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David Helfand Presents "New Windows" on Birth and Death in the Milky Way

Peering through a veil of gas and dust to the far side of the galaxy, astronomers are presenting a striking new view of stellar birth and death in the Milky Way. Speaking on Jan. 9 at the annual meeting of the American Astronomical Society in Washington, DC, Professor David J. Helfand of Columbia University presented radio and X-ray images of dozens of new star birthsites and stellar mortuaries, which promise to provide a complete census of the most massive stars in the galaxy.

For astronomers studying such diverse subjects as the formation of black holes, the chemical enrichment of the galaxy and the continuous transformation of gas to stars and back again, the new images and catalogs offer important new constraints on models for the galaxy's ecosystem and the evolution of its constitutents.

The new survey of the Milky Way is the most sensitive ever conducted, using the world's most powerful radio telescope, the Very Large Array in New Mexico, coupled with the largest X-ray telescope ever built, the European Space Agency's XMM-Newton Observatory. While much of the galaxy is invisible to even the largest optical telescopes as a result of vast dusty clouds suffusing the galactic disk, both radio waves and energetic X-radiation can penetrate these clouds, giving astronomers a clear view to the edge of the Milky Way, over 60,000 light years away. And although stars spend most of their lives pumping out energy in the easily obscured visible part of the spectrum, the violent events surrounding their formation and demise generate copious radiation in the radio and X-ray bands, making these channels particularly well-suited for a census of birth and death in the galaxy.

Helfand is working with two of his students, Elise Laird and Alyson Brooks, and colleagues Robert H. Becker of the University of California at Davis and Richard L. White of the Space Telescope Science Institute, as well as a team at Leicester University in the United Kingdom headed by Bob Warwick and Mike Watson. Helfand has already mapped a 13-degree swath along the central plane of the galaxy with the VLA at wavelengths of 20 cm and 90 cm. The radio images have a sensitivity more than a factor of 30 better than previous maps, as well as providing 10 times the image sharpness. Fifteen X-ray images in the same region, each covering an area about the size of the full moon, have already been obtained, and several dozen more are scheduled for observation in the coming months. Combining the two datasets provides a perfectly matched filter for discovering the sites of stars that have explosively self-destructed, while a comparison of the radio maps with recently released infrared images obtained by the Air For ce's MSX satellite show several hundred locations where massive stars are forming today.

Most of the stars in the Milky Way are similar in size and mass to our Sun. While these stars dominate the stellar population, they are largely passive spectators to the dramatic events shaping the composition and structure of the galaxy, events controlled by the most massive stars. These stellar Leviathans, with masses from 10 to 100 times that of the Sun, produce all 84 chemical elements heavier than oxygen (and much of the carbon and oxygen as well). This freshly made material spews forth in the titanic explosions that mark the stars deaths in supernovae. These stars are also the parents of all of the neutron stars and black holes in the galaxy, and they dominate the structure of interstellar space, sculpting it with powerful winds and searing radiation during their lifetimes, and blasting it aside as they expire. Given their dominant role in regulating the formation of new stellar generations and driving the chemical enrichment of the galaxy, it is essential to understand fully the demographics of mas sive stars if we are to build successful models for our galaxy's evolution.

The new survey is designed to provide a complete picture of this crucial population. Stars are born in cocoons of dust and gas which are heated and ionized, respectively, when a new star turns on. The warming dust radiates infrared light, while the hydrogen atoms, stripped of their only electrons, glow brightly in the radio band. By matching the IR and radio images, the location of new massive stars are immediately revealed. Helfand showed examples of isolated objects containing perhaps only one or two stars and large complexes in which hundreds of stars are required to explain the energy output. Over 350 discrete regions, most identified for the first time, are seen in the surveyed region which, to date, covers only 15 percent of the inner galaxy.

The explosive demise of massive stars also leaves its mark on the new images. The outer layers of the star, expanding at 20,000 miles per second, sweep up and heat interstellar material to temperatures of 10-20 million degrees, as well as accelerating some of the particles to nearly the speed of light. The result is a glowing ring in the radio sky filled with X-ray emitting hot gas. In addition to the eight supernova remnants previously known in this region, the new maps show dozens of radio arcs and circles marking the sites of distant explosions. In one instance, a blob of radio emission thought to be driven by an unseen spinning neutron star shows the unmistakable signs of such a central engine in the luminous X-rays emanating from its core. Despite covering less than a quarter of the region so far, the X-ray images reveal over 200 new sources of high-energy radiation. Some of these are distant quasars seen through the murk of the galaxy and others are nearby solar-type stars with greatly enhanced suns pot activity. But the team has shown that those two known populations of X-ray sources cannot account for number of objects seen, strongly suggesting that white dwarfs, neutron stars and black holes tearing matter from their companion stars are responsible for the remainder of the objects detected.

"The clarity with which these new images reveal the formation and destruction of the most massive stars in the galaxy gives us confidence that a complete census is possible," Helfand said. "These data, all of which will be publicly accessible over the Web, will provide an important resource for astronomers around the world as they build ever more precise models of supernovae, star formation and the chemical evolution of the Milky Way."

The Very Large Array is operated by the National Radio Astronomy Observatory with support from the National Science Foundation. The XMM-Newton Observatory is a project of the European Space Agency. The authors' work is supported in part by grants from the National Aeronautic and Space Administration, the National Science Foundation, the Department of Energy and the British Particle Physics and Astrophysics Research Council.

The images of regions of the Milky Way below are each about the size of the full Moon. The dark splotches are regions in which massive stars are being born, and the colored objects are the remnants of supernova explosions which mark the deaths of these same types of stars. The radio, infrared, and X-ray imaging of the same region allows us to separate for the first time these different types of sources in a systematic manner all along the midplane of the Galaxy.

At least four new supernova remnant shells are evident in this small patch of the Galaxy (colored arcs).

In the lower right is the remnant of an explosion which took place 60,000 years ago, clear across the Galaxy at a distance of over 60,000 light years. The light only reached Earth 725 years ago, marking this as one of the youngest such objects in the Galaxy. Last year, Columbia astronomers discovered a rapidly spinning neutron star inside this remnant which allowed us to date it. In the upper left is a large complex of star formation regions.

The bright supernova remnant in the lower left is apparent in the radio/Infrared image. Our X-ray image also detects this source for the first time. At the lower right is a complex of star forming regions interspersed with what appears to be old supernova remnant shells.

Published: Jan 09, 2002
Last modified: Sep 18, 2002

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