The Explosion Models and Progenitors oThe Explosion Models and Progenitors o

f Type Ia Supernovaef Type Ia Supernovae

Wang Xiao FengWang Xiao Feng

NAOCNAOC

2003. 10. 212003. 10. 21

OutlinesOutlines

Introduction of SN Taxonomy Introduction of SN Taxonomy Observational constraints on the basic modeObservational constraints on the basic mode

ls of SNe Ia ls of SNe Ia Composition of exploding WDs Composition of exploding WDs Mass, the birth and propagation of thermonMass, the birth and propagation of thermon

uclear flame of exploding WDsuclear flame of exploding WDs Progenitor systems of SNe IaProgenitor systems of SNe Ia

SN classificationsSN classifications Taxonomy Chart Spectroscopic classifications

Observational constraintsObservational constraints Observational characteristics of SNe IaObservational characteristics of SNe Ia The most abundant elements of hydrogen and helium in the universe fail to aThe most abundant elements of hydrogen and helium in the universe fail to a

ppear in the spectra, and a substantial wide absorption lines of intermediate-ppear in the spectra, and a substantial wide absorption lines of intermediate-mass elements (O-Ca) dominate the spectra near maximum lightmass elements (O-Ca) dominate the spectra near maximum light

The light curve peak lasts for several days, and displays exponential decline The light curve peak lasts for several days, and displays exponential decline at late timeat late time

Most of SNe Ia show relatively similar spectra and light curves shapes, but Most of SNe Ia show relatively similar spectra and light curves shapes, but definite departures from the canonical events have also been observed. definite departures from the canonical events have also been observed.

SNe Ia explosion have been detected in galaxies of all Hubble typesSNe Ia explosion have been detected in galaxies of all Hubble types

Light curves and spectra of differLight curves and spectra of different SNe Iaent SNe Ia

What can be inferred from observations?What can be inferred from observations?

The absence of the emissions of hydrogen requires that the atmosphere of the eThe absence of the emissions of hydrogen requires that the atmosphere of the exploding star contains no hydrogen or very few (i.e. < 0.1 ), which point towarxploding star contains no hydrogen or very few (i.e. < 0.1 ), which point towards highly evolved compact objects.ds highly evolved compact objects.

The kinematic energy per mass, ½(~ 10,000 km sThe kinematic energy per mass, ½(~ 10,000 km s-1-1))22, inferred from the velocitie, inferred from the velocities of the ejecta in the explosion is the same magnitude as the energy of fusing cas of the ejecta in the explosion is the same magnitude as the energy of fusing carbon and oxygen into Fe-group elements. The shape of SN Ia light curves follorbon and oxygen into Fe-group elements. The shape of SN Ia light curves follow very well with the energy model of radioactive-decay (w very well with the energy model of radioactive-decay (5656Ni-Ni-5656Co-Co-5656Fe)Fe)

The appearance in elliptical galaxies with their old populations hints at significaThe appearance in elliptical galaxies with their old populations hints at significant that nuclear processing must take place before explosion.nt that nuclear processing must take place before explosion.

The fact that the event is explosive suggests the existence of degenerate matter The fact that the event is explosive suggests the existence of degenerate matter

SN 2002fk in SN 2002fk in NGCNGC

A pinpoint of light from a type Ia supernova that exploded more than 10 A pinpoint of light from a type Ia supernova that exploded more than 10 billion years ago. The supernova was revealed by digitally subtracting before and aftbillion years ago. The supernova was revealed by digitally subtracting before and aft

er images of a faint elliptical galaxy that appears in the HST deep Field image.er images of a faint elliptical galaxy that appears in the HST deep Field image.

SNe Ia represent the thermonuclear disruption of SNe Ia represent the thermonuclear disruption of mass accreting WD. Almost all researchers in this mass accreting WD. Almost all researchers in this field have reached an unanimous consensus on thifield have reached an unanimous consensus on this basic model of SN Ia explosion. However, the s basic model of SN Ia explosion. However, the precise nature of the hydrodynamical models and precise nature of the hydrodynamical models and the progenitor systems are still controversial.the progenitor systems are still controversial.

