arXiv subjects

Y. -Z. Qian

Publications and source records attributed to Y. -Z. Qian.

At least 37 records · Page 2Linked to original sources

arXiv2003

Neutron star mergers versus core-collapse supernovae as dominant r-process sites in the early Galaxy

The astrophysical nature of r-process sites is a long standing mystery and many probable sources have been suggested in the past, among them lower-mass core-collapse supernovae (in the range 8 - 10 Msol), higher-mass core-collapse supernovae (with masses > 20 Msol) and neutron star mergers. In this work, we present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major r-process sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. We conclude that, due to the lack of reliable iron and r-process yields as function of progenitor mass, it is not possible at present to distinguish between the lower-mass and higher-mass supernovae scenarios within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as the dominant r-process source, since their low rates of occurrence would lead to r-process enrichment that is not consistent with observations at very low metallicities. Additionally, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in [r-process/Fe] ratios at later times which is much too large compared to observations.

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arXiv2003

Stellar Sources for Heavy r-Process Nuclei

(Abridged) The stellar sites and the complete mechanism of r-process nucleosynthesis are still unresolved issues. From consideration of the observed abundances in metal-poor stars, it is proposed that the production of the heavy r-process nuclei (with mass numbers A>130) is not related to the production of the Fe group elements or of the elements with lower atomic numbers: Na, Mg, Al, Si, Ca, Sc, and Ti. This requires that the production of the heavy r-nuclei not occur in supernovae with extended shell structure, but be associated with either bare neutron stars or SNe II in the range of 8-10 solar masses. From the observations of stars with [Fe/H]=-3 but with high abundances of r-elements, it is clear that these r-process enrichments cannot represent the composition of the interstellar medium from which the stars were formed, but must represent very local contamination from binary companions. Further evidence for very high enrichments of s-process elements in metal-poor stars also requires binary systems for explanation. We propose that the accretion-induced collapse (AIC) of a white dwarf into a neutron star in a binary system may be associated with the production of the heavy r-nuclei and may provide occasional coupling of high r-process and high s-process enrichments in the envelopes of low-mass stars with low [Fe/H]. If we assume that the bulk of the heavy r-nuclei are produced in AIC events, then these events would have produced about 1.6 billion neutron stars in the Galaxy.

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arXiv2003

Abundance Analysis of HE2148-1247, A Star With Extremely Enhanced Neutron Capture Elements

Abundances for 27 elements in the very metal poor dwarf star HE2148-1247 are presented, including many of the neutron capture elements. We establish that HE2148-1247 is a very highly s-process enhanced star with anomalously high Eu as well, Eu/H about half Solar, demonstrating the large addition of heavy nuclei at [Fe/H] = -2.3 dex. Ba and La are enhanced by a somewhat larger factor and reach the solar abundance, while Pb significantly exceeds it. Ba/Eu is ten times the solar r-process ratio but much less than that of the s-process, indicating a substantial r-process addition as well. C and N are also very highly enhanced. We have found that HE2148-1247 is a radial velocity variable. The C, N and the s-process element enhancements thus presumably were produced through mass transfer from a former AGB binary companion. The large enhancement of heavy r-nuclides also requires an additional source as this is far above any inventory in the ISM at such low [Fe/H]. We further hypothesize that accretion onto the white dwarf from the envelope of the star caused accretion induced collapse of the white dwarf, forming a neutron star, which then produced heavy r-nuclides and again contaminated its companion. (abridged)

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arXiv2002

The Prompt Inventory from Very Massive Stars and Elemental Abundances in Ly Alpha Systems

It has been proposed that very massive stars (VMSs) dominated heavy element production until a metallicity threshold corresponding to [Fe/H]=-3 was reached. This results in a prompt (P) inventory of elements, the abundances of which were determined from observations of Galactic halo stars with [Fe/H]=-3. We calculate Omega_P(E) from the P inventory for a large number of elements in the IGM. Using the available data on Omega(E_ion) for C IV, O VI, and Si IV in Ly alpha systems, we find that the ionization fractions calculated from Omega(E_ion)/Omega_P(E) are, within reasonable uncertainties, compatible with the values estimated from ionization models for Ly alpha systems. This agreement appears to hold from z=0.09 to 4.6, indicating that the bulk of the baryonic matter remains dispersed with a fixed chemical composition. We conclude that the P inventory was established in the epoch prior to z=4.6. The dispersal of processed baryonic matter to the general IGM is considered to be the result of energetic VMS explosions that disrupted baryonic aggregates until the metallicity threshold was reached to permit normal astration. The formation of most galaxies is considered to have occurred subsequent to the achievement of this metallicity threshold in the IGM.

