Publications of the Astronomical Society of the Pacific, 110:637±659, 1998 June ᭧ 1998. The Astronomical Society of the Paci®c. All rights reserved. Printed in U.S.A. Invited Review

Gamma-Ray Line Emission from Radioactive in and Galaxies

Roland Diehl Max-Planck-Institut fuÈr extraterrestrische Physik, Giessenbachstrasse 1, D-85740 Garching, Germany; [email protected] and F. X. Timmes Center for and Space Sciences University of California at San Diego, La Jolla, CA 92093; and Institute for Theoretical Physics, University of California at Santa Barbara, Santa Barbara, CA 93106; [email protected] Received 1997 October 29; accepted 1998 January 26

ABSTRACT. Our modern laboratory of has expanded to encompass parts of the universe, or at least our Galaxy. rays emitted by the decays of radioactive nuclei testify to the production of isotopes through nuclear processes in astrophysical events. We collect measurements of the Galactic g-ray sky in spectral lines attributed to the decay of radioactive 7Be, 22Na, 26Al, 44Ti, 56Ni, 57Ni, and 60Fe. We organize and collate these measurements with models for the production sites in novae, supernovae, stellar interiors, and interstellar cosmic-ray interactions. We discuss the physical processes and the spatial distribution of these production sites, along with models of the chemical evolution of the Galaxy. Highlights of measurements made in the last decade include detailed images of the Galaxy in 26Al radioactivity and detection of 56Co and 57Co from SN 1987A, 44Ti from Cas A, and possibly 56Ni from SN 1991T. The 26Al mapping of recent Galactic may be considered as a new view on the entire ensemble of massive stars in the Galaxy. The local region shows prominent radioactive emission from well-known stellar clusters, but the absence of g-rays from the closest Wolf- Rayet , WR 11, in the region is puzzling. SN 1987A studies in g-rays measure the radioactive powering of the curve directly, which will be particularly important for the dim late phase powered by 44Ti. The 57Ni/56Ni isotopic ratio determinations from g-rays provide additional guidance for understanding SN 1987A's complex light curve and now appear to be uniformly settling to about twice the solar ratio. Cas A 44Ti production as measured through g-rays presents the interesting puzzle of hiding the expected, coproduced, and large 56Ni radioactivity. Core-collapse supernova models need to parameterize the inner boundary conditions of the supernova in one way or another, and now enjoy another measurement of the ejecta that is de®nitely originating from very close to the dif®cult regime of the mass cut between ejecta and compact remnant. Other relevant measurements of cosmic element abundances, such as observations of atomic lines from the outer shells of the production sites or meteoritic analysis of interstellar grains, complement the rather direct measurements of penetrating g-rays, thus enhancing the observational constraints of models.

1. INTRODUCTION that the alpha rays were identical with the nuclei of Radioactivity was discovered a little more than a century ( 1905; Rona 1978), and that the beta rays ago when Henri included potassium and uranium were (Becquerel 1900; Badash 1979). By 1912 it was sulfates as part of a photographic emulsion mixture (Becquerel shown that the g-rays had all the properties of very energetic 1896). He soon found that all uranium compounds and the electromagnetic (see, e.g., Allen 1911), but a full metal itself were ªlight sources,º with an intensity proportional appreciation of the physics underlying the measurements took to the amount of uranium present; the chemical combination another two decades (Compton 1929). had no effect. Two years later, Pierre and Marie coined We now understand as transitions between the term ªradioactiveº for those elements that emitted such different states of atomic nuclei, transitions that are ultimately ªBecquerel rays.º A year later, demonstrated attributed to electroweak interactions. Measurement of the de- that at least three different kinds of radiation are emitted in the cay products has grown into an important tool of experimental decay of radioactive substances. He called these ªalpha,º physics. On Earth, it forms the basis of radioactive dating ªbeta,º and ªgammaº rays in an increasing order of their ability through high-precision isotopic analysis, in tree rings, terrestrial to penetrate (Rutherford 1899; Feather 1973). It took a rocks, and meteoritic samples, to name just a few examples few more years for Rutherford and others to conclusively show (Rolfs & Rodney 1989). Radioactive material throughout the

637 638 DIEHL & TIMMES distant universe may be studied in detail by measuring the the asymptotic giant branch; (3) explosive hydrogen burning material's g-ray line spectrum. These g-ray lines identify a ( 12 # 10 8 K) in massive stars and on the surfaces speci®c , and the abundance of the distant material di- of white dwarfs (i.e., novae); and (4) hydrostatic and explosive rectly relates to the measured g-ray line intensity (Clayton carbon and neon burning in massive stars. Any 26Al produced 1982). These g-rays are also unaffected by the intervening by stars is ejected in part by strong winds (Wolf-Rayet, as- matter once the radioactive nucleus has left its dense production ymptotic giant branch stars) and in full amount by explosions site and gone into the interstellar medium. The characteristic (supernova and nova). Besides being formed in stars, 26Al can half-life of an isotope constitutes an exposure timescale of the also in principle be produced by reactions of high- sky in a speci®c g-ray line. Given reasonable event cosmic rays on a range of nuclei (mainly silicon, alu- and yields, the cumulative radioactivity minum, and magnesium), although at substantially lower ef- from many events (26Al, 60Fe, and to a lesser extent 22Na and ®ciency. In fact, 26Al has recently been reported discovered in 44Ti), and individual events (44Ti, 56Ni, 57Ni, and perhaps 7Be, cosmic-ray composition studies with the spacecraft 22Na) can be examined. (Simpson & Connell 1998). By the late 1970s various international collaborations had Once produced, 26Al decays with a half-life of 7.5 # 10 5 yr launched experiments on stratospheric balloons and space sat- from its J p ϭ 5ϩ ground state by bϩ-decay (82% of the time) ellites to explore cosmic rays and X-ray, and g-ray sources and eϪ capture to the J p ϭ 2ϩ of 26Mg. This excited (Murthy & Wolfendale 1993). The ®rst g-ray line found was state then falls to the J p ϭ 0ϩ ground state of 26Mg, emitting reported by Haymes et al. (1975), and their discovery triggered a 1.809 MeV g-ray . 20 years of often controversial measurements and interpreta- Most of the different ways to synthesize 26AlÐType II su- tions of radiation studies. Several addi- pernovae, Wolf-Rayet stars, AGB stars, and classical no- tional lines were discovered shortly thereafter, including the vaeÐcan, at least according to their respective proponents,

HEAO C measurement of line g-rays from 2±3 M, of the produce suf®cient quantities Galaxy-wide. But they cannot all trace isotope 26Al from the central region of ∼the Galaxy (Ma- make the advertised amounts, or else there would be too much honey et al. 1982). After those pioneering missions provided 26Al in the Galaxy. Each prospective source has its advantages new measurements with tantalizing implications, the Compton and dif®culties. We discuss their nature brie¯y, addressing a (CGRO) was launched in 1991 April few of the physical aspects involved. to explore the g-ray sky for the ®rst time over a wide range The treatment of convection, and its implications, constitute of g-ray , including the regime of nuclear lines from one problem area common to several source types. The con- radioactivity. A delightful summary of the CGRO's history and vective coupling between mass zones in both the oxygen-neon relationship to the previous missions, by Kniffen, Gehrels, & and hydrogen shell-burning regions of massive stars can si- Fishman (1998), complements the description of the CGRO's multaneously bring light reactants and seed nuclei into the hot goals, instrumentation, and ®rst achievements by Gehrels et al. zone to aid in the synthesis, and through the same process (1993). Today, collaborative international agencies have nu- remove the fragile product from the high- region merous ongoing and planned projects to deepen speci®c studies where it might otherwise be destroyed. Convective burning in in the area of g-ray (see, e.g., Winkler et al. 1997; the oxygen-neon shell of a 20 M, star has been modeled in Kurfess et al. 1998). two dimensions by Bazan & Arnett (1994). They ®nd that large- In the following review, attention is chie¯y focused on the scale plume structures dominate the velocity ®eld, physically 's g-ray line emission that originates from the ra- caused by the inertia contained in moving mass cells. As a dioactive isotopes 7Be, 22Na, 26Al, 44Ti, 56Ni, 57Ni, and 60Fe. The result, signi®cant mixing beyond the boundaries de®ned con- reviews by Lingenfelter & Ramaty (1978) and Ramaty & Lin- ventionally by mixing-length theory brings fresh fuel into the genfelter (1995) provide excellent supplementary material. convective region (ªconvective overshootº), which may cause local hot spots of nuclear burning. This is different from the situation encountered in spherically symmetric computations. 2. PRODUCTION OF RADIOACTIVE ISOTOPES We will have to await improvements in those very resource, The nucleosynthetic processes in stars responsible for the demanding calculations to assess the inadequacies of mixing- production of 26Al have been summarized by Clayton & Leising length theory and its application in the detailed one-dimen- (1987), Prantzos & Diehl (1996), and MacPherson, Davis, & sional models currently de®ning the baseline. Chemical inhom- Zinner (1995). In stars, 26Al is produced by capture ogeneities create gradients of chemical composition, which pro- reactions, mainly on 25Mg, and is destroyed by eϩ decay and vide yet another driver of mixing (ªsemiconvectionº). (n, p), (n, a), and (p, g) reactions. The ®nal yield from any Large-scale mass motions and local burning conditions are source is temperature sensitive. 26Al can be produced during likely to change the quantitative yields of many isotopes from (1) hydrostatic hydrogen burning in the core of massive ( 11 any single massive star. However, any nonmonotonic and/or ≥ M,) stars; (2) hydrostatic hydrogen burning in the hydrogen nature of the nucleosynthetic yields as a function of shell of low- and intermediate-mass ( 9 M ) stars while on stellar mass tends to be smoothed out by integration over a ≤ ,

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 639

Ϫ4 stellar population, with its initial mass function contributing to pernova. For example, producing more than 10 M, of ra- the observable g-ray emission. Thus, the general features of dioactive 44Ti is dif®cult in spherically symmetric models that the integrated yields, as determined from spherically symmetric are exploded with a piston, but making much less in such models, may remain a useful approximation. models is easy. Ejection of any 44Ti is especially sensitive to Rotation may also have large systematic effects. In massive how much mass falls back onto the remnant. To achieve a stars, the amount of mixing in the hydrogen envelope is in- nearly constant kinetic energy of the ejecta (as supernova rem- creased, thus processing more 25Mg into 26Al. The helium core nant observations suggest), the explosion energy in M տ 25 26 mass may be larger as a result, thus leading to more Al in M, presupernova models must be steadily increased in order the larger neon-oxygen layers (Meynet & Maeder 1997; Langer to overcome the increased of the mantle. How- et al. 1997). Rotating or not, complete loss of the hydrogen ever, even in the 35 and 40 M, stars the presupernova density and helium layers, either through mass loss as a single star or pro®le may still cause nearly all the produced 44Ti to fall back through the effects of close binary evolution (Braun & Langer onto the compact remnant. Unless the explosion mechanism, 1995), could affect the quantitative yields of most of the pre- for unknown reasons, provides a much larger characteristic supernova abundances. Recent improvement of the physical energy in more massive stars, it appears likely that stars larger 26 44 ingredients in Wolf-Rayet Al production models provides than about 30 M, will have dramatically reduced Ti yields some consolidation of hydrostatic nucleosynthesis in massive and leave massive remnants (M 10 M,) that become black stars (Arnould, Paulus, & Meynet 1997; Meynet et al. 1997). holes. If, however, the explosion≥is energetic enough (perhaps These studies point out that combinations of metallicity and overly energetic) more 44Ti is ejected. Rotation may modify mass loss can increase the ejected mass of 26Al by 2. The this picture signi®cantly by breaking the spherical symmetry 26Al abundance present in the hydrogen shell is ejec∼ted un- of the explosion. Jets that are enriched in 44Ti may be induced modi®ed in the supernova explosion or, in more massive stars in the polar regions and still remain in agreement with energy during their Wolf-Rayet phase, by a stellar wind. arguments given above (Nagataki et al. 1997). The 26Al synthesized in the oxygen-neon shell may be sig- The origin and evolution of an accreting white dwarf that ni®cantly enhanced because of operation of the process becomes a classical novae is essentially unknown, yet has im- (Woosley et al. 1990). liberated by n spallation of 20Ne portant consequences on any g-ray signal that may originate capture on 25Mg to produce 26Al. When the oxygen-neon shells from novae. For example, the composition of the nuclear burn- are located closer to the collapsing core, a higher n ¯ux is ing region is often assumed to be a 50-50 mixture of accreted encountered. This may enhance the 26Al yield by 50%. The material and dredged-up white dwarf material. Typically, the relative importance of the n process contribution d∼epends on accreted material is taken to have a solar composition, and the the n energy spectrum. The peak m and t neutrino temperature white dwarf material is assumed to be an oxygen-neon-mag- in real supernovae is somewhat uncertain (Myra & Burrows nesium mixture in mass proportions of 0.3 : 0.5 : 0.2 (see, e.g.,

1990), in spite of measurements from SN 1987A (Arnett et al. Politano et al. 1995). Recent evolutionary models of a 10 M, 1989). Few transport calculations have been carried out long star (Ritossa, Garcia-Berro, & Iben 1996), however, suggest enough (at least 3 s) and with suf®cient energy resolution to that oxygen-neon-magnesium ratios of 0.5 : 0.3 : 0.05 might see the hardening of the neutrino spectrum that occurs as the be more appropriate, although the detailed abundances could proto± star cools (although see Burrows, Hayes, & Fry- vary substantially with initial stellar mass. Since the yields of xell 1995; Mezzacappa et al. 1998). Nevertheless, these un- 26Al and 22Na from nova are sensitive to the initial 25Mg and certainties should be seen in perspective: un- 20Ne abundances, if this latter white dwarf composition is con- certainties are substantial on the 25Mg (p, g) production reaction ®rmed, then it eliminates most of the necessary seed material as well as on the destructions through 26Al(p, g), in addition from which radioactive isotopes may be synthesized. Uncer- the rapidly decaying 26Al state at 228 keV excitation energy tainty in binary star evolution and the binary mass distribution becomes an important agent in oxygen burning. function correspond to uncertainty in the fraction of classical

The genesis of the central compact remnant in core-collapse novae that originate from տ8 M, main-sequence stars (Kolb supernovae bears on several interesting physical problems, & Politano 1997). Finally, all nova models predict ejected some of which may be constrained by radioactivity observa- masses that are too small by an order of magnitude when com- tions. Formation of a appears likely for main- pared with observations (see, e.g., Hernanz et al. 1996). As the sequence masses below 19 M,, while a probably white dwarf becomes more massive, less material is accreted forms for main-sequence∼masses above this regime (Timmes, before the fuel ignites, and the total mass of matter lifted to Woosley, & Weaver 1996b; Woosley & Timmes 1996). The escape velocity, 22Na and 26Al in particular, is smaller. If the energy of the explosion, placement of the mass cut, and how inconsistency between model and observed ejected masses is much mass falls back onto the remnant shortly after the ex- resolved by resorting to a less massive (M Շ 1.1 M,) white plosion, all modify this neutron star±black hole bifurcation dwarf, then the 26Al yields are expected to increase owing to point. Each of these processes also affects the mass of nucle- the lower burning temperatures (JoseÂ, Hernanz, & Coc 1997). osynthetic products ejected from the inner regions of the su- However, should a more violent explosion be required, and be

1998 PASP, 110:637±659 640 DIEHL & TIMMES

tained otherwise. We will discuss g-ray line emission from radioactive isotopes on Galactic scales and from individual sources in the following.

