Thermonuclear Runaway Research Articles

The Impact of Nova Outbursts on the Chemical Abundance of the Interstellar Medium

Nova outbursts are the results of thermonuclear runaways, which occur when sufficient material accretes on the surfaces of white dwarfs (WDs). Using the MESA code, we construct a detailed grid for carbon-oxygen and oxygen-neon-magnesium novae. By employing population synthesis methods, we conduct a statistical analysis of the distribution of novae in the Milky Way. In our models, on average, a typical nova system may undergo about 8000 eruptions and the Galactic nova rate is ∼130 yr−1. The C, N, and O elements in nova ejecta are strongly affected by the mixing degree between WD core and accreted material. Our results show that the average value of 12C/13C in nova ejecta is about an order of magnitude lower than that on the surface of a red giant, that for 16O/17O is about 5 times lower, and that for 14N/15N is about 1.5 times lower. The annual yields of 13C , 15N, and 17O from nova ejection are larger than those from AGB stars. This indicates that compared to a red giant, nova eruptions are a more important source of the odd-numbered nuclear elements of 13C , 15N, and 17O in the Galactic interstellar medium.

Supernovae and superbursts triggered by dark matter clumps

Cosmologies in which dark matter clumps strongly on small scales are unfavorable to terrestrial detectors that are as yet unexposed to the clumps. I show that subhectometer clumps could trigger thermonuclear runaways by scattering on nuclei in white dwarf cores (carbon and oxygen) and neutron star oceans (carbon), setting off type-Ia-like supernovae and x-ray superbursts, respectively. I consider two scenarios: “dark clusters” that are essentially microhalos and “long-range dark nuggets,” essentially macroscopic composites, with long-range Yukawa baryonic interactions that source the energy for igniting explosions. I constrain dark clusters, weighing between the Planck mass and asteroid masses, and long-range dark nuggets over a wider mass range spanning 40 orders of magnitude. These limits greatly complement searches proposed by myself and co-workers [] for scattering interactions of dark clumps in neutron stars, cosmic rays, and prehistoric minerals. Published by the American Physical Society 2024

Burst-induced spin variations in the accreting magnetic white dwarf PBC J0801.2–4625

ABSTRACT PBC J0801.2–4625 is an intermediate polar with a primary spin frequency of 66.08 d−1 and an unknown orbital period. The long-term All Sky Automated Survey for Supernovae (ASAS-SN) light curve of this system reveals four bursts, all of which have similar peak amplitudes (∼2 mag) and durations (∼2 d). In this work, we primarily study the timing properties of this system’s 2019 February burst, which was simultaneously observed by both ASAS-SN and the Transiting Exoplanet Survey Satellite (TESS). Pre-burst, a frequency of 4.064 ± 0.002 d−1(5.906 ± 0.003 h period), likely attributed to the binary orbit, is identified in addition to previous measurements for the white dwarf’s spin. During the burst, however, we find a spin frequency of 68.35 ± 0.28 d−1. Post-burst, the spin returns to its pre-brust value but with a factor 1.82 ± 0.05 larger amplitude. The burst profile is double-peaked, and we estimate its energy to be 3.3 × 1039 erg. We conclude that the burst appears most consistent with thermonuclear runaway (i.e. a 'micronova'), and suggest that the spin variations may be an analogue to burst oscillations (i.e. 'micronova oscillations'). However, we also note that the above findings could be explained by a dwarf nova outburst. With the available data, we are unable to distinguish between these two scenarios.

Simulating Lateral H/He Flame Propagation in Type I X-ray Bursts

X-ray bursts are the thermonuclear runaway of a mixed H/He layer on the surface of a neutron star. Observations suggest that the burning begins locally and spreads across the surface of the star as a flame. Recent multidimensional work has looked in detail at pure He flames spreading across a neutron star. Here we report on progress in multidimensional modeling of mixed H/He flames and discuss the challenges.

Type Ia supernovae in the age of JWST: Finding the ‘right’ questions and the path to answers

Understanding the Physics of thermonuclear explosions of a White Dwarf star (WD), so called Type Ia Supernovae (SNe Ia), provide a playground for modern physics, computational methods, and are a key to modern cosmology. We identify new and investigate a variety of observational signatures of underlying physical processes related to the thermonuclear runaway, the flame propagation and the environment. Being intrinsically multi-dimensional phenomena, probing the physics requires multi-dimensional radiation-hydrodynamics and MHD simulations. For this task, we developed and employed methods for photon transport for the X-, gamma- and of low energy and of positrons under non-LTE conditions. We identify signatures in the light curves and spectra, in particular, line profiles and polarization spectra. Consistent treatment of high energy processes is critical. Therefore, our framework and results can be used directly a variety of scenarios for SNe Ia including merging WDs and explosions of sub-Chandrasekhar mass WDs. Current simulations have limitations but, nevertheless, when combined with recent JWST and VLT observations solutions emerge to many of decade old problems on the ignition process, flame physics and thermonuclear explosion.

