The nature of the silicon carbide in carbon star outflows
The nature of the silicon carbide in carbon star outflows
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DOI：
10.1093/mnras/288.2.431
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发表时间：
1997-06
影响因子：
4.8
通讯作者：
A. Speck;M. Barlow;C. Skinner
中科院分区：
文献类型：
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作者：
A. Speck;M. Barlow;C. Skinner
我们给出了32颗确定或候选碳星的7.5-13.5 μ mUKIRT CGS 3光谱。除了极端碳星星AFGL 3068，之前已知的唯一一颗碳星星显示出11 μm碳化硅（SiC）的吸收特征，我们还发现了另外三个显示出SiC净吸收的来源的例子，即IRAS 02408 + 5458，AFGL 2477和AFGL 5625。我们调查的碳星星SiC的矿物学和它的关系，陨石尘埃通过使用χ2-最小化例程，以适应观察到的SiC功能，和实验室的光学常数，已公布的各种SiC样品。除R For外，所有观察到的SiC特征都与α-SiC晶粒最佳拟合。除V414 Per外，所有8-13 μm色温> 1200 K（对应于范围底端的质量损失率）的光源最好用纯发射的μ-SiC拟合，而所有8-13 μm色温< 1200 K（对应于更高的质量损失率）的光源，除了一个之外，都最好用自吸收的a-SiC发射拟合。SiC特征为净吸收的四个光源（具有最低的8-13 μm色温，因此可能具有最高的质量损失率）也很好地符合自吸收α-SiC发射，但具有更高的光学深度。鉴于β-SiC是陨石中最常见的形式，我们已经在所有这些碳星的拱壳中寻找β-SiC的证据。然而，我们的观测没有提供明确的证据证明这些恒星周围存在β-SiC，所有观察到的SiC特征都可以用α-SiC晶粒来解释。我们在观察到的SiC发射特征中发现的自吸收以前在辐射传输建模中没有考虑在内，因此过去可能低估了流出物中存在的SiC的量。
We present 7.5-13.5 μm UKIRT CGS3 spectra of 32 definite or candidate carbon stars. In addition to the extreme carbon star AFGL 3068, the only carbon star previously known to show the 11-μm silicon carbide (SiC) feature in absorption, we have discovered three further examples of sources that show SiC in net absorption, namely IRAS 02408 + 5458, AFGL 2477 and AFGL 5625. We investigate the mineralogy of carbon star SiC and its relationship to meteoritic dust by using a χ2-minimization routine to fit the observed SiC features, and laboratory optical constants that have been published for a variety of SiC samples. With the exception of R For, all of the observed SiC features are best fitted by α-SiC grains. Excluding V414 Per, all of the sources with 8–13 μm colour temperatures > 1200 K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by μ-SiC in pure emission, whereas all but one of the sources with 8–13 μm colour temperatures < 1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed a-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8–13 μm colour temperatures and therefore presumably the highest mass-loss rates) are also well fitted by self-absorbed α-SiC emission, but with higher optical depths. Given that β-SiC is the form most commonly found in meteorites, we have searched for evidence of β-SiC in the circumstellar shells of all these carbon stars. However, our observations provide no unambiguous evidence for the presence of β-SiC around these stars, with all of the observed SiC features being best explained in terms of α-SiC grains. The self- absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modelling, and so the amount of SiC present in the outflows has probably been underestimated in the past.