The most massive stars in the universe collapse to form black holes at the end of their lives. Theory suggests that such black holes can’t have much more than about 10 times the sun’s mass, but a team has now identified one that tips the scales at 15.65 times the sun’s mass. Another research group has tentative evidence of an even bigger beast.
The findings may call into question current predictions about the evolution and ultimate fate of heavyweight stars.
Jerome Orosz of San Diego State University and his colleagues studied a pair of closely orbiting stars in the nearby galaxy M33. As seen from Earth, each star periodically passes in front of the other, and the durations of these eclipses revealed that one of the partners must be extremely compact. From visible-light observations, the researchers determined that the other star has a mass 70 times that of the sun. Knowing that star’s mass and orbit, they calculated that the compact partner weighs 15.65 solar masses. That would mean it must be a black hole—in fact, the heaviest stellar-mass black hole ever to be precisely measured, Orosz and his colleagues report in the Oct. 18 Nature.
The pairing of such a black hole with a massive normal star “is very difficult to explain using stellar evolutionary models,” the team asserts. The more massive a star, the shorter its life span. So if the two stars in M33 were born around the same time, the one that reached the end of its life first and collapsed to become the black hole must have started out with a greater mass than the surviving normal star. The collapsed star’s initial mass would probably have been about 100 times that of the sun, suggests Orosz.
This may pose a puzzle, however, because such a massive star would generate a fierce wind that would carry off a significant fraction of its mass. Earlier in its evolution, moreover, the heavy star would have briefly ballooned in size and spilled material onto its lower-mass partner. This mass loss could make it difficult for the star to remain heavy enough to form the giant black hole.
Theorist Vicky Kalogera of Northwestern University in Evanston, Ill. says that the finding might not be a problem. However, the possible detection of an even bigger black hole, reported in the Nov. 1 Astrophysical Journal Letters, poses a challenge, she says.
A team led by Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., examined X-ray emissions from a spot in the nearby galaxy IC 10 and found that the radiation waxes and wanes every 34.4 hours. The most likely source of the X rays is a bright, heavy star eclipsed by a more compact body, the researchers say. To explain the observations, the heavy star’s partner would have to be a black hole 25 to 34 times as massive as the sun.
The likely existence of huge stellar black holes, along with observations of massive stars, suggests that stellar winds drive off only one-third to one-tenth as much mass as researchers had previously supposed, says Nathan Smith of the University of California, Berkeley. An analysis he recently posted online at http://xxx.lanl.gov/abs/0710.3430 suggests that tamer winds would mean that the predicted fate of massive stars “may need to be revised.” Unexpectedly heavy stellar-mass black holes may be abundant in the modern-day universe, Smith suggests.