3. INTEGRATED NUCLEOSYNTHESIS The High-Energy Astronomical Observatory C (HEAO C) discovery of the 1.809 MeV g-ray line from radioactive 26Al in the Galaxy (Mahoney et al. 1982), which had been antici- pated from theoretical considerations (Clayton 1971, 1982; Ar- nett 1977; Ramaty & Lingenfelter 1977; Lingenfelter & Ra- maty 1978), opened new doors in g-ray astronomy. Measurements of various radioactive decays in the Milky Way Fig. 1.ÐReported 1.809 MeV ¯ux values for the inner Galaxy, from eight offer a global proof of the idea that heavy-element formation experiments. The length of each vertical stripe is set by the reported uncer- is an ongoing process and is still happening inside stars (Bur- tainties. A value of 4 # 10Ϫ4 cmϪ2 sϪ1 radϪ1 appears plausible given bidge et al. 1957; Cameron 1957). In particular, radioactive ∼ all experiments, although some discrepancies will have to be investigated decay times of 106 yr (as for 26Al and 60Fe) probe a timescale (adapted from Diehl et al. 1998). that is short com∼pared to galactic evolution, thus testifying to the occurrence of relatively recent nucleosynthesis events achieved by increased mixing of core material, then the higher throughout the Galaxy. The 26Al and 60Fe isotopes provide this burning temperatures are expected to decrease the 26Al mass unique observational window, since massive-star nucleosyn- ejected. While these examples show that our understanding of thesis is expected to produce these trace elements in quantities the physics is rather incomplete, the thermonuclear runaway suf®cient to yield g-ray line ¯uxes above instrumental threshold model for classical novae has been a ®rst-order success. A sensitivities. For typical yields of 10Ϫ4 M per event, ap- novae event within 1 kpc would provide several important , proximately 10,000 individual event∼s will contribute to the line diagnostics (radioactive and otherwise) for re®ning the mixing emission from the Galaxy, typical ¯uxes of degree-sized source and energetics aspects of the model. regions are 10Ϫ5 photons cmϪ2 sϪ1. Gamma-ray astronomy seeks to constrain the astrophysical ∼ origin site(s) of radioactive isotopes in the Galaxy, and on 26 smaller spatial scales, regions of coherent star formation. Some- 3.1. Al in the Galaxy times the dominant origin site (supernovae, novae, Wolf-Rayet Eight experiments over the 15 years since its discovery by stars, AGB stars, cosmic rays) of a Galactic radioactivity are HEAO C (Mahoney et al. 1982) have reported detecting g-ray not immediately clear from the observations, but are valuable emission from radioactive 26Al (see review by Prantzos & Diehl to know. The differences between any derived spatial distri- 1996). Instrumental capabilities differ substantially, but the in- butions of the competing sources are often not large enough tegrated ¯ux measured from the general direction of the inner to provide a crisp discriminant, especially since young popu- Galaxy, integrated over latitude and the inner radian in lon- lation stars relate to several of the source types (Prantzos & gitude, has been used to roughly compare results. We sum- Diehl 1996). Increasing the resolution to subdegree domains marize the ¯ux values in Figure 1, including the results re- should provide a better test of various questions (e.g., do AGB viewed by Prantzos & Diehl (1996) plus the recent Gamma-Ray stars give a 1.809 MeV glow that trails the spiral arms?), but Imaging Spectrometer (GRIS) measurement (Naya et al. 1996). that option must await the next generation of g-ray . All measurements are consistent with values 4 # 10Ϫ4 pho- Are there other measurable quantities that might help distin- tons cmϪ2 sϪ1 radϪ1, as indicated by the horiz∼ontal line. Note guish between competing sources? One method is to look for that the determination method varies between the instruments, correlated productions with other radioactive isotopes. For ex- and in particular the ¯ux values for the nonimaging instruments ample, g-rays from radioactive 60Fe may be a very good dis- depend on the assumed spatial distribution: All nonimaging criminant for the contested origin site of Galactic 26Al for two instruments essentially assume the same (or equivalent) smooth main reasons: (1) Type II supernovae produce comparable spatial distribution narrowly following the plane of the Galaxy amounts of 26Al and 60Fe, while the other potential candidates as it had been derived from COS B measurements of Galactic produce signi®cantly smaller amounts of 60Fe. (2) Even more g-rays in the 100 MeV regime. The distribution of 1.809 MeV important, the ª26Al follows 60Feº model can be tested with emission seems signi®cantly different, however. Maximum en- present generation g-ray spectrometers. For individual events, tropy deconvolution images derived from measurements by the other physical parameters are often observed in great detail, so Compton (COMPTEL) aboard the CGRO show spa- the general nature of the origin site is usually clear. In these tial structure in the emission (Fig. 2). The ridge of the Galactic individual events g-ray line measurements can provide speci®c plane dominates, but there is asymmetry in the emission pro®le constraints in themselves, or complement any constraints ob- along the disk, and there are several prominent regions of emis-

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 641

Fig. 2.ÐCOMPTEL 1.8 MeV all-sky image, derived from 5 yr of observation through maximum entropy deconvolution (adapted from Oberlack 1998) sion such as Vela and Cygnus. All estimates of the absolute tribution) is proportional to the 1.809 MeV map in all signif- 26Al mass in the Galaxy rest on assumptions about the spatial icant detail over the entire plane of the Galaxy (KnoÈdlseder distribution of the sources, since the 1.809 MeV measurements 1998). Assuming a standard initial mass function, this calcu- themselves do not carry distance information. The COMPTEL lation reproduces the expected massive-star population and the team ®tted a wide range of models for candidate source spatial supernova rate from both maps consistently, if Wolf-Rayet stars distributions to their data. When localized regions of emission from high-metallicity regimes in the inner Galaxy provide the beyond the inner Galaxy are excluded, then all axisymmetric bulk of 26Al (KnoÈdlseder 1998). 26 model ®ts yield a Galactic mass of Ӎ2 M, (Diehl et al. 1995; The evidence above may be taken to constrain Al contri- KnoÈdlseder et al. 1996a; Diehl et al. 1998). The extent of spiral- butions from classical novae, where a smooth distribution of arm emission can be estimated if a composite model of disk the emission with a pronounced peak in the central bulge region emission plus emission along spiral arms is adopted and com- would be expected. The upper limit for such contributions is 26 pared with the disk-only model. Spiral structure appears sig- probably 1 M, of Al. On the other hand, Ne-rich novae in 26 ni®cant, including between 1.1 M, and all of the Al (Diehl our Galaxy may occur more frequently in the disk and hence et al. 1997; KnoÈdlseder et al. 1996a). follow the Galactic distribution of massive stars more closely If massive stars are the candidate sources, they were thought than the overall white dwarf distribution. In this case, differ- to follow the molecular gas distribution, and thus would be entiating nova sources from massive-star sources will rely on represented by CO survey data (Dame et al. 1987). Although the consistency of the calculated yields with other lines of generally compatible with the 26Al map, other tracers were evidence (such as another radioactive isotope), assisted by found to provide a better ®t. One of these tracers is a semi- 1.809 MeV line-shape arguments. analytical model of spiral-arm structure based on H ii region From the apparent (yet not very signi®cant) 26Al ¯ux in- data but re®ned by free- measurements from consistencies visible in Figure 1, studies of more local 26Al signal dispersions. This tracer shows ridges similar to the 26Al contributions have been revived: The low value from COMP- 35Њ, along with a prominent feature in TEL is mainly based on Galactic plane emission, while theע map at longitudes Carina (l ϭ 280 ⅙), which supports the connection to a young large ®eld-of-view instruments (100Њ±160Њ) of GRIS and Solar stellar population (Taylor & Cordes 1993; Chen et al. 1997). Maximum Mission (SMM) mainly sampled the sky along the This indicates that tracers which measure the energy input plane of the ecliptic with relatively more exposure of the high- into the interstellar medium from massive stars appear to be latitude sky; those instruments may capture large-scale ¯ux of an approximate representation of the 26Al source distribution. low surface brightness that COMPTEL's image failed to cap- Examples are warm dust maps such as the long- ture. 26Al emission from the solar vicinity had been predicted Cosmic Background Explorer (COBE) DIRBE maps, or the long ago (see, e.g., Mor®ll & Hartquist 1985; Blake & Dearborn far- cooling lines of the ionized interstellar medium 1989) but was discarded when COMPTEL's image had shown (e.g., Cϩ) (Diehl et al. 1998). In a recent multifrequency image dominating emission along the plane of the Galaxy. Local con- comparison it was demonstrated that a map of the tributions to the overall emission may exist: the existence of power from massive stars (free-free emission) as derived from two at 100 pc distances ( and R CrA), but the COBE DMR map (after correction for the synchrotron con- also the nearby∼Gould Belt structure dominated by B stars, and

1998 PASP, 110:637±659 642 DIEHL & TIMMES massive-star activity signposts such as Loop I, attributed to the cloud complex at Ӎ2±5 kpc distance (Grabelsky et al. 1987), ⅙ nearby Sco-Cen association, suggest that the 500 pc envi- houses the prominent 140 M, h Carinae star (l ϭ 288 ), and ronment of the may well have experienced∼a higher than shows the densest concentration of young open clusters along average star formation and supernova activity since 50 million the plane of the Galaxy. KnoÈdlseder et al. (1996b) discuss that years ago (see, e.g., PoÈppel 1997). In view of the ve∼ry different 26Al production within these clusters as part of the Car OB1 instrumental techniques, each with substantial systematic un- association may relate to the observed 1.809 MeV feature at certainties, the ¯ux measurements must be consolidated and l ϭ 286 ⅙. This feature is consistent with a for the ensured to be comparable, before such speculations are pursued. 4Њ resolution COMPTEL instrument, thus spatially more con- Imaging of MeV g-rays is far from straightforward because ®ned than the originally estimated signature from analysis of of the high instrumental backgrounds and the complex g-ray the Milky Way's spiral structure (Prantzos 1993a, 1993b). It detection methods. Consistency checks between different tech- is also interesting that some of the 1.809 MeV image structures niques have shown that some of the spikiness of the apparent that fail to align with spiral arms do coincide with directions emission in the COMPTEL result can be an artifact of analysis toward nearby associations of massive stars (Diehl et al. 1995; techniques (see, e.g., KnoÈdlseder 1998). Nevertheless, signif- KnoÈdlseder et al. 1998). Patchiness in such a nucleosynthetic icant emission from the Cygnus, Carina, and Vela regions ap- snapshot might be expected from the clustering of formation pears consolidated. In the Vela region, there was some hope environments of massive stars (Elmegreen & Efremov 1996). to detect 26Al from one single source, the nearby Vela supernova If viewed from the outside, the Milky Way might also display remnant (Oberlack et al. 1994; Diehl et al. 1995). Recent results the signs of massive-star populations in the form of H ii regions show, however, that the main 1.809 MeV feature is signi®cantly arranged like beads on a string along spiral arms, such as are offset from the Vela supernova remnant. The offset may be observed in M31 from Ha emission analysis (Williams et al. caused by emission superimposed from a newly discovered 1995) or in M51 from heated dust seen in infrared continuum young supernova remnant, and/or by OB associations and shell- at 15 mm (Kessler et al. 1996). Interstellar absorption and source like features at larger distances (Oberlack 1998). A direct cal- confusion prevents such mapping within the Milky Way, ibration of core-collapse supernova nucleosynthesis, which was unfortunately. Therefore, detailed investigations of the COMP- a tantalizing prospect 2 years ago (Diehl et al. 1995), does not TEL image systematic uncertainty, that is, a quantitative limit appear as feasible. The other prominent candidate source in the to arti®cial bumpiness of the imaging algorithm, will be im- Vela region is the binary system g2 Velorum, representing the portant for such interpretation of 1.809 MeV emission. Such Wolf-Rayet star WR 11 closest to the Sun with an O star concerns also apply for other instruments and future measure- companion (van der Hucht et al. 1988). Recent Hipparcos par- ments of large segments of the Galactic plane. allax measurements suggest that this binary system is at a dis- In a recent balloon ¯ight, GRIS drift scanned the Galactic tance of 250±310 pc, which is closer than previous estimates center region with its 100Њ ®eld of view, and detected the ע of 300±450 pc (van der Hucht et al. 1997; Schaerer, Schmutz, 1.809 MeV line at 6.8∼j signi®cance with a ¯ux of 4.8 & Grenon 1997). At this closer distance the absence of a signal 0.7 # 10Ϫ4 photons cmϪ2 sϪ1 radϪ1 (Naya et al. 1996). The from g2 Velorum in the COMPTEL 1.8 MeV data is unex- main surprise of this measurement is the width of the astro- pected, particularly since recent models have increased the ex- physical line pro®le, which was signi®cantly broader than the pected 26Al yields for this object (Meynet et al. 1997). Mod- instrumental resolution of the germanium detector, and reported keV. This line width is much larger than 1.4 ע i®cation of the 26Al ejected from WR 11 caused by the O star as DE ϭ 5.4 companion seems inadequate to account for the discrepancy expected from Galactic rotation ( 1 keV; Gehrels & Chen (Braun & Langer 1995; Langer et al. 1997). 1996), which dominates above the≤broadening from random About 80% of the prominent 1.809 MeV emission associated motions in the interstellar medium (Ramaty & Lingenfelter with the Cygnus region can be understood in terms of the 1977, 1995). It is presently dif®cult to understand how such expected 26Al signal from known sources (del Rio et al. 1996). high-velocity motion could be maintained over the million year One may be concerned with this high fraction, since 26Al decays timescale of 26Al decay. Thermal broadening by a very hot on a timescale longer than the observable features of supernova phase of the interstellar medium ( 108 K) with long cooling remnants and Wolf-Rayet winds prevail. It has been suggested times ( 105 yr), or a kinetic broad∼ening at high average ve- that the 26Al from this region attributed to ªseenº sources should locities∼( 500 km sϪ1), seems required. Either case requires be multiplied by a factor of 1±10 to account for ªunseenº extremely∼ low-density phases of the interstellar medium on sources. The latest COMPTEL images show structures that large spatial scales (Chen et al. 1997). Alternatively, one can suggestively align with the Cygnus superbubble and Cyg OB1. hypothesize massive, high-speed dust grains rich in 26Al to Further analysis may be able to separate source regions spatially explain the measurement (Chen et al. 1997). Further obser- and in particular assess the signi®cance of emission from the vations of the line-shape details are required to examine any prominent group of Wolf-Rayet stars in this region. spatial variations in the line broadening. Spectral resolution of The Carina region (l ϭ 282 ⅙±295⅙) presents a tangential 2 keV is required for such a study. Although the International view along a spiral arm, identi®ed through a large molecular G∼amma-Ray Astrophysics Laboratory (INTEGRAL) may still