1100 days in the life of the supernova 2018ibb

Stars with zero-age main sequence masses between 140 and 260 M⊙ are thought to explode as pair-instability supernovae (PISNe). During their thermonuclear runaway, PISNe can produce up to several tens of solar masses of radioactive nickel, resulting in luminous transients similar to some superluminous supernovae (SLSNe). Yet, no unambiguous PISN has been discovered so far. SN 2018ibb is a hydrogen-poor SLSN at z = 0.166 that evolves extremely slowly compared to the hundreds of known SLSNe. Between mid 2018 and early 2022, we monitored its photometric and spectroscopic evolution from the UV to the near-infrared (NIR) with 2–10 m class telescopes. SN 2018ibb radiated > 3 × 1051 erg during its evolution, and its bolometric light curve reached > 2 × 1044 erg s−1 at its peak. The long-lasting rise of > 93 rest-frame days implies a long diffusion time, which requires a very high total ejected mass. The PISN mechanism naturally provides both the energy source (56Ni) and the long diffusion time. Theoretical models of PISNe make clear predictions as to their photometric and spectroscopic properties. SN 2018ibb complies with most tests on the light curves, nebular spectra and host galaxy, and potentially all tests with the interpretation we propose. Both the light curve and the spectra require 25–44 M⊙ of freshly nucleosynthesised 56Ni, pointing to the explosion of a metal-poor star with a helium core mass of 120–130 M⊙ at the time of death. This interpretation is also supported by the tentative detection of [Co II] λ 1.025 μm, which has never been observed in any other PISN candidate or SLSN before. We observe a significant excess in the blue part of the optical spectrum during the nebular phase, which is in tension with predictions of existing PISN models. However, we have compelling observational evidence for an eruptive mass-loss episode of the progenitor of SN 2018ibb shortly before the explosion, and our dataset reveals that the interaction of the SN ejecta with this oxygen-rich circumstellar material contributed to the observed emission. That may explain this specific discrepancy with PISN models. Powering by a central engine, such as a magnetar or a black hole, can be excluded with high confidence. This makes SN 2018ibb by far the best candidate for being a PISN, to date.

Hydrodynamic Simulations of Oxygen–Neon Classical Novae as Galactic 7Li Producers and Potential Accretion-induced Collapse Progenitors* *We dedicate this paper to the memory of R. Mark Wagner. Friend, collaborator, exemplary observational astronomer, and contributor to studies of classical novae and transient objects for more than 40 years. He will be missed.

We report on studies of classical nova (CN) explosions where we follow the evolution of thermonuclear runaways (TNRs) on oxygen–neon (ONe) white dwarfs (WDs). Using NOVA, a 1D hydrodynamic computer code, we accrete solar matter until the TNR is ongoing and then switch to a mixed composition. This approach is guided by the results of multidimensional studies of TNRs in WDs, which find that sufficient mixing with WD core material occurs after the TNR is well underway, and levels of enrichment of the CNONeMg elements are reached that agree with observations of CN ejecta abundances. Because the amount of accreted material is inversely proportional to the oxygen abundance, by first accreting solar matter, the amount of accreted material is larger than in those simulations with an initially enriched composition. We vary the mass of the WD (from 0.6 M ⊙ to 1.35 M ⊙) and the composition of the mixed materials. Our results show large enrichments of 7Be in the ejected gases, implying that ONe CNe and CO CNe may be responsible for a significant fraction (∼100 M ⊙) of the Galactic 7Li (∼1000 M ⊙). The production of 22Na and 26Al in CN explosions and the γ-ray emission predicted by our simulations are discussed. The WDs in all our simulations eject less material than they accrete and we predict that the WD is growing in mass as a consequence of the CN outburst. ONe CNe, therefore, may be an important channel for accretion-induced collapse events.

