Explosive Nucleosynthesis Based on the Accreted Envelope Has Been Already Built Up, Are 1D Presupernova Evolution

arXiv:astro-ph/0103418v1 26 Mar 2001 rnsa.Tpclvlcte fsproa ejecta supernovae of velocities neu- 10 a Typical some or are hole black star. a either tron remnant as leave which o ftewiedafi ece.Tpclveloc- 10 Typical some between reached. are is ejecta dwarf novae on of white mass ities accreted the critical of the top time every explosion expected phe- an is since recurrent a supernovae), are to in (contrary explosions dwarf nomena These white the a system. of of explosion binary shells the accreted of H-rich result external the are novae Classical jce asssm M some masses ejected I bc r xldn asv tr (M stars massive exploding type of are explo- (supernovae Ib/c) the supernovae sys- after II, collapse binary remnant Core a close leave in sion. not dwarfs do which type white tems, of exploding are supernovae Ia, or supernovae, Thermonuclear novae. and classical both (core-collapse) supernovae, gravitational Galaxy: and emis- the thermonuclear the in for gamma-rays responsible of are sion explosion Two of astronomy. types nucle- explosive main gamma-ray of for sites the relevant review osynthesis will I paper this In su- abun- novae; nucleosynthesis, observations; dances. reactions, rays: nuclear gamma pernovae; words: Key un- also remaining will reviewed. modelling still be supernova the and nova gamma-ray as in the well certainties on as ejecta features, the the emission of of influence properties The dynamic individual explosions. from supernova emphasis emission and nova special gamma-ray a ensuing these with the in presented, on nucleosynthesis be of will explosive review scenarios A of supernovae. results and events, recent novae explosive classical as during such as- synthesized gamma-ray are for trophysics relevant nuclei radioactive Many 4 ms km .INTRODUCTION 1. − ABSTRACT 1 nrisivle 10 involved energies , ⊙ . dfiiNxs /rnCpta -,004Breoa Spain Barcelona, 08034 Capità, 2-4, C/Gran Nexus, Edifici 1 ntttdEtdsEpcasd aauy,IEEC, Catalunya, de Espacials d’Estudis Institut XLSV NUCLEOSYNTHESIS EXPLOSIVE 2 nttt eCeca e sai,CSIC Espacio, del Ciencias de Instituto ∼ > 51 2 n 10 and r and erg 10M .Hernanz M. ⊙ ), 3 aete eetbei niiulobjects. individual 57 in detectable them make and ftesbCadaehrtp poie htthey that (provided type sub-Chandrasekhar supernovae the thermonuclear in of it synthesized but be supernovae, also core-collapse can in produced mainly o eetrve) hr-ie stps uhas such isotopes, 1998, Short-lived Timmes, and review). Diehl recent 56 (see a lifetime depend- for distinguished, their be on can ing isotopes of types Two in e rol n ent19 n entand reviews). Meynet recent and for 1997 1999 Meynet Arnould, evolu- by and hydrostatic Arnould nuclei their see radioactive during tion; eject winds, stellar Wolf- can strong the contri- (which or the stars proceedings) (see Rayet these stars (non- Mowlavi, AGB by other the bution in like isotopes discuss sites, radioactive won’t explosive) of I synthesis elements. the other) Galaxy (and the radioactive of quan- enrichment in this which final since the know ejected, determine to be will finally tity well the will as star in the crucial that of part is from it evolution phases; star explosive massive dis- the to of case pre-explosive important stage the the during is nucleosynthesis In the it tinguish scenarios. supernovae, these core-collapse radioac- in of these of proceeds synthesis nuclei the be- how tive 3 discuss and will 2 I sections low, In novae. of classical in case synthesized the at in exclusively, not least (although explosions supernova in ( → → 7 r umrzdi al .Tretpso decay of types ( Three captures electron 1. occur: table can chains in summarized supernovae, explo- in are or novae during in either synthesized nucleosynthesis, sive isotopes radioactive The 57 ewe 10 between ms km .SNHSSO AIATV ISOTOPES RADIOACTIVE OF SYNTHESIS 2. 56 Li), iaepoue naltpso supernovae; of types all in produced are Ni Ni, Ni 1,2 Ni, 60 26 60 − → β 57 Co 1 gand Mg o aelftmssoteog setbe1 to 1) table (see enough short lifetimes have Co, 56 + nrisivle 10 involved energies , i(n hi daughters their (and Ni Co, → eas( decays 57 − NSPROAEXPLOSIONS SUPERNOVA IN Co 3 57 60 n 10 and Ni, i.Tefis i stpsi h table the in isotopes six first The Ni). 22 → Na 56 44 Co 57 Ti, 26 → − Fe, l,weestels w are two last the whereas Al), 5 → 26 M 22 44 land Al ⊙ e and Ne) 56 Ti 45 . Fe, r n jce masses ejected and erg → 56 44 oand Co 60 Sc e r produced are Fe) 44 β − cand Sc 56 → Ni eas( decays 57 44 → Co), Ca, 56 7 iand Ni 44 Be 56 iis Ti 60 26 44 Co, Al Fe Ti → 1 2 exist; see discussion of SNeIa types below). 60Co Sandie et al. 1988, Cook et al. 1988, Rester et al. is produced directly and from 60Fe decay, with 60Fe 1989). One surprising fact was the appearence of belonging to the long-lived isotopes group. these lines only 200 days after the explosion (Matz et al., 1988), much earlier than expected. This has Long-lived radioactive isotopes, such as 26Al and been interpreted as a sign of some early extra mixing 60Fe, have lifetimes long enough to make them unde- of 56Co in order to transport this isotope into regions tectable in individual sources, because the nuclei can of low gamma-ray optical depth (see, e.g., Pinto and be quite far away from their source and mixed with Woosley, 1988, Leising 1988, Bussard, Burrows and those coming from other explosions (since the life- The, 1989, Leising and Share, 1990). The line pro- time is longer than the typical period between two files observed with GRIS (Teegarden et al. 1989, succesive explosions in the Galaxy). For these iso- Tueller et al. 1990) have also put constraints on theo- topes, only the accumulated emission in the Galaxy retical models of supernova explosions, since spheric- can be observed and used as diagnostic of models and ity and homogeneity of the ejecta were incompatible of the Galactic distribution of the sources. The same with the observed fluxes and widths of the 56Co lines. classification scheme applies to isotopes synthesized in novae; in this case, 7Be belongs to the short-lived Another crucial gamma-ray observation of short- group, whereas 22Na belongs to both of them (see lived isotopes in SN 1987A was the detection of section 3 below). gamma-ray radiation from 57Co decay (between 50 and 136 keV), with OSSE on the CGRO (Kurfess et A very recent and interesting compilation of papers al. 1992). The deduced 57Co content (for models about astronomy with radioactivities can be found with low gamma-ray optical depth, see Kurfess et al. in Diehl and Hartmann (1999). 