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 643 have insuf®cient energy resolution to make complete velocity maps of the 1.809 MeV emission along the plane of the Galaxy, the brightest features can probably have their velocity centroids determined well enough to place them on the Galactic rotation curve and thus derive a distance to the features. Surveys that combine velocity information and Galactic latitude extent could then examine the existence and nature of any Galactic ªfoun- tainsº and ªchimneysº from possible ªventingº of 26Al into the Galactic halo. The COMPTEL-measured latitude width may constrain the mean velocity of observed 26Al below the GRIS- suggested value for an assumed young and hence narrow pop- ulation of sources and isotropic expansion over 106 yr (Oberlack 1998). In any case, the large line width measured by GRIS, although inconsistent with the HEAO C line width limit of 3 keV, needs con®rmation, since it could have pro- found im≤plications for our understanding of the interstellar me- dium in the Galaxy.

3.2. 60Fe in the Galaxy Physically, 60Fe should be a good discriminant of different source types generating 26Al, because massive stars produce 26Al and 60Fe in the same regions and in roughly comparable amounts (Fig. 3). Shown are abundances as a function of mass inside a 25 M,, solar metallicity, Type II supernova model (Woosley & Weaver 1995) at the end of the presupernova ev- olution (dashed curve) and the ®nal, postexplosion abundances (solid curves). While the 26Al production occurs in the hydrogen shell and the oxygen-neon shell, 60Fe is produced in He shell burning and at the base of the oxygen-neon shell. Most im- portant is that the majority of both 26Al and 60Fe are produced, mainly during the presupernova evolution, between 3 and 6

M,. These two isotopes should have similar spatial distribu- 26 60 Fig. 3.ÐMass pro®les of Al (top) and Fe (bottom) for a 25 M, model tions after the explosion of these stars. (adapted from Timmes et al. 1995). 26Al and 60Fe are produced in different ratios in stars of different mass (Fig. 4). Massive stars heavier than 25 M, tend to synthesize more 26Al than 60Fe in the Woosley &∼Weaver are shown in Figure 4. SMM reported an upper limit of (1995) models, while the two isotopes are produced in roughly 8.1 # 10Ϫ5 photons cmϪ2 sϪ1 for the 1.173 MeV 60Co line over equal amounts below 25 M,. An estimate for the injection rate the central radian of Galactic longitude, giving an upper limit 60 60 into the Milky Way is the steady state event rate times the of 1.7 M, of Fe (Leising & Share 1994). This Fe mass is 26 Ϫ4 average mass ejected per event; taking M( Al) 10 M,, consistent with the expectations given above and is 20% of 60 Ϫ5 ∼ 26 ∼ M( Fe) 4 # 10 M,, and 2 core-collapse supernovae per the Al ¯ux. Recently, the GRIS (Naya et al. 1998), the Ori- ∼ Ç 26 ∼ Ϫ1 Ç 60 century, one has M( Al) 2.0 M, Myr and M( Fe) 0.8 ented Scintillation Spectrometer Experiment (OSSE) aboard the Ϫ1 ∼ ∼ M, Myr . More re®ned chemical evolution calculations sug- CGRO (Harris 1998), and COMPTEL (Diehl et al. 1998) teams gest that Type II supernovae are responsible for a steady state have reported upper limit 60Fe/26Al ratios. If the stringent GRIS 26 60 M, of Fe measurements are con®rmed, then the initial model estimates 0.9 ע M, of Al and 1.7 1.1 ע abundance of 2.2 in the Galaxy (Timmes et al. 1995). Once 60Fe can be unam- for the total Galactic ¯ux ratio might be too large. But there biguously detected with g-ray telescopes, we can test the hy- are uncertainties of 2 in the models, from nuclear cross sec- pothesis of the supernova origin of 26Al, since 60Fe from other tions, explosion ene∼rgy uncertainties (affecting the large 60Fe sources is negligible. Thus, the 60Fe ¯ux map is then expected contribution from explosive He burning), and chemical evo- to follow the 26Al distribution, and the 60Fe/26Al line ¯ux ratio lution models. The qualitative picture of 26Al and 60Fe tracing -the 26Al image (see Fig. 2) will have each other to a good degree of precision still presents an im :10% ע should be 16% the same morphology as the 60Fe image but will be dimmer. portant observational challenge, even more so with a smaller A few recent 60Fe measurements and ¯ux ratios with 26Al ¯ux ratio.

1998 PASP, 110:637±659 644 DIEHL & TIMMES

Fig. 4.ÐTop: Decay of 60Fe showing the important -parity levels and g-ray photons. Bottom left: Mass of 26Al and 60Fe ejected vs. main-sequence progenitor mass (adapted from Timmes et al. 1995), assuming no mass loss. Bottom right: 60Fe/26Al ratio predicted from supernova models and the preliminary ratio derived from recent measurements.

4. SHORTER LIVED RADIOACTIVITIES Weaver 1995). Production in Type Ib supernovae is more uni- 4.1. 44Ti form because the models converge to a common presupernova In massive stars, stable 44Ca is produced chie¯y, almost ex- mass in the narrow range 2.3±3.6 M,. All of the models, whose 51 clusively, as radioactive 44Ti in the ªa-rich freeze-out.º This ejecta all have 10 of kinetic energy at in®nity, predict 44 ∼ Ϫ5 process occurs when material initially in nuclear statistical equi- Ti yields between 1 and 15 # 10 M,. Typical values are Ϫ5 librium, and a relatively low density, is cooled rapidly enough 3 # 10 M, for the Type II models, or twice that value for ∼ that the free a-particles do not have time to merge back into the Type Ib models. the iron group by the relatively inef®cient triple-a reaction. Thielemann, Nomoto, & Hashimoto (1996) have also ex- Thus, the distribution of nuclei cools down in the presence of amined the detailed nucleosynthesis of core-collapse super- an anomalously large concentration of a-particles (Woosley, novae, and they ®nd, in general, that larger amounts of 44Ti Arnett, & Clayton 1973; Hix & Thielemann 1996). No other are ejected. The differences may be caused by how the explo- 44Ca production process is compatible with the large observed sion is simulated from expansion of an arti®cial high-entropy 48Ca/46Ca ratio. Representative yields of 44Ti from solar bubble, but partially also by the nuclear reaction rates em- metallicity Type II and Type Ib supernova models are shown ployed, or the progenitor structure. Thielemann et al. explode in Figure 5 (Woosley & Weaver 1995; Woosley, Langer, & their stellar models by depositing thermal energy deep in the

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 645

Asymmetries in the explosion mechanism could drive an enhancement of 44Ti in the ejecta (Nagataki et al. 1997). Regions of a larger entropy material could develop behind an asymmetric shock front and incur a larger 44Ti production, per- haps by as much as an order of magnitude. It is possible that core-collapse supernovae have asymmetric explosions. The high birth velocity observed for pulsars (Lyne & Lorrimer 1994) is often taken as evidence for some small asymmetry in the explosion of core-collapse supernovae, although B-®eld or n-wind asymmetry provide alternative explanations. Rotation and magnetic ®elds are a common characteristic of massive stars, and each could induce asymmetries. Differences in the neutrino ¯uxes, seeded by intermittence in the convective ¯ows, could also drive an asymmetry. Consistent modeling of such asymmetry has not been achieved yet, and in particular the measured nickel isotope ratios provide a tight constraint on the entropy/neutron ratio in the inner nucleosynthesis region (F. Thielemann 1997, private communication). Type Ia supernovae could also be important sources of 44Ti if the sub-Chandrasekhar model turns out to be viable. Here a

0.6±0.9 M, carbon-oxygen white dwarf accretes helium at a Ϫ8 Ϫ1 rate of a few times 10 M, yr from a binary companion. (Tutukov, Yungelson, & Iben 1992; Iben & Livio 1993). When

0.15±0.20 M, of helium has been accreted in this model, a helium detonation is initiated at the base of the accreted layer. This helium detonation compresses the carbon-oxygen core, which triggers a detonation there as well (Livne & Glasner 1991; Woosley & Weaver 1994; Livne & Arnett 1995). The broadband photometry implied by these models is generally bluer than the observed photometry of Type Ia supernovae; on Fig. 5.Ð44Ti yields from Type II and Ib supernovae (adapted from Timmes the other hand, these models come closer to Type Ia spectro- et al. 1996a). scopic data. The iron group nucleosynthesis is acceptable in these models, and the production factors for 44Ca relative to neutronized core (which later becomes the neutron star). This its fraction range from 200 to 3000. Depending gives a larger entropy to the innermost zones than what a upon how frequently these sub-Chandrasekhar mass white momentum- (piston-) driven explosion would impart and, in dwarfs explode, the large production factors suggest that these principle, eject more material. A larger entropy also ensures a types of thermonuclear events might be the principal origin of more vigorous alpha-rich freeze-out, and thus a larger 44Ti pro- 44Ca, rather than the typical core-collapse event. This leaves duction. Thielemann et al. sum the ejecta from the outside of the 44Ti observation from Cas A as a puzzle. the star inward and place the mass cut (which is arti®cial in There are three g-ray lines one can use to examine or detect all models) at the position where suf®cient 56Ni is produced to the decay of 44Ti; the 67.9 and 78.4 keV lines from the 44Sc explain the observations. Woosley & Weaver place the piston de-excitation cascade and the 1.157 MeV line as 44Ca decays at a suitable ®rst approximation to mass cut and then follow to its stable ground state. These transitions are shown in Figure an explosion trajectory. It is the difference between injecting 6 with their respective spins, parities, and energies. The half- momentum or energy in modeling the explosion that to life of 44Ti, used to translate observed g-ray ¯uxes into a su- the two groups following different adiabatic paths (Aufder- pernova mass of 44Ti, has been surprisingly uncertain (Fig. 6). heide, Baron, & Thielemann 1991). What self-consistent ex- Measured values over the last 20 years range from 39 to 66 plosion models do, exploding via neutrino heating and multi- years, with a trend toward around 44 years for methods de- dimensional convection, might be re¯ective of one-dimensional termining the activity, and scattering around 58 years for meth- momentum-driven explosions, one-dimensional energy-driven ods following the decay curve. The half-life is dif®cult to mea- explosions, or some intermediate case. Hence, the differences sure because the number of 44Ti nuclei one can obtain is small between the two groups in the amount of 44Ti ejected is an and the half-life is large (relative to Avogadro's number and example of the spread one obtains due to uncertainties in mod- the available laboratory time, respectively). The recent mea- eling the Type II explosion mechanism. surement through the activity method with a mixed 44Ti/22Na

1998 PASP, 110:637±659 646 DIEHL & TIMMES

Fig. 6.ÐDecay of 44Ti (top) and measurements of its half-life (bottom) beam by the Notre Dame group (GoÈrres et al. 1998) appears kpc), young (explosion in A.D. 1668±1680), and wide (physical convincing, and agrees with two careful new measurements diameter 4 pc), making it one of the prime sites for studying from the decay curve (Ahmad et al. 1998; Norman et al. 1998). the compo∼sition and early behavior of a supernova remnant. It may only be equaled as SN 1987A unfolds. The discovery of ע This should ®nally settle this issue, with a half-life of 60 1 yr. 1157 keV g-rays from the 300 yr old Cas A supernova rem- The Cas A supernova remnant is relatively close (2.8±3.7 nant (Iyudin et al. 1994; se∼e Fig. 7) was a scienti®c surprise,

