Quark-Novae Ia in the Hubble diagram: implications for dark energy

The accelerated expansion of the Universe was proposed through the use of Type-Ia supernovae （SNe） as standard candles. The standardization depends on an empirical correlation between the stretch/color and peak luminosity of the light curves. The use of Type-Ia SNe as standard candles rests on the assumption that their properties （and this correlation） do not vary with redshift. We consider the possibility that the majority of Type-Ia SNe are in fact caused by a Quark-Nova detonation in a tight neutron-star-CO-white-dwarf binary system, which forms a Quark-Nova Ia （QN-Ia）. The spin-down energy injected by the Quark-Nova remnant （the quark star） contributes to the post-peak light curve and neatly explains the observed correlation between peak luminosity and light curve shape. We demonstrate that the parameters describing QN-Ia are NOT constant in redshift. Simulated QN-Ia light curves provide a test of the stretch/color correlation by comparing the true distance modulus with that determined using SN light curve fitters. We determine a correction between the true and fitted distance moduli, which when applied to Type-Ia SNe in the Hubble diagram recovers the ΩM = 1 cosmology. We conclude that Type-Ia SNe observations do not necessitate the need for an accelerating expansion of the Universe （if the observed SNe Ia are dominated by QNe Ia） and by association the need for dark energy.     

Testing the distance-duality relation with data from galaxy clusters and type Ia supernovae

We test the distance-duality(DD) relation by combining the angular diameter distance DA provided by two galaxy cluster samples compiled by De Filippis et al.(the elliptical β model) and Bonamente et al.(the spherical β model),and the luminosity distance DL from Constitution and Union2 type Ia supernova(SNe Ia) datasets. To obtain DL associated with the observed DA at the same redshift,we smooth the noise of the SNe Ia in a model-independent way,obtain the evolutionary curve of DL and,finally,test the DD rel...     

The gravitational wave emission of double white dwarf coalescences

Type Ia Supernovae(SNe Ia) are widely used as standard candles to probe the Universe. However,how these fierce explosions are produced itself is still a highly debated issue. There are mainly two popular models for SNe Ia: the double-degenerate scenario and the single-degenerate scenario. The doubledegenerate scenario suggests that SNe Ia are produced by the coalescence of two degenerate white dwarfs,while the single-degenerate scenario suggests that the continuous accretion of a single degenerate white dwarf from its normal stellar companion will finally lead to a disastrous explosion when it is over-massive,resulting in an SN Ia. The rapid development of the gravitational wave astronomy sheds new light on the nature of SNe Ia. In this study, we calculate the gravitational wave emissions of double white dwarf coalescences and compare them with the sensitivities of several upcoming detectors. It is found that the gravitational wave emissions from double white dwarf mergers in the local universe are strong enough to be detected by LISA. We argue that LISA-like gravitational wave detectors sensitive in the frequency range of 0.01 — 0.1 Hz will be a powerful tool to test the double-degenerate model of SNe Ia, and also to probe the Universe.     

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-la supemovae (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 flat universe with the cosmological constant, we measureΩM = 0.28-0.05 0.04, which is consistent with Riess et al. The transition redshift is ZT = 0.60-0.08 0.06. 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.     

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.     

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

The amount of 56Ni produced in Type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia lumi-nosities with their light curves. Based on an empirical relation between the 56Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56Ni 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 Chandraseldaar-mass model (with a 56Ni 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 56Ni to power the light curve. This is also consistent with the find-ing 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 △m15 < 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.     

Disk-Corona Model in Active Galactic Nuclei： an Observational Test

We compiled a sample of 98 radio-quiet active galactic nuclei observed by ASCA, Chandra, XMM-Newton, INTEGRAL and Swift with the aim of testing the formation of hot corona and the magnetic shear stress operating in a disk-corona system. We found a strong correlation between the hard X-ray luminosity, bolometric luminosity L Bol and Eddington luminosity LEdd, in the sense that the fraction f of hard X-ray to the bolometric luminosity is inversely proportional to the Eddington ratio. This correlation favors the shear stress tensor being of the form of trφ∝ Pgas, with which the disk-corona structure is stable.     