1992) was such that the original ratio 57Ni/56Ni produced in the explosion should be about 1.5 times the solar 57Fe/56Fe ratio . Observations up to now show 56 2.1. Observational clues the change of slope related to the sequence of Co- 57Co decays (see figure 3 in Timmes et al. 1996). Future UVOIR observations would possibly be able Gamma-ray astronomy provides an unique opportu- to show the light curve powering from 44Ti. The SPI nity to detect radioactive isotopes in individual ob- instrument onboard INTEGRAL has some possibili- jects, giving a proof of ongoing nucleosynthesis in ties to detect the gamma-ray emission from this 44Ti, them. In the case of supernovae, another tool for the which would provide a unique proof of the nucleosyn- determination of the amount of radioactive nuclei in thesis in core collapse supernovae and an important the ejecta is the bolometric light curve (UVOIR, from link between the UVOIR and the gamma-ray obser- ultraviolet, optical and infrared). In addition to the vations. Coming back to 57Co it was first thought well known fact that 56Co (daughter of 56Ni) powers that the SN 1987A 57Co content deduced from OSSE the early evolution of the light curve (56Ni mass can observations was not enough to power the available be determined from luminosity at maximum), 57Co is bolometric light curve, since 5 times solar 57Fe/56Fe responsible for powering the light curve from around ratio was required (Suntzeff et al. 1992) and alterna- day 1000 after maximum. Later on 44Ti will pro- tive mechanisms to power the bolometric light curve vide a floor to the bolometric light curve and 22Na were suggested (Clayton et al. 1992). However, more and 60Co could also play a minor role, depending on recent observations seem to require a smaller amount the supernova type and the specific yields of these of 57Co-decay to power the light curve, in agreement radioactivities (see, e.g., Woosley, Pinto and Hart- with the 57Fe/56Fe ratio deduced from OSSE obser- mann, 1989, Timmes et al. 1996, Diehl and Timmes vations (see figure 9 in Diehl and Timmes 1998). 1998). The excitement induced by the above mentioned These two types of observational approaches to the gamma-ray observations (together with many other radioactive content in the ejecta had been possible observations at other wavelength ranges) has led the for only one object so far: the supernova 1987A, theorists to suggest different possibilities for mixing which exploded 13 years ago in the LMC, only 55 kpc both during the explosion and the ejection phases away from us. This was a type II supernova, which (to quote only a few of the early works, see e.g., Ar- is not the most favorable case to look for gamma- nett, Fryxell and Müller 1989, Benz and Thielemann ray emission, since its is much more opaque and has 1990, Fryxell, Arnett and Müller 1991, Herant and a smaller content of the most relevant radioactivites Benz 1992, and also the general reviews of SN 1987A than SNIa (see below); however, its very short dis- from Arnett et al. 1989 and McCray 1993 and refer- tance allowed for detection of its gamma-ray emission ences therein). and also for a follow-up until very late times of its light curve (through photometry in the UVBRIJHK Another detection (tentative) of gamma-ray emission bands). from a supernova, with COMPTEL on CGRO, was that of SN 1991T (which was an overluminous SNIa), Gamma-ray lines from 56Co decay, at 847 and 1238 in NGC 4527 at around 17 Mpc distance. In that keV, were detected in SN 1987A with the GRS in- case, a marginal detection of the 847 keV line was strument of the SMM satellite (Matz et al. 1988) reported (Morris et al. 1995, 1997), leading to a and confirmed by several ballon-borne instruments prediction of a quite large 56Ni mass, implying that (i.e., Teegarden et al. 1989, Mahoney et al. 1988, all the white dwarf mass should have been inciner- 3 ated to 56Ni (in contradiction with current theoreti- 2.2. Thermonuclear supernovae (SNeIa) cal models). More recently, upper limits to the fluxes of the 847 and 1238 keV lines from 57Co-decay in the type Ia supernova 1998bu, in NGC 3368 at around The defining characteristic of SNeIa is the lack of 8 Mpc distance, have been deduced from COMP- hydrogen in their spectra, as well as the presence of TEL observations (Georgii et al. 2000). Although no a P Cygni feature related to SiII, λ6335, at max- detection has been obtained, these limits are restric- imum light (Wheeler and Harkness 1990); in gen- tive enough to constrain some of the available models eral, intermediate-mass elements (O, Mg, Si, S, Ar, Ca) appear in the spectrum near maximum light of SNeIa nucleosynthesis (like a sub-Chandrasekhar −1 mass model from Nomoto et al. 1997). with high velocities (8000-30000 km s ). SNeIa are quite homogeneous from the observational point of view (i.e, ∼90% of all SNeIa have similar spec- There is another important observation of gamma- tra, light