Hear that? Listen closely. It's the sound of thousands of rabid vampire enthusiasts squealing in anticipation of two Twilight stars invading SF's very own Stonestown Galleria today. Actors Ashley Greene and Kellan Lutz, who play Robert Pattinson's onscreen siblings, Alice and Emmett Cullen, will be appearing at the mall today to promote the November 20 release of the Twilight sequel, New Moon. But don't expect to get up close and personal with the vamps on your way to the food court. Fans lined up Halloween morning to snag the 500 wristbands for separate Nordstrom and Hot Topic meet-and-greets, and organizers said no more are available. If you're willing to stand around in the Macy's parking lot though, you might catch a glimpse of the question-and-answer session set for 7:30 PM. Security is expected to be extremely tight today after 3,000 overzealous fans caused a near-riot at Stonestown last November, when Pattinson appeared for an autograph session. Police ended up canceling the event due to the unruly crowd. Pattinson wisely stayed away this year, but you may want to pack some protective gear anyway, just in case.