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arXiv2002

Neutrino-Induced Fission and r-Process Nucleosynthesis

An r-process scenario with fission but no fission cycling is considered to account for the observed abundance patterns of neutron-capture elements in ultra-metal-poor stars. It is proposed that neutrino reactions play a crucial role in inducing the fission of the progenitor nuclei after the r-process freezes out in Type II Supernovae. To facilitate neutrino-induced fission, the proposed r-process scenario is restricted to occur in a low-density environment such as the neutrino-driven wind from the neutron star. Further studies to develop this scenario are emphasized.

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arXiv2001

Determination of Nucleosynthetic Yields of Supernovae and Very Massive Stars from Abundances in Metal-Poor Stars

(Abridged) We determine the yields of Na to Ni for Type II supernovae (SNe II) and the yield patterns of the same elements for Type Ia supernovae (SNe Ia) and very massive (>100 M_sun) stars (VMS) using a phenomenological model of stellar nucleosynthesis and the data on a number of stars with -4<[Fe/H]<-3, a single star with [Fe/H]=-2.04, and the sun. We consider that there are two distinct kinds of SNe II: the high-frequency SNe II(H) and the low-frequency SNe II(L). We also consider that VMS were the dominant first-generation stars formed from big bang debris. The yield patterns of Na to Ni for SNe II(H), II(L), and Ia and VMS appear to be well defined. It is found that SNe II(H) produce almost none of these elements, that SNe II(L) can account for the entire solar inventory of Na, Mg, Si, Ca, Ti, and V, and that compared with SNe II(L), VMS underproduce Na, Al, V, Cr, and Mn, overproduce Co, but otherwise have an almost identical yield pattern. A comparison is made between the yield patterns determined here from the observational data and those from ab initio models of nucleosynthesis in SNe II and VMS. The evolution of the other elements relative to Fe is shown to involve three distinct stages, the earliest of which is directly related to the problems of early aggregation and dispersion of baryonic matter. It is argued that the VMS contributions should represent the typical composition of dispersed baryonic matter in the universe.

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arXiv2001

A Model for Abundances in Metal-Poor Stars

It is argued that the abundances of r-process related elements in stars with -3<[Fe/H]<-1 can be explained by the contributions of three sources. The sources are: the first generations of very massive (>100 solar masses) stars that are formed from Big Bang debris and are distinct from SNII, and two types of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe (presumably leaving behind black holes). The L events are of low frequency and produce Fe and dominantly light (A<130) r-elements (essentially none above Ba). By using the observed abundances in two ultra-metal-poor stars and the solar r-abundances, the initial or prompt inventory of elements produced by the first generations of very massive stars and the yields of H and L events can be determined. The abundances of a large number of elements in a star can then be calculated from the model by using only the observed Eu and Fe abundances. To match the model results and the observational data for stars with -3<[Fe/H]<-1 requires that the solar r-abundances for Sr, Y, Zr, and Ba must be significantly increased from the standard values. Whether the solar r-components of these elements used here to obtain a fit to the stellar data can be reconciled with those obtained from solar abundances by subtracting the s-components calculated from models is not clear.

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arXiv2001

Abundances in the Uranium-Rich Star CS 31082-001

The recent discovery by Cayrel et al. of U in CS 31082-001 along with Os and Ir at greatly enhanced abundances but with [Fe/H]=-2.9 strongly reinforces the argument that there are at least two kinds of SNII sources for r-nuclei. One source is the high-frequency H events responsible for heavy r-nuclei (A>135) but not Fe. The H-yields calculated from data on other ultra-metal-poor stars and the sun provide a template for quantitatively predicting the abundances of all other r-elements. In CS 31082-001 these should show a significant deficiency at A<135 relative to the solar r-pattern. It is proposed that CS 31082-001 should have had a companion that exploded as an SNII H event. If the binary survived the explosion, this star should now have a compact companion, most likely a stellar-mass black hole. Comparison of abundance data with predicted values and a search for a compact companion should provide a stringent test of the proposed r-process model. The U-Th age determined by Cayrel et al. for CS 31082-001 is, to within substantial uncertainties, in accord with the r-process age determined from solar system data. The time gap between Big Bang and onset of normal star formation only allows r-process chronometers to provide a lower limit on the age of the universe.

