WHEN an international collaboration
began a search for dark matter in the Milky Way Galaxy, measuring
a stars mass was not the goal they had in mind. But as the
team examined more closely the massive compact halo objects (MACHOs)
they had found, they realized that all the information they needed
for measuring a single, isolated stellar mass was right in front
the mass of a star or a satellite such as planets or moons used
to be a relative affair: How does the presence of one body affect
the position of the other? The movement of the Sun as the Earth
and other planets revolve around it is largely a function of their
relative masses. The same is true for our Moon as it revolves around
our home planet. New discoveries about the existence of planets
around other stars are inferred by observing wobble in a stars
position. More wobble means more mass is revolving around it. Careful
observations of the wobble can reveal how many planets are acting
on the star.
Astrophysicist Kem Cook, who leads the Livermore contribution to
the team, notes, Measuring the mass of a star has never been
possible when it was in physical isolation. Always before, we were
looking at two bodiesthe Sun and Earth or binary stars. Now
we can do just one, but it takes lots of time and data.
step-by-step guide to gravitational microlensing. (a) Light
from the source star (green) is deflected by the lens (red),
creating two enlarged and distorted images (blue). (b) As the
lens passes in front of the source, the pair of images becomes
first larger and then smaller, so that the observed light flux
(blue curve) becomes brighter and then fainter.
The Critical Image
1992 to 2000, the MACHO collaboration, including researchers from
the U.S., Australia, Chile, Germany, Britain, and Canada, searched
the outer regions of the Milky Way for MACHOs using the Large Magellanic
Cloud (LMC) as a backdrop (see the box below The
Search for MACHOs). They were looking for events in which
the gravitational field of a MACHO came between their detector and
a distant star, causing the distant star to brighten significantly.
Gravity acts as a lens in a process called microlensing. Six years
of observational data have been analyzed thus far, revealing 17
microlensing events in the Milky Ways halo, including the
first event ever positively identified.
interest was the brightest event, known as LMC-5. The distant, or
source, star became at least 15 times brighter when it was behind
the microlensing object. The unmagnified color of the patch of sky
occupied by both the lens and the source star was redder before
and after the peak of the microlensing event. Researchers suspected
that the source star was a blue star in the LMC that, because of
atmospheric blurring, was confused with a red star, whose color
and brightness suggested a red dwarf star in the Milky Way.
characterize the source stars of all microlensing events, the team
obtained images taken by the Hubble Space Telescope, which operates
well above Earths atmosphere. The 1999 Hubble image for the
LMC-5 region, taken 6.3 years after the peak of the LMC-5 microlensing
event in February 1993, revealed a faint red object in addition
to the source star. The two stars were still so close to one another
that even on the Hubble image, they appeared slightly blended.
team went back to reexamine the details of the LMC-5 microlensing
curve. By analyzing perturbations in the curve caused by Earths
movement around the Sun, they were able to predict the direction
that the lens would move across the sky. They found that the red
star shown in the Hubble image was in the place that their calculations
predicted and must be the lens. But the team didnt stop there.
three-color composite image from the Hubble Space Telescope
(HST) of the brightest microlensing event. The microlensing
source star is the blue star at the right of the figure, which
is partially blended with a much redder object (indicated by
the arrow) displaced by 0.134 arcseconds. The directions of
motion of the lens on the sky derived from the HST (–92
degrees) and from the unconstrained parallax (par) fit (–100
degrees) are shown.
Adding It All Up
Over 30 years ago, researchers suggested that gravitational microlensing
could be used to measure the masses of nearby stars, although no
one had yet seen a microlensing event in action.
Data collected in microlensing events are insufficient to measure
the mass of the lens: The duration is proportional to the mass of
the lens, the relative distances of the lens and the source star
from Earth, and the motion of the lens across the line of sight
of the source star.
LMC-5, much more information was available, thanks to the image
from the Hubble Space Telescope. If we assume that the red
star is the microlens and the other star is the source star, then
we have a physical measurement on the sky of the motion of the lens, says
team had already determined the apparent direction and motion across
the line of sight of the lens from the distortion in the
light curve due to the motion of the Earth. Combining that information
with the known distance to the Large Magellanic Cloud, which is
very shallow, and the time between the original image and the Hubble
image, allowed them to determine the distance the lens had traveled
and, hence, its velocity. With the Hubble imagethe first ever
of a microlens scooting across the heavensthe team had all
of the elements needed to determine the distance of the lens and
its mass. They found that the mass of the LMC-5 lens was about
the mass of the Sun.
compact halo objects (MACHOs) are thought to be one
kind of the invisible dark matter that surrounds and
permeates our galaxy and other galaxies like it. Astronomers
have determined that the Milky Way Galaxy must be much
more massive than the amount of mass that is visible.
Without the additional mass, the galaxy would fly apart.
In fact, as much as 90 percent of the Milky Ways
mass cannot be detected with available techniques. There
is no question that dark matter exists. Finding it is
studies suggested that MACHOs would produce gravitational
microlensing, a process by which the gravity of one
object comes between an observer and a distant star
and causes the light of the distant star to be briefly
magnified. Because a microlensing event requires that
the two bodies be lined up almost perfectly, microlensing
events are quite rare. Although a star could act as
a microlens, the relative lack of stars in the outer
regions of the Milky Way made it likely that any microlenses
seen would prove to be MACHOs.
the Large Magellanic Cloud (LMC) Galaxy as a backdrop
of distant, or source, stars, researchers from around
the world searched for MACHO microlensing events with
a special camera developed at Livermore.
Scientists monitored millions of stars in the LMC at
Mount Stromlo Observatory in Canberra, Australia, from
1992 to 2000. This observatory was chosen because the
LMC is only visible from the southern hemisphere.
observing whether the brightness of any
source stars varied over time as a gravitational
lens passed between the star and the
observatorys detector, the team
identified numerous microlensing events
toward the Large Magellanic Cloud. Their
data to date indicate that between 8
and 50 percent of the Milky Ways
mass is in the form of MACHOs.
Hubble Space Telescope images to more
closely examine the source stars of LMC
microlensing events, the team has thus
far found that only LMC-5 involved a
visible star acting as a lens. All the
rest were apparently caused by the gravitational
field of a MACHO.
research came to a screeching halt at
the Mount Stromlo Observatory in January
2003 when a raging bush fire destroyed
many buildings, including four telescopes,
computers, and a spectrograph being constructed
for the 8.2-meter Gemini North telescope
in Mauna Kea, Hawaii. The fire destroyed
about one-third of Australias astronomical
A Better Way Soon
even better way to measure the mass of stars will be available
or so when National Aeronautics and Space Administration launches
the Space Interferometry Mission (SIM). SIM will use optical interferometry
to measure the tiny, apparent motion of stars, which is caused
by microlensing, to determine the masses and distances of stars
much greater accuracy than previously possible. Measuring the mass
of individual stars with data from SIM will be a veritable walk
in the park.
Key Words: gravitational microlensing, massively compact halo objects
For further information contact Kem Cook (925) 423-4634 (firstname.lastname@example.org).
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