[论文解读] Infrared observations of the flaring maser source G358.93-0.03 -- SOFIA confirms an accretion burst from a massive young stellar object

B. Stecklum, V. Wolf+25 more|arXiv (Cornell University)|Jan 5, 2021

Astrophysics and Star Formation Studies参考文献 102被引用 61

一句话总结

本研究利用SOFIA/FIFI-LS的远红外观测，确认了大质量年轻恒星对象（MYSO）G358.93-0.03存在一次吸积爆发，该爆发在尘埃连续谱辐射中表现为显著的光度增加。此次爆发是迄今已知持续时间最短、亮度最低的，触发了爆发性喷出的II型甲醇脉泽，验证了脉泽喷发作为大质量恒星形成中周期性吸积的可靠指标。

ABSTRACT

Class II methanol masers are signs of massive young stellar objects (MYSOs). Recent findings show that MYSO accretion bursts cause flares of these masers. Thus, maser monitoring can be used to identify such bursts. Burst-induced SED changes provide valuable information on a very intense phase of high-mass star formation. In mid-January 2019, a maser flare of the MYSO G358.93-0.03 was reported. ALMA and SMA imaging resolved the core of the star forming region and proved the association of the masers with the brightest continuum source MM1. However, no significant flux rise of the (sub)mm dust continuum was found. Thus, we performed NIR imaging with GROND and IFU spectroscopy with FIFI-LS aboard SOFIA to detect possible counterparts to the (sub)mm sources, and compare their photometry to archival measurements. The comparison of pre-burst and burst SEDs is of crucial importance to judge whether a luminosity increase due to the burst is present and if it triggered the maser flare. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our RT modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the multi-epoch SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous MYSO accretion burst so far.

研究动机与目标

• 验证在大质量年轻恒星对象（MYSO）G358.93-0.03中存在吸积爆发，该爆发由爆发性喷出的II型甲醇脉泽推断得出。
• 利用近红外和远红外测光，确定脉泽喷发是否由中心原恒星的光度增加驱动。
• 通过多 epoch 谱存在函数（SEDs）的辐射转移建模，约束爆发参数（如峰值光度、持续时间及能量）。
• 研究爆发后包层的加热与冷却演化 timescales。
• 探讨爆发引起的原恒星包层变化与观测到的脉泽位置迁移及喷发之间的关联。

提出的方法

• 利用2.2米MPG/ESO望远镜上的GROND仪器获取近红外（NIR）成像，测量短波长处的爆发前与爆发时的流量。
• 利用SOFIA的FIFI-LS仪器在两个观测 epoch（爆发早期和约16个月后）进行远红外（FIR）积分场光谱观测，测量MM1源在889–1532 µm波段的流量。
• 将观测到的FIR流量与赫歇尔（Herschel）和ALMA/SMA的档案数据进行比较，检测到显著的流量增加，表明中心原恒星光度上升。
• 利用多波段测光，通过去卷积技术排除其他源的贡献，构建爆发前、爆发中和爆发后的SED。
• 对尘埃连续谱SED进行辐射转移建模，推导出爆发参数，如光度、持续时间及质量吸积速率。
• 利用干涉测量（ALMA/SMA）数据分离MM1源，确认其与脉泽喷发的关联，实现流量的精确去混淆。

实验结果

研究问题

• RQ1 G358.93-0.03的6.7 GHz甲醇脉泽喷发是否对应于中心MYSO红外光度的可检测增加？
• RQ2 通过多 epoch SED 建模推导出的吸积爆发关键参数（光度、持续时间、能量）是什么？
• RQ3 爆发后包层的加热与冷却 timescales 与理论预期相比如何？
• RQ4 瞬态包层盘在爆发动力学和脉泽重新激活中扮演何种角色？
• RQ5 脉泽喷发在多大程度上可作为大质量恒星形成中周期性吸积的可靠示踪？

主要发现

• SOFIA/FIFI-LS检测到MM1源的远红外流量显著增加，889 µm处的流量比赫歇尔测量值高出约2.5倍，证实了光度上升。
• 爆发峰值光度达到约1.5 × 10⁴ L⊙，持续时间约1.5年，是迄今已知持续时间最短、亮度最低的吸积爆发。
• SED的辐射转移建模揭示了MYSO周围存在一个瞬态包层盘，可能在调制爆发过程中发挥了关键作用。
• 包层的加热 timescale 估计为约0.5年，冷却 timescale 为约1.5年，与大质量原恒星爆发驱动加热的模型一致。
• 脉泽喷发及脉泽斑点的向外传播直接与爆发的热辐射前驱波相关，证实了辐射泵浦机制。
• 爆发后16个月，SED仍显示较高的流量，表明包层环境存在持续的热弛豫阶段。