Long gamma-ray bursts (GRBs) are associated with the deaths of only the most massive stars and occur relatively rarely in spiral galaxies such as our own Milky Way, according to research published online in Nature this week.
That's good news, because a nearby gamma-ray burst could wreak havoc on Earth by destroying the ozone layer in the upper atmosphere. But researchers already knew that a gamma-ray burst near enough to threaten our planet was highly unlikely. They are more interested in the implications of the new study for understanding how gamma-ray bursts and supernovae are triggered by the collapse of massive stars.
"The fundamentally interesting observation is that the stars that explode as gamma-ray bursts have a different distribution from stars that explode as supernovae. Gamma-ray bursts tend to take place in galaxies not like ours," said Stephen Thorsett, professor of astronomy and astrophysics at the University of California, Santa Cruz, and a coauthor of the Nature paper.
Thorsett first described the terrestrial implications of a nearby gamma-ray burst in a 1995 paper, but he said the likelihood of such an event now appears to be very small.
The new paper is a comprehensive analysis of observations of gamma-ray bursts and supernovae made by NASA's Hubble Space Telescope over the past ten years. The Hubble observations show that long-duration GRBs--those lasting more than one to two seconds--tend to occur in small irregular galaxies where stars are deficient in the heavier elements. Spiral galaxies like the Milky Way tend to be rich in heavier elements.
The study involved a large team of researchers led by Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Md. Stan Woosley, professor of astronomy and astrophysics at UCSC and a leading expert on gamma-ray bursts and supernovae, is also a coauthor of the paper.
The paper looks at long-duration gamma-ray bursts and type II or core-collapse supernovae, both of which result from the violent deaths of massive stars. When such a star exhausts its fuel, the core collapses and explodes. The new findings shed light on the conditions that determine whether the collapse will generate a brilliant supernova or an even more powerful gamma-ray burst (GRB).
Fruchter's team used Hubble to examine the environments of 42 long-duration GRBs and 16 core-collapse supernovae. The researchers found that most of the long GRBs in the sample were detected in small, faint, misshapen galaxies. Such "irregular" galaxies are usually deficient in heavier elements. Only one of the GRBs was spotted in a spiral galaxy. By contrast, the hosts of supernovae were divided equally between spiral and irregular galaxies.
The researchers also found that long GRBs are far more concentrated on the brightest regions of their host galaxies where the most massive stars reside. Supernovae, on the other hand, occur throughout their host galaxies.
"The discovery that long-duration GRBs lie on the brightest regions of their host galaxies suggests that they come from the most massive stars--20 or more times as massive as our Sun," Fruchter said. "Their occurrence in small irregulars implies that only stars that lack heavy chemical elements [elements heavier than hydrogen and helium] tend to produce long-duration GRBs."
Galaxies build up a stockpile of heavier chemical elements through the ongoing evolution of successive generations of stars. Massive stars abundant in heavy elements are unlikely to trigger GRBs because they may lose too much material through stellar "winds" off their surfaces before they collapse and explode. When this happens, the stars don't have enough mass left to produce the proper conditions that would trigger GRBs.
These findings fit with the predictions of Woosley's "collapsar model," which describes how a long GRB is produced when a massive star collapses to form a black hole. The more massive a star is when it dies, the more likely it is to form a black hole. And stars with low metallicity lose less mass as they burn and are more massive and rotate more rapidly when they die, Woosley said.
"The core of the star has to be rotating rapidly to make a gamma-ray burst, and as stars lose mass by blowing away material in stellar winds, they rotate more slowly," Woosley said.
When the core of a massive star collapses to form a black hole, the energy from the collapse escapes along a narrow jet, which burns its way through the remnants of the star. The formation of directed jets that concentrate energy along a narrow beam explains why GRBs are so powerful.
GRBs can be divided into two classes: short bursts, which last between milliseconds and about two seconds and produce very high-energy radiation, and long bursts, which last between two and tens of seconds and create less energetic gamma rays. Short bursts are believed to arise from collisions between two compact objects, such as neutron stars.
The Hubble Space Telescope is an international cooperative project between NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington, D.C.
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Note to reporters: You may contact Thorsett at (831) 459-5170 or thorsett@ucolick.org and Woosley at (831) 459-2976 or woosley@ucolick.org.
