Accretion-induced Collapse Of White Dwarfs Research Articles

Multimessenger emission from the accretion-induced collapse of white dwarfs

ABSTRACT We present fully general relativistic 3D numerical simulations of accretion-induced collapse (AIC) of white dwarfs (WDs). We evolve three different WD models (non-rotating, rotating at 80 per cent and 99 per cent of the Keplerian mass shedding limit) that collapse due to electron capture. For each of these models, we provide a detailed analysis of their gravitational waves (GWs), neutrinos, and electromagnetic counterpart and discuss their detectability. Our results suggest that fast rotating AICs could be detectable up to a distance of 8 Mpc with third-generation GW observatories, and up to 1 Mpc with LIGO. AIC progenitors are expected to have large angular momentum due to their accretion history, which is a determining factor for their stronger GW emission compared to core-collapse supernovae (CCSNe). Regarding neutrino emission, we found no significant difference between AICs and CCSNe. In the electromagnetic spectrum, we find that AICs are two orders of magnitude fainter than type Ia supernovae. Our work places AICs as realistic targets for future multimessenger searches with third generation ground-based GW detectors.

Formation of millisecond pulsars with long orbital periods by accretion-induced collapse of white dwarfs

ABSTRACT Accretion-induced collapse (AIC) of massive white dwarfs (WDs) has been proposed as an important way for the formation of neutron star (NS) systems. An oxygen-neon (ONe) WD that accretes H-rich material from a red-giant (RG) star may experience the AIC process, eventually producing millisecond pulsars (MSPs), known as the RG donor channel. Previous studies indicate that this channel can only account for MSPs with orbital periods $\gt 500\, \rm d$. It is worth noting that some more MSPs with wide orbits ($60\!-\!500\, \rm d$) have been detected by recent observations, but their origin is still highly uncertain. In this work, by employing an adiabatic power-law assumptions for the mass-transfer process, we performed a large number of complete binary evolution calculations for the formation of MSPs through the RG donor channel in a systematic way. We found that this channel can contribute to the observed MSPs with orbital periods in the range of $50\!-\!1200\, {\rm d}$, and almost all the observed MSPs with wide orbits can be covered by this channel in the WD companion mass versus orbital period diagram. This work indicates that the AIC process provides a viable way to form MSPs with wide orbits.

X-Ray Transients from the Accretion-induced Collapse of White Dwarfs

Abstract The accretion-induced collapse (AIC) of a white dwarf in a binary with a nondegenerate companion can sometimes lead to the formation of a rapidly rotating and highly magnetized neutron star (NS). The spin-down of this NS can drive a powerful pulsar wind (PW) and bring out some detectable multi-wavelength emissions. On the one hand, the PW can evaporate the companion in a few days to form a torus surrounding the NS. Then, due to the blockage of the PW by the torus, a reverse shock can be formed in the wind to generate intense hard X-rays. This emission component disappears in a few weeks’ time, after the torus is broken down at its inner boundary and scoured into a very thin disk. On the other hand, the interaction between the PW with an AIC ejecta can lead to a termination shock of the wind, which can produce a long-lasting soft X-ray emission component. In any case, the high-energy emissions from deep inside the system can be detected only after the AIC ejecta becomes transparent for X-rays. Meanwhile, by absorbing the X-rays, the AIC ejecta can be heated effectively and generate a fast-evolving and luminous ultraviolet (UV)/optical transient. Therefore, the predicted hard and soft X-ray emissions, associated by an UV/optical transient, provide a clear observational signature for identifying AIC events in current and future observations (e.g., AT 2018cow).

