Contributions of type II and Ib/c supernovae to Galactic chemical evolution

Type II and Ib/c supernovae （SNe II and Ib/c） have made major stellar nu- cleosynthetic contributions to the inventories of stable nuclides during chemical evolu- tion of the Galaxy. A case study is performed here with the help of recently developed numerical simulations of Galactic chemical evolution in the solar neighborhood to un- derstand the contributions of SNe II and Ib/c by comparing the stellar nucleosynthetic yields obtained by two leading groups in this field. These stellar nucleosynthetic yields differ in terms of their treatment of stellar evolution and nucleosynthesis. The formu- lation describing Galactic chemical evolution is developed with the recently revised solar metallicity of -0.014. Furthermore, the recent nucleosynthetic yields of stellar models based on the revised solar metallicity are also used. The analysis suggests that it could be difficult to explain, in a self-consistent manner, the various features asso- ciated with the elemental evolutionary trends over Galactic timescales by any single adopted stellar nucleosynthetic model that incorporates SNe II and Ib/c.

A Study of Magnetized White Dwarf+Helium Star Binary Evolution to Type Ia Supernovae

The white dwarf(WD)+helium(He)star binary channel plays an important role in the single degenerate scenario for the progenitors of type Ia supernovae(SNe Ia).Previous studies on the WD+main sequence star evolution have shown that the magnetic fields of WDs may significantly influence their accretion and nuclear burning processes.In this work we focus on the evolution of magnetized WD+He star binaries with detailed stellar evolution and binary population synthesis(BPS)calculations.In the case of magnetized WDs,the magnetic fields may disrupt the inner regions of the accretion disk,funnel the accretion flow onto the polar caps and even confine helium burning within the caps.We find that,for WDs with sufficiently strong magnetic fields,the parameter space of the potential SN Ia progenitor systems shrinks toward shorter orbital periods and lower donor masses compared with that in the non-magnetized WD case.The reason is that the magnetic confinement usually works with relatively high mass transfer rates,which can trigger strong wind mass loss from the WD,thus limiting the He-rich mass accumulation efficiency.The surviving companion stars are likely of low-mass at the moment of the SN explosions,which can be regarded as a possible explanation for the non-detection of surviving companions after the SNe or inside the SN remnants.However,the corresponding birthrate of Galactic SNe Ia in our high-magnetic models is estimated to be~(0.08–0.13)×10^(-3)yr^(-1)(~0.17–0.28×10^(-3)yr^(-1)for the non-magnetic models),significantly lower than the observed Galactic SN Ia birthrate.

The Progenitors of Type Ia Supernovae with Asymptotic Giant Branch Donors

TypeⅠa supernovae(SNe Ia)are among the most energetic events in the universe.They are excellent cosmological distance indicators due to the remarkable homogeneity of their light curves.However,the nature of the progenitors of SNeⅠa is still not well understood.In the single-degenerate model,a carbon-oxygen white dwarf(CO WD)could grow its mass by accreting material from an asymptotic giant branch(AGB)star,leading to the formation of SNe Ia when the mass of the WD approaches to the Chandrasekhar-mass limit,known as the AGB donor channel.In this channel,previous studies mainly concentrate on the wind-accretion pathway for the mass-increase of the WDs.In the present work,we employed an integrated mass-transfer prescription for the semidetached WD+AGB systems,and evolved a number of WD+AGB systems for the formation of SNe Ia through the Roche-lobe overflow process or the wind-accretion process.We provided the initial and final parameter spaces of WD+AGB systems for producing SNe Ia.We also obtained the density distribution of circumstellar matter at the moment when the WD mass reaches the Chandrasekhar-mass limit.Moreover,we found that the massive WD+AGB sample AT 2019qyl can be covered by the final parameter space for producing SNe Ia,indicating that AT 2019qyl is a strong progenitor candidate of SNe Ia with AGB donors.

