New Astronomy Reviews 44 (2000) 315–320www.elsevier.nl / locate /newar

Constraints on stellar yields and SNe from gamma-ray lineobservations

a,b¨ ¨Jurgen KnodlsederaINTEGRAL Science Data Centre, Chemin d’Ecogia 16, 1290 Versoix, Switzerland

bCentre d’Etude Spatiale des Rayonnements, 31028 Toulouse Cedex 4, France

Abstract

Gamma-ray line observations provide a versatile tool for studies of nucleosynthesis processes and supernova physics. Inparticular, the observation of radioactive species in the interstellar medium probes recent nucleosynthesis activity on varioustime-scales for different kinds of sources. Considerable progress in gamma-ray instrumentation during the last decades hasled to the discovery of several cosmic gamma-ray lines. In this review, recent observational results are presented and theirastrophysical implications are discussed. Prospects of gamma-ray line astronomy will be explored in view of the futureINTEGRAL mission. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Gamma-ray astronomy; Nucleosynthesis; Supernovae; Chemical evolution

1. Introduction material is continuously growing, and is starting toprovide interesting constraints on nucleosynthesis

During the last decade, the field of gamma-ray line processes.astronomy has made important progress. On the one Most gamma-ray lines are produced in desexcita-hand, the explosion of the nearby supernova SN tion transitions between nuclear levels. These gam-1987A in the Large Magellanic Cloud provided us ma-ray lines may be subdivided into two categorieswith a bright source of nuclear gamma-ray lines, due that are defined by the channel that led to the nuclearto the decay of freshly produced radioactive isotopes. excitation: (a) radioactive decay into an excited stateOn the other hand, a new generation of space-borne of the daughter isotope, or (b) nuclear interactionstelescopes, such as COMPTEL and OSSE on the and reactions. Radioactive decays are eventuallyCompton Gamma-Ray Observatory, and TGRS on accompanied by positron emission, resulting in a 511

1 2WIND, provided for the first time sufficient sensitivi- keV gamma-ray line from e e annihilation. Otherty, angular and spectral resolution for a comprehen- mechanisms leading to positron production involvesive study of galactic gamma-ray lines. Still, gamma- compact objects, where high densities or strong

1 2ray line astronomy is plagued by a dominating magnetic fields favour e e pair production.instrumental background, induced by cosmic-raybombardment in the hostile space environment,reducing line detections to significancies below 2. Radioactivity linestypically 5–10 s. Consequently, uncertainties onobserved line fluxes or profiles are still quite im- Gamma-ray lines from radioactivities demandportant. Nevertheless, the available observational several conditions to be observable. First, a hot and

1387-6473/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S1387-6473( 00 )00046-4

¨316 J. Knodlseder / New Astronomy Reviews 44 (2000) 315 –320

dense medium with sufficiently low entropy is radioactivity lines implies various time-scales. Forrequired to allow for the synthesis of fresh radio- lifetimes that are short compared to the event

57 57isotopes. Such a medium can be found in stellar frequency ( Ni– Co), individual transient gamma-interiors, at the base of the accreted envelope of ray line sources are expected, mainly in the form ofwhite dwarfs in close binary systems, or even in supernovae or novae. For lifetimes of the order of

22 44accretion disks around compact objects. The nuclear the event frequency ( Na– Ti), several ratherreaction networks in operation are characteristic for steady gamma-ray line sources are expected fromthe composition, density and temperature at the supernova remnants or recent nova events. Forburning site, hence, the observation of isotopic lifetimes that are long compared to the event fre-

26 60abundance patterns provides direct insight into the quency ( Al– Fe), a superposition of numerousnucleosynthesis conditions. Second, the fresh radio- individual gamma-ray line sources will lead to aisotopes have to be removed quickly from the diffuse glow of gamma-ray line emission. Addition-formation site to prevent destruction by nuclear ally, the radioisotopes may travel considerable dis-reactions or natural decay. This generally implies tances away from the production sites before theyconvection followed by mass ejection, either in the decay (|10–100 pc), leading to intrinsically extend-form of stellar winds or explosions, and requires ed sources. Hence, diffuse galaxywide emission is

26 60lifetimes of at least several days, better several expected for Al and Fe (see Prantzos & Diehl,months. Additionally, nucleosynthesis sites are gen- 1996; Diehl & Timmes, 1998 for recent reviews).erally optically thick to gamma-rays, hence, escapeof the radioisotopes to optically thin regions is 2.1. SN 1987A – a nucleosynthesis laboratorymandatory for gamma-ray line observations. Conse-quently, radioisotopes can probe stellar convection The explosion of SN 1987A in the Large Magel-and ejection processes, providing important infor- lanic Cloud was a great opportunity for gamma-raymation about the involved stellar physics. Third, the line astronomy. For the first time, a supernovalifetime has to be short enough and the abundance of explosion occurred close enough to be in reach ofthe isotope has to be high enough to assure a available gamma-ray telescopes. During core col-