OPTICAL TRANSIENTS POWERED BY MAGNETARS: DYNAMICS, LIGHT CURVES, AND TRANSITION TO THE NEBULAR PHASE

ABSTRACT Millisecond magnetars can be formed via several channels: core collapse of massive stars, accretion-induced collapse of white dwarfs (WDs), double WD mergers, double neutron star (NS) mergers, and WD-NS mergers. Because the mass of ejecta from these channels could be quite different, their light curves are also expected to be diverse. We evaluate the dynamic evolution of optical transients powered by millisecond magnetars. We find that the magnetar with a short spin-down timescale converts its rotational energy mostly into the kinetic energy of the transient, while the energy of a magnetar with a long spin-down timescale goes into radiation of the transient. This leads us to speculate that hypernovae could be powered by magnetars with short spin-down timescales. At late times the optical transients will gradually evolve into a nebular phase because of the photospheric recession. We treat the photosphere and nebula separately because their radiation mechanisms are different. In some cases the ejecta could be light enough that the magnetar can accelerate it to a relativistic speed. It is well known that the peak luminosity of a supernova (SN) occurs when the luminosity is equal to the instantaneous energy input rate, as shown by Arnett. We show that photospheric recession and relativistic motion can modify this law. The photospheric recession always leads to a delay of the peak time t pk ?> relative to the time t × ?> at which the SN luminosity equals the instantaneous energy input rate. Relativistic motion, however, may change this result significantly.

Can eccentric binary millisecond pulsars form by accretion-induced collapse of white dwarfs?

Binary radio pulsars are generally believed to have been spun up to millisecond periods (i.e. recycling) via mass accretion from their donor stars, and they are the descendants of neutron star low-mass X-ray binaries. However, some studies indicate that the formation of pulsars from the accretion-induced collapse (AIC) of accreting white dwarfs (WDs) cannot be excluded. In this work, we use a population-synthesis code to examine if the AIC channel can produce eccentric binary millisecond pulsars (BMSPs) in the Galaxy. Our simulated results indicate that only when the natal MSPs receive a relatively strong kick (>rsim 100 km s −1), can the AIC channel produce ∼10–180 eccentric (e > 0.1) BMSPs in the Galaxy, most of which are accompanied by a helium star. Such a kick seems to be highly unlikely in the conventional AIC process, hence the probability of forming eccentric BMSPs via the AIC channel can be ruled out. Even if a high kick is allowed, the AIC channel cannot produce eccentric BMSPs with an orbital period of ≳20 d. Therefore, we propose that the peculiar BMSP PSR J1903+0327 cannot be formed by the AIC channel. However, the AIC evolutionary channel may produce some fraction of isolated MSPs, and even submillisecond pulsars if they really exist.

Nickel-rich outflows produced by the accretion-induced collapse of white dwarfs: light curves and spectra

The accretion-induced collapse (AIC) of a white dwarf to form a neutron star can leave behind a rotationally supported disc with mass of up to ∼ 0.1 M⊙. The disc is initially composed of free nucleons but as it accretes and spreads to larger radii, the free nucleons recombine to form helium, releasing sufficient energy to unbind the remaining disc. Most of the ejected mass fuses to form 56Ni and other iron group elements. We present spherically symmetric radiative transfer calculations of the transient powered by the radioactive heating of this ejecta. We estimate the ejecta composition using nucleosynthesis calculations in the literature and explore the sensitivity of our results to uncertainties in the ejecta kinematics. For an ejecta mass of 10−2 M⊙ (3 × 10−3 M⊙), the light curve peaks after ≲ 1 d with a peak bolometric luminosity ≃ 2 × 1041 erg s−1 (≃ 5 × 1040 erg s−1); the decay time is ≃ 4 (2) d. Overall, the spectra redden with time reaching U−V≃ 4 after ≃ 1 d; the optical colours (B−V) are, however, somewhat blue. Near the peak in the light curve, the spectra are dominated by Doppler-broadened Nickel features, with no distinct spectral lines present. At d, strong calcium lines are present in the infrared, although the calcium mass fraction is only ∼ 10−4.5. If rotationally supported discs are a common byproduct of AIC, current and upcoming transient surveys such as the Palomar Transient Factory should detect a few AIC per year for an AIC rate of ∼ 10−2 of the Type Ia rate. We discuss ways of distinguishing AIC from other rapid, faint transients, including. Ia's and the ejecta from binary neutron star mergers.

