arXiv subjects

Y. -Z. Qian

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

At least 19 recordsLinked to original sources

arXiv2021

Impact of Dark Photon Emission on Massive Star Evolution and Pre-Supernova Neutrino Signal

We study the effects of additional cooling due to the emission of a dark matter candidate particle, the dark photon, on the final phases of the evolution of a $15\,M_\odot$ star and resulting modifications of the pre-supernova neutrino signal. For a substantial portion of the dark photon parameter space the extra cooling speeds up Si burning, which results in a reduced number of neutrinos emitted during the last day before core collapse. This reduction can be described by a systematic acceleration of the relevant timescales and the results can be estimated semi-analytically in good agreement with the numerical simulations. Outside the semi-analytic regime we find more complicated effects. In a narrow parameter range, low-mass dark photons lead to an increase of the number of emitted neutrinos because of additional shell burning episodes that delay core collapse. Furthermore, relatively strong couplings produce a thermonuclear runaway during O burning, which could result in a complete disruption of the star but requires more detailed simulations to determine the outcome. Our results show that pre-supernova neutrino signals are a potential probe of the dark photon parameter space.

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arXiv2018

Three-Dimensional Simulations of Neutrino-Driven Core-Collapse Supernovae from Low-Mass Single and Binary Star Progenitors

We present a suite of seven 3D supernova simulations of non-rotating low-mass progenitors using multi-group neutrino transport. Our simulations cover single star progenitors with zero-age main sequence masses between $9.6 M_\odot$ and $12.5 M_\odot$ and (ultra)stripped-envelope progenitors with initial helium core masses between $2.8 M_\odot$ and $3.5 M_\odot$. We find explosion energies between $0.1\,\mathrm{Bethe}$ and $0.4\,\mathrm{Bethe}$, which are still rising by the end of the simulations. Although less energetic than typical events, our models are compatible with observations of less energetic explosions of low-mass progenitors. In six of our models, the mass outflow rate already exceeds the accretion rate onto the proto-neutron star, and the mass and angular momentum of the compact remnant have closely approached their final value, barring the possibility of later fallback. While the proto-neutron star is still accelerated by the gravitational tug of the asymmetric ejecta, the acceleration can be extrapolated to obtain estimates for the final kick velocity. We obtain gravitational neutron star masses between $1.22 M_\odot$ and $1.44 M_\odot$, kick velocities between $11\, \mathrm{km}\, \mathrm{s}^{-1}$ and $695\, \mathrm{km}\, \mathrm{s}^{-1}$, and spin periods from $20\, \mathrm{ms}$ to $2.7\,\mathrm{s}$, which suggests that typical neutron star birth properties can be naturally obtained in the neutrino-driven paradigm. We find a loose correlation between the explosion energy and the kick velocity. There is no indication of spin-kick alignment, but a correlation between the kick velocity and the neutron star angular momentum, which needs to be investigated further as a potential point of tension between models and observations.

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arXiv2015

Estimating the evolution of gas in the Fornax dwarf spheroidal galaxy from its star formation history: an illustrative example

We propose that detailed data on the star formation history of a dwarf spheroidal galaxy (dSph) may be used to estimate the evolution of the total mass M_g(t) for cold gas in its star-forming disk. Using Fornax as an illustrative example, we estimate its M_g(t) and the corresponding net gas flow rate Delta F(t) assuming a global star formation rate psi(t) = lambda_*(t)[M_g(t)/M_sun]^alpha consistent with observations of nearby galaxies. We identify the onset of the transition in Delta F(t) from a net inflow to a net outflow as the time t_sat at which the Fornax halo became a Milky Way satellite and estimate the evolution of its total mass M_h(t) at t < t_sat using the median halo growth history in the LambdaCDM cosmology and its present mass within the half-light radius derived from observations. We examine three different cases of alpha = 1, 1.5, and 2, and justify the corresponding lambda_*(t) by comparing the gas mass fraction f_g(t) = M_g(t)/M_h(t) at t < t_sat with results from simulations of gas accretion by halos in a reionized universe. We find that the Fornax halo grew to M_h(t_sat) ~ 2x10^9 M_sun at t_sat ~ 5 or 8 Gyr, in broad agreement with previous studies using data on its stellar kinematics and its orbital motion. We describe qualitatively the evolution of Fornax as a satellite and discuss potential extension of our approach to other dSphs.

