Galactic mass-losing AGB stars probed with the IRTS. I.

AGB mass-losing sources are easy to identify and to characterize in the near-infrared range (1-5 mu m). We make use of the near-infrared data acquired by the Japanese space experiment IRTS to study a sample of sources detected in the 2 celestial strips surveyed by the IRTS. Mass-loss rates and distances are estimated for 40 carbon-rich sources and 86 oxygen-rich sources of which 8 are probably of S-type. Although the sample is small, one sees a dependence of the relative contribution of the two kinds of sources to the replenishment of the interstellar medium (ISM) on the galactocentric distance. E.g. from 6 to 8 kpc, oxygen-rich sources in our sample contribute 10-12 times as much as carbon rich sources, whereas from 10 to 12 kpc, the latters contribute 3-4 times as much as the formers. Therefore, one would expect a gradient in the composition of the ISM between 6 and 12 kpc from the Galactic Centre, especially in its dust component. Most of the replenishment (>50%) by AGB stars is due to sources with mass-loss rate larger than 10^-6 M_sun yr^-1.