Testing the distance-duality relation with a combination of cosmological distance observations

We propose an accurate test of the distance-duality (DD) relation, η = DL(z)(1 + z)-2/DA(z) = 1 (where DL and DA are the luminosity distances and angular diameter distances, respectively), with a combination of cosmological observational data of Type Ia Supernovae (SNe Ia) from the Union2 set and the galaxy cluster sample under an assumption of the spherical model. In order to avoid bias brought on by redshift non-coincidence between observational data and to consider redshift error bars of both clusters an...     

Testing X-ray measurements of galaxy cluster gas mass fraction using the cosmic distance-duality relation

We propose a consistency test for some recent X-ray gas mass fraction(fgas) measurements in galaxy clusters, using the cosmic distance-duality relation, ηtheory = DL(1 + z) 2/DA, with luminosity distance(DL) data from the Union2 compilation of type Ia supernovae. We set ηtheory ≡1, instead of assigning any redshift parameterizations to it, and constrain the cosmological information preferred by fgasdata along with supernova observations. We adopt a new binning method in the reduction of the Union2 data, in order to minimize the statistical errors. Four data sets of X-ray gas mass fraction, which are reported by Allen et al.(two samples), LaRoque et al. and Ettori et al., are analyzed in detail in the context of two theoretical models of fgas. The results from the analysis of Allen et al.’s samples demonstrate the feasibility of our method. It is found that the preferred cosmology by LaRoque et al.’s sample is consistent with its reference cosmology within the 1σ confidence level. However, for Ettori et al.’s fgas sample, the inconsistency can reach more than a 3σ confidence level and this dataset shows special preference to an Λ = 0 cosmology.     

Velocity Distance of the Open Cluster M11

On the reasonable hypothesis that the internal motions of member stars of a cluster are random and isotropic, a method which can be used to estimate the velocity distance of the cluster and its uncertainty is developed. The velocity distance so determined is an absolute distance estimate, and is completely independent of the (widely used) luminosity distance, which is a relative distance estimate. Using the published high-accuracy observational data of radial velocities and proper motions of the stars in the open cluster M11 region, we have determined the distance of M11 to be 1.89 ± 0.52kpc. This is in quite good agreement with the published luminosity distances of the cluster. We briefly discuss the problems concerned, including the sources of errors in the method and its applicable range.     

Cosmological constraint on Brans-Dicke Model

We combine new Cosmic Microwave Background(CMB) data from Planck with Baryon Acoustic Oscillation(BAO) data to constrain the Brans-Dicke(BD) theory,in which the gravitational constant G evolves with time.Observations of type Ia supernovae(SNe Ia) provide another important set of cosmological data,as they may be regarded as standard candles after some empirical corrections.However,in theories that include modified gravity like the BD theory,there is some risk and complication when using the SNIa data because their luminosity may depend on G.In this paper,we assume a power law relation between the SNIa luminosity and G,but treat the power index as a free parameter.We then test whether the difference in distances measured with SNIa data and BAO data can be reduced in such a model.We also constrain the BD theory and cosmological parameters by making a global fit with the CMB,BAO and SNIa data set.For the CMB+BAO+SNIa data set,we find 0.08 × 10~(-2) ζ 0.33 × 10~(-2) at the 68% confidence level(CL) and-0.01 × 10~(-2) ζ 0.43 × 10~(-2) at the 95% CL,where ζ is related to the BD parameter ω by ζ = ln(1 + 1/ω).     

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 × 10-3yr-1. This indicates that the double-detonation model only produces part of the SNe Ia. 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.     

The reionization of He II and the temperature evolution of the intergalactic medium

A number of observations suggest that He II in the intergalactic medium(IGM) was fully ionized at z ～ 3, probably by quasi-stellar objects(QSOs). Here we construct a simple model of a QSO to study the reionization of He II and the corresponding thermal evolution of the IGM. We assume that QSOs are triggered by major mergers of dark matter halos, and the luminosity evolution of individual QSOs is described by an initial accretion stage with a constant Eddington ratio and then a powerlaw decay driven by long term disk evolution or fueling. Once a QSO is triggered, it immediately ionizes its surrounding area as an ionized bubble. The resulting changes in size and volume of the bubble are determined by the luminosity evolution of the central QSO. With the emergence of more and more bubbles, they eventually overlap each other and finally permeate the whole universe. During the He II reionization,the IGM temperature increases due to the photoheating by the ionization processes.Applying the bubble model and considering various heating and cooling mechanisms,we trace the thermal evolution of the IGM and obtain the average IGM temperature as a function of redshift, which is very consistent with observations. The increase in IGM temperature due to the reionization of He II may be determined more accurately in the future, which may put robust constraints on the QSO model and the physics of He II reionization.     

