Shell Flashes Research Articles

Formation of long-period post-common-envelope binaries

Context. The vast majority of close binaries containing a compact object, including the progenitors of supernovae Ia and at least a substantial fraction of all accreting black holes in the Galaxy, form through common-envelope (CE) evolution. Despite this importance, we struggle to even understand the energy budget of CE evolution. For decades, observed long-period post-CE binaries have been interpreted as evidence of additional energies contributing during CE evolution. We have recently shown that this argument is based on simplified assumptions for all long-period post-CE binaries containing massive white dwarfs (WDs). The only remaining post-CE binary star that has been claimed to require contributions from additional energy sources to understand its formation is KOI 3278. Aims. Here, we address in detail the potential evolutionary history of KOI 3278. In particular, we investigate whether extra energy sources, such as recombination energy, are indeed required to explain its existence. Methods. We used the 1D stellar evolution code MESA to carry out binary evolution simulations and searched for potential formation pathways for KOI 3278 that are able to explain its observed properties. Results. We find that KOI 3278 can be explained if the WD progenitor filled its Roche lobe during a helium shell flash. In this case, the orbital period of KOI 3278 can be reproduced if the CE binding energy is calculated taking into account gravitational energy and thermodynamic internal energy. While the CE evolution that led to the formation of KOI 3278 must have been efficient – that is, most of the available orbital energy must have been used to unbind the CE – recombination energy is not required. Conclusions. We conclude that currently not a single observed post-CE binary requires one to assume that energy sources other than gravitational and thermodynamic energy are contributing to CE evolution. KOI 3278, however, remains an intriguing post-CE binary as, unlike its siblings, understanding its existence requires highly efficient CE ejection.

Hydrodynamic models of pulsation period evolution in R Hydrae

ABSTRACT Pulsation period evolution during the helium–shell flash in the Mira variable R Hya is investigated using consistent stellar evolution and non-linear stellar pulsation computations. The initial and time-dependent inner boundary conditions for the equations of radiation hydrodynamics describing non-linear stellar oscillations were determined using a grid of TP–AGB model sequences with initial masses on the main sequence 1.5M⊙ ≤ MZAMS ≤ 5.0M⊙ and the initial metallicity Z = 0.014. The setup of initial conditions for hydrodynamic models corresponds to ≈100 yr prior to the maximum of the helium–shell luminosity and ensures that the stellar envelope of the evolution model is under both hydrostatic and thermal equilibrium. Solution of the equations of hydrodynamics allowed us to determine the temporal variation of the pulsation period Π(t) during ≈500 yr. Within this time interval R Hya is a fundamental mode pulsator. The period temporal dependencies Π(t) calculated for the AGB star models at the beginning of the third dredge-up phase and with masses $4.4M_{\odot }\lessapprox M\lessapprox 4.5M_{\odot }$ are in agreement with observational estimates of the period of R Hya obtained during last two centuries. The mean radius of R Hya pulsation models at the end of the XX century ($470 R_{\odot }\lessapprox \bar{R}\lessapprox 490 R_{\odot }$) agrees with observational estimates obtained using the interferometric angular diameter measurements.

Study of an Isolated Dust Structure Nearby the White Dwarf WD0011-399 Using IRIS, AKARI, and WISE Data

