Supernova 1987A's Keyhole:A Long-lived Jet-pair in the Final Explosion Phase of Core-collapse Supernovae

I further study the manner by which a pair of opposite jets shape the“keyhole”morphological structure of the core-collapse supernova(CCSN)SN 1997A,now the CCSN remnant(CCSNR)1987A.By doing so,I strengthen the claim that the jittering-jet explosion mechanism accounts for most,likely all,CCSNe.The“keyhole”structure comprises a northern low-intensity zone closed with a bright rim on its front and an elongated low-intensity nozzle in the south.This rim-nozzle asymmetry is observed in some cooling flow clusters and planetary nebulae that are observed to be shaped by jets.I build a toy model that uses the planar jittering jets pattern,where consecutive pairs of jets tend to jitter in a common plane,implying that the accreted gas onto the newly born neutron star at the late explosion phase flows perpendicular to that plane.This allows for a long-lived jet-launching episode.This long-lasting jet-launching episode launches more mass into the jets that can inflate larger pairs of ears or bubbles,forming the main jets'axis of the CCSNR that is not necessarily related to a possible pre-collapse core rotation.I discuss the relation of the main jets'axis to the neutron star's natal kick velocity.

Rotating Massive Strangeon Stars and X-Ray Plateau of Short GRBs

Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the remnants of binary strangeon star mergers could even be long-lived massive strangeon stars.We study rigidly rotating strangeon stars in the slowly rotating approximation,using the Lennard-Jones model for the equation of state.Rotation can significantly increase the maximum mass of strangeon stars with unchanged baryon numbers,enlarging the mass-range of long-lived strangeon stars.During spin-down after merger,the decrease of radius of the remnant will lead to the release of gravitational energy.Taking into account the efficiency of converting the gravitational energy luminosity to the observed X-ray luminosity,we find that the gravitational energy could provide an alternative energy source for the plateau emission of X-ray afterglow.The fitting results of X-ray plateau emission of some short gamma-ray bursts suggest that the magnetic dipole field strength of the remnants can be much smaller than that of expected when the plateau emission is powered only by spin-down luminosity of magnetars.

Variable Stars in the 50BiN Open Cluster Survey.Ⅲ.NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way.Variable stars in open clusters provide a unique way to accurately probe the internal structure,temporal and dynamical evolutionary stages of individual stars and the host cluster.The most powerful tool for such studies is time-domain photometric observations.This paper follows the route of our previous work,concentrating on a photometric search for variable stars in NGC 884.The target cluster is the companion of NGC869,forming the well-known double cluster system that is gravitationally bound.From the observation run in 2016 November,a total of 9247 B-band CCD images and 8218Ⅴ-band CCD images were obtained.We detected a total of 15 stars with variability in visual brightness,including five Be stars,three eclipsing binaries,and seven of unknown types.Two new variable stars were discovered in this work.We also compared the variable star content of NGC 884 with its companion NGC 869.

Prospects of the Multi-channel Photometric Survey Telescope in the Cosmological Application of Type Ia Supernovae

The Multi-channel Photometric Survey Telescope(Mephisto)is a real-time,three-color photometric system designed to capture the color evolution of stars and transients accurately.This telescope system can be crucial in cosmological distance measurements of low-redshift(low-z,z■0.1)Type Ia supernovae(SNe Ia).To optimize the capabilities of this instrument,we perform a comprehensive simulation study before its official operation is scheduled to start.By considering the impact of atmospheric extinction,weather conditions,and the lunar phase at the observing site involving the instrumental features,we simulate light curves of SNe Ia obtained by Mephisto.The best strategy in the case of SN Ia cosmology is to take the image at an exposure time of 130 s with a cadence of 3 days.In this condition,Mephisto can obtain hundreds of high-quality SNe Ia to achieve a distance measurement better than 4.5%.Given the on-time spectral classification and monitoring of the Lijiang 2.4 m Telescope at the same observatory,Mephisto,in the whole operation,can significantly enrich the well-calibrated sample of supernovae at low-z and improve the calibration accuracy of high-z SNe Ia.

Phases of Dense Matter in Supernovae and Neutron Stars

We study the properties of dense matter at finite temperature with various proton fractions for use in supernova simulations. The relativistic mean-field theory is used to describe homogeneous nuclear matter, while the Thomas-Fermi approximation is adopted to describe inhomogeneous matter. We also discuss the equation of state of neutron star matter at zero temperature in a wide density range. The equation of state at high densities can be significantly softened by the inclusion of hyperons.

