$\ion{H}{i}$ and CO in the circumstellar environment of the S-type star RS Cancri

Abstract

This paper presents interferometric and single-dish CO observations along with HI data obtained for the oxygen-rich semi-regular variable RS Cnc, in order to probe its circumstellar environment at different scales. With the Plateau de Bure Interferometer and the IRAM 30-m telescope, we detect both the CO(1-0) and the CO(2-1) rotational lines from RS Cnc. The line profiles are composite, with two components of half-width ~2 km/s and ~8 km/s respectively. Whereas the narrow velocity component seems to originate from an equatorial disk in the central part of the CO envelope, the broad component reveals a bipolar structure, with a north-south velocity gradient. In addition, we obtain new HI data on the source and around it in a field of almost 1 square degree. The HI line is centered at v_LSR = 7 km/s in agreement with CO observations. A new reduction process reveals a complex extended structure in the northwest direction, of estimated size ~18 arcmin, with a PA (~310 degrees) opposite to the direction of the stellar proper motion (~140 degrees). We derive an HI mass of ~3 10^-2 M_sun for this structure. Based on a non-spherical simulation, we find that this structure is consistent with arising from the interaction of the star undergoing mass loss at an average rate of ~10^-7 M_sun.yr^-1 over ~2-3 10^5 years with the interstellar medium. This work illustrates the powerful complementarity of CO and HI observations with regard to a more complete description of circumstellar environments around AGB stars.