Composition of exploding WDsComposition of exploding WDs

WDs form at the end of the evolution of stars whose original masses are less than 8 MWDs form at the end of the evolution of stars whose original masses are less than 8 M sun.sun.A stA star can always lose a large fraction of its material by ejecting outer layers into space at the fiar can always lose a large fraction of its material by ejecting outer layers into space at the final stages of evolution. The mass of a remaining WD is always less than the Chandrasekhanal stages of evolution. The mass of a remaining WD is always less than the Chandrasekhar limit, 1.4 Mr limit, 1.4 Msunsun, above which a hydrostatic equilibrium of degenerate matter is impossible. , above which a hydrostatic equilibrium of degenerate matter is impossible.

An isolated WD is stable and almost inert, because its temperature is not high enough to indAn isolated WD is stable and almost inert, because its temperature is not high enough to induce any substantial nuclear reactions. This isolated dead star can exist almost indefinitely, sluce any substantial nuclear reactions. This isolated dead star can exist almost indefinitely, slowly cooling down to black dwarf as it radiates its energy into space. No supernova explosiowly cooling down to black dwarf as it radiates its energy into space. No supernova explosi

on will ensueon will ensue. . A very different fate awaits a WD that accretes mass from a close binary companion star. OA very different fate awaits a WD that accretes mass from a close binary companion star. O

bservations show, however, that more than 50% of all stars are not isolated. They belong to bservations show, however, that more than 50% of all stars are not isolated. They belong to groups of two or three stars that orbit a common center of mass. In a close binary system, a groups of two or three stars that orbit a common center of mass. In a close binary system, a WD can increase its own mass by accreting material from its companion star. Such systems WD can increase its own mass by accreting material from its companion star. Such systems are considered to be the most probable SN Ia progenitors.are considered to be the most probable SN Ia progenitors.

In principle, the WDs that accretes to the explosion could be compoIn principle, the WDs that accretes to the explosion could be composed of He, of C-O or of O-Ne-Mg. sed of He, of C-O or of O-Ne-Mg.

(i) helium (He) WDs, composed almost entirely of helium , form as th(i) helium (He) WDs, composed almost entirely of helium , form as the degenerate cores of low-mass giants (M <2 Me degenerate cores of low-mass giants (M <2 Msun.sun. ) which lose their h ) which lose their h

ydrogen envelope before helium can ignite;ydrogen envelope before helium can ignite;

(ii) carbon-oxygen(C-O) WDs, composed of about 20% C and 80% o(ii) carbon-oxygen(C-O) WDs, composed of about 20% C and 80% oxygen, form as the cores of asymptotic giant branch (AGB) stars or naxygen, form as the cores of asymptotic giant branch (AGB) stars or naked helium burning stars that lose their envelope before carbon ignitioked helium burning stars that lose their envelope before carbon ignition; n;

(iii) oxygen-neon-magnesium (O-Ne-Mg) WDs, composed of heavier (iii) oxygen-neon-magnesium (O-Ne-Mg) WDs, composed of heavier combinations of elements, form from giants that ignite carbon in their combinations of elements, form from giants that ignite carbon in their cores. cores.

The fate of accreting WDsThe fate of accreting WDs

Mass, ignition and propagation of flames of eMass, ignition and propagation of flames of exploding WDsxploding WDs

Chandrasekhar-mass model Chandrasekhar-mass model When the central density approach the critical value (2When the central density approach the critical value (2101099 g cm g cm-3-3), the thermonuclear reaction rate exceeds ), the thermonuclear reaction rate exceeds

the energy loss neutrino the ignition of the energy loss neutrino the ignition of 1212C + C + 1212C takes place in the center due to the compressional heating. TC takes place in the center due to the compressional heating. The release energy further increases the temperature, thus further accelerating thermonuclear reactions. This prhe release energy further increases the temperature, thus further accelerating thermonuclear reactions. This process is slowed down by neutrino cooling and by the convective and conductive heat exchange. Nevertheless, ocess is slowed down by neutrino cooling and by the convective and conductive heat exchange. Nevertheless, the temperature in the WD center rises and reaches the point where the energy release overwhelms the energy the temperature in the WD center rises and reaches the point where the energy release overwhelms the energy outflow. Under the condition of strong electron degeneracy , the thermonuclear reaction after ignition is unstaoutflow. Under the condition of strong electron degeneracy , the thermonuclear reaction after ignition is unstable and explosive. The basic physics mechanism is clear:ble and explosive. The basic physics mechanism is clear:

the the Fermi Fermi pressure of degenerate gas is not sensitive to the change of temperature. At the initial phase, all of epressure of degenerate gas is not sensitive to the change of temperature. At the initial phase, all of energy released by nuclear reaction is used to increase the temperature, while the pressure remains almost unchnergy released by nuclear reaction is used to increase the temperature, while the pressure remains almost unchanged. As a result the strong temperature-dependent nuclear reaction rate increases rapidly (i.e. anged. As a result the strong temperature-dependent nuclear reaction rate increases rapidly (i.e. T T and and i is usually very large) which would further increase the temperature circularly, and the reaction becomes events usually very large) which would further increase the temperature circularly, and the reaction becomes eventually thermal runaway until degeneracy disappears. In the C-O core of electron degeneracy, it is expected that ually thermal runaway until degeneracy disappears. In the C-O core of electron degeneracy, it is expected that nuclear burning is explosive after carbon ignition since the energy released at the burning point can not be taknuclear burning is explosive after carbon ignition since the energy released at the burning point can not be taken away immediately. en away immediately.

when the degenerate C-O is ignited, the burning is explosive. The burning front will propagate into the surrowhen the degenerate C-O is ignited, the burning is explosive. The burning front will propagate into the surrounding fuels by subsonic deflagration or supersonic detonation, which depends on the overpressure produced unding fuels by subsonic deflagration or supersonic detonation, which depends on the overpressure produced by the burning material. by the burning material.

Based on the above two basic propagation modes of thermonuclear buBased on the above two basic propagation modes of thermonuclear burning, different hydrodynamical models have been proposed: rning, different hydrodynamical models have been proposed:

Prompt detonation : Arnett proposed the first hydodynamical model thPrompt detonation : Arnett proposed the first hydodynamical model that the thermonuclear burning start from a detonation wave, which burnat the thermonuclear burning start from a detonation wave, which burns the whole star with a supersonic velocity. The resulting nuclear synths the whole star with a supersonic velocity. The resulting nuclear synthesis is contradict with observations (without intermediate-mass elemenesis is contradict with observations (without intermediate-mass elements) ts)

Pure deflagration: (Nomoto W7 model, 20%-30% of Vc, one of the mPure deflagration: (Nomoto W7 model, 20%-30% of Vc, one of the most successful model, which can give reasonable nuclear synthesis resuost successful model, which can give reasonable nuclear synthesis results, e.g. Large amounts of intermediate mass elements Ca-S-Si, O-Ne-lts, e.g. Large amounts of intermediate mass elements Ca-S-Si, O-Ne-Mg etc. problems: overproduction Mg etc. problems: overproduction of neutron-rich isotopic elements (of neutron-rich isotopic elements (5858Ni, Ni, 5454Fe, Fe, 5454Cr), e.g. W7 model giveCr), e.g. W7 model gives s 5858Ni, Ni, 5454Cr, 4-5 times higher than solar isotopes. Cr, 4-5 times higher than solar isotopes.

The overproduction of Fe-isotopesThe overproduction of Fe-isotopes

Delayed detonationDelayed detonationInspired by the terrestrial combustion experiments that turbulence deflagrations can sometimInspired by the terrestrial combustion experiments that turbulence deflagrations can sometimes be observed to undergo spontaneous transitions to detonations (DDT), it was suggested tes be observed to undergo spontaneous transitions to detonations (DDT), it was suggested that this process may occur in the late phase of a Mhat this process may occur in the late phase of a Mchch-explosion. The delayed detonation mod-explosion. The delayed detonation mod

els assume that the early propagation of the deflagration is as low as a few percent of the souels assume that the early propagation of the deflagration is as low as a few percent of the sound speed required to preexpand the star, followed by a transition to a shock-driven, supersonind speed required to preexpand the star, followed by a transition to a shock-driven, supersonic detonation mode that produces large amounts of high-velocity intermediate-mass elements. c detonation mode that produces large amounts of high-velocity intermediate-mass elements. Many 1D simulations have demonstrated that the delayed detonation models can reproduce Many 1D simulations have demonstrated that the delayed detonation models can reproduce well the features of observed SN Ia spectra and light curves, as well as reasonable nucleosynwell the features of observed SN Ia spectra and light curves, as well as reasonable nucleosynthesis. However, the physical mechanisms by such DDTs occur are unclear.thesis. However, the physical mechanisms by such DDTs occur are unclear.

Pulsational delayed detonationPulsational delayed detonationIf the initial deflagration phase fails to release energy to unbind the star and no DDT takes plIf the initial deflagration phase fails to release energy to unbind the star and no DDT takes place during the expansion, the star undergoes a large amplitude of pulsation by one or more tiace during the expansion, the star undergoes a large amplitude of pulsation by one or more times. The following contraction may trigger a detonation by compression heating, eventually mes. The following contraction may trigger a detonation by compression heating, eventually the WD is completely disrupted. the WD is completely disrupted.

