3. Spectral signatures of a collapsing cloud
There are several theoretical studies of line formation in dense cores undergoing collapse (e.g. Leung & Brown 1977; Leung 1978; Walker et al. 1986; Adelson & Leung 1988; Zhou 1992; Zhou et al. 1993; Walker et al. 1994; Myers et al. 1995; Zhou 1995; Choi et al. 1995). The general results of these studies applied to an emergent line profile from a spherically symmetric cloud with mass infall in the radial direction and an excitation temperature which is increasing inwards, are the following: (1) As the optical depth increases, the line profile progresses from a symmetric single-peaked profile to a double-peaked profile with the blue-shifted component being the brighter. This structure is caused by a self-absorption dip at the centre of the profile. At intermediate optical depths this dip is not pronounced and the profile remains single-peaked but is skewed to blue-shifted velocities. (2) The degree of self-absorption and asymmetry is greatest at the cloud center, and decreases outward. (3) In the case of an expanding cloud a similar sequence of line profiles is obtained with increasing optical depth but in this case the line profile is skewed to red-shifted velocities. (4) Rotation can also produce asymmetric line profiles. However, contrary to the cases with radial movements, the profiles are mirror images of each other relative to the rotation axis. Along the line of sight towards the cloud center, the rotation has no effect and the profiles are symmetric.
A line profile with a brighter red-shifted component is, hereafter said, to have "anti-infall" asymmetry, whether or not this asymmetry is due to expansion.
© European Southern Observatory (ESO) 1997