During the asymptotic giant branch (AGB) phase, a major fraction of mass is spread by the stars (0.6 - 10 M⊙) in the interstellar medium (ISM) in the form of dust. In the left phase of the post-main sequence evolution in the Hertzsprung-Russell (HR) diagram, these dust are found to be surrounding the white dwarfs (WD). Some small fraction of dust is also formed in the circumstellar shells and cavities around WD which is usually a source of infrared (IR) excess. The formation and evolution of such IR dust structures are the results of high-pressure events such as the Helium shell flash (thermal pulse) and stellar winds, and such structures are crucial in the study of interaction phenomena in ISM. This research project is focused on the study of an isolated dust structure near the white dwarf WD0011-399 located at R.A. (J2000) 00h 13m 47.48s and Dec. (J2000) -39° 370 24.2800 using Improved Reprocessing of the IRAS (IRIS), AKARI and Wide-field Infrared Survey Explorer (WISE) surveys from SkyView Virtual Observatory along with SIMBAD Astronomical Database and Gaia Archive of ESA. The size of the cavity under study is 7.43 pc×2.90 pc, 2.99pc×0.99pc, and 1.26×0.53pc with an inclination angle of 71.12°, 75.73°, and 68.48° respectively in IRIS, AKARI and WISE data suggesting the cavity is neither a face-on nor an edge-on. The relative flux density of the region has been studied through pixel extraction of Far-Infrared (FIR) images and dust color temperature along with dust mass has been calculated. Using IRIS data, the temperature of the isolated region is found between a maximum value of 36.82±4.30 K to a minimum of 22.59±2.32 K with an offset of 14.23 K. The average temperature of the region is 28.22±0.18 K. Following similar procedures for AKARI data, the temperature is found between a maximum of 26.37±3.56 K and a minimum of 16.32±1.47 K with an offset of 10.06K. The average temperature is 19.25±0.15 K. Similarly, using WISE data, the temperature is found between a maximum of 353.72±18.54 K and a minimum of 307.24±4.69 K with an offset of 46.48K. The average temperature is 316.62±0.81 K. High value of offset temperature suggests that the cavity might be evolving with disruptions from background radiative sources. Approximately Gaussian distribution of the temperature in all the surveys implies that the region might be tending towards local thermodynamic equilibrium. The total mass of the structure is estimated to be around 0.03±1.5×10−6M⊙, 1.6×10−3±1.4×10−5 M⊙, and 10−7±3×10−10 M⊙ using IRIS, AKARI, and WISE data respectively. The Jean’s mass of the structure is calculated assuming the structure is a non-degenerate gas, which is estimated around 2001.93M⊙, 178.44M⊙, and 121.47M⊙ respectively using IRIS, AKARI, and WISE data. Since Jean’s mass is much greater than the mass of the structure there seems no possibility of star formation within the region of interest. The color maps illustrate identical distribution for all wavelengths, however, no significant relation is observed between dust color temperature and dust mass.

Pre-explosion Helium Shell Flash in Type Ia Supernovae

I study the possibility that within the frame of the core degenerate (CD) scenario for type Ia supernovae (SNe Ia) the merger process of the core of the asymptotic giant branch (AGB) star and the white dwarf (WD) maintains an envelope mass of ≈0.03 M ⊙ that causes a later helium shell flash. I estimate the number of pre-explosion helium shell flash events to be less than a few per cent of all CD scenario SNe Ia. A helium shell flash while the star moves to the left on the HR diagram as a post-AGB star (late thermal pulse—LTP) or along the WD cooling track (very LTP—VLTP) causes the star to expand and become a “born again” AGB star. Merger remnants exploding while still on the AGB form hydrogen-polluted peculiar SNe Ia, while an explosion inside an inflated born-again star results in an early flux excess in the light curve of the SN Ia. The fraction of systems that might show an early flux excess due to LTP/VLTP is <few × 10−4 of all SNe Ia, much below the observed fraction. In the frame of the CD scenario SNe Ia with early flux excess result from SN ejecta collision with planetary nebula fallback gas, or from mixing of 56Ni to the outer regions of the SN ejecta. Ongoing sky surveys might find about one case per year where LTP/VLTP influences the SN light curve.

Detached Shell Carbon Stars: Tracing Thermal Pulses on the Asymptotic Giant Branch

Abstract We consider whether the subset of carbon-rich asymptotic giant branch (AGB) stars that exhibit detached, expanding circumstellar shells may reveal the past histories of these stars as having undergone helium shell flashes (thermal pulses) on the AGB. We exploit newly available Gaia parallaxes and photometry, along with archival infrared photometry, to obtain refined estimates of the luminosities of all (12) known detached shell carbon stars. We examine the relationship between these luminosities and the estimated dynamical ages (ejection times) of the detached shells associated with the 12 stars, which range from ∼1000 to ∼30,000 yr. When arranged according to detached shell dynamical age, the (implied) luminosity evolution of the known detached shell carbon stars closely follows the predicted “light curves” of individual thermal pulses obtained from models of AGB stars. The comparison between data and models suggests that detached shell carbon stars are descended from ∼2.5 to 4.0 M ⊙ progenitors. We conclude that detached shell carbon stars may serve as effective tracers of the luminosity evolution of AGB thermal pulses.

