First detection of rotational CO line emission in an RGB star
  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).
