Long-lived Stars Research Articles

Complexity Growth Patterns in the Stelliferous Era of Big History

A previous study compared the symmetries of the cooling after the Big Bang and the development of life, humans, and civilization on Earth. A simple model of the relative rates of change depicts two cones joined at their bases representing the Big Bang expansion and the acceleration of complexity development on earth through life evolution, human evolution, and civilization development. This indicates points of rapid change at both ends corresponding to at the Big Bang and our current technological and demographic change. However, this depiction oversimplifies complexity growth, specifically at where the transitions from the Universe’s decelerating physical complexity development and Earths accelerating complex adaptive systems. This transition is important in both organization and energy flow. In this paper, the build-up of potential variety in elements and chemicals to later build planets and life is the focus. These are mostly derived from stellar formation through the conversion of gravitational and nuclear potential energy into elements through nuclear fusion. Other’s models have been employed to explore the dynamics of this buildup from the first Population I stars, which were large and short-lived, through the Population II stars of galactic spherical bulges and haloes, to the formation of smaller, long-lived stars with a high amount of non-Hydrogen and Helium atoms, called “metals”. The rates of these dynamics during the transition from cosmic development to terrestrial evolution on Earth are explored here.

Features and prospects for kilonova remnant detection with current and future surveys

We study the observable spectral and temporal properties of kilonova remnants (KNRs) analytically, and point out quantitative differences with respect to supernova remnants. We provide detection prospects of KNRs in the context of ongoing radio surveys. We find that there is a good chance to expect tens of these objects in future surveys with a flux threshold of $ 0.1$ mJy. Kilonova remnants from a postulated population of long-lived supermassive neutron star remnants of neutron star mergers are even more likely to be detected, as they are extremely bright and peak earlier. For an ongoing survey with a threshold of sim 1 mJy, we expect to find tens to hundreds of such objects if they are a significant fraction of the total kilonova (KN) population. Considering that there are no such promising KN candidates in presently ongoing surveys, we constrain the fraction of these extreme KN to be no more than 30 percent of the overall KN population. This constraint depends sensitively on the details of ejecta mass and external density distribution.

The Origin of the Coherent Radio Flash Potentially Associated with GRB 201006A

Rowlinson et al. recently claimed the detection of a coherent radio flash 76.6 minutes after a short gamma-ray burst (GRB). They proposed that the radio emission may be associated with a long-lived neutron star engine. We show through theoretical and observational arguments that the coherent radio emission, if real and indeed associated with GRB 201006A and at the estimated redshift, is unlikely to be due to the collapse of the neutron star, ruling out a blitzar-like mechanism. Instead, we show if a long-lived engine was created, it must have been stable with the radio emission likely linked to the intrinsic magnetar activity. However, we find that the optical upper limits require fine-tuning to be consistent with a magnetar-driven kilonova: we show that neutron-star engines that do satisfy the optical constraints would have produced a bright kilonova afterglow that should already be observable by the Very Large Array or MeerKAT (for ambient densities typical for short GRBs). Given the optical limits and the current lack of a kilonova afterglow, we instead posit that no neutron star survived the merger, and the coherent radio emission was produced far from a black hole central engine via mechanisms such as synchrotron maser or magnetic reconnection in the jet—a scenario consistent with all observations. We encourage future radio follow-up to probe the engine of this exciting event and continued prompt radio follow-up of short GRBs.