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arXiv2013

New Primary Mechanisms for the Synthesis of Rare 9Be in Early Supernovae

We present two new primary mechanisms for the synthesis of the rare nucleus 9Be, both triggered by nu-induced production of 3H followed by 4He(3H,gamma)7Li in the He shells of core-collapse supernovae. For progenitors of ~8M_sun, 7Li(3H,n_0)9Be occurs during the rapid expansion of the shocked He shell. Alternatively, for ultra-metal-poor progenitors of ~11-15M_sun, 7Li(n,gamma)8Li(n,gamma)9Li(e^-anti-nu_e)9Be occurs with neutrons produced by 4He(anti-nu_e,e^+n)3H, assuming a hard effective anti-nu_e spectrum from oscillations (which also leads to heavy element production through rapid neutron capture) and a weak explosion (so the 9Be survives shock passage). We discuss the associated production of 7Li and 11B, noting patterns in LiBeB production that might distinguish the new mechanisms from others.

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arXiv2013

Reexamining the temperature and neutron density conditions for r-process nucleosynthesis with augmented nuclear mass models

We explore the effects of nuclear masses on the temperature and neutron density conditions required for r-process nucleosynthesis using four nuclear mass models augmented by the latest atomic mass evaluation. For each model we derive the conditions for producing the observed abundance peaks at mass numbers A ~ 80, 130, and 195 under the waiting-point approximation and further determine the sets of conditions that can best reproduce the r-process abundance patterns (r-patterns) inferred for the solar system and observed in metal-poor stars of the Milky Way halo. In broad agreement with previous studies, we find that (1) the conditions for producing abundance peaks at A ~ 80 and 195 tend to be very different, which suggests that, at least for some nuclear mass models, these two peaks are not produced simultaneously; (2) the typical conditions required by the critical waiting-point (CWP) nuclei with the N = 126 closed neutron shell overlap significantly with those required by the N=82 CWP nuclei, which enables coproduction of abundance peaks at A ~ 130 and 195 in accordance with observations of many metal-poor stars; and (3) the typical conditions required by the N = 82 CWP nuclei can reproduce the r-pattern observed in the metal-poor star HD 122563, which differs greatly from the solar r-pattern. We also examine how nuclear mass uncertainties affect the conditions required for the r-process and identify some key nuclei including76Ni to 78Ni, 82Zn, 131Cd, and 132Cd for precise mass measurements at rare-isotope beam facilities.

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arXiv2011

A Long, Cold, Early r-process? Neutrino-induced Nucleosynthesis in He Shells Revisited

We revisit a neutrino-driven r-process mechanism in the He shell of a core-collapse supernova, finding that it could succeed in early stars of metallicity < solar/1000, at relatively low temperatures and neutron densities, producing A ~ 130 and 195 abundance peaks over ~ 10-20 s. The mechanism is sensitive to the neutrino emission model and to neutrino oscillations. We discuss the implications of an r-process that could alter interpretations of abundance data from metal-poor stars, and point out the need for further calculations that include effects of the supernova shock.

