Mass loss from evolved stars. IV - The dust-to-gas ratio in the envelopes of Mira variables and carbon stars

The gas and grain loss rates have been calculated for a sample of 40 evolved giant stars, consisting of 18 carbon stars and 22 oxygen-rich (OH/IR and Mira) stars, for which good observations exist. The analysis shows that the gas-to-dust ratio is roughly constant in circumstellar envelopes, with values of ∼ 160 by mass for oxygen-rich stars and ∼ 400 by mass for carbon-rich stars, over a wide range of mass-loss rates. The gas-to-dust ratio in the Galaxy thus appears to be determined by the available heavy element abundances. The carbon stars form a very homogeneous population; the available data suggest progenitor masses for these stars of ∼ 2 Msun.