HCN detection in the Kreutz sungrazing comet C/2026 A1 (MAPS) shortly before its disruption

Abstract

Context. Kreutz sungrazing comets are characterised by extremely small perihelion distances (q < 3.45 R). These objects are of particular interest because their intense activity and fragmentation can expose material from deeper within the nucleus, providing unique access to their bulk chemical composition rather than merely their surface layers. Aims. The discovery of C/2026 A1 (MAPS) three months before its perihelion offered a rare opportunity to investigate the composition and activity of a Kreutz sungrazing comet. We present the first millimetre wavelength study of this type of comet. Methods. We observed C/2026 A1 (MAPS) with the IRAM 30-m telescope from April 1 to April 3 2026 at heliocentric distances rh = 0.10-0.23 au, the three days preceding its perihelion and full disintegration. We analysed the molecular emissions using non-local thermodynamic equilibrium radiative transfer models to derive molecular production rates. Results. We report the first HCN detection in a Kreutz sungrazer through its (3-2) transition at 265.886 GHz, enabling us to characterise the variability of its activity. We witnessed two outbursts, on April 1 and April 3. The second one, occurring the day before the disintegration of C/2026 A1 (MAPS), led to the release of 0.4% to 1.2% of the total cometary mass, thus suggesting a pre-disruption before entering the Roche limit region. We also obtained upper limits for HNC, CS, H2CO, CO, OCS and CH3OH