Strongly screened electron capture rates of chromium isotopes in presupernova evolution

Taking into account the effect of electron screening on electron energy and electron capture threshold energy, by using the method of Shell-Model Monte Carlo and random phase approximation theory, we investigate the capture rates of chromium isotopes with strong electron screening according to the linear response theory screening model. Strong screening rates can decrease by about 40.43% （e.g., for 6Cr at T9 = 3.44, Ye = 0.43）. Our conclusions may be helpful to researches on supernova explosions and related numerical simulation methods.

Influence of Supernova SN Ia Rate and the Early Star Formation Rate on the Galactic Chemical Evolution

Based on the recently developed numerical approach to understand the formation and the chemical evolution of the milky-way galaxy in the solar neighborhood we study the influence of the supernova type SN Ia rates on the galactic chemical evolution. Supernova SN Ia plays an important role in producing the iron inventory of the galaxy. We also study the dependence of the chemical evolution on the star formation rate prevailing during the initial one billion years of the evolution of the galaxy. This era marks the formation of the galactic halo and the thick disk. A comparison of the elemental abundance distributions of the dwarf stars in the solar neighborhood is made among the various models simulated in the present work. In order to explain the majority of the observed elemental evolutionary trends, specifically those related with the galactic evolution of iron and oxygen, it would be essential to incorporate a major component of prompt SN Ia to the galactic evolution. The prompt SN Ia would produce significant fraction of SN Ia within the initial ~100 million years from the time of star formation. The essential requirement of prompt SN Ia would result in a significant enhancement of SN Ia rates during the earliest epoch of the galaxy. The elemental evolutionary trends also favor an enhancement in the star formation rate during the initial one billion years of the galaxy at least by a factor of three compared to the trend prevailing during the latter evolutionary time of the galaxy.

Supernova Explosion in the Process of Stellar Evolution

Supernova explosion is a peculiar astronomical phenomenon,which has aroused great interest and extensive research by astronomers.At present,however,it is erroneously believed that the supernova explosion is a splendid funeral of a dying star.In fact,new astronomical observations and the author’s theoretical studies have shown that in the process of a star’s never-ending evolution,numerous supernova explosions may occur,and supernova explosion will run through many stages of stellar evolution.Whenever the atmosphere of the star becomes very dense and its surface is covered with a thick layer of material again,a thermonuclear reaction can be ignited again,causing a supernova explosion.Especially,when the star has undergone a number of supernovae explosions and its surface is covered with multiple layers of supernova explosion remnants,the core of the massive star may undergo sudden gravitational collapse,causing a violent supernova;when a star is impacted by a falling celestial body,it also produces violent supernova explosions.

Double-detonation model of type Ia supernovae with a variable helium layer ignition mass

Although Type Ia supernovae （SNe Ia） play an important role in the study of cosmology, their progenitors are still poorly understood. Thermonuclear explosions from the helium double-detonation sub-Chandrasekhar mass model have been considered as an alternative method for producing SNe Ia. By adopting the assumption that a double detonation occurs when a He layer with a critical ignition mass accumulates on the surface of a carbon-oxygen white dwarf （CO WD）, we perform detailed binary evolution calculations for the He double-detonation model, in which a He layer from a He star accumulates on a CO WD. According to these calculations, we obtain the initial parameter spaces for SNe Ia in the orbital period and secondary mass plane for various initial WD masses. We implement these results into a detailed binary population synthesis approach to calculate SN Ia birthrates and delay times. From this model, the SN Ia birthrate in our Galaxy is ～0.4 - 1.6 × 10^-3 yr^-1. This indicates that the double-detonation model only produces part of the SNe la. The delay times from this model are ～ 70 - 710 Myr, which contribute to the young population of SNe Ia in the observations. We found that the CO WD ＋ sdB star system CD-30 11223 could produce an SN Ia via the double-detonation model in its future evolution.

