An international team of astronomers reports the discovery of a star, HE1327-2326, which sets a new record for being the most heavy element-deficient star ever found. Its chemical composition, as measured with the Subaru Telescope High Dispersion Spectrograph, provides evidence of nucleosynthesis by the first generations of stars in the universe, and places new constraints on their masses and metal enrichment history in the very early universe.
The first generation of stars are believed to have formed several hundred million years after the Big Bang, which occurred almost 14 billion years ago. These stars were part of the transition from a universe that consisted only of hydrogen and helium gas to one that contains a variety of elements and objects including stars and galaxies (Note 1). Recent theoretical studies of the first stars to form in the universe suggest the formation of super-massive stars (several hundred times heavier than the Sun and not seen in the present-day Milky Way galaxy). In addition, the theories do not predict the formation of low-mass stars like the Sun in the early universe. There is, however, no clear observational evidence for these predictions to date.
One approach to this problem is to investigate very old stars in our galaxy. These contain only small amounts of heavy elements, in particular iron. Their abundance patterns constrain the nucleosynthesis models of first-generation stars and their mass distribution. First-generation low-mass stars, which contain essentially no heavy elements, may also be found among such iron-deficient stellar populations (Note 2).
The astronomers conducting the observational program focused on these very old stars (Note 3) discovered that HE1327-2326 (Note 4) had the lowest iron abundance ever seen. It was first identified as a metal-poor candidate through the Hamburg/ESO survey, carried out with the European Southern Observatory 1.5-meter Schmidt Telescope. The star’s extremely low abundance of heavy elements was measured through spectroscopy with the ESO 3.6-meter telescope. The Subaru observation using the High Dispersion Spectrograph (HDS), coupled with photometry from the MAGNUM Telescope, revealed that the star’s iron abundance is only 1/250,000 that of the Sun, but the carbon and nitrogen abundance ratios relative to iron are remarkably high. These are common properties with another iron-deficient star, HE0107-5240, which was found in 2001. This result suggests that the metal-enrichment histories of these two stars are quite different from that of other low-metal stars. The elemental abundance pattern of HE1327-2326 measured with Subaru/HDS, and comparisons with that of HE0107-5240, provide new understanding of the nucleosynthesis of first-generation stars and their formation processes (Note 5).
A possible scenario to explain the chemical abundance patterns of these stars is to assume the existence of “peculiar” supernovae that provided only small amounts of heavy elements like iron. In this case, then, the star we are currently observing should be a “second generation” star “seeded” with heavy elements by a first-generation supernova. Supernova models proposed by astronomers in the University of Tokyo explain the chemical abundance patterns of the two objects (Note 6). According to these models, the progenitors were not supermassive stars, but stars with several tens of solar masses.
An alternative possibility is that HE1327-2326 is a first-generation star formed from the initial gas component of the very early universe. If so, then the heavy elements found in this object could be the result of pollution by interstellar matter containing heavy elements. Yet another process is required to explain the high abundances of light elements such as carbon (Note 7).
Although the chemical abundance patterns of the star discovered by the present study is not yet completely understood, the abundances observed with the Subaru Telescope provide strong constraints on the formation scenarios of most iron-deficient stars. Further detailed observations of this object, as well as theoretical studies on stellar evolution and formation, will promote our understanding of the characteristics of the first stars in the universe.
These results will be published in the April 15, 2005, issue of Nature.
[Notes mentioned in the press release can be found at the website mentioned above.]
The full text, supplementary images and notes are available on the following web page: http://subarutelescope.org/PressRelease/2005/04/13/index.html