New models for the rapid evolution of the central star of the Stingray Nebula

Abstract We present stellar evolution calculations from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase for models of initial mass 1.2 M⊙ and 2.0 M⊙ that experience a Late Thermal Pulse (LTP), a helium shell flash that occurs following the AGB and causes a rapid looping evolution between the AGB and PN phase. We use these models to make comparisons to the central star of the Stingray Nebula, V839 Ara (SAO 244567). The central star has been observed to be rapidly evolving (heating) over the last 50 to 60 years and rapidly dimming over the past 20–30 years. It has been reported to belong to the youngest known planetary nebula, now rapidly fading in brightness. In this paper we show that the observed timescales, sudden dimming, and increasing Log(g), can all be explained by LTP models of a specific variety. We provide a possible explanation for the nebular ionization, the 1980’s sudden mass loss episode, the sudden decline in mass loss, and the nebular recombination and fading.

Asymmetrical Mass Ejection from Proto-white Dwarfs and the Formation of Eccentric Millisecond Pulsar Binaries

Abstract Binary millisecond pulsars (MSPs) are believed to have descended from low-mass X-ray binaries (LMXBs), which have experienced substantial mass transfer and tidal circularization. Therefore, they should have very circular orbits. However, the discovery of several eccentric binary MSPs (with eccentricity e ∼ 0.01–0.1) challenges this standard picture. Three models have been proposed thus far based on accretion-induced collapse of massive white dwarfs (WDs), neutron star–strange star transition, and formation of circumbinary disks. All of them are subject to various uncertainties and are not entirely consistent with observations. Here, we propose an alternative model that takes into account the influence of thermonuclear flashes on proto-WDs. We assume that the flashes lead to asymmetrical mass ejection, which imparts a mild kick to the proto-WDs. By simulating orbital changes of binary MSPs with multiple shell flashes, we show that it is possible to reproduce the observed eccentricities, provided that the kick velocities are around a few km s−1.

Searching for the most powerful thermonuclear X-ray bursts with the Neil Gehrels Swift Observatory

We searched for thermonuclear X-ray bursts from Galactic neutron stars in all event mode data of the Neil Gehrels Swift Observatory collected until March 31, 2018. In particular, we are interested in the intermediate-duration bursts (shell flashes fueled by thick helium piles) with the ill-understood phenomenon of strong flux fluctuations. Nine such bursts have been discussed in the literature to date. Swift is particularly suitable for finding additional examples. We find and list a total of 134 X-ray bursts; 44 are detected with BAT only, 41 with XRT only, and 49 with both. Twenty-eight bursts involve automatic slews. We find 12 intermediate-duration bursts, all detected in observations involving automatic slews. Five show remarkably long Eddington-limited phases in excess of 200 s. Five show fluctuations during the decay phase; four of which are first discussed in the present study. We discuss the general properties of the fluctuations, considering also 7 additional literature cases. In general two types of fluctuations are observed: fast ones, with a typical timescale of 1 s and up and downward fluctuations of up to 70%, and slow ones, with a typical timescale of 1 min and only downward fluctuations of up to 90%. The latter look like partial eclipses because the burst decay remains visible in the residual emission. We revisit the interpretation of this phenomenon in the context of the new data set and find that it has not changed fundamentally despite the expanded data set. It is thought to be due to a disturbance of the accretion disk by outflowing matter and photons, causing obscuration and reflection due to Thompson scattering in an orbiting highly ionized cloud or structure above or below the disk. We discuss in detail the most pronounced burster SAX J1712.6–3739. One of the bursts from this source is unusual in that it lasts longer than 5600 s, but does not appear to be a superburst.

Production of Silicon on Mass-increasing White Dwarfs: Possible Origin of High-velocity Features in Type Ia Supernovae

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon-oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 $M_\odot$, 1.2 $M_\odot$, and 1.35 $M_\odot$ CO WDs accreting helium-rich matter with several mass-accretion rates ranging from $1 \times 10^{-7}~M_\odot$ yr$^{-1}$ to $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$. For the $1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer developed as helium burning ash and consisted of 40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by weight. For a higher mass accretion rate of $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and 0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). No substantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si II HVFs, its characteristics are consistent with that of mass increasing WDs. We also discuss possible Ca production on very massive WDs ($ \gtrsim 1.38~M_\odot$).