Accretion in the recurrent nova T CrB: Linking the superactive state to the predicted outburst

Context. T CrB (NOVA CrB 1946) is a famous recurrent nova with a recurrence timescale of 80 years. Aims. We aim to estimate the colours, luminosity, and mass-accretion rate for T CrB (NOVA CrB 1946) during and after the superactive state. Methods. We performed and analysed UBV photometry of the recurrent nova T CrB. Results. For the hot component of T CrB, we find average dereddened colours of (U − B)0 = −0.70 ± 0.08 and (B − V)0 = 0.23 ± 0.06, which correspond to an effective temperature of 9400 ± 500 K and an optical luminosity of 40 − 110 L⊙ during the superactive state (2016–2022). After the end of the superactive state, the hot component became significantly redder, (U − B)0 ≈ −0.3 and (B − V)0 ≈ 0.6 in August 2023, and its luminosity decreased markedly to 20 − 25 L⊙ in April–May 2023, and to 8 − 9 L⊙ in August 2023. The total mass accreted during the superactive state from 2014 to 2023 is ∼2 × 10−7 M⊙. Conclusions. This is a significant fraction of the mass required to cause a thermonuclear runaway (TNR). Overall our results support a model in which a large accretion disc acts as a reservoir with increased accretion rate onto the central white dwarf during disc high states, ultimately leading to a TNR explosion, which now seems to be imminent.

Search for sub-TeV Neutrino Emission from Novae with IceCube-DeepCore

The understanding of novae, the thermonuclear eruptions on the surfaces of white dwarf stars in binaries, has recently undergone a major paradigm shift. Though the bolometric luminosity of novae was long thought to arise directly from photons supplied by the thermonuclear runaway, recent gigaelectronvolt (GeV) gamma-ray observations have supported the notion that a significant portion of the luminosity could come from radiative shocks. More recently, observations of novae have lent evidence that these shocks are acceleration sites for hadrons for at least some types of novae. In this scenario, a flux of neutrinos may accompany the observed gamma rays. As the gamma rays from most novae have only been observed up to a few GeV, novae have previously not been considered as targets for neutrino telescopes, which are most sensitive at and above teraelectronvolt (TeV) energies. Here, we present the first search for neutrinos from novae with energies between a few GeV and 10 TeV using IceCube-DeepCore, a densely instrumented region of the IceCube Neutrino Observatory with a reduced energy threshold. We search both for a correlation between gamma-ray and neutrino emission as well as between optical and neutrino emission from novae. We find no evidence for neutrino emission from the novae considered in this analysis and set upper limits for all gamma-ray detected novae.

No Surviving SN Ia Companion in SNR 0509-67.5: Stellar Population Characterization and Comparison to Models

The community agrees that Type Ia supernovae arise from carbon/oxygen white dwarfs undergoing thermonuclear runaway. However, the full progenitor system and the process that prompts the white dwarf to explode remain unknown. Most current models suggest that the white dwarf explodes because of interaction with a binary companion that may survive the process and remain within the resulting remnant of the exploded star. Furthermore, both the pre-supernova interaction process and the explosion of the primary are expected to imprint a significant departure from ordinary stellar radii and temperatures onto the secondary, making the star identifiable against the unrelated stellar population. Identification of a surviving companion inside an SN Ia remnant might confirm a specific corresponding SN Ia progenitor channel based on the identity of the companion. We conducted a surviving companion search of the Type Ia remnant SNR 0509−67.5 based in the Large Magellanic Cloud. The well-constrained distance to and foreground extinction of the Large Magellanic Cloud allow for Bayesian inference of stellar parameters with low correlation and uncertainties. We present a deep catalog of fully characterized stars interior to SNR 0509−67.5 with radii, effective temperatures, and metallicities inferred using combined Hubble Space Telescope photometric observations across multiple visits. We then compile a list of surviving companion models appropriate for the age of the remnant (roughly 400 yr after the explosion). We compare these predictions with the inferred stellar parameters and conclude that none of the stars are consistent with the predicted signatures of a surviving companion.

The multiwavelength view of shocks in the fastest nova V1674 Her

ABSTRACT Classical novae are shock-powered multiwavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t2 = 1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV γ-rays to cm-band radio using coordinated Fermi-LAT, NuSTAR, Swift, and VLA observations supported by optical photometry. Fermi-LAT detected short-lived (18 h) 0.1–100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 ± 0.4) × 10−6 photons cm−2 s−1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kTshock = 4 keV. The lack of a detectable 6.7 keV Fe Kα emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin–Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova.

