arXiv 2109.050482021-09-10astro-ph.SRastro-ph.GA

渐近巨星分支星长蛇座W的CO包层的脉泽辐射

Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

W. H. T. Vlemmings, T. Khouri, D. Tafoya

备注：: Accepted for publication in A\&A, 14 pages, 14 figures
发表信息：: A&A 654, A18 (2021)
DOI：: 10.1051/0004-6361/202141656

摘要：

观测渐近巨星分支（AGB）恒星周围的CO辐射是确定气体质量损失率的主要方法。虽然辐射转移模型已经表明，CO的分子水平可以成为温和的反转，导致脉泽发射，CO脉泽发射还有待于证实的观测。高分辨率观测AGB星周围的CO辐射，现在有亮度温度的灵敏度，以检测可能的弱CO脉泽发射。我们使用阿塔卡马大型毫米/亚毫米阵列（ALMA）进行的高角分辨率观测，观察富氧AGB恒星W Hya周围CO $J=3-2$发射的小尺度结构。我们发现CO脉泽辐射放大了恒星的连续光谱，其光学厚度为$τ\approx-0.55$。脉泽主要放大恒星的边缘，因为从扩展的恒星大气中吸收的CO $J=3-2$在恒星中心最强。CO脉泽速度对应于先前观察到的高频H $_2$ O脉泽和OH脉泽的可变分量，OH脉泽被确定为放大的恒星图像。这意味着脉泽起源于加速区域之外，并限制了速度分布，因为我们发现粒子数反转主要发生在星周包层内部。我们发现，反转可以解释的辐射场在4.6 $μ$ m和CO脉泽发射的存在是一致的估计质量损失率W Hya。然而，泵浦机制需要在扩展大气中吸收和发射谱线之间的复杂相互作用。在W Hya的拱星包层中的尘埃的过量不足以显著地贡献于4.6 $μ$ m处所需的辐射场。分子线之间的相互作用，导致泵送可以限制未来的多层次的CO观测。

英文摘要：

Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO $J=3-2$ emission around the oxygen-rich AGB star W Hya. We find CO maser emission amplifying the stellar continuum with an optical depth $τ\approx-0.55$. The maser predominantly amplifies the limb of the star because CO $J=3-2$ absorption from the extended stellar atmosphere is strongest towards the centre of the star. The CO maser velocity corresponds to a previously observed variable component of high-frequency H$_2$O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 $μ$m and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 $μ$m. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

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