On the Association of GW190425 with Its Potential Electromagnetic Counterpart FRB 20190425A

Recent work by Moroianu et al. has suggested that the binary neutron star (BNS) merger GW190425 might have a potential fast radio burst (FRB) counterpart association, FRB20190425A, at the 2.8σ level of confidence with a likely host galaxy association, namely UGC10667. The authors argue that the observations are consistent with a long-lived hypermassive neutron star (HMNS) that formed promptly after the BNS merger and was stable for approximately 2.5 hr before promptly collapsing into a black hole. Recently, Bhardwaj et al. conclusively associated FRB20190425A with UGC10667, potentially providing a direct host galaxy candidate for GW190425. In this work, we examine the multimessenger association based on the spacetime localization overlaps between GW190425 and the FRB host galaxy UGC10667 and find that the odds for a coincident association are O(5) . We validate this estimate by using a Gaussian process density estimator. Assuming that the association is indeed real, we then perform Bayesian parameter estimation on GW190425 assuming that the BNS event took place in UGC10667. We find that the viewing angle of GW190425 excludes an on-axis system at p(θ v > 30°) ≈ 99.99%, highly favoring an off-axis system similar to GRB 170817A. We also find a slightly higher source frame total mass for the binary, namely, mtotal=3.42−0.11+0.34M⊙ , leading to an increase in the probability of prompt collapse into a black hole and therefore disfavors the long-lived HMNS formation scenario. Given our findings, we conclude that the association between GW190425 and FRB20190425A is disfavoured by current state-of-the-art gravitational-wave analyses.

Mass segregation and velocity dispersion as evidence for a dark star cluster

ABSTRACT A dark star cluster (DSC) is a system in which the cluster potential is dominated by stellar remnants, such as black holes and neutron stars having larger masses than the long-lived low-mass stars. Due to mass segregation, these remnants are located in the central region of the cluster and form a dark core. We expect that at a few kpc from the Galactic Centre, the efficient evaporation of the lower-mass stars caused by the strong tidal force exposes the dark core, because the dynamical properties of the DSC are dominated by the remnants. Due to the invisibility of the remnants, finding a DSC by observation is challenging. In this project, we use N-body simulations to obtain models of DSCs and try to discern observables that signify a DSC. We consider four observables: the mass spectrum, the observational mass density profile, the observational velocity dispersion profile and the mass segregation. The models show that a DSC typically exhibits two distinct characteristics: for a given mass in stars and a given half-light radius, the expected velocity dispersion is underestimated when only visible stars are considered, and there is a lack of measurable mass segregation among the stars. These properties can be helpful for finding DSCs in observational data, such as the Gaia catalogue.

Secular Outflows from Long-Lived Neutron Star Merger Remnants

We study mass ejection from a binary neutron star merger producing a long-lived massive neutron star remnant with general-relativistic neutrino-radiation hydrodynamics simulations. In addition to outflows generated by shocks and tidal torques during and shortly after the merger, we observe the appearance of a wind driven by spiral density waves in the disk. This spiral-wave-driven outflow is predominantly located close to the disk orbital plane and have a broad distribution of electron fractions. At higher latitudes, a high electron-fraction wind is driven by neutrino radiation. The combined nucleosynthesis yields from all the ejecta components is in good agreement with Solar abundance measurements.

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.

Effective yields as tracers of feedback effects on metallicity scaling relations in the EAGLE cosmological simulations

ABSTRACT Effective yields, yeff, are defined by fundamental galaxy properties (i.e. stellar mass M⋆, gas mass Mgas, and gas-phase metallicity). For a closed-box model, yeff is constant and equivalent to the mass in metals returned to the gas per unit mass locked in long-lived stars. Deviations from such behaviour have been often considered observational signatures of past feedback events. By analysing eagle simulations with different feedback models, we evaluate the impact of supernovae (SNe) and active galactic nuclei (AGNs) feedback on yeff at redshift z = 0. When removing supermassive black holes (BHs) and, hence, AGN effects, in simulations, galaxies are located around a plane in the M⋆–Mgas–O/H parameter space (being O/H a proxy for gas metallicity, as usual), with such a plane roughly describing a surface of constant yeff. As the ratio between BH mass and M⋆ increases, galaxies deviate from that plane towards lower yeff as a consequence of AGN feedback. For galaxies not strongly affected by AGN feedback, a stronger SN feedback efficiency generates deviations towards lower yeff, while galaxies move towards the opposite side of the plane (i.e. towards higher values of yeff) as SN feedback becomes weaker. Star-forming galaxies observed in the Local Universe are located around a similar 3D plane. Our results suggest that the features of the scatter around the observed plane are related to the different feedback histories of galaxies, which might be traced by yeff.

