The First Photometric and Spectroscopic Study of Contact Binary V2840 Cygni

The first photometric,spectroscopic and period variation studies of neglected short-period eclipsing binary V2840Cygni are presented.High mass ratio contact binaries(HMRCBs),especially those in the weak-contact configuration,are vital when probing the evolutionary models of contact binaries(CBs) using stellar parameters.The photometric solutions reveal the weak-contact nature of V2840 Cygni with a high mass ratio(~1.36),motivating us to investigate the nature of such binaries.The period variation study of V2840 Cygni spanning 15 yr shows a secular period decrease at a rate of ~5.5 × 10^(-7) day yr^(-1),indicating mass transfer between the components.The superimposed cyclic variation provides a basic understanding of the possible third body(P_(3)- 8 yr,m_(3)- 0.51 M_(⊙)).Following the derived parameters,the evolution of the system is discussed based on the thermal relaxation oscillation(TRO) model.It is found that V2840 Cygni falls in a special category of HMRCBs,which validates TRO.To characterize the nature of HMRCBs,a catalog of 59 CBs with high mass ratios has been compiled along with their derived parameters from the literature.For all the HMRCBs in the study,a possible correlation between their contact configuration and observed period variations for relative log J_(rel) is discussed.The spectroscopic study of V2840 Cygni provides evidence of the presence of magnetic activity in the system and the existence of ongoing mass transfer which is additionally deduced from the period variation study.The LAMOST spectra of 17 HMRCBs are collected to interpret the stellar magnetic activity in such systems.

On the Nature of Jets from a Main Sequence Companion at the Onset of Common Envelope Evolution

I consider a flow structure by which main sequence companions that enter a common envelope evolution(CEE)with giant stars might launch jets even when the accreted gas has a sub-Keplerian specific angular momentum.I first show that after a main sequence star enters the envelope of a giant star the specific angular momentum of the accreted gas is sub-Keplerian but still sufficiently large for the accreted gas to avoid two conical-like openings along the two opposite polar directions.I suggest that the high-pressure zone that the accreted gas builds around the main sequence equatorial plane accelerates outflows along these polar openings.Most of the inflowing gas is deflected to the polar outflows,i.e.,two oppositely directed jets.The actual mass that the main sequence star accretes is only a small fraction,≈0.1,of the inflowing gas.However,the gravitational energy that this gas releases powers the inflow-outflow streaming of gas and adds energy to the common envelope ejection.This flow structure might take place during a grazing envelope evolution if it occurs,during the early CEE and possibly in some post-CEE cases.This study increases the parameter space for main sequence stars to launch jets.Such jets might shape some morphological features in planetary nebulae,add energy to mass removal in CEE and power some intermediate luminosity optical transients.

Period Investigation on Two W UMa Binaries HH UMa and V1175 Her

HH UMa and V1175 Her are two W UMa contact binary systems whose periods were reported as undergoing secular increase.In this paper,we improved their period analyses with a more extensive database of eclipse timings,finding that both periods show cyclic variation.The cyclic variation could be attributed to a Light Travel Time Effect induced by a third body.Both circular orbit and eccentric orbit cases were considered.For HH UMa,the cyclic variation with a period of around 20 yr has been detected,which may be caused by a third body with the mass larger than 0.23 M_(⊙).However,no parabolic variation was detected in its O−C curve,implying the balance of the mass transfer between the two components and the angular momentum loss from the binary system.As to V1175 Her,a long-term period increase superposed on a periodic oscillation was detected.The period increase with a rate of about dP/dt=2×10^(-7)day yr^(−1)indicates the mass transfer from the less massive component to its companion.The cyclic variation of about 7.5 yr could be caused by a hierarchical third body with a minimal mass exceeding 0.46M_(⊙)orbiting around the central binary.This mass is larger than that of the less massive component of the binary,which means that the secondary component was not replaced by the third body during early stellar interactions,implying that it keeps original dynamical information.By removing angular momentum from the central binary system,the tertiary component has played a significant role in the formation of contact binaries.

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.

