Mass-losing stars in the South Galactic Cap

Abstract

Observations are presented for 162 late-type stars in the South Galactic Cap (b < -30°) which were selected on the basis of their IRAS 25/12-µm flux ratios as high-mass-loss candidates. JHKL photometry (over 1100 observations) was obtained for all of the stars, BV(RI)C photometry for 78 of them and optical spectra for 51.154 of these stars are non-Mira M or S stars, of which many, and possibly all, are semi-regular variables. Of the remaining eight IRAS objects, three are T Tauri stars, three are interacting binaries and two are carbon stars. A few of the M giants and both of the carbon stars have circumstellar envelopes of the type more normally associated with Mira variables. These include two 1612-MHz OH maser sources. It is suggested that such stars may have been Miras in the recent past, but are currently out of the instability strip owing to a recently experienced helium-shell flash. Alternatively, some of them could be binary stars, but there is as yet no evidence for the second star. The near-infrared colours of the M giants are compared with those of similar stars in the Bulge. They are similar to those of the inner Bulge and unlike those found in either the outer Bulge or the globular clusters. The kinematics and Galactic distribution of the M giants indicate that they are probably from a mixed population and that they could be associated with Miras with a range of periods. A comparison of the observed colours with those derived from models indicates a range of metallicity with the bulk of stars slightly more metal-rich than the Sun. It also reveals significant numbers of stars with colours outside the predicted range, possible due to the effects of circumstellar reddening or to inadequacies in the models. The most metal-rich examples of the M stars have high mass-loss rates for non-Miras |$(\sim 10^{-6}\enspace \text M_\odot\enspace \text {yr}^{-1})$| and there are far fewer of them outside than inside the solar circle. This may be due to a metallicity gradient. These stars have a scaleheight of more than 500 pc. The two carbon stars have unusual colours and detached shells. One of them, R Scl, is shown to vary with a period of 379 d superimposed on a possible second period of about 2300 d. The other carbon star, which is more distant and previously unknown, may also have double-period variations.