curves and peak absolute magnitudes), al- ray lines related to short and medium-lived radioac- though some differences exist (i.e., subluminous ex- tivities in supernovae: the discovery of 44Ti emission plosions, like SN1991bg and SN1992K, and overlu- at 1157 keV in the Cas A supernova remnant (Iyudin minous ones, like SN1991T). SNeIa appear in both et al. 1994; see reviews from Diehl and Timmes 1998 elliptical and spiral galaxies and, therefore, their pro- and Knödlseder, these proceedings). Again the ob- genitors should be long-lived. These facts all to- servations in gamma-rays are in some way puzzling, gether suggest that the thermonuclear disruption of because the amount of 44Ti deduced from observa- mass-accreting carbon-oxygen (CO) white dwarfs is tions implies a 56Ni content (according to theoretical responsible for these explosions. Already in the six- models of supernovae nucleosynthesis) which should ties, Hoyle and Fowler (1960) suggested that ther- have originated a very bright supernova, in contrast monuclear burning in an electron-degenerate stellar with the absence of historical records (Timmes et al. core might be responsible for type I supernova (there 1996). Observations in gamma-rays push forward was no subclassification at the epoch) explosions, the theoretical models, in order to balance all the with the explosion energy coming from the ther- available possibilities and to consider new ones. monucler burning of CO into higher mass elements (see also, e.g., the pioneering works by Arnett 1969, A different kind of information is obtained from the Hansen and Wheeler 1969). It was also suggested at 26 the epoch that the early supernova luminosity might observations of long-lived radioactivities ( Al and 56 60Fe). In this case, what is seen is not the ongoing have its origin on the radioactive decay of Ni (Col- nucleosynthesis in a particular object, but the inte- gate and McKee 1969), which was already known to grated nucleosynthesis in the Galaxy. Up to now, be a product of supernova nucleosynthesis, and that this has been possible for the 1809 keV 26Al emis- gamma-ray lines should be emitted from those ex- sion. The 26Al map obtained with the COMPTEL plosions (Clayton, Colgate and Fishman, 1969). But instrument onboard the Compton Gamma-Ray Ob- the particular scenario where the explosion occurs servatory CGRO (Diehl et al. 1995, 1997, Oberlack (see, e.g., Livio 1999) and the physics of the flame et al. 1996, Knödlseder 1997, 1999, Plüschke et al. itself (see, e.g., Hillebrandt and Niemeyer 2000) are these proceedings) has posed interesting questions far from being understood. about the origin of the galactic 26Al (see, e.g., review from Prantzos and Diehl, 1996). It provides a direct Two types of progenitors have been suggested so and unique insight on the integrated nucleosynthe- far, concerning the mass of the exploding CO white sis during the last 106 years. Some regions of en- dwarf: Chandrasekhar and sub-Chandrasekhar mass hanced emission have been discovered (Cygnus, Ca- models. In the Chandrasekhar mass models, a CO rina, Vela), indicating the presumable link between white dwarf explodes when reaching that mass, with 26Al emission and massive star formation, as well as central carbon ignition propagating outwards being the relationship with spiral structure of the Galaxy responsible for the explosion. The main problems re- (Diehl et al. 1996, Knödleseder et al. 1996a,b, Diehl lated to this model are the uncertainties concerning et al. 1999, Knödlseder, these proceedings). For burning propagation (deflagration, detonation, de- 60Fe, a similar map should be observed by INTE- layed detonation, see below), but also the scenario GRAL, because the sources of this isotope are the is unclear. Either a double degenerate (merging of same as those of 26Al, being the yields smaller by two CO white dwarfs) or a single degenerate scenario some factor (Timmes et al. 1995, Diehl et al., 1997). is possible. In all cases, the growth to the Chan- drasekhar mass is problematic, because both mass loss (through nova episodes, for instance) and ac- It is worth mentioning that the integrated nucleosyn- cretion induced collapse (if the initial mass is high thesis of 44Ti and 22Na may also be seen, if instru- enough and/or the white dwarf is made of oxygen ments are sensitive enough. The future 44Ti and and neon) should be avoided (see, e.g., Canal, Is- 22Na maps will provide a precious information about ern and Labay 1990, 1992, Canal et al. 1990, Is- their sources, i.e., supernovae (mainly core collapse ern, Canal and Labay 1991, Nomoto and Kondo ones) and novae, respectively. 1991, Bravo and Garc´ıa-Senz 1999). In addition, for the double degenerate scenario there is a problem of statistics: there are not enough double white dwarf systems with sufficiently short period and total mass in excess of the Chandrasekhar mass able to explode 4 in less than the Hubble time and to explain the galac- To overcome this problem, delayed detonations were tic SNeIa rate. In fact there wasn’t any observed suggested (Khokhlov 1991a). There are two situ- system fulfilling these conditions until the very re- ations in which a deflagration to detonation tran- cent discovery of KPD 1939+2752 (Maxted, Marsh sition (DDT) could occur in supernovae (see, e.g., and North, 2000), which is the first SNIa progenitor Khokhlov, Oran and Wheeler, 1997): DDT could candidate observed. occur directly or as a result of a previous expansion. For instance, in the pulsation delayed detonation, In the sub-Chandrasekhar mass models, a CO white a first slow deflagration is quenched because of the dwarf of low-mass (0.6-0.8 M⊙) accretes helium expansion of the white dwarf, which subsequently (∆MHe ∼ 0.1−0.2 M⊙), reaching a final mass smaller