Past and Future of a Type Ia Supernovae Progenitor Candidate HD 265435

As one of the most useful cosmological distance indicators,type Ia supernovae(SNe Ia)play an important role in the study of cosmology.However,the progenitors of SNe Ia are still uncertain.It has been suggested that carbonoxygen white dwarf(CO WD)+He subgiant systems could produce SNe Ia through the double-degenerate(DD)model,in which the He subgiant transfers He-rich matter to the primary CO WD and finally evolves to another CO WD.Recently,a CO WD+He star system(i.e.,HD 265435)has been discovered to be a new SNe Ia progenitor candidate based on the DD model.The orbital period of the system is about 0.0688 days,and the masses of the CO WD and the He star are 1.01±0.15 M_(⊙) and 0.63_(-0.12)^(+0.13)M_(⊙),respectively.In this work,we evolve a large number of primordial binaries to the formation of CO WD+He star systems and investigate the evolutionary history of HD265435.We find that HD 265435 may originate from a primordial binary that has a 5.18 M_(⊙) primary and a3.66 M_(⊙) secondary with an initial orbital period of 5200 days.The CO WD+He star system would be formed after the primordial binary experiences two common-envelope ejection processes.We also find that HD 265435 would evolve to a double WD system with a total mass of 1.58 M⊙after a stable mass-transfer process,and the double WD system would merge driven by gravitational wave radiation.We estimate that it would take about 76 Myr for HD 265435 to form an SN Ia.In addition,HD 265435 would be a potential target of space-based gravitational wave observatories(e.g.,LISA,Taiji and TianQin).

ASAS J174406+2446.8 is identified as a marginal-contact binary with a possible cool third body

ASAS J174406+2446.8 was originally found as aδScuti-type pulsating star with the period P=0.189068 d by ASAS survey.However,the LAMOST stellar parameters reveal that it is far beyond the red edge of pulsational instability strip on the log g-T diagram ofδScuti pulsating stars.To understand the physical properties of the variable star,we observed it by the 1.0-m Cassegrain reflecting telescope at Yunnan Observatories.Multi-color light curves in B,V,R_c and I_c bands were obtained and are analyzed by using the W-D program.It is found that this variable star is a shallow-contact binary with an EB-type light curve and an orbital period of 0.3781 d rather than aδScuti star.It is a W-subtype contact binary with a mass ratio of 1.135(±0.019)and a fill-out factor of 10.4%(±5.6)%.The situation of ASAS J174406+2446.8 resembles those of other EB-type marginal-contact binaries such as UU Lyn,ⅡPer and GW Tau.All of them are at a key evolutionary phase from a semi-detached configuration to a contact system predicted by the thermal relaxation oscillation theory.The linear ephemeris was corrected by using 303 new determined times of light minimum.It is detected that the O-C curve shows a sinusoidal variation that could be explained by the light-travel-time effect via the presence of a cool red dwarf.The present investigation reveals that some of theδScuti-type stars beyond the red edge of pulsating instability strip on the log g-T diagram are misclassified eclipsing binaries.To understand their structures and evolutionary states,more studies are required in the future.

Photometric analysis and evolutionary stages of the contact binary V2790 Ori

A photometric analysis and evolutionary stages of the contact binary V2790 Ori are presented.The BV RC observations were carried out at the Thai National Observatory. The photometric light curves were fitted to provide fundamental parameters, required to examine evolutionary stages of the binary. The results indicate that V2790 Ori is a W-type contact system with a mass ratio of q = 2.932. The orbital period increase is found at a rate of d P/dt = 1.03×10^-7 d yr^-1. This implies that a rate of mass transfer from the secondary component to the primary one is dm2/dt =6.31×10^-8 M⊙yr^-1. Furthermore, we find that from the detached phase to the contact phase, the amount of mass that the evolved secondary component has lost is 1.188±0.110 M⊙, i.e., mass lost by the system is 0.789±0.073 M⊙and mass transfer to the primary is0.399±0.037 M⊙. Since the time of the first overflow, the angular momentum loss is found to be 72.2% of JFOF, causing the orbit and Roche surface to shrink until the present time.

