I understand factually that H$\delta$ lines are most prominent in type A stars and less so in more extreme types of stars on the H-R diagram. However I was wondering the reason for why they are not prominent in more extreme type stars such as type O or type F. For type F stars I believe (this may not be correct) none of the hydrogen is in the 4th excited state (or very few are) so there aren't many H$\delta$ balmer lines. However I am very unsure of why the EW H$\delta$ is low in type O stars. Can someone please explain this, Thanks!
H$\delta$ absorption is formed when hydrogen in the level $n=2$ is excited to $n=6$.
To get strong H$\delta$ absorption lines you need large amounts of hydrogen in the first excited state $n=2$ and a radiation field that contains large numbers of photons with an energy equal to the difference between the $n=6$ and $n=2$ states.
These requirements are satisfied in stars with photospheres between about 8,000K and 15,000K, and we call these early-F and A-type stars.
At cooler temperatures most of the hydrogen is in the ground state, there are few photons with sufficient energy (3eV) to be absorbed and the H$\delta$ line weakens. At much higher temperatures (in O-stars for example) then none of the hydrogen is in the $n=2$ level, and in fact most of it is ionised.