Axisymmetric general relativistic simulations of the accretion-induced collapse of white dwarfs

(Abridged.) The accretion-induced collapse (AIC) of a white dwarf (WD) may lead to the formation of a protoneutron star and a collapse-driven supernova explosion. This process represents a path alternative to thermonuclear disruption of accreting white dwarfs in Type Ia supernovae. Neutrino and gravitational-wave (GW) observations may provide crucial information necessary to reveal a potential AIC. Motivated by the need for systematic predictions of the GW signature of AIC, we present results from an extensive set of general-relativistic AIC simulations using a microphysical finite-temperature equation of state and an approximate treatment of deleptonization during collapse. Investigating a set of 114 progenitor models in rotational equilibrium, with a wide range of rotational configurations, temperatures and central densities, we extend previous Newtonian studies and find that the GW signal has a generic shape akin to what is known as a "Type III" signal in the literature. We discuss the detectability of the emitted GWs, showing that the signal-to-noise ratio for current or next-generation interferometer detectors could be high enough to detect such events in our Galaxy. Some of our AIC models form massive quasi-Keplerian accretion disks after bounce. In rapidly differentially rotating models, the disk mass can be as large as ~0.8-Msun. Slowly and/or uniformly rotating models produce much smaller disks. Finally, we find that the postbounce cores of rapidly spinning white dwarfs can reach sufficiently rapid rotation to develop a nonaxisymmetric rotational instability.

Formation of binary millisecond pulsars by accretion-induced collapse of white dwarfs

It has generally been assumed that neutron stars (NSs) that become millisecond pulsars (MSPs) originated in a core-collapse supernova. The possibility of formation by accretion-induced collapse (AIC) of an oxygen/neon white dwarf (WD) has largely been ignored or considered negligible. Here, we demonstrate that population synthesis calculations with generic assumptions yield birthrates of binary MSPs via AIC that are comparable to and can exceed those for core collapse. Allowing both modes for NS formation, we estimate birthrates and orbital period distributions and compare these with observations of binary MSPs and low-mass X-ray binaries. Our calculations show that both the core-collapse and AIC routes lead to populations of binary systems that can be identified with X-ray binaries while transferring matter and binary MSPs at the end of an accretion phase. The estimated birthrates indicate that the often-neglected AIC route cannot be ignored. It also appears that this route can provide a better match to the period distributions of some types of binary MSPs. In particular, it appears to be the major route to the orbital period distribution of long-period (a few days or greater) systems with helium WD companions under certain model assumptions. The birthrate problem confronting the low- and intermediate-mass X-ray binaries remains but can still be resolved by invocation of irradiation-driven limit cycles.

Nickel-rich outflows from accretion discs formed by the accretion-induced collapse of white dwarfs

A white dwarf (WD) approaching the Chandrasekhar mass may in several cases undergo accretion-induced collapse (AIC) to a neutron star (NS) before a thermonuclear explosion ensues. It has generally been assumed that AIC does not produce a detectable supernova (SN). If, however, the progenitor WD is rapidly rotating (as may be expected due to its prior accretion), a centrifugally supported disk forms around the NS upon collapse. We calculate the subsequent evolution of this accretion disk using time-dependent height-integrated simulations with initial conditions taken from the AIC calculations of Dessart et al. (2006). Initially, the disk is cooled by neutrinos and its composition is driven neutron-rich (electron fraction Ye ~ 0.1) by electron captures. However, as the disk viscously spreads, it is irradiated by neutrinos from the central proto-NS, which dramatically alters its neutron-to-proton ratio. We find that electron neutrino captures increase Ye to ~ 0.5 by the time that weak interactions in the disk freeze out. Because the disk becomes radiatively inefficient and begins forming alpha-particles soon after freeze out, powerful winds blow away most of the disk's remaining mass. These Ye ~ 0.5 outflows synthesize up to a few times 1e-2 Msun in 56Ni. As a result, AIC may be accompanied by a radioactively powered SN-like transient that peaks on a timescale of ~ 1 day. Since few intermediate mass elements are likely synthesized, these Ni-rich explosions should be spectroscopically distinct from other SNe. PanSTARRs and the Palomar Transient Factory should detect a few AIC transients per year if their true rate is ~1/100 of the Type Ia rate, and LSST should detect hundreds per year. High cadence observations (< 1 day) are optimal for the detection and follow-up of AIC (abridged).