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 647

Fig. 7.ÐMeasurement of 44Ti from Cas A (adapted from Iyudin et al. 1997)

Ϫ5 44 because supernova models had indicated 3 # 10 M, of Ti would be ejected (see Fig. 5), which tr∼anslates into a g-ray Fig. 8.ÐMass pro®les of 44Ti and 56Ni for a 25 M core-collapse supernova intensity generally below instrument ¯ux sensitivities. Recent , model (adapted from Hoffman et al. 1995). analysis of COMPTEL data supports a 5 j detection of Cas A 10Ϫ5 photons cmϪ2 sϪ1 in the 1.157 MeV line # 0.9 ע at 4.2 Ϫ4 44 Ϫ4 (Iyudin et al. 1997), implying 2.4 # 10 M, of Ti. Con- as reported ( 10 M,) would probably mean the ejection of ∼ 56 version of the measured ¯ux into mass limits must account for at least 0.05 M, of Ni. With or without a hydrogen envelope, the uncertainties in the 44Ti half-life, distance to the event, and the supernova would then have a peak luminosity brighter than the precise time of the explosion. the 1042 ergs sϪ1 observed for SN 1987A. If no interstellar Data taken with the OSSE instrument, when best-®tted to absorption would attenuate the optical light curve, the Cas A all three 44Ti g-ray lines, give an (insigni®cant) ¯ux value of supernova should have had a peak apparent magnitude of Ϫ4, 10Ϫ5 photons cmϪ2 sϪ1 for Cas A (The et al. 1996). easily recognizable on the sky (Timmes et al. 1996a). Cas A # 1.4 ע 1.7 The uncertainty of systematic errors in the COMPTEL ¯ux was not widely reported as such; some 10 mag of visual ex- determination, coupled with low OSSE upper limits, confused tinction is required to make the g-ray 44Ti measurements con- the community for a while, but the two instruments now appear sistent with these historical records (or rather their absence). to yield values that are consistent within their respective un- There may indeed have been such a large visual extinction certainties (Iyudin et al. 1994, 1997; SchoÈnfelder et al. 1996; to Cas A at the time of the explosion. At a distance of 3 kpc The et al. 1995, 1996). Measurements of the 68 and 78 keV the optical extinction in the plane of the Galaxy has lon∼g been photons by the Rossi X-Ray Timing Explorer result in a mar- estimated to be 4±6 mag. If Cas A was embedded in a dusty 10Ϫ5 photons cmϪ2 region, or experienced signi®cant mass loss that condensed into # 1.95 ע ginal ¯ux measurement at 2.87 sϪ1 (Rothschild et al. 1997). Since cosmic-ray interactions in dust grains before the explosion, the extinction could have been the produce ¯uorescence lines at 74 and 85 even larger. Measurements of the X-ray scattering halo around keV, along with an activation line present at 66 keV, Cas A from the RoÈntgensatellite (ROSAT) and the Japanese the detection of 44Ti lines has been more dif®cult than expected Asuka satellite (ASCA) offer some compelling evidence for a for this experiment. Additional measurements are necessary. larger reddening correction. ROSAT reports that the scattering 44 From the g-ray observations one may constrain the Ti half- halo is unusually low for the derived NH values (NH ϭ 1.8 # 22 Ϫ2 life by assuming the supernova model yields, a distance to Cas 10 cm ), while this NH is twice as large as an AV ϭ 5 usually A of 3.4 kpc, and the year of the explosion as 1680. This implies (Predehl & Schmitt 1995). This can be understood if procedure also tends to favor the larger 44Ti half-∼lives, too. extra material is distributed close to Cas A, which is relatively The abundance of 44Ti and 56Ni as a function of mass inside dust-free and corresponds to an additional optical extinction of a 25 M, star is shown in Figure 8 (Timmes et al. 1996a). The AV ϭ 5 at the time of the supernovaÐjust about what is needed. mass cut is shown as the solid vertical line. Everything interior While hypotheses such as that the explosion took place in a to the mass cut becomes part of the neutron star, everything dense molecular cloud or that an optically thick cloud occulted exterior may be ejected, depending on how much mass falls Cas A at the time of the explosion cannot be ruled out, sec- back onto the neutron star during the explosion. Regardless, if ondary evidence suggest these scenarios are unlikely. A perhaps 44Ti is ejected, so is 56Ni. The isotope 56Ni is made with 44Ti, more natural hypothesis for this local material is the dusty shell in the same mass zone regime, and is 3 orders of magnitude of material ejected prior to the explosion as a Type Ib super- more abundant than 44Ti. A large quantity of 56Ni ejected means nova. Measurements of X-ray emission from the remnant by a bright supernova. How bright? An abundance of 44Ti as large ASCA (see, e.g., Holt et al. 1994) can best be explained by Ӎ6

1998 PASP, 110:637±659 648 DIEHL & TIMMES

M, of circumstellar material ejected by the presupernova star during its supergiant phase (Borkowski et al. 1996). The supernova shock could have destroyed much of the dust as it propagated through the debris and the material surrounding the Cas A supernova. This scenario explains the lack of optical detection, excess neutral hydrogen column density, dust-free and metal-rich debris, and ejection of 10Ϫ4 M of 44Ti (Hart- ∼ , mann et al. 1997). The present nature of the compact remnant in Cas A is ambiguous, with the actual mass at the onset of carbon burning being critical. Deep X-ray imaging around Cas A does not reveal the synchrotron nebula one might expect around a neu- tron star (Ellison et al. 1994). Deep infrared images taken under excellent seeing conditions have set strict magnitude limits on the presence of a stellar remnant (van den Bergh & Pritchet 1986; Fesen & Becker 1991). Recent ISO observations provide a hint toward presupernova dust north of the bright ring of Fig. 9.ÐBolometric light curve of SN 1987A blast-wave±heated dust (Lagage et al. 1996) and supports some local extinction from comparison of infrared to optical images per 1000 days; Suntzeff 19981), and suggest 10Ϫ4 M of 44Ti (C. Cesarsky 1997, private communication). The region around , (Fransson & Kozma 1998), the value used in Figure 9 when the center of Cas A simply appears void of any detectable stars. showing the thermal luminosity deriving from the decay of So observationally, a black hole may have formed. This would 44 various radioactive isotopes. The solid line is the total lumi- make the ejection of a lot of Ti more dif®cult, but not an nosity, assuming that radioactive decay is the sole power impossibly rare event. The black hole mass would need to be source. This energy input may be dif®cult to resolve uniquely, not too far above the critical gravitational mass of probably however, if the atomic processes that convert nuclear decay less than 2 M,. energy into optical-infrared luminosity are no longer operating Exclusive of any energy input from a pulsar, accreting com- in steady state (Clayton et al. 1992; Fransson & Kozma 1993). pact object or circumstellar interaction, SN 1987A is an epoch Ϫ4 44 Notice that 10 M, of Ti is inferred to have been ejected where the dominant energy source should be from the decay in SN 1987A ∼(a Type II event) and in Cas A (probably a Type 44 of Ti. The thermal luminosity, mostly from eϩ kinetic energy, Ib event). This agreement may be fortuitous, but it may be is expected to be evidence for the core-collapse explosion mechanism being well regulated. For a distance of 50 kpc to SN 1987A and a half-life of 60 t 2 Ϫ4 44 36 0 yr, 10 M, of Ti would produce a g-ray line ¯ux of 2 # L 44 ϭ 4.1 # 10 1 Ϫ exp Ϫk 44 f 0 ϩ 1.3 Ϫ6 Ϫ2 Ϫ1 { [ ( t ) ] } 10 photons cm s . This line ¯ux is too small for the spec- trometers aboard CGRO, possibly too small for INTEGRAL t M( 44 Ti) instruments, but large enough that it might be detected by next- # exp Ϫ t ergs sϪ1, ( ) [ Ϫ4] generation g-ray missions. Spherically symmetric models of t44 1.0 # 10 SN 1987 A show that the 44Ti that manages to be ejected has a very low speed, typically less than about 1000 km sϪ1. This 4 Ϫ2 where f 0 ϭ 7.0 # 10 g cm is the column depth at a ®ducial would result in very narrow lines. Observations of SN 1987A, 6 2 Ϫ1 time t0 ϭ 10 s, k 44 ϭ 0.04 cm g is an estimate of the ef- especially the broad infrared lines of nickel, early appearance 44 fective opacity, and t44 is the mean Ti lifetime (Woosley, of X-rays, and smoothness of the bolometric light curve, all Pinto, & Hartmann 1989). Note that changes in the amount of argue for mixing of 56Ni out to velocities between 2000 and 44Ti ejected produce a linear shift of the light curve. Bolometric 4000 km sϪ1 (see, e.g., Arnett et al. 1989). The 44Ti may be light curves of SN 1987A derived from broadband photometry similarly mixed. It is interesting to know if it is, for it may (Suntzeff et al. 1992; Suntzeff 1998; Bouchet & Danziger 1993) give us some valuable information about the explosion mech- and the above theoretical considerations are shown in Figure anism and multidimensional mixing. 9 for the 500±3500 day period. There are no data points past day 2000, since most of the emission is in the far-infrared wavelength region, where it is not easily observed. Preliminary 1 See also Suntzeff, N. B. 1997, in Supernovae: Their Causes and Conse- ®ts to the latest UVBRIJHK light curves show that the cooling quences, ed. A. Burrows, K. Nomoto, & F. Thielemann; www.itp.ucsb.edu/ time of the remnant is longer than previously thought (Շ1 mag online/supernova/snovaetrans.html.

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 649

4.2. 7Be and 22Na

Classical novae are the most plausible sources for production of possibly detectable 478 keV g-rays from the decay of ra- dioactive 7Be, and the 1.275 MeV g-ray line from the decay of 22Na. Other sources such as red giant stars and supernovae also synthesize 7Be, but the isotope cannot be transported with suf®cient rapidity into regimes that are transparent to g-rays. The explosion of a massive star also produces 22Na in poten- tially detectable amounts, but classical novae remain a favored candidate due to their relative proximity (known Galactic events) and their relative of occurrence ( 30 yrϪ1 in the Galaxy). Each of these two isotopes probes diffe∼rent phases of the thermonuclear runaway model. Both have also been a target of g-ray astronomy for quite some time (Hoyle & Clayton 1974). Nuclei of mass A ϭ 7 are formed predominantly through the 3He(a, g)7Be reaction (Fig. 10), whether in classical nova ex- plosions, hydrostatic red giant envelopes, or primordial nucle- osynthesis scenarios. In novae, the 3He seed material originates mainly from the accreted material and to a much lesser extent from incomplete hydrogen burning (®rst two steps of the hy- drogen-burning proton-proton chain). Competing with the 3He(a, g) reaction in the consumption of 3He is 3He(3He, 2p)4He reaction. This third step of the proton-proton chain is always faster, except at very low 3He abundances. Accordingly, the ®nal 7Be yields from novae display a logarithmic, not a linear, dependence on the initial 3He content (Bof®n et al. 1993). A solar 3He mass fraction in the accreted envelope cannot generate a 7Li (7Be) abundance that greatly exceeds the solar mass fraction. After 7Be is produced, it is destroyed mainly by proton cap- tures to form 8B. This destruction is quite ef®cient below tem- peratures of 108 K and densities above 100 g cmϪ3. At hotter temperatures∼the of ∼8B reduces the overall 8 9 destruction somewhat. Including the leakage of B to C is 7 22 7 Fig. 10.Ð Be and production processes (top) and Na mea- important, since it reduces the Be abundances by allowing a surements from individual novae (bottom). ¯ow out of any 7Be-8B equilibrium (Bof®n et al. 1993). Once the nuclear reactions cease, any remaining 7Be will b-decay to 7Li with a half-life of 76 days, and produce the 487 keV photon metric models of classical novae that incorporate time-depen- that is a target of g-ray astronomy. Assuming a nova ejects a dent convection and the accretion phase tend to give 7Be yields Ϫ4 7 Ϫ12 Ϫ10 total mass of 10 M,, of which the Be mass fraction is between 10 and 10 M, depending on the mass and com- 5 # 10Ϫ6, the 487 keV ¯ux is shown as a function of time in position of the white dwarf and accreted material (Politano et Figure 10 for a distance to the nova of 0.5 kpc. These values al. 1995). Two-dimensional hydrodynamic models that examine may not be typical, and preferences for other values are easily the evolution of a fully convective hydrogen-rich envelope dur- accommodated; the ¯ux scales linearly with the total ejected ing the earliest stages of the thermonuclear runaway are be- mass and 7Be mass fraction while scaling with the inverse ginning to be calculated and should help clarify some of these square of the distance. For the conditions shown, a space-borne uncertainties in the future (Shankar & Arnett 1994; Glasner, spectrometer with a sensitivity of 10Ϫ5 photons cmϪ2 sϪ1 will Livne, & Truran 1997). be able to study such novae for about 3 months. The effect of the white dwarf mass and composition also Convective processing and mixing timescales in nova en- plays a crucial role in the resulting nucleosynthesis. Less mas- velopes are critical issues. Signi®cant g-ray signals from 7Li sive white dwarfs generally support a larger accreted envelope can only occur when convection can transport freshly synthe- mass before ignition occurs. More massive envelopes ignite sized 7Be into a cooler and less dense region. Spherically sym- material at the core-envelope interface at a larger density (de-

