Subaru Telescope, NAOJ

Discovery of the Most Metal-deficient Star Ever Found: Studying Nucleosynthesis Signatures of the First Stars

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 14, 2005, issue of Nature.

		Figure 1 : The position of HE1327-2326 on a constellation chart

		Figure 2 : A color image of HE1327-2326 with the MAGNUM Telescope. The MAGNUM photometry data were used to estimate the temperature of this star. The background is a color composite of DSS images (STScI and AAO/ROE).

		Figure 3 : Spectra of HE1327-2326 and the Sun. The upper panel shows low-resolution spectra covering the optical wavelength range, with a spectral image of the Sun. The lower panel shows an ultraviolet range spectrum of HE1327-2326 obtained with the Subaru Telescope, comparing with a solar one. There are few absorption features of Fe and CH molecule in HE1327-2326, while the solar spectrum is covered by numerous absorption lines by a variety of elements.

		Figure 4 : Two scenarios proposed for the chemical composition of HE1327-2326. The first stars formed from the gas containing only H and He remained after the Big Bang. In the first scenario, the supernova explosion by the first generation massive stars (2A) polluted the interstellar material with heavy elements (3A), and then low-mass stars, including HE1327-2326, have formed (4A). In this case, a key problem is understanding how the supernova models reproduce the chemical composition of HE1327-2326. An alternative scenario is that a binary system containing the low mass star HE1327-2326 formed from gas containing no heavy elements. The slightly massive companion synthesizes light elements like carbon and the material is transferred to the secondary star HE1327-2326 (2B). The primary star has already evolved to a faint white dwarf. The small amounts of heavy elements like iron in HE1327-2326 are explained by the accretion from interstellar matter later (3B). In this case, formation of low-mass stars in the first generation is indicated.

(Note 1)
The current standard models predict the formation of massive (and/or super-massive) stars in advance of formation of larger structures like galaxies. The first generation massive stars are expected to be important sources of ultraviolet photons that re-ionized the universe after atoms had first formed. These massive stars would also have contributed to the metal enrichment in the earliest stages, and have affected the formation of next generations of stars.

(Note 2)
Since the lifetime of massive stars are at most several million years, the first ones have already terminated their lives through supernova explosions. However, they provided newly synthesized heavy elements in the surrounding interstellar gas, from which low-mass stars would have formed. These lower-mass stars could survive until now because their lifetimes can be as long as the age of the universe. Searches for such old, low-mass stars, and follow-up investigations of elemental abundances, enable us to characterize nucleosynthesis in the first generation of massive stars, which will constrain their mass distribution. Moreover, if low-mass stars have directly formed from the initial gas remaining after the Big Bang, they will be found as stars containing only hydrogen and helium. Until 2001, several stars with iron abundances about 1/10,000 of the Sun were known. The absence of objects with lower iron abundances had been regarded as possible evidence that no stars formed from the initial primordial gas. However, the star HE0107-5240, discovered in 2001, has an iron abundance more than an order of magnitude lower than previously known stars (ESO press release). Several scenarios have been proposed to interpret this object. One model suggested that this object is a first-generation star, and its heavy elements accreted from the interstellar matter. Another model proposes that this star is a second-generation star affected by peculiar supernovae that have yielded only small amount of iron group elements. These models are still being debated.

(Note 3)
This study is a collaboration of astronomers in National Astronomical Observatory of Japan, University of Tokyo, Hokkaido University, Tokai University, Australian National University, Hamburg University (Germany), Michigan State University (USA), Uppsala Astronomical Observatory (Sweden), and The Open University (UK)

(Note 4)
The distance to HE1327-2326 has not yet been measured, but is estimated to be at most 4,000 light-years away. The age and mass of this object is also unknown, but the deficiency of heavy elements indicates that this star was born in the very early universe, suggesting that its age is about 13 billion years and its mass is slightly lower than that of the Sun.

(Note 5)
An important difference between HE1327-2326 and HE0107-5240 is the evolutionary status: while the latter is an aging red giant star, the former is still an unevolved star, indicating essentially no effect of its internal nuclear processes. This clearly excludes the possibility that carbon and nitrogen are produced inside the object itself. Some important differences of the chemical abundance ratios also exist between the two objects, such as the magnesium/iron abundance ratio. The high abundance of strontium (an element heavier than iron) found in HE1327-2326 is an unexpected result, and provides a new probe to investigate the production of heavy elements in the early universe.

(Note 6)
The model was proposed by Umeda and Nomoto (2003, Nature 422, 871). Their model explains the differences of the abundance ratios like magnesium/iron between HE1327-2326 and HE0107-5240.

(Note 7)
A possible mechanism to provide lighter elements like carbon is the nucleosynthesis in an intermediate-mass star (several solar masses) that is a companion of the star the research team is currently observing. When the primary star reaches the latest stages of evolution, material contaminated by the yields of nucleosynthesis inside it might have moved to the surface of the secondary star. The primary star has already evolved to become a faint white dwarf that is not currently observable. Such material transfer is known in many binary systems, although there is still no signature of binarity for HE1327-2326.

April 13, 2005