CATACLYSMIC VARIABLES DISCOVERED IN HEART OF GLOBULARCLUSTER
CAMBRIDGE, Mass.--The first observations of faint starsdeep within the heart of a globular cluster by the HubbleSpace Telescope (HST) provides strong new evidence forbinary star systems known as cataclysmic variables (CVs),according to a team of astronomers led by JonathanGrindlay of the Harvard-Smithsonian Center forAstrophysics and Adrienne Cool of UC Berkeley. Theirdiscovery, announced today at the American AstronomicalSociety meeting in Pittsburgh, PA, reveals new informationabout both CVs and the evolution of globularclusters--enormous, spherical collections of stars thoughtto contain some of the oldest stars in the Universe.
Grindlay, Cool, and colleagues from Harvard, Yale, andIndiana University used the Faint Object Spectrograph onHST to obtain spectra on three faint stars in NGC 6397, aglobular cluster about 6,000 light-years from Earth. Eachspectrum bore the telltale signature of a cataclysmicvariable--a white dwarf star that is pulling mass off alarge, low-mass companion star in close orbit around it. White dwarfs are the super-dense, burned-up remains ofstars that were previously the size of the Sun. CVs mayform during near collisions between stars, scientistsbelieve. These binary star systems are well-knownelsewhere in the galaxy, but had never before beenidentified spectroscopically deep inside a globularcluster.
Other compact binary systems contain not white dwarfs butneutron stars--the ultra-dense remnants of stars that weremuch more massive than the Sun. While the recentlydiscovered objects in NGC 6397 are most likely CVs, theymight instead be neutron star binaries in a very lowluminosity (or quiescent) state, according to theresearchers.
That scenario would only deepen the mystery surroundingCVs, however, since astronomers have long suspected theirpresence inside clusters. The multitudes of stars packedclosely together in clusters should favor CV formation. And, faint x-ray emissions from the center of NGC 6397,detected by Grindlay and Cool with the ROSAT x-raytelescope, provided futhur evidence.
With the repaired HST, the scientists were finally able topinpoint individual faint stars inside the clusters--apreviously impossible task with other ground-andspace-based telescopes. "HST can focus the light andpoint the telescope with such precision that it can screenout the surrounding bright stars, allowing us to get thespectra of individual faint stars deep in the cluster,"says Grindlay. What the spectra actually reveal is theso-called accretion disk, or huge disk of matterspiralling down onto the compact white dwarf star from itscompanion.
"The luminosity we are detecting is powered bygravitational energy, not nuclear fusion," explainsGrindlay. (Nuclear fusion generates the light emanatingfrom normal stars). In this case, the accretion diskcompletely overwhelms the light from the low-mass companionstar, he adds. The changing nature of this accretion iswhy they are known as variables.
The spectra also allow the scientists to limit the sizeand mass of the low-mass companion star. White dwarfs areknown to be roughly the size of the Earth (with a radiusof about 10,000 km), while the radii of the accretiondisks in CVs are nearly 10 times as big. The secondarystar orbiting the white dwarf in at least one of theprobable CVs in NGC 6397 is probably less than half themass of the Sun, the scientists estimate.
Finding the first direct spectroscopic evidence for CVs inthe core of a globular cluster is important because it supports the view that close binary stars form insignificant numbers in the crowded, collision-prone coresof these massive collections of stars, according to theresearchers. In addition, scientists suspect that compactbinaries may actually prevent the center of a globularcluster from collapsing down and forming a massive blackhole.
"Now, we can begin to really understand what makes aglobular cluster tick," says Grindlay. Such information,he adds, is fundamental to understanding the formation andsurvival of globular clusters which, in turn, may help usunderstand the creation of the galaxy itself.
For more information, contact: Jonathan Grindlay (617)495-7204, or Adrienne Cool (510) 642-5016.