Dark energy in five-dimensional Brans-Dicke cosmology with dimensional reduction

We explore a 5D Brans-Dicke scalar cosmology by conjecturing that the four-dimensional Hubble parameter varies as H = εφs,ε∈ R and s is some unknown power index and that the extra-dimensions compactify as the visible dimensions expand as b(t) ≈ ax(t) ,x ∈ R-. We mainly discuss the case x =-1. For critical values of ε close to unity,it was observed that the acceleration of the universe occurs at redshift close to z = 0.8 which indicates that in our model,accelerated expansion of the universe began only recen...     

The clustering of QSOs and the dark matter halos that host them

The spatial clustering of QSOs is an important measurable quantity which can be used to infer the properties of dark matter halos that host them.We construct a simple QSO model to explain the linear bias of QSOs measured by recent observations and explore the properties of dark matter halos that host a QSO.We assume that major mergers of dark matter halos can lead to the triggering of QSO phenomena,and the evolution of luminosity for a QSO generally shows two accretion phases,i.e.,initially having a constant Eddington ratio due to the self-regulation of the accretion process when supply is sufcient,and then declining in rate with time as a power law due to either diminished supply or long term disk evolution.Using a Markov Chain Monte Carlo method,the model parameters are constrained by fitting the observationally determined QSO luminosity functions（LFs）in the hard X-ray and in the optical band simultaneously.Adopting the model parameters that best fit the QSO LFs,the linear bias of QSOs can be predicted and then compared with the observational measurements by accounting for various selection efects in diferent QSO surveys.We find that the latest measurements of the linear bias of QSOs from both the SDSS and BOSS QSO surveys can be well reproduced.The typical mass of SDSS QSOs at redshift 1.53 6×1012h 1M⊙and the typical mass of BOSS QSOs at z².4 is²×1012h 1M⊙.For relatively faint QSOs,the mass distribution of their host dark matter halos is wider than that of bright QSOs because faint QSOs can be hosted in both big halos and smaller halos,but bright QSOs are only hosted in big halos,which is part of the reason for the predicted weak dependence of the linear biases on the QSO luminosity.     

Model-Synthesized Rate of Type Ia Supernovae and its Influence on the Chemical Enrichment of the ISM

Using Hurley＇s rapid binary stellar evolution code, we have studied the model-synthesized rate of Type Ia Supernovae （SNe Ia） and its influence on the chemical enrichment of the interstellar medium ejected by stellar populations. We adopt two popular scenarios, i.e., single degenerate scenario （SD） and double degenerate scenario （DD）, for the progenitors of SNe Ia to calculate the rates of SNe Ia. Rates calculated in this work agree with that of Hachisu et al. and Han ＆ Podsiadlowski, but are different from that usually adopted in chemical evolution models of galaxies. We apply the rates of SNe Ia to the chemical enrichment （especially Fe enrichment）, then compare the results with previous studies. As known SNe Ia slightly affect the enrichment of C, N, O and Mg elements, while significantly affect the enrichment of Fe. We find that the occurrence and the value of the Fe enrichment in our models are earlier and smaller than that commonly adopted in chemical evolution models. We also study the evolution of [Mg/Fe] ratios, which are almost reciprocals of the Fe enrichment. The study may provide constraints on the free parameters of chemical evolution models of galaxies and evolutionary population synthesis.     