Litte Ni but a subtantially amount of Si and Ca. Weak SN Ia explosion Litte Ni but a subtantially amount of Si and Ca. Weak SN Ia explosion

SubChandrasekhar-mass modelSubChandrasekhar-mass model

C-O WD below the Chandradekhar mass do not reach the critical density and teC-O WD below the Chandradekhar mass do not reach the critical density and temperature for explosive carbon burning by accretion, and therefore need to be imperature for explosive carbon burning by accretion, and therefore need to be ignited by an external trigger. In this model, also known as Edge Lit Detonatiognited by an external trigger. In this model, also known as Edge Lit Detonation(ELD), the first nuclear ignition takes place near the bottom of the accretion hn(ELD), the first nuclear ignition takes place near the bottom of the accretion helium layer of about 0.15-0.20Melium layer of about 0.15-0.20Msunsun. A prompt detonation propagates outwards t. A prompt detonation propagates outwards t

hrough the helium, while an inward non-burning pressure wave compressed the hrough the helium, while an inward non-burning pressure wave compressed the C-O core which ignites off-center and derives a second detonation outwards thrC-O core which ignites off-center and derives a second detonation outwards through the C-O core. Owing to the difference between the nuclear kinematics of ough the C-O core. Owing to the difference between the nuclear kinematics of carbon and helium burning these models have a composition structure that is fucarbon and helium burning these models have a composition structure that is fundamentally different from that of carbon ignitors. ndamentally different from that of carbon ignitors. 44He burns to He burns to 1212C by the slow C by the slow triple alpha process and as soon as triple alpha process and as soon as 1212C is formed it rapidly captures alpha particC is formed it rapidly captures alpha particles to form les to form 5656Ni, so the original helium layer ends up as a high-velocity mixture Ni, so the original helium layer ends up as a high-velocity mixture of of 5656Ni and leftover Ni and leftover 44He. He.

The ELD models are mainly favored by required statistics, since less mass needThe ELD models are mainly favored by required statistics, since less mass needs to be accreted, and WD does not need to be extremely massive. s to be accreted, and WD does not need to be extremely massive.

Turbulent flame surfaceTurbulent flame surface

The WD Near the Chandrasekhar limitThe WD Near the Chandrasekhar limit

When the mass of the WD approaches the When the mass of the WD approaches the MMchch, the pressure of degener, the pressure of degener

ate electrons could not resist the gravity. Any small mass increase resuate electrons could not resist the gravity. Any small mass increase results in a substantial contraction of the star, and this increases the density lts in a substantial contraction of the star, and this increases the density and temperature in the center of C-O core rapidly. The energy balance and temperature in the center of C-O core rapidly. The energy balance near the center is determined by the neutrino losses and the compressinear the center is determined by the neutrino losses and the compressional heating. onal heating.

The evolution of entropy and temperature near the core of the WDs mThe evolution of entropy and temperature near the core of the WDs may be affected by the convective URCA process (a convectively driveay be affected by the convective URCA process (a convectively driven electron capture-beta decay cycle leading to neutrino-antineutrino lon electron capture-beta decay cycle leading to neutrino-antineutrino losses). sses).

Progenitors of SN IaProgenitors of SN IaWhat are the progenitor systems (or pre-supernovae) of exploding SNe Ia, and how they evolve towards explosion? What are the progenitor systems (or pre-supernovae) of exploding SNe Ia, and how they evolve towards explosion? This is the key problem in stellar evolution and remains unresolved yet. In contrast to SN II from collapsing of This is the key problem in stellar evolution and remains unresolved yet. In contrast to SN II from collapsing of massive stars for which in two cases the progenitor Stars were identified and some of its properties could be inferred massive stars for which in two cases the progenitor Stars were identified and some of its properties could be inferred directly from observations before explosion, e.g. SN 1987A in LMC and SN 1993J in M81, attempting to identify directly from observations before explosion, e.g. SN 1987A in LMC and SN 1993J in M81, attempting to identify the progenitors of Sne Ia is a difficult task since they are most likely faint compact dwarf stars. Therefore we can but the progenitors of Sne Ia is a difficult task since they are most likely faint compact dwarf stars. Therefore we can but surmise their progenitors and give the potential candidates by indirect means, namely, matching some parameters surmise their progenitors and give the potential candidates by indirect means, namely, matching some parameters indirectly derived from the explosion to the observations. indirectly derived from the explosion to the observations.

