, to reevaluate its dynamical history and spectroscopic data in order to determine the extent of its similarities with C/2016 R2.Dynamical historyUsing two independent dynamical models, MERCURY and REBOUND, we investigated the past trajectory of comet C/1908 R1 by simulating 1000 clones derived from its orbital covariance matrix. Both models revealed that C/1908 R1 has no close encounters with the giant planets, and had likely been stored in the Oort cloud at about 100,000 au, supporting its classification as a dynamically new comet. Despite a hyperbolic orbit with an average eccentricity of 1.16 ± 0.27, the calculated excess velocity v∞ of 0.83 km/s is much lower than those of known interstellar objects, reinforcing its Solar System origin. We compared this to C/2016 R2's dynamical history [5], but while C/1908 R1 remained dynamically isolated, C/2016 R2 experienced significant gravitational interactions with Jupiter, resulting in a chaotic and unpredictable orbit, though likely an Oort cloud origin.Spectroscopic analysisWe were able to obtain access to historical photographic plates of Comet C/1908 R1 preserved at the Meudon Observatory, covering observations from October 16 to November 29, 1908, including early spectroscopic data by A. de la Baume Pluvinel [6,7] and A. Bernard [8] from the Juvisy and Meudon observatories, respectively. These were digitized using the NAROO center's high-precision scanner (Fig.1). Despite the historical nature of the data and inherent challenges such as lack of historical information, potential atmospheric extinction misestimation due to lack of standard star identification in some cases, non-linear response of the photographic plates, and optical aberration from the objective prism, we were able to produce high-quality FITS files for detailed analysis and modeling. Spectral extraction involved calibrating wavelength using lines from the comet or reference stars and conducting flux calibration with standard stars such as Vega and Capella. We determined the N2+/CO+ ratios for the combined nights of October 31 and November 1, achieving a fit with a ratio of 7.7%, and on November 28, a slightly lower ratio of 7.2%, indicating consistency in our spectral analysis despite variations in observational conditions and the quality of the plates. Moreover, the analysis confirmed the presence of weak CN bands and subdued dust emissions, traits shared with Comet C/2016 R2, suggesting a low dust-to-gas ratio and a unique evolutionary path for these bodies. These findings, coupled with the identification of previously unrecognized C3 emissions in historical spectra, underscore remarkable compositional similarities between C/1908 R1 and C/2016 R2, hinting at their classification within a rare comet type. However, the question about the exact water ice composition of C/1908 R1 remains open, due to observational limitations of the time.Tail MorphologyObservations using early photographic methods documented rapid morphological changes in the comet’s tail, exhibiting cycles of brightening, mass ejection, and detachment (TDEs) [9]. We find a recurring interval of approximately 15 days, suggesting influences beyond mere nucleus rotation. These changes correlate with solar activity, as evidenced by concurrent auroral displays, highlighting the tail's sensitivity to solar wind due to its volatile and ionic composition.ConclusionsComet C/1908 R1 (Morehouse) presents a compelling study of a dynamically new object, preserved in a pristine state since its origins at the outer edge of the Oort cloud, highlighted by both dynamical models and spectral analysis. Dynamical simulations show no close encounters with giant planets, affirming its status as an untouched relic from the Solar System's early days, while spectral analysis uncovers a unique composition, rich in N2+ and CO+, similar to that observed in Comet C/2016 R2. Despite significant insights, the absence of definitive water ice data leaves its complete classification tentative. With the rarity of blue comets, these findings underscore the importance of integrating historical data with modern scientific techniques.Figure 1: The 1908 spectral plate obtained on October 31 and November 1, 1908, when the comet was 1.07 au from the Sun (left) and the resulting modeling of the extracted spectrum (right), corresponding to the tail region. [1] Adam J. McKay et al., AJ, 158.3, 128 (Sept. 2019), p. 128.[2] C. Opitom et al., A&A 624 (Apr. 2019), A64, ISSN: 1432-0746.[3] S. E. Anderson et al., MNRAS, 515.4 (Oct. 2022), pp. 5869–5876.[4] V. Robert et al., A&A,652, A3 (Aug. 2021), A3.[5] S. E. Anderson et al., MNRAS, 524.4 (Oct. 2023), pp. 5182–5195.[6] A. de La Baume Pluvinel and F. Baldet, Societe Astronomique de France (Jan. 1908), pp. 532–534.[7] A. de La Baume Pluvinel and F. Baldet, AJ, 34 (Sept. 1911), p. 89.[8] A. Bernard and H. Deslandres, Societe Astronomique de France (Jan. 1908), pp. 29–31.[9] A.S. Eddington, MNRAS, (Feb. 1909), pp. 97–112.

