Comet C/2025 K1 (ATLAS) is deeply depleted of C2
ATel #17351; Federico Manzini (Stazione Astronomica di Sozzago), Alessandra Mura (Padova University, INAF-OAPd), Andrea Reguitti (INAF-OAPd), Paolo Ochner (Padova University, INAF-OAPd), Virginio Oldani (Stazione Astronomica di Sozzago), Luigi R. Bedin (INAF-OAPd), Rachele Ottaviani (Padova University), Vittorio Andreoli (Osservatorio Astronomico Monte Baldo)
on 22 Aug 2025; 16:24 UT
Credential Certification: Luigi R. Bedin BEDIN (luigi.bedin@inaf.it)
The non periodic comet C/2025 K1 was discovered by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) search program in Hawaii on May 24, 2025 (MPEC 2025-PD8). It is expected to reach a magnitude of 7.5 around perihelion on October 8, when it will pass close to the Sun at only 0.334 AU. The eccentricity of the orbit of comet C/2025 K1 is 1.000294; the orbital elements indicate it as a dynamically new comet. A series of 3 spectra of the comet, each in the nights of August 03, 11, 13 and 15 were acquired with the 1.22 m Galileo Telescope (UniPD) at the Asiago Astrophysical Observatory-Italy (MPC-IAUC code 046). The comet was between 1.525 to 1.310 AU from the Sun, and between 0.636 to 0.579 AU from the Earth, with a high velocity vector near SW due to the geometric conditions of the observations. The spectra show lines due to CN, C3, NH2 and OI. The emission lines due to C2 are reliably detectable only at 5164 Angstrom. The spectra appear flat in all observing sessions. Therefore, the comet C/2025 K1 appears significantly depleted in the carbon-chain molecules (A'Hearn et al., Icarus, 1995; A. Cochran et al., Icarus, 2011). The flux of the CN (0-0) violet band near 3880A was measured in the spectra of August 3 and August 13 within rectangular apertures and the integrated band profile was fitted with a Gaussian function. The integrated fluxes and CN production rates are in the format [Date;r(AU);flux(erg s^-1 cm^-2);Q_CN(molecules s^-1);aperture;t_exp(s)]: [2025 August 03.855;1.52;(3.03+/-1.10)e^-13;(4.38+/-1.41)e^+23;3.41"x61.5";3600] and [2025 August 13.841;1.34;(5.08+/-2.21)e^-13;(9.95+\-4.35)e^+23;2.56"x34.0";2400].The CN production rate was calculated using a standard Haser model, adopting scale lengths from Randall et al. (BAAS, 1992) and fluorescence efficiencies from Schleicher (AJ, 2010). The CN production rate increased by a factor of about 2.3 between the two observations, consistent with the decrease in heliocentric distance. Both values are below the CN production rate expected for a typical comet at 1.5 AU, indicating unusually low CN activity. Figures and captions are available at: https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2025K1/
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