1998 PASP, 110:637±659 650 DIEHL & TIMMES generacy) and achieve a higher peak temperature. This results sical novae. The category ªneon novaº has been suggested for in stronger outbursts with shorter evolutionary timescales and nova events that display strong neon features (i.e., show large extends the nuclear activity toward higher Z nuclei (Jose & metal enrichments) at optical (Starr®eld et al. Hernanz 1998). While isotope production is thus generally en- 1996). For neon-novae, which are expected to eject the largest hanced when the initial mass of the underlying white dwarf is 22Na masses, the average COMPTEL upper limit ¯ux of 3 # increased, less material is accreted before the fuel ignites in 10Ϫ5 photons cmϪ2 sϪ1 translates into an upper limit on the 22 Ϫ8 more massive white dwarfs, and the total mass of matter lifted ejected Na mass of 3.7 # 10 M, (Iyudin et al. 1995). out of the gravitational well is smaller. In addition, the less 12C Spherically symmetric models of classical novae that incor- that is present during the runaway, the less nuclear energy is porate a more self-consistent treatment of the hydrodynamic released by the CNO cycle at a given temperature, and the less evolution and re®ned initial white dwarf compositions tend to 7 22 Ϫ8 light isotope production there is. Estimates for mass of Li give Na yields of 10 M, for the most favorable neon- ejected in the carbon-oxygen white dwarf models are almost novae cases (Starr®e∼ld et al. 1996). In general, these recent an order of magnitude larger than the corresponding (same core improvements to the modeling have reduced the 22Na yield by mass) oxygen-neon white dwarf models. It should also be em- about an order of magnitude compared to the older models phasized that the mass accretion rate and the initial white dwarf (Weiss & Truran 1990; Starr®eld et al. 1992, 1993), which luminosity (or temperature) may also in¯uence the results. lacked these re®nements and were at distinct odds with the More violent outbursts are obtained when lower mass accretion COMPTEL measurements. It may still be that present nova rates or lower initial luminosities are adopted, since a higher models are biased from the observations of a few exceptional level of degeneracy is attained (Jose & Hernanz 1998). The events and that the typical neon-novae ejects much less mass expected effect on the resulting nucleosynthesis is an extension into the interstellar medium than the exceptional events. of the nuclear activity toward heavier species as the mass ac- cretion rate or the initial luminosity decreases because of the 4.3. 56Ni and 57Ni higher temperatures achieved in the envelope. There have been no reported detections of 478 keV line A goal of observational g-ray astronomy for the last three emission from either the cumulative effects of many novae decades has been detection of line radiation from the decay of ( 100) near the region, or individual novae. radioactive 56Ni and 56Co produced in supernovae (Clayton, ∼ However, SMM reported a 3 j upper limit ¯ux of 1.7 # Colgate, & Fishman 1969; see Fig. 11). Type Ia events are 10Ϫ4 photons cmϪ2 sϪ1, assuming a point source of constant favored over the other supernova classes because they produce intensity at the Galactic center (Harris, Leising, & Share 1991). an order of magnitude more 56Ni than the other types ( 0.6 Ϫ3 ∼ They also reported 3 j upper limit ¯uxes of 2.0 # 10 photons M,; Thielemann, Nomoto, & Yokoi 1986) and because they cmϪ2 sϪ1 from Nova Aql 1982, 8.1 # 10Ϫ4 photons cmϪ2 sϪ1 expand rapidly enough to allow the g-rays to escape before all from Nova Vul 1984, and 1.1 # 10Ϫ3 photons cmϪ2 sϪ1 from the fresh radioactivity has decayed. Even so, detection of these Nova Cen 1986. These results imply upper limit 7Be abun- events has been dif®cult to achieve. dances that are about an order of magnitude above even the Type Ib supernovae synthesize 5±10 times less 56Ni than a most optimistic theoretical expectations, and thus are not very typical Type Ia, but expand almost as rapidly as Type Ia su- constraining. pernovae. Thus, their signal is intermediate between Type II The radioactive isotope 22Na is synthesized in classical novae and Type Ia. The g-ray line detection of any Type II supernova at the interface between the white dwarf and the accreted en- outside the Local Group is very improbable. The best-studied velope. How much carbon, oxygen, magnesium, and neon is supernova of all in radioactive line emission was the Type II dredged up from the white dwarf or accreted from the binary supernova SN 1987A (e.g., Arnett et al. 1989), mainly because companion is a critical issue for determining the strength of of its proximity. Detection of 56Co and 57Co lines from SN any 22Na g-ray signal. The chief nuclear ¯ows that create and 1987A by many experiments gave the ®rst extragalactic g-ray destroy 22Na are discussed in detail by Higdon & Fowler (1987) line signal from radioactive isotopes. The early appearance of and Coc et al. (1995), along with the uncertainties in several 56Co g-radiation presented evidence for enhanced mixing of key reaction rates. As the ejecta cools and expands, and nuclear supernova products within the envelope. burning ceases ( 7 days), 22Na decays with a half-life of 2.6 Later, OSSE reported detection of 57Co radiation from SN p ∼ ϩ 22 Ϫ4 Ϫ2 Ϫ1 yr from the J ϭ 3 state to a short-lived excited state of Ne, 1987A, with a measured ¯ux of 10 photons cm s be- emitting a 1.275 MeV g-ray line in the process (Fig. 10). tween 50 and 136 keV (Kurfess e∼t al. 1992). For models with Gamma-ray line emission at 1.275 MeV from classical novae low 57Co optical depths, the observed g-ray ¯ux suggested that in the Milky Way remains to be positively detected. SMM, the 57Ni/56Ni ratio produced by explosion was about 1.5±2.0 OSSE, and COMPTEL have, however, reported upper limits times the solar system ratio of 57Fe/56Fe (Clayton et al. 1992; for several individual novae (Leising 1993; Iyudin et al. 1995; see Fig. 9). Estimates of SN 1987A's bolometric luminosity at see Fig. 10). The 2 j upper limit ¯uxes reported by COMPTEL optical and infrared wavelengths at day 1500, while indicating put tight constraints on thermonuclear runaway models of clas- that 57Co was indeed powering the light curve at that time,

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 651

Fig. 11.ÐDecay of 56Ni to 56Fe and 57Ni to 57Fe, illustrating the most signi®cant spin-parity levels and g-ray photons. The most important transitions for g-ray line astronomy and supernova diagnostics are marked with an asterisk. seemed to indicate a 57Ni/56Ni ratio that was 5 times the solar 1996), and the light curve evolved unusually slowly. The clas- 57Fe/56Fe ratio (Suntzeff et al. 1992; Dwek et al. 1992). How- si®cation as a peculiar Type Ia event is based on the absence ever, preliminary ®ts to the latest UVBRIJHK light curves show of the silicon lines so typical in early Type Ia spectra. In ad- that the cooling time of the remnant is longer than previously dition, the Fe iii lines were unusually strong at early epochs. thought (Շ1 mag per 1000 days; Suntzeff 1998), reducing the Since no other iron group lines were observed at this time, this demand for heating from radioactive 57Co. These preliminary iron was probably not a fresh nucleosynthetic product. The 56 57 ®ts give 0.069 M, of Co and 0.0033 M, of Co (Fransson spectra did become more typical of Type Ia events at later & Kozma 1993, 1998; see Fig. 9). This gives a epochs when the expanding debris became more transparent. 57Ni/56Ni ratio of twice the solar 57Fe/56Fe ratio and is in rea- Detection of high-velocity ( 13,000 km sϪ1) iron and nickel sonable agreement with the ratio implied by the OSSE in the outer layers of SN 19∼91T favors models in which the measurements. subsonic ¯ame front propagates larger distances from the white Only one Type Ia supernova has been seen in g-rays, SN dwarf core before making the transition to a detonation. These 1991T in NGC 4527 (Morris et al. 1995, 1998; see Fig. 12). types of delayed-detonation models are also consistent with the This galaxy is 17 Mpc distant (determined from Cepheids) velocity pro®le of most of the other ejecta (silicon, ) and in the direc∼tion of the Virgo Cluster. The supernova was seen in SN 1991T. Shigeyama et al. (1993) suggest that de- 56 unusually bright at maximum (0.7 M, of Ni; Hoȯich et al. tection in the early light curve of the 812 keV g-ray line from

1998 PASP, 110:637±659 652 DIEHL & TIMMES

Fig. 12.ÐSN 1991T spectrum as measured by COMPTEL, after subtraction of a background model (adapted from Morris et al. 1995, 1998). Fig. 13.ÐHistogram of inferred 26Al/27Al for interstellar grains of various types (wide hatched columns) and solar system objects (narrow ®lled columns), the decay of 56Ni (Fig. 11) would be direct evidence for de- adapted from MacPherson et al. (1995). layed-detonation models, since the line cannot be seen when the 56Ni is embedded deeper in other categories of Type Ia models. However, sub-Chandrasekhar mass models of Type Ia are inferred to have formed in circumstellar atmospheres or, in supernova, especially those of white dwarfs in a binary system, some cases, in nova or supernova explosions. Because their are discussed as well. These models may be favorable for eject- compositions re¯ect the isotopic and chemical signatures of ing a larger than average 56Ni mass and seek to explain some their sources, presolar grains provide direct information about of the other early light-curve peculiarities as arising from in- stellar evolution and nucleosynthesis, mixing processes in stars, teractions of the supernova debris with the thick disk of material the physical and chemical conditions of stellar atmospheres, that surrounded the merger. and the chemical evolution of the Galaxy (Anders & Zinner 56 1993; Ott 1993). The tentative COMPTEL detection of the Co decay g-rays 26 22 indicates that this isotope was present in the outer envelope, There are several interesting parallels between Al, Na, 44 60 g and thus supports extensive mixing scenarios. The COMPTEL Ti, and Fe in -ray astronomy and in the laboratory study of presolar meteoritic grains. Enhanced 26 24 measurement converts into a surprisingly large 56Ni mass, how- Mg/ Mg ratios in the calcium-aluminum (Ca-Al) rich inclusions of the Allende me- ever, between 1.3 M (for a distance of 13 Mpc; Morris et al. , teorite were the ®rst evidence for live 26Al in the early solar 1995) and 2.3 M, for the 17 Mpc favored currently (P. Ruiz- Lapuente 1997, private communication). This requires that al- system (Lee, Papanastassiou, & Wasserburg 1977). Present most all of the Chandrasekhar mass white dwarf must be turned compilations (MacPherson et al. 1995), with over 1500 data points derived from various types of meteoritic samples, con- into radioactive 56Ni. The OSSE upper limits (Leising et al. ®rm the enhancements in 26 1995) may indicate that the 56Co line ¯ux derived by Morris Mg in these old and stable inclusions et al. (1995) is too high, although the detection itself is con- of solar system material and show a homogeneous isotopic ratio 26Al/27Al of 5 # 10Ϫ5. This is interpreted as the in situ ®rmed at the same 3±4 j signi®cance (Morris et al. 1998). 26 ∼ 56 decay of Al in the early protosolar droplets (MacPherson et More detections of Type Ia supernovae in Ni are required to 26 clarify how typical SN 1991T was. COMPTEL and OSSE will al. 1995), mainly from the strong correlation of Mg excess with aluminum abundances of the samples. This is direct ev- hopefully remain ready to observe any nearby events (for es- 26 timates see below). idence of an injection of (at least) Al into the solar nebula shortly before solar system formation. The inferred 26Al/27Al Ϫ5 5. ASSOCIATED ASTROPHYSICAL CHALLENGES ratio of 5 # 10 is substantially larger than the ratio of ∼Ϫ6 2±3 # 10 estimated from g-ray measurements (Diehl et al. 5.1. Meteoritic Grains 1995). Although the nucleosynthetic processes occurring in differ- Figure 13 compares the inferred initial 26Al/27Al ratio dis- ent stars generally result in a wide range of isotopic compo- tributions of presolar grains (corundum, graphite, and silicon sitions, by far most of the material from the many stellar sources carbide) with the total population of data for aluminum-rich that contributed to the protosolar cloud was thoroughly proc- material (mostly Ca-Al rich inclusions) that represents solar essed and mixed, which resulted in the essentially isotopically system aluminum. There is very little overlap between the two homogeneous solar system we know today. A small fraction populations. This ensures that these types of meteoritic samples of the original material, in the form of presolar dust grains, constitute different observational windows that need not cor- survived solar system formation and became trapped in prim- relate. Rather, each distribution carries imprints of the speci®c itive meteorites. From their highly unusual isotopic composi- grain formation or even early solar system processes (see tions, relative to that of the solar system, these presolar grains MacPherson et al. 1995).

