Colour–magnitude diagrams ( CMD ) of the SMC star cluster NGC 419 , derived from HST/ACS data , reveal a well-delineated secondary clump located below the classical compact red clump typical of intermediate-age populations .
We demonstrate that this feature belongs to the cluster itself , rather than to the underlying SMC field .
Then , we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He-ignition in stars just massive enough to avoid e ^ { - } -degeneracy settling in their H-exhausted cores .
The main red clump instead is made of the slightly less massive stars which passed through e ^ { - } -degeneracy and ignited He at the tip of the RGB .
In other words , NGC 419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H-exhausted cores .
At this particular moment of a cluster ’ s life , the colour distance between the main sequence turn-off and the red clump ( s ) depends sensitively on the amount of convective core overshooting , \Lambda _ { c } .
By coupling measurements of this colour separation with fits to the red clump morphology , we are able to estimate simultaneously the cluster mean age ( 1.35 _ { -0.04 } ^ { +0.11 } Gyr ) and overshooting efficiency ( \Lambda _ { c } = 0.47 _ { -0.04 } ^ { +0.14 } ) .
Therefore , clusters like NGC 419 may constitute important marks in the age scale of intermediate-age populations .
After eye inspection of other CMDs derived from HST/ACS data , we suggest that the same secondary clump may also be present in the LMC clusters NGC 1751 , 1783 , 1806 , 1846 , 1852 , and 1917 .