On the Formation and Interaction of Multiple Supermassive Stars in Cosmological Flows

Supermassive primordial stars with masses exceeding ∼105 M ⊙ that form in atomically cooled halos are the leading candidates for the origin of high-redshift quasars at z > 6. Recent numerical simulations, however, find that multiple accretion disks can form within a halo, each of which can potentially host a supermassive star. We investigate the formation and evolution of secondary supermassive stars in atomically cooled halos, including strong variations in their accretion histories driven by gravitational interactions between their disks and those surrounding the primary supermassive stars in each halo. We find that all secondary disks produce long-lived supermassive stars under sustained rapid accretion. We also find, however, that the majority of secondary supermassive stars do undergo at least one protracted quiescent accretion phase, during which time they thermally relax and may become powerful sources of ionizing feedback. In many halos, the two satellite disks collide, suggesting that the two stars can come into close proximity. This may induce additional mass exchange between them, leading to a great diversity of possible outcomes. These range from coevolution as main-sequence stars to main sequence—black hole pairs and black hole—black hole mergers. We discuss the likely outcome for these binary interactions based on the evolutionary state of both supermassive stars at the end of our simulations, as well as prospects for their future detection by current and next-generation facilities.

Jets from Neutron-Star Merger Remnants and Massive Blue Kilonovae.

We perform three-dimensional general-relativistic magnetohydrodynamic simulations with weak interactions of binary neutron-star (BNS) mergers resulting in a long-lived remnant neutron star, with properties typical of galactic BNS and consistent with those inferred for the first observed BNS merger GW170817. We demonstrate self-consistently that within ≲30 ms postmerger magnetized (σ∼5-10) incipient jets emerge with asymptotic Lorentz factor Γ∼5-10, which successfully break out from the merger debris within ≲20 ms. A fast (v≲0.6c), magnetized (σ∼0.1) wind surrounds the jet core and generates a UV/blue kilonova precursor on timescales of hours, similar to the precursor signal due to free neutron decay in fast dynamical ejecta. Postmerger ejecta are quickly dominated by magnetohydrodynamically driven outflows from an accretion disk. We demonstrate that, within only 50ms postmerger, ≳2×10^{-2}M_{⊙} of lanthanide-free, quasispherical ejecta with velocities ∼0.1-0.2c is launched, yielding a kilonova signal consistent with GW170817 on timescales of ≲5 d.

Ab-initio General-relativistic Neutrino-radiation Hydrodynamics Simulations of Long-lived Neutron Star Merger Remnants to Neutrino Cooling Timescales

We perform the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling timescale. We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (∼20 ms postmerger). Electron flavor antineutrino luminosity dominates over electron flavor neutrino luminosity at early times, resulting in a secular increase of the electron fraction in the outer layers of the remnant. However, the two luminosities become comparable ∼20–40 ms postmerger. A dense gas of electron antineutrinos is formed in the outer core of the remnant at densities ∼1014.5 g cm−3, corresponding to temperature hot spots. The neutrinos account for ∼10% of the lepton number in this region. Despite the negative radial temperature gradient, the radial entropy gradient remains positive, and the remnant is stably stratified according to the Ledoux criterion for convection. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible, stable, rigidly rotating configurations. The remnant is differentially rotating, but it is stable against the magnetorotational instability. Other MHD mechanisms operating on longer timescales are likely responsible for the removal of the differential rotation. Our results indicate the remnant massive neutron star is thus qualitatively different from a protoneutron stars formed in core-collapse supernovae.

