David Aguilar
(617) 495-7462

Christine Pulliam
(617) 495-7463


CfA Press Release
 Release No.: 03-13
For Release: 10:00 a.m. CDT, May 28, 2003

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://www.cfa.harvard.edu/news/archive/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.

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016

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