DWT Power Spectrum of the Two-Degree Field Galaxy Redshift Survey

The power spectrum of the two-degree Field Galaxy Redshift Survey (2dFGRS) sample is estimated with the discrete wavelet transform (DWT) method. The DWT power spectra within 0.035 < k < 2.2 h Mpc-1 are measured for three volume-limited samples de-fined in consecutive absolute magnitude bins -19 ～-18, -20～ -19 and -21～-20. We show that the DWT power spectrum can effectively distinguish ACDM models of σs = 0.84 and σ8 = 0.74. We adopt maximum likelihood method to perform three-parameter fitting of the bias parameter b, pairwise velocity dispersion σpv and redshift distortion parame-ter β = Ω0.6m/b to the measured DWT power spectrum. The fitting results state that in a σ8= 0.84 universe the best-fit values of σm given by the three samples are mutually consis-tent within the range 0.28 ～ 0.36, and the best fitted values of σpv are 398 35-27475 37-29 and 550 ± 20km s-1 for the three samples, respectively. In the model of σ8 = 0.74, our three samples give very different values of Ωm. We repeated the fitting using the empirical formula of redshift distortion. The result of the model of low σ8 is still poor, especially, one of the best-fit values of σpv is as large as 103 km s-1. We also repeated our fitting by incorporating a scale-dependent galaxy bias. This gave a slightly lower value of Ωm. Differences between the models of σ8 = 0.84 and σ8 = 0.74 still exist in the fitting results. The power spectrum of 2dFGRS seems to disfavor models with low amplitude of density fluctuations if the bias parameter is assumed to be scale independent. For the fitting value of Ωm to be consistent with that given by WMAP3, strong scale dependence of the bias parameters is needed.     

Past and future of the central double-degenerate core of Henize 2–428

It has been suggested that Type Ia supernovae(SNe Ia) could be produced in the conditions of the violent merger scenario of the double-degenerate model, in which a thermonuclear explosion could be produced when a double carbon-oxygen white dwarf(CO WD) merges. It has been recently found that the nucleus of the bipolar planetary nebula Henize 2–428 consists of a double CO WD system that has a total mass of^1.76 M⊙, a mass ratio of^1 and an orbital period of^4.2 h, which is the first and only discovered progenitor candidate for an SN Ia predicted by the violent merger scenario. In this work, we aim to reproduce the evolutionary history of the central double CO WD of Henize 2–428. We find that the planetary nebula Henize 2–428 may originate from a primordial binary that has a^5.4 M⊙primary and a^2.7 M⊙secondary with an initial orbital period of^15.9 d. The double CO WD was formed after the primordial binary experienced two Roche-lobe overflows and two common-envelope ejection processes.According to our calculations, it takes about^840 Myr for the double CO WD to merge and form an SN Ia driven by gravitational wave radiation after their birth. To produce the current status of Henize 2–428,a large common-envelope parameter is needed. We also estimate that the rate of SNe Ia from the violent merger scenario is at most 2.9 × 10^-4 yr^-1, and that the delay time is in the range of^90 Myr to the Hubble time.     

Effects of Redshift on the Classifying Criteria of BL Lacertae Objects

We have collected a sample of 70 BL Lacs (33 radio-selected BL Lacs and 37 X-ray selected BL Lacs) with multi-waveband data for investigating the classifying criteria of BL Lacertae Objects. For each source, we estimate its luminosities in radio, optical and X-ray, the broad-band spectral index from radio to X-ray and the peak frequency of the syn- chrotron emission, and make a statistical analysis of the data obtained. Our main results are as follows: (1) The broad-band spectral index and the peak frequency have no correlation with the redshift, while they are inversely correlated with each other and they could be regarded as equivalent classifying criteria of BL Lac objects. (2) There are significant effects of the luminosity/redshift relation on the observed luminosity distribution in our sample, hence, if the radio luminosity is to be used as a classifying criterion of BL Lac objects, it should not be regarded as equivalent to the broad-band spectral index or the peak frequency. (3) Our re- sults supply a specific piece of evidence for the suggestion that the use of luminosities always introduces a redshift bias to the data and show that the location of the peak frequency is not always linked to the luminosity of any wave band.     

Numerical simulation of the Hall effect in magnetized accretion disks with the Pluto code

We investigate the Hall effect in a standard magnetized accretion disk which is accompanied by dissipation due to viscosity and magnetic resistivity.By considering an initial magnetic field,using the PLUTO code,we perform a numerical magnetohydrodynamic simulation in order to study the effect of Hall diffusion on the physical structure of the disk.Current density and temperature of the disk are significantly modified by Hall diffusion,but the global structure of the disk is not substantially affected.The changes in the current densities and temperature of the disk lead to a modification in the disk luminosity and radiation.