Galactic Gamma-Ray Line EmissionFrom Radioactive IsotopesRoland DiehlMax-Planck Institut f�ur extraterrestrische PhysikGiessenbachstr. 1, D-85740 Garching, GermanyFrank X. TimmesAstronomy & AstrophysicsUniversity of California, Santa Cruz CA 95064, USAAbstract. Measurements of the Galactic -ray sky in lines attributed to theradioactive decay of 7Be, 22Na, 26Al, 44Ti, 56Ni, and 60Fe are collected, organized,and collated with models for the production sites/mechanisms, spatial distribu-tions, and the chemical evolution of these isotopes.INTRODUCTIONRadioactivity was discovered a little more than a century ago when HenriBecquerel included potassium and uranium sulfates as part of a photographicemulsion mixture (Becquerel 1896). He soon found that all uranium com-pounds and the metal itself were \light sources", the intensity was propor-tional to the amount of uranium present, with chemical combination havingno e�ect. Two years later, Pierre and Marie Curie coined the term \radioac-tive" for those elements that emitted such \Becquerel-rays". A year later,Ernest Rutherford demonstrated that at least three di�erent kinds of radi-ation are emitted in the decay of radioactive substances. He called these\alpha", \beta", and \gamma" rays in an increasing order of their ability topenetrate matter (Rutherford 1899, Feather 1973). It took a few more yearsfor Rutherford and others to conclusively show the alpha rays were identicalwith the nuclei of helium atoms (Rutherford 1905; Rona 1978), and the betarays were electrons (Becquerel 1900; Badash 1979). By 1912 it was shownthat the gamma rays had all the properties of very energetic electromagneticradiation (e.g., Allen 1911).c 1997 American Institute of Physics 1

2 We now understand radioactive decay as transitions between di�erent statesof atomic nuclei, transitions that are ultimately attributed to electroweak in-teractions. Measurement of the decay products has grown into an importanttool of experimental physics: On Earth, it forms the basis of radioactive dat-ing through high precision isotopic analysis, in tree rings, terrestrial rocks,and meteoritic samples, to name just a few (Rolfs & Rodney 1989). Dis-tant radioactive material throughout the universe may be studied in detail bymeasuring the material's gamma-ray line spectrum. These gamma-ray linesidentify a speci�c isotope, and the abundance of the distant material directlyrelates to the measured gamma-ray line intensity (Clayton 1982). These -rays are also una�ected by the intervening matter once the radioactive nucleushas left its dense production site and gone into the interstellar medium. Thecharacteristic half-life constitutes an exposure time scale of the sky in a spe-ci�c -ray line. Given reasonable event frequencies and stellar nucleosynthesisyields, the cumulative radioactivity from many events (26Al, 60Fe, and to alesser extent 22Na and 44Ti), and individual events (44Ti, 56Ni, and perhaps7Be, 22Na) can be examined.By the late 1970s various international collaborations had launched exper-iments on various stratospheric balloons and space satellites and to explorecosmic rays, X-ray, and -ray sources (Murthy & Wolfendale 1993). The �rstgamma-ray line discovery had been reported (Haymes et al. 1975), severaldiagnostic lines were reported in the years after. Following these pioneeringmissions the Gamma Ray Observatory was set out to explore the gamma-raysky for the �rst time over a wide range of gamma-ray energies, including theregime of nuclear lines from radioactivity. A delightful summary of the his-tory and relationship to the previous missions by Kni�en, Gehrels, & Fishman(1997) complements the description of the Observatory goals, the instrumen-tation and �rst achievements by Gehrels et al. (1995). Today collaborativeinternational agencies have numerous ongoing and planned projects to deepenspeci�c studies in the area of gamma-ray astronomy (ref. recent workshopproceedings edited by Winkler et al. (1997) and Kurfess et al. (1997)).In the following review, attention is chie y focused on the Milky Way'sgamma-ray line emission that originates from the radioactive isotopes 7Be,22Na, 26Al, 44Ti, 56Ni, and 60Fe.PRODUCTION OF RADIOACTIVE ISOTOPESThe nucleosynthetic processes in stars responsible for the production of26Al have been summarized by Clayton & Leising (1987), Prantzos & Diehl(1996), and MacPherson, Davis & Zinner (1995). In stars, 26Al is produced byproton capture reactions, mainly on 25Mg, and is destroyed by e+ decay, (n,p),(n,�), and (p, ) reactions. The �nal yield from any source is temperaturesensitive. 26Al can be produced during: (1) hydrostatic hydrogen burning in

3the core of massive (� 11 M�) stars; (2) hydrostatic hydrogen burning inthe hydrogen shell of low- and intermediate mass (� 9 M�) stars while onthe asymptotic giant branch; (3) explosive hydrogen burning (temperatures> 2�108 K) in massive stars and on the surfaces of white dwarfs (i.e., novae);and (4) hydrostatic and explosive carbon and neon burning in massive stars.Any 26Al produced by stars is ejected in part by strong winds (Wolf-Rayet,asymptotic giant branch stars), and in full amount by explosions (supernovaand nova). Besides being formed in stars, 26Al can also in principle be producedby spallation reactions of high-energy cosmic rays on a range of nuclei (mainlysilicon, aluminum and magnesium), although at substantially lower e�ciency.Most of the di�erent ways to synthesize 26Al, Type II supernovae, Wolf-Rayet stars, AGB stars, and classical novae, can all, at least according totheir respective protagonists, produce su�cient quantities Galaxy-wide. Sothey cannot all make � 2 M�, or else there would be too much 26Al in theGalaxy. Each prospective source has it's advantages and di�culties. Wediscuss their nature brie y, addressing a few of the physical aspects involved.The treatment of convection, and its implications, constitutes one problemarea common to several source types. The convective coupling between masszones in both the oxygen-neon and hydrogen shell burning regions of mas-sive stars can simultaneously bring light reactants and seed nuclei into thehot zone to aid in the synthesis, and through the same process remove thefragile product from the high temperature region where it might otherwise bedestroyed. Convective burning in the oxygen-neon shell of a 20 M� star hasbeen modeled in two-dimensions by Bazan & Arnett (1994). They �nd thatlarge-scale plume structures dominate the velocity �eld, physically caused bythe inertia contained in moving mass cells. As a result, signi�cant mixingbeyond the boundaries de�ned conventionally by mixing-length theory (con-vective overshoot) brings fresh fuel into the convective region causing localhot spots of nuclear burning. This is very di�erent from the situation encoun-tered in spherically symmetric computations. Additionally, chemical inhomo-geneities create gradients of chemical composition which result in additionalmixing (semi-convection). Large scale mass motions and local burning condi-tions are likely to change the quantitative yields of many isotopes from anysingle massive star. However, any non-monotonic and/or stochastic nature ofthe nucleosynthetic yields as a function of stellar mass tend to be smoothedout by integration over an initial mass function of the stellar population con-tributing to the observable gamma-ray emission. Thus, the general features ofthe integrated yields, as determined from spherically symmetric models, mayremain intact.Rotation may have also have large systematic e�ects. In massive stars, theamount of mixing in the hydrogen envelope is increased, thus processing more25Mg into 26Al. The helium core mass may be larger as a result, thus leadingto more 26Al in the larger neon-oxygen layers (Meynet & Maeder 1997; Langeret al. 1997). Rotating or not, complete loss of the hydrogen and helium

