Why is の used in this sentence?

Question

I encountered the following sentence in a book.

私は彼に今晩来るのか聞いた。

Okay, I guess the meaning is easy enough: I asked him if he would come tonight. This の, however, was a slight surprise. Grammatically speaking, since it comes after 来る, it has to be nominalization of some kind. But I have no idea why か would require any kind of nominalization when it’s used for indirect speech questions.

In fact, if I had to say something like this, I would most likely say:

彼に今晩来るかどうか聞いた。

Or even just:

彼に今晩来るか聞いた。

I thought of three (rather unsatisfying) explanations. First one: の (or のか) is a common interrogation particle in casual speech, so one could say to a close friend: 今晩来るの（か）？ However, I highly doubt that the の in the sentence is this kind of の, since it does not sound like direct speech is quoted (at all).

My second idea would be that this の is an emphatic particle, just like when we end a sentence with のだ・んだ. But why use it here? I have no idea.

The third option is regular nominalization, which is used here for some reason. So the sentence would be (literally): I asked him if the fact that he will come is. But then, why would this structure be used at all?

Answer 1

This の is the same thing with のだ, as in your second option. The third one is partially correct too, since the expression would be literally translated "it is that".

The usage of のだ (non-question form of のか) has been a hot topic of debate, despite, or because of its generality among Japanese utterance. The currently widely agreed formulation is that using のだ indicates "relation" or "reference" to existing shared premises in the context. This is a fairly vague idea, but if you limit the scope down to のだ with yes-no question, the explanation would be greatly simplified.

The simple yes-no question V（ますか）？ asks the trueness of the fact, while のだ-fied yes-no question Vの（ですか）？ that of the judgment.

For example:

(You bring a bag of liquorice candies in front of your friends, and say...)
○ 食べる？ Do you (wanna) eat?
× 食べるの？ You (wanna) eat, is it true?

(You start to eat them by yourself, then notice that one of your friends is staring at you...)
○ 食べる？ Do you (wanna) eat?
○ 食べるの？ You (wanna) eat, is it true (i.e. what I see in your face)?

(Finally they say they want to try them...)
× 食べる？ Do you (wanna) eat?
○ 食べるの？ You (wanna) eat, is it true (i.e. what I hear)?

So, the implication of 彼に今晩来るのか聞いた is that you asked him in order to know if you'd be able to take his attendance for granted for an already-known schedule. Of course, it has little difference with 来るか（どうか）聞いた in usual situations.

One thing is that using のだ in indirect speech doesn't necessarily reflect that the speaker uses the word in real conversation, because the context base may vary depending on the hearer:

じゃあ、彼に今晩のパーティー来るのか聞いてみるね。
Okay, then I ask him if he'll come to the party tonight.

☎「もしもーし、今晩パーティーやるんだけど、来る（× の）？」
Hello, we gonna have a party tonight. Are you coming?