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arXiv2009

Chemical Evolution of the Juvenile Universe

Only massive stars contribute to the chemical evolution of the juvenile universe corresponding to [Fe/H]<-1.5. If Type II supernovae (SNe II) are the only relevant sources, then the abundances in the interstellar medium of the juvenile epoch are simply the sum of different SN II contributions. Both low-mass (~8-11M_sun) and normal (~12-25M_sun) SNe II produce neutron stars, which have intense neutrino-driven winds in their nascent stages. These winds produce elements such as Sr, Y, and Zr through charged-particle reactions (CPR). Such elements are often called the light r-process elements, but are considered here as products of CPR and not the r-process. The observed absence of production of the low-A elements (Na through Zn including Fe) when the true r-process elements (Ba and above) are produced requires that only low-mass SNe II be the site if the r-process occurs in SNe II. Normal SNe II produce the CPR elements in addition to the low-A elements. This results in a two-component model that is quantitatively successful in explaining the abundances of all elements relative to hydrogen for -3<[Fe/H]<-1.5. This model explicitly predicts that [Sr/Fe]>-0.32. Recent observations show that there are stars with [Sr/Fe]<-2 and [Fe/H]<-3. This proves that the two-component model is not correct and that a third component is necessary to explain the observations. This leads to a simple three-component model including low-mass and normal SNe II and hypernovae (HNe), which gives a good description of essentially all the data for stars with [Fe/H]<-1.5. We conclude that HNe are more important than normal SNe II in the chemical evolution of the low-A elements, in sharp distinction to earlier models. (Abridged)

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arXiv2008

Abundances in Metal-Poor Stars and Chemical Evolution of the Early Galaxy

We have attributed the elements from Sr through Ag in stars of low metallicities ([Fe/H] < -1.5) to charged-particle reactions (CPR) in neutrino-driven winds, which are associated with neutron star formation in low-mass and normal supernovae (SNe) from progenitors of ~ 8 to 11 M_sun and ~ 12 to 25 M_sun, respectively. Using this rule and attributing all Fe production to normal SNe, we previously developed a phenomenological two-component model, which predicts that [Sr/Fe] > -0.32 for all metal-poor stars. This is in direct conflict with the high-resolution data now available, which show that there is a great shortfall of Sr relative to Fe in many stars with [Fe/H] < -3. The same conflict also exists for the CPR elements Y and Zr. We show that the data require a stellar source leaving behind black holes and that hypernovae (HNe) from progenitors of ~ 25 to 50 M_sun are the most plausible candidates. If we expand our previous model to include three components (low-mass and normal SNe and HNe), we find that essentially all of the data are very well described by the new model. The HN yield pattern for the low-A elements from Na through Zn (including Fe) is inferred from the stars deficient in Sr, Y, and Zr. We estimate that HNe contributed ~ 24% of the bulk solar Fe inventory while normal SNe contributed only ~ 9% (not the usually assumed ~ 33%). This implies a greatly reduced role of normal SNe in the chemical evolution of the low-A elements.

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arXiv2008

Abundances of Sr, Y, and Zr in Metal-Poor Stars and Implications for Chemical Evolution in the Early Galaxy

Studies of nucleosynthesis in neutrino-driven winds from nascent neutron stars show that the elements from Sr through Ag with mass numbers A~88-110 are produced by charged-particle reactions (CPR) during the alpha-process in the winds. Accordingly, we have attributed all these elements in stars of low metallicities ([Fe/H]<-1.5) to low-mass and normal supernovae (SNe) from progenitors of ~8-11M_sun and ~12-25M_sun, respectively, which leave behind neutron stars. Using this rule and attributing all Fe production to normal SNe, we previously developed a phenomenological two-component model, which predicts that [Sr/Fe]>-0.32 for all metal-poor stars. The high-resolution data now available on Sr abundances in Galactic halo stars show that there is a great shortfall of Sr relative to Fe in many stars with [Fe/H]<-3. This is in direct conflict with the above prediction. The same conflict also exists for two other CPR elements Y and Zr. The very low abundances of Sr, Y, and Zr observed in stars with [Fe/H]<-3 thus require a stellar source that cannot be low-mass or normal SNe. We show that this observation requires a stellar source leaving behind black holes and that hypernovae (HNe) from progenitors of ~25-50M_sun are the most plausible candidates. (Abridged)

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arXiv2007

Where, oh where has the r-process gone?