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 653

An indication that live 60Fe also existed when the meteorites the solar nebula experienced an enrichment in relatively short- solidi®ed is found from isotopic analysis of the Chernvony Kut lived radioactive isotopes. Whether this enrichment is caused meteorite (Shukolyukov & Lugmair 1993). The measured ex- by the hypothesized triggering events (if they occurred) or some cess of 60Ni, after alternative modes of production such as unconsidered process during formation of the Sun remains an spallation and (n, g) reactions on 59Co could be eliminated, active area of research. leads to a 60Fe/56Fe ratio at the time of iron-nickel fractionation Shu et al. (1997) discuss the idea that the chondrules and of 7.5 # 10Ϫ9. This is consistent with the inferred 10 million Ca-Al rich inclusions found in chondritic meteorites might have yea∼r hiatus before the formation of the Ca-Al rich inclusions been formed as solids entrained and melted in the bipolar wind in the Allende meteorite, which has a much larger 60Fe/56Fe that results from the interaction of the accreting protosolar neb- ratio at the time of fractionation of 1.6 # 10Ϫ6 (Birck & Lug- ula and the magnetosphere of the young proto-Sun. Aerody- mair 1988). However, the possibili∼ty that some of the 60Ni in namic sorting and a mechanical selection for molten droplets the Ca-Al rich inclusions is fossil rather than from the in situ that rain back onto the disk at planetary distances explain the decay of 60Fe cannot be excluded. size distributions and patterns of element segregation that we Despite their rarity, noble gases have played a key role in observe in carbonaceous and ordinary chondrites. Cosmic-ray establishing the interstellar origin of diamond, graphite, and generated in the ¯ares that accompany the general mag- silicon carbide. Neon has three stable isotopes, permitting com- netic activity of the inner region may irradiate the precursor plex mixtures to be resolved into their components. One of the rocks before they are launched in the bipolar wind. Under components, Ne-E, exists in two varieties that differ in both certain scaling assumptions for the ef®ciency of the process in thermal release temperature of the neon and the density of the protostars, Shu et al. ®nd that cosmic-ray bombardment can carrier, and hence are designated ªHº (high) and ªLº (low). generate the short-lived 26Al, 41Ca, and 53Mn at High-precision measurements show that Ne-E(H) is not radi- their inferred meteoritic levels. Simply put, this mechanism ogenic 22Ne from the decay of 22Na, but primary neon. The captures material trying to get on the Sun, heats it up, irradiates strongest evidence for a parentless origin of Ne-E(H) comes it with cosmic rays, and dumps it back onto the disk farther from direct measurement of 160 individual silicon carbide out (Glanz 1997). It is far from clear, however, that such low- grains by gas extraction.∼In each case, the 22Ne is accom- energy cosmic rays can explain most of the complex isotopic panied by an amount of 4He that is a close match with the patterns found in chondrites. Additional measurements are nec- expected helium-shell composition of stars with a mass Շ3 essary to see if the isotopic anomalies are best explained by

M, and Fe/H near solar (Nichols et al. 1993; Lewis, Amari, such a single mechanism. & Anders 1993). No such helium accompanies Ne-E(L) from graphite spherules, and so for Ne-E(L), a major contribution 5.2. from Radioactivities from radioactive 22Na is a viable, even preferred explanation (Gallino et al. 1990; & Clayton 1992; Zinner 1997). Radioactive decays can generate positrons whenever the en- Large excesses in 44Ca have been observed in four low- ergy level of the daughter is below the energy level of density graphite grains and ®ve silicon carbide grains of Type the parent by more than the 1.022 MeV thresh- X extracted from the Murchison meteorite and have been shown old. Interesting numbers of positrons are produced from the to be related to the radioactive decay of 44Ti (Nittler et al. 1996; bϩ-decay of 26Al, 44Ti, 56Co and the distinct nova products 13N Hoppe et al. 1996; see Fig. 6). Because 44Ti is produced only and 18F. Annihilation produces two 511 keV photons if the in supernovae, these grains must have a supernova origin. positron and electron spins point in opposite directions, or three Moreover, the silicon, carbon, , aluminum, oxygen, and photons in a continuous energy spectrum if the spins are par- isotopic compositions of these large grains (11 mm) allel. The three-photon annihilation process usually involves re¯ect the isotopic composition expected from a Type II su- formation of , which then decays before being col- pernova source (Nittler et al. 1996). This is also strong evidence lisionally destroyed because of the low densities of interstellar that these grains are supernova condensates and provides ev- space. As a result, the fraction of positronium radiation in the idence for deep and heterogeneous mixing of different super- total annihilation signal carries information about the ther- nova regions, including the nickel core. modynamic properties of the annihilation environment. A cold, Extensive high-precision measurements of other meteorites neutral environment results in positronium fractions of have found many other anomalous abundance ratios, and these 0.9±0.945 (Bussard, Ramaty, & Drachman 1979; Brown, Lev- are generally attributed to isotopes that were still radioactive enthal, & Mills 1986), while larger positronium fractions tend during the decoupling of the solar nebula from the interstellar to indicate annihilation in warmer environments, 5 # 10 3 K or medium 4.6 Gyr ago (see, e.g., Harper 1996). These measure- hotter. The positrons produced by the interesting radioactive ments have led to hypotheses of the solar system formation decays have average kinetic energies of MeV, and thus are being related to a nearby event such as a supernova or an AGB relativistic. Deceleration and thermalizat∼ are more likely star (Cameron 1993; Wasserburg et al. 1995). In other words, than annihilation in ¯ight, so that the positron lifetime in in-

1998 PASP, 110:637±659 654 DIEHL & TIMMES terstellar space before annihilation is 105 yr (Lingenfelter, 5.3. Galactic Nucleosynthesis and Supernova Rates ∼ Chan, & Ramaty 1993; Ramaty & Lingenfelter 1995). This thermalization process means intrinsically narrow 511 keV line Direct measurement of the Milky Way's supernova rate is widths that are related to the annihilation environment rather notoriously dif®cult. Various methods have been attempted, than to the positron production environment. such as O±B2 star counts within 1 kpc of the Sun, radio su- In addition to annihilation from radioactive decays, positron pernova remnant counts, g-ray ¯uxes from the decay of 44Ti, annihilation is expected from the disks around accreting com- neutrino burst detections, and pulsar birthrate estimates. These pact remnants, from the jets caused by dynamo action of ac- indirect methods incur large errors and only yield upper limits creting compact sources, and from g-g reactions in strong mag- for cases based on incomplete observations. Galactic supernova netic ®elds. How can these various signals be differentiated? rates have therefore been based on extragalactic supernova Positrons from radioactive decays usually annihilate in the dif- searches. These estimates depend upon morphological type, fuse interstellar medium where the thermalization lifetime is Hubble constant, completeness of the surveys, determination long. Positrons produced around compact objects should an- of reliable control times, and the uncertain luminosity of the Galaxy. The total luminosity, Ha, and far-infrared ¯uxes nihilate near the compact objects, since the density is usually of the Milky Way remain uncertain, in contrast to those of most much larger in those environments. This is expected to result Local Group galaxies. Nevertheless, using the extragalactic es- in a more localized and possibly time-variable signal. Such timates with a total Galactic blue luminosity of 2.3 # 1010 L , annihilation spectra may also contain such recognizable sig- , a Hubble constant of 75 km sϪ1 MpcϪ1, and a Sbc Galactic natures as a high-energy tail above 511 keV from annihilation morphology, the Galactic core-collapse supernova rate is es- in ¯ight, a blueshift from positron jets moving toward the ob- timated to be 2.4±2.7 per century, while the Type Ia rate is server, a when originating from sources with a large estimated to be 0.3±0.6 per century (van den Bergh & McClure gravitational ®eld, or a distinctly smaller positronium fraction. 1994; Tammann, LoÈf¯er, & SchroÈder 1994). Thus, the total Observations of the 511 keV line from positron annihilation supernova rate is about 3 per century and the ratio of core show a steady diffuse component from the Galactic disk su- collapse to thermonuclear events is about 6. perimposed upon a time-variable point source located near the There are six known local supernovae: Lupus (SN 1006) is Galactic center (Johnson, Harden, & Haymes 1972; Ramaty, considered to have been a Type Ia event, Cas A (SN 1680) Skibo, & Lingenfelter 1994). OSSE measurements of the an- and Tycho (SN 1572) were most likely Type Ib supernovae, nihilation radiation can be analyzed in terms of plausible spatial and the Crab (SN 1054), SN 1181, and Kepler (SN 1604) were distribution models that aim to separate the disk component probably Type II events. This listing may be complete to 4 ∼ from the Galactic bulge component (Kinzer et al. 1996; Smith, kpc. For an exponential disk with a 4 kpc scale length extending Purcell, & Leventhal 1998). This decomposition ®nds anni- between 3 and 15 kpc, about 9% of all Galactic OB stars will hilation rates of 1043 eϩ sϪ1 for the disk and 2.6 # 10 43 eϩ sϪ1 be located within 4 kpc of the Sun. At a radial distance of 8.5 for the bulge. Almost all of the annihilation luminosity from kpc, the number of supernovae with massive progenitors within the Galactic disk may be explained by radioactive sources. 4 kpc of the Sun may be estimated as 0.09 # 0.85 # 3 2.3 per millennium (van den Bergh & Mc∼Clure 1994). This i∼s -is assigned to 26Al, with the remainder par 5% ע About 16% close to the observed number that are known to have occurred titioned between 44Ti, 56Co, and old stellar population products within 4 kpc of the Sun during the last 2000 years. However, (Timmes et al. 1996a; Lingenfelter et al. 1993). Kinzer et al. the statistical uncertainty in the frequency with which super- (1996) determined a positronium fraction of 0.94±1.0 for the novae of different types occur in galaxies of different Hubble inner Galaxy, suggesting that the contribution from compact class is large, and this agreement may be fortuitous. sources might be small. However, this constraint strongly de- The total 44Ti line ¯ux originating from the central regions pends on the environment of the compact sources; Ramaty et of the Milky Way was sought by the large ®eld-of-view spec- al. (1994) point out that the entire bulge component could be trometers aboard the HEAO 3 and SMM satellites. Analysis of explained from 1E 1740.7Ϫ2942 alone if positrons are not the data taken with HEAO 3 gave an upper limit of 2 # rapidly annihilated in a target close to that compact source. 10Ϫ4 photons cmϪ2 sϪ1 on the Galactic 67.9 and 78.4 keV Hernanz et al. (1996) estimate from their simulations of clas- emission (Mahoney et al. 1992). Searching through nearly 10 sical novae that the peak 511 keV emission reaches 10Ϫ2 (D/ years of data taken by SMM gave an upper limit of 8 # ∼ 1 kpc)2 photons cmϪ2 sϪ1 for a period of about 7 hr after the 10Ϫ5 photons cmϪ2 sϪ1 for the inner 150Њ of the Milky Way in outburst. This would make detections from distances as far as the 1.157 MeV line (Leising & Share 1994); the imaging the Galactic center feasible, if timing with the observations COMPTEL instrument sets upper limits below 2 # 10Ϫ5 pho- were fortunate. Overall, however, the nova contribution to the tons cmϪ2 sϪ1 for the known historical events (Dupraz et al. diffuse 511 keV glow of the Galaxy is expected to be low 1997). (Lingenfelter & Ramaty 1989; Ramaty & Lingenfelter 1995). A ®rst COMPTEL 1.07±1.25 MeV background-subtracted

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 655 sky map is shown in Figure 14 (Dupraz et al. 1997). Even though only part of the data has been used in this analysis and the potentially more sensitive background modeling from ad- jacent energies has not been employed, ®rst conclusions can be drawn: Cas A is the only historic event detected (at 5 j now; see discussion above). Upper limits to the 44Ti line∼¯ux from Tycho and Kepler are about half the Cas A signal. Even Fig. 14.ÐA ®rst COMPTEL map of the Galactic plane in the 1.157 MeV with the longest 44Ti half-lives measured, Lupus, Crab, and SN line from 44Ti (adapted from Dupraz et al. 1997), using the ®rst 3 yr of data. 1181 are far too old to have signi®cant amounts of 44Ti re- Star symbols mark locations of historical supernova 1181, Tycho, Cas A, Kepler, Lupus, and Crab (from left to right). The Cas A signal in these data maining to be detected. No previously unknown event is found, corresponds to 3.5 j signi®cance; likelihood contour levels are in 1 j steps even though COMPTEL's sensitivity should provide complete for known sources, 0.75 j for search of new sources. sampling of the last century beyond the distance of the Galactic ∼ center. range of 0.4±3.5 M from present data, g-ray measurements Core-collapse supernovae and Chandrasekhar mass ther- , cannot help resolve the various systematic uncertainties present monuclear supernovae tend to account for about one-third of in the classical methods (e.g., galaxy type, Hubble constant, the solar 44Ca abundance, with most of the production attrib- sample completeness). Future measurements of the position and utable to Type II events. The mass of 44Ca produced as itself shape of the 1.809 MeV g-ray line may provide the basis for is comparable to the 44Ca synthesized from the radioactive de- a three-dimensional deconvolution of the apparent emission cay of 44Ti. Even at this 1 solar level, the sky should contain 3 (Gehrels & Chen 1996). Note that the measurement of a 6 keV several mean Type II sup∼ernova remnants bright enough to be wide 1.809 MeV line by GRIS (Naya et al. 1996) suggests that seen in their 44Ti afterglow. However, no search (HEAO 3, any 0.1±0.9 keV line shifts expected from Galactic rotation SMM, COMPTEL, OSSE) has produced a detection of 44Ti line may be drowned in (yet to be found) broadening processes and emission from any young, previously unknown, and visually hence not allow a direct mapping of the emission. obscured remnants. We simply do not see the expected number of supernova remnants emitting g-rays from the decay of ra- 5.4. Other Galaxies dioactive 44Ti. After eliminating alternative modes of increasing the 44Ca production (e.g., larger yields), the conclusion becomes In undertaking searches for g-ray line signals from extra- almost inescapable. The solar abundance of 44Ca is apparently galactic supernovae, it is useful to have some guidelines as to due to rare events with exceptionally high 44Ti yields. This what might be expected. With recent catalogs of supernova conclusion was already derived from the SMM upper limits to events, and distances based upon some assumptions regarding 44Ti g-rays (Leising & Share 1994). Rare events could mean the peak absolute magnitude of supernovae, estimates of the Type II events where the explosion energy is large enough to detectable event rate as a function of g-ray telescope sensitivity minimize the mass of 44Ti that falls back onto the compact can be made. Vigorous ground-based programs that search, remnant, or Type Ia events that manage to explode a sub- discover, classify, and catalog supernovae are essential pre- Chandrasekhar mass white dwarf. Mapping the Galaxy in 44Ti requisites in studying the g-ray emission in detail and, hence, should help to resolve the unknown rate of these rare events; studying the explosion in detail. they would still mark bright spots in the 1157 keV sky. The Data on more than 1000 extragalactic supernovae are given COMPTEL survey will provide the ultimate database; the result in the Sternberg Astronomical Institute Supernova Catalogue2 from the ®rst three mission years (Dupraz et al. 1997) still and the Asiago Supernova Catalogue.3 About 50% of the su- appears inconclusive in this respect. pernovae in either catalog lack or have an uncertain supernova Integrated nucleosynthesis measurements, such as the 1.809 type identi®cation; but certainly the brightest and more recent MeV 26Al observations, can be a useful measure of the Galactic events do have supernova types assigned. All Type Ia events supernova rates. If a dominating origin of 26Al from massive are assumed to have a peak absolute bolometric magnitude of stars is adopted (Prantzos & Diehl 1996), then the g-ray data M ϭ Ϫ19.0. This standard candle assumption may be chal- combined with nucleosynthesis yields provide independent lenged on both observational (see, e.g., Saha et al. 1996) and measures of the massive star formation rate in the Galaxy. The theoretical (see, e.g., Hoȯich et al. 1996) grounds, but it remains core-collapse supernova rate determined in this way appears a useful ®rst approximation. Distances to individual supernovae consistent with various classical rate determination methods, are then calculated from the cataloged magnitudes by the stan- such as on Ha measurements and supernova records in ªsim- dard formula log10 D ϭ (m Ϫ M ϩ 5)/5, with no reddening cor- ilarº distant galaxies (Timmes et al. 1997). Crucial to such an rections applied. analysis is the integrated radioactive mass inferred from the g- ray measurements and attributed to massive stars and the spatial 2 Tsvetkov, Pavlyuk, & Bartunov 1997; www.sai.msu.su/groups/sn. distribution of the emission in the Galaxy. But given the wide 3 Cappellaro, Barbon, & Turatto 1997; www.pd.astro.it/supern.