Context. The long-period comet C/1908 R1 (Morehouse) is distinguished by its early spectroscopic tail photography, which uncovered notably intense emission bands of N-2(+) and CO+, similar to the unusual characteristics of the atypical blue comet C/2016 R2 (Pan-STARRS). Aims. To probe potential parallels with C/2016 R2 further, we revisited the historical spectroscopic plates of C/1908 R1 while leveraging the New Astrometric Reduction of Old Observations (NAROO) project's advanced sub-micrometric scanner. Methods. We first reviewed the intensity ratio method, followed by a comprehensive spectroscopic analysis of the original historical plates to determine the comet's composition. Our analysis also encompassed an evaluation of C/1908 R1's dynamic trajectory using an N-body integrator and a detailed examination of tail morphology records. Results. Our findings suggest that C/1908 R1 experienced no significant close encounters as it crossed the inner Solar System, anchoring its origins directly in the Oort Cloud and allowing us to ascertain that this was its inaugural voyage near the Sun. We determined a N-2(+)/CO+ ratio of similar to 7% along with a dust-poor composition, particularities it shares with C/2016 R2. Moreover, by synthesizing observations of the tail's structure over the three-month period of visibility, we uncovered a link between tail dislocation events and aurora borealis sightings on Earth. This association underscores the comet tail's heightened sensitivity to solar wind fluctuations due to its volatile makeup. Conclusions. The comet C/1908 R1 (Morehouse) emerges as one of the most unaltered relics of our Solar System's formation, offering another instance of a C/2016 R2-analogous comet. This underscores the importance of preserving and reexamining historical astronomical datasets, not only for historical significance but as a critical resource for contemporary scientific advancement.

Context. CO is an abundant species in comets, creating CO+ ion with emission lines that can be observed in the optical spectral range. A good modeling of its fluorescence spectrum is important for a better measurement of the CO+ abundance. Such a species, if abundant enough, can also be used to measure the 12C/13C isotopic ratio. Aims. This study uses the opportunity of a high CO content observed in the comet C/2016 R2 (PanSTARRS), which created bright CO+ emission lines in the optical range, to build and test a new fluorescence model of this species and to measure the 12C/13C isotopic ratio in this chemical species for the first time with ground-based observations. Methods. Thanks to laboratory data and theoretical works available in the scientific literature, we developed a new fluorescence model both for 12CO+ and 13CO+ ions. The 13CO+ model can be used for coadding faint emission lines and to obtain a sufficient signal-to-noise ratio to detect this isotopologue. Results. Our fluorescence model provides a good modeling of the 12CO+ emission lines, allowing us to publish revised fluorescence efficiencies. Based on similar transition probabilities for 12CO+ and 13CO+, we derive a 12C/13C isotopic ratio of 73±20 for CO+ in comet C/2016 R2. This value is in agreement with the Solar System ratio of 89±2 within the error bars, but is also consistent with the 12C/13C ratio in local interstellar medium (68±15).

Context: CO is an abundant species in comets, creating CO$^+$ ion with emission lines that can be observed in the optical spectral range. A good modeling of its fluorescence spectrum is important for a better measurement of the CO$^+$ abundance. Such a species, if abundant enough, can also be used to measure the $^{12}$C/$^{13}$C isotopic ratio. Aims: This study uses the opportunity of a high CO content observed in the comet C/2016 R2 (PanSTARRS), that created bright CO$^{+}$ emission lines in the optical range, to build and test a new fluorescence model of this species and to measure for the first time the $^{12}$C/$^{13}$C isotopic ratio in this chemical species with ground-based observations. Methods: Thanks to laboratory data and theoretical works available in the scientific literature we developed a new fluorescence model both for $^{12}$CO$^+$ and $^{13}$CO$^+$ ions. The $^{13}$CO$^+$ model can be used for coadding faint emission lines and obtain a sufficient signal-to-noise ratio to detect this isotopologue. Results: Our fluorescence model provides a good modeling of the $^{12}$CO$^+$ emission lines, allowing to publish revised fluorescence efficiencies. Based on similar transition probabilities for $^{12}$CO$^+$ and $^{13}$CO$^+$ we derive a $^{12}$C/$^{13}$C isotopic ratio of 73$\pm$20 for CO$^+$ in comet C/2016 R2. This value is in agreement with the solar system ratio of 89$\pm$2 within the error bars, making the possibility that this comet was an interstellar object unlikely.

