Bujarrabal V, Castro-Carrizo A, Alcolea J, Santander-García M, Van Winckel H, Sánchez Contreras C
Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, E-28803 Alcalá de Henares, Spain.
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406, Saint Martin d'Hères, France.
Astron Astrophys. 2016 Sep;593. doi: 10.1051/0004-6361/201628546. Epub 2016 Sep 29.
We aim to study the rotating and expanding gas in the Red Rectangle, which is a well known object that recently left the asymptotic giant branch (AGB) phase. We analyze the properties of both components and the relation between them. Rotating disks have been very elusive in post-AGB nebulae, in which gas is almost always found to be in expansion.
We present new high-quality ALMA observations of CO =6-5 and HCN =4-3 line emission and results from a new reduction of already published CO =3-2 data. A detailed model fitting of all the molecular line data, including previous maps and single-dish observations of lines of CO, CII, and CI, was performed using a sophisticated code that includes an accurate nonlocal treatment of radiative transfer in 2D. These observations (of low- and high-opacity lines requiring various degrees of excitation) and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties, particularly in the determination of the boundaries of the CO-rich gas and some properties of the outflow.
We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 , and that of the CO-rich outflow is around ten times smaller. High temperatures of ≳ 100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between 5000 and more than 20000 yr. The angular momentum of the disk is found to be high, ~ 9 AU km s, which is comparable to that of the stellar system at present. Our observations of HCN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ≲ 60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to an efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.
我们旨在研究红矩形星云（一个最近离开渐近巨星分支（AGB）阶段的著名天体）中旋转且膨胀的气体。我们分析了这两个成分的性质以及它们之间的关系。在AGB后星云（其中气体几乎总是处于膨胀状态）中，旋转盘一直非常难以捉摸。
我们展示了新的高质量阿塔卡马大型毫米/亚毫米波阵列（ALMA）对CO（6 - 5）和HCN（4 - 3）谱线发射的观测结果，以及对已发表的CO（3 - 2）数据进行新处理后的结果。使用一个复杂的代码对所有分子谱线数据进行了详细的模型拟合，该代码包括对二维辐射传输的精确非局部处理，这些数据包括之前的图谱以及对CO、CII和CI谱线的单 dish观测。这些观测（针对需要不同激发程度的低透明度和高透明度谱线）以及相应的建模使我们能够深化对星云性质的分析。我们还强调了不确定性，特别是在确定富含CO气体的边界和外流的一些性质方面。
我们确认了存在一个旋转的赤道盘和一股外流，外流主要由离开盘的气体形成。盘的质量约为0.01 ，而富含CO的外流质量大约小十倍。大多数成分的温度都高于100K。通过比较质量值，我们大致估算了旋转盘的寿命，发现约为10000年。考虑到其他一些AGB后复合星云的数据，我们发现AGB后恒星周围盘的寿命通常在5000年至超过20000年之间。发现盘的角动量很高，约为9 天文单位·千米/秒，这与目前恒星系统的角动量相当。我们对HCN的观测显示出特别宽的速度弥散，并表明这种分子仅在距离恒星系统小于60天文单位的内开普勒盘中丰富。我们认为HCN是在一个致密的光解离区域（PDR）中形成的，这是由于已知由恒星系统产生的紫外线过量，遵循了星际介质PDR和围绕年轻恒星的盘所熟知的化学机制。我们进一步认为，这种紫外线过量可能导致多环芳烃（PAHs）和其他含碳大分子的有效形成和激发，其发射在光学对应体中非常强烈。
