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Debris From Massive Star Turns Into a Laboratory for Atomic Physics

In 1600 A.D., some nine years before Galileo looked through the first astronomical telescope, medieval astronomers noted a sudden brightening of one of the stars in the summer constellation of Cygnus the Swan. They did not understand at the time that they were witnessing the last stages in the life of a massive star. The star, P Cygni, had just ejected a huge shell of material into space, a phenomenon that mankind has observed only twice in recorded history. These shell ejections are the final stages in the evolution of a massive star on its way to becoming a supernova, which occurs when the core of the massive star collapses and destroys the star in a huge explosion that is bright enough to be visible across most of the visible universe.

Astronomers Nathan Smith (Colorado) and Patrick Hartigan (Rice) were studying the debris left over from the 1600 A.D. eruption of P Cygni with telescopes at Kitt Peak Observatory near Tucson AZ when they discovered that the shell emits dozens of spectral lines of ionized iron in the near-infrared. These lines provide important data concerning the mass, density, composition, and dynamics of the outburst which help constrain how these stars deposit material back into the interstellar medium, out of which new stars will someday form. The new data made it possible to separate the fast-moving shell from the light of the star (see figure), which in turn made it possible to determine the age of the shell, learn how it moves and identify shock waves in the flow. But the large number of iron lines enabled a completely different, and unexpected, area of research to proceed.

"We suddenly realized we were looking at a huge laboratory for atomic physics", Hartigan explained. "None of the atomic parameters of these emission lines can be measured in Earth-based labs because the densities are too high even in the best vacuums for the lines to be visible. But in the vastness of space, there is enough low density material for the lines to be quite bright." Using the observed brightness of the emission lines, Smith and Hartigan were able to measure the first transition probabilities for the lines, which differ significantly in some cases from difficult theoretical estimates. The new atomic parameters for the iron emission lines are extremely important for many nebular objects, because they make it possible to determine how dust absorbs light as the light passes through the interstellar medium. "Without the atomic parameters it is impossible to interpret what we observe", said Hartigan. "These results will finally enable us to study dusty regions like those around newly-formed stars, and understand better how the disks and outflows in these objects behave."

 

feshell.gif
The Iron Shell of P Cygni. The dark band at the center of the image is the spectrum of the star. The shell appears offset from the star both along the vertical axis (distance) and the horizontal axis (velocity). The size and speed of the shell imply that it was ejected in the 1600 A.D. outburst.