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Recent observations have found a growing number of hypervelocity stars with speeds of ≈1500 − 2500 km s⁻¹that could have only been produced through thermonuclear supernovae in white dwarf binaries. Most of the observed hypervelocity runaways in this class display a surprising inflated structure: their current radii are roughly an order of magnitude greater than they would have been as white dwarfs filling their Roche lobe. While many simulations exist studying the dynamical phase leading to supernova detonation in these systems, no detailed calculations of the long-term structure of the runaways have yet been performed. We used an existing AREPOhydrodynamical simulation of a supernova in a white dwarf binary as a starting point for the evolution of these stars with the one-dimensional stellar evolution code MESA. We show that the supernova shock is not energetic enough to inflate the white dwarf over timescales longer than a few thousand years, significantly shorter than the 10^5 − 6year lifetimes inferred for observed hypervelocity runaways. Although they experience a shock from a supernova less than ≈0.02 R_⊙away, our models do not experience significant interior heating, and all contract back to radii of around 0.01 R_⊙within about 10⁴years. Explaining the observed inflated states requires either an additional source of significant heating or some other physics that is not yet accounted for in the subsequent evolution. 

Abstract The precise origin of Type Ia supernovae (SNe Ia) is unknown despite their value to numerous areas in astronomy. While it is a long-standing consensus that they arise from the explosion of a carbon/oxygen white dwarf, the exact progenitor configurations and explosion mechanisms that lead to SNe Ia are still debated. One popular theory is the double detonation, in which a helium layer, accreted from a binary companion, detonates on the surface of the primary star, leading to a converging shock-induced detonation of the underlying core. It has recently been seen in simulations that a helium-rich degenerate companion may undergo its own explosion triggered by the impact from the ejecta of the primary star. We show 2D simulations that approximate a white dwarf undergoing a double detonation, which triggers the explosion of the degenerate companion, leading to either a triple or quadruple detonation. We also present the first multidimensional radiative transfer results from the triple and quadruple detonation scenario. We find that within a range of mass configurations of the degenerate binary, the synthetic light curves and spectra of these events match observations as well as theoretical models of isolated double detonations do. Notably, double and quadruple detonations that are spectrally similar and reach the same peak brightnesses have drastically different ejecta masses and produce different amounts of Si- and Fe-group elements. Further understanding of this scenario is needed in order to determine if at least some observed SNe Ia actually originate from two stars exploding. 

Abstract Type Ia supernovae (SNe Ia), critical for studying cosmic expansion, arise from thermonuclear explosions of white dwarfs, but their precise progenitor pathways remain unclear. Growing evidence supports the “double-degenerate scenario,” where two white dwarfs interact. The absence of nondegenerate companions capable of explaining the observed SN Ia rate, along with observations of hypervelocity white dwarfs, interpreted as surviving companions of such systems, provide compelling evidence for this scenario. Upcoming millihertz gravitational-wave observatories like the Laser Interferometer Space Antenna (LISA) are expected to detect thousands of double-degenerate systems, though the most compact known candidate SN Ia progenitors produce marginally detectable signals. Here, we report observations of ATLAS J1138-5139, a binary white dwarf system with an orbital period of just 28 minutes. Our analysis reveals a 1M_☉carbon–oxygen white dwarf accreting from a high-entropy helium-core white dwarf. Given its mass, the accreting carbon–oxygen white dwarf is poised to trigger a typical-luminosity SN Ia within a few million years, to evolve into a stably transferring AM Canum Venaticorum (or AM CVn) system, or undergo a merger into a massive white dwarf. ATLAS J1138-5139 provides a rare opportunity to calibrate binary evolution models by directly comparing observed orbital parameters and mass-transfer rates closer to merger than any known SN Ia progenitor. Its compact orbit ensures detectability by LISA, demonstrating the potential of millihertz gravitational-wave observatories to reveal a population of SN Ia progenitors on a Galactic scale, paving the way for multimessenger studies offering insights into the origins of these cosmologically significant explosions. 

We survey our understanding of classical novae—nonterminal, thermonuclear eruptions on the surfaces of white dwarfs in binary systems. The recent and unexpected discovery of GeV gamma rays from Galactic novae has highlighted the complexity of novae and their value as laboratories for studying shocks and particle acceleration. We review half a century of nova literature through this new lens, and conclude the following: ▪  The basics of the thermonuclear runaway theory of novae are confirmed by observations. The white dwarf sustains surface nuclear burning for some time after runaway, and until recently, it was commonly believed that radiation from this nuclear burning solely determines the nova's bolometric luminosity. ▪  The processes by which novae eject material from the binary system remain poorly understood. Mass loss from novae is complex (sometimes fluctuating in rate, velocity, and morphology) and often prolonged in time over weeks, months, or years. ▪  The complexity of the mass ejection leads to gamma-ray-producing shocks internal to the nova ejecta. When gamma rays are detected (around optical maximum), the shocks are deeply embedded and the surrounding gas is very dense. ▪  Observations of correlated optical and gamma-ray light curves confirm that the shocks are radiative and contribute significantly to the bolometric luminosity of novae. Novae are therefore the closest and most common interaction-powered transients. 

