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This 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. [ABSTRACT FROM AUTHOR]

For the first time the data on the eight confirmed or candidate carbon (C) stars found mainly from objective-prism plates are presented. By using the Catalina database of lightcurves, we find that all these stars are pulsating, allowing a distance to be estimated through the K-band Period-Luminosity (PL) relation. This relation does not depend on spectral type (M or C) and distances are reliable even for C candidates. Seven stars are more than 10 kpc from the galactic plane, suggesting they do not belong to the galactic disk. We also find one star located at about 180 kpc from the Sun, being one of the most distant star in the Galaxy. Many of these new C stars are relatively blue. Some comments are also provided on seven other known halo carbon stars for which either a pulsation period is obtained, or because they were not included in previous works on halo C stars. [ABSTRACT FROM AUTHOR]

In this paper, second in this series, we discuss the nature of 66 faint carbon (C) stars which have been discovered by scrutinizing the plates of the First Byurakan Survey (FBS). These plates display lowresolution spectra of objects located at high Galactic latitudes and have a limiting magnitude of about V = 16. Our sample of 66 objects is part of a total of 120 stars found in the FBS and confirmed spectroscopically to be C stars. These 66 objects are those which show early-type spectra (not N-type). To better characterize these objects, medium-resolution CCD spectra were obtained and are exploited for them all, together with consideration of their 2MASS near-infrared (NIR) colors and their optical variability. First, we establish criteria for getting a spectral classification by using our medium-resolution spectra. Then, 57 objects show spectral features which are typical of CH-giants, while four can be classified as a R-type stars. Five objects in our sample are reported to be probably carbon dwarfs according to previous studies. We derive effective temperatures from photometry. Finally, the optical variability of our objects are studied by using the data of the Catalina Sky Survey (CSS). It is found that the vast majority do not display variability. However, for some of them, the phased light curve may indicate the presence of a secondary component. We estimate the detection range (in kpc) for each class of carbon stars detected in the FBS. Finally, our studies of C stars found at high galactic latitude are discussed in the context of the Gaia mission. [ABSTRACT FROM AUTHOR]

The goal of this paper is to present an optical variability study of the comparatively faint carbon (C) stars which have been discovered by searching the First Byurakan Survey (FBS) low-resolution (lr) spectral plates at high Galactic latitudes using recent wide-area variability databases. The light curves from the Catalina Sky Survey (CSS) and Northern Sky Variability Survey (NSVS) databases were exploited to study theit variability nature. In this paper, first in this series, the variability classes are presented for 54 N-type Asymptotic Giant Branch (AGB) C stars. One finds that 9 stars belongs to the group of Mira-type, 43 are Semi-Regular (SR), and 2 stars are Irregular (Irr)-type variables. The variability types of 27 objects has been established for the first time. K-band absolute magnitudes, distances, and height from the Galactic plane were estimated for all of them. We aim to better understand the nature of the selected C stars through spectroscopy, 2MASS photometric colors, and variability data. Most of the tools used in this study are developed within the framework of the Astronomical Virtual Observatory. [ABSTRACT FROM AUTHOR]

The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. [ABSTRACT FROM AUTHOR]

We present new observations of seven late-type stars previously discovered in the First Byurakan Survey (FBS). These observations prove that they belong to the family of carbon (C) stars. Seventy-nine similar FBS C stars were previously known. The seven objects under analysis were presented in the FBS lists as M or C star candidates. Among the seven objects, six objects are confirmed by low-resolution spectra of the Hamburg Quasar Objective-Prism Survey. We also present for five of them moderate-resolution spectra obtained at optical wavelengths with a spectrograph equipped with a CCD detector. Three objects can be classified as N-type C stars. One object is an early CH-type C star. Most likely, the star FBS 1339+117 belongs to the group of carbon dwarfs. We estimate distances of these seven new C stars either by using their red-band magnitudes, or by using their near-infrared 2MASS J-KS color and KS-band magnitudes. [ABSTRACT FROM AUTHOR]

Using recent astronomical databases, we investigate the characteristics of a list of new Galactic carbon (C) stars. These stars were discovered on the First Byurakan Spectral Sky Survey (FBS) plates and constitute the second part of our search for such objects. This second list of FBS C stars contains 44 objects. Slit spectra obtained by us with the BAO 2.6-m telescope confirm the carbon-rich nature of all of them. The list comprises 12 N-type carbon stars and 32 CH-type stars. We consider spectral types, B and R magnitudes retrieved from the USNO-B1.0 catalog, and JHKs near-infrared photometry for stars extracted from the 2MASS point-source catalog. The R magnitudes of our objects are in the range 10 to 14. We derive distance estimates for all objects and find that most of the stars are located between approximately 3 and 20 kpc from the Sun. Their heights above or below the Galactic plane are in the range 1.5 to 13.0 kpc. [ABSTRACT FROM AUTHOR]

The absolute K-band magnitude determined for the object FBS 0137+400 is typical for Asymptotic Giant Branch (AGB) carbon stars. The large J - K color index as well as large amplitude variability (ΔK > 0.4) and the presence of Hα in the emission indicate also that this star is a mass-losing long-period Carbon Mira variable with thick circumstellar shell. The IRAS LRS spectrum indicates the presence of a dust shell surrounding this object. Monitoring of the K-band magnitude is necessary for the determination of the pulsation period of FBS 0137+400. [ABSTRACT FROM AUTHOR]