8 Individual envelopes

U Cam is a carbon-rich star that has probably experienced a change in its mass loss (Oloffson et al. 1990). The ¹²CO radial visibility profile and the integrated flux profile have revealed a detached shell of 10$^{\prime\prime}$ radius surrounding the central star. Recent observations with the PdBI in the HCN ($J=1\rightarrow 0$) line emission support this picture (Lindqvist et al. 1996).

04307+6210 is classified as carbon-rich M-star (Groenewegen 1994, formerly associated to IRC+60144, an optically visible star of 12 magnitude). The ¹²CO($J=1\rightarrow 0$) radial visibility profile and the spiky integrated emission profile suggest that the star is surrounded by an inner $\sim 8''$ and an outer $\sim 34''$ shell, one of the two largest in our sample. This leads to the conclusion that variations in the mass loss of 04307+6210 have already taken place.

RS Cnc is classified as an MS giant, a thermally pulsing AGB star whose surface is enriched from dredged up material. The star for which Ake and Johnson (1988) suggest a binary nature, is surrounded by a thick envelope with thermal line emission detected in several molecules (HCN, CS, SiO...). The position-velocity diagram in both lines testifies the clear bipolar nature of the ¹²CO envelope. The spiky emission profile which does not appear to follow the velocity gradient of the broader peak, recalls the position-velocity diagram of the broad and narrow emission components observed in X Her (Kahane & Jura 1996). These properties are indications that the envelope does not show the expected spherical structure in isotropic expansion, being possibly in a phase of strong evolution.

CIT 6 is a carbon-rich star, the most intense and one of the most extended sources in our catalog. The envelope appears slightly asymmetric but no systematic variation with the velocity of the emission centroid is found. Our interferometric maps show the CO emission to be distributed in two components, in a compact one and in a weak and extended halo, both centered on the same position. Other sources show a similar behaviour (see for instance, IRC+10011, 04307+6210, IRC+20326, RAFGL 2155, IRC+20370, $\chi$ Cyg, and RAFGL3068), although less clearly.

Y CVn is classified as a carbon-rich star, the brightest J-type star known in the optical. Izumiura et al. (1996) report the detection with ISOPHOT of a very large, slightly asymmetric, detached dust shell surrounding the central star. Though the envelope is not asymmetric (f<0.1) according to our analysis, the ($J=2\rightarrow 1$) position-velocity diagram show presumably the kinematical signature of a faint asymmetry developping in the innermost regions around the star.

RAFGL 2155 is a carbon-rich star which shows a systematic behaviour in the emission centroid position as a function of velocity, for both transitions, with a general displacement from south-east to north-west of a few arcsec. As in other sources, this behaviour probably corresponds to the presence of a circumstellar component in bipolar expansion.

IRC+10420 is an oxygen-rich star surrounded by a heavy cool envelope. The nature of the star is still controversial: Hvrinak et al. (1989) propose a post-AGB star evolving towards a young planetary nebula (PN), Oudmajer et al. (1994) a very luminous hypergiant undergoing an extremely rapid evolution towards a cool Wolf-Rayet star. The complex width and velocity-position diagrams suggest a very clumpy envelope, quite different from a spherical shell. The object is probably the precursor of very excited and structured PN, as NGC6302 and NGC7027. We determined the distance according to the method described in Sect. 7.7. We found two possible estimates $D=6.12\,$kpc, which is close to the value commonly adopted, and $D=0.76\,$kpc (see also Kastner et al. 1995).

M1-92 also known as Minkowski's Footprint, is a well studied proto-planetary nebula. Both, the ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) observations reveal the clear bipolar morphology of the 6$^{\prime\prime}$ dense shell, remnant of the AGB stage. Recent interferometric observations in the ($J=1\rightarrow 0$) line emission bear evidence of two 3$^{\prime\prime}$ wide cavities along the nebular axis, probably formed by the passage of a bipolar bow shock (Bujarrabal et al. 1994b, 1997). This supports the idea that stellar wind interaction is the process that very likely dominates the circumstellar dynamics of post-AGB stars and of M1-92 in particular.

T Cep is an oxygen-rich Mira variable. The ¹²CO($J=2\rightarrow 1$) velocity position diagram (the ¹²CO($J=1\rightarrow 0$) data is probably sensitivity limited) shows the north-west south-east signature of an axial velocity gradient, the velocity increasing steadily with distance from the central star.

RAFGL 3068 is a well-known carbon-rich star (Sopka et al. 1989) with strong emission ¹²CO profiles of nearly parabolic shape. The radial ¹²CO($J=1\rightarrow 0$) visibility profile is characteristic of a large outer circumstellar nebulosity surrounding a compact inner envelope. The outer shell is presumably detached (this is suggested by the ringing features in the visibility profile) and strongly bipolar. The position angle which is almost fixed at $\sim -30\,$deg is likely to be due to an uneven sampling of the uv-plane.

RAFGL 3099 is a carbon-rich Mira variable. Though the ¹²CO($J=1\rightarrow 0$) emission traces essentially a spherical morphology, as suggested by the visibility profile, the ($J=2\rightarrow 1$) emission is slightly asymmetric. Moreover, channel maps obtained at the nominal velocity sampling of the interferometer show a linear but faint signature in the position velocity diagrams typical of an expanding circumstellar envelope with axial symmetry (south-east - north-west).

R Cas is an oxygen-rich star presenting an aspherical configuration (Tuthill et al. 1994), probably related to the influence of a binary companion. The star is surrounded by a large, optically thick circumstellar envelope whose radial ¹²CO($J=1\rightarrow 0$) visibility profile show little if no deviation from circular symmetry (Bujarrabal et al. 1994a). The data, however, reveal a remarkable kinematical continuity (south-east - north-west) in both the ¹²CO($J=1\rightarrow 0$) and ¹²CO($J=2\rightarrow 1$) position velocity diagrams which clearly supports the idea that the circumstellar dynamics is confined to an axially symmetric structure.