What makes the core of some chromospheric lines such as the transition at 8542 A of the infrared Ca II triplet appear in emission in the dark umbra of sunspots?
The emission is from the chromosphere and can be seen in contrast against the strong absorption from the cooler photosphere of the spot.
A similar thing is now happening in Rho Cas where emission from a shell of material is showing in emission within some absorption lines as the stars photosphere cools.
In addition there is enhanced chromospheric heating above the sunspots due to the concentration of magnetic flux enhancing the emission.
Regards Andrew