Follow-Up Imaging and Spectroscopic Observations of Comet C/2025 A6
Left: Image of comet C/2025 A6, taken on the morning of 1 October 2025 at the University Observatory in Großschwabhausen with the CTK-II in the R-band (15x DIT=120s).
The diameter of the coma is about 150000km, and the tail length shown in the image measures about 1 million km. The direction towards the Sun and the comet's heliocentric velocity are indicated with arrows.
Right: Color-composite image of comet C/2025 A6, taken on the evening of 13 October 2025 at the University Observatory in Großschwabhausen with the CTK-II in the B-, V-, R-band (each 4x DIT=60s).
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.
Spectra of the central coma of comet C/2025 A6, recorded with the Échelle spectrograph FLECHAS on the 90cm reflector telescope at the University Observatory in Großschwabhausen on 13 and 18 October 2025 (each 3x DIT=600s). The FLECHAS spectra show emission lines originating from atoms and molecules in the comet's coma. In the wavelength range between 4500 and 7000Å the most prominent emission features are the emission bands of C2 (that also cause the greenish color of the comet's coma) and the emission lines of [OI], and NH2. In addition strong emission lines of Na are detected in both observing epochs. Besides these emission features also absorption lines e.g. those of H, Na or Mg, the latter indicated here with black arrows, are detected in the spectra of C/2025 A6, which are observable due to the reflection of sunlight from dust particles in the coma of the comet. While the emission features of C2, [OI], and NH2 all remain stable at the 25%-level, the equivalent width of the Na emission lines increases significantly by more than a factor of 2 between the observation epochs while the comet's distance from the Sun decreased from 0.79 to 0.71 au.
M. Mugrauer 2025 - All imaging and spectroscopic data are available upon request.
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