用VEGA/CHARA干涉仪对天津四和参宿七的H α谱线形成区的时间、空间和光谱分辨率
Time, spatial, and spectral resolution of the Halpha line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer
摘要：
BA型超巨星是最明亮的恒星之一。它们在河外环境中是可以观测到的，因此可能是精确的距离指标。BA超巨星参宿七（B8 Ia）和天津四（A2 Ia）的H α线的发射活动表明存在局部的时间依赖的质量喷射。在这里，我们采用光学干涉法研究在这些恒星环境中的H α线形成区域。在CHARA干涉仪上安装了可见光复合器VEGA，利用S1 S2列阵基线（34 m）获得了H α的高空间分辨率（0.001 arcsec）和高光谱分辨率（R= 30000）观测。在2008年和2009年对天津四进行了六次独立观测，2009年对参宿七进行了两次观测。我们用一维非LTE辐射传输代码CMFGEN分析了这个数据集，并使用Balmer线和连续光子评估了风对可见光和近红外干涉特征的影响。我们观察到参宿七和天津四的H α能见度下降，这表明线形成区域扩大（1.5-1.75 R*）。我们观察到天津四在SiII 6371线的能见度显著下降。我们见证了天津四的微分相位的时间变化，这意味着一个不均匀和不稳定的星周环境，而没有这样的变化是在微分vibrium。天津四的辐射传输模型，考虑到星风质量损失，相当好地解释了观测到的Halalpha能见度的下降。根据观测到的微分衰变率，我们估计天津四的质量损失率变化不到5%。
英文摘要：
BA-type supergiants are amongst the most optically-bright stars. They are observable in extragalactic environments, hence potential accurate distance indicators. Emission activity in the Halpha line of the BA supergiants Rigel (B8Ia) and Deneb (A2Ia) is indicative of presence of localized time-dependent mass ejections. Here, we employ optical interferometry to study the Halpha line-formation region in these stellar environments. High spatial- (0.001 arcsec) and spectral- (R=30 000) resolution observations of Halpha were obtained with the visible recombiner VEGA installed on the CHARA interferometer, using the S1S2 array-baseline (34m). Six independent observations were done on Deneb over the years 2008 and 2009, and two on Rigel in 2009. We analyze this dataset with the 1D non-LTE radiative-transfer code CMFGEN, and assess the impact of the wind on the visible and near-IR interferometric signatures, using both Balmer-line and continuum photons. We observe a visibility decrease in Halpha for both Rigel and Deneb, suggesting that the line-formation region is extended (1.5-1.75 R*). We observe a significant visibility decrease for Deneb in the SiII6371 line. We witness time variations in the differential phase for Deneb, implying an inhomogeneous and unsteady circumstellar environment, while no such variability is seen in differential visibilities. Radiative-transfer modeling of Deneb, with allowance for stellar-wind mass loss, accounts fairly well for the observed decrease in the Halpha visibility. Based on the observed differential visibilities, we estimate that the mass-loss rate of Deneb has changed by less than 5%.
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