Michael Kuhn (U. Hertfordshire): "The Evolution of a Young Stellar Accretion Outburst from V1741 Sgr"

Many young stellar objects exhibit variable accretion rates from circumstellar discs. Accretion outbursts represent some of the most extreme variability, which may contribute significantly to stellar mass assembly. Nevertheless, the triggers of these events and the optimal categorisations for them remain uncertain. In this talk, I will discuss photometric and spectroscopic monitoring of a year-long outburst from the young star V1741 Sgr and what this tells us about the structure of the source. This young star, on the outskirts of the Lagoon Nebula, brightened by ~3 mag over ~2 months in mid-2022, stayed in its bright state until early 2023, then faded erratically over ~4 months in mid-2023. Spectra from near the peak (October 2022) showed an EX Lup-type (EXor) spectrum, with strong emission from H I, He I, and Ca II lines and CO bands. At this stage, spectroscopic absorption features indicated a temperature of ~4750 K with low gravity. However, by April 2023, with the outburst beginning to dim, strong TiO absorption appeared, indicating a cooler ~3600 K temperature. However, the TiO absorption disappeared by August 2023, once the star returned to quiescence. I argue that the observed brightening requires a continuum emitting area larger than the stellar surface, likely from optically thick circumstellar gas with cooler surface layers producing the absorption features. Cooling of the circumstellar gas would explain the appearance of TiO, which subsequently disappeared once this gas had faded and the stellar spectrum reemerged.