A search for metal-poor stars pre-enriched by pair-instability supernovae I. A pilot study for target selection from Sloan Digital Sky Survey

We report on a pilot study on identifying metal-poor stars pre-enriched by Pair-Instability Supernovae（PISNe）.Very massive,first generation（Population Ⅲ） stars（140 M⊙≤M≤260 M⊙）end their lives as PISNe,which have been predicted by theories,but no relics of PISNe have been observed yet.Among the distinct characteristics of the yields of PISNe,as predicted by theoretical calculations,are a strong odd-even effect,and a strong overabundance of Ca with respect to iron and the solar ratio.We use the latter characteristic to identify metal-poor stars in the Galactic halo that have been pre-enriched by PISNe,by comparing metallicites derived from strong, co-added Fe lines detected in low-resolution（i.e.,R=λ/△λ～2000）spectra of the Sloan Digital Sky Survey（SDSS）,with metallicities determined by the SDSS Stellar Parameters Pipeline（SSPP）.The latter are based on the strength of the CaⅡ K line and assumptions on the Ca/Fe abundance ratio.Stars are selected as candidates if their metallicity derived from Fe lines is significantly lower than the SSPP metallicities.In a sample of 12 300 stars for which SDSS spectroscopy is available,we have identified 18 candidate stars.Higher resolution and signal-to-noise ratio spectra of these candidates are being obtained with the Very Large Telescope of the European Southern Observatory and the XSHOOTER spectrograph,to determine their abundance patterns,and to verify our selection method.We plan to apply our method to the database of several million stellar spectra to be acquired with the Guo Shou Jing Telescope （LAMOST）in the next five years.

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).

Searching For Wide Binary Stars with Non-coeval Components in the Southern Sky

We have completed our observational program to search for wide binary systems with non-coeval components in the southern sky and report our results here.The final set of four systems was spectroscopically investigated in this paper.No binary systems with components of different ages were found among them.Taking into account our previous studies,we estimate the fraction of such binaries(i.e.,binaries formed,presumably,by capture)to be not higher than 0.06%.The study will be continued on the northern sky.

Revise Thermal Winds of Remnant Neutron Stars in Gamma-Ray Bursts

It seems that the wealth of information revealed by the multi-messenger observations of the binary neutron star(NS)merger event,GW170817/GRB 170817A/kilonova AT2017gfo,places irreconcilable constraints to models of the prompt emission of this gamma-ray burst(GRB).The observed time delay between the merger of the two NSs and the trigger of the GRB and the thermal tail of the prompt emission can hardly be reproduced by these models simultaneously.We argue that the merger remnant should be an NS(last for,at least,a large fraction of 1 s),and that the difficulty can be alleviated by the delayed formation of the accretion disk due to the absorption of high-energy neutrinos emitted by the NS and the delayed emergence of effective viscosity in the disk.Further,we extend the consideration of the effect of the energy deposition of neutrinos emitted from the NS.If the NS is the central object of a GRB with a distance and duration similar to that of GRB 170817A,thermal emission of the thermal bubble inflated by the NS after the termination of accretion may be detectable.If our scenario is verified,it would be of interest to investigate the cooling of nascent NSs.

The Variability and Radial Velocity of Planetary Nebula Central Stars

The extremely accurate estimates of stellar variability and radial velocity in the Gaia Data Release 3(Gaia DR3)have enabled us to examine the close binarity and radial velocity(RV)of central stars(CSs)of planetary nebulae(PNe).This study is twofold:(1)searching for new close binary CS candidates to better understand how binarity affects the formation and evolution of PNe;and(2)extending the sample size of known RVs of PNe in order to understand their kinematics and the dynamics of the Milky Way.As a target sample,we used all true,possible,and likely PNe available in the literature.Then,we looked for their matched Gaia DR3 sources that provide measurements of variability and RV.As a result,we detected the first large collection of trustworthy photometric variability of 26 symbiotic stars and 82 CSs.In this CS group,there are 24 sources already classified as true close binary CSs in the literature.Hence,we discovered 58 new close binary CS candidates.This close binary(CB)sample represents more than half of what is currently available in the literature.In addition,we identified the radial velocities for 51 PNe.To our knowledge,24 of these were measured for the first time.The RV measurements predicted by Gaia,based on the Doppler shift of the CS absorption lines,and those derived from nebular emission lines,show satisfactory agreement except for a few extremely high-velocity PNe.