Abstract We explore the observational appearance of the merger of a low-mass star with a white dwarf (WD) binary companion. We are motivated by recent work finding that multiple tensions between the observed properties of cataclysmic variables (CVs) and standard evolution models are resolved if a large fraction of CV binaries merge as a result of unstable mass transfer. Tidal disruption of the secondary forms a geometrically thick disk around the WD, which subsequently accretes at highly super-Eddington rates. Analytic estimates and numerical hydrodynamical simulations reveal that outflows from the accretion flow unbind a large fraction ≳90% of the secondary at velocities ∼500–1000 km s −1 within days of the merger. Hydrogen recombination in the expanding ejecta powers optical transient emission lasting about a month with a luminosity ≳10 38 erg s −1 , similar to slow classical novae and luminous red novae from ordinary stellar mergers. Over longer timescales the mass accreted by the WD undergoes hydrogen shell burning, inflating the remnant into a giant of luminosity ∼300–5000 L ⊙ , effective temperature T eff ≈ 3000 K, and lifetime ∼10 4 –10 5 yr. We predict that ∼10 3 –10 4 Milky Way giants are CV merger products, potentially distinguishable by atypical surface abundances. We explore whether any Galactic historical slow classical novae are masquerading CV mergers by identifying four such post-nova systems with potential giant counterparts for which a CV merger origin cannot be ruled out. We address whether the historical transient CK Vul and its gaseous/dusty nebula resulted from a CV merger. 

Abstract Study of the double-detonation Type Ia supernova scenario, in which a helium-shell detonation triggers a carbon-core detonation in a sub-Chandrasekhar-mass white dwarf (WD), has experienced a resurgence in the past decade. New evolutionary scenarios and a better understanding of which nuclear reactions are essential have allowed for successful explosions in WDs with much thinner helium shells than in the original, decades-old incarnation of the double-detonation scenario. In this paper, we present the first suite of light curves and spectra from multidimensional radiative transfer calculations of thin-shell double-detonation models, exploring a range of WD and helium-shell masses. We find broad agreement with the observed light curves and spectra of nonpeculiar Type Ia supernovae, from subluminous to overluminous subtypes, providing evidence that double detonations of sub-Chandrasekhar-mass WDs produce the bulk of observed Type Ia supernovae. Some discrepancies in spectral velocities and colors persist, but these may be brought into agreement by future calculations that include more accurate initial conditions and radiation transport physics. 

Abstract Type Ia supernovae (SNe Ia) are securely understood to come from the thermonuclear explosion of a white dwarf as a result of binary interaction, but the nature of that binary interaction and the secondary object is uncertain. Recently, a double white dwarf model known as the dynamically driven double-degenerate double-detonation (D6) model has become a promising explanation for these events. One realization of this scenario predicts that the companion may survive the explosion and reside within the remnant as a fast moving (V_peculiar> 1000 km s⁻¹), overluminous (L> 0.1L_⊙) white dwarf. Recently, three objects that appear to have these unusual properties have been discovered in the Gaia survey. We obtained photometric observations of the SN Ia remnant SN 1006 with the Dark Energy Camera over four years to attempt to discover a similar star. We present a deep, high-precision astrometric proper-motion survey of the interior stellar population of the remnant. We rule out the existence of a high-proper-motion object consistent with our tested realization of the D6 scenario (V_transverse> 600 km s⁻¹withm_r< 21 corresponding to an intrinsic luminosity ofL> 0.0176L_⊙). We conclude that such a star does not exist within the remnant or is hidden from detection by either strong localized dust or the unlikely possibility of ejection from the binary system almost parallel to the line of sight. 

We present the first results of a comprehensive supernova (SN) radiative-transfer (RT) code-comparison initiative (StaNdaRT), where the emission from the same set of standardised test models is simulated by currently used RT codes. We ran a total of ten codes on a set of four benchmark ejecta models of Type Ia SNe. We consider two sub-Chandrasekhar-mass (M_tot= 1.0M_⊙) toy models with analytic density and composition profiles and two Chandrasekhar-mass delayed-detonation models that are outcomes of hydrodynamical simulations. We adopt spherical symmetry for all four models. The results of the different codes, including the light curves, spectra, and the evolution of several physical properties as a function of radius and time are provided in electronic form in a standard format via a public repository. We also include the detailed test model profiles and several Python scripts for accessing and presenting the input and output files. We also provide the code used to generate the toy models studied here. In this paper, we describe the test models, radiative-transfer codes, and output formats in detail, and provide access to the repository. We present example results of several key diagnostic features.