Jin An, a doctoral student working as part of the PLANET Collaboration, measured the mass of a dim binary star system 6,500 light-years (38 quadrillion miles) away, when it passed in front of a brighter star on the far side of the galaxy.
An will describe the technique Tuesday, January 8, at the American Astronomical Society meeting in Washington, DC. The same technique could be used to find dark matter within our galaxy, and might even help account for the missing mass of the universe, which has so far eluded detection by the most powerful instruments, including the Hubble Space Telescope.
"This work relates to the dark matter problem, but not necessarily the dark matter problem you've heard about," said Andrew Gould, An's thesis advisor. "Normally people look for dark matter in the outer 'halo' region of the galaxy, but in this case, we're looking inside the galaxy, in the central bulge and surrounding disk, to find out what's there. For instance, we know a large portion of the disk is made up of bright stars, but we don't know much about the population of black holes, or stars that are too dim to see. That's why gravitational microlensing is useful."
Gravitational microlensing occurs when a massive dark object in space, like a planet, dim star, or black hole, crosses in front of a luminous source star in the background. The object's strong gravitational pull bends the light rays from the star and magnifies them like a lens. Here on Earth, we see the star get brighter as the lens crosses in front of it, and then fade as the lens gets farther away.
In May 2000, a binary star system about one fourth of the distance to the center of our Milky Way galaxy crossed in front of a luminous source star lying near or beyond the galactic center. The unusually long passing took approximately 200 days, and gave the PLANET team chance to test the mass-calculation method that An and Gould had been working on.
PLANET, which stands for Probing Lensing Anomalies NETwork, is an international organization of astronomers who monitor microlensing events from telescopes in Chile, Australia, and South Africa.
An and Gould's method relies on the observer being able to gauge the position of a microlensing event from two separate locations that are distant from each other. Ideally, one measurement would be taken from Earth and another from a satellite in orbit around the sun, such as NASA's Space Interferometer Mission (SIM), set to launch in 2009.
But even without SIM at their disposal, An and Gould were still able to test their method, because the Earth moved far enough in its orbit over the 200 days to give the astronomers the two distant points of reference they needed.
Astronomers call this type of effect "parallax." Of the roughly 1000 microlensing events recorded over the last decade, about a dozen were long enough to detect this parallax effect, but these detection alone did not permit mass measurements.
"What allowed us to measure the mass," An said, " was that parallax was detected in a binary lens. Unlike single lenses, binary lenses can have multiple sharp peaks of brightness, where the source brightens about 10 times in a few hours or days. We got the extra information we needed for the mass measurement by studying the structure of these peaks."
The special event, dubbed EROS BLG-2000-5, gave the PLANET team the chance to test a mass-measurement technique that will be useful much more often once SIM is launched. "We've proven that our technique works for binary microlensing events, but once SIM is launched, we'll be able to do it for single events too," Gould said.
The mass that An and the PLANET team calculated for the binary star system in EROS BLG-2000-5 suggested that the stars were what astronomers call red dwarfs.
"These are probably garden variety stars," Gould said, "and so nothing to write home about. But the point is, once we can put this technique into mass production, we can make a representative census of all the objects in our galaxy, both dark and luminous."
Contact: Jin An, (614) 292-1892; jinhan@astronomy.ohio-state.edu
During the AAS meeting, An can be reached at the Windsor Park Hotel at (202) 483-7700.
Andrew Gould, (614) 292-1892; gould@astronomy.ohio-state.edu
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu