Nucleosynthetic signatures of the first stars

The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium present within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models have been suggested to explain the origin of the abundance pattern of the giant star HE0107-5240, which hitherto exhibited the highest deficiency of heavy elements known. Here we report the discovery of HE1327-2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE0107-5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE1327-2326, however, challenge existing theoretical understanding: no model predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars.     

核心坍缩超新星中心黑洞超吸积在星系尺度上的贡献

核心坍缩超新星(Core-Collapse Supernova,CCSN)是大质量恒星演化末期的爆发现象,产生了宇宙中大多数的中子星和恒星级黑洞等致密天体.爆发可能伴随着强磁场中子星或黑洞超吸积引发的剧烈长时标伽马射线暴.CCSN还被认为是宇宙重元素的主要来源之一.本综述介绍了我们近期对CCSN中心黑洞超吸积过程的系列研究成果,主要包括研究了大质量星系中心附近伽马射线暴余辉阶段,因大量暗物质粒子湮灭电子注入而引发的光变和能谱的形态变化,探讨了其作为暗物质探测手段的可能性;研究了坍缩星框架下,中微子主导吸积流外流对核合成的贡献,及对太阳临近空间、(活动)星系等化学组分和演化的影响;最后,从数值模拟角度讨论了CCSN起源的致密天体质量分布,给出了低质量间隙可能起源于CCSN爆发能量分布的结论.     

The formation of the first low-mass stars from gas with low carbon and oxygen abundances

The first stars in the Universe are predicted to have been much more massive than the Sun. Gravitational condensation, accompanied by cooling of the primordial gas via molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass it undergoes a slow, quasi-hydrostatic contraction without further fragmentation; lower-mass stars cannot form. Here we show that as soon as the primordial gas--left over from the Big Bang--is enriched by elements ejected from supernovae to a carbon or oxygen abundance as small as approximately 0.01-0.1 per cent of that found in the Sun, cooling by singly ionized carbon or neutral oxygen can lead to the formation of low-mass stars by allowing cloud fragmentation to smaller clumps. This mechanism naturally accommodates the recent discovery of solar-mass stars with unusually low iron abundances (10(-5.3) solar) but with relatively high (10(-1.3) solar) carbon abundance. The critical abundances that we derive can be used to identify those metal-poor stars in our Galaxy with elemental patterns imprinted by the first supernovae. We also find that the minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background.     

Measurement of stellar age from uranium decay

The ages of the oldest stars in the Galaxy indicate when star formation began, and provide a minimum age for the Universe. Radioactive dating of meteoritic material and stars relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232Th (half-life 14 Gyr) has been used to date Galactic stars, but it decays by only a factor of two over the lifetime of the Universe. 238U (half-life 4.5 Gyr) is in principle a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously not been detected in stars. Here we report a measurement of this line in the very metal-poor star CS31082-001, a star which is strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H) = -13.7 +/- 0.14 +/- 0.12 yields an age of 12.5 +/- 3 Gyr, though this is still model dependent. The observation of this cosmochronometer gives the most direct age determination of the Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly precise lower limit to the age of the Universe.     

通过贫金属星研究宇宙的增丰历史

贫金属星对于我们了解元素起源和宇宙的化学演化具有关键意义。本文将描述这些稀有天体的发现历史,分析它们的光谱确定其表面化学丰度的方法,以及如何利用它们的丰度模式来阐释其起源与演化。     

Imprints of fast-rotating massive stars in the Galactic Bulge

The first stars that formed after the Big Bang were probably massive, and they provided the Universe with the first elements heavier than helium ('metals'), which were incorporated into low-mass stars that have survived to the present. Eight stars in the oldest globular cluster in the Galaxy, NGC?6522, were found to have surface abundances consistent with the gas from which they formed being enriched by massive stars (that is, with higher α-element/Fe and Eu/Fe ratios than those of the Sun). However, the same stars have anomalously high abundances of Ba and La with respect to Fe, which usually arises through nucleosynthesis in low-mass stars (via the slow-neutron-capture process, or s-process). Recent theory suggests that metal-poor fast-rotating massive stars are able to boost the s-process yields by up to four orders of magnitude, which might provide a solution to this contradiction. Here we report a reanalysis of the earlier spectra, which reveals that Y and Sr are also overabundant with respect to Fe, showing a large scatter similar to that observed in extremely metal-poor stars, whereas C abundances are not enhanced. This pattern is best explained as originating in metal-poor fast-rotating massive stars, which might point to a common property of the first stellar generations and even of the 'first stars'.     

