THE ASTROSPHERE OF THE ASYMPTOTIC GIANT BRANCH STAR CIT 6

Abstract

We have discovered two extended half-ring structures in a far-ultraviolet image taken with the GALEX satellite of the well-known mass-losing carbon star CIT 6 (RW LMi). The northern (southern) ring is brighter (fainter) with a diameter of about 15 arcmin (18 arcmin). These structures most likely represent the astrosphere resulting from the shock interaction of CIT 6's molecular wind with the Warm Interstellar Medium, as it moves through the latter. These data provide a direct estimate of the size of CIT 6's circumstellar envelope that is a factor ~20 larger than previous estimates based on CO millimeter-wave line data. We find that CIT 6 has been undergoing heavy mass-loss for at least 93,000 yr and the total envelope mass is 0.29 Msun or larger, assuming a constant mass-loss rate of 3.2 x 10^(-6) Msun/yr. Assuming that the shock front has reached a steady-state and CIT 6's motion relative to the ISM is in the sky-plane, we measure the termination-shock standoff distance directly from the image and find that CIT 6 is moving at a speed of about 39 (0.17 cm^(-3)/n_ISM)^(1/2) km/s through the interstellar medium around it. However, comparisons with published numerical simulations and analytical modelling shows that CIT 6's forward shock (the northern ring) departs from the parabolic shape expected in steady-state. We discuss several possible explanations for this departure.