The rich molecular environment of the luminous blue variable star AFGL 2298^★,★★

¹ ISDEFE, Beatriz de Bobadilla 3, 28040 Madrid, Spain
² Centro de Astrobiología (CSIC-INTA), Ctra. M-108, km. 4, 28850 Torrejón de Ardoz, Spain
e-mail: ricardo.rizzo@cab.inta-csic.es
³ INAF – Osservatorio Astrofísico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
⁴ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, 8320000 Santiago, Chile

Received: 23 May 2023
Accepted: 17 July 2023

Abstract

Context. Luminous blue variable (LBV) stars represent a short-lived stage in the late evolution of the most massive stars. Highly unstable, LBVs exhibit dense stellar winds and episodic eruptions that produce complex circumstellar nebulae, the study of which is crucial for properly constraining the impact of these sources at a Galactic scale from a structural, dynamical, and chemical perspective.

Aims. We aim to investigate the molecular environment of AFGL 2298, an obscured Galactic LBV that hosts a highly structured circumstellar environment with hints of multiple mass-loss events in the last few 10⁴ a.

Methods. We present spectral line observations of AFGL 2298 at 1 and 3 mm performed with the IRAM 30 m radio telescope.

Results. We report the detection of several carbon- and nitrogen-bearing species (CO, ¹³CO, C¹⁸O, C¹⁷O, HCO⁺, HCN, HNC, H¹³CO⁺, CN, N₂H⁺, and C₂H) in the surroundings of AFGL 2298. We identified three velocity components that clearly stand out from the Galactic background. The morphology, kinematics, masses, and isotopic ratios, together with a comparative study of the fractional abundances, lead us to suggest that two of these components (36 and 70 km s⁻¹) have a stellar origin. The other component (46 km s⁻¹) most likely traces swept-up interstellar material, and probably also harbours a photon-dominated region.

Conclusions. We provide the first inventory of the circumstellar molecular gas around AFGL 2298. Our results are compatible with the hypothesis of former mass-loss events produced before the one that created the infrared nebula. The chemistry of this LBV suggests the presence of ejected stellar material, and also swept up gas. These findings will help us to better understand the mass-loss history of this class of evolved massive stars, which is important given that they heavily influence the overall chemical evolution of the Galaxy.