Two evolutionary scenarios have been proposed, including:Two evolutionary scenarios have been proposed, including: A single degenerate (SD) scenario, i.e., accretion of hydrogen-rich matter via mA single degenerate (SD) scenario, i.e., accretion of hydrogen-rich matter via m

ass transfer from binary companion (Nomoto 1982). The strong wind from accreass transfer from binary companion (Nomoto 1982). The strong wind from accreting WD plays a key role, which yields important age and metallicity effects on tting WD plays a key role, which yields important age and metallicity effects on the evolution. he evolution.

A double degenerate (DD) scenario, i.e., merging of double C-O WDs with a coA double degenerate (DD) scenario, i.e., merging of double C-O WDs with a combined mass exceeding the Chandrasekhar mass limit mbined mass exceeding the Chandrasekhar mass limit MMchch (Iben & Tutukov 198 (Iben & Tutukov 198

4; Webbink 1984) 4; Webbink 1984)

Two evolutionary channels for SD modelTwo evolutionary channels for SD model

WD+RG (symbiotic system)WD+RG (symbiotic system)WD+MS system(super-soft system)WD+MS system(super-soft system)

SD progenitors are theoretically favored even though it is very difficult for SD progenitors are theoretically favored even though it is very difficult for hydrogen-accreting WDs to reach the Chandrasekhar limit. They consist of a hydrogen-accreting WDs to reach the Chandrasekhar limit. They consist of a low-mass WD accreting material (H or He) from the companion star until low-mass WD accreting material (H or He) from the companion star until

either it reaches either it reaches MMchch oror a a layerlayer of helium has formed on top of C-O core that of helium has formed on top of C-O core that

can ignite and possibly drive a burning front into carbon and oxygen fuels. can ignite and possibly drive a burning front into carbon and oxygen fuels. Critical accretion rate(steady hydrogen burning):Critical accretion rate(steady hydrogen burning):

MM⊙⊙/yr/yr

If the accretion rate exceeds the critical value, it would lead to form an If the accretion rate exceeds the critical value, it would lead to form an extended H-rich common envelope around the WD since the material accreted extended H-rich common envelope around the WD since the material accreted is larger than that consumed. If the accretion rate is low, undergo repeated is larger than that consumed. If the accretion rate is low, undergo repeated nova outburst. The mass of WD will not grow at all. At a moderate accretion nova outburst. The mass of WD will not grow at all. At a moderate accretion rate, helium flash and give rise to sub-Ch explosions. rate, helium flash and give rise to sub-Ch explosions.

60.75 10 ( 0.40)WDb

MM M

M

． ．

The main problem with this scenarioThe main problem with this scenario

At SN Ia explosion, ejecta would collide with the CSM, At SN Ia explosion, ejecta would collide with the CSM,

which produce shock waves propagating both outward and which produce shock waves propagating both outward and

inward. At the shock front, particle accelerations take place to inward. At the shock front, particle accelerations take place to

cause radio emissions. Hot plasmas in the shocked materials cause radio emissions. Hot plasmas in the shocked materials

emit X-rays. The CSM ahead of the shock is ionized by X-emit X-rays. The CSM ahead of the shock is ionized by X-

rays and produce recombination Ha emissions, which is rays and produce recombination Ha emissions, which is

inconsistent with SN Ia spectral observations.inconsistent with SN Ia spectral observations.

Accretion problem?Accretion problem?

The first case for the detection of Ha emission in SN Ia (2002The first case for the detection of Ha emission in SN Ia (2002ic by Hamuy et al)ic by Hamuy et al)

Spectroscopic evolution of SN 2002ic. a, This sequence shows five spectra of SN 2002ic (in AB magnitudes) obtained between 2002 Nov. 29 and

2003 Feb. 1 UT with the Las Campanas Observatory Baade 6.5-m and du Pont 2.5-m telescopes, and the Steward Observatory Bok 2.3-m telescope. Arbitrary offsets have been added to the spectra for clarity. The spectra are +6, +10, +34,+47, and +70 days from estimated maximum light. We attempted to remove the 8 two most prominent telluric lines (indicated with the circled plus signs symbols), but some residuals are evident. The top spectrum shows the Si II λ6355 feature that defines the Ia class, as well as prominent Fe III absorption features at 4200 and 4900 Å. The absence of the He/Na feature at 5900 Å in the spectral evolution rules out a type Ib/c classification. b, A comparison between the 29 Nov 2002 (+6 days) observation of SN 2002ic and the spectrum of the type Ia SN 1991T obtained at an epoch of +4 days, shows that both spectra are quite similar, except that the features in SN 2002ic are all diluted in strength.

