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Public Affairs Department
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar{at}cfa.harvard.edu

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
clafon{at}cfa.harvard.edu

For Release: 10:00 a.m. CDT, May 28, 2003

Release No.: 03-13

Harvard Continues Legacy of Cepheid Discoveries

Ninety years ago, Harvard astronomer Henrietta Leavitt spent years
painstakingly examining thousands of sky photographs to search for
and study variable stars. Her laborious hunt led to the discovery of
the period-luminosity relationship for Cepheid variables. Leavitt's
name was immortalized because the correlation of Cepheid brightness
to pulsation period means they can be used as yardsticks to measure
the distances to other galaxies. 

Today, another woman astronomer at what has since become the
Harvard-Smithsonian Center for Astrophysics is continuing Leavitt's
pioneering work on Cepheids. Smithsonian astrophysicist Nancy Remage
Evans and her colleagues used the unique capabilities of NASA's
Hubble Space Telescope to study Cepheids in known binary systems.
Their goal was to find the masses of the Cepheids -- a fundamental
stellar property. Instead, they announced today at the 202nd meeting
of the American Astronomical Society that they have uncovered a
hidden, third star in the Cepheid system Y Carinae. More
surprisingly, of the 14 total star systems with the most complete
orbital information, 7 were found to be likely triple systems, a
remarkably high percentage. 

"Our finding is of particular interest because it contributes to our
knowledge of whether massive stars are formed singly, in pairs, or in
multiple systems," Evans says. "Surprises like the third star in the
Y Car system are providing us with a much more complete picture of
whether stars occur singly or in groups." 

Cepheids are important to astronomers for their key role as
extragalactic distance indicators. Cepheids are variable stars that
regularly brighten and dim as they pulsate rhythmically. Their
pulsation period is proportional to their intrinsic brightness -- the
longer the period, the brighter the star. A comparison of intrinsic
to apparent brightness yields the distance to the Cepheid. 

From ground-based telescopes, the Y Car system is dominated by light
from the Cepheid. Only velocity studies showed the existence of a
second, hidden companion star. By observing in the ultraviolet
(possible only from satellites), Evans and her colleagues were able
to measure the velocity of the companion, Y Carinae B, which is
hotter than the Cepheid and, hence, dominates in the ultraviolet.

Using the Space Telescope Imaging Spectrograph (STIS) on the Hubble
Space Telescope (HST), they found that the velocity of the Cepheid's
companion didn't match predictions.

"At first, we thought we must have made a mistake," says co-author
Kenneth Carpenter (NASA Goddard Space Flight Center). "We designed
our observations very carefully to get the most accurate velocities
possible, which is why we used the HST/STIS. When we didn't find a
reasonable velocity for the companion, we talked extensively with
colleagues and STIS instrument scientists to be sure we understood
the performance of the instrument and had properly interpreted the
data. Finally, we concluded that we were seeing a real effect.
Something -- a third, hidden star -- was giving Y Car B a tug."

"The discovery process we've gone through with the Y Car system
reminds me of opening a Russian nesting doll. Every time you look
inside, you find something new that you never would have guessed was
there," adds Evans. 

The final picture of the Y Carinae system is that of an approximately
4.7 solar mass Cepheid star in orbit with a companion that is itself
a binary. That binary consists of a 2.5 solar mass star and an unseen
third star that is perhaps close to the Sun's mass. While the Cepheid
and the binary orbit at a respectable distance of at least 200
million miles (a bit more than the Sun-Mars distance), the two stars
in the close binary are only perhaps a tenth or a hundredth as far
apart. Nevertheless, the two binaries should be far enough from each
other to avoid tidal interactions, at least until Y Car B evolves off
the main sequence and swells to become a red giant.

"That's when things will really get interesting!" Evans predicts.

The team plans to continue work on similar Cepheid systems, both to
improve our knowledge and understanding of such systems, and to
measure the Cepheid masses where possible.

NOTE TO EDITORS: A high-resolution artwork image is available at
      http://cfa-www.harvard.edu/press/pr0313image.html

In addition to Evans and Carpenter, participating researchers were
Richard Robinson (Johns Hopkins University), Francesco Kienzle
(Geneva Observatory), who provided ground-based velocities from the
CORALIE instrument, and Anne Dekas (Harvard University).

This research was supported by STScI grant HST-GO-09146.01-A and by
the Chandra X-ray Center NASA Contract NAS8-39073.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into six research divisions
study the origin, evolution, and ultimate fate of the universe.

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