1 Introduction

The ultimate stage of the red giant evolution, namely their transition from the Asymptotic Giant Branch (AGB) to the Planetary Nebulae (PNe), and the mechanisms and chronology of events driving them to expell the bulk of their mass in less than a million years (in some cases 10000 years), constitutes one of the less known, albeit most exciting topics in stellar evolution. The whole evolution of the morphological, dynamical and chemical layout of envelopes from the AGB to the PNe stage through the state of a proto-planetary nebula (PPN), strongly depends on the massive ejection of stellar material during this stage. Such a copious mass loss process necessarily leads to substantial changes in the stellar structure and appearance especially when the amount of mass ejecta becomes comparable to the mass of the star. Since the dominant process which brings back recycled stellar material to the interstellar medium is thought to be the formation of PNe, the mass loss process also greatly affects the composition of the interstellar gas and dust, its enrichment in heavy elements and the interstellar isotopic ratios.

During their high mass loss phase, the AGB stars are surrounded by dust and gas expanding envelopes which, if thick enough, are readily detectable in molecular radio line emission. In particular, high resolution observations of the rotational transitions of ¹²CO, in the millimetric and submillimetric ranges, yield unique data on the envelope geometry, mass loss rate and dynamics.

Since the middle of the eighties, a large number of surveys, using the largest available single dish radiotelescopes, have been devoted to searches for ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) emission in the circumstellar envelopes of evolved stars, on the basis of various selection criteria. See for instance the pioneering work of Knapp & Morris (1985), the compilation of ¹²CO and HCN data by Loup et al. (1993), and, among the recent large surveys, that of Olofsson et al. (1993). The first targets of these studies were the brightest infrared sources or less opaque envelopes around well known evolved stars. Since 1988, the IRAS colour-colour diagram (van der Veen & Habing 1988) has been extensively used to increase the sample of dusty AGB stars, based on their characteristic infrared colours, it has also proved to be a powerful tool to select various classes of objects. Surveys based on CO observations at higher frequencies are still rare (Young 1995; Stanek et al. 1995). Presently, the number of circumstellar envelopes detected in CO radio emission around evolved stars (AGB, proto- and Planetary Nebulae) is larger than 700.

 

Figure 1: Distribution of the sample of AGB and post-AGB stars (listed in Table 4) in the IRAS colour-colour diagram for O ($\circ$), C ($\bullet$), S ($\diamond$) and indeterminate ($\ast$) chemical types. Regions defined by van der Veen & Habing (1988) are drawn as landmarks. S12, S25 and S60 are the IRAS-PSC flux densities, not colour corrected

Due to the limited sensitivity and spatial resolution of radiotelescopes, the maps of CO emission around evolved stars are much less numerous (around 50) and essentially devoted to the most evolved objects (PPN or PN), which have generally been selected for some peculiar morphology (bipolar, ring like, ...). Since the first maps, obtained at the end of the eighties (see e.g. Healy & Huggins 1988), most observations have been done with single dish telescopes.

Recently several selected objects have been extensively mapped by interferometric techniques (see for instance Bujarrabal et al. 1997). To our knowledge, atlases of CO maps, obtained and analyzed in uniform conditions, around relatively large samples of evolved stars, are however almost inexistent. The pioneering work of Bujarrabal & Alcolea (1991) presents ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) mapping, with the IRAM 30 m telescope, of the circumstellar gas around 10 evolved stars of various chemical and variability types. More recently, the envelopes of 18 evolved stars, ranging from AGB to PN and showing various chemical types, have been mapped with the Caltech Submillimeter Observatory in ¹²CO($J=3\rightarrow 2$) (Stanek et al. 1995).

We present here an atlas of ¹²CO($J=1\rightarrow 0$) and ¹²CO($J=2\rightarrow 1$) maps of a sample of 46 AGB and post-AGB stars based on IRAM 30 m telescope and interferometer observations. The ¹²CO($J=1\rightarrow 0$) data come both from the interferometer and the 30 m telescope and are combined to circumvent the missing short-spacing problem. To get some additional information on CO excitation in the envelopes, we have also obtained fully sampled ¹²CO($J=2\rightarrow 1$) maps with the 30 m telescope. The stars sample is presented in Sect. 2, the 30 m telescope observations and ¹²CO($J=2\rightarrow 1$) data analysis in Sects. 3 and 4, the interferometer observations and ¹²CO($J=1\rightarrow 0$) data analysis in Sects. 5 and 6. Statistical results on various envelope parameters (flux, size, mass loss rate, ...) derived from these observations are described in Sect. 7 and an individual presentation of peculiar objects is given in Sect. 8. In Sect. 9 we draw our conclusions.