Carbon stars with silicate dust shells - II. More stars with enhanced 13C(Jstars)

Abstract

The seven previously-studied carbon stars with silicate dust are all J stars with [12]-[25]>0.7; they comprise most of the known J stars in this colour range. An additional sample of 41 optically-identified carbon stars with [12]-[25]≥0.7 has revealed 12 new J stars, while two more were found in a survey of red IRAS sources without previous optical identifications. Two of the new sample were classified as having silicates on the basis of IRAS LRS spectra, but one has very weak silicate emission while the other probably has carbon-rich dust. The probability of variability of the new J stars is low, confirming an earlier result. The presence of NaD emission as well as an unusual energy distribution in one of these can be understood in terms of the disc model of these stars. The remainder of the sample includes three CS stars and another six stars with strong NaD absorption but weak C2 bands. These may be understood as stars with C/O≥1, whose dust shells have an intermediate composition which causes less obscuration in the optical waveband than a shell of similar extent but with the higher C/O of more typical carbon stars. Ordinary strong-banded carbon stars are greatly under-represented among visible carbon stars with [12]-[25]>0.7 as compared to visually selected carbon stars in general. Other evidence that ordinary carbon stars do produce discs in some cases leads to the suggestion that this occurs only on the upper AGB as the result of continuing mass loss which obscures the star at visible wavelengths.