4layers, either through mass loss as a single star or through the e�ects of closebinary evolution (Braun & Langer 1995), could a�ect the quantitative yieldsof most of the presupernova abundances. Recent improvement of the physicalingredients in Wolf-Rayet 26Al production models provide some consolidationof hydrostatic nucleosynthesis in massive stars (Arnould et al. 1997; Meynetet al. 1997). These studies point out that combinations of metallicity andmass loss can increase the mass of 26Al ejected by � 2. The 26Al abundancepresent in the hydrogen shell is ejected unmodi�ed in the explosion or, in moremassive stars during their Wolf-Rayet phase, by a stellar wind.The 26Al synthesized in the oxygen-neon shell may be enhanced by � 30%due to operation of the neutrino-process (Woosley et al. 1990). Protonsliberated by �-spallation of 20Ne capture on 25Mg to produce 26Al. When theoxygen-neon shells are located closer to the collapsing core, a larger higher�- ux is encountered. This may enhance the 26Al yield by � 50%. Therelative importance of the �-process contribution depends on the � energyspectrum: the peak � and � neutrino temperature is somewhat uncertain(Myra & Burrows 1990), in spite of the SN 1987A measurements (Arnett etal. 1989). Few transport calculations have been carried long enough (at least3 s) with su�cient energy resolution to see the hardening of the neutrinospectrum that occurs as the proto-neutron star cools.The genesis of the central compact remnant in core-collapse supernovaebears on several interesting physical problems, some of which may be con-strained by radioactivity observations. Below main sequence masses of 19 M�,formation of a neutron star appears likely. Modern nuclear equations of statesuggest that above 19 M�, the neutron star becomes a black hole. The en-ergy of the explosion, placement of the mass cut, and how much mass fallsback onto the remnant can all modify this neutron star - black hole bifur-cation point. Each of these processes also dramatically a�ects the mass ofthe nucleosynthesis products ejected from the inner regions of the supernova.For example, producing more than 10�4 M� of radioactive 44Ti is di�cult inmodels, but making much less is easy. Ejection of any 44Ti is sensitive to howmuch mass falls back onto the remnant. To achieve a nearly constant kineticenergy of the ejecta, the explosion energy in M�25 M� presupernova modelsmust be steadily increased in order to overcome the increased binding energyof the mantle. However, even in the 35 and 40 M� stars the density pro�lemay still cause nearly all the 44Ti produced to fall back onto the compact rem-nant. Unless the explosion mechanism, for unknown reasons, provides a muchlarger characteristic energy in more massive stars, it appears likely that starslarger than about 30 M� will have dramatically reduced 44Ti yields and leavemassive remnants (M�10 M�) which become black holes. If, however, theexplosion is energetic enough (perhaps overly energetic) more 44Ti is ejected.The genesis and evolution of an accreting white dwarf which eventuallybecomes a classical novae is essentially unknown, yet has important conse-quences on the Galactic -ray signal (Kolb & Politano 1997). For example,

5uncertainty in the binary mass distribution function for nova progenitors cor-responds to an uncertainty in the fraction of white dwarfs that originate from�8 M� main-sequence stars. Detection of strong neon lines in nova envelopeshas suggested the category "neon-nova" (Starr�eld et al. 1996). Derived fromabout a dozen well-measured nova events, the fraction of metal-enriched neon-novae is typically quoted as 25-30%. The material composition of the nuclearburning region is usually assumed to be a 50-50 mixture of accreted material(solar composition) and dredged up white dwarf material. Typically the whitedwarf material is assumed to oxygen-neon-magnesium in mass proportions of0.3:0.5:0.2 (e.g. Politano et al. 1995). Evolution of a 10 M� star, however,suggests mass ratios 0.5:0.3:0.01 might be more appropriate (Ritossa et al.1996). If con�rmed, this eliminates most of the necessary seed material fromwhich 26Al is synthesized, since the nova yields of 26Al and 22Na are sensi-tive to the initial 25Mg and 20Ne abundances. Inconsistencies between themass ejected by neon-novae models and observed total ejected masses (mod-els are too low by an order of magnitude, e.g. Hernanz et al. 1996) showthat our understanding of the nova process is incomplete, in spite of the �rst-order success of the thermonuclear runaway model. If the white dwarf hasM � 1.1 M�, then less accreted matter is needed to trigger a classical novaoutburst. Correspondingly, the amount of matter is expected to be less. If theinconsistency between model and observed ejected masses is resolved by use ofa M < 1.1 M� white dwarf, then the 26Al yields are expected to increase dueto the lower burning temperatures. If resolution depends upon larger massesbeing ejected through increased mixing of core material, then higher burningtemperatures is expected to decrease of mass of 26Al ejected, as compared topresent baseline models in the 1.15-1.35 M� range.Gamma-ray astronomy seeks to constrain the astrophysical origin site(s) ofradioactive isotopes in the Galaxy, and on smaller spatial scales correspond-ing to regions of coherent star formation. The di�erences between the derivedspatial distributions of the competing sources are often not large enough toprovide a crisp discriminant, especially since young population stars relate toseveral source types (Prantzos & Diehl 1996). Nevertheless, spatial resolu-tion of -ray line measurements in sub-degree domains should provide sucha test (e.g. AGB stars resulting in a 1.809 MeV glow trailing spiral arms).Are there other measurable quantities that might help distinguish betweenthe competing sources? Yes, one may look for the associated production ofother radioactive isotopes. For example, -rays from radioactive 60Fe may bea very good discriminant for the contested origin site of Galactic 26Al for twomain reasons: (1) Type II supernovae produce comparable amounts of 26Aland 60Fe, while the other potential candidates produce signi�cantly smalleramounts of 60Fe. (2) Even more importantly, the \26Al follows 60Fe" modelhas consequences that can be measured with present generation -ray spec-trometers. For individual events, on the other hand, -ray measurements canprovide speci�c constraints to detailed models, complementing other physical

6parameters which are often observed for these in great detail. We separatelydiscuss these two aspects of -ray line measurement impacts in the following.INTEGRATED NUCLEOSYNTHESISThe HEAO-C discovery of the 1.809 MeV gamma-ray line from radioactive26Al in the Galaxy (Mahoney et al. 1982), which had been anticipated fromtheoretical considerations (Clayton 1971, 1982; Ramaty & Lingenfelter 1977),opened new doors in gamma-ray astronomy. Measurements of various ra-dioactive decays in the Milky Way o�ers a global proof of the idea that heavyelement formation occurs continuously and mainly inside stars (Burbidge etal., 1957, Cameron 1957). In particular, radioactive decay times of �106 y(as for 26Al and 60Fe) probe a time scale which is short compared to galacticevolution, thus testifying to the occurrence of relatively recent nucleosynthe-sis events throughout the Galaxy. The 26Al and 60Fe isotopes provide thisunique observational window, as massive-star nucleosynthesis is expected toproduce these trace elements in quantities su�cient to yield -ray line uxesabove instrumental threshold sensitivities. For typical yields of �10�4 M�per event, approximately 10000 individual events will contribute to the lineemission from the Galaxy, resulting in a typical ux of �10�5 ph cm�2 s�1.Aluminum 26 in the GalaxyEight experiments over the 15 years since the pioneering detection have re-ported detecting -ray emission from radioactive 26Al (see review by Prantzos& Diehl 1996). Instrumental capabilities di�er substantially, but the inte-grated ux measured from the general direction of the inner Galaxy, integratedover latitude and the inner radian in longitude, has been used to roughly com-pare results. We summarize the ux values in Fig. 1, including the resultsreviewed by Prantzos & Diehl (1996) plus the recent GRIS measurement (Nayaet al. 1996). All measurements are consistent with values � 4�10�4 ph cm�2s�1/rad, as indicated by the horizontal line. Note that the determinationmethod varies between the instruments, and in particular the ux values forthe non-imaging instruments depend on the assumed spatial distribution: Allnon-imaging instruments essentially assume the same (or equivalent) smoothspatial distribution derived from COS-B measurements of Galactic gamma-rays in the 100 MeV regime. The distribution of 1.809 MeV emission seemssigni�cantly di�erent, however. Maximum entropy deconvolution images de-rived from the COMPTEL Telescope measurements show spatial structurein the emission (Fig. 2). The ridge of the Galactic plane dominates, butthere is asymmetry in the emission pro�le along the disk, and there are sev-eral prominent regions of emission such as Vela and Cygnus. All estimates ofthe absolute 26Al mass in the Galaxy rest on assumptions about the spatial

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FIGURE 1. Reported 1.809 MeV ux values for the inner Galaxy, from 8 experiments.The length of each vertical stripe is set by the reported uncertainties. A value of � 4�10�4ph cm�2s�1rad�1 appears plausible given all experiments, although some discrepancies willhave to be investigated.distribution of the sources, as the 1.809 MeV measurements themselves donot carry distance information. The COMPTEL team �tted several modelsof candidate source distributions to the COMPTEL data, such as CO surveydata (Dame et al. 1987), analytical models based on exponential disks, andH II region data evaluated in the context of spiral-arm structure (Taylor &Cordes 1993). When localized regions of emission beyond the inner Galaxyare excluded, then all axisymmetric model �ts yield a Galactic mass of 2-3 M�(Diehl et al. 1995, Kn�odlseder et al. 1996a, Diehl et al. 1997). The extentof spiral-arm emission can be estimated if a composite model of disk emis-sion plus emission along spiral-arms is adopted and compared to the disk-onlymodel. Spiral structure appears signi�cant, including between 1.1 M� and allof the 26Al (Diehl et al. 1997).Yet, imaging of MeV gamma-rays is far from straightforward, both due tothe high instrumental backgrounds, and the complex gamma-ray detectionmethods. Consistency checks between di�erent techniques have shown thatsome of the spikyness of the apparent emission in the COMPTEL result canbe instrumental (e.g., Kn�odlseder et al. 1996a). Nevertheless, signi�cantemission from the Cygnus, Carina, and Vela regions appears consolidated. Inthe Vela region, there was some hope to detect 26Al from one single source,the nearby Vela supernova remnant (Oberlack et al. 1994; Diehl et al. 1995).