Tracing the first stars with fluctuations of the cosmic infrared background

The deepest space- and ground-based observations find metal-enriched galaxies at cosmic times when the Universe was less than 1 Gyr old. These stellar populations had to be preceded by the metal-free first stars, known as 'population III'. Recent cosmic microwave background polarization measurements indicate that stars started forming early--when the Universe was < or =200 Myr old. It is now thought that population III stars were significantly more massive than the present metal-rich stellar populations. Although such sources will not be individually detectable by existing or planned telescopes, they would have produced significant cosmic infrared background radiation in the near-infrared, whose fluctuations reflect the conditions in the primordial density field. Here we report a measurement of diffuse flux fluctuations after removing foreground stars and galaxies. The anisotropies exceed the instrument noise and the more local foregrounds; they can be attributed to emission from population III stars, at an era dominated by these objects.     

A stellar relic from the early Milky Way

The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts; if totally metal-free ('population III') stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun, leading to the suggestion that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist--that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.     

Supernova explosions in the Universe

During the lifetime of our Milky Way galaxy, there have been something like 100 million supernova explosions, which have enriched the Galaxy with the oxygen we breathe, the iron in our cars, the calcium in our bones and the silicon in the rocks beneath our feet. These exploding stars also influence the birth of new stars and are the source of the energetic cosmic rays that irradiate us on the Earth. The prodigious amount of energy (approximately 10(51), or approximately 2.5 x 10(28) megatonnes of TNT equivalent) and momentum associated with each supernova may even have helped to shape galaxies as they formed in the early Universe. Supernovae are now being used to measure the geometry of the Universe, and have recently been implicated in the decades-old mystery of the origin of the gamma-ray bursts. Together with major conceptual advances in our theoretical understanding of supernovae, these developments have made supernovae the centre of attention in astrophysics.     

Low-mass relics of early star formation

The earliest stars to form in the Universe were the first sources of light, heat and metals after the Big Bang. The products of their evolution will have had a profound impact on subsequent generations of stars. Recent studies of primordial star formation have shown that, in the absence of metals (elements heavier than helium), the formation of stars with masses 100 times that of the Sun would have been strongly favoured, and that low-mass stars could not have formed before a minimum level of metal enrichment had been reached. The value of this minimum level is very uncertain, but is likely to be between 10(-6) and 10(-4) that of the Sun. Here we show that the recent discovery of the most iron-poor star known indicates the presence of dust in extremely low-metallicity gas, and that this dust is crucial for the formation of lower-mass second-generation stars that could survive until today. The dust provides a pathway for cooling the gas that leads to fragmentation of the precursor molecular cloud into smaller clumps, which become the lower-mass stars.     

Accretion of low-metallicity gas by the Milky Way

Models of the chemical evolution of the Milky Way suggest that the observed abundances of elements heavier than helium ('metals') require a continuous infall of gas with metallicity (metal abundance) about 0.1 times the solar value. An infall rate integrated over the entire disk of the Milky Way of approximately 1 solar mass per year can solve the 'G-dwarf problem'--the observational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range. This infall dilutes the enrichment arising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with time. However, in other spiral galaxies, the low-metallicity gas needed to provide this infall has been observed only in associated dwarf galaxies and in the extreme outer disk of the Milky Way. In the distant Universe, low-metallicity hydrogen clouds (known as 'damped Ly alpha absorbers') are sometimes seen near galaxies. Here we report a metallicity of 0.09 times solar for a massive cloud that is falling into the disk of the Milky Way. The mass flow associated with this cloud represents an infall per unit area of about the theoretically expected rate, and approximately 0.1-0.2 times the amount required for the whole Galaxy.     

First-generation black-hole-forming supernovae and the metal abundance pattern of a very iron-poor star

It has been proposed theoretically that the first generation of stars in the Universe (population III) would be as massive as 100 solar masses (100 M(O)), because of inefficient cooling of the precursor gas clouds. Recently, the most iron-deficient (but still carbon-rich) low-mass star--HE0107-5240--was discovered. If this is a population III star that gained its metals (elements heavier than helium) after its formation, it would challenge the theoretical picture of the formation of the first stars. Here we report that the patterns of elemental abundance in HE0107-5240 (and other extremely metal-poor stars) are in good accord with the nucleosynthesis that occurs in stars with masses of 20-130 M(O) when they become supernovae if, during the explosions, the ejecta undergo substantial mixing and fallback to form massive black holes. Such supernovae have been observed. The abundance patterns are not, however, consistent with enrichment by supernovae from stars in the range 130-300 M(O). We accordingly infer that the first-generation supernovae came mostly from explosions of approximately 20-130 M(O) stars; some of these produced iron-poor but carbon- and oxygen-rich ejecta. Low-mass second-generation stars, like HE0107-5240, could form because the carbon and oxygen provided pathways for the gas to cool.     

