The 2022-2023 accretion outburst of the young star V1741 Sgr

Abstract: V1741 Sgr (= SPICY 71482/Gaia22dtk) is a Classical T Tauri star on the outskirts of the Lagoon Nebula. After at least a decade of stability, in mid-2022, the optical source brightened by ~3 mag over two months, remained bright until early 2023, then dimmed erratically over the next four months. This event was monitored with optical and infrared spectroscopy and photometry. Spectra from the peak (October 2022) indicate an EX Lup-type (EXor) accretion outburst, 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 T ~ 4750 K with low-gravity lines (e.g., Ba II and Sr II). By April 2023, with the outburst beginning to dim, strong TiO absorption appeared, indicating a cooler T ~ 3600 K temperature. However, once the source had returned to its pre-outburst flux in August 2023, the TiO absorption and the CO emission disappeared. When the star went into outburst, the source's spectral energy distribution became flatter, leading to bluer colours at wavelengths shorter than ~1.6 microns and redder colours at longer wavelengths. The 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. Additional contributions to the outburst spectrum may include blue excess from hotspots on the stellar surface, emission lines from diffuse gas, and reprocessed emission from the dust disc. Cooling of the circumstellar gas would explain the appearance of TiO, which subsequently disappeared once this gas had faded and the stellar spectrum reemerged.

• The paper details the EX Lup-type accretion outburst of V1741 Sgr through comprehensive optical and infrared spectroscopic and photometric analysis.
• It identifies a two-phase brightness evolution with a 3-magnitude rise and a variable decline, tracking temperature changes from ~4750 K to ~3600 K.
• The study underscores the role of optically thick circumstellar gas and hotspots in the outburst, offering insights to refine models of young stellar object accretion.

An Analysis of the 2022--2023 Accretion Outburst of V1741 Sgr

The paper under discussion examines the accretion outburst of the Classical T Tauri star V1741 Sgr, located on the periphery of the Lagoon Nebula. This event, cataloged as Gaia22dtk, marked the first recorded outburst from this source. Specifically, the research utilizes a blend of optical and infrared spectroscopy, along with photometric data, to capture a detailed progression of the 2022--2023 outburst.

Summary of Key Observations

1. Event Timeline and Characteristics: The outburst of V1741 Sgr was observed to have begun in mid-2022, with a brightening of approximately 3 magnitudes over two months. The star remained luminously stable until early 2023 before dimming erratically over the ensuing months, finally returning to its pre-outburst brightness by August 2023. This timeline reflects a typical EX Lup-type (EXor) accretion event characterized by two phases—initial peak brightness followed by a variable decline.
2. Spectroscopic Features: During the peak of the outburst, the star exhibited distinct spectroscopic features synonymous with EX Lup-type stars. These include strong H\,{\sc i}, He\,{\sc i}, and Ca\,{\sc ii} emission lines, as well as CO bands. Initial observations indicated a temperature of approximately 4750 K, signified by low-gravity absorption features such as those from Ba\,{\sc ii} and Sr\,{\sc ii}. As the star began to fade, a transition to cooler temperatures (~3600 K) was observed, suggested by the appearance of TiO absorption bands.
3. Impacts and Implications: The outburst caused substantial brightening in the optical spectrum, necessitating an emitting area much larger than the stellar surface. This was attributed to optically thick circumstellar gas with cooler surface layers contributing to the absorption features. Additional contributors to the flux during the outburst may have included hotspots and diffuse gas emissions, supplemented by reprocessed light from the surrounding dust disc.
4. Stellar and Environmental Context: The paper situates V1741 Sgr within the broader field of pre-main-sequence stars, leveraging Gaia astrometry to establish the star's location relative to the Lagoon Nebula. Parallaxes and proper motions confirm its association with the nebula, marking it as a relatively isolated T Tauri star with a notably high pre-outburst accretion rate, estimated at $\sim 2 \times 10^{-8} M_\odot \text{yr}^{-1}$.

Implications for Future Research

The findings enrich our comprehension of EX Lup-type outbursts, contributing to the spectroscopic diversity recognized in young stellar objects (YSOs). The ability to precisely monitor the spectral evolution from optical to infrared wavelengths offers detailed insights into the mechanisms of accretion and the interactions within circumstellar disks. Such detailed case studies enhance the comparative analyses necessary for advancing theoretical models on the lifecycle and behavior of YSOs.

The identification and characterization of outbursts in stars like V1741 Sgr stand to sharpen the parameters used in stellar models while underscoring the dynamic processes that govern YSO systems. This paper serves as an important contribution, particularly in cataloging the spectral characteristics of such events, thereby furnishing astronomers an enhanced framework for interpreting forthcoming observational data. The increasing frequency and resolution of sky-monitoring surveys promise to unearth additional events, drawing finer distinctions within the accretion outburst category and refining the overall understanding of these celestial phenomena.