Hyperactive comets are a small group of comets whose activity are higher than expected. They seem to emit more water than they should based on the size of their nucleus and comet 46P/Wirtanen is one of them. Investigating its activity and composition evolution could provide clues about its origins and formation region in the Solar nebulae. Given the exceptional close approach in 2018 of comet 46P to the Earth, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion. We used both TRAPPIST telescopes to monitor the comet for almost a year with broad-band and narrow-band filters. We derived the production rates of five gaseous species, e.g. OH, NH, CN, C$_3$ and C$_2$, using a Haser model as well as the A($\theta$)f$\rho$, dust proxy parameter. The comet was also observed with two optical high resolution spectrographs UVES and ESPRESSO mounted on the 8-m ESO VLT to measure the isotopic ratios of C and N, the oxygen forbidden lines ratios and the NH$_2$ ortho-to-para ratios. We followed during almost a year the rise and decline of the production rates of different species as well as the dust activity of 46P on both pre- and post-perihelion. Relative abundances with respect to CN and OH along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. We determined the rotation period of the nucleus using high cadence observations and long series of CN images on several nights, and we obtained a value of (9.18$\pm$0.05) hr at perihelion. Using high resolution spectra of 46P coma, we derived C and N isotopic ratios of 100$\pm$20 and 150$\pm$30 and a green-to-red forbidden oxygen [OI] lines ratio of 0.23$\pm$0.02. We measured a NH$_2$ ortho-to-para ratio of 3.31$\pm$0.03 and derived an ammonia ratio of 1.19$\pm$0.03 corresponding to a spin temperature of 27$\pm$1 K.

Io is the most volcanically active body in the Solar System. This volcanic activity results in the ejection of material into Io's atmosphere, which may then escape from the atmosphere to form various structures in the jovian magnetosphere, including the plasma torus and clouds of neutral particles. The physical processes involved in the escape of particles - for example, how the volcanoes of Io provide material to the plasma torus - are not yet fully understood. In particular, it is not clear to what extent the sodium jet, one of the sodium neutral clouds related to Io, is a proxy of processes that populate the various reservoirs of plasma in Jupiter's magnetosphere. Here, we report on observations carried out over 17 nights in 2014-2015, 30 nights in 2021, and 23 nights in 2022-2023 with the TRAPPIST telescopes, in which particular attention was paid to the sodium jet and the quantification of their physical properties (length, brightness). It was found that these properties can vary greatly from one jet to another and independently of the position of Io in its orbit. No clear link was found between the presence of jets and global brightening of the plasma torus and extended sodium nebula, indicating that jets do not contribute straightforwardly to their population. This work also demonstrates the advantage of regular and long-term monitoring to understanding the variability of the sodium jet and presents a large corpus of jet detections against which work in related fields may compare.

