Cool carbon stars in the halo: A new survey based on 2MASS

We present the first results of a new survey for finding cool N-type carbon (C) stars in the halo of the Galaxy. Candidates were first selected in the 2MASS Second Incremental Release database with JHK(s) colours typical of red AGB C stars and K(s) < 13. and subsequently checked through medium resolution slit spectroscopy. We discovered 27 new C stars phis one known previously and two similar objects in the Fornax and Sculptor dwarf galaxies. We determine and discuss the properties of our sample. including optical and near-infrared colours, radial velocities, as well as Ha emission and variability that are frequent. all these characteristics being compatible with an AGB C-type classification. Surprisingly, of the 30 studied objects. 8 were found to have small but measurable proper motions (p) in the USNO-B1.0 catalogue, ranging over 8 < mu < 21 mas yr(-1) and opening the possibility that some objects could perhaps he dwarf carbon stars. Yet, a detailed analysis based on comparison with the sample of known carbon dwarfs leads us to consider these p as incompatible with the broader picture suggested by the other data taken as a whole. So, we adopt the view that all objects are of AGB type, i.e. luminous and distant. Because the stream of Sagittarius dwarf galaxy is known to be the dominant Source of luminous C stars in the halo, we chose to determine distances for our sample by scaling them oil the 26 known AGB C stars of the Sgr galaxy itself, which are found to be, in the K(s)-band. similar to 0.5 mag less luminous than the average LMC C stars for a given J - K(s) colour. The obtained distances of our halo stars range from 8 to 80 kpc from the Sun. Then, examination of position and radial velocities show that about half belong to the Sgr stream. Our findings Suggest that numerous AGB C stars remain to be discovered in the halo. Long term K(s)-band monitoring would be of great value to ascertain distance estimates through the period-luminosity relation, because a large fraction of our sample is probably made of Mira variables.