A DOUBLE DUST SHELL SURROUNDING THE CARBON STAR U-ANTLIAE

We have investigated the N-type carbon star U Ant in high resolution I RAS images. We find that the star shows two extended dust shell compon ents and that these two shells are also clearly present in the origina l survey scan data. We have fitted a double dust shell model with sphe rical symmetry to the data to obtain the physical quantities of the sh ells. The inner dust shell component is related directly to the detach ed 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 co unterpart. The projected separation of the inner edges of the two shel ls, which is insensitive to any of the model parameters, is derived to be 141''-148''. Our results together with the CO observations show th at the mass loss rate varies by two orders of magnitude along the AGE 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 pu lses along the AGE evolution of this star. They are 324pc, 1.0x1O(4)ye ars, 0.80M(circle dot), and 9.4x10(3)L(circle dot) for the first therm al pulse stage, and 436pc, 1.4x10(4)years, 0.77M(circle dot), and 1.7x 10(4)L(circle dot) for the full amplitude thermal pulse stage, assumin g an expansion velocity of 21 km s(-1) for both shells. The implied pr ogenitor mass of U Ant is 3-5 M-circle dot. This method can be applied to other AGB stars with a multiple dust shell to be detected in futur e observations, which provides a way to determine reliable physical qu antities of AGE stars.