Ghostly ring found circling dead star
An international team of scientists has found a strange ring around a dead star by using images taken by NASA's Spitzer space telescope. This star, called SGR 1900+14, belongs to a class of objects known as magnetars. According to NASA, a magnetar is 'a highly magnetized neutron star and the remnant of a brilliant supernova explosion signaling the death throes of a massive star.' So far, about a dozen magnetars have been found. An amazing thing about these stellar objects is their magnetic field. One of the researchers said that 'magnetars possess magnetic fields a million billion times stronger than the magnetic field of the Earth.' But read more...
The image above "shows a ghostly ring extending seven light-years across around the corpse of a massive star. The collapsed star, called a magnetar, is located at the exact center of this image. NASA's Spitzer Space Telescope imaged the mysterious ring around magnetar SGR 1900+14 in infrared light. The magnetar itself is not visible in this image, as it has not been detected at infrared wavelengths (it has been seen in X-ray light). [...] This composite image was taken using all three of Spitzer's science instruments. The blue color represents 8-micron infrared light taken by the infrared array camera, green is 16-micron light from the infrared spectograph, and red is 24-micron radiation from the multiband imaging photometer." (Credit: NASA/JPL-Caltech) Here is a link to a NASA page where you can find larger versions of this picture.
This research project has been led by Stefanie Wachter, research scientist at NASA's Spitzer Science Center at the California Institute of Technology.
One of her collaborators was Donald Figer, professor at Rochester Institute of Technology (RIT). Here is what he says about magnetars. "'Out of 400 billion stars in our galaxy, there are about a dozen magnetars that we know of. Discovering the ring is groundbreaking because it discovers some other phenomenon associated with, and physically near, a magnetar. And when you know so little about an object, each new morsel you can gather up is very important. [...] Magnetars possess magnetic fields a million billion times stronger than the magnetic field of the Earth,' Figer says. The magnetic field of a magnetar is one petagauss (10 to the 15th or 1,000,000,000,000,000 gauss) while, in comparison, Earth's magnetic field strength registers at 0.5 gauss, the Sun at one gauss and a sunspot at about 1,000 gauss."
According to a news release of the University of Chicago, "CSI: Milky Way team works scene of dead star," Wachter also worked with Vikram Dwarkadas, a Senior Research Associate in Astronomy & Astrophysics at the University of Chicago. Here are some of his comments. "'It's the first time something like this has ever been seen around a magnetar,' Dwarkadas said. Magnetars come from massive stars that have exploded as a core-collapse supernova. 'These stars are at least eight times the mass of the sun, or more massive than that,' he said. Magnetars interest astrophysicists because of their mysterious and unusual characteristics. When massive stars collapse, they usually form compact objects called neutron stars or black holes. 'We have no idea why some neutron stars are magnetars and some are not,' Dwarkadas said."
So what's the origin of this ring? Here is what says Wachter. "We think that the ring was created when a giant flare from the SGR (soft gamma repeater) carved a cavity into the dusty environment surrounding the magnetar, thus naturally explaining why the ring is centered on the magnetar."
The team has published its results in Nature under the title "An infrared ring around the magnetar SGR 1900+14" (Volume 453, Number 7195, Pages 563-696, May 29, 2008). Here is the editor's summary, "Magnetars: Ring of fire." "Magnetars are rotating neutron stars with magnetic fields at least an order of magnitude stronger than those of 'normal' radio pulsars. There are only about a dozen magnetars known to date and their evolutionary history is not yet firmly established. Circumstantial evidence links them to very massive stars as progenitors; now the discovery of an infrared ring or shell around magnetar SGR 1900+14 (a soft gamma repeater) confirms that link. Observations from the Spitzer telescope are consistent with the presence of a dust-free cavity in the magnetar environment -- possibly produced by the giant flare emitted by the source in August 1998 -- strongly suggesting an association with a cluster of massive stars."
Here is the end of the abstract. "The appearance and energetics of the ring are difficult to interpret within the framework of the progenitor's stellar mass loss or the subsequent evolution of the supernova remnant. We suggest instead that a dust-free cavity was produced in the magnetar environment by the giant flare emitted by the source in August 1998. Considering the total energy released in the flare, the theoretical dust-destruction radius matches well with the observed dimensions of the ring. We conclude that SGR 1900+14 is unambiguously associated with a cluster of massive stars, thereby solidifying the link between magnetars and massive stars."
Sources: Rochester Institute of Technology news release, May 28, 2008; and various websites
You'll find related stories by following the links below.