“Unlink” an attachment from its parent?

Hi,

I added an image to a post. Now, that file is “attached” to the post. When I output the content of the following, I get its attachments:

$args = array(
	'post_type' => 'attachment',
	'numberposts' => null,
	'post_status' => null,
	'post_parent' => $post->ID
	);
$attachments = get_posts($args);

However, even if I delete the image from the post, the image remains “attached” to the post. Is there a way to “unlink” an image from its parent post without having to delete the file?

Rémi