New Carbon Stars in the Catalina Catalog of Periodic Variables

International audienceThis study is about carbon stars detected in a program of spectral type determinations for a large amount of optically faint periodic variables. These variables are taken from the Catalina Survey Data Release-1 (CSDR1) data set. As much as 967 objects were studied. Spectral classification uses the objective-prism scanned plates of the Hamburg Quasar Survey (HQS) and of the Hamburg/ESO Survey (HES). Spectra from the Sloan Digital Sky Survey (SDSS) and LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) are also exploited. The majority of the variables in CSDR1 have not been known before. While the majority of stars are found to be F, G, K, M giants and dwarfs, a few dozens of known carbon (C-type) stars are recovered. Six are new. Some supplementary spectra obtained with the Byurakan Observatory 2.6-m telescope are shown. We study in more detail eight C-type stars. Their periods are in the range 120 – 400 days and their V-band magnitude are 14 – 16.5mag. With the assumption that they are classical asymptotic giant branch (AGB) stars and obey the Miras period-luminosity (P-L) relation, large distances (5 up to 100 kpc) are obtained. Three of them are angularly close to C stars attributed to the Sgr tidal leading arm. However, one of these seven stars, CRTS J092231.7+510740, shows significant proper-motion, as given by the recent Sloan catalog. If this motion is confirmed, e.g., by Gaia, and it is a dwarf C star, which is supported by its colors, it is at about 100 pc from the Sun. But in this case its periodic variability remains to be explained. Our work supports the view that the known sample of distant halo C stars can be increased by our method and that great care must be adopted in distance determination