pulses and recontracts, causing a detonation upon than the Chandrasekhar mass. Provided the accre- recollapse (Khokhlov 1991b). The propagation of −8 −1 tion rate is moderate (around 10 M⊙ yr ), there the detonation wave through the pre-expanded star is He ignition on the top of the CO core. This ig- produces the required intermediate mass elements in nition causes an outward propagating He-detonation the outer layers at densities lower than ∼ 107g cm−3 wave (basically transforming He into Ni at high ve- (which are not synthesized in detonations at larger locity) and an inward propagating pressure wave. densities). In these models, the problem of over- The last one finally provoques a carbon ignition production of highly-neutronized nuclei is alleviated (central or off-center), which leads to an outward but not solved (Khokhlov 1991a, b). Models of de- carbon-detonation incinerating all the white dwarf, layed detonations in 2D, both of the first deflagration and synthesizing intermediate-mass elements, in ad- phase and of the subsequent detonation phase, have dition to Ni (see, e.g., Livne 1990, Livne and Glas- been performed by Arnett and Livne (1994a, b); they ner 1991, Woosley and Weaver 1994). Therefore, show that the first slow deflagration is insufficient to in this model (called “indirect double detonation”, unbind the star, that a pulsation of large amplitude IDD, or “edge lit detonation”, ELD) there is an is generated and that reignition occurs after the first outer layer of high-velocity Ni and He above the contraction phase. intermediate-mass elements, which does not exist in the Chandrasekhar-mass models. Sub-Chandrasekar In summary, there is a general consensus about the mass models are not considered as good SNeIa pro- fact that, in order to explain spectroscopic observa- genitors nowadays, because of both observational tions, burning should proceed subsonically (deflagra- and theoretical problems; observational: the high ve- tion) in the inner core (where densities are large, i.e., locity Ni above intermediate mass elements is not ρ > 108 g cm−3), whereas burning becomes super- seen in the spectra; theoretical: the He-driven car- sonic (detonation) in the outer lower density zones bon detonation is very model dependent (see for in- (see examples of models in Bravo et al. 1993, Höflich stance the 3D models from Garc´ıa-Senz, Bravo and and Khokhlov 1996, Bravo et al. 1996, Woosley Woosley 1999). But it is still a possibility that sub- 1997). But the way in which the deflagration to deto- Chandrasekar mass models explain some sublumi- nation transition (DDT) occurs is not yet clear, (see, nous SNeIa, like SN1991bg (see, e.g., Ruiz-Lapuente, e.g., discussions in recent papers by Niemeyer and Canal and Burkert 1997). Woosley 1997, Niemeyer 1999, Lisewski, Hillebrandt and Woosley 2000, and in the review by Hillebrandt In summary, the bulk of normal SNeIa are as- and Niemeyer 2000). There is also ample debate sumed to be exploding Chandrasekhar-mass CO about the way in which the initial burning occurs: white dwarfs, but there is still room for the sub- flame instabilities, flame-turbulence interactions (see Chandrasekhar mass models to explain some peculiar review about turbulence and thermonuclear burning objects. Therefore, whether SNeIa come from sin- by Hillebrandt and Niemeyer 1997, and references gle or double-degenerate scenarios and whether they therein). come from carbon or helium plus carbon ignition are not closed issues (see, e.g., the recent paper from All 1D models (which were the unique ones avail- Branch 2000). able up to the nineties and still are the only ones to include complete nucleosynthesis) rely on prescrip- The main problems still remaining on the modeling tions based on some parametrization of the flame of SNeIa affect the ignition process and the flame speed and, in the case of delayed detonations, of propagation. Different possibilities exist: deflagra- the deflagration-detonation transition -DDT- densi- tion (subsonic flame speed), detonation (supersonic) ties. Different groups work in models of thermonu- and a combination of both (delayed detonation). A clear SNIa and their nucleosynthesis, including the detonation with densities larger than ∼ 107 g cm−3 radioactivities. It is out of the scope of this pa- is not a viable mechanism, since all the star would be per to mention even a small fraction of them, but incinerated to Ni, without synthesis of intermediate- a small sample can be useful to show the main re- mass elements. On the contrary, if the density is sults and the main caveats still remaining (see the lower, intermediate-mass elements are synthesized, recent books Thermonuclear Supernovae, edited by in agreement with the observations. Concerning de- Ruiz-Lapuente, Canal and Isern, 1997, and Type flagrations, they produce nucleosynthesis at veloc- Ia Supernovae: Theory and Cosmology, edited by ities in general agreement with the observed spec- Niemeyer and Truran, 2000). Nomoto and cowork- tra, but some neutronized isotopes (such as 54Fe, ers have computed detailed nucleosynthesis in carbon 54Cr and 58Ni) are overproduced in amounts incom- deflagration supernovae (Nomoto, Thielemann and patible with the chemical evolution of the Galaxy. Yokoi, 1984, Thielemann, Nomoto and Yokoi, 1986), 5 and also in other types of explosive carbon burn- 2.3. Core-collapse supernovae ing (such as delayed detonations, with parametrized ignition densities and deflagration-detonation transition -DDT- densities, see Iwamoto et al. 1999). All supernova types except type Ia’s (i.e., type II, Ib/c) are explained by the explosion of massive stars. The yields of radioactive isotopes are mainly af- > fected by the DDT density (i.e., synthesized mass Stars with initial masses (M∼ 10 