Formation of Extremely Low-mass White Dwarf Binaries

Motivated by the discovery of a handful of pulsating, extremely low mass white dwarfs (ELM WDs, mass $M \lesssim 0.17\, M_\odot$) which likely have WD companions, this paper discusses binary formation models for these systems. Formation of an ELM WD by unstable mass transfer (MT) or a common envelope (CE) event is unlikely. Stable Roche-lobe overflow with conservative MT produces only $M \gtrsim 0.2\, M_\odot$. This paper discusses the formation of ELM WD using angular momentum losses by magnetic braking. The initially more massive star becomes the companion WD through a CE event. The initially less massive star becomes the ELM WD as an evolved donor in a Cataclysmic Variable binary. Evolutionary models are constructed using the Modules for Experiments in Stellar Astrophysics (MESA), with ELM WD progenitors in the range $M_d=1.0-1.5\, M_\odot$ and WD companions in the range $M_{\rm a} \simeq 0.4-0.9\, M_\odot$. A prescription to reduce magnetic braking for thin surface convection zones is included. Upon the thinning of the evolved donor's envelope, the donor star shrinks out of contact and MT ceases, revealing the ELM WD. Systems with small masses have previously been suggested as possible AM CVN's. Systems with large masses, up to the limit $M \simeq 0.18\, M_\odot$ at which shell flashes occur on the WD cooling track, will tend to expand out to orbital periods $P_{\rm orb} \gtrsim 15\, {\rm hr}$. In between this range, ELM WD may become pulsators both as pre-WD and on the WD cooling track. Brickhill's criterion for convective mode driving is used to estimate the location of the blue edge of the g-mode instability strip.

A Millennium-long Evolution of the 1 yr Recurrence Period Nova—Search for Any Indication of the Forthcoming He Flash

Abstract We present 1500 cycles of hydrogen shell flashes on a white dwarf (WD) for a mass accretion rate of yr−1, the mass ejection of which is calculated consistently with the optically thick winds. This model mimics the 1 yr recurrence period nova M31N 2008-12a. Through these hydrogen flashes a helium ash layer grows in mass and eventually triggers a helium nova outburst. Each hydrogen flash is almost identical, and there is no precursor for the forthcoming He flash either in the outburst or in the quiescent until the next He flash suddenly occurs. Thus, M31N 2008-12a is a promising candidate of He novae, outbursting in any time within a millennium. The prompt X-ray flash of He nova lasts as short as 15 minutes, with the X-ray luminosity being about half of the Eddington luminosity, making the observation difficult. In the very early phase of a He flash, the uppermost H-rich layer is convectively mixed into the deep interior and most of the hydrogen is consumed by nuclear burning. In comparison with hydrogen shell flashes of M31N 2008-12a, we expect the forthcoming He nova to have a very short prompt X-ray flash (15 minutes), a very bright optical/near-IR peak (∼3.5 mag brighter than M31N 2008-12a), a much longer nova duration ( yr), and a longer supersoft X-ray source phase (40–50 days or more).

Accretion-induced Collapse from Helium Star + White Dwarf Binaries

Abstract Accretion-induced collapse (AIC) occurs when an O/Ne white dwarf (WD) grows to nearly the Chandrasekhar mass ( ), reaching central densities that trigger electron captures in the core. Using Modules for Experiments in Stellar Astrophysics (MESA), we present the first true binary simulations of He star + O/Ne WD binaries, focusing on a He star in a 3 hr orbital period with O/Ne WDs. The helium star fills its Roche lobe after core helium burning is completed and donates helium on its thermal timescale to the WD, , which is a rate high enough that the accreting helium burns stably on the WD. The accumulated carbon/oxygen ashes from the helium burning undergo an unstable shell flash that initiates an inwardly moving, carbon burning flame. This flame is only quenched when it runs out of carbon at the surface of the original O/Ne core. Subsequent accumulation of fresh carbon/oxygen layers also undergo thermal instabilities, but no mass loss is triggered, which allows , and then triggers the onset of AIC. We also discuss the scenario of accreting C/O WDs that experience shell carbon ignitions to become O/Ne WDs, and then, under continuing mass transfer, lead to AIC. Studies of the AIC event rate using binary population synthesis should include all of these channels, especially this latter channel, which has been previously neglected but might dominate the rate.

