CO observations and mass loss of MS- and S-stars

Abstract

We present (12) CO J = 1-0 and 2-1 observations of 14 S-stars, and report 6 new detections. Two stars were observed in the (13) CO J = 1-0 and 2-1 lines, and one tentative 2-1 detection is reported. A compilation is presented of all CO observations of S-stars. The stars in this sample are separated into ``intrinsic'' and ``extrinsic'' S-stars, based on direct observation of the Technetium line, or infrared properties. The dust mass loss rate per unit distance is derived from IRAS 60 mu m data taking into the fact that for small mass loss rates the observed flux is an overestimate of the excess emission due to dust. The gas mass loss rate per unit distance is derived from CO data. Distances and luminosities are estimated, partly from hipparcos parallax data. The largest mass loss rate derived is that for W Aql with (0.8-2.0) x 10(-5) {M_sun} yr(-1) , and the lowest is that for o Ori with <1.2 x 10(-9) {M_sun} yr(-1) . The S-stars without Tc have smaller mass loss rates, than those with Tc. Diagrams showing mass loss rate, dust-to-gas ratio and expansion velocity versus pulsation period are presented, and compared to similar data for carbon- and oxygen-rich Miras. The S-Miras stand not out in any way from the C- or O-Miras in these diagrams. In the diagram with expansion velocity versus pulsation period, the S-SRs span the same range in velocity as the S-Miras, but they have periods which are about a factor of 2.5 shorter. This was previously noted for O-rich SRs. As in that case, the most straightforward explanantion is that the SRs among the S-stars pulsate in a higher order pulsation mode. Based on data from the ESA hipparcos astrometry satellite.