Constraining the long-lived supramassive neutron stars by magnetar boosted kilonovae

ABSTRACT Kilonovae are optical transients following the merger of neutron star binaries, which are powered by the r-process heating of merger ejecta. However, if a merger remnant is a long-lived supramassive neutron star supported by its uniform rotation, it will inject energy into the ejecta through spin-down power. The energy injection can boost the peak luminosity of a kilonova by many orders of magnitudes, thus significantly increasing the detectable volume. Therefore, even if such events are only a small fraction of the kilonova population, they could dominate the detection rates. However, after many years of optical sky surveys, no such event has been confirmed. In this work, we build a boosted kilonova model with rich physical details, including the description of the evolution and stability of a proto neutron star, and the energy absorption through X-ray photoionization. We simulate the observation prospects and find the only way to match the absence of detection is to limit the energy injection by the newly born magnetar to only a small fraction of the neutron star rotational energy, thus they should collapse soon after the merger. Our result indicates that most supramassive neutron stars resulting from binary neutron star mergers are short lived and they are likely to be rare in the Universe.

On the possibility to detect gravitational waves from post-merger supermassive neutron stars with a kilohertz detector

ABSTRACT The detection of a secular post-merger gravitational wave (GW) signal in a binary neutron star (BNS) merger serves as strong evidence for the formation of a long-lived post-merger neutron star (NS), which can help constrain the maximum mass of NSs and differentiate NS equations of state. We specifically focus on the detection of GW emissions from rigidly rotating NSs formed through BNS mergers, using several kilohertz GW detectors that have been designed. We simulate the BNS mergers within the detecting limit of LIGO-Virgo-KARGA O4 and attempt to find out on what fraction the simulated sources may have a detectable secular post-merger GW signal. For kilohertz detectors designed in the same configuration of LIGO A+, we find that the design with peak sensitivity at approximately 2 kHz is most appropriate for such signals. The fraction of sources that have a detectable secular post-merger GW signal would be approximately $0.94{\!-\!}11~{{ \rm per\ cent}}$ when the spin-downs of the post-merger rigidly rotating NSs are dominated by GW radiation, while it would be approximately $0.46{\!-\!}1.6~{{ \rm per\ cent}}$ when the contribution of electromagnetic (EM) radiation to the spin-down processes is non-negligible. We also estimate this fraction based on other well-known proposed kilohertz GW detectors and find that, with advanced design, it can reach approximately $12{\!-\!}45~{{ \rm per\ cent}}$ for the GW-dominated spin-down case and $4.7{\!-\!}16~{{\rm per\ cent}}$ when both the GW and EM radiations are considered.

The Influence of Disk Composition on the Evolution of Stars in the Disks of Active Galactic Nuclei

Disks of gas accreting onto supermassive black holes, powering active galactic nuclei (AGN), can capture stars from nuclear star clusters or form stars in situ via gravitational instability. The density and thermal conditions of these disks can result in rapid accretion onto embedded stars, dramatically altering their evolution in comparison to stars in the interstellar medium. Theoretical models predict that, when subjected to sufficiently rapid accretion, fresh gas replenishes hydrogen in the cores of these stars as quickly as it is burned into helium, reaching a quasi-steady state. Such massive, long-lived (“immortal”) stars may be capable of dramatically enriching AGN disks with helium, and would increase the helium abundance in AGN broad-line regions relative to that in the corresponding narrow-line regions and hosts. We investigate how the helium abundance of AGN disks alters the evolution of stars embedded therein. We find, in agreement with analytical arguments, that stars at a given mass are more luminous at higher helium mass fractions, and so undergo more radiation-driven mass loss. We further find that embedded stars tend to be less massive in disks with higher helium mass fractions, and that immortal stars are less common in such disks. Thus, disk composition can alter the rates of electromagnetic and gravitational wave transients as well as further chemical enrichment by embedded stars.

How much metal did the first stars provide to the ultra-faint dwarfs?