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arXiv2001

Diverse Supernova Sources for the r-Process and Abundances in Metal-Poor Stars

The dispersion and mean trends of r-process abundances in metal-poor stars are discussed based on a model of diverse supernova sources for the r-process. This model is unique in that its key parameters are inferred from solar system data independent of stellar observations at low metallicities. It is shown that this model provides a good explanation for the observed dispersion and mean trend of Eu abundances over -3 < [Fe/H] < -1. It is also shown that this model provides a means to discuss r-abundances in general. For example, the Ag abundance in any metal-poor star with observed Eu and Fe abundances can be calculated from the model. This approach is demonstrated with success for two stars and can be further tested by future Ag data. The dispersion and mean trend of Ag abundances in metal-poor stars are also calculated for comparison with future observations.

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arXiv2000

Evolution of O Abundance Relative to Fe

We present a three-component mixing model for the evolution of O abundance relative to Fe, taking into account the contributions of the first very massive (> 100 solar masses) stars formed from Big Bang debris. We show that the observations of O and Fe abundances in metal-poor stars in the Galaxy by Israelian et al. and Boesgaard et al. can be well represented both qualitatively and quantitatively by this model. Under the assumption of an initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to result from the addition of O and Fe only from type II supernovae (SNII) to the prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result suggests that protogalaxies with low ``metallicities'' should exhibit high values of (O/Fe). The C/O ratio produced by the first very massive stars is expected to be much less than 1 so that all the C should be tied up as CO and that C dust and hydrocarbon compounds should be quite rare at epochs corresponding to [Fe/H] < -3.

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arXiv2000

A Model of Metallicity Evolution in the Early Universe

We apply the phenomenological model used to explain the abundances of Fe and r-process elements in very metal-poor stars in the Galaxy to [Fe/H] of damped Ly alpha systems. It is assumed that the first stars formed after the Big Bang were very massive and promptly enriched the interstellar medium to [Fe/H] ~ -3, at which metallicity formation of normal stars took over. Subsequent Fe enrichment was provided by Type II supernovae. The range of [Fe/H] at a given redshift z for damped Ly alpha systems is explained by the time t* after the Big Bang at which normal star formation started in an individual protogalactic system. The average t* is approx 80% the age of the universe for damped Ly alpha systems at z approx 1.5 to 4.5, indicating a long delay between the Big Bang and the turn-on of protogalaxies. It is inferred that a substantial fraction of the total baryonic matter may not have been aggregated into protogalaxies where normal star formation had occurred down to z ~ 1.5. The data near z = 2.2 suggest that the rate of turn-on of protogalaxies was initially very low and slowly reached a maximum at ~ 3 Gyr after the Big Bang. This may be important in understanding the rate of formation of quasars.

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arXiv2000

Supernovae versus Neutron Star Mergers as the Major r-Process Sources

I show that recent observations of r-process abundances in metal-poor stars are difficult to explain if neutron star mergers (NSMs) are the major r-process sources. In contrast, such observations and meteoritic data on Hf182 and I129 in the early solar system support a self-consistent picture of r-process enrichment by supernovae (SNe). While further theoretical studies of r-process production and enrichment are needed for both SNe and NSMs, I emphasize two possible direct observational tests of the SN r-process model: gamma rays from decay of r-process nuclei in SN remnants and surface contamination of the companion by SN r-process ejecta in binaries.

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arXiv1999

Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars

We present a model to explain the wide range of abundances for heavy r-process elements (mass number A > 130) at low [Fe/H]. This model requires rapid star formation and/or an initial population of supermassive stars in the earliest condensed clots of matter to provide a prompt or initial Fe inventory. Subsequent Fe and r-process enrichment was provided by two types of supernovae: one producing heavy r-elements with no Fe on a rather short timescale and the other producing light r-elements (A < or = 130) with Fe on a much longer timescale.

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arXiv1999

Sterile neutrinos and supernova nucleosynthesis

A light sterile neutrino species has been introduced to explain simultaneously the solar and atmospheric neutrino puzzles and the results of the LSND experiment, while providing for a hot component of dark matter. Employing this scheme of neutrino masses and mixings, we show how matter-enhanced active-sterile neutrino transformation followed by active-active neutrino transformation can solve robustly the neutron deficit problem encountered by models of r-process nucleosynthesis associated with neutrino-heated supernova ejecta.