Predicting Gravitational Waves from Jittering-jets-driven Core Collapse Supernovae

I estimate the frequencies of gravitational waves from jittering jets that explode core collapse supernovae(CCSNe)to crudely be 5–30 Hz,and with strains that might allow detection of Galactic CCSNe.The jittering jets explosion mechanism(JJEM)asserts that most CCSNe are exploded by jittering jets that the newly born neutron star(NS)launches within a few seconds.According to the JJEM,instabilities in the accreted gas lead to the formation of intermittent accretion disks that launch the jittering jets.Earlier studies that did not include jets calculated the gravitational frequencies that instabilities around the NS emit to have a peak in the crude frequency range of 100–2000Hz.Based on a recent study,I take the source of the gravitational waves of jittering jets to be the turbulent bubbles(cocoons)that the jets inflate as they interact with the outer layers of the core of the star at thousands of kilometers from the NS.The lower frequencies and larger strains than those of gravitational waves from instabilities in CCSNe allow future,and maybe present,detectors to identify the gravitational wave signals of jittering jets.Detection of gravitational waves from local CCSNe might distinguish between the neutrino-driven explosion mechanism and the JJEM.

The Study of the Physical Properties and Energy Sources of Five Luminous Type Ibc Supernovae

In this paper,we study five luminous supernovae(LSNe)Ibc(SN 2009ca,ASASSN-15mj,SN 2019omd,SN 2002ued,and SN 2021bmf)whose peak absolute magnitudes M_(peakare)≈-19.5 to-21 mag by fitting their multi-band light curves(LCs)with different energy source models.We find that SN 2009ca might be powered by the^(56)Ni model since the required^(56)Ni mass(0.56 M_(⊙))is comparable to those of energetic SNe Ic,while the rest four SNe cannot be accounted for the^(56)Ni model since their derived^(56)Ni masses are(?)1 M_(⊙)or the ratios of the^(56)Ni mass to the ejecta mass are larger than 0.2.This indicates that some LSNe might be powered by^(56)Ni decay,while most of them need additional energy sources.We then use the magnetar plus^(56)Ni model and the fallback plus^(56)Ni model to fit the LCs of the four LSNe that cannot be explained by the^(56)Ni model,finding that the two models can account for the four SNe,and the derived parameters are comparable to those of LSNe or superluminous SNe in the literature,if they were(mainly)powered by magnetars or fallback.We suggest that the magnetar plus^(56)Ni model is more reasonable than the fallback plus^(56)Ni model,since the validity of the fallback plus^(56)Ni model depends on the value of accretion efficiency(η)and favors a largeηvalue,and the magnetar plus^(56)Ni model yields smallerχ^(2)/dof values.It should be pointed out that,however,the fallback plus^(56)Ni model is still a promising model that can account for the four SNe in our sample as well as other LSNe.

The Progenitors of Type Ia Supernovae with Asymptotic Giant Branch Donors

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

The Energy Sources and the Explosion Mechanism of Ca-rich Supernova PTF 10iuv

In this paper,we perform the detailed modeling for the light curves(LCs)of PTF 10iuv which is a calcium-rich(Ca-rich)supernova(SN)to constrain the physical properties of its ejecta and the energy sources,as well as the explosion mechanism.We find that the^(56)Ni model and the56Ni plus circumstellar interaction model fail to explain the LCs,while the four-element(^(56)Ni,^(48)Cr,^(52)Fe,and^(44)Ti)model can account for the LCs.The ejecta mass of PTF10iuv derived by the model(1.52_(-0.25)^(+0.34)M_(⊙))is consistent with that of the merger of a sub-Chandrasekhar mass white dwarf.The early-time LCs were mainly powered by^(56)Ni whose mass is~0.03 M_(⊙),while the contributions of^(48)Cr and^(52)Fe can be neglected.The derived^(44)Ti mass(~0.25 M_(⊙))is~1.8 times the upper limit of the derived^(44)Ti mass of Ca-rich SN 2005E.We suggest that subtracting the contributions of the host-galaxy,which are unknown,and including the flux from other long-lived elements(e.g.,^(57)Co,^(55)Fe,^(60)Co)can reduce the amount of^(44)Ti,and that this value can be regarded as an upper limit.