56 57sufficient radioactive decay activity that is in reach lapse, substantial amounts of Ni and Ni areof modern gamma-ray telescopes. produced, which subsequently decay under gamma-

56,57 56,57These constraints result in a list of candidate ray lines emission to Co and finally to Feisotopes that may actually be accessible to gamma- (cf. Table 1). The production of these isotopes inray line astronomy (cf. Table 1). The observation of supernova explosions has been indirectly inferred

Table 1Gamma-ray lines from radioactivities that may be accessible to gamma-ray astronomy (ordered by ascending lifetime). Theoreticalnucleosynthesis yield estimates are quoted for different source types

Isotope Lifetime t Lines (keV) Typical yields (M )(

WR SN Ia SN Ib/c SN II Nova57 23 23Ni 2.14 days 1378 0.02 5 10 5 1056Ni 8.5 days 158, 812 0.5 0.1 0.159 25 25Fe 64.2 days 1099, 1292 5 10 5 10

7 27 27 211Be 77 days 478 10 5 10 5 1056 †Co 112 days 847, 1238 0.5 0.1 0.157 23 23Co 392 days 122 0.02 5 10 5 1022 † 28 26 26 29Na 3.76 years 1275 10 10 10 5 1060 25 25Co 7.61 years 1173, 1333 10 1044 † 25 25 25Ti 87 years 1157 10 5 10 5 1026 † 6 24 25 25 28Al 10 years 1809 10 5 10 5 10 1060 6 23 25 25Fe 2.2 10 years 1173, 1333 5 10 5 10 5 10

† Positron emitters.

¨J. Knodlseder / New Astronomy Reviews 44 (2000) 315 –320 317

44from lightcurve characteristics, reflecting the respec- product Ca. Due to the penetrating power of44tive decay times. The direct observation of the gamma-rays, Ti lines from recent supernova events

56gamma-ray lines from Co (Matz et al., 1988) and throughout the Galaxy can reach the Earth and,57Co (Kurfess et al., 1992) in SN 1987A was a therefore, unveil yet unknown young supernovabrilliant confirmation of this interpretation. The remnants.observed relative intensities of the gamma-ray lines Proof of the principle was recently achieved by the

56 57 57 56from Co and Co indicated a Ni / Ni ratio that first observation of a 1.157 MeV gamma-ray linewas between 1.5 and 2 times the solar ratio of from the 320-year-old Cas A supernova remnant,57 56Fe / Fe, consistent with core collapse supernova using the COMPTEL telescope (Iyudin et al., 1994).models (Woosley & Hoffman, 1991). Note that Cas A almost escaped visual detection, but

56Surprisingly, the Co lines could be detected only it was accidentally included in Flamsteed’s sky atlas6 months after the explosion, at an epoch where as a star of the sixth magnitude. This fact is indeedstandard onion-shell supernova expansion models very puzzling since nucleosynthesis models predict

56 44still predicted a substantial gamma-ray opacity for large amounts of Ni being co-produced with Ti,the envelope. The gamma-ray line lightcurves pre- which would have resulted in a visual peak mag-

56 msented clear evidence that Co was found over a nitude of 24 for Cas A. Although a possiblelarge range of optical depths, with a small fraction at extinction of 10 mag could solve this problem, it isvery low depth (Leising & Share, 1990). It is yet to be confirmed observationally. Alternative

44probable that some fragmentation of the ejecta, and explanations invoke ionisation of Ti or an44acceleration of the emitting radioactivity, are re- asymmetric explosion. Ionisation would prevent Ti

quired to explain the observations. The acceleration decaying by orbital electron capture and, hence,hypothesis is supported by various gamma-ray line- falsify the relation between observed gamma-ray line

44profile measurements, all of which indicated line flux and the present Ti mass, relaxing the con-56widths of the order of 1% FWHM, corresponding to straint on the amount of Ni produced during the

21Doppler velocities of 3000 km s (Mahoney et al., explosion (Mochizuki et al., 1999). An asymmetric44 561988; Rester et al., 1989; Tueller et al., 1990). explosion could increase Ti over Ni production

Measurements of the bolometric SN 1987A light- in the high entropy alpha-rich freezeout along the44curve indicate that some Ti was also produced polar directions (Nagataki et al., 1998).

