When and why does a protostar start to radiate again?

Question

I'm currently the formation processes of stars in Carroll's "An Introduction into Modern Astrophysics" and Bennett's "The Essential Cosmic Perspective" and I'm currently stumped with the following and can't seem to find an answer:

After the fragmentation processes of a protostar formation are complete, the protostar has - by definition - an optically thick shell and it's compression is essentially adiabatic.

The (proto)star now moves towards the Hayashi track, where it's surface area decreases slightly while the surface temperature stays nearly constant before transitioning towards the main sequence in which nuclear fusion starts in the star's core.

However, is this at the same time the star starts to radiate again, because the shell is discarded by the huge amount of force released in the fusion? If this is the case, isn't the luminosity of the star $L = 0$ before fusion takes place, effectively being a horizontal line in the Hertzsprung-Russell-Diagramm and decidedly not moving on the Hayashi track?

If this isn't the point of time where radiation can escape the start, what else is?

Answer 1

On the Hayashi track the surface area decreases enormously. The protostar contracts at almost constant surface temperature. The contraction results in gravitational potential energy heating the protostellar interior and supplying the protostellar luminosity. In some cases, accretion from a disc also contributes some luminosity.

I am not sure what you mean by an optically thick "shell". Class I protostars are still accreting from an optically thick envelope and are not generally shown on an HR diagram. However their (infrared) luminosities can be very large - powered by accretion and contraction. Protostars on the Hayashi track are either class II or class III protostars (or pre main sequence stars). These have visible photospheres and any circumstellar matter is confined to a disc.