Follow-Up Imaging and Spectroscopic Observations of Comet C/2025 R2

Right: Color-composite image of comet C/2025 R2, taken on the evening of 18 October 2025 at the University Observatory in Großschwabhausen with the CTK-II in the B-, V-, and R-band (each 10x DIT=20s). The diameter of the coma is about 40000 km. The direction towards the Sun and the comet's heliocentric velocity are indicated with arrows. The greenish color of the comet's coma, which is caused by strong C₂-emission (Swan-bands), is clearly visible in the color-composite image. The faint tail of the comet points eastwards.

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Spectra of the central part of the coma of comets C/2025 R2 (black spectrum, 1x DIT=600s), and C/2025 A6 (grey spectrum, 3x DIT=600s, shown for comparison), recorded consecutively on the evening of 18 October 2025 with the Échelle spectrograph FLECHAS, which is installed on the 90cm reflector telescope at the University Observatory in Großschwabhausen. The FLECHAS spectrum of C/2025 R2 shows strong emission lines originating from atoms and molecules in the comet's coma. In the wavelength range between 5000 and 7000Å the most prominent emission features are those of C₂ (that also cause the greenish color of the comet's coma), [OI], and NH_2. In contrast to C/2025 A6 in the spectrum of C/2025 R2 no emission lines of Sodium are detected. During the spectroscopic observations, C/2025 R2 was at a heliocentric distance of 0.95 au, while C/2025 A6 was only 0.71 au from the Sun.

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M. Mugrauer 2025 - All imaging and spectroscopic data are available upon request.