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FIGURE 2. COMPTEL 1.8 MeV all-sky image, derived from 5 years of observationthrough Maximum Entropy deconvolution (Oberlack et al., 1997)Recent results show the main 1.809 MeV feature is signi�cantly o�set fromthe Vela supernova remnant. A direct calibration of core-collapse supernovanucleosynthesis, which was a tantalizing prospect two years ago (Diehl et al.1995), does not appear feasible as superimposed emission from other sourcesat larger distances must be considered signi�cant (Oberlack et al. 1997).Another candidate source in the Vela region is the Wolf Rayet binary system 2 Velorum, holding the closest known Wolf Rayet star WR11 (van der Hucht1988). Recent Hipparcos parallax measurements suggest this binary systemis at a distance of 250-310 pc, which is even closer than previous estimates of300-450 pc (van der Hucht et al. 1997; Schaerer, Schmutz & Grenon 1997).At this close of a distance the absence of a detection from 2 Velorum inthe COMPTEL data is unexpected, particularly since recent models suggestlarger yields for this object (Meynet et al. 1997).About 80% of the prominent 1809 keV emission associated with the Cygnusregion can be understood in terms of the expected 26Al signal from knownsources (del Rio et al. 1996). One may be worried about this high percentage,since 26Al decays on a time scale longer than the emission features of super-nova remnants and Wolf-Rayet winds used to make this inference. The 26Alattributed to 'seen' sources should be multiplied by a factor � 1 to accountfor 'unseen' sources. The latest COMPTEL images show structures whichappear consistent with the Cygnus superbubble and Cyg OB1, but furtheranalysis is required on this data to investigate the signi�cance of emissionfrom Wolf-Rayet stars in this region.The Carina region shows the densest concentration of young open clusters

9along the plane of the Galaxy. Kn�odlseder et al. (1996b) discuss how this con-centration may relate to the observed 1.809 MeV feature, which is spatiallymore con�ned than originally estimated from analysis of the Milky Way's spi-ral structure (Prantzos 1993ab). It is also interesting that some of the 1809keV image structures which fail to align with spiral arms, do coincide with di-rections towards nearby associations of massive stars (Diehl 1995; Kn�odlsederet al. 1997). Patchiness in such a nucleosynthetic snapshot might be expectedfrom the clustering of formation environments of massive stars (Elmegreen &Efremov 1996). If viewed from the outside, the Milky Way might also dis-play the signs of massive star populations in the form of HII regions arrangedlike beads on a string along spiral arms, such as observed in M31 from H�emission analysis (Williams et al. 1995), or in M51 from heated dust seen ininfrared continuum at 15 �m (Kessler et al. 1996). Interstellar absorptionand source confusion prevents such mapping within the Milky Way, unfortu-nately. Therefore, detailed investigations of the COMPTEL image systematicuncertainty, that is, a quantitative limit to arti�cial bumpiness of the imagingalgorithm, will be important for such interpretation of 1.809 MeV emission.Such concerns also hold for di�erent instruments and future measurements of� 20�segments of the Galactic plane with sub-degree resolutions.In a recent balloon ight, the Gamma Ray Imaging Spectrometer (GRIS)drift scanned the Galactic center region with its �100� �eld of view, and de-tected the 1.809 MeV line at 6.8� with a ux of 4.8�0.7 � 10�4 ph cm�2s�1/rad (Naya et al. 1996). The main surprise of this measurement is thewidth of the astrophysical line pro�le, which was signi�cantly broader thanthe instrumental resolution of the Ge detector, and reported as �E = 5.4�1.4keV. This line width is much larger than expected from Galactic rotation (�1 keV; Gehrels & Chen 1996), which dominate above the broadening fromrandom motions in the interstellar medium (Ramaty & Lingenfelter 1977). Itis presently di�cult to understand how such motion could be maintained overthe million year time scale of 26Al decay. Thermal broadening by a very hotphase of the interstellar medium (� 108 K) with long cooling times (� 105years), or a kinetic broadening at high average velocities (� 500 km/s) seemsrequired. Either case requires extremely low density phases of the interstellarmedium on large spatial scales (Chen et al. 1997). Alternatively one canhypothesize massive, high speed, dust grains rich in 26Al to explain the mea-surement (Chen et al. 1997). Further observations of the line shape detailsare required to examine any spatial variations in the line broadening. Spectralresolution of � 2 keV is required for such a study. Although INTEGRAL maystill have insu�cient energy resolution to make complete velocity maps of the1.809 MeV emission along the plane of the Galaxy, the brightest features canprobably have their velocity centroids determined well enough to place themon the Galactic rotation curve and thus derive a distance to the features. Sur-veys which combine velocity information and Galactic latitude extent couldthen examine the existence and nature of any Galactic \fountains" and \chim-

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Pre-SNFIGURE 3. Mass pro�les of 26Al (left) and 60Fe (right) for a 25 M� model.neys" from possible \venting" of 26Al into the Galactic halo. In any case, thelarge line width measured by GRIS, although inconsistent with the HEAO-Cline width limit of �3 keV, needs con�rmation, since it could have profoundimplications to our understanding of the interstellar medium in the Galaxy.Iron 60 in the GalaxyPhysically, 60Fe should be a good discriminant of di�erent source typesgenerating 26Al, because massive stars produce 26Al and 60Fe in the sameregions and in roughly comparable amounts (Fig. 3). Shown are abundancesas a function of mass inside a 25 M�, solar metallicity, Type II supernovamodel (Woosley & Weaver 1995) at the end of the presupernova evolution(dashed) and the �nal, post explosion abundances (solid curves; Timmes etal. 1995). While the 26Al production occurs in the hydrogen shell and theoxygen-neon shell, 60Fe is produced in He shell burning and at the base ofthe oxygen-neon shell. Most important is that the majority of both 26Al and60Fe are produced, mainly during the presupernova evolution, between 3-6 M�.These two isotopes should have similar spatial distributions after the explosionof the star.Certainly 26Al and 60Fe are produced in di�erent ratios in stars of di�erentmass (Fig. 4). Massive stars heavier than � 25 M� tend to synthesize more26Al than 60Fe in the Woosley & Weaver models, while the two isotopes areproduced in roughly equal amounts below 25 M�. An estimate for the injectionrate into the Milky Way is the steady-state event rate times the average massejected per event. Taking M(26Al) � 10�4 M�, M(60Fe) � 4�10�5 M� and �2 core-collapse supernovae per century, one has _M(26Al) � 2.0 M�/Myr and_M(60Fe) � 0.8 M�/Myr. More re�ned chemical evolution calculations suggest

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FIGURE 4. Mass of 26Al and 60Fe ejected versus main{sequence progenitor mass (left),assuming no mass loss. 26Al to 60Fe ratio: range predicted from supernova models (line)and measurements.that Type II supernovae are responsible for 2.2 � 1.1 M� of 26Al and 1.7 � 0.9M� of 60Fe in the Galaxy. These values, when coupled with the two isotopesbeing synthesized in the same physical region, gives rise to several predictions.When 60Fe is unambiguously detected with gamma-ray telescopes, the 60Fe uxmap will peak towards the inner Galaxy, the 60Fe emission will follow the 26Aldistribution (adding weight to assertions that Type II events make most ofthe Galactic 26Al), the 60Fe and 26Al hot spots will overlap, and the 60Fe/26Al ux ratio will be 16 � 10%. To be explicit, the 26Al image (see Fig. 2) will bethe 60Fe image; the 60Fe skymap will have the same morphology, but will bedimmer.A few recent 60Fe measurements and ux ratios with 26Al are shown inFig. 4. SMM reported an upper limit of 8.1�10�5 ph cm�2 s�1 for the 1.173MeV 60Co line over the central radian of Galactic longitude, giving an upperlimit of 1.7 M� of 60Fe (Leising & Share 1994). This 60Fe mass is consistentwith the expectations given above, and is � 20% of the 26Al ux. Recently, theGRIS (Naya et al. 1997), OSSE (Harris 1997), and COMPTEL (Diehl et al.1997) teams have reported upper limit 60Fe/26Al ratios. If the stringent GRISmeasurements are con�rmed, then the initial model estimates for the totalGalactic ux ratio might be too large. The qualitative picture of 26Al and 60Fetracing each other to a good degree of precision still presents an importantobservational challenge, even more so with a smaller ux ratio.

