AGB Stars with High Mass Loss Rates in the Bulge of our Galaxy (Abstract)

Abstract

Using suitable criteria for the IRAS flux density ratios F25/F12 and F60/F25 a subset of AGB star candidates with high mass loss rates (OH/IR stars) were selected. These stars, which are at the end of their mass loss phase, clearly show a disk and a bulge component. The bulge component was studied by selecting those stars with 1 < 10° and 2° < b < 10°.For all of these stars a total luminosity and a ‘redness’ were determined. Luminosities were determined by integrating under two Planck curves, one fitting the 12 to 25μm and one fitting the 25 to 60μm ratios and assuming a distance to the galactic centre of 8.7 Kpc. The observed luminosity histogram shows that the majority of the objects have luminosities between 2000 and 7500 Lo.The ‘redness’, defined as the fraction of the total flux emitted at μ > 25 μm, is well correlated with the relative depth of the 9.7 μm silicate absorption as found from LRS spectra of over twenty nearby OH/IR stars, figure 1, which in turn is correlated with mass loss rates as model spectra show (Bedijn, priv. comm.). The redness of a star is thus a measure of its mass loss rate. Because the time scale for mass loss at the very end of the AGB is much smaller than the corresponding time scale for the growth of the core mass, both core masses and luminosities can be taken constant during this phase.From the redness distribution within one bin in the luminosity histogram the mass loss distribution follows directly and thus the mass loss evolution for stars with the same initial ZAMS masses. The mass loss evolution at the end of the AGB appeared to be exponential in time (M ∽ exp(t/τ) in agreement with earlier results obtained from OH-data of OH/IR stars in the disk (Baud, 1983).The duration of this phase was found to be a few times 104 years, during which the mass loss increases from to 10-5 to 10-4 Mo yr-1. Here it was assumed that 20% of the ZAMS mass was ejected in the preceeding low mass loss phase (Reimers wind).For these calculations initial ZAMS masses of 1.5-2.5 Mo were assumed, corresponding with the observed luminosities and assuming solar abundances. The bulge stars, however, can be expected to be metal rich which will bring this mass range down.