Cooling star shows evidence of helium shell flash

Cooling star shows evidence of helium shell flash

Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

A large number of extremely low-mass helium white dwarfs (ELM WDs) have been discovered in recent years. The majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass X-ray binary (LMXB) or a cataclysmic variable (CV) system. Here, we investigate the formation of these objects through the LMXB channel with emphasis on the proto-WD evolution in environments with different metallicities. We study, for the first time, the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-WDs and on the cooling properties of the resulting WDs. We present state-of-the-art binary stellar evolution models computed with MESA for metallicities between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 M$_{\odot}$. Our results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience hydrogen shell flashes. The occurrence of these flashes produces a clear dichotomy in the cooling timescales of ELM WDs, which has important consequences e.g. for the age determination of binary millisecond pulsars. Rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated ELM proto-WDs and therefore plays a key role in determining their surface chemical abundances. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime, a crucial ingredient for understanding the newly observed ELM proto-WD pulsators. The hydrogen envelope at detachment, although small compared to the total mass of the WD, contains enough angular momentum such that the spin frequency of the resulting WD on the cooling track is well above the orbital frequency.

SHORTEST RECURRENCE PERIODS OF FORCED NOVAE

ABSTRACT We revisit hydrogen shell burning on white dwarfs (WDs) with higher mass accretion rates than the stability limit, , above which hydrogen burning is stable. Novae occur with mass accretion rates below the limit. For an accretion rate &gt; , a first hydrogen shell flash occurs followed by steady nuclear burning, so the shell burning will not be quenched as long as the WD continuously accretes matter. On the basis of this picture, some persistent supersoft X-ray sources can be explained by binary models with high accretion rates. In some recent studies, however, the claim has been made that no steady hydrogen shell burning exists even for accretion rates &gt; . We demonstrate that, in such cases, repetitive flashes occurred because mass accretion was artificially controlled. If we stop mass accretion during the outburst, no new nuclear fuel is supplied, so the shell burning will eventually stop. If we resume mass accretion after some time, the next outburst eventually occurs. In this way, we can design the duration of outburst and interpulse time with manipulated mass accretion. We call such a controlled nova a “forced nova.” These forced novae, if they exist, could have much shorter recurrence periods than “natural novae.” We have obtained the shortest recurrence periods for forced novae for various WD masses. Based on the results, we revisit WD masses of some recurrent novae, including T Pyx.

GROWING WHITE DWARFS TO THE CHANDRASEKHAR LIMIT: THE PARAMETER SPACE OF THE SINGLE DEGENERATE SN Ia CHANNEL

ABSTRACT Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a SN Ia? We explore the range of accretion rates that allow a WD to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach 1.4M ⊙—the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, especially in the outer layers. This key result yields helium burning that is quasi-steady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, self-consistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.

Cool white dwarf companions to four millisecond pulsars

We report on photometric and spectroscopic observations of white dwarf companions to four binary radio millisecond pulsars, leading to the discovery of companions to PSRs J0614-3329, J1231-1411 and J2017+0603. We place limits on the brightness of the companion to PSR J0613-0200. Optical spectroscopy of the companion to PSR J0614-3329 identifies it as a DA type white dwarf with a temperature of Teff=6460+-80 K, a surface gravity log g=7.0+-0.2 cgs and a mass of Mwd=0.24+-0.04 Msun. We find that the distance to PSR J0614-3329 is smaller than previously estimated, removing the need for the pulsar to have an unrealistically high gamma-ray efficiency. Comparing the photometry with predictions from white dwarf cooling models allows us to estimate temperatures and cooling ages of the companions to PSRs J0613-0200, J1231-1411 and J2017+0603. We find that the white dwarfs in these systems are cool Teff<4000 K and old >5 Gyr. Thin Hydrogen envelopes are required for these white dwarfs to cool to the observed temperatures, and we suggest that besides Hydrogen shell flashes, irradiation driven mass loss by the pulsar may have been important.

