Observations carried out at Carnegie Science’s Las Campanas Observatory contributed to a study of an unusual phenomenon in a young star: the presence of extremely hot water vapor in the disk of material surrounding it during an episode of sharply increased brightness. The study, led by astronomer Zhen Guo of the Institute of Physics and Astronomy at the University of Valparaíso (IFA UV), analyzed V557 Mon, a young star located in the Rosette Nebula, more than 4,500 light-years from Earth. The research combined observations obtained with different telescopes and at several wavelengths to follow the evolution of an outburst that began in late 2024 and lasted approximately one year.
Las Campanas contributed to two components of the observational follow-up. Using the FIRE infrared spectrograph, installed on the 6.5-meter Baade Telescope, the team obtained spectra of V557 Mon during the outburst and again as the star began to fade. These observations made it possible to examine the inner region of the disk surrounding the star and to study different molecular species present during the event.
The near-infrared spectra revealed numerous emission lines and bands, including those produced by carbon monoxide and water vapor. By comparing observations obtained at different times, the researchers were able to track how the physical conditions of the material changed as the outburst evolved. The models indicate molecular temperatures between 2,000 and 3,000 K, associated with an inner disk that was temporarily heated by the sharp increase in the accretion of material onto the star.
The detection is unusual. Water is commonly present in the disks of gas and dust surrounding young stars, but observing strong water-vapor emission bands at temperatures approaching 3,000 K is much less common. The study reports that this type of emission, with these characteristics, had not previously been observed in a Class II young star during one of these eruptive episodes.
V557 Mon belongs to a group of young stars known as EXor variables. During these episodes, the amount of material falling from the disk onto the star increases abruptly, causing the star to brighten and changing the conditions in the innermost regions of the system. In this case, the peak accretion rate reached several tens of times the level measured when the star was in its quiescent state.
The observations obtained with FIRE were particularly useful for studying this transformation. The instrument provides near-infrared spectra over a wavelength range of approximately 0.8 to 2.5 microns, where several of the molecular features analyzed in the study are found. The Magellan Baade Telescope, which hosts FIRE, is one of the two 6.5-meter Magellan Telescopes located at Las Campanas Observatory.
The study also included photometric observations obtained with the one-meter Swope Telescope. Graduate students from IFA UV participated directly in these observations as part of their training in observational astronomy, collecting data on V557 Mon through different filters. The Swope measurements were incorporated into the dataset used to reconstruct changes in the star’s brightness and to complement its monitoring over time.
The combination of spectroscopy and photometry allowed the team to study not only which molecules were present, but also how their temperatures changed as V557 Mon faded. The results suggest that, during the outburst, the inner disk became hotter and moved closer to the star, temporarily reaching conditions similar to those found in stellar systems at earlier stages of evolution.
The result also illustrates the value of coordinated follow-up observations of variable astronomical phenomena. V557 Mon was initially identified through monitoring and classification systems for variable sources and was subsequently observed with telescopes operated by several institutions. At Las Campanas, the combination of the spectroscopic capabilities of Magellan Baade and photometric monitoring with Swope provided information at different stages of the event.
The research brings together scientists and students from institutions in Chile and other countries and has been accepted for publication in Astronomy & Astrophysics. The paper, titled Hot H₂O emission during an outburst on a classical T Tauri star, is led by Zhen Guo and includes researchers and students from the Institute of Physics and Astronomy at the University of Valparaíso, together with international collaborators.