Mass loss of Mira stars associated with a star formation field in the M16 and M17 region

We examine the possibility to obtain a reasonable limit for the stellar mass in a sample of Mira stars located in a star forming region, associated with the nebulosities M16 and M17. We use a set of historic photographic plates in I filter (0.68/0.88μm), together with more recent data from the IRAS (12, 25, 60μm fluxes) and 2MASS (JHK magnitudes) missions. We estimate, through modeling a spherically symmetric circumstellar envelope with constant outflow, the 10μm opacity, and this allows us to compute the mass loss rate in dust. By taking into account all photometric bands, as well as the spatial position of each star with respect to the nebula, we are able to compute the mass loss in gas and therefore the gas to dust ratio. The estimated value and analytic model suggest the presence of silicates in the dust, although uncertainties remain on the grain dimensions, which would decrease the ratio. The value for the ratio we estimate is typical of intermediate-mass stars (2.5/5 M<SUB>sun</SUB>). The stars in the sample are all O-rich, with light periods longer than 250-300 days, and this again agrees with their status of intermediate mass stars.