We present a review of the possible sources for r-process nuclei. It is known that there is as yet no self-consistent mechanism to provide abundant neutrons for a robust r-process in the neutrino-driven winds from nascent neutron stars. We consider that the heavy r-nuclei with mass numbers A>130 (Ba and above) cannot be produced in the neutrino-driven winds. Nonetheless, the r-process and the neutrino-driven winds may be directly or indirectly related by some unknown additional mechanism, which, for example, could provide ejecta with very short dynamic timescales of <0.004 s. This undetermined mechanism must supply a neutron source within the same general stellar sites that undergo core collapse to produce the neutron star. Observational data on low-metallicity stars in the Galactic halo show that sites producing the heavy r-nuclei do not produce Fe or any other elements between N and Ge. Insofar as a forming neutron star is key to producing the heavy r-nuclei, then the only possible sources are supernovae resulting from collapse of O-Ne-Mg cores or accretion-induced collapse of white dwarfs, neither of which produce the elements of the Fe group or those of intermediate mass (above C and N). Using a template star with high enrichments of heavy r-nuclei and another with low enrichments we develop a two-component model based on the abundances of Eu (from sources for heavy r-nuclei) and Fe (from Fe core-collapse supernovae). This model gives very good quantitative predictions for the abundances of all the other elements in those metal-poor stars with [Fe/H]<-1.5 for which the Eu and Fe abundances are known. (Abridged)

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arXiv2007

r-Process Nucleosynthesis in Shocked Surface Layers of O-Ne-Mg Cores

We demonstrate that rapid expansion of the shocked surface layers of an O-Ne-Mg core following its collapse can result in r-process nucleosynthesis. As the supernova shock accelerates through these layers, it makes them expand so rapidly that free nucleons remain in disequilibrium with alpha-particles throughout most of the expansion. This allows heavy r-process isotopes including the actinides to form in spite of the very low initial neutron excess of the matter. We estimate that yields of heavy r-process nuclei from this site may be sufficient to explain the Galactic inventory of these isotopes.

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arXiv2005

Metal Enrichment of the Intergalactic Medium and Production of Massive Black Holes

A model for the chemical evolution of the intergalactic medium (IGM) is presented using theoretical yields of very massive stars (VMSs) and Type II supernovae (SNe II). It is shown that if [Si/C] is indeed as high as ~0.7 in the IGM, then VMSs associated with pair-instability supernovae (PI-SNe) in low-mass halos at high redshift must produce at least 50% of the Si. The remainder is from later galactic outflows of SN II debris, which also provide most of the C and O. Both sources are required to account for the metal inventory in the IGM. The early VMS production must continue until redshift z~15 so that the efficiency of VMS formation per low-mass halo is significantly below unity. Contributions from the later galactic outflows mainly occur at z~4-6. Using a Salpeter initial mass function, we infer that the number of VMSs producing massive black holes (MBHs) is 0.72 times the the number of VMSs associated with PI-SNe. The amount of metals (particularly Si) in the IGM that is attributable to PI-SNe is thus closely coupled with the total mass of MBHs produced in epochs prior to galaxy formation. Production of ~50% of the Si in the IGM by PI-SNe corresponds to an early inventory of MBHs that is comparable to the global mass budget of the central supermassive black holes in present-day galaxies. (Abridged)

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arXiv2005

Nuclear physics and astrophysics of the r-process

Some nuclear and astrophysical aspects of the r-process are discussed. Particular attention is paid to observations of abundances in metal-poor stars and their implications for the astrophysical site and yield patterns of the r-process. The effects of supernova neutrinos and related nuclear processes on the yield patterns are explored. The uncertainties in the theoretical nuclear input for the r-process are discussed and the need for experimental data is emphasized.

