The CS (21) and (32) spectra towards CS-C exhibit the classical double-peaked asymmetric spectral signature of inward motion (see e.g. Zhou 1992; Myers et al. 1996). Before discussing the implications of the discovery it is necessary to exclude other causes for the line shapes. In principle, the line profile can be caused by an unrelated cold absorption layer right in front of CS-C as discussed e.g. in the case of IRAS 162193-2422 (Menten et al. 1987). Because the number of dense cores in a cirrus cloud is low and the double-peaked lines are only found towards CS-C it is however very unlikely that we observe two unrelated dense cores right on the same line of sight. The velocity of the optically thin C18O (10) line and the CS (54) line, which is in between the two peaks of the lower transitions, is further proof for that. The most likely explanation is thus that the CS (21) and (32) lines are self-absorbed and thus double-peaked, and the (54) line is single-peaked due to its low optical depth.
The fact that the self-absorption is spatially concentrated on CS-C suggests that the core is centrally condensed as expected for a bound object. We are thus possibly observing the contraction of a part of MCLD 123.5+24.9. To estimate the inward speed of the gas I make use of the simple two-layer model presented by Myers et al. (1996). This model assumes two uniform regions of equal temperature and velocity dispersion, , which are the front and rear halves of a centrally condensed cloud contracting with an inward velocity, . In this model, can be derived from the observed line profile. I assume that is that of the CS (54) transition km s-1. The inward velocity is then estimated to be between km s-1 and 0.010 km s-1 for the (21) and the (32) transition, resp. Due to the much better signal-to-noise ratio of the (32) spectra the latter value is more reliable. It is similar low as that found for L 1544 (Tafalla et al. 1998). The mass infall rate can be estimated from with m being the mean molecular mass and n the observed volume density (Myers et al. 1996). For CS-C yr-1.
There is now evidence for inward motion in a large number of dense cores in star-forming regions (e.g. Gregersen et al. 1997, Mardones et al. 1997). Class 0 and 1 protostars have mainly been selected as targets for those studies. Only two starless dark core have been described so far with evidence for inward motion (Tafalla et al. 1998; Williams & Myers 1999). Whether or not the three dense condensations in MCLD 123.5+24.9 and especially CS-C are associated with low-temperature compact dust sources is unknown so far. Recently, Bernard et al. (1999) have observed this region in different infrared and submm bands and found dust emission with very low dust temperatures ( K) arising from extended cirrus as well as from a compact source. The angular resolution of their observations (2´) is however insufficient to resolve the cloud. Clearly, high-angular resolution studies of the dust continuum are required to resolve this issue.
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999