Follow-Up Imaging and Spectroscopic Observations of Comet C/2025 A6
ATel #17456; M. Mugrauer, K.-U. Michel, B. Dincel, E. Loesch
on 27 Oct 2025; 20:34 UT
Credential Certification: Markus Mugrauer (markus@astro.uni-jena.de)
Referred to by ATel #: 17462
We carried out spectroscopic follow-up observations of comet C/2025 A6 on 13 and 18 October 2025 with the Échelle spectrograph FLECHAS, which is installed at the Nasmyth port of the 90 cm reflector telescope of the University Observatory Jena. The instrument was operated in its 2x2 binning mode, which provides a spectral resolving power of R~7400. Beside the scientific data, flatfield and arc lamp spectra were always taken before each comet observation to calibrate the scientific spectra. A total integration time on target of 30 minutes was used in both observation epochs. Throughout the spectroscopy of the comet, the fiber of the spectrograph, which has a diameter of 3.9 arcsec in the sky, was centered on the coma of C/2025 A6, to which the telescope was guided to correct for the rapid angular motion of the comet. In the wavelength range between 4500 and 7000 Å we have detected several emission features in the spectra of C/2025 A6, with those of C2, [OI], NH2, and Na being the most prominent ones. Due to the velocity of the comet relative to the observation site, the detected emission features are blueshifted (by about 36 and 13 km/s for 13 and 18 October 2025, respectively), which agrees well with the expected RV values based on the current orbital solution of the comet. While the emission features of C2, [OI] and NH2 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, with the comet's distance from the Sun decreasing from approximately 0.79 to 0.71 au. In addition to the emission lines, the spectra of C/2025 A6 also show absorption lines due to sunlight reflection on dust particles in the comet's coma. In order to measure the apparent brightness of C/2025 A6, its position in the sky and to study its morphology, the comet was also imaged before and simultaneously to the spectroscopic observations in the B-, V-, and R-band with the CCD-camera CTK-II, which is operated at the 25 cm Cassegrain auxiliary telescope of the observatory.
Follow-Up Imaging and Spectroscopic Observations of Comet C/2025 A6 - Online Data