The non-explosive stellar merging origin of the ultra-massive carbon-rich white dwarfs

ABSTRACT We have investigated the origin of a sub-class of carbon-polluted white dwarfs (DQ) originally identified as the “hot DQ” white dwarfs. These objects are relatively hot ($10\, 000\lesssim T_{\rm eff}\lesssim 25\, 000$ K), have markedly higher carbon abundance (C-enriched), are more massive (M ≳ 0.8 M⊙) than ordinary DQs (M ∼ 0.6 M⊙), and display high space velocities. Hence, despite their young appearance their kinematic properties are those of an old white dwarf population. The way out of this dilemma is to assume that they formed via the merging of two white dwarfs. In this paper, we examine the observed characteristics of this population of “C-enriched” DQ white dwarfs and confirm that nearly half of the 63 known objects have kinematic properties consistent with those of the Galactic thick disc or halo. We have also conducted population synthesis studies and found that the merging hypothesis is indeed compatible with observations. Studies of this sub-class of white dwarfs have important implications for our understanding of Type Ia Supernovae (SNeIa), commonly used to determine the expansion history of the Universe, since the same formation channel applies to both kinds of objects. Hence, probing the properties of these white dwarfs that failed to explode may yield important constraints to the modelling of the mechanisms leading to a thermonuclear runaway.

A catalogue of unusually long thermonuclear bursts on neutron stars

ABSTRACT Rare, energetic (long) thermonuclear (Type I) X-ray bursts are classified either as intermediate-duration or ‘supern’ bursts, based on their duration. Intermediate-duration bursts lasting a few to tens of minutes are thought to arise from the thermonuclear runaway of a relatively thick (≈1010 g cm−2) helium layer, while superbursts lasting hours are attributed to the detonation of an underlying carbon layer. We present a catalogue of 84 long thermonuclear bursts from 40 low-mass X-ray binaries, and defined from a new set of criteria distinguishing them from the more frequent short bursts. The three criteria are: (1) a total energy release longer than 1040 erg, (2) a photospheric radius expansion phase longer than 10 s, and (3) a burst time-scale longer than 70 s. This work is based on a comprehensive systematic analysis of 70 bursts found with INTEGRAL, RXTE, Swift, BeppoSAX, MAXI, and NICER, as well as 14 long bursts from the literature that were detected with earlier generations of X-ray instruments. For each burst, we measure its peak flux and fluence, which eventually allows us to confirm the distinction between intermediate-duration bursts and superbursts. Additionally, we list 18 bursts that only partially meet the above inclusion criteria, possibly bridging the gap between normal and intermediate-duration bursts. With this catalogue, we significantly increase the number of long-duration bursts included in the MINBAR and thereby provide a substantial sample of these rare X-ray bursts for further study.

Pre-maximum Evolution of the Classical Nova YZ Reticuli

The pre-maximum evolution of a nova is the last frontier in studying such objects. YZ Reticuli 2020 is the only nova whose X-ray flash was detected. The X-ray flash occurs immediately after the onset of thermonuclear runaway, so its physical properties impose severe constraints on the nova model. We discuss what we can learn from the early phase observations.

Physics of nova outbursts: A theoretical model of classical nova outbursts with self-consistent wind mass loss

Abstract We present a model for one cycle of a classical nova outburst based on a self-consistent wind mass loss accelerated by the gradient of radiation pressure, i.e., so-called optically thick winds. Evolution models are calculated by a Henyey code for a 1.0 $M_{\odot }$ white dwarf with a mass-accretion rate of 5 × 10−9 $M_{\odot }$ yr−1. The outermost part of the hydrogen-rich envelope is connected to a steadily moving envelope where optically thick winds occur. We confirm that no internal shock waves occur at thermonuclear runaway. The wind mass-loss rate reaches a peak of 1.4 × 10−4 $M_{\odot }$ yr−1 at the epoch of the maximum photospheric expansion, where the photospheric temperature decreases to log Tph (K) = 3.90. Almost all of the accreted mass is lost in the wind. The nuclear energy generated in hydrogen burning is lost in a form of photon emission (64%), gravitational energy (lifting up the wind matter against gravity, 35%), and the kinetic energy of the wind (0.23%). A classical nova should be very bright in a far-UV (100–300 Å) band for one day just after the onset of thermonuclear runaway (∼ 25 d before the optical maximum). In the decay phase of the nova outburst, the envelope structure is very close to that of a steady-state solution.

