High Mass-Loss Carbon Stars and the Evolving Interstellar Medium

The aim of the survey was to measure the relative amounts of the isotopes 12C and 13C in the stellar envelopes, and see how these ejecta will affect the isotopic ratio in surrounding gas clouds. It has always been puzzling that the 12C/13C ratio of the Sun is 89, but that in local clouds is only about 60-70; evolution due to stellar input is an obvious answer, but it has not been quantitatively measured.

In May 1995 we observed the 12CO and 13CO J=2-1 lines of all the accessible high mass-loss carbon stars within about 1 kpc of the Sun. The sample of 10 stars (with individual mass-loss rates greater than 10(-5) M(solar) per year) provides about 80 % of the carbon-rich mass-return within this distance, and should be complete except for very southern sources (below -33 degrees Declination). Examples of the spectra are shown in the figure. The top plot shows spectra of CRL3068, which has one of the highest mass-loss rates, and consequently moderately optically thick emission (tau = 5 for CO J=2-1). The bottom plot shows the spectra of IRC+00365, which has average mass-loss for the sample, but an unusually fast wind (37 km s-1). This source had very weak 13CO emission, of only 13 mK, and the spectrum took 35 minutes of integration to detect.

Figure 1: Spectra of CRL3068 and IRC+00365. The 12CO and 13CO J=2-1 spectra are shown by light and heavy lines respectively. The same scales are used, to show the differences in intensity and wind speed (total velocity range of the emission = 2 v(wind)).

The line intensities were modelled using a radiative transfer code, based on large velocity gradients within the expanding envelope. The 12CO/13CO abundance range for the stellar sample was found to be 25 +/- 13, much more 13C-rich than the solar value of 89. The total rate of ejection into the ISM is 1.5 x 10(-10) M(solar) per year, per square pc of the Galactic plane, and this means the time to totally replace all the gas mass already present would be 9 x 10(10) years. The process is speeded up by the ejecta from O-rich stars, which supply an equal amount of mass-loss (but with 2-5 times lower carbon abundances). The net result is that, since the Sun formed, 4.6 x 10(9) years ago, the ISM has become considerably richer in 13C. Based on our data, the 12C/13C ratio should have decreased from 89 to about 72 (+/- 8). Since the measured ISM ratios are about 60-70, this shows that carbon (and oxygen) stars can in fact explain the isotopic evolution of the gas.

In future observations, we hope to examine the isotopic differences between carbon stars and oxygen stars, plus perhaps cool Wolf-Rayet stars. These data provide a picture of the different nucleosynthesis processes going on, as a function of initial stellar mass, and may explain why intermediate mass stars diverge into the O-rich and C-rich paths.

Jane Greaves & Wayne Holland

(JAC)