We report the discovery of J1521 - 3538 , a bright ( V = 12.2 ) , very metal-poor ( [ Fe/H ] = -2.8 ) strongly r -process enhanced field horizontal branch star , based on a high-resolution , high signal-to-noise Magellan/MIKE spectrum .
J1521 - 3538 shows the largest r -process element over-abundance in any known r -process-enhanced star , with [ Eu/Fe ] = +2.2 , and its chemical abundances of 22 neutron-capture elements closely match the scaled solar r -process pattern .
J1521 - 3538 is also one of few known carbon-enhanced metal-poor stars with r -process enhancement ( CEMP- r stars ) , as found after correcting the measured C abundance for the star ’ s evolutionary status .
We propose to extend the existing classification of moderately enhanced ( +0.3 \leq \mbox { [ Eu / Fe ] } \leq + 1.0 ) r -I and strongly r -process enhanced ( \mbox { [ Eu / Fe ] } > +1.0 ) r -II stars to include an r -III class , for r -process stars such as J1521 - 3538 , with \mbox { [ Eu / Fe ] } > +2.0 and \mbox { [ Ba / Eu ] } < -0.5 , or \geq 100 times the solar ratio of europium to iron .
Using cosmochronometry , we estimate J1521 - 3538 to be 12.5 \pm 5 Gyr and 8.9 \pm 5 Gyr old , using two different sets of initial production ratios .
These ages are based on measurements of the Th line at 4019 Å and other r -process element abundances .
This is broadly consistent with the old age of a low-mass metal-poor field red horizontal branch star . \object [ 2MASS J15213995-3538094 ] J1521 - 3538 likely originated in a low-mass dwarf galaxy that was later accreted by the Milky Way , as evidenced by its highly eccentric orbit .