Multidimensional Simulations of the Accretion‐induced Collapse of White Dwarfs to Neutron Stars

We present 2.5-dimensional radiation-hydrodynamics simulations of the accretion-induced collapse (AIC) of white dwarfs, starting from two-dimensional rotational equilibrium configurations, thereby accounting consistently for the effects of rotation prior to and after core collapse. We focus our study on a 1.46 and a 1.92 M☉ a model. Electron capture leads to the collapse to nuclear densities of these cores a few tens of milliseconds after the start of the simulations. The shock generated at bounce moves slowly, but steadily, outward. Within 50-100 ms, the stalled shock breaks out of the white dwarf along the poles. The blast is followed by a neutrino-driven wind that develops within the white dwarf, in a cone of ~40° opening angle about the poles, with a mass loss rate of (5-8) × 10-3 M☉ s-1. The ejecta have an entropy on the order of (20-50)kB baryon-1 and an electron fraction that is bimodal. By the end of the simulations, at ≳600 ms after bounce, the explosion energy has reached (3-4) × 1049 ergs and the mass has reached a few times 10-3 M☉. We estimate the asymptotic explosion energies to be lower than 1050 ergs, significantly lower than those inferred for standard core collapse. The AIC of white dwarfs thus represents one instance where a neutrino mechanism leads undoubtedly to a successful, albeit weak, explosion. We document in detail the numerous effects of the fast rotation of the progenitors: the neutron stars are aspherical; the ``νμ'' and e neutrino luminosities are reduced compared to the νe neutrino luminosity; the deleptonized region has a butterfly shape; the neutrino flux and electron fraction depends strongly upon latitude (a la von Zeipel); and a quasi-Keplerian 0.1-0.5 M☉ accretion disk is formed.

Low-mass bare strange stars

Strange stars with low masses are suggested to exist in reality, the origin of which could be via accretion-induced collapse of white dwarfs. Such a strange star is likely bare, and would thus spin very fast, even to a period of <0.1 ms. Strange stars with low masses may differ from those with solar masses in various astrophysical appearances. Observations to test this “low-mass” idea are proposed.

Trans‐Relativistic Blast Waves in Supernovae as Gamma‐Ray Burst Progenitors

We investigate the acceleration of shock waves to relativistic velocities in the outer layers of exploding stars. By concentrating the energy of the explosion in the outermost ejecta, such trans-relativistic blast waves can serve as the progenitors of gamma-ray bursts (GRBs); in particular, the baryon-loading problem that plagues many models of GRBs is circumvented. Postshock acceleration is effective in boosting the kinetic energy in relativistic ejecta. We present physically motivated analytic expressions to describe trans-relativistic blast waves in supernovae, and we validate these expressions against numerical simulations of test problems. Investigating the effect of stellar structure on mass ejection, we find that relativistic ejecta are enhanced in more centrally condensed envelopes, e.g., for radiative envelopes, when the luminosity approaches the Eddington limit. Convenient formulae are presented with which to estimate the production of relativistic ejecta from a given progenitor. We apply our analytic and numerical methods to a model of SN 1998bw, finding significantly enhanced relativistic ejecta compared to previous studies. We propose that GRB 980425 is associated with SN 1998bw and may have resulted from an approximately spherical explosion producing ~10-6 M☉ of mildly relativistic ejecta with mean Lorentz factor ~ 2, which then interacted with a dense circumstellar wind with a mass-loss rate of approximately a few times 10-4 M☉ yr-1. A highly asymmetric explosion is not required. An extreme model of hypernova explosions in massive stars is able to account for the energetics and relativistic ejecta velocities required by many of the observed cosmological GRBs. However, the most energetic bursts require asymmetric expulsion of ejecta, perhaps caused by rotationally flattened progenitors. We present simplified models and simulations of explosions resulting from accretion-induced collapse of white dwarfs and phase transitions of neutron stars. While we find increased energies in relativistic ejecta compared to previous studies, these explosions are unlikely to be observed at cosmological distances with current detectors, unless extreme explosion energies and asymmetries are invoked.

