NASA Satellite Detects Massive Star Partner
Scientists using NASA’s Far Ultraviolet Spectroscopic Explorer
satellite made the first direct detection of a companion star of Eta
Carinae. Eta Carinae is one of the most massive and unusual stars in
the Milky Way galaxy. The detection was made possible by the high
temperature of the companion star and the unique sensitivity of the
satellite at the shortest ultraviolet wavelengths.
Eta Carinae is an unstable star thought to be rapidly approaching the
final stage of its life. It is clearly visible from the southern
hemisphere and has been the subject of intense studies for decades.
This mysterious star is located about 7,500 light-years from Earth in
the constellation Carina. Scientists thought a companion star in
orbit around Eta Carinae might explain some of its strange
properties, but researchers lacked direct evidence a companion star
existed.
“Until now, Eta Carinae’s partner has evaded direct detection,” said
Dr. Rosina Iping, a research scientist at Catholic University of
America in Washington. “This discovery significantly advances our
understanding of the enigmatic star.”
Evidence that Eta Carinae might be a double star system was inferred
from a repeating pattern of changes in visual, X-ray, radio and
infrared light over approximately 5 ? years. Astronomers thought a
second star in a 5 ? year orbit around Eta Carinae might cause the
repeated changes in its light. The strongest indirect evidence
supporting the double star theory is that once every 5 ? years, the
X-rays coming from the system disappear for about three months. Eta
Carinae is too cool to generate X-rays, but it continuously blasts a
flow of gas into space as a stellar wind at about 300 miles per
second.
If its companion has a similar wind, their stellar winds would collide
with enough force to generate the X-rays. This collision region must
lie somewhere between the two stars.
As Eta Carinae moves in its orbit, it passes in front of the region
where the winds collide, as viewed from Earth. When this occurs, Eta
Carinae eclipses the X-rays once every 5 ? years, causing them to
disappear. The last X-ray eclipse began on June 29, 2003. The 5 ?
year orbit places the companion star only about 10 times farther from
Eta Carinae than Earth is from the sun. Eta Carinae is too far away
for telescopes to distinguish two stars in such a close orbit.
Another way to find evidence of a double-star system would be to
detect the light of the second star, which in this case is much
fainter than Eta Carinae. Several scientists searched for light from
Eta Carinae’s companion using ground-based telescopes, but none
succeeded. Because the companion is thought to be much hotter than
Eta Carinae, astronomers reasoned it should be brighter at shorter
wavelengths like ultraviolet light. However, it still escaped
detection when it was searched for using the ultraviolet capabilities
of the Hubble Space Telescope.
Iping and her collaborators used the satellite to detect the
companion, because it can see even shorter ultraviolet wavelengths
than Hubble. The team observed the far-ultraviolet light from Eta
Carinae with the satellite on June 10, 17 and 27, 2003, right before
the expected X-ray eclipse. While the far ultraviolet light from Eta
Carinae was seen in the observations from June 10 and 17, it vanished
on the 27, two days before the X-ray eclipse.
The disappearance of far ultraviolet light so close to the X-ray
eclipse implies when Eta Carinae eclipsed the X-rays, it also
eclipsed the companion star. The far-ultraviolet light observed prior
to the eclipse was from the hotter companion, because Eta Carinae is
too cool to emit much far-ultraviolet light.
“This far ultraviolet light comes directly from Eta Carinae’s
companion star, the first direct evidence that it exists,” said Dr.
George Sonneborn. He is Far Ultraviolet Spectroscopic Explorer
Project Scientist at NASA’s Goddard Space Flight Center, Greenbelt,
Md. “The companion star is much hotter than Eta Carinae, settling a
long-standing mystery about this important star.”
This discovery will be published today in the Astrophysical Journal
Letters. Authors include Iping, Sonneborn and Ted Gull of Goddard;
Derck Massa of SGT Inc., Greenbelt, Md.; and John Hiller of the
University of Pittsburgh. The project is a NASA Explorer mission
developed in cooperation with the French and Canadian space agencies
by Johns Hopkins University, Baltimore, University of Colorado,
Boulder, and University of California, Berkeley. Goddard manages the
program for NASA’s Science Mission Directorate.