WD+RG systems as the progenitors of type Ia supernovae

Type Ia supernovae （SNe Ia） play an important role in the study of cosmic evolution, especially in cosmology. There are several progenitor models for SNe Ia proposed in the past years. By considering the effect of accretion disk instability on the evolution of white dwarf （WD） binaries, we performed detailed binary evolution calculations for the WD ＋ red-giant （RG） channel of SNe Ia, in which a carbon--oxygen WD accretes material from a RG star to increase its mass to the Chandrasekhar mass limit. According to these calculations, we mapped out the initial and final parameters for SNe Ia in the orbital period-secondary mass （log Pi_ M2^i） plane for various WD masses for this channel. We discussed the influence of the variation of the duty cycle value on the regions for producing SNe Ia. Similar to previous studies, this work also indicates that the long-period dwarf novae offer possible ways for producing SNe Ia. Meanwhile, we find that the surviving companion stars from this channel have a low mass after the SN explosion, which may provide a means for the formation of the population of single low-mass WDs （〈0.45 M⊙）.

Color excesses of type Ia supernovae from the single-degenerate channel model

The single degenerate model is the most widely accepted progenitor model of type Ia supernovae （SNe Ia）, in which a carbon-oxygen white dwarf （CO WD） accretes hydrogen-rich material from a main sequence or a slightly evolved star （WD＋MS） to increase its mass, and explodes when its mass approaches the Chandrasekhar mass limit. During the mass transfer phase between the two components, an optically thick wind may occur and the material lost as wind may exist as circumstellar material （CSM）. Searching for the CSM around a progenitor star is helpful for discriminating different progenitor models of SNe Ia. In addition, the CSM is a source of color excess. The purpose of this paper is to study the color excess produced from the single-degenerate progenitor model with an optically thick wind, and reproduce the distribution of color excesses of SNe Ia. Meng et al. systemically carded out binary evolution calculations of the WD ＋MS systems for various metallicities and showed the parameters of the systems before Roche lobe overflow and at the moment of supernova explosion in Meng ＆ Yang. With the results of Meng et al., we calculate the color excesses of SNe Ia at maximum light via a simple analytic method. We reproduce the distribution of color excesses of SNe Ia by our binary population synthesis approach if the velocity of the optically thick wind is taken to be an order of magnitude of 10km s^-1. However, if the wind velocity is larger than 100km s^-1, the reproduction is bad.

Distribution of ^(56)Ni Yields of Type Ia Supernovae and its Implication for Progenitors

The amount of 56↑Ni produced in Type Ia supernova （SN Ia） explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the 56↑Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56↑Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56↑Ni masses for different SNe Ia could vary by a factor of ten （e.g., MNi = 0.1 - 1.3 M⊙）, which cannot be explained in terms of the standard Chandrasekhar-mass model （with a 56↑Ni mass production of 0.4 - 0.8 M⊙）. Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich （with MNi 〉 0.8 M⊙） SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56↑Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △ml5 〈 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia （with MNi 〈 0.2 M⊙） may invoke the sub- Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.

The Evolution of Advanced Merger (U)LIRGs on the Color-Stellar Mass Diagram

Based on a sample of 79 local advanced merger （adv-merger） （U）LIRGs, we search for evidence of quenching processes by investigating the distributions of star formation history indicators （EW（Ha）, EW（HfiA） and D,（4000）） on the NUV-r color-mass and SFR-M, diagrams. The distributions of EW（Ha） and Dn（4000） on the NUV-r color-mass diagram show clear trends that at a given stellar mass, galaxies with redder NUV-r colors have smaller EW（Ha） and larger Dn （4000）. The reddest adv-merger （U）LIRGs close to the green valley mostly have Dn（4000）〉 1.4. In addition, in the SFR-M, diagram, as the SFR decreases, the EW（Ha） decreases and the Dn （4000） increases, implying that the adv-merger （U）LIRGs on the star formation main sequence have more evolved stellar populations than those above the main sequence. These results indicate that a fraction of the adv-merger （U）LIRGs have already exhibited signs of fading from the starburst phase and that the NUV-r reddest adv-merger （U）LIRGs are likely at the initial stage of post-starbursts with an age of - 1 Gyr, which is consistent with the gas exhaustion time-scales. Therefore, our results offer additional support for the fast evolutionary track from the blue cloud to the red sequence.