A triple star origin for T Pyx and other short-period recurrent novae

ABSTRACT Recurrent novae are star systems in which a massive white dwarf accretes material at such a high rate that it undergoes thermonuclear runaways every 1–100 yr. They are the only class of novae in which the white dwarf can grow in mass, making some of these systems strong Type Ia supernova progenitor candidates. Almost all known recurrent novae are long-period ($P_{\mathrm{orb}} \gtrsim 12\, \mathrm{h}$) binary systems in which the requisite mass supply rate can be provided by an evolved (sub-)giant donor star. However, at least two recurrent novae are short-period ($P_{\mathrm{orb}} \lesssim 3\, \mathrm{h}$) binaries in which mass transfer would normally be driven by gravitational radiation at rates three to four orders of magnitude smaller than required. Here, we show that the prototype of this class – T Pyxidis – has a distant proper motion companion and therefore likely evolved from a hierarchical triple star system. Triple evolution can naturally produce exotic compact binaries as a result of three-body dynamics, either by Kozai–Lidov eccentricity cycles in dynamically stable systems or via mass-loss-induced dynamical instabilities. By numerically evolving triple progenitors with physically reasonable parameters forward in time, we show explicitly that the inner binary can become so eccentric that mass transfer is triggered at periastron, driving the secondary out of thermal equilibrium. We suggest that short-period recurrent novae likely evolved via this extreme state, explaining their departure from standard binary evolution tracks.

Localized thermonuclear bursts from accreting magnetic white dwarfs

Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs1-3. It has been predicted4-6 that localized thermonuclear bursts on white dwarfs can also take place, similar to type-I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately strong magnetized white dwarf from a low-mass companion, have been observed on several occasions in the past 40 years7-11. During these bursts, the optical/ultraviolet luminosity increases by a factor of more than three in less than an hour and fades in around ten hours. Fast outflows have been observed in ultraviolet spectral lines7, with velocities of more than 3,500 kilometres per second, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae and in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy of approximately 10-6 times thanthose of classical nova explosions (micronovae) and bear a strong resemblance to type-I X-ray bursts12-14. We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically confined accretion columns as a viable explanation.

Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs

ABSTRACT Rapid bursts at optical wavelengths have been reported for several accreting white dwarfs. In these bursts, the optical luminosity can increase by up to a factor of 30 in less than an hour, before fading on time-scales of several hours, and the energy release can reach ~1039 erg (‘micronovae’). Several systems have also shown these bursts to be semirecurrent on time-scales of days to months, and the temporal profiles of these bursts strongly resemble those observed in Type-I X-ray bursts in accreting neutron stars. It has been suggested that the observed micronovae may be the result of localized thermonuclear runaways in the surface layers of accreting white dwarfs. Here we propose a model in which the magnetic confinement of accretion streams on to the accreting magnetic white dwarf may trigger localized thermonuclear runaways. The model proposed to trigger micronovae appears to favour magnetic systems with both a high white dwarf mass and a high mass-transfer rate.

Mixing fraction in classical novae

Context. Classical novae are powered by thermonuclear runaways occurring on the surface of accreting white dwarfs (WDs). In the observations, the enrichments of heavy elements in nova ejecta have been detected, indicating a mixing process between the accreted matter and the matter from the outer layers of the underlying WDs prior to nova outbursts. However, the mixing fraction in classical novae is still uncertain. Aims. The purpose of this article is to investigate some elemental abundance ratios during nova outbursts that can be used to estimate the WD mixing fraction in classical novae. Methods. By considering different WD mixing fractions with the stellar evolution code Modules for Experiments in Stellar Astrophysics, we carried out a series of simulations of nova outbursts, in which the initial CO WD masses range from 0.7−1.0 M⊙. Results. We identified four elemental abundance ratios (i.e. (H + He)/∑CNO, (H + He)/Ne, ∑CNO/Mg, and ∑CNO/Si) that satisfy the conditions for determining the WD mixing fraction, in which (H + He)/∑CNO is the most suitable mixing meter. We also estimated the WD mixing fraction in some representative classical novae. Additionally, we found that a higher metallicity (i.e. higher WD mixing fraction) is preferentially accompanied by a longer t2 (the time of decline by two magnitudes from peak luminosity) during nova outbursts. Our results can be used to constrain the mixing process in classical novae.

A search for cool molecular gas in GK Persei and other classical novae

Detecting molecular line emission from classical nova remnants has the potential to reveal information on the composition of the ejecta, in particular accurate isotopic ratios in the matter processed by a thermonuclear runaway. We conducted searches toward more than 100 classical novae for emission in lines of CO or HCN molecules using single-dish telescopes and interferometric arrays at millimeter and submillimeter wavelengths. The survey demonstrates that classical novae, young or old, are not strong sources of molecular emission at submillimeter or millimeter wavelengths. Additionally, we mapped CO emission around Nova Persei 1901 (GK Per), earlier claimed to be of circumstellar origin. Our measurements indicate that the observed emission is from the interstellar medium. Although no molecular emission at millimeter or submillimeter wavelengths has been found in classical novae, it is still likely that some will be detected with high-sensitivity interferometers such as ALMA.