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arXiv2005

Sources for Metals in the Intergalactic Medium

We present a discussion of possible sources of C, O, and Si in the IGM using the yields of VMSs and SNe II. The chemical evolution of the IGM is considered based on analytical phenomenological models of hierarchical structure formation. Two regimes are considered: one for gas expulsion by VMSs and SNe II in low-mass halos prior to dissociation of H2 molecules and reionization, and the other for later SN II-driven outflows from intermediate-mass halos. We use recent data on the abundances of C, O, and Si in the IGM inferred from two UVB models. We show that the results from a UVB model including quasars only cannot be explained by existing stellar models. To account for the results, in particular [Si/C], from a softer UVB model requires VMSs to provide more than 15% of the C in the IGM. The preferred scenario is that VMSs in low-mass halos provided between 15% and 60% of the C in the regime of very high redshift (z>15) and that galactic outflows provided the remainder during later epochs (4<z<6). Thus, there is a large gap in z between metal production by VMSs very early in the chemical evolution of the IGM and subsequent contributions from galactic outflows. (Abridged)

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arXiv2004

The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements

We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~2A) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H] < -2.5dex. For several hundred confirmed metal-poor giants brighter than B~16.5mag (most of them from the HES), ``snapshot'' spectra (R~20,000; S/N~30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, 8 from the HES, and 4 comparison stars. We discovered two new r-II stars, CS29497-004 ([Eu/Fe] = 1.64 +/- 0.22) and CS29491-069 ([Eu/Fe] = 1.08 +/- 0.23). A first abundance analysis of CS29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.

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arXiv2004

Hierarchical Structure Formation and Chemical Evolution of Galaxies

We present an analytical and phenomenological model for metal enrichment in halos based on hierarchical structure formation. This model assumes that astration of normal stellar populations along with SNe II already occurs at very high redshift. For halos that are not disrupted by SN II explosions, the chemical evolution of the gas and stars is explicitly determined by the rate of gas infall as compared with the astration rate and the corresponding rate of metal production by SNe II per H atom in the gas. This model provides a good description of the data on [Fe/H] for damped Ly alpha systems over a wide range of redshift 0.5 < z < 5. For all halos not disrupted by SN II explosions, if there is a cessation of gas infall, the metallicities of stars follow a bimodal distribution. This distribution is characterized by a sharp peak at the value of [Fe/H] corresponding to the time of infall cessation and by a broad peak at a higher value of [Fe/H] corresponding to the subsequent period of astration during which the bulk of the remaining gas forms stars. Such a distribution may be compared to that observed for the Galactic halo stars. If the gas in a halo is rapidly lost upon cessation of infall, then an assemblage of stars with a very sharply-defined [Fe/H] value will be left behind. This assemblage of stars may be accreted by a larger system and become a globular cluster of the larger system. We also discuss the masses and metallicities of the globular clusters in this model. (Abridged)

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arXiv2003

The r-Process: Recent Progress and Needs for Nuclear Data

Several nuclear physics issues essential to understanding the r-process are discussed. These include validity of the waiting-point approximation, strength of closed neutron shells in neutron-rich nuclei far from stability, and effects of neutrino interaction with such nuclei. The needs for nuclear data in resolving these issues are emphasized.

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arXiv2003

Hierarchical Structure Formation and Chemical Evolution of Damped Ly alpha Systems

We present a model for chemical evolution of damped Ly alpha systems considering production of metals by SNe II and infall associated with hierarchical structure formation. The growth of metallicity in these systems is a reflection of the competition between astration and infall. The apparent late turn-on of these systems is due to the late cut-off of infall. The wide range in [Fe/H] at a given redshift is explained by the range of the times for onset of star formation and the range of the times for infall cessation in different systems. The observed lower bound of [Fe/H] = -3 follows from the very rapid initial rise of [Fe/H] subsequent to onset of star formation. To reach [Fe/H] = -3 from a metal-free initial state requires only about 30 Myr so that the probability of observing lower [Fe/H] values is very small.

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