The long-period comet C/2017 K2 (PanSTARRS) was discovered in 2017 at a large heliocentric distance of 16 au (Wainscoat et al. 2017). Pre-discovery images from 2013 show that K2 was even active at a record distance of ~24 au from the Sun (Jewitt et al. 2017) well beyond the snow line, indicating that, most probably, CO and CO2 ices - the most abundant species after water - might drive its activity. CO was indeed detected in K2’s coma in the sub-mm range at a heliocentric distance of 6.7 au (Yang et al. 2021) and K2 was claimed to be a CO-rich comet. Detecting comets at such large distances is becoming more frequent, but it is still a rare occasion to study a well preserved comet surface coming directly from the Oort Cloud or on a several million years orbit, and especially if it is of a rare type. K2 will reach its perihelion on 2022 December 19 (Rh=1.8 au, Δ=2.5 au) and become a bright target in automn with good observing conditions from the Southern hemisphere. We have started an observing campaign on May 8 (Rh=3.2 au), 2022 with UVES at the ESO VLT to obtain high resolution and good SNR optical spectra to characterize the detailed coma composition of its daughter species before and after K2 perihelion. We report here about the first epochs before perihelion. UVES was setup with a slit width of 0.45" (length of 10") to provide a resolving power of 80.000, and we selected two different settings (DIC#1 346/580 and DIC2 437/860) to cover the whole optical range (304-1040 nm) at each epoch in only two long exposures on the same night. These spectra will allow us to compare K2 - characterized by its unusual distant activity - to other well studied comets in the optical and particularly using the same instrument since 20 years by the Liège comet team. These spectra will allow us to measure the detailed composition of its coma: the production rates of the daughter species (OH, CN, C2 etc.) to check among other things if the comet is a C-chain depleted or normal comet (A'Hearn et al. 1995), to link those production rates with those from the parent species observed in the IR (see CRIRES+ poster by Lippi et al.), to search for CO+ and CO2+ lines to check if K2 is a CO-rich comet like the unique CO-N2-rich blue comet C/2016 R2 (PanSTARRS) (Opitom et al. 2019), to measure the ratio of the [OI] lines to estimate the CO/H2O ratio (Decock et al. 2015), and if the comet is bright enough to measure the isotopic ratios of the light elements (12C/13C and 14N/15N from the CN isotopologues), to measure the ortho- to para- ratio of NH2, and search for faint FeI and NiI lines which are a new and puzzling component of the cometary coma (Manfroid et al. 2021).

1. Introduction C/2016 R2 (PanSTARRS) was a surprising comet. Detected on September 7, 2016 by Pan-STARRS it showed an unusual composition when it became a bright comet at the end of 2017 and the beginning of 2018. It developed a coma at large (~6 au) heliocentric distance and observations showed that it had a highly unusual composition: no water molecules (or OH radical) could be detected, and the abundances of the usual radicals (CN, C2, C3) were unusually low, with a surprising coma composition dominated by CO, CO2 and N2 molecules with bright CO+ and N2+ emission lines in the visible range. A high CO production rate of about 1029 molecules s-1 was measured (Biver et al. 2018; Wierzchos & Womack 2018) as well as a high CO2 production rate (CO2/CO=1.1 from Opitom et al. 2019), and a high ratio N2/CO varying between 0.06 and 0.09 (Biver et al. 2018; Cochran & McKay 2018a,b; Opitom et al. 2019; Venkataramani et al. 2020). The detection of such bright N2+ emission lines in this comet highlighted the necessity of a good modeling of the N2+ fluorescence spectrum in comets. The high-quality spectra published by Opitom et al. (2019) provided a good opportunity to test such a model. This model also permits to compute the fluorescence spectrum of the 14N15N+ species, leading to the possibility of future measurements of the 14N/15N isotopic ratio in the N2 molecules, one of the main constituant of the solar nebula. 2. Observations The spectra used for this work have been obtained with the UVES spectrograph mounted on the ESO 8.2 m UT2 telescope of the VLT. Three different observing nights have been used, corresponding to February 11, 13 and 14, 2018. One single exposure of 4800 s of integration time was obtained during each night and we used a 0.44” wide slit, providing a resolving power R~80,000. The slit length was 8” corresponding to about 14,500 km at the distance of the comet (geocentric distance of 2.4 au). The average heliocentric distance was 2.76 au. Opitom et al. (2019) describe in more details the data processing. From the 2D spectra having a spatial extension of 30 rows, each of them corresponding to a different cometocentric distance, we extracted different 1D spectra for each night. These spectra were then averaged for similar cometocentric distances allowing a detailed comparison of these spectra at different cometocentric distances, the furthest one corresponding to 2x4 rows at the two extremities of the slit (i.e. at a cometocentric distance varying between 4800 and 6600 km). 3. Modeling the N2+ fluorescence spectrum We developed a new fluorescence model for modeling our observational spectra. The transition involved in this spectrum is the first negative group, i.e. the B2Σu+ → X2+Σg+ electronic transition with the (0,0) bandhead appearing near 3914 Å. We considered the first three vibrational levels (v = 0; 1; 2) for both X2+Σg+ and B2Σu+ state, each of them with all the rotational levels from N = 0 to 40. N2+ having no permanent dipole moment, the pure rotational and vibrational transitions are forbidden (or have a very low probability, through quadrupolar transitions, not taken into account in our model). For that reason it takes a long time for this species to reach its fluorescence equilibrium because it needs a few tens of absorption / emission cycles between the X2+Σg+ and B2Σu+ states to reach this equilibrium. A comparison of the spectrum obtained on the nucleus with the one obtained at the edges of the slit revealed clear differences due to different rotational relative populations. For that reason we decided to model the N2+ fluorescence spectrum with a Monte-Carlo simulation. Such a computational method allows to compute a spectrum at different times from an initial relative population distribution. Our model starts with a Boltzmann relative population distribution of 80 K (representing an estimate of the kinetic temperature in the inner coma) and uses 10,000 s of evolution time. We managed to explain satisfactorily the observed N2+ emission spectrum. Fig. 1 presents a close up view around the (0,0) bandhead. This work, presented in more details in Rousselot et al. (2022) also allowed to compute accurate fluorescence efficiencies. Figure 1: Comparison of the observed VLT UVES spectrum of comet C/2016 R2 (blue) obtained at the ends of the slit with our N2+ model (red). 4. 14N15N+ fluorescence spectrum Our modeling of the N2+ fluorescence spectrum can be used to compute the 14N15N+ fluorescence spectrum, leading to the possibility of measuring the 14N/15N isotopic ratio in N2 molecules. We will present such a spectrum as well as a search for this isotopologue in the C/2016 R2 spectra. Such comets are rare but future observations will reveal other comets similar in composition to C/2016 R2. With future observing facilities now under construction (such as the ESO ELT) 14N/15N measurements for N2 molecules will probably become possible, leading to new constraints on this isotopic ratio. References Biver N., et al., 2018, A&A 619, A127 Cochran A. L. & McKay, A. J. 2018a, ApJ, 856, L10 Cochran A. L. & McKay A. J., 2018b, ApJ, 854, L20 Opitom C., et al. 2019, A&A, 624, A64 Rousselot P., et al., 2022, A&A, in press Venkataramani K., et al., 2020, MNRAS, 495, 3559 Wierzchos K. & Womack M. 2018, AJ, 156, 134