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FIGURE 5. 44Ti yields from Type II and Ib supernovae.SHORTER LIVED RADIOACTIVITIESTitanium 44In massive stars, stable 44Ca is produced chie y, though not exclusively,as radioactive 44Ti in the \�-rich freeze-out". This process occurs when ma-terial initially in nuclear statistical equilibrium, and a relatively low density,is cooled rapidly enough that the free �-particles do not have time to mergeback into the iron group by the relatively ine�cient triple-� reaction. Thus,the distribution of nuclei cools down in the presence of an anomalously largeconcentration of �-particles (Hix & Thielemann 1996). Representative yieldsof 44Ti from solar metallicity Type II and Type Ib supernova models are shownin Fig. 5 (Woosley & Weaver 1995; Woosley, Langer & Weaver 1995). Pro-duction in Type Ib supernovae is more uniform because the models convergeto a common presupernova mass in the narrow range 2.3�3.6 M�. All of themodels, whose ejecta all have � 1051 erg of kinetic energy at in�nity, predict44Ti yields between 1-15 �10�5 M�. Typical values are � 3�10�5 M� for theType II models, or twice that value for the Type Ib models.Thielemann, Nomoto & Hashimoto (1996) have also examined the detailednucleosynthesis of core collapse supernovae, and they �nd, in general, thatlarger amounts of 44Ti are ejected. The di�erences may be due to the nuclearreaction rates employed, or how the explosion is simulated, or the progenitorstructure. Thielemann et al. explode their stellar models by depositing ther-mal energy deep in the neutronized core (which later becomes the neutronstar). This gives a larger entropy to the innermost zones than what a mo-

13mentum (piston) driven explosion would impart and, in principle, eject morematerial. A larger entropy also ensures a more vigorous alpha�rich freezeout, and thus a larger 44Ti production. Thielemann et al. sum the ejecta fromthe outside of the star inwards and place the mass cut (which is arti�cial inall models) at the position where su�cient 56Ni is produced to explain theobservations. Woosley & Weaver place the piston at a suitable �rst approxi-mation to mass cut and then follow an explosion trajectory. It is the di�erencebetween injecting momentum or energy in modeling the explosion that leadsto the two groups following di�erent adiabatic paths (Aufderheide, Baron &Thielemann 1991). What self-consistent explosion models do, exploding vianeutrino heating and multidimensional convection, might be re ective of onedimensional momentum driven explosions, one dimensional energy driven ex-plosions, or some intermediate case. Hence, the di�erences between the twogroups in the amount of 44Ti ejected is an example of the spread one obtainsdue to uncertainties in modeling the Type II explosion mechanism.There are 3 -ray lines one can use to examine or detect the decay of 44Ti;the 67.9 and 78.4 keV lines from the 44Sc de-excitation cascade, and the 1.157MeV line as 44Ca decays to it's stable ground state. These transitions areshown in Fig. 6 with their respective spins, parities, and energies. The half-life of 44Ti, used to translate observed -ray uxes into a supernova mass of44Ti, is surprisingly uncertain (Fig. 6). Values range from 39 to 66 years, witha trend towards around 44 years for methods determining the activity, andscattering around 58 years for methods following the decay curve. Why is thehalf-life so di�cult to measure? The basic reasons are that the number of 44Tinuclei one can obtain is small, and the half-life is large.The Cas A supernova remnant is relatively close (2.8�3.7 kpc), young (ex-plosion in 1668�1680) and wide (physical diameter � 4 pc), making it oneof the premier sites for studying the composition and early behavior of asupernova remnant. It may only be equaled as SN 1987A unfolds. The dis-covery of 1157 keV gamma-rays from the � 300 year-old Cas A supernovaremnant (Iyudin et al. 1994; Fig. 7) was a scienti�c surprise, because su-pernovae models had indicated � 3�10�5 M� of 44Ti would be ejected (seeFig. 5), which translates into a -ray intensity generally below instrument ux sensitivities. Recent analysis of COMPTEL data supports a 5� detectionof Cas A at 4.8 � 0.9 �10�5 ph cm�2 s�1 in the 1.157 MeV line, implying1.0-4.0�10�4 M� of 44Ti (Iyudin et al. 1997). Conversion of the measured ux into the mass limits takes into account the uncertainties in the 44Ti half-life, distance to the event, and the precise time of the explosion.Data taken with the OSSE instrument, when best-�t to all three 44Tigamma-ray lines, gives an (insigni�cant) ux value of 1.7 � 1.4 � 10�5 phcm�2 s�1 for Cas A (The et al. 1996). The uncertainty of systematic errorsin the COMPTEL ux determination, coupled with low OSSE upper limits,confused the community for a while, but the two instruments now appear toyield values that are consistent within their respective uncertainties (Iyudin et

14FIGURE 6. 44Ti decay scheme, and measurements of the half life.al. 1994, 1997; Sch�onfelder et al. 1996; The et al. 1995, 1996). Measurementsof the 68 and 78 keV photons by the Rossi X-Ray Timing Explorer (RXTE)result in a measured ux of 2.87 � 1.95 � 10�5 ph cm�2 s�1 (Rothschild etal. 1997). Due to activation of radioactive daughters by cosmic rays, however,the detection of 44Ti lines has been more di�cult than expected also for thisinstrument. Additional measurements are necessary.From the -ray observations one may constrain the 44Ti half-life by assumingthe supernova model yields, a distance to Cas A of 3.4 kpc, and the year of theexplosion as � 1680. This procedure tends to favor the larger 44Ti half-lives.The abundance of 44Ti and 56Ni as a function of mass inside a 25 M� star isshown in Fig. 8. The mass cut is shown as the solid vertical line. Everythinginterior to the mass cut becomes part of the neutron star, everything exteriormay be ejected, depending on how much mass falls back onto the neutron starduring the explosion. Regardless, if 44Ti is ejected, so is 56Ni (Timmes et al.1996). The isotope 56Ni is made with 44Ti, in the same mass zone regime,and is 3 orders of magnitude more abundant than 44Ti. A large quantityof 56Ni ejected means a bright supernova. How bright? An abundance of44Ti as large as reported (�10�4 M�) would probably mean the ejection of atleast 0.05 M� of 56Ni. With or without a hydrogen envelope, the supernovawould then have a peak luminosity brighter than the 1042 erg/s observedfor SN 1987A. If no interstellar absorption would attenuate the optical lightcurve, the Cas A supernova should have had a peak apparent magnitude of

15 Cas A COMPTEL Phase 1-5

Energy (keV)

Cou

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-100

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FIGURE 7. 44Ti measurement from Cas A (Iyudin et al. 1997)-4, easily recognizable on the sky. Cas A was not widely reported as such;some 10 magnitudes of visual extinction is required to make the -ray 44Timeasurements consistent with these historical records.There may indeed have been such a large visual extinction to Cas A at thetime of the explosion. At a distance of � 3 kpc the optical extinction in theplane of the Galaxy has long been estimated to be 4-6 magnitudes. If Cas Awas embedded in a dusty region or experienced signi�cant mass loss whichcondensed into dust grains before the explosion, the extinction could havebeen even larger. Measurements of the X-ray scattering halo around Cas Afrom the ROSAT and ASCA satellites o�er some compelling evidence for a1.8 2 2.2 2.4 2.6

-4

-3

-2

-1

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log

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ut

FIGURE 8. 44Ti and 56Ni mass pro�les for a 25 M� core collapse supernova model.