M⊙) don’t end 56 57 their lives as white dwarfs. Succesive phases of ther- of Ni ranges from 0.55 to 0.77 M⊙, and Ni from −3 −2 monuclear burning (C, Ne, O, Si) give as a result a 9.6x10 to 1.98x10 M⊙, in Iwamoto et al.’s models). These yields are larger than those from core star with an “onion-skin” structure, where a central collapse supernovae (see below) and distributed in iron core is surrounded by shells made of elements less opaque zones, since there isn’t much mass above of progressively lower atomic mass. The chemical them. This makes type Ia supernovae better tar- composition along the star is the following (see, e.g., gets for INTEGRAL than SNeII (but see section figure 10.8, corresponding to a 25 M⊙ star, in Ar- 2.1 for observational results). In the context of nett 1996): Fe core, Si-burning zone (made mainly gamma-ray astronomy, it is important to stress that of elements from Si to Ni, without O), O-burning sub-Chandrasekhar mass models synthesize larger zone (O, Si-Ca), Ne-burning zone (Ne, Mg and O, amounts of 44Ti than Chandrasekhar mass ones (see, no C), C-burning zone (C and O, Ne and Mg), ra- e.g., Woosley and Weaver, 1994). diative He-burning zone (He, C and O), convective He zone, inert part of old He core (interior to H- burning shell), material above the H-burning shell Gamma-ray spectra of SNeIa for the different models (plus some inert zones associated with the Si, O, Ne provide important signatures of the explosion mecha- and C-burning zones and just outside them). Once nism, although unfortunately there isn’t much obser- the Fe-core reaches the Chandrasekhar mass, it be- vational data to compare with (see previous section). comes unstable and collapses to form a neutron star. Prospects for SNeIa explosion mechanism identifica- The gravitational energy released (∼ 1054 erg) dur- tion with gamma-rays have been analyzed recently ing core collapse is responsible for the ensuing su- by Gómez-Gomar et al. (1998a), with a special em- pernova explosion, but it is not yet completely un- phasis on detectability with the instruments that will derstood how the conversion of this potential energy be onboard INTEGRAL (see also Burrows and The into kinetic energy proceeds (only 0.1% of the avail- 1990, Höflich, Khokhlov and Müller 1994, Kumagai able potential energy is needed). and Nomoto 1997, Höflich, Wheeler and Khokhlov 1998). Baade and Zwicky (1934) were the first to suggest 56 that the gravitational energy released during the for- Lines from Ni-decay (158, 750, 812 keV) are promi- mation of a neutron star could produce a supernova nent during the first days after the explosion, but explosion. Colgate and White (1966) built a super- they disappear very fast, because of the short 56Ni- 56 57 nova model, considering that the transfer of energy lifetime. Lines from Co (847, 1238 keV) and Co takes place by the emission and deposition of neutri- (122, 136 keV) appear later and have longer dura- nos; Wilson (1971) showed that the electron capture tions. The most intense lines are those at 847, 1238, neutrino burst was not strong enough to eject ma- 812 and 158 keV, in addition to the annihilation line terial. The Weinberg-Salam model of electroweak at 511 keV. The strongest line is always the 847 keV interactions opened new possibilities of neutrino in- one (detectable up to 11-16 Mpc with SPI on INTE- 56 teractions with matter (neutral currents). In 1974, GRAL), whereas the 158 keV line (from Ni-decay) Freedman noticed the importance of neutral currents is the most interesting to discriminate between mod- in the physics of core collapse supernovae; as a re- els. The 158 keV line is narrower and, therefore, sult of the increased cross section of core material detectable at longer distances with SPI, than an- 56 to neutrinos, these particles are trapped during the other Ni-line (at 812 keV), despite being fainter. collapse. It was shown by Bethe et al. in 1979 that It is almost undetectable in pure deflagration mod- one of the consequences of neutrino trapping is that els, whereas it is even stronger than the 1238 line the entropy of the core changes little during collapse in detonation models. Another interesting signature (it remains low), leaving the collapse continue up to of the models is the ratio between the 847 keV and nuclear densities. Further compression is prevented the 158 keV line fluxes (200 days after maximum by the repulsive component of the strong interac- and at maximum, respectively), because it provides tion (stiffness of nuclear matter), leading to the core information about the ratio between total 56Ni in 56 bounce. A shock wave is generated at its boundary the ejecta and Ni in the external layers: the late and propagates outwards. But it has been shown emission at 847 keV comes from 56Co-decay (com- 56 56 that the energy of this shock is mainly invested in the ing from Ni-decay), while only the Ni present in photodisintegration of heavy nuclei and in neutrino the outermost shells is responsible for the 158 keV losses; therefore, the shock stalls and the explosion line flux (see Gómez-Gomar et al. 1998a for details). via the so called “prompt mechanism” is unsuccess- Finally, line profiles will also provide important in- ful. In the “delayed mechanism”, there is a revival formation allowing for discrimination between the of the stalled shock because of neutrino heating be- models, for explosions at distances short enough (see hind the shock (Bethe and Wilson 1985). However, again Gómez-Gomar et al. 1998a). the explosion energy does not reach easily the necessary 1051 erg. Further works introduced the effect of convective instabilities, caused by a negative 6

