First detection of rotational CO line emission in an RGB star

Abstract

Context: For stars with initial masses below about 1 solar mass, the mass loss during the first red giant branch (RGB) phase dominates mass loss in the later asymptotic giant branch (AGB) phase. Nevertheless, mass loss on the RGB is still often parameterised by a simple Reimers law in stellar evolution models. Aims: To try to detect CO thermal emission in a small sample of nearby RGB stars with reliable Hipparcos parallaxes that were shown to have infrared excess in an earlier paper. Methods: A sample of five stars was observed in the CO J=2-1 and J=3-2 lines with the IRAM and APEX telescopes. Results: One star, the one with the largest mass-loss rate based on the previous analysis of the spectral energy distribution, was detected. The expansion velocity is unexpectedly large at 12 km/s. The line profile and intensity are compared to the predictions from a molecular line emission code. The standard model predicts a double-peaked profile, while the observations indicate a flatter profile. A model that does fit the data has a much smaller CO envelope (by a factor of 3), and a CO abundance that is two times larger and/or a larger mass-loss rate than the standard model. This could indicate that the phase of large mass loss has only recently started. Conclusions: The detection of CO in an RGB star with a luminosity of only about 1300 solar luminosities and a mass-loss rate as low as a few 10(-9) solarmasses/yr is important and the results also raise new questions. However, ALMA observations are required in order to study the mass-loss process of RGB stars in more detail, both for reasons of sensitivity (6 hours of integration in superior weather at IRAM were needed to get a 4sigma detection in the object with the largest detection probability), and spatial resolution (to determine the size of the CO envelope).