16larger reddening correction (NH=1.8�1022 cm�2 attributed to an additionalmaterial component that must be close to Cas A, and relatively dust-free (Pre-dehl & Schmitt 1995). This corresponds to an additional optical extinctionof AV=5 at the time of the supernova, just about what is required. Whilehypotheses such as the explosion took place in a dense molecular cloud, or aoptically thick cloud occulted Cas A at the time of the explosion cannot beruled out, secondary evidence suggest these scenarios are unlikely. A perhapsmore natural hypothesis for this local material is the dusty shell of materialejected prior to the explosion as a Type Ib supernova. The supernova shockwave could have destroyed the dust as it propagated through the debris andthe material surrounding the Cas A supernova. This may have resulted ina dimmer supernova, explaining the lack of optical detection, and it wouldnaturally explain the excess neutral hydrogen column density (Hartmann etal. 1997).The present nature of the compact remnant in Cas A is ambiguous, withthe actual mass at the onset of carbon burning being critical. Deep X-rayimaging around Cas A does not reveal the synchrotron nebula one mightexpect around a neutron star (Ellison et al. 1994). Deep infrared imagestaken under excellent seeing conditions have set strict magnitude limits onthe presence of a stellar remnant (van den Bergh & Pritchet 1986; Fesen &Becker 1991). The region around the center of Cas A simply appears void ofany detectable stars. So observationally, a black hole may have formed. Thiswould make the ejection of a lot of 44Ti more di�cult, but not an impossiblyrare event. The black hole mass would need to be not too far above the criticalgravitational mass, probably less than 2 M�.Beryllium 7 and Sodium 22Classical novae are the most plausible sources for detection of the 478 keVgamma-ray line from the decay of radioactive 7Be, and the 1.275 MeV gamma-ray line from the decay of 22Na. Other sources such as red giant stars andsupernovae also synthesize 7Be, but the isotope cannot be transported withsu�cient rapidity into regimes that are transparent to -rays. The explo-sion of a massive star also produces 22Na in potentially detectable amounts,but classical novae remain a favored candidate due to their relative proximity(known Galactic events) and their relative frequency of occurrence (� 30/yearin the Galaxy). Each of these two isotopes probe di�erent phases of the ther-monuclear runaway model. Both have also been a target of -ray astronomyfor quite some time (Hoyle & Clayton 1974).Nuclei of mass A=7 are formed predominantly through the 3He(�, )7Bereaction (Fig. 9), whether in classical novae explosions, hydrostatic red giantenvelopes, or primordial nucleosynthesis scenarios. In novae, the 3He seedmaterial originates mainly from the accreted material and to a much lesser

170 100 200 300

0

Time (days)

D=1/2 kpcFIGURE 9. 7Be decay scheme and production processes (left), and 22Na measurementsfrom individual nova (right).extent from incomplete hydrogen burning (�rst two steps of the pp chain).Competing with this reaction in the consumption of 3He is 3He(3He,2p)4Hereaction. This third step of the pp chain is always faster, except at very low3He abundances. As such, the �nal 7Be yields from novae display a logarithmic,not a linear, dependence on the initial 3He content (Bo�n et al. 1993). Asolar 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 captures to form 8B.This destruction is quite e�cient below temperatures of �108 K and densitiesabove �100 g/cm3. At hotter temperatures the photodisintegration of 8Breduces the overall destruction somewhat. Including the leakage of 8B to 9C isimportant, since it reduces the 7Be abundances by allowing a ow out of any7Be-8B equilibrium (Bo�n et al. 1993). Once the nuclear reactions cease anyremaining 7Be will beta decay to 7Li with a half-life of 76 days, and producethe 487 keV photon that is a target of -ray astronomy. Assuming a novaejects a total mass of 10�4 M�, of which the 7Be mass fraction is 5�10�6,the 487 keV ux is shown as a function of time in Fig. 9 for a distance tothe novae of 0.5 kpc. These values may not be typical and preferences forother values are easily accommodated; the ux scales linearly with the totalejected mass and 7Be mass fraction, while scaling with the inverse square ofthe distance. For the conditions shown, a space-borne spectrometer with asensitivity of 10�5 ph cm�2 s�1 will be able to study such novae for about 3months.Convective processing and mixing timescales in nova envelopes are criti-

18cal issues. Signi�cant -ray signals from 7Li can only occur when convec-tion can transport freshly synthesized 7Be into a cooler and less dense re-gions. Spherically symmetricmodels of classical novae which incorporate time-dependent convection and the accretion phase tend to give 7Be yields between10�12-10�10 M� depending on the mass and composition of the white dwarfand accreted material (Politano et al. 1995). Two-dimensional hydrodynamicmodels that examine the evolution of a fully convective hydrogen-rich enve-lope during the earliest stages of the thermonuclear runaway are beginning tobe calculated, and should help clarify some of these uncertainties in the future(Shankar & Arnett 1994; Glasner, Livne, & Truran 1997).There have been no reported detections of 487 keV line emission from eitherthe cumulative e�ects of many novae (�100) near the Galactic center region, orindividual novae. However, SMM reported a 3� upper limit ux of 1.7�10�4ph cm�2 s�1, assuming a point source of constant intensity at the GalacticCenter (Harris, Leising & Share 1988). They also reported 3� upper limit uxes of 2.0�10�3 ph cm�2 s�1 from Nova Aql 1982, 8.1�10�4 ph cm�2 s�1from Nova Vul 1984, and 1.1�10�3 ph cm�2 s�1 from Nova Cen 1986. Theseresults imply upper limit 7Be abundances that are about an order of magnitudeabove even the most optimistic theoretical expectations.The radioactive isotope 22Na is synthesized in classical novae at the inter-face between the white dwarf and the accreted envelope. How much carbon,oxygen, magnesium and neon is dredged up from the white dwarf, or accretedfrom the binary companion is a critical issue for determining the strength ofany 22Na -ray signal. The chief nuclear ows that create and destroy 22Na isdiscussed in detail by Higdon & Fowler 1987 and Coc et al. 1995, along withthe uncertainties in several key reaction rates. As the ejecta cools, expands,and nuclear burning ceases (� 7 days), 22Na decays with a half-life of 2.60years from the J�=3+ state to a short-lived excited state of 22Ne, emitting a1.275 MeV gamma-ray line in the process.Gamma-ray line emission at 1.275 MeV from classical novae in the MilkyWay remains to be positively detected. SMM, OSSE, and COMPTEL have,however, reported upper limits for several individual nova (Leising 1993;Iyudin et al. 1995; Fig 9). The 2� upper limit uxes reported by COMPTELputs tight constraints on thermonuclear runaway models of classical novae.For neon-novae, which are expected to eject the largest 22Na masses, the av-erage COMPTEL upper limit ux of 3�10�5 ph cm�2 s�1 translates into anupper limit on the ejected 22Na mass of 3.7�10�8 M� (Iyudin et al. 1995).Spherically symmetric models of classical novae which incorporate a moreself-consistent treatment of the hydrodynamic evolution and re�ned initialcompositions tend to give 22Na yields of �10�8 M� for the most favorableneon-novae cases (Starr�eld et al. 1996). In general, these recent improve-ments to the modeling have reduced the 22Na yield by about an order ofmagnitude compared to the older models (Starr�eld et al. 1992) that lackedthese re�nements and were at distinct odds with COMPTEL measurements.

