The lighting that's behind the model really just sets an amazing shading that makes it seem that she has an aura about her. I simply love how her eyes are just so beautiful in this due to all the contrasting elements. It's altogether a really great photo. The way that the natural curves of her body are shown because of the lighting really just accentuates the model's beauty. The natural lighting and the model's features really makes the photo seem almost like a painting. It's really just stellar how this could be done, at first I couldn't tell if it were a photo or a painting and that's what peaked my interest, making me read into the picture more. Simply marvelous.
I'm not sure why you're using the word "chaste" for these pictures. A person that is chaste abstains from sex and/or only has sex with their partner. It relates to sexual activity, not nudity and nudity isn't equivalent to sex. So... I'm not sure what you're trying to say with this.
I guess I know, instead. It's the portrait itself suggesting chastity, in the sense of innocency. The expression of the model, her eye gaze suggest curiosity instead of aggressive passion (as most nude portraits suggest indeed in deviantart). The innocent curiosity of an underage girl, fascinated by the novelty of posing nude. Or just revealing herself to someone.