Antiope Occultation Yields Double Bonanza

By: J. Kelly Beatty September 9, 2011 6

There are two kinds of astro-chasers in this world. Members of the better-known breed, the self-proclaimed "umbraphiles", journey to exotic destinations in the hope of standing in the Moon's shadow during total solar eclipse.

Here's how the binary asteroid 90 Antiope looks when recorded by the giant Keck telescope in Hawaii. This infrared image was made on July 15, 2011, just four days before Antiope occulted the star LQ Aquarii.

W. Merline & others

The other group chases down occultations (brief cover-ups) of stars by asteroids. These occultations aren't nearly as flashy as the solar corona, but they can be just as rewarding — and sometimes have far more scientific value.

An excellent case in point was July 19th's occultation of the star LQ Aquarii by the much dimmer asteroid 90 Antiope. Three circumstances set this occultation apart from the scores of other such events predicted this year: (1) the star is relatively bright, making it easy to see in even relatively small telescopes; (2) Antiope is a big, close-bound pairing with nearly identical halves about 55 miles (90 km) across; and (3) the predicted track crossed a well-populated chunk of the Far West.

This was such a high-profile opportunity that members of the International Occultation Timing Association agreed to hold their annual meeting in Rocklin, California — directly in the event's path. Expectations were high, buoyed by the prospect of clear weather along much of the track.

IOTA members weren't the only observers who'd gathered for the event. European astronomers, who've studied this unusual asteroid for many years, mounted a substantial campaign coordinated by François Colas (Paris Observatory) and Franck Marchis (SETI Institute) . "The point of this occultation," Marchis explains, "was to confirm the shape of the components, confirm the presence of the crater, and find which component has it."

More than 50 observations (colored lines) obtained on July 19, 2011, create the chords that silhouette the like-sized halves of asteroid 90 Antiope. The lines are broken where the star LQ Aquarii disappeared on the right side of each body and reappeared on the left. Note the indentation (a crater?) on the lower component. Click here for a larger view.

David Dunham / IOTA

I'm happy to report that Antiope did not disappoint. As the plot at right shows, observers collected more than 50 well-timed observations, spread out (by design) across and along the track in such a way that the collective blink-outs define the sizes and shapes of the two components with better fidelity than is possible by even the mightiest of professional telescopes. In particular, the southern one is missing a big chunk along its eastern (trailing) limb that's perhaps a crater. Interestingly, earlier observations had hinted that this deformity exists.

Based on images made a few days beforehand with one of the giant Keck telescopes, a team led by astronomer William Merline (Southwest Research Institute) predicted that observers along the track's centerline might see LQ Aquarii clipped by both halves of Antiope. But in reality the gap was wider than expected. "A few observers in the middle had no occultation by either component," explains David Dunham, IOTA's president. "We're debating whether to call these central miss observations 'Moses chords' or 'Hercules chords', for threading the Pillars of Hercules to gain new knowledge."

Initial reports suggested that the recorded light curves of LQ Aquarii revealed the star itself to be a double. "We're still debating the reality of the secondary star," Dunham says. "But for sure, the red giant primary has a significant angular diameter, causing all events to be gradual, lasting almost a second or more."

This year's meeting of the International Occultation Timing Association (IOTA) was held in Rocklin, California, so that attendees would already be in the path of the stellar occultation by asteroid 90 Antiope on July 19, 2011.

David Dunham / IOTA

One highly anticipated bonus from the July event was supposed to be a live webcast of the star's disappearance and reemergence as seen through one of the 18 small telescopes set up by diehard occultation observer Scotty Degenhardt of Columbia, Tennessee. Although I and many others tuned in as the event time neared, my monitor went blank a few minutes before the spectacle. "Fear not!" Degenhardt says. "I was recording while broadcasting," and he's uploaded what we all missed in a YouTube video (the occultation itself begins about 7 minutes into the 9½-minute clip).

Dunham and others are still poring over the Antiope results. But this occultation surely ranks among the most remarkable and scientifically valuable in recent years.

Here's what I find ironic about all this: Back in the 1970s, Dunham and others argued strenuously — but in vain — trying to convince professional astronomers that a few occultations had revealed the existence of binary asteroids. Then the Galileo spacecraft discovered that asteroid 243 Ida has a tiny companion (later named Dactyl), and professionals' resistance quickly waned. Today more than 200 multiple asteroids are known.

You can click here to learn more about IOTA and here for predictions of upcoming occultations that you might be able to observe. And you'll find several articles right here on SkyandTelescope.com about how to view and record occultations.

Comments

John Wharton

September 9, 2011 at 1:49 pm

I recall doing a visual timing of an asteroid in the early 80s from a darkened football stadium in Yukon, Oklahoma. Wayne Wyrick and I each were at a 'scope and independently saw the hint of a double blink, and passed our timings and observations along to IOTA. We never heard if any other observers had similar observations, and our subsequent musings to IOTA about the possibility of a binary were fairly well dismissed, as I recall. Thirty years later it's nice to know we might have been right.