19FIGURE 10. SN1991T spectrum as measured by COMPTEL, after subtraction of a back-ground model (Morris et al., 1995, and these proceedings).It may still be that present nova models are biased from the observations of afew exceptional events, and that the typical neon-novae ejects much less massinto the interstellar medium than the exceptional events.Nickel 56A goal of observational -ray astronomy, since at least Clayton, Colgate& Fishman in 1969, has been detection of line radiation from the decay ofradioactive 56Ni and 56Co produced in Type Ia supernovae. Type Ia eventsare favored over the other supernova classes because they produce a lot of56Ni (� 0.6 M�; Thielemann et al. 1986), and they expand rapidly enough toallow the -rays to escape before all the fresh radioactivity has decayed. TypeIa supernovae are favored over the other classes because they produce a lot of56Ni. Even so, detection of these events has been di�cult to achieve. Type Ibsupernovae synthesize 5 to 10 times less 56Ni than a typical Type Ia, but theyalso expand rapidly. The best studied supernova of all in radioactive line emis-sion was the Type II supernova SN 1987A (e.g., Arnett et al. 1989), but suchstudies were possible only because of its proximity. The -ray line detectionof any Type II supernova outside the Local Group is very improbable.Only one Type Ia supernova has been seen in -rays, SN 1991T in NGC 4527(Lichti et al. 1994). This galaxy is 10-13 Mpc distant and in the direction ofthe Virgo cluster. The supernova was unusually bright at maximum (0.7 M�of 56Ni; H�o ich et al. 1996) and the light curve evolved unusually slow. De-tection of high velocity (�13000 km/s) iron and nickel in the outer layersof SN 1991T favors models where the subsonic ame front propagates largerdistances from the white dwarf core before making the transition to a detona-tion. These types of delayed-detonation models are also consistent with thevelocity pro�le of most of the other ejecta (silicon, calcium) seen in SN 1991T.Shigeyama et al. (1993) suggest that detection of the 812 keV -ray line fromthe decay of 56Ni is direct evidence for delayed-detonation models, as the line

20cannot be seen when the 56Ni is embedded deeper in the supernova, as it tendsto be in other categories of Type Ia models.The tentative COMPTEL detection of the 56Co decay -rays (Morris et al.1995, Fig. 10) indicates this isotope was present in the outer envelope, andthus supports extensive mixing scenarios. At a distance of 13 Mpc, however,the COMPTEL measurement converts into a surprisingly large 56Ni mass of1.3 M�. This requires that almost all of the Chandrasekhar mass white dwarfmust be turned into radioactive 56Ni. The OSSE upper limits (Leising et al.1995) may indicate that the 56Co line ux derived by Morris et al. (1995) isoptimistic, although the detection itself seems signi�cant. More detections ofType Ia supernovae in 56Ni are required to clarify how typical SN 1991T was.Estimates of how many more Type Ia events might be expected are discussedbelow. COMPTEL and OSSE will hopefully remain ready to observe anynearby events.ASSOCIATED ASTROPHYSICAL CHALLENGESMeteoritic GrainsAlthough the nucleosynthetic processes occurring in di�erent stars generallyresult in a wide range of isotopic compositions, by far most of the materialfrom the many stellar sources that contributed to the proto-solar cloud wasthoroughly processed and mixed, which resulted in the essentially isotopicallyhomogeneous solar system we know today. However, a small fraction of theoriginal material, in the form of presolar dust grains, survived solar systemformation and was trapped in primitive meteorites. From their highly un-usual isotopic compositions, compared to that of the solar system as a whole,these presolar grains are inferred to have formed in circumstellar atmospheresor, in some cases, in nova or supernova explosions. Because their compo-sitions re ect the isotopic and chemical signatures of their sources, presolargrains provide information about stellar evolution and nucleosynthesis, mixingprocesses in stars, the physical and chemical conditions of stellar atmospheres,and the chemical evolution of the Galaxy (Anders & Zinner 1993; Ott 1993).There are several interesting parallels between 26Al, 60Fe and 44Ti in -rayastronomy and the laboratory study of presolar meteoritic grains. Enhanced26Mg/24Mg ratios in the Ca-Al rich inclusions of the Allende meteorite were�rst evidence for live 26Al in the early solar system (Lee, Papanastassiou, &Wasserburg 1977). Present compilations (MacPherson et al. 1995), with over1500 data points derived from various types of meteoritic samples, con�rmthat the enhancements in 26Mg are observed in the oldest and most stableinclusions of solar system material, and homogeneously show an isotopic ratio26Al/27Al of � 5 10�5. This is interpreted as in situ decay of 26Al in thoseearly droplets (MacPherson et al. 1995), mainly from the strong correlation of

210

5

10

15

20

25

0-1-2-3-4-5-6-7

inferred initial isotopic ratio: log(26Al/27Al)

nu

mb

er o

f sa

mp

les

(IS

gra

ins)

0

50

100

150

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250

300

350CorundumGraphiteSiC-XSiC-MainSolar System Objects

nu

mb

er o

f sa

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(So

l.Sys

.Ob

j.)

FIGURE 11. Histogram of inferred 26Al/27Al for various grain types (narrow bars) andsolar system objects (wide bars), after MacPherson et al. (1995).26Mg excess with aluminum abundances of the samples. This is direct proof ofan injection of (at least) 26Al into the solar nebula shortly before solar systemformation. The inferred 26Al/27Al ratio of � 5�10�5, is larger than the ratioof 2-3�10�6 estimated from -ray measurements (Diehl et al. 1995).Figure 11 compares the inferred initial 26Al/27Al ratio distributions of corun-dum, graphite and silicon carbide grains with the total population of data foraluminum-rich material (mostly Ca-Al rich inclusions) that represents solarsystem aluminum. There is very little overlap between the interstellar grainsand the solar system objects. This reassures that these types of meteoriticsamples constitute di�erent observational windows which may not be expectedto correlate, but rather carry imprints of the speci�c grain formation event,or early solar system processes, respectively (see MacPherson et al. 1995).An indication that live 60Fe also existed when the meteorites solidi�ed isfound from isotopic analysis of the Chernvony Kut meteorite (Shukolyukov& Lugmair 1993). The measured excess of 60Ni, after alternative modes ofproduction such as spallation and (n, ) reactions on 59Co could be elimi-nated, leads to a 60Fe/56Fe ratio at the time of iron-nickel fractionation of �7.5�10�9. This is consistent with the inferred 10 million year hiatus beforethe formation of the Ca-Al rich inclusions in the Allende meteorite, which hasa much larger 60Fe/56Fe ratio at the time of fractionation of � 1.6�10�6 (Birck& Lugmair 1988). However, the possibility that some of the 60Ni in the Ca-Alrich inclusions is fossil rather than from the in situ decay of 60Fe cannot beexcluded. Large excesses in 44Ca, from the radioactive decay of 44Ti (see Fig.6), have been observed in four low-density graphite grains and �ve silicon car-

22bide grains of type X extracted from the Murchison meteorite (Nittler et al.1996; Hoppe et al. 1996). Because 44Ti is only produced in supernovae, thesegrains must have a supernova origin. Moreover, the silicon, carbon, nitrogen,aluminum, oxygen, and titanium isotopic compositions of these large grains(>1 micron) require a Type II supernova source. This is strong evidence thatthese grains are supernova condensates and provides evidence for deep andheterogeneous mixing of di�erent supernova regions, including the nickel core.Extensive high-precision measurements of other meteorites discovered manyother anomalous ratios attributed isotopes that were still radioactive duringthe decoupling of the solar nebula from the interstellar medium 4.6 Gy ago(e.g., Harper 1996). Combined arguments lead to hypotheses of the solarsystem formation being related to a nearby event such as a supernova oran AGB star (Cameron 1993; Wasserburg et al. 1995). The solar nebulaexperienced enrichment above interstellar abundances in relatively shortlivedradioactive species, either from the triggering events (if they occurred), orfrom some previously unconsidered process during formation of the sun.Shu et al. (1997) discuss the idea that the chondrules and Ca-Al rich in-clusions found in chondritic meteorites might have been formed as solids en-trained and melted in the bipolar wind that results from the interaction of theaccreting proto-solar nebula and the magnetosphere of the young protosun.Aerodynamic sorting and a mechanical selection for molten droplets that rainback onto the disk at planetary distances explain the size distributions andpatterns of element segregation that we observe in carbonaceous and ordinarychondrites. Cosmic ray ions generated in the ares that accompany the generalmagnetic activity of the inner region may irradiate the precursor rocks beforethey are launched in the bipolar wind. Under certain scaling assumptions forthe e�ciency of the process in protostars, Shu et al. �nd that cosmic-ray bom-bardment can generate the short-lived radionuclides 26Al, 41Ca, and 53Mn attheir inferred meteoritic levels. Simply put, this mechanism captures materialtrying to get on the sun, heats it up, irradiates it with cosmic rays, and dumpsit back onto the disk farther out (Glanz 1997). It is far from clear, however,that such low-energy cosmic rays can explain most of the complex isotopicpatterns found in chondrites. Additional measurements are necessary to see ifthe morphological and isotopic anomalies are best explained by such a singlemechanism. Positrons from RadioactivitiesRadioactive decays can generate positrons whenever the energy level of thedaughter atom is more than the 1.022 MeV pair production threshold be-low the energy level of the parent. Some interesting amounts are positronsand produced from the �+ decay of 56Co, 57Co, 26Al, 44Sc, and the distinctnova products 13N and 18F. Annihilation produces two 511 keV photons if

