1997A&A...323..449I

We have investigated the N-type carbon star U Ant in high resolution IRAS images. We find that the star shows two extended dust shell components and that these two shells are also clearly present in the original survey scan data. We have fitted a double dust shell model with spherical symmetry to the data to obtain the physical quantities of the shells. The inner dust shell component is related directly to the detached gas envelope detected in the mm-wave CO lines in previous studies, while the outer shell has an inner radius of about 3' but has no CO counterpart. The projected separation of the inner edges of the two shells, which is insensitive to any of the model parameters, is derived to be 141"-148". Our results together with the CO observations show that the mass loss rate varies by two orders of magnitude along the AGB evolution. We are able to deduce the distance, interpulse period, core mass, and luminosity of the star selfconsistently, assuming that the two detached shells are related directly to two consecutive thermal pulses along the AGB evolution of this star. They are 324pc, 1.0x10⁴years, 0.80M_☉, and 9.4x10³L_☉ for the first thermal pulse stage, and 436pc, 1.4x10⁴years, 0.77M_☉, and 1.7x10⁴L_☉ for the full amplitude thermal pulse stage, assuming an expansion velocity of 21km/s for both shells. The implied progenitor mass of U Ant is 3-5M_☉. This method can be applied to other AGB stars with a multiple dust shell to be detected in future observations, which provides a way to determine reliable physical quantities of AGB stars.