Binary Population Synthesis

Binary interactions lead to the formation of intriguing objects,such as compact binaries,supernovae,gamma ray bursts,X-ray binaries,pulsars,novae,cataclysmic variables,hot subdwarf stars,barium stars and blue stragglers.To study the evolution of binary populations and the consequent formation of these objects,many methods have been developed over the years,for which a robust approach named binary population synthesis(BPS)warrants special attention.This approach has seen widespread application in many areas of astrophysics,including but not limited to analyses of the stellar content of galaxies,research on galactic chemical evolution and studies concerning star formation and cosmic re-ionization.In this review,we discuss the role of BPS,its general picture and the various components that comprise it.We pay special attention to the stability criteria for mass transfer in binaries,as this stability largely determines the fate of binary systems.We conclude with our perspectives regarding the future of this field.

Photometric investigations on two totally eclipsing contact binaries:V342 UMa and V509 Cam

By analyzing two sets of complete BV Rc Ic light curves for V342 UMa and three sets of complete BV Rc Ic light curves for V509 Cam, we determined that the two systems are both W-subtype contact binaries and that V342 UMa manifests a shallow contact configuration, while V509 Cam exhibits a medium contact configuration. Given that both of them are totally eclipsing binaries, the physical parameters derived only by the photometric light curves are reliable. Meanwhile, the period changes of the two targets were analyzed based on all available eclipsing times. We discovered that V342 UMa shows long-term period decrease with a rate of-1.02(±0.54)× 10^-7 d yr^-1 and that V509 Cam displays long-term period increase with a rate of 3.96(±0.90)× 10^-8 d yr^-1. Both the conservative mass transfer and angular momentum loss via magnetic stellar winds can be used to interpret the long-term period decrease of V342 UMa. The longterm period increase of V509 Cam can be explained by mass transfer from the less massive star to the more massive one. The absolute parameters of the two binaries were estimated according to their Gaia distances and our derived photometric solution results. This method can be extended to other contact binaries without radial velocities but with reliable photometric solutions. Their evolutionary states were investigated and we found that they reveal properties that are identical to other W-subtype contact systems.

The first photometric investigations of the G-type shallow contact binary IO Cnc

IO Cnc was classified to be a new G-type(G0)W UMa-type eclipsing binary system.Our first multicolor photometric solutions show that IO Cnc is a new W-subtype shallow contact binary with a fill-out factor of f=16.1%and a low mass ratio of q=3.12(or 1/q=0.32).During orbital period investigations,a cyclic variation and a downward parabolic variation with a rate of(-1.28±0.43)×10^(-7) d yr^(-1) was discovered in the observed–calculated(O-C)curve.The cyclic variation was analyzed by the light travel time effect(LTTE)via a potential red dwarf companion star,an orbital semi-major axis shorter than 4.88±0.82 AU was obtained.Finally,we collect physical parameters of a sample of 50 G-type shallow contact binaries(f≤20%),it is suggested that most of the G-type shallow contact binaries are undergoing a longterm and periodic orbital period changes,especially more systems show long-term decreases.The long-term orbital period decrease indicates that IO Cnc is in a mass transferring from the more massive component to the less massive one.With the long-term decrease of the orbital period,this shallow contact binary will evolve into a deeper contact one.

Evolution Toward the Observational Features of a Stripped Envelope TypeⅡb Supernova in a Binary System

TypeⅡb supernovae(SNeⅡb)that have a thin layer of hydrogen left in their outer envelope have been believed to belong to core collapse supernovae.Mass transfer via Roche lobe overflow can significantly change the nucleosynthesis and surface chemical elements of the progenitors of SNeⅡb.We aim to explore what conditions a close binary can meet with the observational features of SNeⅡb.We find that an observed low mass SNⅡb cannot be produced by a low mass isolated star with M<20,M_(⊙)due to the existence of a thick hydrogen envelope regardless of rotation.Binaries dominate as progenitors in the mass interval(i.e.,M<20 M_(⊙))considered in this paper.The 16 M_(⊙),primary with a 14 M_(⊙) companion in a binary system with~10 days <P_(orb) <720 days can reproduce observational features of SNe Ⅱb(i.e.,T_(eff),log L/L_(⊙),M_(He),M_(H),etc.).With the decrease of the hydrogen-rich envelope mass,the radius of the progenitor shrinks.The associated types of SNⅡb progenitors from RSGs and YSGs to BSGs are closely related to the amount of hydrogen left in the envelopes.Rotation can bring the production of the CNO reaction to the stellar surface at an early phase,which would explain the nitrogen-rich circumstellar material of SN 1993 J and can also explain the large He/H ratio of supernova ejecta.Rotation can increase the corresponding region of the orbital period which can produce an SNⅡb.

