V557 Mon：在一颗爆发的年轻恒星上观测到的热水蒸气
Hot water emission during an outburst in a classical T Tauri star
AI总结：
本研究观测玫瑰星云的年轻恒星V557 Mon，发现其爆发期间峰值吸积率达6.3×10⁻⁷ M☉/yr，探测到热水蒸气发射，揭示EXor爆发时热分子内盘瞬态形成，分子激发温度与恒星亮度正相关。
AI中文摘要：
本文介绍了通过盖亚科学警报系统利用盖亚时序数据识别的、位于玫瑰星云（Rosette Nebula）中的一颗爆发年轻恒星的观测结果。我们旨在研究V557 Mon在其爆发及随后衰减过程中的亮度和吸积率演化，此外还通过监测分子发射特征追踪爆发期间内吸积盘的演化。我们汇编了来自盖亚（Gaia）、ZTF以及多台1米级地面望远镜的多波段测光时序数据，并获取了覆盖爆发和衰减阶段多个 epoch 的光学和近红外光谱。恒星参数由宁静态颜色/光谱及光谱能量分布（SED）拟合得出，我们还测量了质量吸积率并对分子发射带进行了模型拟合。自2024年末以来，V557 Mon经历了为期一年的爆发，符合EXor变异性特征。根据宁静态测光，V557 Mon的光谱型为M1，消光量AV=1.8±0.3 mag，与一颗质量为0.4-0.5 M☉、年龄为2 Myr的恒星相符。我们的多epoch光谱和u波段测光显示，其爆发期间的峰值吸积率为6.3×10⁻⁷ M☉/yr，约为宁静态的70倍。我们报告了热水蒸气发射带的探测结果，同时还有TiO、VO和CO发射特征。利用ExoMol模型，我们测得内盘温度在爆发衰减阶段从3000 K降至2000 K。我们报告了一颗低质量II型年轻恒星（YSO）近期发生的EXor爆发，观测结果揭示了在EXor事件期间，由可变水蒸气发射追踪的热分子内盘的瞬态形成，且分子激发温度与整体恒星亮度呈正相关。
英文摘要：
In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time series data. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. We compiled multi-band photometric time series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour/spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of AV = 1.8+_0.3 mag, consistent with a 0.4-0.5 M_sol star at an age of 2 Myr. Our multi-epoch spectra and u-band photometry indicate a peak accretion rate of 6.3x10^(-7)M_sol/yr during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.
