Three-micron spectra of AGB stars and supergiants in nearby galaxies
Three-micron spectra of AGB stars and supergiants in nearby galaxies
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附近星系中 AGB 恒星和超巨星的三微米光谱
DOI：
10.1051/0004-6361:20042305
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发表时间：
2005
影响因子：
6.5
通讯作者：
L. Waters
中科院分区：
文献类型：
--
作者：
M. Matsuura;A. Zijlstra;J. V. Loon;I. Yamamura;A. Markwick;P. Whitelock;P. Woods;J. Marshall;M. Feast;L. Waters;L. Waters
利用大、小麦哲伦星云（LMC和SMC）和人马座矮椭球星系中AGB星（富碳和富氧）和M型超巨星的红外L波段光谱，研究了恒星分子带与金属丰度的关系。富氧星的光谱包括SiO带，富碳AGB星的光谱包括乙炔（C2 H2）、CH和HCN带。乙炔的等效宽度被发现是高的，即使在低金属。高C2 H2丰度可以用低金属碳星的高碳氧比（C/O）来解释。相比之下，HCN的等效宽度较低：不到一半的河外碳星显示3.5微米的HCN带，只有少数LMC星显示高HCN等效宽度。HCN丰度受氮和碳元素丰度的限制。合成氮的量取决于初始质量，高光度（即高初始质量）的恒星可能具有高HCN丰度。CH带在河外和银河系的碳星中都有发现。一颗SMC后AGB星星，SMC-S2，显示出3.3微米的PAH带。这是第一次从SMC后AGB星星中检测到PAH带，证实了PAH可以在这些低金属含量的恒星中形成。没有一颗富氧的大麦哲伦星系恒星显示出SiO带，除了一个可能的低质量光谱。SiO带的等效宽度的限制低于LMC金属丰度高达30 A的预期。几种可能的解释进行了讨论，主要是基于脉动和星周尘埃的影响。观测结果表明，LMC和SMC碳星的质量损失率可能与银河系碳星一样高，因为有更多的碳原子可用，可以形成更多的碳质尘埃。另一方面，缺乏一氧化硅意味着在低金属丰度的富氧恒星中，尘埃较少，质量损失率较低。并讨论了对ISM粉尘富集的影响。
The dependence of stellar molecular bands on the metallicity is studied using infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C2H2 abundance can be explained with a high carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra-galactic carbon stars show the 3.5 µm HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity (i.e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra-galactic and Galactic carbon stars. One SMC post-AGB star, SMC-S2, shows the 3.3 µm PAH band. This first detection of a PAH band from an SMC post-AGB star confirms PAHs can form in these low-metallicity stars. None of the oxygen-rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30 A for LMC metallicity. Several possible explanations are discussed, mostly based on the effect of pulsation and circumstellar dust. The observations imply that LMC and SMC carbon stars could reach mass-loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich stars. The effect on the ISM dust enrichment is discussed.