entropy gradient, in order to deliver energy to the range between 13 and 25 M⊙, with initial metallic- shock (see, e.g., Bethe, 1990, Herant, Benz and Col- ities, Z, equal to solar (see Nakamura et al. 1999 gate 1992, Herant et al. 1994, Bethe 1995, Janka for the effect of low Z). Woosley and Weaver (1995) and Müller, 1995, Burrows, Hayes and Fryxell, 1995, studied the range 11-40 M⊙, for Z=0 and Z between −4 to quote only a few of the papers dealing with this 10 and Z⊙. Si, O, Ne and C explosive burning oc- topic). Convection aids the explosion because it in- cur when the shock wave crosses the corresponding creases the efficiency at which neutrino energy is de- zones in the pre-supernova (see above for the descrip- posited (material that rises cools and converts energy tion of its structure). A brief description of the re- from neutrino deposition into kinetic energy, instead sults concerning the synthesis of radioactive isotopes of re-radiating it as neutrinos) and also reduces the follows. energy required to launch the explosion (by reducing 56 57 the pressure at the accretion shock) (see recent re- Ni and Ni are produced when either oxygen or > views by Fryer, 2000, Burrows 2000, and references silicon-rich layers with low neutron excess (Ye ∼ therein). The handling of this process is very model 0.498) are heated to temperatures above 4x109 K dependent: treatement of neutrino transport, multi- (explosive O- and Si-burning). They are produced dimensional aspects. Also the structure of the stellar whether the material ejected is alpha-rich or not, al- core before collapse (i.e., the presupernova model) though 57Ni synthesis is favored in alpha-rich freeze- are important for the final outcome of the explosion. out; this happens when material, initially in nuclear statistical equilibrium (NSE) at relatively low den- Fortunately, nucleosynthesis during core collapse su- sity, is cooled so rapidly that the free alpha particles pernova explosions can be computed without a com- do not have time to merge via the 3α reaction and, plete knowledge of the explosion mechanism itself. therefore, matter cools down in the presence of a As in the case of thermonuclear supernovae, all the large concentration of α-particles, which modify the details of the physics involved in the explosion are final composition (with respect to the normal freeze- not required to have an approximate, but quite good out). 44Ti is also produced during α-rich freeze-out when compared with the observations, idea of which from NSE in the hottest and deepest layers ejected are the main nucleosynthetic yields of core collapse during the explosion. Therefore, the yields of these supernovae. Two steps are needed to compute SNII radioactive isotopes are very sensitive to the mass- (and Ib/c) yields: nucleosynthesis during the massive cut location (Woosley and Hoffman 1991, Hoffman et star evolution (i.e., pre-supernova phase) and explo- al. 1995, Woosley and Weaver 1995, Timmes et al. sive burning when a shock wave crosses the mantle 1996). For example, stars with masses larger than 30 56 57 44 surrounding the collapsing core. M⊙ don’t eject any Ni (nor Ni and Ti) if the kinetic energy (at infinity) is around 1.2x1051 erg. If There are different ways to simulate the explosion this energy is enhanced, the mass-cut is lowered and artificially. One is by means of a “thermal bomb”, some 56Ni (and 57Ni and 44Ti) are ejected. Ejected 56 44 i.e., injecting thermal energy inside the Fe core, in a masses of Ni are around 0.1 M⊙ and those of Ti −5 −4 way such that the ejecta attains the desired kinetic between ∼ 10 and 10 M⊙. Similar results are energy, ∼ 1051erg (see, e.g., Thielemann, Nomoto obtained by Thielemann et al. (1996), except for and Hashimoto, 1996). Another alternative is the the larger amounts of 44Ti, probably because of the injection of momentum, through a piston, inward- different way of simulating the explosion, which pos- moving during the infall previous to the explosion, sibly injects larger entropy in the inner shells and and outward-moving during the explosion, with a favors a larger ejected mass and an enhanced α-rich velocity such that the desired kinetic energy of the freeze-out (see, e.g., Aufderheide, Baron and Thiele- ejecta is obtained (see, e.g., Woosley and Weaver, mann 1991 and Hoffman et al. 1999). 1995). The mass cut between the collapsing core and the ejecta determines the amount of mass ejected 26Al is another important radioactive isotope which (and that of 56Ni and other radioactive isotopes, in is produced in core collapse supernovae (and in other particular). In the “thermal bomb” method, they ad- scenarios) through the 25Mg(p,γ) reaction. 26Al just it taking into account the relationship between yields depend on pre-supernova evolution (H- and supernova progenitor masses and 56Ni masses ejected O-Ne burning shells) and on the explosion. Two fac- deduced from some observations. In the piston ap- tors enhance 26Al production during the explosion: proach, the mass cut is obtained from the choice of explosive burning in O-Ne shells and ν-spallation re- piston position and energy; a mass cut located out- actions on 20Ne, 16O, 23Na, 24Mg, which liberate side the piston is often obtained (for a discussion protons that are captured by 25Mg. It is important of the differences between both models, including an to stress that another important long-lived radioiso- analysis of the influence of the nuclear reaction rates, tope, 60Fe, is coproduced with 26Al in the same re- see Hoffman et al., 1999). In summary, both groups gions within SNII (this isotope is synthesized by neu- have performed calculations of detailed nucleosyn- tron captures on 56,58Fe in the O-Ne burning shell thesis by inducing the core-collapse supernova explo- and in the base of the He-burning shell, both pre- sion on massive stars (previously evolved following all explosively and explosively). Therefore, these nuclei the nucleosynthesis phases). Other groups have per- should have similar spatial distributions in the ejecta formed studies of massive star evolution, but there is (Timmes et al. 1995). The 26Al/60Fe ratio depends no room in this short review to mention all of them. on the mass of the pre supernova: 26Al/60Fe is larger than 1 for M larger than 25 M⊙ and similar to 1 for The masses studied by Thielemann et al. (1996) smaller masses. The typical yields of 26Al are 10−4 7

