Cernicharo J, Marcelino N, Agúndez M, Guélin M
ICMM. CSIC. Group of Molecular Astrophysics. C/ Sor Juana Inés de la Cruz 3. Cantoblanco, 28049 Madrid, Spain.
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA.
Astron Astrophys. 2015 Mar;575. doi: 10.1051/0004-6361/201424565. Epub 2015 Mar 3.
Thermally-pulsating AGB stars provide three-fourths of the matter returned to the interstellar medium. The mass and chemical composition of their ejecta largely control the chemical evolution of galaxies. Yet, both the mass loss process and the gas chemical composition remain poorly understood. We present maps of the extended CO and CO emissions in IRC+10216, the envelope of CW Leo, the high mass loss star the closest to the Sun. IRC+10216 is nearly spherical and expands radially with a velocity of 14.5 km s. The observations were made On-the-Fly with the IRAM 30 m telescope; their sensibility, calibration, and angular resolution are far higher than all previous studies. The telescope resolution at = 1.3 mm (11″ HPBW) corresponds to an expansion time of 500 yr. The CO emission consists of a centrally peaked pedestal and a series of bright, nearly spherical shells. It peaks on CW Leo and remains relatively strong up to = 180″. Further out the emission becomes very weak and vanishes as CO gets photodissociated. As CO is the best tracer of the gas up to , the maps show the mass loss history in the last 8000 yr. The bright CO shells denote over-dense regions. They show that the mass loss process is highly variable on timescales of hundreds of years. The new data, however, do not support previous claims of a strong decrease of the average mass loss in the last few thousand years. The over-dense shells are not perfectly concentric and extend farther to the N-NW. The typical shell separation is 800-1000 yr in the middle of the envelope, but seems to increase outwards. The shell-intershell brightness contrast is ≥3. All those key features can be accounted for if CW Leo has a companion star with a period ≃800 yr that increases the mass loss rate when it comes close to periastron. Higher angular resolution observations are needed to fully resolve the dense shells and measure the density contrast. The latter plays an essential role in our understanding of the envelope chemistry.
热脉动渐近巨星分支（AGB）恒星为星际介质提供了四分之三的物质。它们抛射物质的质量和化学成分在很大程度上控制着星系的化学演化。然而，质量损失过程和气体化学成分仍然知之甚少。我们展示了IRC+10216（最接近太阳的高质量损失恒星——CW Leo的包层）中扩展的一氧化碳（CO）和一氧化碳发射图。IRC+10216几乎是球形的，以14.5千米每秒的速度径向膨胀。观测是使用IRAM 30米望远镜进行的实时扫描；其灵敏度、校准和角分辨率远高于以往所有研究。在1.3毫米（11″半高全宽）波长处的望远镜分辨率对应于500年的膨胀时间。CO发射由一个中心峰值基座和一系列明亮的、近乎球形的壳层组成。它在CW Leo处达到峰值，在高达180″的距离内仍相对较强。再往外，发射变得非常微弱，并随着CO被光解离而消失。由于CO是高达[具体数值]的气体的最佳示踪剂，这些图展示了过去8000年的质量损失历史。明亮的CO壳层表示过密区域。它们表明质量损失过程在数百年的时间尺度上高度可变。然而，新数据并不支持之前关于在过去几千年平均质量损失大幅下降的说法。过密壳层并非完全同心，并且向西北偏北方向延伸得更远。在包层中部，典型的壳层间距为800至1000年，但似乎向外增大。壳层与壳层间的亮度对比度≥3。如果CW Leo有一颗周期约为800年的伴星，当它接近近日点时会增加质量损失率，那么所有这些关键特征都可以得到解释。需要更高角分辨率的观测来完全分辨致密壳层并测量密度对比度。后者在我们对包层化学的理解中起着至关重要的作用。