Light curves of SN 2002icLight curves of SN 2002ic

Spectroscopic comparison between SN 2002ic and SN 1997cy.spectrum of SN 2002ic taken on Jan. 9 (~47 days after maximum light, which corresponds to ~67 days after explosion for an assumed time of 20 days between explosion and peak brightness) compared to that of the type IIn SN1997cy taken 71 days after explosion13, which is assumed to coincide with thedetection of GRB 97051412. The striking similarity between these two objects suggests that some SNe IIn are the result of thermonuclear explosions of whitedwarfs surrounded by a dense CSM instead of core collapse in massive stars.

Why was hydrogen not detected before?Why was hydrogen not detected before? One of the key questions posed by the recent observations oOne of the key questions posed by the recent observations o

f Hamuy et al. (2003): why hydrogen has been detected only f Hamuy et al. (2003): why hydrogen has been detected only in case of SN 2002ic since there exist about 100 spectra of Sin case of SN 2002ic since there exist about 100 spectra of Sne Ia? ne Ia?

The main problem of this scenario is that one would expect tThe main problem of this scenario is that one would expect to observe o observe a range a range of of Ha lines in Sne Ia, depending on the aHa lines in Sne Ia, depending on the amount of circunstellar material (in turn, determined primarilmount of circunstellar material (in turn, determined primarily by the mass of the AGB star), rather than detecting a relatiy by the mass of the AGB star), rather than detecting a relatively strong line in only one case (it is also hard to believe thavely strong line in only one case (it is also hard to believe that this is the first progenitor system containing an AGB star). t this is the first progenitor system containing an AGB star).

Instead, it might be proposed that the total absence of Ha liInstead, it might be proposed that the total absence of Ha lines in all pre-SN2002ic Sne Ia observed to date argues that nes in all pre-SN2002ic Sne Ia observed to date argues that SN 2002ic represents rather rare cirmustances, not a WD aSN 2002ic represents rather rare cirmustances, not a WD accreting from the wind of an AGB star.ccreting from the wind of an AGB star.

Merging of two WDsMerging of two WDs

There is no question that binary WD systems are an There is no question that binary WD systems are an expected outcome of binary star evolution. Double WD expected outcome of binary star evolution. Double WD systems having short enough periods are expected to systems having short enough periods are expected to merge as a result of angular momentum losses via merge as a result of angular momentum losses via gravitational wave radiation (GWR) in a time:gravitational wave radiation (GWR) in a time:

RR

RR

RRRR

ff

GWR MM

MMP

MMMM

Ayrt

21

3/121

3/87

2121

48)(108

)(

105.1)(

Evolutionary scenarios for two merging WDsEvolutionary scenarios for two merging WDs

Problems with DD modelProblems with DD model

Probabilities of realizationProbabilities of realization

Some progress has been made in the search of DD binary systems. For example, Saffer etal. (1998) found 18 in 153 Some progress has been made in the search of DD binary systems. For example, Saffer etal. (1998) found 18 in 153

field WDs and subdwarf B stars. Based on N-body simulations, Shara & Hurley (2002) find a remarkably enhanced field WDs and subdwarf B stars. Based on N-body simulations, Shara & Hurley (2002) find a remarkably enhanced production rate(~15 times) in star clusters of very short period, massive DD systems due to dynamical interactions. production rate(~15 times) in star clusters of very short period, massive DD systems due to dynamical interactions.

If the enhancement mechanism works, the frequency number would not be a problem.If the enhancement mechanism works, the frequency number would not be a problem. Accretion-induced collapse ?Accretion-induced collapse ?

Compressional heating effect will trigger C-ignition off center of C-O core (makes c-o core become mixture of O-NCompressional heating effect will trigger C-ignition off center of C-O core (makes c-o core become mixture of O-N

e-Mg). Electrons has been captured by e-Mg). Electrons has been captured by 2424Mg, decreasing the electron pressureMg, decreasing the electron pressure. .

The End The End

Thanks for listening!Thanks for listening!