44during the explosion. The expected 1.157 MeV Evidence for another galactic Ti source wasgamma-ray line intensities are actually too weak for recently found in the Vela region, where no young

44current telescopes, but the decay of Ti is suffi- supernova remnant was known before (Iyudin et al.,ciently slow that it will be observable by future, 1998). Triggered by this discovery, a re-analysis ofmore sensitive, instruments. Hence, SN 1987A still ROSAT X-ray data indeed revealed a spherical

44remains an interesting nearby laboratory for studies structure at the position of the new Ti source, nowof explosive nucleosynthesis processes. identified as the RX J0852.0-4622 supernova rem-

44nant (Aschenbach, 1998). Although the Ti observa-44 ¨2.2. Ti – unveiling recent supernova tion is only marginal (Schonfelder et al., 2000), it is

the first time that gamma-ray line observationsThe census of recent galactic supernova events is triggered the discovery of a new supernova remnant.

exclusively based on historic records of optical Again, no optically bright display has been recorded44observations and amounts to six events during the for RX J0852.0-4622—are Ti-producing super-

last 1000 years. Due to galactic absorption and novae optically faint (or obscured)?observational bias, this census is incomplete by far.

44 26Gamma-ray line observations of the Ti isotope 2.3. Al – recent galactic star-formation historyhave the potential to considerably increase the

44statistics. Ti is believed to be exclusively produced Intense galactic gamma-ray line emission at 1.80926by supernova events, and its production can be MeV, attributed to the radioactive decay of Al, has

inferred from the solar abundance of the decay been reported by numerous instruments (see Prantzos

¨318 J. Knodlseder / New Astronomy Reviews 44 (2000) 315 –320

26& Diehl, 1996 for a review). In principle, Al could poorly determined rate. There are indications forbe produced in appreciable amounts by a variety of enhanced star formation between 3 and 6 kpc,sources, such as massive mass losing stars (mainly coinciding with the molecular ring structure, as seenduring the Wolf-Rayet phase), Asymptotic Giant in CO data (Dame, 1987). Enhanced star formationBranch stars (AGBs), novae (mainly of ONe sub- is also seen in the solar neighbourhood (8–9 kpc),type), and core collapse supernovae. Considerable which probably corresponds to the local spiral arm

26uncertainties that are involved in the modelling of structure. However, the radial Al profile is proba-nucleosynthesis processes, mainly due to the poorly bly not directly proportional to the radial star-forma-

26known physics of stellar convection, do not allow for tion profile since Al nucleosynthesis may dependa theoretical determination of the dominant galactic on metallicity. It will be important to determine this26Al sources. metallicity dependence in order to extract the true

The 1.809 MeV gamma-ray line has now, for the star-formation profile from gamma-ray line data.first time, been imaged using the COMPTEL tele- Valuable information about the metallicity depen-

¨scope (Knodlseder, 1994; Diehl et al., 1995; Ober- dence will come from a precise comparison of the¨lack et al., 1996; Knodlseder et al., 1999). The 1.809 MeV longitude profile to the profile of free-

COMPTEL image shows an intense, asymmetric ¨free emission (Knodlseder, 1999). Additionally, ob-60ridge of diffuse galactic 1.809 MeV emission with a servations of gamma-ray lines from Fe, an isotope

prominent localised emission enhancement in the that is only believed to be produced during super-Cygnus region (cf. Fig. 1). Additional hints for nova explosions, can help to distinguish between

26emission peaks along the galactic plane can be hydrostatically and explosively produced Al and,understood as fingerprints of the galactic spiral therefore, can help to disentangle the metallicitypattern. Globally, the distribution of 1.809 MeV dependencies for the different candidate sources.gamma-ray line emission very closely follows thedistribution of galactic free-free emission

¨(Knodlseder et al., 1999). Since galactic free-freeemission is an excellent tracer of the massive star 3. Annihilation linepopulation (M . 20 M ), the close correlation sug-i (

26gests that Al is mainly produced by this population The 511-keV gamma-ray line due to annihilation¨(Knodlseder, 1999). Consequently, due to the short of positrons and electrons in the interstellar medium

26lifetime of massive stars, Al becomes an excellent has been observed by numerous instruments (seetracer of recent galactic star formation. Harris et al., 1998 and references therein). At least

26The radial Al mass density distribution illus- two galactic emission components have been iden-trates that the bulk of galactic star formation occurs tified so far: an extended bulge component and a diskat distances of less than 6 kpc from the galactic component. Indications of a third component,centre (cf. Fig. 1). Star formation is also present situated above the galactic centre, have been reportedwithin the central 3 kpc of the Galaxy, although at a (Purcell et al., 1997; Harris et al., 1998), but this

26¨Fig. 1. Left: COMPTEL 1.809 MeV gamma-ray line allsky map (Knodlseder et al., 1999). Right: Radial Al mass density profile¨(Knodlseder, 1997).

