UCO/Lick's Automated Supernova Search Successful
November 11, 1999
FOR IMMEDIATE RELEASE
Contact:
Robert Sanders
UC Berkeley Public Information Office
510-643-6998
rls@pa.urel.berkeley.edu
BERKELEY, CA: A supernova dubbed 1999em, recently discovered at Lick Observatory, may become one of the century’s best observed supernovae, according to Alexei Filippenko, the University of California, Berkeley, astronomer whose group found the exploding star. The supernova is a coup for Filippenko’s automated supernova search, which employs a robotic telescope, the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory, to take nightly pictures of large numbers of nearby galaxies in search of new points of light. 1999em is the 30th supernova that he and his team have discovered so far this year--twice what any other search for nearby supernovae has turned up in a 12-month period.
“This has the potential to become the best observed supernova since the one in 1987,” said Filippenko, professor of astronomy at UC Berkeley and director of the supernova search. He’s referring to the most spectacular supernova of the century, a naked-eye exploding star that wowed astronomers and casual observers alike in 1987.
The new supernova was photographed by KAIT just before dawn on Friday, Oct. 29, and postdoctoral fellow Weidong Li immediately e-mailed news of it around the world. Six hours later, astronomers at the Beijing Astronomical Observatory confirmed it was a very bright Type II supernova located in a nearby galaxy called NGC 1637, in the constellation Eridanus near its border with Orion. Though SN 1999em outshines the core of its own galaxy and is extremely bright compared to normal stars, because it is 25 million light years away it isn’t visible to the naked eye.
Astronomers nevertheless jumped at the chance to observe an explosion in progress. The Chandra X-ray Observatory turned its eye on the supernova on Nov. 2, while the Hubble Space Telescope wheeled around to look at the brightening explosion on Nov. 5. “Supernovae expand quickly and cool quickly, so each day we delay observing the supernova it has changed irretrievably,” Filippenko said. “We caught this really early, only a day or two after the explosion. We were lucky.”
Discoveries like 1999em make UC Berkeley’s automated supernova search exciting, Li said. From the top of Mt. Hamilton outside San Jose, California, the telescope--a 30-inch reflector--photographs a few thousand galaxies every three to five days. “Among the galaxies that we monitor, we are discovering essentially every exploding star within about 100 million light years of Earth,” Filippenko said. The only ones they miss are those too near other bright objects, like galactic centers, or those buried in haze near the horizon.
The point of the automated supernova search is to learn as much as possible about the physics of exploding stars. To do that, Filippenko, Li and their colleagues need to find them shortly after they explode so they can chart the course of brightening and dimming, and also obtain spectra--the range and brightness of colors in the star’s light.
The 50 supernovae they have found since the telescope came online in 1997 are giving them information on the types of supernovae in the cosmic zoo and providing clues to the fate of the cosmos. Filippenko and an international crew of astronomers--the High-Z Supernova Search Team--last year drew upon the distances of Type Ia supernovae to conclude, along with a competing team, that the expansion of the universe is accelerating. The Type Ia supernovae were used as a “standard candle” to estimate galactic distances.
Though none of the supernovae employed in that study were found by KAIT, which looks only for nearby supernovae, Filippenko has been compiling data on nearer Type Ia supernovae to bolster confidence in the use of these supernovae to estimate distance. In the December issue of Astronomical Journal, Filippenko and his colleagues will report a worrisome finding - nearby supernovae might take longer than more distant supernovae to reach their maximum brightness.
“Is this an indication of inherent differences between the nearby supernovae astronomers think they understand, and the older and more distant ones?,” Filippenko asks. “If so, can we really say that their maximum brightness is the same, and use them as a standard candle?”
“Perhaps we will find that we can explain this possible difference, and that there really is no error in our study of distant supernovae,” Filippenko said. “But what if distant supernovae are mostly weird and not like nearer ones at all?”
One reason Li and Filippenko are excited about 1999em is that it gives them a chance to test a method for using Type II supernovae as distance indicators, providing an independent means of checking their earlier results. “Our group has been waiting for this type of supernova for a while, to test a technique for calculating the distance to Type II supernovae,” Li said. “This bright object provides us with a very good opportunity--I’ll be observing this every night for a long time.”
Ultimately, Type II supernovae--thought to be the collapsing iron core of a massive star that leaves behind a neutron star or black hole--could be distance indicators that will tell us the structure of the universe.
Li oversees the robotic telescope’s day-to-day operation, though he visits the mountain only about once a month. Every night computers check the weather, open the dome, point and focus the telescope and take digital pictures. A mere two seconds later, the image is in Li’s computer at UC Berkeley, where it is compared automatically to earlier pictures of the same region of sky. Images with new points of light are flagged, so that the next morning, a crew of six undergraduates can double-check them. In winter, the KAIT can take as many as a thousand images a night, which results in an average of three supernovae discovered each month.
“We didn’t come up with the idea, but we are the most successful effort to date, and the only currently existing, fully automated supernova search,” Filippenko said. “With the supernovae we find, we are doing a lot of science. ”
Though the robotic telescope discovered only one new supernova the year it was turned on, it found 19 last year. That improvement is largely due to Li, who took over in 1997 after arriving from the Beijing Astronomical Observatory, where he had directed their automated supernova search. UC Berkeley engineer and astronomer Richard Treffers made significant additional improvements in the hardware.
Li’s main scientific interest, now that the KAIT runs reliably without constant oversight, is how Type Ia supernovae differ from one another. When a supernova is discovered, spectra of the star often are obtained at the Shane three-meter telescope at Lick Observatory, and photometry is carried out by KAIT itself. These data give Li a chance to study the details of Type Ia supernovae. Surprisingly, he finds that Type Ia supernovae are not all that similar. More than one-third are peculiar, in that they are brighter or dimmer than the “average ” Type Ia, or their spectra show unusual stellar composition. “People say that the percentage of peculiar supernovae is quite small, less than 10 percent,” Li said. “In fact, we’ve found that it is nearly 40 percent. That is really surprising, and has many implications for how we think about Type Ia supernovae.”
The high peculiarity rate implies that not all Type Ia supernovae explode the same way. Physicists picture them as white dwarfs that steal matter from a larger companion until their mass exceeds a certain limit, at which point the star becomes unstable and is consumed in a gigantic thermonuclear explosion. “These are the events that produce the iron in your red blood cells and the silicon in rocks, ” Filippenko said.
Type Ia supernovae may differ in mass or composition, so that the underlying physics of the explosion is slightly different, he said. Only by studying a large number of supernovae will Filippenko, Li and their colleagues be able to pinpoint these differences. “If we study a few in gory detail and as many as possible in a somewhat superficial degree, we can probe an area of astrophysics that has not been probed before,” Filippenko said.
KAIT operations have been supported by the Sylvia and Jim Katzman Foundation, which donated funds at a critical time in the telescope’s development, as well as the National Science Foundation, the University of California, the Hewlett-Packard Company, Sun Microsystems, Photometrics Ltd., AutoScope Corporation and Lick Observatory.
Editor’s Notes:
Alex Filippenko can be reached at 510-642-1813 or alex@astro.berkeley.edu.
Weidong Li is at 510-643-8973 or wli@astro.berkeley.edu.
Digitized photographs are available at http://astron.berkeley.edu/~bait/kait.html .The top rows of photos show KAIT, as well as SN 1999em (marked with an arrow) in the galaxy NGC 1637 (25 million light years away). Any of these photos (and others on the webpage) may be downloaded and used in news stories.