1998 PASP, 110:637±659 656 DIEHL & TIMMES

All the various Type Ia models tend to produce peak 847 keV line ¯uxes in the 2±10 # 10Ϫ5 photons cmϪ2 sϪ1 range at

Dp ϭ 10 Mpc, reaching this maximum 75 days after the ex- plosion (Chan & Lingenfelter 1991; Kho∼khlov, MuÈller, & HoÈf- lich 1993; see Fig. 11). The peak 1.238 MeV line ¯ux is slightly smaller at the same distance (due to a smaller branching ratio), and tends to lie in the 1.5±6 # 10Ϫ5 photons cmϪ2 sϪ1 range. Here we simply assume all Type Ia supernovae have a peak Ϫ5 Ϫ2 Ϫ1 g-ray ¯ux of Fp ϭ 3 # 10 photons cm s at Dp ϭ 10 Mpc. Peak g-ray ¯uxes are then calculated by the ¯ux ratio F ϭ 2 Fp (Dp /D) . Catalog completeness, volumes sampled, and lim- itations of the assumptions are important concerns. These are discussed in detail by Timmes & Woosley (1997), who also report a similar analysis for core-collapse supernovae. A total of 90 Type Ia supernovae brighter than 16th apparent magnitude are listed in these catalogs (Fig. 15). As long-term search, discovery, and classi®cation programs continue to ma- ture, the signi®cance of average detection and classi®cation rate being roughly constant ( 5 yrϪ1 for a 16th apparent mag- nitude cut) may be evaluated∼. These implied rates are lower bounds for several reasons; a large visual extinction by dust from the parent galaxy may hide events, search programs are becoming more ef®cient, and unclassi®ed events may include some Type Ia supernovae. The apparent magnitude distribution for the 81 events dis- covered and identi®ed as Type Ia supernova since 1966 are shown with the -scale histogram and right y-axis in Figure 15. Also shown are the six brightest Type Ia supernovae that occurred within this time span. The apparent magnitudes may be converted into distances and peak g-ray ¯ux values (see top and bottom ordinates) by making the standard candle assump- Fig. 15.ÐTop: Number of events discovered and identi®ed as Type Ia su- pernovae vs. the year of the explosion. Bottom: Gamma-ray ¯ux sensitivity tions described above. Integrating the histogram and normal- of 1966±1996 Type Ia events (adapted from Timmes & Woosley 1997). izing to the time frames' average Type Ia supernova rate gives the g-ray event rate as a function of the g-ray line ¯ux. This can be compared with instrumental sensitivities. For example, ray lines, will take considerably more sensitivity. These rate a next-generation instrument such as the proposed ATHENA estimates should be regarded as lower bound guidelines, to aid, mission, which has a ¯ux sensitivity of 2 # 10Ϫ6 photons for example, in the design of future instruments. cmϪ2 sϪ1 to 5000 km sϪ1 broadened lines, sh∼ould easily observe Type Ia events in the Virgo (18.2 Mpc), Fornax (18.4 Mpc), 6. CONCLUDING REMARKS and possibly the Hydra (41 Mpc) galaxy clusters at a rate of The search for an overarching paradigm that encompasses 1 yrϪ1 (roughly 10%±30% of all Type Ia events within 100 measurements of g-ray emission from radioactive isotopes, the Mpc). The transparency of galaxies to g-rays may allow g-ray solar abundances, isotopic abundance measurements of presolar telescopes that posses suf®cient sensitivity to meteoritic grain, stellar nucleosynthesis calculations, and Ga- provide a direct measure of absolute supernova rates. INTE- lactic chemical evolution has seen some major advances during GRAL's spectrometer sensitivity of 6 # 10Ϫ6 photons cmϪ2 the last decade. Data obtained with the CGRO and ion micro- sϪ1 (Winkler 1995), for a 10 day obs∼ervation, could detect all probes in meteoritic analyses have provided key steps on this Type Ia supernovae out to 20 Mpc and allow a rate near 1 journey. Yet, our search remains unful®lled. Our goal is noble: Type Ia event yrϪ1. Instrume∼nt with broad line ¯ux sensitivities to explain the origin of every isotope in every location (and larger than 1.5 # 10Ϫ5 photons cmϪ2 sϪ1, such as present-day time) in the universe. The next few steps toward this goal instruments, are probably limited to detecting Type Ia super- include accurate determination of the Galactic 26Al and 60Fe novae within 10 Mpc. These numbers indicate the minimum masses, ®nding the exceptional events that eject 44Ti, 60Fe, 7Be, sensitivity for∼detection. Actually studying the emission in de- and 22Na, and putting the connection with the yields of these tail so as to learn, e.g., about the physics of the explosion by isotopes from stellar nucleosynthesis on solid ground. New the velocity structure and velocity distribution of the 56Ni g- instrumentation, analysis methods, and theoretical advances,

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 657 with their often con¯icting constraints, will yield exciting ad- Lingenfelter, Juan Naya, Nikos Prantzos, Reuven Ramaty, Stan ventures in these future studies. Woosley, and Ernst Zinner. This work has been supported by the Max Planck Gesellschaft (R. D.), the National Science Foundation under grant PHY94-07194 to the Institute for The- Useful discussions with many colleagues are re¯ected in this oretical Physics at UC Santa Barbara (R. D. and F. X. T.), and article, in particular Don Clayton, Claes Fransson, Dieter Hart- a Compton Gamma Ray Observatory Postdoctoral Fellowship mann, Rob Hoffman, Peter Hoȯich, Mark Leising, Richard (F. X. T.).

REFERENCES

Ahmad, I., et al. 1998, Phys. Rev. Lett., in press Dupraz, C., et al. 1997, A&A, in press Allen, S. J. 1911, Phys. Rev., 34, 296 Dwek, E., Moseley, S. H., Glaccum, W., Graham, J. R., Loewenstein, Anders, E., & Zinner, E. 1993, Meteoritics, 28, 490 R. F., Silverberg, R. F., & Smith, R. K. 1992, ApJ, 389, 21 Arnett, W. D. 1977, Ann. NY Acad. Sci., 302, 90 Ellison, D. C., et al. 1994, PASP, 106, 780 Arnett, W. D., Bahcall, J. N., Kirshner, R. P., & Woosley, S. E. 1989, Elmegreen, B., G., & Efremov, Y. N. 1996, ApJ, 466, 802 ARA&A, 27, 629 Feather, N. 1973, Lord Rutherford (London: Priory Press) Arnould, M., Paulus, G., & Meynet, G. 1997, A&A, 321, 452 Fesen, R. A., & Becker, R. H. 1991, ApJ, 371, 621 Aufderheide, M., Baron, E., & Thielemann, F.-K. 1991, ApJ, 370, 630 Fransson, C., & Kozma, C. 1993, ApJ, 408, L25 Badash, L. B. 1979, Radioactivity in America: Growth and Decay of ÐÐÐ. 1998, in The Fifth CTIO/ESO/LCO Workshop, SN 1987A: a Science (Baltimore: Johns Hopkins Univ. Press) Ten Years After, ed. M. M. Phillips & N. B. Suntzeff (San Francisco: Bazan, G., & Arnett, D. 1994, ApJ, 433, L41 ASP), in press Becquerel, H. 1896, Compt. Rend., 122, 420 Gallino, R., Busso, M., Picchio, G., & Raiteri, C. M. 1990, Nature, ÐÐÐ. 1900, Compt. Rend., 130, 809 348, 298 Birck, J. L., & Lugmair, G. W. 1988, Earth Planet. Sci. Lett., 90, 131 Gehrels, N., & Chen, W. 1996, A&AS, 120, 331 Blake, J. B., & Dearborn, D. S. P. 1989, ApJ, 338, L17 Gehrels, N., Fichtel, C. E., Fishman, G. J., Kurfess, J. D., & SchoÈn- Bof®n, H. M. J., Paulus, G., Arnould, M., & Mowlavi, N. 1993, A&A, felder, V. 1993, Sci. Am., 269, 68 279, 173 Glanz, J. 1997, Science, 276, 1789 Borkowski, K. J., Szymkoviak, A. E., Blondin, J. M., & Sarazin, Glasner, S. A., Livne, E., & Truran, J. W. 1997, 475, 754 C. L. 1996, ApJ, 466, 866 GoÈrres, J., et al. 1998, Phys. Rev. Lett., in press Bouchet, P., & Danziger, I. J. 1993, A&A, 273, 451 Grabelsky, D. A., Cohen, R. S., Bronfman, L., Thaddeus, P., & May, Braun, H., & Langer, N. 1995, A&A, 297, 483 J. 1987, ApJ, 315, 122 Brown, B. L., Leventhal, M., & Mills, A. P., Jr. 1986, Phys. Rev. A, Harper, C. E. 1996, ApJ, 466, 1026 33, 2281 Harris, M. J. 1998, in AIP Conf. Proc. 410, Fourth Compton Symp. Brown, L. E., & Clayton, D. D. 1992, Science, 258, 970 on Gamma-Ray Astronomy and Astrophyics, ed. C. Dermer, J. Burbidge, E. M., Burbidge, G. R., Fowler, W. A., & Hoyle, F. 1957, Kurfess, & M. Strickman (New York: AIP), 1079 Rev. Mod. Phys., 29, 547 Harris, M. J., Leising, M. D., & Share, G. H. 1991, ApJ, 375, 216 Burrows, A., Hayes, J., & Fryxell, B. A. 1995, ApJ, 450, 830 Hartmann, D. H., et al. 1997, Nucl. Phys. A, 621, 83 Bussard, R. W., Ramaty, R., & Drachman, R. J. 1979, ApJ, 228, 928 Haymes, R. C., Walraven, G. D., Meegan, C. A., Hall, R. D., Djuth, Cameron, A. G. W. 1957, Chalk River Report, CRL-41 F. T., & Shelton, D. H. 1975, ApJ, 201, 593 ÐÐÐ. 1993, in Protostars and Planets III, ed. E. H. Levy & J. Lunine Hernanz, M., JoseÂ, J., Coc, A., & Isern, J. 1996, ApJ, 465, L27 (Tucson: Univ. Arizona Press), 47 Higdon, J. C., & Fowler, W. A. 1987, ApJ, 317, 750 Chan, K., & Lingenfelter, R. 1991, ApJ, 368, 515 Hix, W. R., & Thielemann, F. K. 1996, ApJ, 460, 869 Chen, W., et al. 1997, in The Transparent Universe: Proc. Second Hoffman, R. D., Woosley, S. E., Weaver, T. A., Timmes, F. X., East- INTEGRAL Workshop, ed. C. Winkler, T. J.-L. Courvoisier, & P. man, R. G., & Hartmann, D. H. 1995, in The Gamma-Ray Sky Durouchoux (ESA SP-382), 105 with Compton GRO and SIGMA, ed. M. Signore, P. Salati, & G. Clayton, D. D. 1971, Nature, 234, 291 Vedrenne (Dordrecht: Kluwer), 267 ÐÐÐ. 1982, in Essays in Nuclear Astrophysics, ed. C. A. Barnes, Hoȯich, P., Khokhlov, A., Wheeler, J. C., Phillips, M. M., Suntzeff, D. D. Clayton, & D. N. Schramm (Cambridge: Cambridge Univ. N. B., & Hamuy, M. 1996, ApJ, 472, 81 Press), 401 Holt, S. S., Gotthelf, E. V., Tsunemi, H., & Negoro, H. 1994, PASJ, Clayton, D. D., Colgate, S. A., & Fishman, G. 1969, ApJ, 220, 353 46, L151 Clayton, D. D., & Leising, M. D. 1987, Phys. Rep., 144, 1 Hoppe, P., Strebel, R., Eberhardt, P., Amari, S., & Lewis, R. S. 1996, Clayton, D. D., Leising, M. D., The, L.-S., Johnson, W. N., & Kurfess, Science, 272, 1314 J. D. 1992, ApJ, 399, L141 Hoyle, F., & Clayton, D. D. 1974, ApJ, 191, 705 Coc, A., Mochkovitch, R., Oberto, Y., Thibaud, J.-P., & Vangioni- Iben, I., Jr., & Livio, M. 1993, PASP, 105, 1373 Flam, E. 1995, A&A, 299, 479 Iyudin, A. F. 1997, in Proc. Second INTEGRAL Workshop, The Trans- Compton, A. H. 1929, Naturwissenschaften, 17, 507 parent Universe, ed. C. Winkler, T. J.-L. Courvoisier, & P. Du- Dame, T. M., et al. 1987, ApJ, 322, 706 rouchoux (ESA SP-382), 37 del Rio, E., et al. 1996, A&A, 315, 237 Iyudin, A. F., et al. 1994, A&A, 284, L1 Diehl, R., et al. 1995, A&A, 298, 445 Iyudin, A. F., et al. 1995, A&A, 300, 422 ÐÐÐ. 1998, in AIP Conf. Proc. 410, Fourth Compton Symp. on Iyudin, A. F., et al. 1997, in The Transparent Universe, ed. C. Winkler, Gamma-Ray Astronomy and Astrophyics, ed. C. Dermer, J. Kurfess, T. J.-L. Courvoisier, & P. Durouchox (ESA SP-382) (Noordwijk: & M. Strickman (New York: AIP), 1109 ESA), 39