¨J. Knodlseder / New Astronomy Reviews 44 (2000) 315 –320 319

needs to be confirmed using more sensitive instru- annihilation mainly occurs in the warm neutral orments. ionised interstellar medium (Harris et al., 1998).

The galactic disk component may be explained by26 44radioactive positron emitters, such as Al, Sc and

56Co (Lingenfelter & Ramaty, 1989). Although all of 4. Perspectivesthese isotopes are also gamma-ray line emitters, only26Al will lead to correlated gamma-ray line and 511 Due to continuing progress in instrumentation, thekeV emission since the typical annihilation time scale field of gamma-ray line astronomy has now become

5of some 10 years considerably exceeds the lifetime a new complementary window to the universe. Withof the other isotopes. Consequently, 511 keV line- the COMPTEL and OSSE telescopes on CGRO, theemission is a potential tracer of extinct, short-lived, entire sky has been imaged for the first time in thegalactic radioactivities. light of gamma-ray lines, leading to maps of 511 keV

26The origin of the galactic bulge component is annihilation radiation and Al 1.809 MeV emission.much less clear. Reports of time-variable, possibly New gamma-ray lines have been discovered, such as

44red-shifted 511 keV line features from the galactic the 1.157 MeV line from Ti, or several decay lines56 57centre direction led to the idea of compact objects from Co and Co. Gamma-ray lines probe aspects

being responsible for the galactic bulge component of nucleosynthesis, stellar evolution and supernova(Lingenfelter & Ramaty, 1989). However, recent physics that are difficult to access by other means. Inobservations using more sensitive instruments could addition, they provide tracers of galactic activity andnot confirm any time-variability (Jung et al., 1995; improve our understanding of the interstellar re-Smith et al., 1996; Harris et al., 1998; Cheng et al., cycling processes.1998). Actually, the most plausible source of the The progress will continue. In 2002, ESA’s INTE-galactic bulge component may also be extinct, short- GRAL gamma-ray observatory, which is equippedlived, radioisotopes from an old stellar population with two gamma-ray telescopes, optimised forwith a prominent galactic bulge component. Type Ia high-resolution imaging (IBIS) and high-resolutionsupernovae could be good candidates for such a spectroscopy (SPI) (see http: / / astro.estec.esa.nl /population since they produce appreciable amounts SA-general /Projects / Integral / integral.html) will be

56of Co (a positron emitter) and they are believed to launched. Gamma-ray line astrophysics figuresbelong to the old stellar population. Hence, it may among the prime objectives of this mission. Withturn out that the 511 keV annihilation line is an respect to precedent instruments, the INTEGRALexcellent tool for studies of the galactic SN Ia telescopes provide enhanced sensitivity together withpopulation. improved angular and spectral resolution. In par-

Some information about the annihilation environ- ticular, SPI will map gamma-ray lines with a spectralment is obtained from 511 keV line-shape measure- resolution E /DE | 500, corresponding to Doppler

21ments and the determination of the positronium velocities of | 600 km s . It will provide muchfraction. In fact, positrons and electrons may eventu- more detailed maps of the galactic 511 keV andally form a short-lived hydrogen-like system called 1.809 MeV line emissions, and determine their linepositronium, which decays either into two 511 keV profiles with unprecedented accuracy. Followingphotons or a three-photon continuum below 511 keV. nucleosynthesis theory, the detection of diffuse galactic

60The relative intensities of both components carry gamma-ray line emission from Fe decay is expected,44information about the fraction f of annihilations via and more Ti supernova remnants should be

positronium formation, probing the thermodynamic discovered.and ionisation state of the annihilation environment Further progress is expected from new instrumen-(Guessoum et al., 1991). Recent observations sug- tal concepts, such as a high-resolution Comptongest a positronium fraction of f 5 0.9–1.0 for the telescope (e.g. Johnson et al., 1995; Aprile et al.,galactic bulge component (Kinzer et al., 1996; Harris 1998) or a crystal lens diffraction telescope (Vonet al., 1998). Together with the only moderately Ballmoos et al., 1996). The latter concept has thebroadened 511 keV line width, this indicates that outstanding capacity of providing extremely high

¨320 J. Knodlseder / New Astronomy Reviews 44 (2000) 315 –320

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