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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17754
Message-Id: <20260416070315.E092E288088A@server.astronomerstelegram.org>
Date: Thu, 16 Apr 2026 03:03:15 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17754							     ATEL #17754

Title:	Detection of the H-alpha and H2O+ emission lines in spectra
		of Comet C/2025 R3 (PanSTARRS)
Author:	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)
Queries:	manzini.ff@aruba.it
Posted:	15 Apr 2026; 10:24 UT
Subjects:Optical, Comet

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: : https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2025R3/
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