New observations of cool carbon stars in the halo^*

GRAAL, CNRS and Université Montpellier II, Place Bataillon, 34095 Montpellier, France e-mail: mauron@graal.univ-montp2.fr

Received: 10 January 2008
Accepted: 9 February 2008

Abstract

Aims. We report new results of our search for rare, cool carbon stars located at large distances from the Galactic plane.

Methods. Candidate stars were selected in the 2MASS point source catalogue with JHK_s colours typical of N-type carbon stars, with and with Galactic latitude °. Low resolution slit spectroscopy was carried out on 58 candidates.

Results. Eighteen new carbon stars were discovered. Six are remarkable by showing the two peculiarities of a strong infrared excess at 12 μm and a large height above the Galactic plane, from 1.7 to 6 kpc. The number of C stars with these properties has been increased to 16. Mass-loss rates were tentatively estimated by assuming that all these 16 stars are Miras and by using the correlation between and the K - [12] colour index. It is found that several stars have large mass loss, with a median   of 4  10^-6   yrand a dispersion of about a factor of 3 around this value. It would be desirable to detect their CO emission to see whether, like one object already on the list, they display a very low expansion velocity that could be the signature of AGB mass loss at low metallicity. The distances of our new carbon stars were determined by supposing them to be similar to those of the Sagittarius dwarf galaxy. These distances are relatively uncertain, but they do indicate that eight stars might be more than 30 kpc from the Sun, and two at the unprecedented distance of 150 kpc.