Cool carbon stars in the halo and in dwarf galaxies: Halpha, colours, and variabiity

Abstract

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 Halpha 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 is it 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.

11 pages, 10 figures + one appendix of 26 pages; accepted for publication in Astronomy and Astrophysics

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• A Brown Dwarf Census from the SIMP Survey
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• Properties of carbon stars in the Solar neighbourhood based on Gaia DR2 astrometry
• Carbon Stars in the Satellites and Halo of M31
• 30-micron sources in galaxies with different metallicities
• Emission-line stars in M31 from the SPLASH and PHAT surveys
• The variable V381 Lac and its possible connection with the R CrB phenomenon
• CGCS 6306, another X-ray-emitting asymptotic giant branch star confirmed to be a symbiotic binary