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Astron. Astrophys. 317, L5-L8 (1997)

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5. Discussion

The CS(J =2-1) and HCN(J =1-0, F =0-1) line profiles indicate mass infall towards IRAS 13036-7644. However, a determination of excitation conditions as a function of the cloud radius is necessary before a unique interpretation of the observed line profiles can be given.

However, the 12CO(J =1-0), HCO[FORMULA] (J =1-0) and HCN(J =1-0, F =2-1/1-1) line profiles towards the IRAS source show an "anti-infall" asymmetry. The situation is similar to the HCO[FORMULA] (J =1-0) profiles in the collapse candidate B335 with "anti-infall" asymmetry observed by Frerking et al. (1987). According to their model of B335 the core is collapsing, but the envelope is expanding due to the magnetic flux enhancement caused by ambipolar diffusion out of the core. This model produces HCO[FORMULA] (J =1-0) profiles which are consistent with observations. However, the HCO[FORMULA] (J =3-2) line in B335 shows a profile characteristic of infall (Zhou et al. 1993). It is expected that the J =1-0 transition of HCO[FORMULA] traces much less dense gas (i.e. the envelope) than the J =3-2 transition (e.g. Gibb et al. 1994). The shape of the observed line profile (infall or "anti-infall" asymmetry) is determined by optical depth effects (see Leung and Brown 1977; Leung 1978). This behaviour is well demonstrated by our HCN lines.

[FIGURE]Fig. 5. A cross scan of HCN(J =1-0) spectra across the IRAS source in the east-west direction

One should note that the observed line profile in a collapsing cloud is strongly dependent on the angular resolution of the observations (Zhou 1992). The telescope beam has to be small enough compared to the infall region in order to reveal the infall signature. However, the HCO[FORMULA] (J =1-0) line profile towards the IRAS source would not turn into a profile with infall asymmetry even if observed with a higher angular resolution: the observed HCN(J =1-0) hyperfine components show that both infall and "anti-infall" asymmetry can be seen with the same angular resolution, depending on the optical depth of the transition.

All the lines that trace high density gas are consistent with mass infall towards IRAS 13036-7644. None of the low density gas tracers indicate infall, but some of them show a profile with "anti-infall" asymmetry. Thus the outer parts of the cloud are probably not in a state of collapse. Thorough modelling is required to find out whether the "anti-infall" asymmetry is due to expansion, outflow, or some other reason.

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