60 −5 M⊙ and those of Fe 4x10 M⊙. is that these isotopes are transported by convection to the outer layers of the envelope, during the run- The final yields depend on three aspects: presuper- away, where they subsequently decay (τconv < τdecay) nova evolution, explosion energy and details of the and cause the expansion of the envelope and the in- explosion mechanism (see Diehl and Timmes, 1998 crease in visual luminosity. Second, the decay of the and Thielemann 1999 for recent analyses). The main unstable nuclei originates gamma-ray emission, be- issues concerning presupernova evolution are those cause of either direct emission of gamma-ray photons affecting general stellar evolution of low-mass stars, or positrons (for β+-unstable nuclei), which annihi- plus some specific ones relative to massive stars. For late with electrons. The photons emitted (511 keV, instance, the treatement of convection affects nucle- positronium continuum, 478 and 1275 keV, see table osynthesis; in particular, convective burning in the 1) experience Comptonization in the nova expanding O-shell of massive stars. Models with M=20 M⊙ envelope. Therefore, the emission from novae con- have deserved a particular attention for the theo- sists of lines plus a continuum (see Gómez-Gomar et rists, since they are crucial to understand the mix- al. 1998b, Hernanz et al. 1999, Hernanz et al. these ing of radioactive (and other) isotopes, like 56Ni, in proceedings and references therein). The potential supernova ejecta, which has been deduced from the role of classical novae as gamma-ray emitters had observations of SN1987A (see section 2.1 above). 2D been alredy pointed out many years ago (Clayton models of O-burning (Bazan and Arnett 1994) ob- and Hoyle 1974, Clayton 1981, Leising and Clayton tain significant mixing beyond the boundaries de- 1987). fined by mixing-length convection. What they ob- tain are perturbations in density in the oxygen shell The first available hydrodynamic models of nova ex- that are sufficiently large to “seed” hydrodynamic plosions (Starrfield et al. 1978 and Prialnik et al. instabilities, which will mix the “onion-skin” compo- 1978) realized that there was a need of an initial sition of the presupernova (Bazan and Arnett 1998). enrichment in CNO isotopes both to power the ex- This occurs in precisely the region in which 56Ni is plosion and to explain some observed abundances. explosively produced by oxygen burning behind the Two and three dimensional simulations of the ther- explosion shock. This result poses some problems monuclear runaway of a CO white dwarf, valid when to the models of explosive nucleosynthesis based on the accreted envelope has been already built up, are 1D presupernova evolution. Rotation can also have the only available up to now (Glasner et al., 1997, some effect (see, e.g., works by Meynet and Maeder Kercek et al. 1998, 1999). They predict that en- 1997, Heger, Langer and Woosley 2000), as well as richment proceeds too slowly if the accreted gas has mass-loss during the presupernova evolution, in the nearly solar CNO abundances at the onset of the final yields of radioactive elements. Concerning the thermonuclear runaway, and conclude that fast nova energy of the explosion and the details of the explo- outbursts require huge enrichments of C and O. The sion mechanism, one of the main problems is the lo- mechanism which leads to such enhancements must cation of the mass-cut, which determines how much operate prior to the outburst and has not been mod- mass falls back into the collapsing core (and there- eled up to now. Therefore, it is known that some fore whether it will be a neutron star or a black hole) mixing with core material (either CO or ONe) dur- and how much mass is ejected and with wich compo- ing the accretion phase prior to the runaway should sition (the profile of some isotopes is steep around the occur, but this process has not been modeled yet in mass-cut location and, therefore, the yield is affected a self-consistent way. Another approach to the prob- by it). Therefore, the mass-cut determines crucially lem of initial enrichment comes from the multicycle the final yields of radioactive elements, specially for 1D models (of CO novae only, up to now), from Pri- those produced in the inner regions of the super- alnik and Kovetz (Prialnik and Kovetz 1995, Kovetz nova (56Ni, 57Ni and 44Ti). As mentioned above, the and Prialnik, 1997); diffusion is responsible for the explosion energy, and the corresponding entropy in enrichment, which becomes larger after a number of the inner shells, crucially affect the degree of α-rich flashes. However, the large metallicities and neon freeze-out and, therefore, the yields of the Fe-group abundances observed in some novae are not well re- nuclei and of 44Ti. produced. Another approach is based on 1D models with an initial (parametrized) enrichment, such that the general properties of observed novae (mainly abundances) are well modeled (see for instance Star- 3. CLASSICAL NOVAE rfield et al. 1998, Joséand Hernanz 1998).

