Spitzer observations of acetylene bands in carbon-rich AGB stars in the Large Magellanic Cloud
Spitzer observations of acetylene bands in carbon-rich AGB stars in the Large Magellanic Cloud
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DOI：
10.1111/j.1365-2966.2006.10664.x
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发表时间：
2006-06
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通讯作者：
M. Matsuura;P. Wood;G. Sloan;A. Zijlstra;J. V. Loon;M. Groenewegen;J. Blommaert;M. Cioni;M. Feast;H. Habing;S. Hony;E. Lagadec;C. Loup;J. Menzies;L. Waters;P. Whitelock
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作者：
M. Matsuura;P. Wood;G. Sloan;A. Zijlstra;J. V. Loon;M. Groenewegen;J. Blommaert;M. Cioni;M. Feast;H. Habing;S. Hony;E. Lagadec;C. Loup;J. Menzies;L. Waters;P. Whitelock
我们使用斯皮策太空望远镜 (SST) 上的红外摄谱仪 (IRS) 在 5–38 µm 范围内研究大麦哲伦星云 (LMC) 中富含碳的 AGB 恒星的分子带。所有 26 个低分辨率光谱均显示 7 和 14 µm 处的乙炔 (C2H2) 谱带。这些波长处的氰化氢 (HCN) 谱带非常弱或不存在。这与大麦哲伦地区的低氮丰度是一致的。所观察到的 14 µm C2H2 谱带可以通过 500 K 的激发温度合理地再现。星周尘埃发射没有明显稀释 14 µm C2H2 谱带。这个 14 µm 谱带源自这些高质量损失率恒星的星周包层中的分子气体，这与之前对银河系恒星的发现一致。来自 13.7 µm 波段的 C2H2 柱密度显示气体质量损失率在 3 ×10 -6 至 5 ×10 -5 M ⊙ yr -1 范围内。这与根据光谱能量分布估计的这些恒星的总质量损失率相当。此外，我们还将 LMC 样本的 13.7 µm C2H2 谱带的谱线强度与银河系样本的谱线强度进行了比较。尽管LMC的金属丰度较低，但LMC和银河系恒星的C2H2丰度没有明显差异。这反映了第三次疏浚将自产碳带到表面的效果，导致低金属丰度下的高 C/O 比。
We investigate the molecular bands in carbon-rich AGB stars in the Large Magellanic Cloud (LMC), using the InfraRed Spectrograph (IRS) on board the Spitzer Space Telescope (SST) over the 5–38 µm range. All 26 low-resolution spectra show acetylene (C2H2) bands at 7 and 14 µm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 µm C2H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 µm C2H2 band by circumstellar dust emission. This 14 µm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2H2 column density, derived from the 13.7 µm band, shows a gas mass-loss rate in the range 3 ×10 −6 to 5 ×10 −5 M ⊙ yr −1 . This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 µm C2H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2H2 abundance among LMC and Galactic stars. This reflects the effect of the 3rd dredge-up bringing self-produced carbon to the surface, leading to high C/O ratios at low metallicity.