A probable stellar solution to the cosmological lithium discrepancy

The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed, but found experimentally not to be viable. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time, which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing. We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star.     

Molecular gas in the host galaxy of a quasar at redshift z = 6.42

Observations of molecular hydrogen in quasar host galaxies at high redshifts provide fundamental constraints on galaxy evolution, because it is out of this molecular gas that stars form. Molecular hydrogen is traced by emission from the carbon monoxide molecule, CO; cold H2 itself is generally not observable. Carbon monoxide has been detected in about ten quasar host galaxies with redshifts z > 2; the record-holder is at z = 4.69 (refs 1-3). Here we report CO emission from the quasar SDSS J114816.64 + 525150.3 (refs 5, 6) at z = 6.42. At that redshift, the Universe was only 1/16 of its present age, and the era of cosmic reionization was just ending. The presence of about 2 x 1010 M\circ of H2 in an object at this time demonstrates that molecular gas enriched with heavy elements can be generated rapidly in the youngest galaxies.     

A very energetic supernova associated with the gamma-ray burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (>2 s) gamma-ray bursts (GRBs)-the most luminous of all astronomical explosions-signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the 'collapsar' model.     

弱s过程分量系数C2的研究

利用贫金属星中子俘获元素丰度的模型计算了30颗样品星的分量系数,并详细分析了弱s过程分量系数C2小于零的各种可能原因。     

Discovery of three lead-rich stars

About half of the stable nuclei heavier than iron are believed to be synthesized during the late stages of evolution of stars with masses in the range 0.8-8 solar masses. These elements are then expelled into the interstellar medium through stellar winds after being 'dredged up' towards the surface of the stars. These processes occur when the star is in the 'asymptotic giant branch' (AGB) phase of its life. Nuclei (mainly iron) deep inside the star slowly capture neutrons and progressively build up heavier elements (the 's-process'). For AGB stars that formed early in the history of the Galaxy, and that therefore have very low abundances of elements heavier than helium ('metals'), models predict that the s-process will accumulate synthesized material with atomic weights in the Pb-Bi region. Such stars will therefore have large overabundances of lead relative to other heavy elements. Here we report the discovery of large amounts of lead in three metal-poor stars (HD187861, HD196944 and HD224959). Our analysis shows that these stars are more enriched in lead than in any other element heavier than iron. The excellent agreement between the observed and predicted abundances reinforces our current understanding of the detailed operation of the s-process deep in the interiors of AGB stars.     

The star-formation history of the Universe from the stellar populations of nearby galaxies

The determination of the star-formation history of the Universe is a key goal of modern cosmology, as it is crucial to our understanding of how galactic structures form and evolve. Observations of young stars in distant galaxies at different times in the past have indicated that the stellar birthrate peaked some eight billion years ago before declining by a factor of around ten to its present value. Here we report an analysis of the 'fossil record' of the current stellar populations of 96,545 nearby galaxies, from which we obtained a complete star-formation history. Our results broadly support those derived from high-redshift galaxies. We find, however, that the peak of star formation was more recent--around five billion years ago. We also show that the bigger the stellar mass of the galaxy, the earlier the stars were formed, which indicates that high- and low-mass galaxies have very different histories.     

A supernova origin for dust in a high-redshift quasar

Interstellar dust plays a crucial role in the evolution of the Universe by assisting the formation of molecules, by triggering the formation of the first low-mass stars, and by absorbing stellar ultraviolet-optical light and subsequently re-emitting it at infrared/millimetre wavelengths. Dust is thought to be produced predominantly in the envelopes of evolved (age >1 Gyr), low-mass stars. This picture has, however, recently been brought into question by the discovery of large masses of dust in the host galaxies of quasars at redshift z > 6, when the age of the Universe was less than 1 Gyr. Theoretical studies, corroborated by observations of nearby supernova remnants, have suggested that supernovae provide a fast and efficient dust formation environment in the early Universe. Here we report infrared observations of a quasar at redshift 6.2, which are used to obtain directly its dust extinction curve. We then show that such a curve is in excellent agreement with supernova dust models. This result demonstrates a supernova origin for dust in this high-redshift quasar, from which we infer that most of the dust at high redshifts probably has the same origin.     

恒星典型中子俘获元素丰度观测误差对分量系数的影响

根据贫金属星中子俘获元素丰度的计算模型，研究了典型中子俘获元素丰度的观测误差对代表各核合成过程相对贡献的分量系数的影响，给出了分量系数的变化范围。