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tokens: different sub-types of carbon stars in line indices and multi-band photometric space. Finally, a short conclusion is drawn in Section 5 . 2. DATA 2.1. The LAMOST Data The LAMOST, also called the Guo Shou Jing telescope, is a 4 m reflective Schmidt telescope with 4000 fibers on a 20 square degree focal plane (Cui et al. 2012 ; Zhao et al. 2012 ). The unique design of LAMOST enables it to take 4000 spectra in a single exposure to a limiting magnitude as faint as r = 18 at the resolution R = 1800. The LAMOST survey will finally obtain more than 5 million low-resolution stellar spectra after its five year survey (Deng et al. 2012 ). Previous works have shown that LAMOST observations are biased more to giant than dwarf stars, since the limiting magnitude is relatively brighter (Liu et al. 2014a ; Wan et al. 2015 ). By the end of 2014, the LAMOST team had released the DR2 catalog, which contains 4,136,482 targets including 3,784,461 stars. In DR2, there are about 0.5 million late type giant stars (Liu et al. 2015a ), implying that there should be large numbers of carbon stars. 2.2. Line Indices The typical spectra of carbon stars are characterized by strong C 2 , CN, and CH absorption bands (Secchi 1869 ). Therefore, we measure the line indices of carbon related molecular lines to identify carbon stars. We show several sample LAMOST spectra for typical carbon stars in Figure 1 . The features of the C 2 swan bands at 4737, 5165, and 5635 Å, and red CN molecule bands at 7065 and 7820 Å are prominent in these spectra, while the detailed features are different in different sub-types. Compared to the traditional approach to classifying spectral types, using line indices allows a semi-automatic classification. With multiple spectral line indices, various spectral types may transitionally change from one to another, which naturally reflects the variation of the astrophysical parameters, e.g., effective temperature, surface gravity, and metallicity, from type to type (Liu et al. 2015b ). Figure 1. LAMOST spectra for typical carbon stars. The features of the C 2 swan bands at 4737, 5165, and 5635 Å and red CN molecule bands at 7065 and 7820 Å are indicated. In order to identify carbon stars, we adapt the Lick line indices defined by Worthey ( 1994 ) and Worthey Ottaviani ( 1997 ). For the LAMOST DR2 spectra, we use the measured results from Liu et al. ( 2015b ). The Ba ii (4554 Å), molecular bands C 2 (around 5635 Å), CN (7065 Å), and CN (7820 Å) are also measured based on the wavelength definition listed in Table 1 . Table 1 Line Indices Definition Used in This Work The line index in terms of equivalent width (EW) is defined by the following equation (Worthey 1994 ; Liu et al. 2015a ):where F λ and F C are the fluxes of the spectral line and continuum, respectively, which are functions of wavelength λ . The pseudo-continuum F C is estimated via linear interpolation of the fluxes located in the “shoulder” region on either side of each index bandpass. The line