They also tend to be slow rotators and have an effective temperature between 7000 and 10000 K. The Ap stars (CP2 stars) are characterized by strong magnetic fields, enhanced abundances of elements such as Si, Cr, Sr and Eu, and are also generally slow rotators.
As the name implies, these stars show increased abundances of singly ionized mercury and manganese.
The proposed mechanism causing this stability is the unusually large magnetic field that is generally observed in stars of this type.
[7][3] A much smaller percentage show stronger peculiarities, such as the dramatic under-abundance of iron peak elements in λ Boötis stars.
[8] It was originally proposed that the unusual helium lines were created in a weak shell of material around the star,[9] but are now thought to be caused by the Stark effect.
The phrase chemically peculiar star without further specification usually means a member of one of the hot main sequence types described above.