The Spin-up of a Star Gaining Mass in a Close Binary System on the Thermal Time Scale

We investigate the exchange of mass in a binary system as a channel through which a Be star can receive a rapid rotation.The mass-transfer phase in a massive close binary system in the Hertzsprung-gap is accompanied by the spinning up of the accreting component.We consider a case when the mass of the accreting component increases by 1.5 times.The component acquires mass and angular momentum while in a state of critical rotation.The angular momentum of the component increases by 50 times.Meridional circulation effectively transports angular momentum inside the component during the mass-transfer phase and during the thermal timescale after the end of the mass-transfer phase.As a result of mass transfer,the component acquires the rotation typical of classical Be stars.

Evolution of Close Binary System Parameter Distributions

In this paper,we investigate the orbital and stellar parameters of low-and intermediate-mass close binary systems.We use models,presented in the catalog of Han et al.and calculate parameters of accretors.We also construct distributions of systems along luminosity,semimajor axis and angular momentum,and make some conclusions on their evolution with time.We compare the results with observational data and it shows a good agreement.The set of theoretical models published quite adequately describes the observational data and,consequently,can be used to determine the evolutionary path of specific close binary systems,their initial parameter values and final stages.

An Orbital Period Study of the Contact Binary CK Bootis

Various times of light minima of the contact binary CK Boo are compiled and the correspnoding O C curve is analyzed with Kalimeris at al.’s method (1994). It is shown that the orbital period varies in a quasi sinusoidal form with a period of about 14 years and an amplitude of 0.26 × 10 -5 days while it undergoes a secular period increase dP/dE =+4 97 × 10 -7 days/year. The mechanisms that could explain the changes in the orbital period are also discussed.

Phase-Resolved UV Spectroscopy of V834 Cen and MR Ser in High and Intermediate States

We present a spectroscopic study of two polar systems, V834 Cen and MR Ser observed with IUE (International Ultraviolet Explorer) obtained during the period 1982-1991 and 1982-1987 for both systems respectively, to diagnose the ultraviolet fluxes of C IV 1550 and He II 1640 emission lines originating in the accretion stream during different orbital phases. Two spectra for both systems showing the variations in line fluxes at different orbital phases in high and intermediate states are presented. We concentrated on calculating the line fluxes of C IV & He II emission lines. Our results show that there is spectral variability for the line fluxes at different times, similar to that for optical spectrum of V834 Cen [1] and similar to that for the light curves of MR Ser [2]. We attribute this spectral variability to the variations of mass accretion rate [3]. Also we found that the line fluxes of both CIV and He II for V834 Cen are greater than the line fluxes of MR Ser.

A Photometric Study of the W UMa-type Eclipsing Binary System GSC 0445-1993

Several new light minimum times for the eclipsing binary GSC 0445-1993 have been determined from the observations by Koppelman et al. and the orbital period of this system was revised. A photometric analysis was carried out using the 2003 version of the Wilson-Devinney code. The results reveal that GSC 0445-1993 is a W-type eclipsing binary with a mass ratio of q = 0.323(±0.002) and an over-contact degree of f = 22.8%(±4.2%). A small temperature difference between the components of △T = 135 K and an orbital inclination of i = 65.7°(±0.3°) were obtained. The asymmetry of its light curve (i.e., the O'Connell effect) for this binary star is explained by the presence of a dark spot on the more massive component.

The rms-flux relation of Cyg X-2 in the horizontal branch

We have investigated the relation between the root mean square(rms) variability and the X-ray flux(rms-flux relation) of the Z source Cyg X-2,and as well the energy dependence based on the Rossi X-ray Timing Explorer(RXTE) observations.We currently focus on the horizontal branch(HB),due to the negative correlation in flux of the soft and the hard X-rays.The rms-flux correlation has energy dependence as follows:positive at hard X-rays(above 10 keV) but negative at soft X-rays(below 10 keV).This provides a feature different from the previous one,and may be suggestive of different origins of X-rays below and above 10 keV.Nevertheless,the overall spectrum can be well fitted with a model consisting of a blackbody and Comptonization components,but the fitting results do not reveal any features around 10 keV that could account for such a change in the rms-flux relation.