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LiBeB, Cosmic Rays, and Gamma‐Ray Line Astronomy Author(s): Reuven Ramaty, Elisabeth Vangioni‐Flam, Michel Cassé, and Keith Olive Source: Publications of the Astronomical Society of the Pacific, Vol. 111, No. 759 (May 1999), pp. 651-652 Published by: The University of Chicago Press on behalf of the Astronomical Society of the Pacific Stable URL: http://www.jstor.org/stable/10.1086/316355 . Accessed: 23/05/2014 20:32 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . The University of Chicago Press and Astronomical Society of the Pacific are collaborating with JSTOR to digitize, preserve and extend access to Publications of the Astronomical Society of the Pacific. http://www.jstor.org This content downloaded from 195.78.109.26 on Fri, 23 May 2014 20:32:01 PM All use subject to JSTOR Terms and Conditions

LiBeB, Cosmic Rays, and Gamma‐Ray Line Astronomy

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LiBeB, Cosmic Rays, and Gamma‐Ray Line AstronomyAuthor(s): Reuven Ramaty, Elisabeth Vangioni‐Flam, Michel Cassé, and Keith OliveSource: Publications of the Astronomical Society of the Pacific, Vol. 111, No. 759 (May 1999),pp. 651-652Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/316355 .

Accessed: 23/05/2014 20:32

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The University of Chicago Press and Astronomical Society of the Pacific are collaborating with JSTOR todigitize, preserve and extend access to Publications of the Astronomical Society of the Pacific.

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This content downloaded from 195.78.109.26 on Fri, 23 May 2014 20:32:01 PMAll use subject to JSTOR Terms and Conditions

PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 111 :651È652, 1999 May1999. The Astronomical Society of the PaciÐc. All rights reserved. Printed in U.S.A.(

Conference Highlights

LiBeB, Cosmic Rays, and Gamma-Ray Line Astronomy1

The light elements Li, Be, and B (LiBeB) play a uniquerole in astrophysics. The Li abundance of old halo stars is akey diagnostic of big bang nucleosynthesis (BBN), alongwith 2H and 4He. The essentially constant Li abundance(Li/H^ 2 ] 10~10, Spite plateau) as a function of metal-licity [Fe/H] for low-metallicity stars ([Fe/H]\ [1) isbelieved to be the primordial abundance resulting fromBBN. where Fe/H is[Fe/H]4 log (Fe/H) [ log (Fe/H)

_,

the Fe abundance by number relative to H and is(Fe/H)_

the solar system value.The rare and fragile LiBeB nuclei are not generated in the

normal course of stellar nucleosynthesis and are, in fact,destroyed in stellar interiors, a characteristic that is reÑec-ted in their very low abundances. Cosmic-ray interactionscontribute to their production, but only 6Li, 9Be, and 10Bare entirely cosmic-ray produced. Neutrino-induced spall-ation, 12C(l, l@p)11B, appears to play an important role inthe origin of B by producing the excess 11B needed toaccount for the B isotopic ratio in meteorites, which exceedsthe predictions of all viable cosmic-ray scenarios. Whilereactions on metals (primarily C and O) contribute to all ofthe LiBeB nuclei, reactions of fast a-particles on ambient Heproduce both 7Li and 6Li and are the dominant source ofthe latter. Nucleosynthesis in a variety of other Galacticobjects, including Type II supernovae, novae, and giantstars, produces the bulk of the 7Li at epochs when [Fe/H]exceeds about [1.

Traditionally, the cosmic-ray role in LiBeB evolution wasinvestigated by assuming that at all epochs of Galactic evol-ution cosmic rays with energy spectra similar to thoseobserved in the current epoch are accelerated out of theaverage interstellar medium and interact in the interstellarmedium (ISM), mostly with C, N, and O. This Galacticcosmic ray (GCR) paradigm, however, appears to be inconÑict with recent measurements of Be and B abundancesin low-metallicity halo stars, achieved with the 10 m Kecktelescope and the Hubble Space Telescope. The GCR para-digm predicts a quadratic correlation of Be and B versus Fe,as opposed to the data that show a quasi-linear correlation.As a consequence, the paradigm has been modiÐed (M.Casse, R. Lehoucq, & E. Vangioni-Flam, Nature, 373, 318[1995] ; R. Ramaty, B. Kozlovsky, & R. E. Lingenfelter,

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Conference was held in Paris, France, in 1998 December. Proceedings

will be edited by R. Ramaty, E. Vangioni-Flam, M. and K. OliveCasse� ,and published in the ASP Conference Series.

