VVV-WIT-13: An eruptive young star with cool molecular features

¹ Instituto de Física y Astronomía, Universidad de Valparaíso, ave. Gran Bretaña, 1111, Casilla, 5030 Valparaíso, Chile
² Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, AL10 9AB UK
⁴ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT UK
⁵ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabianska 8, 87-100 Toruń, Poland
⁶ Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso, Chile
⁷ School of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH UK
⁸ Fakultat fur Physik, Universitat Duisburg-Essen, Lotharstraße 1, D-47057 Duisburg, Germany
⁹ Research Institute of Physics, Southern Federal University, Rostov-on-Don, 344090 Russia
¹⁰ INAF – Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
¹¹ Departamento de Fisica y Astronomia, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, 8320000 Santiago, Chile
¹² Vatican Observatory, V00120 Vatican City State, Italy
¹³ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, 02800 Antofagasta, Chile
¹⁴ Departamento de Física, Universidade Federal de Santa Catarina, Trindade, 88040-900 Florianopolís, SC, Brazil
¹⁵ University of Wyoming, 1000 E University Ave, Laramie, WY, USA
¹⁶ Cerro Tololo Inter-American Observatory, National Optical Astronomical Observatory, Casilla 603, La Serena, Chile
¹⁷ Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826 Republic of Korea
¹⁸ Research Institute of Basic Sciences, Seoul National University, Seoul, 08826 Republic of Korea
¹⁹ Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH United Kingdom
²⁰ Department of Physics and Astronomy, University of the Western Cape, Bellville, 7535 South Africa
²¹ Chinese Academy of Sciences South America Center for Astronomy (CASSACA), National Astronomical Observatories, CAS, Beijing, 100101 China
²² Departamento de Astronomía, Universidad de Chile, Las Condes, 7591245 Santiago, Chile
²³ Department of Astronomy, the University of Michigan, 1085 S. University, 323 West Hall, Ann Arbor, MI, 48109-1107 USA
²⁴ Department of Astronomy, Tsinghua University, Beijing, 100084 People’s Republic of China

^⋆ Corresponding author: zhen.guo@uv.cl.

Received: 21 June 2025
Accepted: 16 September 2025

Abstract

Context. Outburst phenomena are observed at different stages of stellar evolution, due to the enhancement of the mass accretion rate on protostars or even stellar merger events. In the case of a young stellar object (YSO), the episodic mass accretion event plays an important role in the pre-main-sequence stellar mass assembly. Here we investigate an infrared eruptive source (RA = 16:53:44.38; Dec = − 43:28:19.47), identified from the decade-long VISTA Variables in the Vía Láctea survey (VVV). We named this target after a group of variable sources discovered by VVV, as VVV-WIT-13, where WIT stands for ‘What is this?’, due to its unique photometric variation behaviour and the mysterious origin of the outburst. This target exhibited an outburst with a 5.7 mag amplitude in the K_s-band, remained on its brightness plateau for 3.5 years, and then rapidly faded to its pre-eruptive brightness afterwards.

Aims. Our aim is to reveal the variable nature and outburst origin of VVV-WIT-13 by presenting our follow-up photometric and spectroscopic observations along with theoretical models.

Methods. We gathered photometric time series in both near- and mid-infrared wavelengths. We obtained near-infrared spectra during the outburst and decaying stages on XSHOOTER/VLT and FIRE/Magellan, and then fitted the detected molecular absorption features using models from ExoMol. We applied 2D numerical simulations to re-create the observables of the eruptive phenomenon.

Results. We observe deep AlO absorption bands in the infrared spectra of VVV-WIT-13, during the outburst stage, along with other more common absorption bands (e.g. CO). Our best-fit model suggests a 600 K temperature of the AlO absorption band. In the decaying stage, the AlO bands disappeared, whilst broad blue-shifted H₂ lines arose, a common indicator of stellar wind and outflow. The observational evidence suggests that the CO and TiO features originate from an outflow or a wind environment.

Conclusions. We find that VVV-WIT-13 is an eruptive young star with instability occurring in the accretion disk. One favoured theoretical explanation of this event is a disrupted gas clump at a distance of 3 au from the source. If confirmed, this would be the first such event observed in real time.