Using Galactic dynamics we have determined the age of the low mass post-T Tauri stars TW Hya Association ( TWA ) .
To do so we applied the method of Ortega et al .
( 2002 , 2004 ) to five stars of the association with Hipparcos measured distances ( TWA 1 , TWA 4 , TWA 9 , TWA 11 , TWA 19 ) .The method is based on the calculation of the past 3D orbits of the stars .
Of these stars only TWA 9 presents a quite different orbit so that it does not appear to be a dynamical member of TWA .
The four remaining stars have a first maximum orbits ’ confinement at the age of -8.3 \pm { 0.8 } Myr which is considered the dynamical age of TWA .
This confinement fixes the probable 3D forming region of TWA with a mean radius of 14.5 pc .
This region is related to the older subgroups of the Sco-Cen OB association , Lower Centaurus Crux ( LCC ) and Upper Centaurus Lupus ( UCL ) , both with a mean age of about 18 Myr .
This dynamical age of TWA and that of the \beta Pic Moving Group ( BPMG ) with an age of 11 Myr , also discussed here , introduce a more precise temporal scale for studies of disks evolution and planetary formation around some stars of these associations .
Using the retraced orbit of the runaway star HIP 82868 we examine the possibility that the formation of TWA was triggered by a supernova ( SN ) explosion .
It is shown that for the four considered TWA stars , the expansion in volume is a factor of five since their origin to the present state .
This is mainly due to the presently more distant star TWA 19 .