Classical novae synthesize many radioactive isotopes, Classical novae explosions are the most common ex- which vary depending on the nova type (which in plosions in the Galaxy. The cause of the explo- turn depends on the type -CO or ONe- of the under- sion is a thermonuclear runaway (TNR) on top of a lying white dwarf). CO novae produce mainly 7Be, white dwarf, ensuing the degenerate burning of the whereas ONe produce 22Na and 26Al. Other radioac- accreted hydrogen (Starrfield 1989, Hernanz & José tivities with shorter lifetimes, such as 13N and 18F 2000). The synthesis of radioactive isotopes in clas- (τ = 862s and 158min, respectively) are produced in sical novae is important for two reasons. First, some similar amounts in CO and ONe novae (see Joséand of the isotopes produced are crucial for the explo- Hernanz, 1998, José, Coc and Hernanz, 1999, Her- sion mechanism itself (i.e., 14O, 15O, 17F with life- nanz et al. 1999 and Hernanz et al., these proceed- −7 −8 13 times 102, 176 and 93s, respectively). The reason ings for details). Typically, 10 − 10 M⊙ of N, 8

−9 18 nd 10 M⊙ of F are ejected in both CO and ONe ex- Arnould M., Meynet G., 1997, in 2 INTEGRAL −10 7 plosions. CO novae also eject 10 M⊙ of Be, and Workshop “The transparent Universe”. ESA SP- −9 22 −8 26 ONe novae 10 M⊙ of Na and 10 M⊙ of Al. 382, Noordwijk, p. 33 The reason of the different explosive nucleosynthesis Baade W., Zwicky F., 1934, Phys. Rev. 45, 138 results in CO and ONe nova types is that some seed nuclei (such as 20Ne, 22Ne, 24,25Mg) are necessary Bazan G., Arnett D., 1994, ApJ 433, L41 to synthesize 22Na and 26Al. That’s because tem- Bazan G., Arnett D., 1998, ApJ 496, 316 peratures attained at the peak of the nova outburst Benz W., Thielemann F.-K., 1990, ApJ 348, L17 are not high enough to break the CNO cycle towards NeNa-MgAl cycles. Bethe H.A., 1990, Rev. Mod. Phys. 62, 801 Bethe H.A., 1995, ApJ 449, 714 The two short-lived isotopes 13N and 18F are crucial for the prompt gamma-ray emission of novae, which Bethe H.A., Wilson J.R., 1985, ApJ 295, 14 is the most intense emission (10−3 phot cm−2 s−1), Bethe H.A., Brown G.E., Applegate J., Lattimer J., but of very short duration (a few days) and appearing 1979, Nucl. Phys. A324, 487 before optical detection. The medium-lived isotopes Branch D., 2000, PASP, in press (astro-ph/0009337) (7Be and 22Na) produce fluxes of around 10−6 and 10−5 phot cm−2 s−1, for distances of 1 kpc. The Bravo E., Garc´ıa-Senz D., 1999, MNRAS 307, 984 prospects for detectability with the SPI instrument Bravo E., TornambéA.,Dom´ınguez I., Isern J., 1996, onboard INTEGRAL are analyzed in Hernanz et al. A&A 306, 811 (these proceedings, and references therein). It is im- 7 Bravo E., Dom´ınguez I., Isern J., Canal R., Höflich portant to remind that, in addition to the Be and P., Labay J., 1993, A&A 269, 187 22Na emission from individual novae, the cumula- tive emission from all the galactic novae can give Burrows A., 2000, Nature 403, 727 important information about the distribution of the Burrows A., The L.S., 1990, ApJ 360, 626 sources, specially if there is only one dominant source Burrows A., Hayes J., Fryxell B.A., 1995, ApJ 450, for that particular isotope. For 22Na, novae are the 830 main individual contributors. Therefore, the detection of galactic 22Na emission, and the corresponding Bussard R.W., Burrows A., The L.S., 1989, ApJ 341, 1275 keV emission map (see Jean et al., 1999, 2000), 401 would be a very valuable tool to study the distri- Canal R., Isern J., Labay J., 1990, Ann. Rev. Astron. bution of novae in the Galaxy, which is very poorly Astrophys., 28, 183 known from optical-UV and IR observations because of interstellar extinction. Finally, 26Al is produced Canal R., Isern J., Labay J., 1992, ApJ 398, L49 in ONe novae in such an amount that makes it quite Canal R., Garc´ıaD., Isern J., Labay J., 1990, ApJ improbable that novae contribute largely to the 26Al 356, L51 content of the Galaxy, as observed through its emis- Clayton D.D., 1981, ApJ 244, L97 sion at 1809 keV. Clayton D.D., Hoyle F., 1974, ApJ 187, L101 Clayton D.D., Colgate S.A., Fishman G.J., 1969, ACKNOWLEDGMENTS ApJ 155, 75 Clayton D.D., Leising M.D., The L.S:, Johnson W.N., Kurfess, J.D., 1992, ApJ 399, L141 Research partially supported by the CICYT-P.N.I.E. (ESP98-1348), by the DGES (PB98-1183-C03-02 Colgate S.A., McKee C., 1969, ApJ 157, 623 and PB98-1183-C03-03) and by the AIHF1999-0140 Colgate S.A., White R.H., 1966, ApJ 143, 626 Cook W. R., Palmer D. M., Prince T. A., Schindler S. M., Starr C. H., Stone E.C., 1988, ApJ 334, L87 REFERENCES Diehl R., Hartmann D., eds., 1999, Astronomy with Aufderheide M.B., Baron E., Thielemann F.-K., Radioactivities, MPE Report 274 1991, ApJ 370, 630 Diehl R., Timmes F.X., 1998, PASP 110, 637 Arnett D., 1969, Astrophys. Space Sci. 5, 180 Diehl et al. 1996, A&A Suppl. 120, 321 Arnett D., 1996, Supernovae and Nucleosynthesis, Diehl et al. 1995, A&A 298, 445 Princeton University Press Diehl et al. 1997, 4th Compton Symp., p. 1114 Arnett D., Livne E., 1994a, ApJ 427, 315 Diehl et al. 1999, Astroph. Lett. & Commun., 38, Arnett D., Livne E., 1994b, ApJ 427, 330 357 Arnett D., Bahcall J.N., Kirshner R.P., Woosley Freedman D.Z., 1974, Phys. Rev. D 9, 1389 S.E., 1989, Ann. Rev. Astron. Astrophys. 27, 629 Fryer C.L., in in Cosmic Explosions, eds. S.S.Holt Arnett D., Fryxell B., Müller E., 1989, ApJ 341, L63 and W.W. Zhang, AIP Conf. Proc. 522 (New Arnett D., Truran J.W., Woosley S.E., 1971, ApJ York), p. 113 165, 87 Fryxell B., Arnett D., Müller E., 1991, ApJ 367, 619 9

Table 1. Radioactive isotopes synthesized in explosive events

Isotope Decay chain Lifetime Line energy (keV) 56Ni 56Ni → 56Co 8.8d 158,812,750,480 56Co 56Co → 56Fe 111d 847,1238 57Ni 57Ni → 57Co → 57Fe (52h)390d 122,136 44Ti 44Ti → 44Sc → 44Ca 89yr(5.4h) 78,68,1157 26Al 26Al → 26Mg 1.0x106yr 1809 60Fe 60Fe → 60Co → 60Ni 2.0x106yr(7.6yr) 1173,1332 7Be 7Be → 7Li 77d 478 22Na 22Na → 22Ne 3.8yr 1275

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