SIMBAD references

We address the age rank of the nearest pre-main-sequence (PMS) associations, using low-resolution spectrophotometry to measure gravity-sensitive indices in these stars. We compare spectral index-colour sequences for the PMS populations and rank them according to the strength of their gravity-sensitive features. The age rank using the gravity method is in general agreement with the ranking of these groups suggested by colour-magnitude (CM) diagram placement. Several of the groups have a kinematic origin in the Oph-Sco-Cen OB association. For three of them, their age rank is also in agreement with epochs resulting from a formation scenario for the OB association.