High Resolution Spectroscopy of Comet C/2023 A3 (Tsuchinshan–ATLAS): insight into Sodium, Potassium and Lithium abundances

Introduction

Comet C/2023 A3 (Tsuchinshan-ATLAS) is a dynamically new Oort Cloud comet discovered by The Asteroid Terrestrial impact Last Alert System (ATLAS) on February 2023. A high-resolution (R=130000) spectrum of C/2023 A3 taken with the PEPSI spectropgraph on the Large Binocular Telescope (LBT) revealed usual cometary molecules and ions (CN, C2, CH, O I, NH2). We also detected Sodium (Na), and the much more rare emission lines of Potassium (K), previously identified only in 67/P, C/2011 L4 (PANSTARRS) [1], C/1965 S1 (Ikeya-Seki) [2], C/1995 O1 (Hale-Bopp) [3], Halley [4] and 67/P [5]. Lithium was not detected. The identification of both Na and K provides an interesting opportunity to investigate the alkali content of C/2023 A3. Since alkalis originate from the nucleus refractory material [1,5], estimating their abundances is key to investigate the composition of the cometary nuclei building blocks.

Observations and Data Analysis

Comet C/2023 A3 was observed on 27 Oct 2024 with the PEPSI spectrograph at LBT, in monocular mode (8.4 m). A 30 min spectrum, with the target fiber centered on the comet nucleus was obtained with spectral resolving power of approx. 130000 from 3800 to 9000A. The comet was at 0.8 AU from the Sun, and 0.8 AU form the Earth, receding with a heliocentric radial velocity of 33.6 km/s and a geocentric radial velocity of 56.2 km/s (ephemeris from JPL Horizons). Absolute flux calibration has been performed using standard star HR7950 and the solar and telluric lines were removed. The resulting 1D continuum removed spectra of C/2023 A3 is shown on Fig. (1). Line intensities were calculated by fitting a gaussian profile. Errors on line intensities were calculated by propagating the errors on the gaussian fit and adding, in quadrature, the standard deviation of the continuum. We measured the Sodium (Na) 5889.95 A and 5895.92 A lines and the Potassium (K) 7664.73 A and 7698.96 A lines. We did not detect any line corresponding to the position of the Lithium (Li) 6707.8 A line, so we provide a 3σ upper limit.

Fig. 1. Continuum removed spectrum of C/2023 A3 obtained on 27 Oct 2024

Fig. 2. Top : spectrum of C/2023 A3 around the K I 7664.9 A line and fitted local continuum (red). Mid: continuum-removed spectrum a best-fit emission line. Bottom: gaussian fit residuals. 
Results

Following the approach of [1] we calculated relative abundances Na/K and Na/Li from the measured line intensities. We obtain a Na/K=145±6 abundance, which becomes Na/K=39±2 once corrected for photoionization [1]. We repeated the process for Lithium, using its 3σ upper upper limit, and get a lower limit on the relative abundance Na/Li≥5861.74, that becomes Na/Li≥1521 after correction for photoionization [1]. Other processes capable of extracting alkalis are thermal desorption, photon-stimulated desorption, solar wind sputtering and micro-meteoroid vaporization, but they do not  alter the relative abundances of the released atoms. 

Conclusions

A high resolution spectrum of C/2023 A3 allowed to estimate its nucleus Na/K ratio and provide lower limits for the Na/Li ratio. The latter,  higher than the solar and chondritc values (103,[6]),  confirm that C/2023 A3 is depleted in Li, similarly to C/1965 S1 (Ikeya-Seki) and C/2011 L4 (PANSTARRS) [1]. While this supports that comets may be depleted in lithium, a higher samples size would be required for a robust conclusion. The Na/K ratio of C/2023 A3 is higher than the solar one (15.5, [6]), contrary to all the other comets in which it was estimated. Higher than solar Na/K ratio are detected in a number of meteorites. This includes chondrites affected by chemical alteration, possibly due to fluid circulation, and their partial melts [7] or achondrites such as cumulate eucrites [8], formed from the crystallization of magma deep inside Vesta [9], and highly metamorphosed and shock-modified mesosiderites and ureilites [8]. Higher than solar Na/K ratios are also expected from aqueous alteration of chondritic material [10] and have been detected in Saturn’s E-rings grains [11] and on Europa’s ice shell [12].
C/2023 A3 Na/K ratio suggests that its nucleus underwent significant chemical alteration. Given the comet size and dynamically new nature, we exclude  endogenous or exogenous alterations occurring during its history in the Oort cloud. Instead, processes such as melt extraction during incipient partial melting and fluid circulations during aqueous alteration, shock metamorphism and/or brine evaporation can potentially fractionate Na from K and result in elevated Na/K ratio. While these are unlikely to occur on a cometary nucleus, they were common on the propotoplanets. 
C/2023 A3 nucleus could therefore be a collisional fragment of a planetesimal that underwent these alterations processes and was ejected in the Oort-cloud by the giant planet instability [13]. As a consequence, C/2023 A3 is the first example of a comet which is not tracing the composition of leftover, primordial Solar System material but the geologic processes occurring on the planetesimals. This result represents the first evidence that comets may form from different parent materials and, as asteroids and meteorites, trace a much greater range of early Solar System processes.
 

Acnowledgments:
The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

References: 
[1] Fulle, M. et al., (2013). ApJL771(2),L21. [2] Preston, G. W.1967ApJ 147 718 [3]Fitzsimmons A. and Cremonese G. 1997IAUCirc.6638 1 [4]Jessberger, E. K., et al., (1988). Nature,332(6166),691–695[5]Wurz, P. et al.,(2015).A&A583,A22. [6] Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009 ARA&A 47,481 [7] Aleon, J. e tal., PNAS, 117(15), 8353–8359 (2020);[9] Hublet, G. et al.,Geochim.Cosmochim.Acta 218,73–97 (2017) [8] Mittlefehldt, D.W. in Treatise in Cosmochemistry vol 1. Meteorites and Cosmochemical Processes, pp. 235–266 (2014) [10] Zolotov, M. Y. Icarus 220(2),713–729(2012) [11] Postberg, F., et al.,Nature 459(7250),1098–1101(2009) [12] Johnson, R.E et al., Icarus 156(1),136–142(2002) [13] Morbidelli, A., et al.,Astron. Astrophys. 583,43(2015)