arXiv 1302.42632013-02-18astro-ph.GA

AGB拱星包层的深部光学成像

Deep optical imaging of AGB circumstellar envelopes

N. Mauron, P. J. Huggins, C. -L. Cheung

备注：: 19 pages, 13 figures, color pictures in Appendix, accepted by A&A
DOI：: 10.1051/0004-6361/201219517

摘要：

我们报告的结果，一个程序的图像扩展拱星信封的渐近巨星分支（AGB）恒星在尘埃散射银河光。我们的目标是表征信封的形状，以探测质量损失的几何形状和隐藏的二进制同伴的存在。观测包括22颗具有高质量损失率的AGB恒星的深光学成像：16颗使用ESO 3.5 m NTT望远镜，其余使用其他望远镜。在15个天体中检测到星周包层，质量损失率> 2 E-6 Msun/年。包层的表面亮度随银河半径的增大而显著减小，这表明星际辐射场存在陡峭的径向梯度。信封的范围从圆形到椭圆形的形状，我们表征它们的椭圆度（E =长/短轴）的等强度轮廓。我们发现，大约50%的包络接近圆形，E < 1.1，其他的更椭圆，大约20%，E > 1.2。我们解释的形状人口的单星和双星的信封是平坦的同伴。E的分布是定性一致的预期的基础上的人口合成模型的二元AGB星。我们还发现，大约50%的样品在中心区域表现出小规模的细长特征。我们将这些解释为光从中心恒星通过极孔逃逸，这也可能是由同伴产生的。我们的信封压扁和极孔的观察点内AGB星的拱星信封的同伴隐藏的人口。这些伴星预计将在过渡到后AGB恒星和行星状星云的形成中发挥重要作用。

英文摘要：

We report results of a program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The goal is to characterize the shapes of the envelopes to probe the mass-loss geometry and the presence of hidden binary companions. The observations consist of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the ESO 3.5 m NTT telescope, and the remainder with other telescopes. The circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6 Msun/year. The surface brightness of the envelopes shows a strong decrease with Galactic radius, which indicates a steep radial gradient in the interstellar radiation field. The envelopes range from circular to elliptical in shape, and we characterize them by the ellipticity (E = major/minor axis) of iso-intensity contours. We find that about 50 percent of the envelopes are close to circular with E < 1.1, and others are more elliptical with about 20 percent with E > 1.2. We interpret the shapes in terms of populations of single stars and binaries whose envelopes are flattened by a companion. The distribution of E is qualitatively consistent with expectations based on population synthesis models of binary AGB stars. We also find that about 50 percent of the sample exhibit small-scale, elongated features in the central regions. We interpret these as the escape of light from the central star through polar holes, which are also likely produced by companions. Our observations of envelope flattening and polar holes point to a hidden population of companions within the circumstellar envelopes of AGB stars. These companions are expected to play an important role in the transition to post-AGB stars and the formation of planetary nebulae.

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