Coulomb corrections to the equation of state of nuclear statistical equilibrium matter: implications for SNIa nucleosynthesis and the accretion-induced collapse of white dwarfs

Coulomb corrections to the equation of state of degenerate matter are usually neglected in high-temperature regimes, owing to the inverse dependence of the plasma coupling constant, Γ, on temperature. However, nuclear statistical equilibrium matter is characterized by a large abundance by mass of large-Z (iron group) nuclei. It is found that Coulomb corrections to the ion ideal gas equation of state of matter in nuclear statistical equilibrium are important at temperatures T≲5–10×109 K and densities ρ≳108 g cm−3. At a temperature T=8.5×109 K and a density ρ=8×109 g cm−3, the neutronization rate is larger by ≳28 per cent when Coulomb corrections are included. However, the conductive velocity of a thermonuclear deflagration wave in C–O drops by ∼16 per cent when Coulomb corrections to the heat capacity are taken into account. The implications for SNIa models and nucleosynthesis, and also for the accretion-induced collapse of white dwarfs, are discussed. Particularly relevant is the result that the minimum density for collapse of a white dwarf to a neutron star is shifted down to 5.5–6×109 g cm−3, a value substantially lower than previously thought.

Nonaxisymmetric Neutral Modes in Rotating Relativistic Stars

We study nonaxisymmetric perturbations of rotating relativistic stars. modeled as perfect-fluid equilibria. Instability to a mode with angular dependence $\exp(im\phi)$ sets in when the frequency of the mode vanishes. The locations of these zero-frequency modes along sequences of rotating stars are computed in the framework of general relativity. We consider models of uniformly rotating stars with polytropic equations of state, finding that the relativistic models are unstable to nonaxisymmetric modes at significantly smaller values of rotation than in the Newtonian limit. Most strikingly, the m=2 bar mode can become unstable even for soft polytropes of index $N \leq 1.3$, while in Newtonian theory it becomes unstable only for stiff polytropes of index $N \leq 0.808$. If rapidly rotating neutron stars are formed by the accretion-induced collapse of white dwarfs, instability associated with these nonaxisymmetric, gravitational-wave driven modes may set an upper limit on neutron-star rotation. Consideration is restricted to perturbations that correspond to polar perturbations of a spherical star. A study of axial perturbations is in progress.

Discovery of ten millisecond pulsars in the globular cluster 47 Tucanae

IN the past four years a total of 13 millisecond pulsars have been found in 12 different globular clusters. These pulsars are believed to be old neutron stars that have been spun up ('recycled') in low-mass X-ray binary systems1 although some may have been formed by the accretion-induced collapse of white dwarfs in binaries2. The globular cluster 47 Tucanae has an especially dense core, and is therefore a likely site for millisecond pulsar formation. Using the Parkes radiotelescope, we have now detected ten addi-tional millisecond pulsars in 47 Tuc, more than half of which are members of binary systems. Almost half of the known millisecond pulsars and more than a quarter of the known binary pulsars now reside in this one cluster.

Conditions for accretion-induced collapse of white dwarfs

Recent discovery of an unexpectedly large number of low-mass binary pulsars (LMBPs) in globular clusters has instigated active discussions on the evolutionary origin of binary pulsars. Prompted by the possibility that at least some of LMBPs originate from accretion-induced collapse (AIC) of white dwarfs, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white dwarfs that can ignite explosive nuclear burning at significantly lower central densities than in the previous models. Even with low critical densities, AIC is still much more likely than explosion for both types of white dwarfs. Possible regions for AIC are presented in a diagram of mass accretion rate vs initial mass of the white dwarfs.