1998 PASP, 110:637±659 658 DIEHL & TIMMES

Johnson, W. N. III, Harnden, F. R., Jr., & Haymes, R. C. 1972, ApJ, Myra, E. S., & Burrows, A. 1990, ApJ, 364, 222 172, L1 Naya, J. E., Barthelmy, S. D., Bartlett, L. M., Gehrels, N., Leventhal, JoseÂ, J., & Hernanz, M. 1998, ApJ, 494, 680 M., Parsons, A., Teegarden, B. J., & Tueller, J. 1996, Nature, 384, JoseÂ, J., Hernanz, M., & Coc, A. 1997, ApJ, 479, L55 44 Kessler, M. F., et al. 1996, A&A, 315, L27 Naya, J., Barthelmy, S. D., Gehrels, N., Parsons, A., Teegarden, B., Khokhlov, A., MuÈller, E., & Hoȯich, P. 1993, A&A, 270, 223 Tueller, J., & Leventhal, M. 1998, in ApJ, in press Kinzer, R. L., Purcell, W. R., Johnson, W. N., Kurfess, J. D., Jung, Nagataki, S., Hashimoto, M., Sato, K., & Yamada, S. 1997, ApJ, 486, G., & Skibo, J. 1996, A&AS, 120, 317 1026 Kniffen, D. A., Gehrels, N., & Fishman, G. 1998, in AIP Conf. Proc. Nichols, R. H., Jr., Amari, S., Hohenberg, C. M., Hoppe, P., & Lewis, 410, Fourth Compton Symp. on Gamma-Ray Astronomy and As- R. S. 1993, Meteoritics, 428, 410 trophyics, ed. C. Dermer, J. Kurfess, & M. Strickman (New York: Nittler, L. R., Amari, S., Zinner, E., Woosley, S. E., & Lewis, R. S. AIP), 524 1996, ApJ, 462, L31 KnoÈdlseder, J. 1998, Ph.D. thesis, CESR/UPS, Toulouse, France Norman, E. B., et al. 1998, in Second Oak Ridge Symp. on Atomic KnoÈdlseder, J., Bennett, K., Bloemen, H., Diehl, R., Hermsen, W., and Nuclear Astrophysics, ed. A. Mezzacappa (Bristol: IOP), in Oberlack, U., Ryan, J., & SchoÈnfelder, V. 1996a, A&AS, 120, 327 press KnoÈdlseder, J., et al. 1998, in Lecture Notes in Physics 506, The Local Oberlack, U. 1998, Ph.D. thesis, Tech. Univ., MuÈnchen Bubble and Beyond, ed. D. Breitschwerdt, Mj. Freyberg, & J. TruÈm- Oberlack, U., et al. 1994, ApJS, 92, 433 per (IAU Collq. 166) (Berlin: Springer), 389 Ott, U. 1993, Nature, 364, 25 KnoÈdlseder, J., et al. 1996b, A&AS, 120, 335 Kolb, U., & Politano, M. 1997, A&A, 319, 909 PoÈppel, W. 1997, Fund. Cosmic Phys., 18, 1 Kurfess, J. D., et al. 1992, ApJ, 399, L137 Politano, M., Starr®eld, S., Truran, J. W., Weiss, A., & Sparks, W. M. Kurfess, J. D., et al. 1998, Low/Medium Energy Gamma-Ray Astro- 1995, ApJ, 448, 807 physics Mission Workshop, internal report Prantzos, N. 1993a, ApJ, 405, L55 Lagage, P. O., Claret, A., Ballet, J., Boulanger, F., Cesarsky, C. J., ÐÐÐ. 1993b, A&A, 97, 119 Cesarsky, D., Fransson, C., Pollock, A. 1996, A&A, 315, L273 Prantzos, N., & Diehl, R. 1996, Phys. Rep., 267, 1 Langer, N., Fliegner, J., Heger, A., & Woosley, S. E. 1997, Nucl. Phys. Predehl, P., & Schmitt, J. H. M. M. 1995, A&A, 293, 889 A, 621, 183 Ramaty, R., & Lingenfelter, R. E. 1977, ApJ, 213, L5 Lee, T., Papanastassiou, D. A., & Wasserburg, G. J. 1977, ApJ, 211, ÐÐÐ. 1995, in The Analysis of Emission Lines: A Meeting in Honor L107 of the 70th Birthdays of D. E. Osterbrock & M. J. Seaton (Cam- Leising, M. D. 1993, A&AS, 97, 299 bridge: Cambridge Univ. Press), 180 Leising, M. D., & Clayton, D. D. 1987, ApJ, 323, 159 Ramaty, R., Skibo, J. G., & Lingenfelter, R. E. 1994, ApJS, 92, 393 Leising, M. D., et al. 1995, ApJ, 450, 805 Ritossa, C., Garcia-Berro, E., & Iben, I., Jr. 1996, ApJ, 460, 489 Leising, M. D., & Share, G. H. 1994, ApJ, 424, 200 Rolfs, C. E., & Rodney, W. S. 1988, Cauldrons in the Cosmos (Chi- Lewis, R. S., Amari, S., & Anders, E. 1993, Geochim. Cosmochim. cago: Univ. Chicago Press) Acta, 289, 970 Rona, E. 1978, How It Came About: Radioactivity, Nuclear Physics, Lichti, G. G., et al. 1994, A&A, 292, 569 Atomic Energy (Oak Ridge: Oak Ridge Associated Univ.) Lingenfelter, R. E., Chan, K. W., & Ramaty, R. 1993, Phys. Rep., Rothschild, R. E., et al. 1997, in AIP Conf. Proc. 410, Fourth Compton 227, 133 Symp. on Gamma-Ray Astronomy and Astrophyics, ed. C. Dermer, Lingenfelter, R. E., & Ramaty, R. 1978, Phys. Today, 31, 40 J. Kurfess, & M. Strickman (New York: AIP), 1089 ÐÐÐ. 1989, ApJ, 343, 686 Rutherford, E. 1899, Phil. Mag., 47, 109 Livne, E., & Arnett, D. 1995, ApJ, 452, 62 ÐÐÐ. 1905, Radio-activity (Cambridge: Cambridge Univ. Press) Livne, E., & Glasner, A. S. 1991, ApJ, 370, 272 ÐÐÐ. 1919, Phil. Mag., 37, 581 Lyne, A. G., & Lorrimer, D. R. 1994, Nature, 369, 127 Saha, A., Sandage, A., Labhardt, L., Tammann, G. A., Macchetto, F. MacPherson, G. J., Davis, A. M., & Zinner, E. K. 1995, Meteoritics, D., & Panagia, N. 1996, ApJS, 107, 693 30, 365 Schaerer, D., Schmutz, W., & Grenon, M. 1997, ApJ, 484, L153 Mahoney, W. A., Ling, J. C., Jacobson, A. S., & Lingenfelter, R. E. SchoÈnfelder, V., et al. 1996, A&AS, 120, 13 1982, ApJ, 262, 742 Shankar, A., & Arnett, D. 1994, ApJ, 433, 216 Mahoney, W. A., Ling, J. C., Wheaton, W. A., & Higdon, J. C. 1992, Shigeyama, T., Kumagai, S., Yamaoka, H., Nomoto, K., & Thiele- ApJ, 387, 314 mann, F. K. 1993, A&AS, 97, 223 Meynet, G., & Maeder, A. 1997, A&A, 321, 465 Meynet, G., Arnould, M., Prantzos, N., & Paulus, G. 1997, A&A, Shu, F. H., Shang, H., Lee, T., Glassgold, A. E. 1997, BAAS, 190, 320, 460 4904 Mezzacappa, A., Calder, A. C., Bruenn, S. W., Blondin, J. M., Guidry, Shukolyukov, A., & Lugmair, G. W. 1993, Science, 259, 1138 M. W., Strayer, M. R., & Umar, A. S. 1998, ApJ, 493, 848 Simpson, J. A., & Connell, J. J. 1998, ApJ, 487, L85 Mor®ll, G. E., & Hartquist, T. W. 1985, ApJ, 297, 194 Smith, D. M., Purcell, W. R., & Leventhal, M. 1998, in AIP Conf. Morris, D. J., Bennet, K., Bloemen, H., Hermsen, W., Lichti, G., Proc. 410, Fourth Compton Symp. on Gamma-Ray Astronomy and McConnell, M. L., Ryan, J. M., & SchoÈnfelder, V. 1995, Proc. 17th Astrophyics, ed. C. Dermer, J. Kurfess, & M. Strickman (New York: Texas Symp. on Rel. Astrophys. and Cosm., ed. H. BoÈhringer, G. AIP), 28 E. Mor®ll, & J. E. TruÈmper (NY Acad. Sci., Vol. 759), 397 Starr®eld, S., et al. 1996, in ASP Conf. Ser. 99, Cosmic Abundances, ÐÐÐ. 1998, in AIP Conf. Proc. 410, Fourth Compton Symp. on ed. S. S. Holt & G. Sonneborn (San Francisco: ASP), 242 Gamma-Ray Astronomy and Astrophyics, ed. C. Dermer, J. Kurfess, Starr®eld, S., Shore, S. N., Sparks, W. M., Sonneborn, G., Truran, J. & M. Strickman (New York: AIP), 1084 W., & Politano, M. 1992, ApJ, 391, 71 Murthy, P. V. R., & Wolfendale, A. W. 1993, Gamma-Ray Astronomy Starr®eld, S., Truran, J. W., Politano, M., Sparks, W. M., Nofar, I., & (Cambridge: Cambridge Univ. Press) Shaviv, G. 1993, Phys. Rep., 227, 223

1998 PASP, 110:637±659 GAMMA-RAY EMISSION FROM RADIOACTIVE ISOTOPES 659

Suntzeff, N. B. 1998, in The Fifth CTIO/ESO/LCO Workshop, SN van den Bergh, S., & Pritchet, C. J. 1986, ApJ, 307, 723 1987A: Ten Years After, ed. M. M. Phillips & N. B. Suntzeff (San van der Hucht, K. A., Hidayat, B., Admiranto, A. G., Supelli, K. R., Francisco: ASP), in press & Doom, C. 1988, A&A, 199, 217 Suntzeff, N. B., Phillips, M. M., Elias, J. H., Walker, A. R., & Depoy, van der Hucht, K. A., et al. 1997, NewA, 2, 245 D. L. 1992, ApJ, 384, L33 Wasserburg, G. J., Gallino, R., Busso, M., & Raiteri, C. M. 1995, Tammann, G. A., LoÈf¯er, W., & SchroÈder, A. 1994, ApJS, 92, 487 ApJ, 440, L101 Taylor, J. H., & Cordes, J. M. 1993, ApJ, 411, 674 Weiss, A., & Truran, J. W. 1990, A&A, 238, 178 The, L. S., et al. 1995, ApJ, 444, 244 Williams, B. F., Schmitt, M. D., & Winkler, P. F. 1995, BAAS, 186, The, L. S., Leising, M. D., Kurfess, J. D., Johnson,W. N., Hartmann, 4911 D. H., Gehrels, N., Grove, J. E., & Purcell, W. R. 1996, A&AS, Winkler, C. 1995, Exp. Astron., 6, 71 120, 357 Winkler, C., et al. 1997, in The Transparent Universe: Proc. Second Thielemann, F.-K., Nomoto, K., & Hashimoto, M. A. 1996, ApJ, 460, INTEGRAL Workshop, ed. C. Winkler, T. J.-L. Courvoisier, & P. 408 Durouchoux (ESA SP-382), 105 Thielemann, F.-K., Nomoto, K., & Yokoi, Y. 1986, A&A, 158, 17 Woosley, S. E., Arnett, W. D., & Clayton, D. D. 1973, ApJS, 26, 231 Timmes, F. X., Diehl, R., & Hartmann, D. H. 1997, ApJ, 479, 760 Woosley, S. E., Hartmann, D. H., Hoffman, R. D., & Haxton, W. C. Timmes, F. X., & Woosley, S. E. 1997, ApJ, 489, 160 1990, ApJ, 356, 272 Timmes, F. X., Woosley, S. E., Hartmann, D. H., Hoffman, R. D., Woosley, S. E., Langer, N., & Weaver, T. A. 1995, ApJ, 448, 315 Weaver, T. A., & Matteucci, F. 1995, ApJ, 449, 204 Woosley, S. E., Pinto, P. A., & Hartmann, D. H. 1989, ApJ, 346, 395 Timmes, F. X., Woosley, S. E., Hoffman, R. D., & Hartmann, D. H. Woosley, S. E., & Timmes, F. X. 1996, Nucl. Phys. A, 606, 137 1996a, ApJ, 464, 332 Woosley, S. E., & Weaver, T. A. 1994, ApJ, 423, 371 Timmes, F. X., Woosley, S. E., & Weaver, T. A. 1996b, ApJ, 457, ÐÐÐ. 1995, ApJS, 101, 181 834 Zinner, E. 1997, in Astrophysical Implications of the Laboratory Study Tutukov, A. V., Yungelson, L. R., & Iben, I., Jr. 1992, ApJ, 386, 197 of Presolar Materials, ed. T. J. Bernatowicz & E. Zinner (New York: van den Bergh, S., & McClure, R. D. 1994, ApJ, 425, 205 AIP), 3

1998 PASP, 110:637±659