Two mass-losing carbon stars in the Galactic halo

Abstract

Observations of two mass-losing carbon stars in the Galactic halo, lRAS 08546 + 1732 and 12560 + 1656, are presented. These objects were discovered serendipitously, and stand out from the usual carbon stars at high galactic latitudes in that they have optical and lRAS colours consistent with current mass-loss. New optical spectra, CO observations, and modelling of the spectral energy distribution and of the CO lines are presented. Luminosities are derived using a period-luminosity relation. From the dust modelling lRAS 08546 + 1732 is found to be at 20 kpc from the Sun (11.3 kpc from the Galactic plane) and has a mass-loss rate of 3.3 x 10-6 Mo yc1• From the CO non-detection we deduce that it is probably oxygen-deficient, corroborating earlier work. lRAS 12560 + 1656 is found to be at 8.0 kpc from the Sun (7.8 kpc from the Galactic plane) and has a mass-loss rate of 1.3 x 10-6 Mo yr-1. The detection of the 12CO J = 2-1 transition in the spectrum of lRAS 12560 + 1656 after an integration time of 10 h makes it probably the longest 12CO integration on a stellar object. The detection itself makes the star one of the most distant stellar objects detected in the CO line. The outflow velocity of 3.2 km S-1 is very low, and the stellar velocity is + 88 km S-1 with respect to the LSR. Modelling of the CO line implies an oxygen abundance of 0.7 dex below solar. We examine existing data on the 'faint high-latitude carbon stars' and identify two additional distant, mass-losing, N-type AGB stars. The nature of halo carbon stars is discussed, and suggestions on how to find more mass-losing halo AGB stars are . presented.