Line-of-sight integration of emissions from planetary and cometary atmospheres is the Abel transform of the emission rate, under the spherical symmetry assumption. Indefinite integrals constructed from the Abel transform integral are useful for implementing remote sensing data analysis methods, such as the numerical inverse Abel transform. We propose analytical expressions obtained by a suitable, non-alternating, series development to compute those indefinite integrals. We establish expressions allowing absolute accuracy control of the convergence of these series and illustrate how this accuracy depends on the number of terms involved in the series computation. We compare the analytical method with numerical computation techniques, which are found to be sufficiently accurate as well. Inverse Abel transform fitting is then tested in order to establish that the expected emission rate profiles can be retrieved from the observation of both planetary and cometary atmospheres. We show that the method is robust, i.e. that it can be applied even when the properties of the observed atmosphere depart from the assumed ones, especially when Tikhonov regularization is included. A first application is conducted over observation of comet 46P/Wirtanen, showing some variability, possibly attributable to an evolution of the contamination by dust and icy grains.

Context. N-2 is rarely found in comets, or when it is present, it is found only in small quantities despite its abundance on the surfaces of different outer Solar System objects. A few comets presented N-2(+) emission lines in their optical spectra. One of them, C/2016 R2, showed an unusually high abundance of this species in 2018, with dozens of bright N-2(+) emission lines. A robust model of the N-2(+) fluorescence in comets would permit us to perform a detailed quantitative analysis of this species and enlarge our comprehension of the surprisingly wide range of N-2 abundances in comets. Aims. The goal of this work is to provide the necessary tools to interpret N-2(+) spectra. Computing production rates for a cometary species necessitates a good knowledge of the number of molecules located along the line of sight of the spectrometer. This in turn requires a good modeling of the emission spectrum with detailed fluorescence efficiencies for the different bands. Methods. We developed a model based on available laboratory data and new theoretical results relative to the N-2(+) to compute the emission spectrum of this species observed in 8.2m Very Large Telescope high-resolution spectra of comet C/2016 R2. Because of some significant differences between spectra obtained on the nucleus and at a cometocentric distance of about 6000 km, it became apparent that a classic fluorescence equilibrium spectrum could not be used. A synthetic spectrum based on a Monte Carlo method and producing spectra at different times from an initial relative population was developed and compared to our observational data. Results. Our modeling of the cometary N-2(+) emission spectrum satisfactorily fits our observed spectra of comet C/2016 R2, leading to the first modeling at high resolution. Different fluorescence efficiency factors are computed.