Numerical simulations of dwarf galaxies have so far failed to reproduce the observed metallicity-luminosity relation, down to the regime of ultra-faint dwarfs (UFDs). We address this issue by exploring how the first generations of metal-free stars (Pop III) could help increase the mean metallicity ([Fe/H]) of those small and faint galaxies. We ran zoom-in chemo-dynamical simulations of 19 halos extracted from a Λ Cold Dark Matter (CDM) cosmological box and followed their evolution down to redshiftz = 0. Models were validated not only on the basis of galaxy global properties, but also on the detailed investigation of the stellar abundance ratios ([α/Fe]). We identified the necessary conditions for the formation of the first stars in mini-halos and derived constraints on the metal ejection schemes. The impact of Pop III stars on the final metallicity of UFDs was evaluated by considering different stellar mass ranges for their initial mass function (IMF), the influence of pair-instability supernovae (PISNe), and their energetic feedback, as well as the metallicity threshold that marks the transition from the first massive stars to the formation of low-mass long-lived stars. The inclusion of Pop III stars with masses below 140 M⊙, and a standard IMF slope of −1.3 does increase the global metallicity of UFDs, although these are insufficient to resolve the tension with observations. The PISNe with progenitor masses above 140 M⊙do allow the metal content of UFDs to further increase. However, as PISNe are very rare and sometimes absent in the faintest UFDs, they have a limited impact on the global faint end of the metallicity-luminosity relation. Despite a limited number of spectroscopically confirmed members in UFDs, which make the stellar metallicity distribution of some UFDs uncertain, our analysis reveals that this is essentially the metal-rich tail that is missing in the models. The remaining challenges are thus both observational and numerical: (i) to extend high-resolution spectroscopy data samples and confirm the mean metallicity of the faintest UFDs; and (ii) to explain the presence of chemically enriched stars in galaxies with very short star formation histories.

On the diversity of magnetar-driven kilonovae

ABSTRACT A non-negligible fraction of binary neutron star mergers are expected to form long-lived neutron star remnants, dramatically altering the multimessenger signatures of a merger. Here, we extend existing models for magnetar-driven kilonovae and explore the diversity of kilonovae and kilonova afterglows. Focusing on the role of the (uncertain) magnetic field strength, we study the resulting electromagnetic signatures as a function of the external dipolar and internal toroidal fields. These two parameters govern, respectively, the competition between magnetic-dipole spin-down and gravitational-wave spin-down (due to magnetic-field deformation) of the rapidly rotating remnant. We find that even in the parameter space where gravitational-wave emission is dominant, a kilonova with a magnetar central engine will be significantly brighter than one without an engine, as this parameter space is where more of the spin-down luminosity is thermalized. In contrast, a system with minimal gravitational-wave emission will produce a kilonova that may be difficult to distinguish from ordinary kilonovae unless early epoch observations are available. However, as the bulk of the energy in this parameter space goes into accelerating the ejecta, such a system will produce a brighter kilonova afterglow that will peak in shorter times. To effectively hide the presence of the magnetar from the kilonova and kilonova afterglow, the rotational energy inputted into the ejecta must be ≲10−3to 10−2Erot. We discuss the different diagnostics available to identify magnetar-driven kilonovae in serendipitous observations and draw parallels to other potential magnetar-driven explosions, such as superluminous supernovae and broad-line supernovae Ic.

Engine-fed kilonovae (mergernovae) – I. Dynamical evolution and energy injection/heating efficiencies