Point-symmetry in SNR G1.9+0.3:A Supernova that Destroyed its Planetary Nebula Progenitor

I analyze a new X-ray image of the youngest supernova remnant(SNR)in the Galaxy,which is the type Ia SNR G1.9+0.3,and reveal a very clear point-symmetrical structure.Since explosion models of type Ia supernovae(SNe Ia)do not form such morphologies,the point-symmetrical morphology must come from the circumstellar material(CSM)into which the ejecta expands.The large-scale point-symmetry that I identify and the known substantial deceleration of the ejecta of SNR G1.9+0.3 suggest a relatively massive CSM of■1M⊙.I argue that the most likely explanation is the explosion of this SN Ia into a planetary nebula.The scenario that predicts a large fraction of SN Ia inside PNe(SNIPs)is the core degenerate scenario.Other SN Ia scenarios might lead to only a very small fraction of SNIPs or none at all.

GRB 210323A:Signature of Long-lasting Lifetime of Supra-massive Magnetar as the Central Engine from the Merger of Binary Neutron Star

Theoretically,a supra-massive neutron star or magnetar may be formed after the merger of binary neutron stars.GRB210323A is a short-duration gamma-ray burst(GRB)with a duration of lasting~1 s.The light curve of the prompt emission of GRB 210323A shows a signal-peaked structure and a cutoff power-law model can adequately fit the spectra with E_p=1826±747.More interestingly,it has an extremely long-lasting plateau emission in the X-ray afterglow with a duration of~10^(4)s,and then follows a rapid decay with a decay slope~3.2.This temporal feature is challenging by invoking the external shock mode.In this paper,we suggest that the observed long-lasting X-ray plateau emission is caused by the energy injection of dipole radiation from supra-massive magnetar,and the abrupt decay following the longlasting X-ray plateau emission is explained by supra-massive magnetar collapsing into a black hole.It is the short GRB(SGRB)with the longest X-ray internal plateau emission powered by a supra-massive neutron star.If this is the case,one can estimate the physical parameters of a supra-massive magnetar,and compare with other SGRBs.We also discuss the possible gravitational-wave emission,which is powered by a supra-massive magnetar and its detectability,and the possible kilonova emission,which is powered by r-process and magnetar spin-down to compare with the observed data.

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.

Amplifying magnetic fields of a newly born neutron star by stochastic angular momentum accretion in core collapse supernovae

I present a novel mechanism to boost magnetic field amplification of newly born neutron stars in core collapse supernovae.In this mechanism,that operates in the jittering jets explosion mechanism and comes on top of the regular magnetic field amplification by turbulence,the accretion of stochastic angular momentum in core collapse supernovae forms a neutron star with strong initial magnetic fields but with a slow rotation.The varying angular momentum of the accreted gas,which is unique to the jittering jets explosion mechanism,exerts a varying azimuthal shear on the magnetic fields of the accreted mass near the surface of the neutron star.This,I argue,can form an amplifying effect which I term the stochastic omega(Sω) effect.In the common αω dynamo the rotation has constant direction and value,and hence supplies a constant azimuthal shear,while the convection has a stochastic behavior.In the Sω dynamo the stochastic angular momentum is different from turbulence in that it operates on a large scale,and it is different from a regular rotational shear in being stochastic.The basic assumption is that because of the varying direction of the angular momentum axis from one accretion episode to the next,the rotational flow of an accretion episode stretches the magnetic fields that were amplified in the previous episode.I estimate the amplification factor of the Sω dynamo alone to be ≈ 10.I speculate that the Sω effect accounts for a recent finding that many neutron stars are born with strong magnetic fields.

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.

On the Rotation Properties of a Post-explosion Helium-star Companion in Type Iax Supernovae

Recent studies have suggested that type lax supernovae(SNe lax) are likely to result from a weak deflagration explosion of a Chandrasekhar-mass white dwarf in a binary system with a helium(He)-star companion.Assuming that most SNe Iax are produced from this scenario,in this work we extend our previous work on the threedimensional hydrodynamical simulation of ejecta-companion interaction by taking the orbital and spin velocities of the progenitor system into account.We then follow the post-impact evolution of a surviving He-star companion by using the one-dimensional stellar evolution code MESA.We aim to investigate the post-explosion rotation properties of a He-star companion in SNe Iax.We find that the He-star companion spins down after the impact due to the angular-momentum loss and expansion caused by the mass-stripping and shock heating during the interaction.This leads to the situation where the surface rotational speed of the surviving companion can drop to one-third of its pre-explosion value when it expands to a maximum radius a few years after the impact.Subsequently,the star shrinks and spins up again once the deposited energy is released.This spin-switching feature of the surviving He-star companions of SNe Iax may be useful for the identification of such objects in future observations.