Hybrid C–O–Ne white dwarfs as progenitors of Type Ia supernovae: dependence on Urca process and mixing assumptions

When carbon is ignited off-centre in a CO core of a super-asymptotic giant branch star, its burning in a convective shell tends to propagate to the centre. Whether the C flame will actually be able to reach the centre depends on the efficiency of extra mixing beneath the C convective shell. Whereas thermohaline mixing is too inefficient to interfere with the C-flame propagation, convective boundary mixing can prevent the C burning from reaching the centre. As a result, a C–O–Ne white dwarf (WD) is formed, after the star has lost its envelope. Such a ‘hybrid’ WD has a small CO core surrounded by a thick ONe zone. In our 1D stellar evolution computations, the hybrid WD is allowed to accrete C-rich material, as if it were in a close binary system and accreted H-rich material from its companion with a sufficiently high rate at which the accreted H would be processed into He under stationary conditions, assuming that He could then be transformed into C. When the mass of the accreting WD approaches the Chandrasekhar limit, we find a series of convective Urca shell flashes associated with high abundances of 23Na and 25Mg. They are followed by off-centre C ignition leading to convection that occupies almost the entire star. To model the Urca processes, we use the most recent well-resolved data for their reaction and neutrino-energy loss rates. Because of the emphasized uncertainty of the convective Urca process in our hybrid WD models of Type Ia supernova (SN Ia) progenitors, we consider a number of their potentially possible alternative instances for different mixing assumptions, all of which reach a phase of explosive C ignition, either off or in the centre. Our hybrid SN Ia progenitor models have much lower C-to-O abundance ratios at the moment of the explosive C ignition than their pure CO counterparts, which may explain the observed diversity of the SNe Ia.

THE FINAL FATE OF STARS THAT IGNITE NEON AND OXYGEN OFF-CENTER: ELECTRON CAPTURE OR IRON CORE-COLLAPSE SUPERNOVA?

In the ONeMg cores of $8.8-9.5~{\rm M}_\odot$ stars, neon and oxygen burning is ignited off-center. Whether the neon-oxygen flame propagates to the center is critical to determine whether these stars undergo Fe core collapse or electron capture induced ONeMg core collapse. We present more details of stars that ignite neon and oxygen burning off-center. The neon flame is established in a similar manner to the carbon flame of super-AGB stars, albeit with a narrower flame width. The criteria for establishing a flame are able to be met if the strict Schwarzschild criterion for convective instability is adopted. Mixing across the interface of the convective shell disrupts the conditions for the propagation of the burning front and instead the shell burns as a series of inward-moving flashes. While this may not directly affect whether the burning will reach the center (as in super-AGB stars), the core is allowed to contract between each shell flash. Reduction of the electron fraction in the shell reduces the Chandrasekhar mass and the center reaches the threshold density for the URCA process to activate and steer the remaining evolution of the core. This highlights the importance of a more accurate treatment of mixing in the stellar interior for yet another important question in stellar astrophysics - determining the properties of stellar evolution and supernova progenitors at the boundary between electron capture supernova and iron core-collapse supernova.

The timescale of low-mass proto-helium white dwarf evolution

A large number of low-mass (< 0.20 M_sun) helium white dwarfs (He WDs) have recently been discovered. The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main-sequence star companion. There appear to be discrepancies between the current theoretical modelling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario remains to be fully understood. Here we investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is to examine the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass. Our calculations are then compared to the most recent observational data. Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in a large number of close-orbit LMXBs. Subsequently, we followed the evolution of the detached low-mass proto-He WDs, including stages with residual shell hydrogen burning and vigorous flashes caused by unstable CNO burning. We find that the time between Roche-lobe detachment until the low-mass proto-He WD reaches the WD cooling track is typically Delta_t_proto = 0.5 - 2 Gyr, depending systematically on the WD mass and therefore on its luminosity. The lowest WD mass for developing shell flashes is ~0.21 M_sun for progenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun). The long timescale of low-mass proto-He WD evolution can explain a number of recent observations, including some MSP systems hosting He WD companions with very low surface gravities and high effective temperatures. We find no evidence for Delta_t_proto to depend on the occurrence of flashes and thus question the suggested dichotomy in thermal evolution of proto-WDs.