ApJ, 456, 525 [1996]) by augmenting the cosmic rays accel-erated out of the average ISM with a metal-enriched com-ponent conÐned predominantly to low energies ([100MeV nucleon~1) and thought to be accelerated out of thewinds of Wolf-Rayet stars and the ejecta of supernovae.More recently, it was suggested (R. E. Lingenfelter, R.Ramaty, & B. Kozlovsky, ApJ, 500, L153 [1998] ; J. C.Higdon, R. E. Lingenfelter, & R. Ramaty, ApJ, 509, L33[1998]) that the cosmic rays themselves are acceleratedmostly out of supernova ejecta rather than the averageISM, implying that the source material of the cosmic rayswould be metal enriched at all epochs of Galactic evolution.Both of these models now converge toward acceleration byshocks in superbubbles, but they di†er in the employedparticle energy spectra, a distinction that could be tested bynuclear gamma-ray line observations. However, the e†ect isonly marginally detectable by present generation gamma-ray telescopes.

Light-element research thus impacts several importantastrophysical problems, speciÐcally BBN, the origin ofcosmic rays, Galactic chemical evolution, and gamma-rayastronomy. These were then the topics of the Conference,and they will be covered in detail in the upcoming Pro-ceedings. Here we summarize some of the highlights.

Critical considerations of the Ñatness of the Spite plateauwere presented by Paolo Molaro. These are essential forestablishing the primordial Li abundance. An importantissue in this context is the amount of Li destruction (if any)in the observed stars. Marc Pinsonneault and Sylvie Vau-clair addressed this problem. Another venue for establishingthe primordial nature of Li in connection with binaries wasdiscussed by Spite. The relationship of the light-FrancÓ oiselement data to BBN was reviewed by Keith Olive.

The 6Li observations in low-metallicity stars werereviewed by Lewis Hobbs. There are now good indicationsthat 6Li is present in such stars, with an abundance of a fewpercent relative to 7Li and a factor of several tens relative toBe. The abundance ratio relative to Be, compared with theexpected ratio from the various cosmic-ray scenarios,implies that 6Li could not have been severely depleted in thestars where it is detected. Consequently, because 6Li is morefragile than 7Li, the 7Li depletion should also be small. Theabundance ratio relative to 7Li shows that cosmic-ray inter-actions could not have made a signiÐcant contribution tothe Li/H of the Spite plateau. All of these reinforce theÐnding that the plateau value indeed represents the correct

651

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652 RAMATY ET AL.

primordial abundance. 6Li so far has been detected in onlytwo stars. As its production history could be quite di†erentfrom that of Be (being very efficiently produced in inter-actions involving only He, unlike Be, which requires thespallation of metals), future observations over a broadrange of metallicities could lead to interesting surprises.

The very important new data on O abundances in low-metallicity stars were presented by Ramon Garcia Lopez(see G. Israelian, R. Garcia Lopez, & R. Rebolo, ApJ, 507,805 [1998]). Contrary to previous data, the new obser-vations, if conÐrmed, show that O/Fe is a monotonicallyincreasing function of decreasing metallicity, reachingvalues that exceed the solar ratio by a factor of D4 at[Fe/H]\ [1.5 and by a full order of magnitude at [Fe/H]\ [3. Some of this increase is due to the absence ofType Ia supernovae in the early Galaxy. The additionalincrease is not well understood : it could be due to low Feyields relative to O in the Ðrst generation of core-collapsesupernovae, or it could possibly be due to mixing e†ectsbecause, as pointed out by J. Audouze & J. Silk (ApJ, 451,L49 [1995]) the ISM of the early Galaxy, being metalenriched by only a small number of core-collapse super-novae, could be quite inhomogeneous. In any case, theenhanced early Galactic O abundance makes cosmic-rayacceleration out of the average ISM more efficient. Thise†ect, coupled with the possible lower Fe yield per super-nova, allowed Brian Fields and Keith Olive to show thatcosmic-ray acceleration out of the average ISM, hithertobelieved untenable, could be viable. Their model alsoimplies a decrease of 6Li/Be as a function of increasingmetallicity, a result which appears to be consistent with thefact that the early Galactic ratio mentioned above probablyexceeds the meteoritic ratio at solar metallicity.

