Carbon and oxygen found around galactic bulge stars

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 16 March, 2009

The Spitzer Space Telescope has detected rare evidence for both carbon and oxygen in the dust surrounding stars in the centre of the Milky Way.

“Carbon and oxygen cannot be produced simultaneously, so the only way for a star to produce both kinds of molecules must be due to a change of chemistry in the central star,” astrophysicist Matthew Bobrowsky of the University of Maryland tells Astronomy Now.

A diagram of the Milky Way showing the location of key features such as the Sun, spiral arms and central bulge stars. Image: ESO.

As a star progresses through its life, burning hotter and hotter, hydrogen gas is converted through nuclear fusion to helium and to progressively heavier elements. The heaviest elements fuse within the burning core of the star and are only exposed to the surface at the end of its life. “The big bang produced only hydrogen and helium,” says Bobrowsky. “Heavier elements like carbon and oxygen only come from getting ‘cooked up’ in stars. Nuclear reactions in stars created the heavier elements found in ‘life as we know it’.”

For stars the size of our Sun, carbon atoms are expelled, along with hydrogen and helium, in the last 50,000 years of their 10 billion year life times. These atoms form a cloud of gas around the star, known as a planetary nebula, that eventually disperses into space to become recycled into new stars, planets or even life-giving ingredients on an Earth-like planet.

For much larger stars, the heavy ingredients, known as metals and including all elements heavier than hydrogen and helium, are thrown off in powerful supernovae explosions. These heavy elements on Earth were created by nuclear fusion reactions in previous generations of stars that expelled those elements into space. Our Solar System formed out of that gas containing all the heavy elements that we now find in Earth and in life on Earth.

Studying the chemistry of galactic bulge stars helps scientists learn how the matter that makes up our Earth and other planets in our Galaxy left its stellar birthplaces long ago. Image: NASA/JPL-Caltech/T.Pyle (SSC).

Using the Spitzer Space Telescope, astronomers peered into the ‘galactic bulge’ of our Milky Way to observe 26 stars and their surrounding planetary nebulae. Stars in the centre of the Milky Way are old and metal-rich with a high abundance of heavy elements. The researchers measured the light emitted by the stars and the surrounding dust and were able to identify carbon compounds based on the wavelengths of light emitted by the stars in 21 of the dust clouds. Furthermore, oxygen was also found in the same clouds, revealing a surprising mixture of ingredients for space dust.

“The only way for a star to produce both kind of molecules or dust grains is as a consequence of a change of chemistry in the central star,” says Bobrowsky. A natural way to produce a mixed chemistry environment around a star is by getting a lot of oxygen rich dust produced in the past locked in a 'circum-binary disc'. This usually occurs in binary stars that later become carbon-rich and produce the carbon-rich material that is observed in the outflow together with the oxygen-rich material present in the disc. “But in the galactic bulge planetary nebulae, there is no indication of binary stars, and we know that the change of chemistry must have taken place very recently as there are no Asymptotic Giant Branch (AGB) stars in the bulge showing any indication of carbon excess in their atmospheres.”

AGB defines the period of stellar evolution undertaken by all low to intermediate mass stars late in their life. These stars have an inert core of carbon and oxygen surrounded by a helium shell, which in turn, is surrounded by a hydrogen shell. Most of the time, fusion of hydrogen is the main source of energy for the stars in this evolutionary phase. “But occasionally, this thin helium layer may become active, that is, start fusing helium to carbon every few thousand years, compared to a total AGB lifetime of ~100,000 years,” says Bobrowsky. “This short phase of helium fusion is known as a 'thermal pulse' and is usually very efficient in bringing processed material from the inner layers of the star to the surface in a process called 'dredge-up'.”

In Bobrowsky and colleagues’ sample of galactic bulge stars, they have reason to believe that the change in chemistry occurred very recently, as a result of such a thermal pulse. “If we want to understand how our Galaxy, and the stars, planets and life in it, came to be the way they are, we need to understand the creation of the chemical elements of which they are composed,” he concludes.

A paper describing the results appears in the current issue of the journal Astronomy and Astrophysics.