a core at times 3-$8\times 10^4$ years prior to the collapse, after an evolution of $2.6\times10^6$ years. Thus in that particular model, L1544 finds itself in the last few percent of its evolution prior to becoming a protostar. While this may be a somewhat too literal interpretation of the model results, it shows that “L1544–type cores” should be relatively rare.

Further progress requires the definition of what is a L1544-like core. One answer is to use current estimates of dust emission and absorption selecting cores of dust extinction upwards of 50 mag. Another approach is to say that cores which show signs of infalling gas (as does L1544, see Williams et al. [@Wil99]; Tafalla et al.[@taf98]) are “L1544-twins”. This latter indicator is complicated by the fact that at the high densities found in the nuclei of cores similar to L1544, many molecular species and in particular CO and CS freeze–out onto dust grain surfaces (see Kramer et al. [@kra99]; Caselli et al. [@cwt99]; Bacmann et al. [@bac02]; Bergin et al. [@berg02]; Jørgensen et al. [@jsv02]; Tafalla et al. [@taf02]); observing such tracers implies observing the low density surrounding envelope. However, recent studies indicate that species whose abundance is linked to 