Long-term evolution and gravitational wave radiation of neutron stars with differential rotation induced by r-modes

In a second-order r-mode theory, Sa and Tome found that the r-mode oscillation in neutron stars （NSs） could induce stellar differential rotation, which naturally leads to a saturated state of the oscillation. Based on a consideration of the coupling of the r-modes and the stellar spin and thermal evolution, we carefully investigate the influences of the differential rotation on the long-term evolution of isolated NSs and NSs in low-mass X-ray binaries, where the viscous damping of the r-modes and its resultant effects are taken into account. The numerical results show that, for both kinds of NSs, the differential rotation can significantly prolong the duration of the r-modes. As a result, the stars can keep nearly a constant temperature and constant angular velocity for over a thousand years. Moreover, the persistent radiation of a quasi-monochromatic gravitational wave would also be predicted due to the long-term steady r-mode oscillation and stellar rotation. This increases the detectability of gravitational waves from both young isolated and old accreting NSs.

Mass-accreting white dwarfs and type Ia supernovae

Type Ia supernovae（SNe Ia） play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs（CO WDs） in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H-and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model（including the WD＋MS channel, the WD＋RG channel and the WD＋He star channel）, the doubledegenerate model（including the violent merger scenario） and the sub-Chandrasekhar mass model.Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.

The C/O ratio of He-accreting carbon-oxygen white dwarfs and type Ia supernovae

Type Ia supernovae(SNe Ia)are thermonuclear explosions of carbon-oxygen white dwarfs(CO WDs),and are believed to be excellent cosmological distance indicators due to their high luminosity and remarkable uniformity.However,there exists a diversity among SNe Ia,and a poor understanding of the diversity hampers the improvement of the accuracy of cosmological distance measurements.The variations of the ratios of carbon to oxygen(C/O)of WDs at explosion are suggested to contribute to the diversity.In the canonical model of SNe Ia,a CO WD accretes matter from its companion and increases its mass till the Chandrasekhar mass limit when the WD explodes.In this work,we studied the C/O ratio for accreting CO WDs.Employing the stellar evolution code MESA,we simulated the accretion of He-rich material onto CO WDs with different initial WD masses and different mass accretion rates.We found that the C/O ratio varies for different cases.The C/O ratio of He-accreting CO WDs at explosion increases with a decreasing initial WD mass or a decreasing accretion rate.The various C/O ratios may,therefore,contribute to the diversity of SNe Ia.

The cooling time of white dwarfs produced from type Iasupernovae

Type Ia supernovae （SNe Ia） play a key role in measuring cosmological parameters, in which the Phillips relation is adopted. However, the origin of the relation is still unclear. Several parameters are suggested, e.g. the relative content of carbon to oxygen （C/O） and the central density of the white dwarf （WD） at ignition. These parameters are mainly determined by the WD＇s initial mass and its cooling time, respectively. Using the progenitor model developed by Meng ＆ Yang, we present the distributions of the initial WD mass and the cooling time. We do not find any correlation between these parameters. However, we notice that as the range of the WD＇s mass decreases, its average value increases with the cooling time. These results could provide a constraint when simulating the SN Ia explosion, i.e. the WDs with a high C/O ratio usually have a lower central density at ignition, while those having the highest central density at ignition generally have a lower C/O ratio. The cooling time is mainly determined by the evolutionary age of secondaries, and the scatter of the cooling time decreases with the evolutionary age. Our results may indicate that WDs with a long cooling time have more uniform properties than those with a short cooling time, which may be helpful to explain why SNe Ia in elliptical galaxies have a more uniform maximum luminosity than those in spiral galaxies.

Are Type Ia Supernovae Reliable Distance Indicators?