23the positron-electron spins point in opposite directions, or 3 photons in acontinuous energy spectrum if the spins are parallel. This 3 photon annihi-lation process usually involves formation of positronium, which then decaysbefore being collisionally destroyed (more likely in the low densities of in-terstellar space). The fraction of positronium carries information about theannihilation environment. A cold, neutral environment results in positroniumfractions of 0.9-0.945 (Bussard, Ramaty & Drachman 1979; Brown, Leventhal& Mills 1986), while larger positronium fractions tend to indicate annihila-tion in warm (5�103 K or hotter) environments. Unbound positrons mayhave kinetic energies up to �MeV, and thus may be relativistic when they areproduced. Deceleration and thermalization are more likely than annihilation-in- ight, so that the positron lifetime in interstellar space before annihilationcan be � 105 y. The thermalization process yield intrinsically narrow 511 keVlinewidths that are related to the annihilation environment rather than thepositron production environment.In addition to the annihilation -ray signals from radioactive decays, thereare also such signals from pair plasma disks around accreting compact rem-nants, plasma jets from dynamo action of accreting compact sources, and - -reactions in strong magnetic �elds. How are these signals di�erentiated?Positrons from radioactive decays usually occur in in intrinsically di�use en-vironments with a long thermalization lifetime in the interstellar medium.Positrons ejected from processes surrounding compact objects, however, areusually annihilated nearby resulting in a recognizable more localized (and timevariable) source of annihilation photons. These regions also have a smallerpositronium fraction than regions where radioactive decays usually occur.Almost all of the Galactic-disk annihilation luminosity may be explainedby radioactive sources (Ramaty, Skibo & Lingenfelter 1994). OSSE measure-ments of the Galactic bulge and disk annihilation components (Kinzer et al.1996; Purcell et al. 1997) can be analyzed in terms of plausible spatial dis-tribution models that aim to separate the disk radioactivity component fromthe Galactic bulge component. The annihilation productions are 1043 e+s�1for the disk and 2.6�1043 e+s�1 for the bulge. About 20% of the disk compo-nent can be assigned to 26Al, based on the 1.809 MeV skymaps (Ramaty et al.1994), and suggests a large contribution from sources related to an old stellarpopulation, such as novae, Type Ia supernovae, or compact X-ray sources.Kinzer et al. (1996) determine a positronium fraction between 0.94 and 1.0in the inner Galaxy, suggesting that the contribution from compact sourcesshould be small. However, this constraint strongly depends on the environ-ment of the compact sources; Ramaty et al. (1994) point out that the entirebulge component luminosity could be explained from 1E1740.7-2942 positronproduction alone, if positrons are not rapidly annihilated in a target close tothat compact source. Hernanz et al. (1996) estimate from their simulationsof classical novae that the peak 511 keV emission reaches �10�2 (D/1kpc)2ph cm�2 s�1 for a short period about 7 hours after the outburst. This would

24make detections out to distances of the Galactic Center feasible, if timing ofthe observations would be fortunate. Overall, however, the nova contributionto the di�use 511 keV glow of the Galaxy is expected to be low (Lingenfelter& Ramaty 1989).Galactic Nucleosynthesis and Supernova RatesDirect measurement of the Milky Way's supernova rate is notoriously di�-cult. Various methods have been attempted, such as O-B2 star counts within1 kpc of the Sun, radio supernova remnant counts, -ray uxes from the decayof 44Ti, neutrino burst detections, and pulsar birth rate estimates. These indi-rect methods only yield upper limits with large error bars. Galactic supernovarates have therefore been based on extragalactic supernova searches. Theseestimates depend upon morphological type, Hubble constant, completeness ofthe surveys, determination of reliable control times, and the uncertain lumi-nosity of the Galaxy. The total blue luminosity, H� and far-infrared uxesof the Milky Way remain uncertain, in contrast to most Local Group galax-ies. Nevertheless, using the extragalactic estimates with a total Galactic blueluminosity of 2.3�1010 L�, a Hubble constant of 75 km/s/Mpc, and a SbcGalactic morphology, the Galactic core-collapse supernova rate is estimatedto be 2.4-2.7 per century while the Type Ia rate is estimated to be 0.3-0.6per century (van den Bergh & McClure 1994; Tammann, L�o�er, & Schr�oder1994). Thus, the total rate is about 3/century and the ratio of core collapseto thermonuclear events is about 6.There are 6 known local supernovae: Lupus (SN 1006) is considered to havebeen a Type Ia event, Cas A (SN 1680) and Tycho (SN 1572) which were mostlikely Type Ib supernovae, and the Crab (SN 1054), SN 1181, and Kepler(SN 1604) were probably Type II events. This listing may be complete to� 4 kpc. For an exponential disk with a 4 kpc scale length extending between3-15 kpc, about 9% of all Galactic OB stars will be located within 4 kpc of theSun. At a radial distance of 8.5 kpc, the number of supernova with massiveprogenitors within 4 kpc of the Sun may be estimated as � 0.09�0.85�3 � 2.3per millennium (van den Bergh & McClure 1994). This is close to the observednumber that are known to have occurred within 4 kpc of the Sun during thelast 2000 years. However, the statistical uncertainty in the frequency withwhich supernova of di�erent types occur in galaxies of di�erent Hubble classis large, and this agreement may be fortuitous.The total 44Ti line ux originating from the central regions of the MilkyWaywas sought by the large �eld of view spectrometers aboard the HEAO3 andSMM satellites. Analysis of the data taken with HEAO3 gave an upper limitof 2�10�4 ph cm�2 s�1 on the Galactic 67.9 & 78.4 keV emission (Mahoney etal. 1992). Searching through nearly 10 years of data taken by SMM gave anupper limit of 8�10�5 ph cm�2 s�1 for the inner 150� of the Milky Way in the

25FIGURE 12. A �rst COMPTEL map of the Galactic plane in the 1.157 MeV line from44Ti (Dupraz et al. 1997), using the �rst 3 years of data. Star symbols mark locations ofhistorical supernovae 1181, Tycho, Cas A, Kepler, Lupus, and Crab (from left to right).The Cas A signal in these data corresponds to 3.5� signi�cance, likelihood contour levelsare in 1� steps for known sources, � 0.75� for search of new sources.1.157 MeV line (Leising & Share 1994), the imaging COMPTEL instrumentsets upper limits below 2�10�5 ph cm�2 s�1 for the known historical events(Dupraz et al. 1997).A �rst COMPTEL 1.07-1.25 MeV background subtracted skymap is shownin Fig. 12 (Dupraz et al. 1997). Even though only part of the data have beenused in this analysis, and the potentially more sensitive background modellingfrom adjacent energies has not been employed, �rst conclusions can be drawn:Cas A is the only historic event detected (at � 5� now, see above). Upperlimits to the 44Ti line ux from Tycho and Kepler are about 1/2 of the Cas Asignal. Even with the longest 44Ti half-lives measured, Lupus, Crab, and SN1181 are far too old to have signi�cant amounts of 44Ti remaining to be bedetected. No previously unknown event is found, even though COMPTEL'ssensitivity should provide complete sampling of the last century beyond thedistance of the Galactic center.Core collapse supernovae and Chandrasekhar mass thermonuclear super-novae tend to account for �1/3 of the solar 44Ca abundance, with most ofthe production attributable to Type II events. The mass of 44Ca producedas itself is comparable to the 44Ca synthesized from the radioactive decay of44Ti. Even at this �1/3 of solar level, the sky should contain several meanType II supernovae remnants bright enough to be seen in their 44Ti after-glow. However, no search (HEAO 3, SMM, COMPTEL, OSSE) has produceda detection of 44Ti line emission from any young, previously unknown, andvisually obscured remnants. We simply do not see the expected number ofsupernovae remnants emitting gamma-rays from the decay of radioactive 44Ti.After eliminating alternative modes of increasing the 44Ca production (e.g.,larger yields), the conclusion becomes almost inescapable. The solar abun-dance of 44Ca is apparently due to rare events with exceptionally high 44Tiyields. This conclusion was already derived from the SMM upper limits to44Ti gamma-rays (Leising & Share 1994). Rare events could mean Type IIevents where the explosion energy is large enough to minimize the mass of44Ti that falls back onto the compact remnant, or Type Ia events that man-