Neutron stars and millisecond pulsars from accretion-induced collapse in globular clusters

This paper examines the limits on the number of millisecond pulsars which could be formed in globular clusters by the generally accepted scenario (in which a neutron star is created by the supernova of an initially massive star and subsequently captures a companion to form a low-mass X-ray binary which eventually becomes a millisecond pulsar). It is found that, while the number of observed low-mass X-ray binaries can be adequately explained in this way, the reasonable assumption that the pulsar luminosity function in clusters extends below the current observational limits down to the luminosity of the faintest millisecond pulsars in the field suggests a cluster population of millisecond pulsars which is substantially larger than the standard model can produce. Alleviating this problem by postulating much shorter lifetimes for the X-ray binaries requires massive star populations sufficiently large that the mass loss resulting from their evolution would be likely to unbind the cluster. It is argued that neutron star formation in globular clusters by accretion-induced collapse of white dwarfs may resolve the discrepancy in birthrates.

Genesis of pulsars in globular clusters

THE birth-rate of radio pulsars in globular clusters (GCs) is thought to be a factor of 10 to 100 times greater than that of their candidate progenitors, low-mass X-ray binaries (LMXBs) of canonical lifetime 108 to 109 yr. This apparent discrepancy1,2 would be removed either if there were more rapid accretion of mass onto the primary in the late stages of LMXB evolution (as suggested by the self-excited wind model3), or if there occurred accretion-induced collapse of white dwarfs in binaries with wide orbits4,5. But the available data (Table 1) indicate that GC pulsars cannot be formed by either of these mechanisms alone. Here we suggest that both processes may be necessary to some extent, the corollary of which is that GC pulsars may have at least two distinct classes of progenitors. Many solitary and binary pulsars with short orbital periods may have descended from rapidly evolving LMXBs, whereas others, especially those with long periods, may have formed from binaries that undergo nuclear evolution of the secondary and eventual accretion-induced collapse of the white-dwarf primary.

Birth of millisecond pulsars in globular clusters

The recent report that two millisecond pulsars were found in the globular cluster 47 Tuc1, together with the previous discoveries in the clusters M282, M43 and M154, indicates that formation of millisecond pulsars in globular clusters is greatly enhanced over the field, where a fourth example has recently been found5. This excess appears to be even larger than that for low-mass X-ray binaries (LMXBs), which show a factor of 100 excess (per unit mass) in globular clusters as opposed to the Galaxy6,7. If the birthrate of millisecond pulsars is indeed greater than that of LMXBs in clusters, then, contrary to the common assumption8, LMXBs cannot be the sole precursors of millisecond pulsars. Here we suggest instead that accretion-induced collapse of white dwarfs in binaries9 can form millisecond pulsars directly, without requiring a precursor LMXB stage10. Ablation of the pre-collapse binary companion by the millisecond pulsar's radiation field11, a process invoked to explain some of the characteristics of the recently discovered eclipsing millisecond pulsar5,12,13, can then yield isolated neutron stars without requiring an additional stellar encounter.

Sustained magnetic fields in binary millisecond pulsars

The two known binary millisecond pulsars1,2 PSR1953 + 29 and PSR1855 + 09 with periods P =6.1ms and 5.4ms, respectively, and the long orbital-period binary radio pulsars3 are believed to have evolved from low-mass X-ray binaries (LMXBs)4. The neutron stars in these binaries are believed to have formed from the accretion-induced collapse of white dwarfs with shorter orbital periods5. These neutron stars have weak magnetic fields (3×108 G≲B≲6×1011 G) and rapid spin (5 ms≲P≲1s), compared with typical isolated pulsars, and are believed to have been spun up by accretion after their magnetic fields had decayed (see ref. 6 for review) on a timescale tD ˜9×106 yr (ref. 7). If correct, the fraction of pulsars which are binary millisecond pulsars is too large8 by a factor of >100. This severe discrepancy is removed if, as we propose here, the magnetic fields of neutron stars do not decay either in such binaries or in general. We also show that, if such neutron stars are formed from the accretion-induced magnetic flux and angular momentum-conserving collapse of white dwarfs, most of them are likely to have been born, and remain, spinning rapidly and to have weak magnetic fields, in agreement with observations of binary millisecond pulsars and LMXBs.