Recent applications of type Ia supernovae （SNe Ia） in cosmology have successfully revealed the accelerating expansion of the universe. However, as distance indicators used in measuring the expansion history of the universe and probing the nature of dark energy, these objects must pass more strict tests. We propose a K-S test to investigate if there exists any systematic bias when deriving the luminosity distances under the standard candle assumption. Two samples, one comprising 71 high-redshift SNe Ia and the other, 44 nearby ones, are used in our investigation. We find that it is likely there exists a bias in the adopted samples, which is probably caused by a systematic error, e.g. in the color parameter used in the luminosity calibration and a bias may be caused by the SN evolution or by varying properties of the dust surrounding the SNe Ia.

The influences of convective overshooting and semiconvection on the chemical evolution of massive stars

In massive stars, convection in the interior is different from that of inter- mediate and small mass stars. In the main-sequence phase of small mass stars, there is a convective core and a radiative envelope, between which are the radiative inter- mediate layers with uneven chemical abundances. Semiconvection would occur in the intermediate layers between the convective core and the homogeneous envelope in massive stars. We treat core convective overshooting and semiconvection together as a process. We found that when decreasing overshooting, the semiconvection is more pronounced. In these two processes, we introduce one diffusive parameter D, which is different from other authors who have introduced different parameters for these two zones. The influences of the turbulent diffusion process on chemical evolution and other quantities of the stellar structure are shown in the present paper.

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae （SNe Ia） and anisotropy of cosmic microwave background （CMB）. The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ＂Gold＂ sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a fiat universe with the cosmological constant, we measure ΩM=0.28-0.05^＋0.04 which is consistent with Riess et al. The transition redshift is zT=0.60-0.08^＋0.04. We also discuss several dark energy models that define w（z） of the parameterized equation of state of dark energy including one parameter and two parameters （w（z） being the ratio of the pressure to energy density）. Our calculations show that the accurately calculated transition redshift varies from zT =0.29-0.06^＋0.07 to zT =0.60-0.08^＋0.06 across these models. We also calculate the minimum redshift zc at which the current observations need the universe to accelerate.

IRAS F21013-0739:a possible evolutionary successor of an ultraluminous infrared galaxy

We present a stellar population synthesis study of a type II luminous infrared galaxy, IRAS F21013-0739. Optical images show clear characteristics of a merger remnant. The H-band absolute magnitude is MH = -25.1, which is -2 times as luminous as L＊ galaxies. Stellar populations are obtained through the stellar synthesis code STARLIGHT. We find that it experienced a recent starburst （SB） phase - 100 Myr ago. By reconstructing the ultraviolet-to-optical spectrum, and adopting Calzetti et al. and Leitherer et al.＇s extinction curves, we estimate the past infrared （IR） luminosities of the host galaxy and find it may have experienced an ultraluminous infrared galaxy phase which lasted for about 100 Myr. Its i-band absolute magnitude is Mi = -22.463, and its spectral type shows type 2 active galactic nucleus （AGN） characteristics. The mass of the supermassive black-hole is estimated to be MBH = 1.6 × 107 M⊙ （lower- limit）. The Eddington ratio Lbol/LEdd is 0.15, which is typical of Palomar-Green （PG） quasars. Both the nuclear SB and AGN contribute to the present IR luminosity budget, and the SB contributes -67%. On the diagram of IR color versus IR/opfical excess, it is located between IR quasars and PG quasars. These results indicate that IRAS F21013-0739 has probably evolved from a ULIRG, and it can possibly evolve into an AGN.

Effects of α-Enhancement on Stellar Evolution

Using Eggleton＇s code, we systematically show the differences in stellar evolution between the results based on the scaled-solar mixture and the a-enhanced metal mixture. As input, the OPAL high temperature opacities are used for log（T/K） 〉 4.00, and the new Wichita State low temperature opacities, for log（T/K） ≤ 4.00, Our calculations cover star masses ranging from 0.25 to 80.0M⊙, spaced at ΔlogM = 0.10 or 0.05. The values of metallicities Z are 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08 and 0.10. For a given Z, the initial hydrogen mass fraction is given by X = 0.76 - 3.0Z. We show that a-enhancement can raise the stellar effective temperature and luminosity, and reduce the evolutionary age. Compared with some previous work, the effects of α-enhancement are more obviously demonstrated in our calculations.