26age to explode from a star with high central density. Mapping the Galaxy in44Ti should help to resolve the unknown rate of these rare events; they wouldstill mark bright spots in the 1157 keV sky. The COMPTEL survey from the�rst three mission years (Dupraz et al. 1997) still appears inconclusive in thisrespect.Integrated nucleosynthesis measurements, such as the 1.809 MeV 26Al ob-servations, can be a useful measure of the Galactic supernova rates. If adominating origin of 26Al from massive stars is adopted (Prantzos & Diehl1996), then the -ray data and nucleosynthesis yields provide independentmeasures of the massive star formation rate in the Galaxy. The core-collapsesupernova rate determined in this way is consistent with various classical ratedetermination methods, such as on H� measurements and supernova recordsin "similar" distant galaxies (Timmes et al. 1997). Crucial to such analyses isthe integrated radioactive mass inferred from the -ray measurements and thespatial distribution of the emission in the Galaxy. Given the wide range of 0.4-3.5 M� from present data, -ray measurements cannot help resolve the varioussystematic uncertainties present in the classical methods (e.g. galaxy type,Hubble constant, sample completeness). Future measurements of the positionand -ray line shape may provide the basis for a 3-dimensional deconvolutionof the apparent emission (Gehrels & Chen 1996). Note that measurement of a6 keV wide 1.809 MeV line by Naya et al. (1996) suggests that any 0.1-0.9 keVline shifts expected from Galactic rotation may not allow a direct mapping ofthe emission. Other GalaxiesIn undertaking searches for -ray line signals from supernovae, it is usefulto have some guidelines as to what might be expected. With recent catalogsof supernova events, and distances based upon some assumptions regardingthe peak absolute magnitude of supernovae, estimates of the detectable eventrate as a function of -ray telescope sensitivity can be made. Vigorous groundbased programs that search, discover, classify, and catalog supernovae areessential prerequisites in studying the emission in detail, and hence, studyingthe explosion in detail.Data on more than 1000 extragalactic supernovae are given in the SternbergAstronomical Institute Supernova Catalogue (Tsvetkov, Pavlyuk, & Bartunov1997), and the Asiago Supernova Catalogue (Cappellaro Barbon, & Turatto1997). About 50% of the supernova in either catalog lack, or have an uncertain,supernova type identi�cation; but certainly the brightest and more recentevents do have supernova types assigned. All Type Ia events are assumed tohave a peak absolute bolometric magnitude of M=-19.0. This standard candleassumption may be challenged on both observational (e.g., Saha et al. 1996)and theoretical (e.g., H�o ich et al. 1996) grounds, but it remains a useful �rst

271940 1960 1980 2000

0

2

4

6

8

10

Year

Num

ber

Dis

cove

red

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entif

ied

as T

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Ia S

N

Sternberg & Asiago SN Catalogs

90 Events with m < 16th mag (d < 100 Mpc)

81 since 1966 => 2.6/year

52 since 1986 => 4.7/year

24 since 1992 => 4.8/year

11 since 1995 => 5.5/year

1972E1994D1991T1989B1981B1971I

Distance (Mpc)

100 30 10 3

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Det

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ble

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ata Since 1966 H

istogram)

Sternberg & Asiago SN Catalogs

Data Since 1966

Data Since 1986

Data Since 1992

OSSE/COMPTEL

INTEGRAL

ATHENAFIGURE 13. Number of events discovered and identi�ed as Type Ia supernovae vs theyear of the explosion (left). Flux sensitivity histogram of 1966�1996 Type Ia events (right).approximation. Distances to individual supernovae are then calculated fromthe catalogued magnitudes by the standard formula log10D=(m - M + 5)/5,with no reddening corrections applied.All the various Type Ia models tend to produce peak 847 keV line uxes inthe 2-10�10�5 ph cm�2 s�1 range at Dp=10 Mpc, reaching this maximum �75 days after the explosion (Chan & Lingenfelter 1991; Khokhlov, M�uller, &H�o ich 1993). The peak 1.283 MeV line ux is smaller at the same distance,due to a smaller branching ratio, and tends to lie in the 1.5-6�10�5 ph cm�2s�1 range. Here we simply assume all Type Ia supernovae have a peak gamma-ray ux of Fp=3�10�5 ph cm�2 s�1 at Dp=10 Mpc. Peak gamma-ray uxesare then calculated by the ux ratio F=Fp(Dp=D)2. Catalog completeness,volumes sampled, and limitations of the assumptions are important concerns.These are discussed in detail by Timmes & Woosley (1997), who also reporta similar analysis for core-collapse supernovae.A total of 90 Type Ia supernovae brighter than 16th apparent magnitudeare listed in these catalogs (Fig. 13). As long-term search, discovery and clas-si�cation programs continue to mature, the signi�cance of an apparent averagedetection and classi�cation rate of roughly 5/year may be evaluated. Theseimplied rates are lower bounds for several reasons; large visual extinctions bydust from the parent galaxy may hide events, search programs are becomingmore e�cient, and unclassi�ed events may include some Type Ia supernovae.The uxes for 81 events discovered and identi�ed as Type Ia supernova since1966 are histogrammed in Fig. 13 (individually listed are the 6 brightest TypeIa supernovae). The apparent magnitudes may be converted into distances and

28peak -ray ux values, making the above standard candle assumptions (see topand bottom ordinates). Integrating the histogram and normalizing to the timeframes' average Type Ia supernova rate gives the -ray event rate versus uxcurve. This can be compared with instrumental sensitivities. An instrumentwith a ux sensitivity of � 2�10�6 ph cm�2 s�1 to 5000 km/s broadenedlines should easily observe Type Ia events in the Virgo (18.2 Mpc), Fornax(18.4 Mpc), and possibly the Hydra (41 Mpc) galaxy clusters at a rate of1/year (roughly 10-30% of all Type Ia events within 100 Mpc). INTEGRAL'sspectrometer sensitivity of � 6�10�6 ph cm�2 s�1 (Winkler 1995), for a 10 dayobservation, could detect all Type Ia supernovae out to � 20 Mpc and allow arate near 1 Type Ia event/year. Present instrument broad line ux sensitivitieslarger than 1.5�10�5 ph cm�2 s�1 are probably limited to detecting Type Iasupernova within �10 Mpc. These rate estimates should be regarded as roughguidelines, to aid, for example, in the design of future instruments.CONCLUDING REMARKSThe search for an overarching paradigm which encompasses measurementsof -ray emission from radioactive isotopes, the solar abundances, presolarmeteoritic grains, stellar nucleosynthesis calculations, and Galactic chemicalevolution has seen major advances during the time of the Compton Observa-tory mission. Yet it remains unful�lled. Our goal is to understand the originof every isotope in every location (and time) in the universe. Next steps to-wards this goal will be to determine the Galactic 26Al and 60Fe amounts, toput the connection of these isotopes with massive stars on solid grounds, andto �nd and understand exeptional events which eject 44Ti, 60Fe, 22Na, helpingto enlighten the variety of nova and supernova event types. We shall use thisunderstanding, as it develops, as a diagnostic for the physical processes andevolution of the universe and its contents. New instrumentation and analy-sis methods, and theory advances, and their often con icting constraints willyield exciting adventures in those future studies.Acknowledgements Useful discussions with many colleagues, in particularDieter Hartmann, Mark Leising, and Stan Woosley, are re ected in this article.This work has been supported by the Max Planck Gesellschaft (RD) and aCompton Gamma Ray Observatory Postdoctoral Fellowship (FXT).REFERENCES1. Allen, S. J., 1911, Phys. Rev., 34, 2962. Anders E. & Zinner E., 1993, Meteoritics, 28, 4903. Arnett, W. D., et al., 1989, ARAA, 27, 6294. Aufderheide, M., Baron, E., & Thielemann, F. -K. 1991, ApJ, 370, 630

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