The Infiuence of Velocity-dependent Correction Factor on Proton Decay Reactions in Massive White Dwarfs

Twenty-five typical massive white dwarfs(WDs)are selected and the proton decay reaction catalyzed by magnetic monopoles(MMs)for these WDs is discussed.A velocity-dependent correction factor strongly affects the cross-section.We find that a strong suppression controls the monopole catalysis of nucleon decay by the correction factor.The maximum number of MMs is captured and the luminosity can be 2.235×10^(21)and 1.7859×10^(32)erg s^(-1)(e.g.,for the O+Ne core mass WD J055631.17+130639.78).The luminosities of most massive WDs agree well with the observations at relatively low temperatures(e.g.,T_(6)=0.1),but can be three and two orders of magnitude higher than those of the observations for model(Ⅰ)and(Ⅱ)at relatively high temperatures(e.g.,T_(6)=10),respectively.The luminosities of model(Ⅰ)are about one order of magnitude higher than those of model(Ⅱ).Since we consider the effect of the number of MMs captured on the mass–radius relation and the suppression of the proton decay by the correction factor,the study by model(Ⅱ)may be an improved estimation.

Proton Decay Reaction in Massive White Dwarfs

Two magnetic monopole models (i.e., model (I, II)) are presented to discuss the energy resources problem based on magnetic monopole catalytic nuclear decay in massive white dwarfs. We find that the luminosities for most of massive white dwarfs increase as the temperature increases. The luminosities of model (II) are agreed well with those of the observations at relativistic high temperature (e.g., T6=1,10), However, the luminosities of the observations can be five orders of magnitude larger than those of model (I).

Why Don’t Cold White Dwarfs Exist?

Why no late type M and much later type N white dwarfs with surface temperatures less than 3000 K had ever been observed? What are the heat sources of these later type white dwarfs? In this paper, we find that the energy source of white dwarfs is the nucleons decay catalyzed by magnetic monopoles.

Formation and Destiny of White Dwarf and Be Star Binaries

The binary systems consisting of a Be star and a white dwarf(Be WDs) are very interesting.They can originate from the binaries composed of a Be star and a subdwarf O or B star(Besd OBs),and they can merge into red giants via luminous red nova or can evolve into double WD potentially detected by the LISA mission.Using the method of population synthesis,we investigate the formation and the destiny of Be WDs,and discuss the effects of the metallicity(Z) and the common envelope evolution parameters.We find that Besd OBs are significant progenitors of Be WDs.About 30%(Z = 0.0001)-50%(Z = 0.02) of Be WDs come from Besd OBs.About 60%(Z = 0.0001)-70%(Z = 0.02) of Be WDs turn into red giants via a merger between a WD and a non-degenerated star.About 30%(Z = 0.0001)-40%(Z = 0.02) of Be WDs evolve into double WDs which are potential gravitational waves of the LISA mission at a frequency band between about 3 × 10^(-3)and 3 × 10^(-2)Hz.The common envelope evolution parameter introduces an uncertainty with a factor of about 1.3 on Be WD populations in our simulations.

The double helium-white dwarf channel for the formation of AM CVn binaries

Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation.However,a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer.Such stable mass transfer between two helium white dwarfs（He WDs） provides one channel for the production of AM CVn binary stars.In previous calculations of double He WD progenitors,the accreting He WD was treated as a point mass.We have computed the evolution of 16 double He WD models in order to investigate the consequences of treating the evolution of both components in detail.We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach.By comparing with observed periods and mass ratios,we redetermine masses of eight known AM CVn stars by our double He WDs channel,i.e.HM Cnc,AM CVn,V406 Hya,J0926,J1240,GP Com,Gaia14 aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240,GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double He WD channel.The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna（LISA） project.

A New Solution in Understanding Massive White Dwarfs

The observed high over-luminous type-Ia supernovae imply the existence of super-Chandrasekhar limit white dwarfs, which raises a challenge to the classical white dwarf theories. By employing the Eddington-inspired Born-Infeld （EiBI） gravity, we reinvestigate the structures and properties of white dwarfs, and find out that the EiBI gravity provides a new way to understand the observations. It is shown that by choosing an appropriate positive Eddington parameter k, a massive white dwarf with mass up to 2.8M can be supported by the equation of state of free electron gas. Unlike the classical white dwarf theory, the maximum mass of the white dwarf sequence in the EiBI gravity is not decided by the mass radius relations, but is decided by the central density, pc = 4.3 × 1014 kg/ms, above which neutronization cannot be avoided and the white dwarf will transform into a neutron star. On the other hand, if the gravity in the massive white dwarf really behaves as the EiBI gravity predicts, then one can obtain a constraint on the Eddington parameter in the EiBI gravity, that is, 87rpokG/c2 ≥ 80 （where po =- 10^18 kg/m3） to support a massive white dwarf with mass up to 2.8M. Moreover, we find out that the fast Keplarian frequency of the massive white dwarf raises a degeneration between the two kinds of compact stars, that is, one cannot distinguish whether the observed massive pulsar is a massive neutron star or a massive white dwarf only through the observed pulse frequency and mass.

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.

Measuring the Fundamental Parameters of Hot Hydrogen- Rich White Dwarfs

This review considers the observations of hot, hydrogen-rich white dwarfstars, with particular reference to measurements of temperature, surface gravity and composition.Spectroscopic data from a variety of wavelength ranges are required for this work and, inparticular, the important contributions from optical, ultraviolet and extreme ultraviolet studiesare discussed. Using the values of T_(eff) and log g determined for an individual white dwarf,estimates of mass and radius might be derived from the theoretical mass-radius relation. The issueof the accuracy of the theoretical mass-radius calculations and the prospects for making empiricaltests using observational data are outlined.

Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

The final outcomes of accreting ONe white dwarfs（ONe WDs） have been studied for several decades,but there are still some issues that are not resolved. Recently,some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper,we aim to investigate whether ONe WDs can experience accretion-induced collapse（AIC） or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics（MESA）,we simulate the longterm evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process,leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However,the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electroncapture induced collapse rather than thermonuclear explosion.

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 mass limit of white dwarfs with strong magnetic fields in general relativity

Recently, U. Das and B. Mukhopadhyay proposed that the Chandrasekhar limit of a white dwarf could reach a new high level （2.58M） if a superstrong magnetic field were considered （Das U and Mukhopadhyay B 2013 Phys. Rev. Lett. 110 071102）, where the structure of the strongly magnetized white dwarf （SMWD） is calculated in the framework of Newtonian theory （NT）. As the SMWD has a far smaller size, in contrast with the usual expectation, we found that there is an obvious general relativistic effect （GRE） in the SMWD. For example, for the SMWD with a one Landau level system, the super-Chandrasekhar mass limit in general relativity （GR） is approximately 16.5% lower than that in NT. More interestingly, the maximal mass of the white dwarf will be first increased when the magnetic field strength keeps on increasing and reaches the maximal value M = 2.48MQ with BD = 391.5. Then if we further increase the magnetic fields, surprisingly, the maximal mass of the white dwarf will decrease when one takes the GRE into account.

Possibility of Searching for Accreting White Dwarfs with the Chinese Space Station Telescope

Accreting WDs are very important for the studies of binary evolution,binary population synthesis and accretion physics.So far,there are a lot of accreting WD binaries with low accretion rates,such as cataclysmic variables,detected by different surveys.However,few accreting WD binaries with high accretion rates have been detected.In this paper,we studied the spectrum properties of accreting WD binaries and investigated whether accreting WD binaries with high accretion rates can be detected by the Chinese Space Station Telescope(CSST).We found that some accreting WD binaries with high accretion rates can be distinguishable from other types of stars with(NUV-y,u-y),(NUV-r,u-g),(NUV-i,u-g),(NUV-z,u-g)and(NUV-y,u-g)color-color diagrams.Therefore,some accreting WD binaries with high accretion rates can be detected by the CSST.

Hydrogen and helium shell burning during white dwarf accretion

Type Ia supernovae（SNe Ia） are believed to be thermonuclear explosions of carbon oxygen（CO） white dwarfs（WDs） with masses close to the Chandrasekhar mass limit. How a CO WD accretes matter and grows in mass to this limit is not well understood, hindering our understanding of SN Ia explosions and the reliability of using SNe Ia as a cosmological distance indicator. In this work, we employed the stellar evolution code MESA to simulate the accretion process of hydrogen-rich material onto a 1.0 M⊙CO WD at a high rate（over the Eddington limit） of 4.3 × 10^-7 M⊙yr^-1. The simulation demonstrates the characteristics of the double shell burning on top of the WD, with a hydrogen shell burning on top of a helium burning shell. The results show that helium shell burning is not steady（i.e.it flashes）. Flashes from the helium shell are weaker than those in the case of accretion of helium-rich material onto a CO WD. The carbon to oxygen mass ratio resulting from the helium shell burning is higher than what was previously thought. Interestingly, the CO WD growing due to accretion has an outer part containing a small fraction of helium in addition to carbon and oxygen. The flashes become weaker and weaker as the accretion continues.

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.

Noncommutative dispersion relation and mass-radius relation of white dwarfs

The equation of state of the electron degenerate gas in a white dwarf is usually treated by employing the ideal dispersion relation.However, the effect of quantum gravity is expected to be inevitably present and when this effect is considered through a non-commutative formulation, the dispersion relation undergoes a substantial modification.In this paper, we take such a modified dispersion relation and find the corresponding equation of state for the degenerate electron gas in white dwarfs.Hence we solve the equation of hydrostatic equilibrium and find that this leads to the possibility of the existence of excessively high values of masses exceeding the Chandrasekhar limit, although the quantum gravity effect is taken to be very small.It is only when we impose the additional effect of neutronization that we obtain white dwarfs with masses close to the Chandrasekhar limit with nonzero radii at the neutronization threshold.We demonstrate these results by giving numerical estimates for the masses and radii of helium, carbon and oxygen white dwarfs.

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.

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.

Iron group nuclei electron capture in super-Chandrasekhar superstrong magnetic white dwarfs

Using the theory of relativistic mean-field effective interactions,the influences of superstrong magnetic fields(SMFs)on electron Fermi energy,binding energy per nucleus and single-particle level structure are discussed in super-Chandrasekhar magnetic white dwarfs.Based on the relativistical SMFs theory model of Potekhin et al.,the electron chemical potential is corrected in SMFs,and the electron capture(EC)of iron group nuclei is investigated by using the Shell-Model Monte Carlo method and Random Phase Approximation theory.The EC rates can increase by more than three orders of magnitude due to the increase of the electron Fermi energy and the change of single-particle level structure by SMFs.However,the EC rates can decrease by more than four orders of magnitude due to increase of the nuclei binding energy by SMFs.We compare our results with those of FFNs(Fuller et al.),AUFDs(Aufderheide et al.)and Nabi(Nabi et al.).Our rates are higher by about four orders of magnitude than those of FFN,AUFD and Nabi due to SMFs.Our study may have important reference value for subsequent studies of the instability,mass radius relationship,and thermal and magnetic evolution of super-Chandrasekhar magnetic white dwarfs.

Maximum mass of magnetic white dwarfs

We revisit the problem of the maximum masses of magnetized white dwarfs （WDs）. The impact of a strong magnetic field on the structure equations is addressed. The pressures become anisotropic due to the presence of the magnetic field and split into parallel and perpendicular components. We first construct stable solutions of the Tolman-Oppenheimer-Volkoff equations for parallel pressures and find that physical solutions vanish for the perpendicular pressure whenB ≥ 10^13 G. This fact estab- lishes an upper bound for a magnetic field and the stability of the configurations in the （quasi） spherical approximation. Our findings also indicate that it is not possible to obtain stable magnetized WDs with super-Chandrasekhar masses because the val- ues of the magnetic field needed for them are higher than this bound. To proceed into the anisotropic regime, we can apply results for structure equations appropriate for a cylindrical metric with anisotropic pressures that were derived in our previous work. From the solutions of the structure equations in cylindrical symmetry we have con- firmed the same bound for B- 10^13 G, since beyond this value no physical solutions are possible. Our tentative conclusion is that massive WDs with masses well beyond the Chandrasekhar limit do not constitute stable solutions and should not exist.

On the DB gap of white dwarf evolution: effects of hydrogen mass fraction and convective overshooting

We investigate the spectral evolution of white dwarfs by considering the effects of hydrogen mass in the atmosphere and convective overshooting above the convection zone. Our numerical results show that white dwarfs with MH- 10^-16 MG show the DA spectral type between 46 000 ≤ Teff≤ 26 000 K and the DO or DB spectral type may appear on either side of this temperature range. White dwarfs with MH - 10^-15 M⊙ appear as DA stars until they cool to Teff - 31 000 K; from then on they will evolve into DB white dwarfs as a result of convective mixing. If MH in the white dwarfs is more than 10-14 M⊙, the convective mixing will not occur when Teff 〉 20 000 K, thus these white dwarfs always appear as DA stars. White dwarfs within the temperature range 46 000 ≤ Teff ≤ 31 000 K always show the DA spectral type, which coincides with the DB gap. We notice the importance of the convective overshooting and suggest that the overshooting length should be proportional to the thickness of the convection zone to better fit the observations.