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.