ABSTRACT A binary neutron star merger is expected to be associated by a kilonova, transient optical emission powered by radioactive decay of the neutron-rich ejecta. If the post-merger remnant is a long-lived neutron star, additional energy injection to the ejecta is possible. In this first paper of a series, we study the dynamical evolution of the engine-fed kilonova (mergernova) ejecta in detail. We perform a semi-analytical study of the problem by adopting a modified mechanical blastwave model that invokes interaction between a Poynting-flux-dominated flow and a non-magnetized massive ejecta. Shortly after the engine is turned on, a pair of shocks would be excited. The reverse shock quickly reaches the wind-acceleration region and disappears (in a few seconds), whereas the forward shock soon breaks out from the ejecta (in 102–103 s) and continues to propagate in the surrounding interstellar medium. Most of the energy injected into the blastwave from the engine is stored as magnetic energy and kinetic energy. The internal energy fraction is fint < 0.3 for an ejecta mass equal to 10−3 M⊙. Overall, the energy injecting efficiency ξ is at most ∼0.6 and can be as small as ∼0.04 at later times. Contrary to the previous assumption, efficient heating only happens before the forward shock breaks out of the ejecta with a heating efficiency ξt ∼ (0.006 − 0.3), which rapidly drops to ∼0 afterwards. The engine-fed kilonova light curves will be carefully studied in Paper II.

Radio Constraints on r-process Nucleosynthesis by Collapsars

The heaviest elements in the universe are synthesized through rapid neutron capture (r-process) in extremely neutron-rich outflows. Neutron star mergers were established as an important r-process source through the multimessenger observation of GW170817. Collapsars were also proposed as a potentially major source of heavy elements; however, this is difficult to probe through optical observations due to contamination by other emission mechanisms. Here we present observational constraints on r-process nucleosynthesis by collapsars based on radio follow-up observations of nearby long gamma-ray bursts (GRBs). We make the hypothesis that late-time radio emission arises from the collapsar wind ejecta responsible for forging r-process elements, and consider the constraints that can be set on this scenario using radio observations of a sample of Swift/Burst Alert Telescope GRBs located within 2 Gpc. No radio counterpart was identified in excess of the radio afterglow of the GRBs in our sample. This gives the strictest limit to the collapsar r-process contribution of ≲0.2 M ⊙ for GRB 060505 and GRB 05826, under the models we considered. Our results additionally constrain energy injection by a long-lived neutron star remnant in some of the considered GRBs. While our results are in tension with collapsars being the majority of r-process production sites, the ejecta mass and velocity profile of collapsar winds, and the emission parameters, are not yet well modeled. As such, our results are currently subject to large uncertainties, but further theoretical work could greatly improve them.

Electromagnetic Counterparts of Binary-neutron-star Mergers Leading to a Strongly Magnetized Long-lived Remnant Neutron Star

We explore the electromagnetic counterparts that will associate with binary-neutron-star mergers for the case that remnant massive neutron stars survive for ≳0.5 s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magnetohydrodynamics simulations with a mean-field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnant neutron stars are significantly amplified by the dynamo effect. We also perform a radiative transfer calculation for kilonovae and find that, for the highly amplified magnetic field cases, the kilonovae can be bright in the early epoch (t ≤ 0.5 d), while it shows the optical emission which rapidly declines in a few days and the very bright near-infrared emission which lasts for ∼10 days. All these features have not been found in GW170817, indicating that the merger remnant neutron star formed in GW170817 might have collapsed to a black hole within several hundreds milliseconds or magnetic-field amplification might be a minor effect.

The Panchromatic Afterglow of GW170817: The Full Uniform Data Set, Modeling, Comparison with Previous Results, and Implications

Abstract We present the full panchromatic afterglow light-curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups and attempted to mitigate these differences to provide a more uniform data set. Simple power-law fits to the uniform afterglow light curve indicate a t 0.86±0.04 rise, a t −1.92±0.12 decline, and a peak occurring at 155 ± 4 days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index (−0.584 ± 0.002) across all epochs. This gives a precise and updated estimate of the electron power-law index, p = 2.168 ± 0.004. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized interstellar medium density, <0.01 cm−3, consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having a magnetic field strength between 1010.4 and 1016 G. Our fits to the afterglow data using an analytical model that includes Very Long Baseline Interferometry proper motion from Mooley et al., and a structured jet model that ignores the proper motion, indicates that the proper-motion measurement needs to be considered when seeking an accurate estimate of the viewing angle.