archive: SETI [ASTRO] Stars In Neighboring Galaxy Offer Clues To Mystery Of

SETI [ASTRO] Stars In Neighboring Galaxy Offer Clues To Mystery Of

Larry Klaes ( )
Wed, 23 Dec 1998 16:31:45 -0500

>X-Authentication-Warning: majordom set sender
to owner-astro using -f
>Date: Wed, 23 Dec 1998 20:28:55 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Stars In Neighboring Galaxy Offer Clues To Mystery Of
Dark Matter
>Reply-To: Ron Baalke <>
>Ohio State University
>Contact: Andrew Gould, (614) 292-1892; B. Scott Gaudi, (614) 292-5413
>Written by Pam Frost, (614) 292-9475;
>December 22, 1998
>COLUMBUS, Ohio -- A binary star system in a nearby galaxy may bring
>astronomers closer to understanding the nature of dark matter, according to
>Ohio State University researchers.
>The location of the binary star system suggests that exotic dark objects
>such as black holes do not make up the majority of mass in the universe.
>"Our findings don't offer definitive proof," said Andrew Gould, associate
>professor of astronomy at Ohio State, "but we can begin to probe that
>possibility. It's very exciting."
>Astronomers spotted the binary system within the Small Magellanic Cloud
>(SMC), a galaxy that orbits our own, when this system's gravity bent the
>rays of light from another star and magnified them like a lens.
>Gravitational lensing is what happens when a massive dark object in space,
>such as a planet, dim star, or black hole, crosses in front of a luminous
>source star in the background. 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. This is what astronomers call a "lensing event."
>Gravitational lensing is one of the few ways astronomers may detect the
>presence of dark, massive objects in our galaxy. If many such objects exist,
>they could account for the missing mass of the universe.
>The dark matter, which may account for up to 99 percent of the mass of the
>universe, has so far eluded detection by the most powerful instruments such
>as the Hubble Space Telescope. Astronomers refer to these unseen dark
>objects as massive compact halo objects, or MACHOs.
>The first lensing event in the SMC was discovered by a group of astronomers
>called the MACHO Collaboration in January 1997. On June 8, 1998, the MACHO
>Collaboration noticed this second lensing event and alerted another group,
>the Probing Lensing Anomalies NETwork (PLANET).
>Eight institutions including Ohio State make up the PLANET Collaboration.
>Members gather data from four observatories: Perth Observatory in Bickley,
>Australia; Canopus Hill Observatory of the University of Tasmania in Hobart,
>Australia; South African Astronomical Observatory Sutherland, South Africa;
>and European Southern Observatory in La Silla, Chile.
>During the two weeks that the light from this latest event grew to peak
>intensity, B. Scott Gaudi, a graduate student in astronomy at Ohio State,
>was coordinating telescope operations. Gaudi sent an announcement of the
>event to interested members of the astronomical community over the Internet,
>and directed the operation of the telescope in South Africa to observe the
>peak brightness of the event. Gaudi also analyzed the telescope data with
>software that he had developed.
>The analysis appears in a paper which has been accepted for publication in
>the Astrophysical Journal. A preprint of the paper currently appears in an
>astronomy preprint server on the Internet. The World Wide Web address for
>the paper is
>The way in which a gravitational lens bends the light rays of its background
>star indicates what type of object causes the lens. In this case, the data
>collected by the PLANET team confirmed that the lens object was a binary
>star system.
>When Gaudi analyzed the data with his software, he was able to calculate the
>speed at which the binary system was moving through space. "That's what
>allowed us to distinguish whether this binary was in the halo -- and
>therefore possibly the dark matter that everybody's looking for -- or
>whether it was in the SMC," said Gaudi.
>"A lens in the SMC would be traveling 10 times slower than a lens in the
>halo," said Gould. "From our calculations, if this lens was in the SMC, it
>was moving at roughly 75 kilometers per second. If it was in the halo, it
>would have been moving much slower, such as 35 kilometers per second. The
>likelihood that a halo object would travel that slowly is negligible."
>The binary star system gave astronomers an especially good view of the
>event, because the interplay between the two stars' gravitational fields
>warped the lens effect and magnified the source star to 100 times its normal
>brightness. "A binary lens causes a distortion in the lens structure," said
>Gould. "Think of it this way: If you were wearing glasses, and you stood
>next to a road on a rainy day, you'd notice that raindrops on the glasses
>distorted round lights of the passing cars into diamond shapes. That's what
>a binary lens does."
>Gould said that while these observations poke some holes in the MACHO
>hypothesis, they don't prove a competing idea, namely that dark mater is
>composed of weakly-interacting massive particles, or WIMPS. "Even if we
>totally ruled MACHOs out, that doesn't mean that dark matter is definitively
>made up of WIMPS. Before we could say that, someone would have to detect
>WIMP particles somehow. Our finding is just a step on the road to solving
>this puzzle."
>Other PLANET member institutions include: the Kapteyn Institute,
>Rijksuniversiteit Groningen, The Netherlands; Space Telescope Science
>Institute, Baltimore, Maryland; South African Astronomical Observatory,
>Capetown, South Africa; University of Canterbury, Christchurch, New Zealand;
>Perth Observatory, Bickley, Western Australia; and Canopus Observatory,
>Hobart, Tasmania, Australia.