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arXiv1999

Stellar Abundances in the Early Galaxy and Two r-Process Components

We present quantitative predictions for the abundances of r-process elements in stars formed very early in the Galactic history using a phenomenological two-component r-process model based on the I129 and Hf182 inventory in the early solar system. This model assumes that a standard mass of the ISM dilutes the debris from an individual supernova. High frequency supernova H events and low frequency supernova L events are proposed in the model with characteristics determined by the meteoritic data on I129 and Hf182. The yields in an H or L event are obtained from these characteristics and the solar r-process abundances under the assumption that the yield template for the high mass (A > 130) nuclei associated with W182 or the low mass (A < or = 130) nuclei associated with I127 is the same for both the H and L events and follows the corresponding solar r-pattern in each mass region. The abundance of Eu, not Fe, is proposed as a key guide to the age of very metal-poor stars. We predict that stars with log epsilon (Eu) = -2.98 to -2.22 were formed from an ISM contaminated most likely by a single H event within the first 10**7 yr of the Galactic history and should have an Ag/Eu abundance ratio less than the corresponding solar r-process value by a factor of at least 10. Many of the very metal-poor stars observed so far are considered here to have been formed from an ISM contaminated by many (about 10) r-process events. Stars formed from an ISM contaminated only by a pure L event would have an Ag/Eu ratio higher than the corresponding solar r-process value but would be difficult to find due to the low frequency of the L events. However, variations in the relative abundances of the low and high mass regions should be detectable in very metal-poor stars.

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arXiv1999

Probing r-Process Production of Nuclei Beyond Bi209 with Gamma Rays

We estimate gamma-ray fluxes due to the decay of nuclei beyond Bi209 from a supernova or a supernova remnant assuming that the r-process occurs in supernovae. We find that a detector with a sensitivity of about 10**(-7) photons/cm**2/s at energies of 40 keV to 3 MeV may detect fluxes due to the decay of Ra226, Th229, Am241, Am243, Cf249, and Cf251 in the newly discovered supernova remnant near Vela. In addition, such a detector may detect fluxes due to the decay of Ac227 and Ra228 produced in a future supernova at a distance of about 1 kpc. As nuclei with mass numbers A > 209 are produced solely by the r-process, such detections are the best proof for a supernova r-process site. Further, they provide the most direct information on yields of progenitor nuclei with A > 209 at r-process freeze-out. Finally, detection of fluxes due to the decay of r-process nuclei over a range of masses from a supernova or a supernova remnant provides the opportunity to compare yields in a single supernova event with the solar r-process abundance pattern.

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arXiv1998

Neutrino fluence after r-process freeze-out and abundances of Te isotopes in presolar diamonds

Using the data of Richter et al. (1998) on Te isotopes in diamond grains from a meteorite, we derive bounds on the neutrino fluence and the decay timescale of the neutrino flux relevant for the supernova r-process. Our new bound on the neutrino fluence F after freeze-out of the r-process peak at mass number A = 130 is more stringent than the previous bound F < 0.045 (in units of 10**37 erg/cm**2) of Qian et al. (1997) and Haxton et al. (1997) if the neutrino flux decays on a timescale tau > 0.65 s. In particular, it requires that a fluence of F = 0.031 be provided by a neutrino flux with tau < 0.84 s. Such a fluence may be responsible for the production of the solar r-process abundances at A = 124-126 (Qian et al. 1997; Haxton et al. 1997). Our results are based on the assumption that only the stable nuclei implanted into the diamonds are retained while the radioactive ones are lost from the diamonds upon decay after implantation (Ott 1996). We consider that the nanodiamonds are condensed in an environment with C/O > 1 in the expanding supernova debris or from the exterior H envelope. The implantation of nuclei would have occurred 10**4-10**6 s after r-process freeze-out. This time interval may be marginally sufficient to permit adequate cooling upon expansion for the formation of diamond grains. The mechanisms of preferential retention/loss of the implanted nuclei are not well understood.

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arXiv1998

Neutrino Transport in Strongly Magnetized Proto-Neutron Stars and the Origin of Pulsar Kicks: I. The Effect of Parity Violation in Weak Interactions

The original paper describes neutrino cooling calculations in the presence of asymmetric neutrino opacities due to parity violation in strong magnetic fields. In early April, through the work of P. Arras and D. Lai at Cornell (see astro-ph/9806285) we became aware that the paper contains a serious flaw: although the neutrino scattering opacities depend asymmetrically on the initial and final neutrino momenta, detailed balance requires that there be no cumulative effect from multiple scatterings in the bulk interior of the proto-neutron star where local thermodynamic equilibrium applies to a good approximation. This paper was therefore withdrawn from ApJ in April. Also in April, we submitted an erratum to ApJ Lett. pointing out this error. (The erratum will appear in the July 1, 1998 issue of ApJ Lett.)

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