You want something more like this, where you define the server as a DNS name (no protocol, port, auth or path):
server = new mongo.Server("paulo.mongohq.com", 10057, {
auto_reconnect: true
});
db = new mongo.Db("confirmed", server, { safe: true });
and then once db is defined:
db.open(function(erreur, db) {
db.authenticate('user', 'name', function(err, result) {
//
});