[论文解读] GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age

T. Paumard, O. Pfuhl+11 more|HAL (Le Centre pour la Communication Scientifique Directe)|Jun 20, 2014

Stellar, planetary, and galactic studies参考文献 55被引用 8

一句话总结

本研究利用甚大望远镜干涉仪（VLTI）上的PIONIER仪器，首次对银河系中心的M1型超巨星GCIRS 7进行了H波段干涉观测。结果揭示其光球直径为1.076±0.093毫角秒（960±92 R☉），亮度呈现约470天和约2700天的周期性变化，并在K波段存在包周辐射。推导出的物理参数及年龄（6.5–10 Myr）证实该恒星是银河系中心近期恒星形成事件的关键示踪体，与其它年龄指标一致。

ABSTRACT

The stellar population in the central parsec of the Galaxy is dominated by an old (several Gyr) population, but young, massive stars dominate the luminosity function. We have studied the most luminous of these stars, GCIRS 7, in order to constrain the age of the recent star formation event in the Galactic Centre and to characterise it as an interferometric reference for observations of the Galactic Centre with the instrument GRAVITY, which will equip the Very Large Telescope Interferometer in the near future. We present the first H-band interferometric observations of GCIRS 7, obtained using the PIONIER visitor instrument on the VLTI using the four 8.2-m unit telescopes. In addition, we present unpublished K-band VLTI/AMBER data, build JHKL light-curves based on data spanning 4 decades, and measured the star's effective temperature using SINFONI spectroscopy. GCIRS 7 is marginally resolved at H-band (in 2013: uniform-disk diameter=1.076+/-0.093mas, R=960+/-92Rsun at 8.33+/-0.35kpc). We detect a significant circumstellar contribution at K-band. The star and its environment are variable in brightness and in size. The photospheric H-band variations are well modelled with two periods: P0~470+/-10 days (amplitude ~0.64mag) and long secondary period LSP~2700-2850 days (~1.1mag). As measured from CO equivalent width, =3600+/-195K. The size, periods, luminosity (=-8.44+/-0.22) and effective temperature are consistent with an M1 supergiant with an initial mass of 22.5+/-2.5Msun and an age of 6.5-10Myr (depending on rotation). This age is in remarkable agreement with most estimates for the recent star formation event in the central parsec. Caution should be taken when using this star as an interferometric reference as it is variable in size, is surrounded by a variable circumstellar environment and large convection cells may form on its photosphere.

研究动机与目标

• 利用GCIRS 7作为关键恒星示踪体，约束银河系中心近期恒星形成事件的年龄。
• 通过多波段干涉与光谱数据，表征GCIRS 7的物理特性，包括尺寸、温度及变异性。
• 评估GCIRS 7作为未来VLTI上GRAVITY仪器高精度干涉观测相位参考或可见度标准星的适用性。
• 通过整合光度、光谱与干涉数据，调和中心星团中相互矛盾的年龄估计。

提出的方法

• 在2003至2013年间，利用甚大望远镜干涉仪（VLTI）的四台8.2米单元望远镜上的PIONIER仪器获取H波段干涉数据。
• 分析未发表的K波段VLTI/AMBER数据，以探测包周贡献与变异性。
• 基于近40年档案数据构建JHKL光变曲线，研究长期亮度变化。
• 利用SINFONI积分场光谱仪测量有效温度，并通过CO等效宽度估算T_eff。
• 采用周期性分析与均匀盘直径拟合方法，建模光球与包周变化。
• 结合光度、温度与尺寸测量结果，通过演化模型推断初始质量与年龄。

实验结果

研究问题

• RQ1 GCIRS 7的真实年龄是多少？其年龄如何约束银河系中心近期恒星形成事件的时间？
• RQ2 GCIRS 7在多大程度上适合作为高精度干涉仪器（如GRAVITY）的相位参考或可见度标准星？
• RQ3 GCIRS 7观测到的光度变异性由何引起？短周期与长周期如何反映其物理结构？
• RQ4 包周环境在贡献观测到的K波段通量与可见度方面有多显著？
• RQ5 GCIRS 7的物理参数（尺寸、温度、光度）是否与初始质量为22.5±2.5 M☉的M1型超巨星演化模型一致？

主要发现

• GCIRS 7在H波段的均匀盘直径为1.076±0.093毫角秒，对应距离8.33±0.35 kpc处的半径为960±92 R☉。
• 该恒星表现出两个明显的光度周期：P₀ ≈ 470±10天（振幅≈0.64等）与P_LSP ≈ 2700–2850天（振幅≈1.1等）。
• 通过CO等效宽度分析测得有效温度为T_eff = 3,600±195 K。
• 绝对星等为<M_bol> = -8.44±0.22，与初始质量为22.5±2.5 M☉的明亮M1型超巨星一致。
• K波段AMBER数据揭示约20%的包周辐射，表明恒星周围存在显著的尘埃或气体发射。
• 推导出的年龄6.5–10 Myr与基于星团赫罗图及其他恒星示踪体的先前估计高度一致，反驳了2.5–5.8 Myr的更年轻年龄主张。