Formation of concentric arcs around evolved stars detected in high rotational transitions of CO?

While they are on the Asymptotic Giant Branch (AGB), stars of solar-type shed their entire envelope through a dusty wind, and evolve toward Planetary Nebulae. This mass loss does not occur at a constant rate. It is generally believed that a moderate rate (of 10^-7 solar masses per year) is followed by a sudden increase in mass loss to a few times 10^-5 solar masses per year. This abrupt increase is usually referred to as the superwind, and is needed to explain the formation of the Planetary Nebula. Recently it became clear that the mass-loss rate of AGB stars is even more variable, with the detection of circumstellar concentric arcs around post-AGB stars (Sahai et al. 1998, ApJ 493, 301; Kwok et al. 1998, ApJ 501, L117; Mauron & Huggins 1999, A&A 349, 203). The origin of these arcs are presumably mass-loss modulations during the mass-loss phase on the AGB. Using JCMT observations of the CO rotational transitions in all available frequency bands, we have studied the gas properties of the outflow of a number of AGB stars and conclude that the observed line strengths can be explained by mass-loss modulations on time scales corresponding to the spacing of the concentric arcs around post-AGB stars.

Figure 1: The concentric arcs around post-AGB star IRAS 17150-3224 observed with HST WFPC2 at 606 nm. The arcs are due to mass-loss modulations and the spacing between them correspond to time scales of 200-1000 years, depending on distance and outflow velocity. We may have witnessed the formation of similar structures in the outflow of AGB stars. Figure adopted from Kwok et al. 1998, ApJ 501, L117.

The mass-loss history of evolved stars can be probed by observing different rotational transitions of CO. While the lower rotational transitions have lower excitation temperatures and are formed in the cooler gas located more outward, the higher transitions trace the inner regions. The formation of the lines in stellar outflows gives rise to distinct line profiles, which depend on the physical properties of the outflow. Many studies have focussed on determination of the mass-loss rate, assuming it is constant, by analyzing the profiles one or two rotational transitions (see e.g. Morris, 1980, ApJ 236, 823). More recently,there have been attempts to include the superwind (as a density jump) in some models (e.g. Justtanont et al. 1996, ApJ 456, 337), but with only a few rotational transitions accessible, modelling a more complex mass-loss history did not seem feasible.

But the situation has changed. The MPIfR/SRON 800 GHz receiver at the JCMT has made the CO(7-6) transition accessible. We have observed this transition for a number of AGB stars and combined it with observations of the lower transitions CO(6-5), CO(4-3), CO(3-2) and CO(2-1) which have been obtained using the other heterodyne detectors available at the JCMT. This set of transitions trace the outflow of evolved stars from a few hundred stellar radii in case of CO(7-6) to a few thousand stellar radii using the CO(2-1) transition. The observed line intensities were not compatible with a constant mass loss rate or a superwind described by a jump in the mass loss of a factor of ~100. The lines due to the highest transitions were observed to be less bright than expected. For one object, WX Psc, we determined the mass loss rate for each line independently. We found that the highest mass loss rate was traced by the CO(2-1) transition, and that the rate decreased with higher J, i.e. when more inward regions were probed. The lowest rate was detected with the CO(6-5) line and followed by a slightly higher value for the CO(7-6) transition. This change in trend was continued when the dust mass-loss rate was taken into account, which originates even further inwards and probes yet higher mass loss.

Figure 2. JCMT observations of the rotational transitions of CO in WX Psc. The parabolic profile is typical for optically thick CO gas in the outflow. The intensities of the CO(4-3), CO(6-5) and CO(7-6) are too low to be consistent with a constant mass-loss rate or a superwind phase.

From the outflow velocity, which could also be derived from the CO line profiles, we could determine the ejection time scale corresponding to the distance between the various line formation regions corresponding to maximum mass loss. We find that this time scale is a few hundred years, and this, combined with the amplitude of the variation, which is about a factor of 50, is consistent with the mass-loss modulations that supposedly occurred in the AGB progenitors of the post-AGB stars that show circumstellar concentric arcs of enhanced emission. Thus, we are able to witness the birth of these arcs by observing a sequence of CO rotational transitions. Although we could perform these calculations only for one object in our sample, it is clear that the line strengths arising from the other AGB stars are also inconsistent with neither a constant outflow nor a superwind phase, but that a more complex mass-loss history is taking place.

Studying the mass-loss history of AGB stars will help us to understand the stellar evolution on the AGB, the shaping of Planetary Nebulae and the formation of galactic dust. Observing the high and low rotational transitions of CO provides a powerful tool to do this.

This work has been submitted to Astronomy & Astrophysics.