A critical discussion of the NLTE e†ects, which are essen-tial for the abundance determinations, particularly that ofB, was given by Dan Kiselman. Douglas Duncan reviewedthe B observations and Dieter Hartmann discussed theneutrino-induced processes in core-collapse supernovae,which in particular lead to the production of 11B. Asalready mentioned, this process provides a plausible expla-nation for the excess 11B measured in meteorites. Stellarevolution, another very important ingredient necessary forunderstanding the implications of the light-element data,was discussed by Marc Pinsonneault. The status of Galacticnuclear gamma-ray line observations, showing that the pre-viously reported observations of Orion are no longer valid,was reviewed by Hans Bloemen. In the absence of nucleargamma-ray data, the detection of broad soft X-ray lines(particularly the lines of O just below 1 keV) resulting fromelectron capture and excitation on fast (D1 MeVnucleon~1) ions could provide independent information onthe existence of low-energy cosmic rays. This topic was dis-

cussed by Vincent Tatische†. The capabilities of thegamma-ray imaging and spectroscopic mission INTE-GRAL, to be launched soon, were discussed by Volker

and Bertrand Cordier.Scho� nfelderCurrent-epoch cosmic-ray observations of the electron

capture radioisotope 59Ni and its decay product 59Co, withan instrument on the currently active Advanced Composi-tion Explorer mission, were presented by Robert Binns.59Ni decays by electron capture with a half-life of 7.6] 104yr. However, the decay is suppressed if the accelerationtimescale is shorter than the lifetime because the atom isstripped as it is accelerated (M. & A. Soutoul, ApJ,Casse�200, L75 [1975]). The fact that much more 59Co than 59Niis observed suggests a delay (D105 yr) between nucleo-synthesis and acceleration. This makes it unlikely thatsupernovae accelerate their own ejecta, but still allowscosmic-ray acceleration from metal-enriched superbubbles,as in the Higdon et al. model mentioned above.

Several theoretical papers on cosmic-ray origin andacceleration mechanisms were presented. Jean-Paul Meyerand Donald Ellison reviewed their previously publishedmodel (J.-P. Meyer, L. O. Drury, & D. C. Ellison, ApJ, 487,182 [1997] ; D. C. Ellison, L. O. Drury, & J.-P. Meyer, ApJ,487, 197 [1997]) in which the current-epoch cosmic raysoriginate from an average ISM of solar composition andinterstellar dust plays an important role in determining theabundances. They also discussed the shortcomings of therecently proposed model (R. E. Lingenfelter, R. Ramaty, &B. Kozlovsky, ApJ, 500, L153 [1998]) in which each super-nova accelerates its own freshly produced refractory metals.Maurice Shapiro reviewed his previously proposed modelbased on the preacceleration of the cosmic rays by coronalmass ejection driven shocks on low-mass, cool stars. Accel-eration in superbubbles was discussed by Andrei Bykov andEtienne Parizot, who emphasized that the conditions in thesuperbubbles that would to lead to cosmic rays with hard-energy spectra at low energies extending up to a cuto† at anenergy that is still nonrelativistic. These are the low-energycosmic rays which have been postulated to produce thebulk of the Be at low metallicities (see E. Vangioni-Flam etal., ApJ, 468, 199 [1996]). On the other hand, as pointed outin the publication of Higdon et al., because these giantsuperbubbles are thought to Ðll up a large fraction of theISM, they are the most likely site for the acceleration of thecosmic rays, which of course show no cuto† up to very highultrarelativistic energies. Thus, it is still not clear whetherthe postulated Galaxy-wide low-energy cosmic-ray com-ponent exists, a question that should be resolved by futuregamma-ray line observations.

In summary, LiBeB research indeed spans a broad rangeof interesting problems that will be covered in the plannedProceedings.

Reuven Ramaty, Elisabeth Vangioni-Flam, and Keith OliveMichel Casse� ,NASA Goddard Space Flight Center ; Institut dÏAstrophysique, Paris ; and University of Minnesota.

1999 PASP, 111 :651È652

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