Detection of the H-alpha and H2O+ emission lines in spectra of Comet C/2025 R3 (PanSTARRS)
ATel #17754; Federico Manzini (Stazione Astronomica di Sozzago), Alessandra C. Mura (UniPd, INAF-OAPd), Paolo Ochner (INAF-OAPd, UniPd), Andrea Farina (UniPd, INAF-OABo), Luigi R. Bedin (INAF-OAPd), Andrea Reguitti (INAF-OAPd), Virginio Oldani (Stazione Astronomica di Sozzago)
on 15 Apr 2026; 10:24 UT
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)
Comet C/2025 R3 (PanSTARRS), a dynamically new comet, is approaching perihelion on 2026 April 20 at a
heliocentric distance of about 0.49 AU (JPL-Horizons,
https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=c%2F2025%20r3).
In recent weeks, the comet has shown a marked increase in brightness, accompanied by the
development of a dense coma and a prominent ion tail. The comet has already been observed at radio
wavelengths as in ATel #17744, where Jiang et al. announced the detection of the 18 cm OH- line.
We started a spectroscopic monitoring campaign on March 28, 2026, with the 1.22-m Galileo telescope at
the Asiago Astrophysical Observatory (University of Padova, Italy). Initial observations were obtained
with a 300 lines/mm diffraction grating, which provided a dispersion of 2.25 A/pix in the spectral range
3500-7900 A. Additional spectra were acquired on 2026 March 31 and April 07 with a 1200 lines/mm
diffraction grating, providing dispersion 0.6 A/pix and a spectral coverage 5700-6900 A, in order to
monitor the red spectral region where emission features due to H2O+ and H-alpha are present.
We have identified in C/2025 R3 (still provisionally) the presence of the H-alpha line with increasing
emission probably in relation to the decreasing distance of the comet from the Sun (Mar 28: 0.730 AU,
March 31: 0.683 AU, April 07: 0.585 AU).
To compare the relative intensity of the H-alpha emission among the three epochs, we used the spectra
divided by the solar analog and normalized to the local continuum in the 5800-5870 A interval. For each
spectrum, the local continuum around H-alpha was estimated from two side bands and interpolated across
the line region. The H-alpha line strength was then measured by integrating the net line profile
after subtraction of the local continuum within a restricted wavelength interval centered on 6562.8 A,
while the broader plotted region also includes the adjacent emission feature.
With this procedure, the relative H-alpha line-strength ratios are:
2026 March 31 / 2026 March 28 = 2.76;
2026 April 07 / 2026 March 28 = 6.01
The presence of H2O+ emission is reported in the same observation sequence of comet C/2025 R3
(PanSTARRS). Identification was performed with the Cometary Atlas of the University of Liege
(Jehin et al. 2025, EPSC presentation;
https://www2.cometa.uliege.be/Cometary_Lineatlas/Cometary_atlas.html), based on the exact
positions of the lines. A close-up of the 6130-6230 A region shows the (0,10,0) H2O+ band. The red
portion of the H2O+ emission is free of both C2 and NH2 contamination.
The spectra of C/2025 R3 collected with the Galileo telescope (UniPd, Italy) will be published on ASIACO
(DOI: https://doi.org/10.5281/zenodo.18861813) and made available to the community.
Calibration details, figures and captions are available at:
