Theoretical Models of Highly Magnetic White Dwarf Stars with Non-Polytropic Equation of State

Super-massive white dwarf (WD) stars in the mass range 2.4 - 2.8 solar masses are believed to be the progenitors of “super-luminous” Type Ia supernovae according to a hypothesis proposed by some researchers. They theorize such a higher mass of the WD due to the presence of a very strong magnetic field inside it. We revisit their first work on magnetic WDs (MWDs) and present our theoretical results that are very different from theirs. The main reason for this difference is in the use of the equation of state (EoS) to make stellar models of MWDs. An electron gas in a magnetic field is Landau quantized and hence, the resulting EoS becomes non-polytropic. By constructing models of MWDs using such an EoS, we highlight that a strong magnetic field inside a WD would make the star super-massive. We have found that our stellar models do indeed fall in the mass range given above. Moreover, we are also able to address an observational finding that the mean mass of MWDs are almost double that of non-magnetic WDs. Magnetic field changes the momentum-space of the electrons which in turn changes their density of states (DOS), and that in turn changes the EoS of matter inside the star. By correlating the magnetic DOS with the non-polytropic EoS, we were also able to find a physical reason behind our theoretical result of super-massive WDs with strong magnetic fields. In order to construct these models, we have considered different equations of state with at most three Landau levels occupied and have plotted our results as mass-radius relations for a particular chosen value of maximum Fermi energy. Our results also show that a multiple Landau-level system of electrons leads to such an EoS that gives multiple branches in the mass-radius relations, and that the super-massive MWDs are obtained when the Landau-level occupancy is limited to just one level. Finally, our theoretical results can be explained solely on the basis of quantum and statistical mechanics that warrant no assumptions regarding stars.

M dwarf stars—the by-product of X-ray selected AGN candidates

X-ray loud M dwarfs are a major source of by-products （contamination） in the X-ray band of the multiwavelength quasar survey. As a by-product, the low dispersion spectra of 22 M dwarfs are obtained in which the spectra of 16 sources are taken for the first time. The spectral types and distances of the sample are given based on spectral indices CaH2, CaH3, and TiO5. The parameter ζ TiO/CaH is calculated to separate the different metallicity classes among dwarfs, subdwarfs and extreme subdwarfs. We also discuss the distributions in the diagrams of log（L x /L bol ）, the ratio between X-ray and bolometric luminosity versus spectral type and infrared colors.

Asteroseismology of DAV white dwarf stars and G29–38

Asteroseismology is a powerful tool used for detecting the inner structure of stars, which is also widely used to study white dwarfs. We discuss the asteroseismology of DAV stars. The period-to-period fitting method is discussed in detail, including its reliability in detecting the inner structure of DAV stars. If we assume that all observed modes of some DAV stars are the l = I cases, the errors associated with model fitting will be always large. If we assume that the observed modes are com- posed of I = 1 and 2 modes, the errors associated with model fitting in this case will be small. However, there will be modes identified as l = 2 that do not have ob- served quintuplets. G29-38 has been observed spectroscopically and photometrically for many years. Thompson et al. made 1 modes identifications in the star through the limb darkening effect. With 11 known I modes, we also study the asteroseismology of G29-38, which reduces the blind l fittings and is a fair choice. Unfortunately, our two best-fitting models are not in line with the previous atmospheric results. Based on factors like only a few observed modes, stability and identification of eigenmodes, identification of spherical degrees, construction of physical and realistic models and so on, detecting the inner structure of DAV stars by asteroseismology needs further development.

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.

Quark-novae in neutron star - white dwarf binaries: a model for luminous (spin-down powered) sub-Chandrasekhar-mass Type Ia supernovae?

We show that, by appealing to a Quark-Nova （QN） in a tight binary system containing a massive neutron star and a CO white dwarf （WD）, a Type Ia explosion could occur. The QN ejecta collides with the WD, driving a shock that triggers carbon burning under degenerate conditions （the QN-Ia）. The conditions in the compressed low-mass WD （MwD 〈 0.9 M） in our model mimic those of a Chandrasekhar mass WD. The spin-down luminosity from the QN compact remnant （the quark star） pro- vides additional power that makes the QN-Ia light-curve brighter and broader than a standard SN-Ia with similar 56Ni yield. In QNe-Ia, photometry and spectroscopy are not necessarily linked since the kinetic energy of the ejecta has a contribution from spin-down power and nuclear decay. Although QNe-Ia may not obey the Phillips relationship, their brightness and their relatively ＂normal looking＂ light-curves mean they could be included in the cosmological sample. Light-curve fitters would be con- fused by the discrepancy between spectroscopy at peak and photometry and would correct for it by effectively brightening or dimming the QNe-Ia apparent magnitudes, thus over- or under-estimating the true magnitude of these spin-down powered SNe-Ia. Contamination of QNe-Ia in samples of SNe-Ia used for cosmological analyses could systematically bias measurements of cosmological parameters if QNe-Ia are numerous enough at high-redshift. The strong mixing induced by spin-down wind combined with the low 56Ni yields in QNe-Ia means that these would lack a secondary maximum in the/-band despite their luminous nature. We discuss possible QNe-Ia progenitors.

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 Remnant of Neutron Star-White Dwarf Merger and the Repeating Fast Radio Bursts

Fast radio bursts (FRBs) at cosmological distances still hold concealed physical origins. Previously Liu (2018) proposes a scenario that the collision between a neutron star (NS) and a white dwarf (WD) can be one of the progenitors of non-repeating FRBs and notices that the repeating FRBs can also be explained if a magnetar formed after such NS-WD merger. In this paper, we investigate this channel of magnetar formation in more detail. We propose that the NS-WD post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new NS or even remains as a hybrid WDNS, depending on the total mass of the NS and WD. In particular, the newly formed NS can be a magnetar if the core of the WD collapsed into the NS while large quantities of degenerate electrons of the WD compressed to the outer layers of the new NS. A strong magnetic field can be formed by the electrons and positive charges with different angular velocities induced by the differential rotation of the newborn magnetar. Such a magnetar can power the repeating FRBs by the magnetic reconnections due to the crustal movements or starquakes.

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.

Periodic Variation Studies of the Two Short Period W UMa-type Eclipsing Binaries: LX Lyn and V0853 Aur

In this paper,new light curves(LCs) of contact eclipsing binary(CEB) systems LX Lyn and V0853 Aur are presented and analyzed by using the 2015 version of the Wilson-Devinney(W-D) code.In order to explain their asymmetric LCs,cool starspots on the components were employed.It is suggested that their fill-out degrees are f=12.0%(LX Lyn) and f=26.3%(V0853 Aur).At the same time,we found that LX Lyn is a W-type eclipsing binary(EB) with an orbital inclination of i=84°.88 and a mass ratio of q=2.31.V0853 Aur is also a W-type CEB with a mass ratio of q=2.77 and an orbital inclination of i= 79°.26.Based on all available times of light minimum,their orbital period changes are studied by using the O-C method.The O-C diagram of LX Lyn reveals a cyclic oscillation with a period of about 14.84 yr and an amplitude of 0.0019 days,which can be explained by the light-travel time effect(LTTE) due to the presence of a third body with a minimum mass of0.06M_⊙.For V0853 Aur,it is discovered that the O-C diagram of the system also shows a cyclic oscillation with a period of 9.64 yr and an amplitude of 0.03365 days.The cyclic oscillation of V0853 Aur can be attributed to the LTTE by means of a third body with a mass no less than 3.77M_⊙.The third body may play an important role in the formation and evolution of these systems.

Kinematics and activity of M dwarfs in LAMOST DR1

We report on the first investigation into kinematics and chromospheric activity of M dwarfs from the Guo Shou Jing Telescope （also called the Large Sky Area Multi-Object Fiber Spectroscopic Telescope - LAMOST） data release one （DR1）. The sample comprises 71 304 M dwarfs. Their fundamental parameters such as spectral types, radial velocities, important molecular band indices and magnetic activities are measured. Their distances are determined by a spectroscopic parallax relation. Space motion （U, V, W） and Galactocentric cylindrical coordinates （R, θ, Z） for the M dwarfs are also computed. We examine velocity dispersion as a function of height from the Galactic plane and find that all three components of velocity dispersion in- crease with height as measured with respect to the Galactic plane. The investigation into chromospheric activities along the height from the Galactic plane confirms that M dwarfs closer to the Galactic plane are more likely to be active. We take a pure kinematical approach to select thin disk stars and thick disk stars from our sample, then to investigate the differences in properties between these two populations. Our analysis is in excellent agreement with previous studies and leads to a better understanding of the structure of the Galactic disk.

A catalog of DB white dwarfs from the LAMOST DR5 and construction of templates

In this study, we employ machine learning to build a catalog of DB white dwarfs(DBWDs) from the LAMOST Data Release(DR) 5. Using known DBs from SDSS DR14, we selected samples of highquality DB spectra from the LAMOST database and applied them to train the machine learning process.Following the recognition procedure, we chose 351 DB spectra of 287 objects, 53 of which were new identifications. We then utilized all the DBWD spectra from both SDSS DR14 and LAMOST DR5 to construct DB templates for LAMOST 1 D pipeline reductions. Finally, by applying DB parameter models provided by D. Koester and the distance from Gaia DR2, we calculated the effective temperatures, surface gravities and distributions of the 3 D locations and velocities of all DBWDs.

Atmospheric regimes and trends on exoplanets and brown dwarfs

A planetary atmosphere is the outer gas layer of a planet. Besides its scientific significance among the first and most accessible planetary layers observed from space, it is closely connected with planetary formation and evolution, surface and interior processes, and habitability of planets. Current theories of planetary atmospheres were primarily obtained through the studies of eight large planets, Pluto and three large moons(Io, Titan, and Triton) in the Solar System. Outside the Solar System, more than four thousand extrasolar planets(exoplanets) and two thousand brown dwarfs have been confirmed in our Galaxy, and their population is rapidly growing. The rich information from these exotic bodies offers a database to test, in a statistical sense, the fundamental theories of planetary climates. Here we review the current knowledge on atmospheres of exoplanets and brown dwarfs from recent observations and theories. This review highlights important regimes and statistical trends in an ensemble of atmospheres as an initial step towards fully characterizing diverse substellar atmospheres, that illustrates the underlying principles and critical problems.Insights are obtained through analysis of the dependence of atmospheric characteristics on basic planetary parameters. Dominant processes that influence atmospheric stability, energy transport, temperature, composition and flow pattern are discussed and elaborated with simple scaling laws. We dedicate this review to Dr. Adam P. Showman(1968–2020) in recognition of his fundamental contribution to the understanding of atmospheric dynamics on giant planets, exoplanets and brown dwarfs.

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.

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.

A Spectroscopic Study of the SU UMa-type Dwarf Nova YZ Cnc during its 2002 Superoutburst

We report on time-resolved spectroscopic observations of the SU Ursae Majoris dwarf nova, YZ Cnc for over llh on two nights during its 2002 January superoutburst. The spectra on the first day only showed absorption-line profiles, while on the second day the lines showed “W” profiles with blue and red troughs. The radial velocity curve of the absorption troughs and emission peaks of H/3 has an amplitude of 49±10 km s^-1 and a phase offset of -0.07±0.04, which are very similar to those measured in quiescence. However, the γ velocity deviates strongly from the systemic velocity measured in quiescence, by some ±60 km s^-1. Large shifts of -70 km s^-1 in the orbital-averaged velocity and -0.09 in the phase are also found in our observations. All these features can be well explained by a precessing, eccentric disk.

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.

1RXS J232953.9+062814: a New SU UMa Dwarf Nova below the Period Minimum

RXS 1232953.9+062814 was identified as a cataclysmic variable by Wei et al. (1999). Four low-resolution spectra of 1RXS J232953.9+062814 were obtained with the 2.16-rn telescope of the National Astronomical Observatories, of which two were at outburst, and two were at quiescence. The system is about 16.8 B and 16.5 V at quiescence, and 12.6 B and 12.6 V at outburst. The quiescent spectra were dominated by double-peaked Balmer emissions, which indicates a hydrogen-rich system with a high-inclination accretion disc. MgH and TiO absorption bands appeared in the quiescent spectrum which implies a companion with a spectral type of early M dwarf. If we take it as a MO dwarf, then the system is located at a distance of 350 pc with a proper motion velocity 150 km s-1. The superhump period of 0.046311 days was confirmed by our V photometry. The short period and the hydrogen-rich nature reveal that this system is another SU Ursae Majoris-type dwarf nova below the period minimum after V485 Centauri. 1RXS J23 2953.9+062814 is one of the most important systems for studying the evolutionary scenario of cataclysmic variables since it is much brighter than V485 Cen.

GSC 4560–02157: a new long-period eclipsing cataclysmic variable star

We study the newly discovered variable star GSC 4560-02157. CCD photometry was performed in 2013-2014 at the Tien Shan Astronomical Observatory,and a spectrum was obtained with the 6-m telescope of the Special Astrophysical Observatory in June, 2014. GSC 4560-02157 is demonstrated to be an eclipsing variable star（P = 0.265359d）. All its flat-bottom primary minima are approximately at the same brightness level, but the star＇s out-of-eclipse brightness and brightness at secondary minima vary considerably（by up to 0.6m） from cycle to cycle. Moreover,there are short-term（time scale of 0.03-0.04 days） small-amplitude brightness variations out of eclipse. This behavior suggests a cataclysmic nature for the star, which is confirmed with a spectrum taken on 2014 June 5. The spectrum shows numerous emissions of the hydrogen Balmer series, He Ⅰ and He Ⅱ.

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.