Table Of ContentAstronomy&Astrophysicsmanuscriptno.arxiv (cid:13)c ESO2010
January10,2010
Predicted gamma-ray line emission from the Cygnus complex
PierrickMartin1,2,Ju¨rgenKno¨dlseder1,2,GeorgesMeynet3,andRolandDiehl4
1 Centred’EtudeSpatialedesRayonnements,CNRS/UPS,9,avenuecolonelRoche,BP44346,31028Toulousecedex4,France
2 Universite´deToulouse(UPS),CentreNationaldelaRechercheScientifique(CNRS),UMR5187,France
3 GenevaObservatory,CH-1290Sauverny,Switzerland
4 MaxPlanckInstitutfu¨rextraterrestrischePhysik,Postfach1312,85741Garching,Germany
Received:10July2009/Accepted:17November2009
0
ABSTRACT
1
0
Context.TheCygnusregionharboursahugecomplexofmassivestarsatadistanceof1.0-2.0kpcfromus.About170Ostarsare
2
distributedoverseveralOBassociations,amongwhichtheCygOB2clusterisbyfarthemostimportantwithabout100-120Ostars.
n Thesemassivestarsinjectlargequantitiesofradioactivenucleiintotheinterstellarmedium,suchas26Aland60Fe,andtheirgamma-
a raylinedecaysignalscanprovideinsightintothephysicsofmassivestarsandcore-collapsesupernovae.
J Aims.PaststudiesofthenucleosynthesisactivityofCygnushaveconcludedthatthelevelof26Aldecayemissionasdeducedfrom
CGRO/COMPTELobservationswasafactor2-3abovethepredictionsbasedonthetheoreticalyieldsavailableatthattimeandon
0
theobservedstellarcontentoftheCygnusregion.Wereevaluatethesituationfromnewmeasurementsofthegamma-raydecayfluxes
1
withINTEGRAL/SPI(presentedinapreviouspaper)andnewpredictionsbasedonrecentlyimprovedstellarmodels.
Methods.Webuiltagridofnucleosynthesisyieldsfromrecentmodelsofmassivestars.Comparedtopreviousworks,ourdatain-
]
E cludesomeoftheeffectsofstellarrotationfor thehigher massstarsandacoherent estimateofthecontributionfromSNIb/c.We
H thendevelopedapopulationsynthesiscodetopredictthenucleosynthesisactivityandcorrespondingdecayfluxesofagivenstellar
populationofmassivestars.
.
h Results.The observed decay fluxesfrom theCygnus complex arefound to beconsistent withthe values predicted by population
p synthesisatsolarmetallicity;andyet,whenextrapolatedtothepossiblesubsolarmetallicityoftheCygnuscomplex,ourpredictions
- failtoaccount fortheINTEGRAL/SPImeasurements. Theobservedextent ofthe1809keV emissionfromCygnus isfoundtobe
o consistentwiththeresultofanumericalsimulationofthediffusionof26AlinsidethesuperbubbleblownbyCygOB2.
r Conclusions.Ourworkindicatesthatthepastdilemmaregardingthegamma-raylineemissionfromCygnusresultedfromanover-
t
s estimateof the1809keV flux of the Cygnus complex, combined withanunderestimate of the nucleosynthesis yields. Our results
a illustratetheimportanceofstellarrotationandSNIb/cinthenucleosynthesisof26Aland60Fe.Theeffectsofbinarityandmetallicity
[ mayalsobenecessarytoaccountfortheobservationssatisfactorily.
1 Key words. Gamma rays: observations – open clustersand associations: individual: Cyg OB – Stars:early-type – ISM:bubbles –
v Nuclearreactions,nucleosynthesis,abundances
2
2
5
1. Introduction thereforeinevitableinastrophysics.Amongstthevariousobser-
1
. vationalapproachesemployedtoexplorethephysicsofmassive
1
stars and their SNe, gamma-ray line astronomy can be a very
0 Conventionally defined as stars with initial masses above 8-10
valuabletool. By studyingthe characteristic decay radiationof
0 and upto about120M , massive stars play a fundamentalrole
⊙ certainradio-nuclidesproducedinthehydrostaticand/orexplo-
1 inastrophysicsbecauseoftheverypowerfulbehaviourtheyex-
siveburningstagesofmassivestars,gamma-raylineastronomy
: hibit during their lives, from the early main-sequence to their
v providesindirectaccesstothephysicalconditionsthatgoverned
Xi finalexplosion(andevenbeyondthankstothecompactobjects their synthesis and ejection in the ISM. Among the few radio-
they leave behind). Usually clustered in OB associations of a
isotopesaccessibletothepresent-daygamma-raytelescopes,the
r few tens to several thousand members, massive stars severely
a long-lived26Aland60Feholdpotentialforinterestingconstraints
affect their environment on the hundred-parsec scale through
ontheevolutionofmassivestars.
their sustained mechanical and radiative energy output: strong
TheGalacticdecayemissionfrom26Alwasextensivelystud-
luminosity in the Lyman continuum, very energetic winds and
iedinthe1990swiththeCGRO/COMPTELinstrumentandthe
masslossepisodes,andsupernovaexplosions.Inadditiontome-
resultant cartography of the 26Al decay signal at 1809keV re-
chanicalenergy,thesestellaroutflowsalsocarrytheproductsof
vealed that most of the emission comes from the inner Galaxy
nuclear burning and thus enrich the interstellar medium (ISM)
and from a few nearby star-forming regions(Diehletal. 1994;
with heavy elements. The physical and chemical evolution of
delRioetal. 1996; Plu¨schkeetal. 2001). The intensity distri-
the Galaxy is driven to a considerable extent by massive stars
bution observed with COMPTEL allowed it to be established
and a wide varietyof Galactic objectsfollow fromtheir evolu-
thatmassivestarsarethedominantsourceof26AlintheGalaxy
tion:supernovae(SNe)andtheirremnants(SNRs),neutronstars
(Prantzos&Diehl 1996; Kno¨dlsederetal. 1999). In the early
orstellar-massblackholes,someofwhichmanifestintheform
2000s, the first detection of the Galactic 60Fe decay signal at
ofpulsarsormagnetars,X-raybinaries,ormicroquasars.
1173 and 1332keV was achieved from RHESSI observations
Athoroughunderstandingoftheprocessesthatruletheevo-
(Smith2004),withameasuredfluxofabout15%ofthe26Alde-
lution of massive stars and their death as core-collapse SNe is
2 PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex
cayflux.Althoughthedecayemissionwastooweaktoachieve ies have been devoted to the nucleosynthesis of 26Al and
a detailed cartography and thus confirm that massive stars are 60Fe in the Cygnus region (see for example delRioetal.
themainsourceof60Fe,thecombinedanalysisof26Aland60Fe 1996; Plu¨schkeetal. 2000; Cervin˜oetal. 2000; Plu¨schkeetal.
decaysignalsappearedasapotentiallystrongconstraintonthe 2002; Kno¨dlsederetal. 2002) and the latest studies agree that
processes that govern the nucleosynthesis in massive stars and the level of the 1809keV emission from Cygnus as deduced
SNe. from CGRO/COMPTEL observations is a factor of 2-3 above
The launch in 2002 of the INTEGRAL gamma-ray obser- the predictions, when the latter are based on the theoretical
vatory,equippedwiththeSPIhigh-resolutionspectrometer,has yields by Meynetetal. (1997), Woosley&Weaver (1995) and
openedthe possibility ofextendingthe analysisof the gamma- Woosleyetal.(1995)andontheobservedstellarcontentofthe
ray line emission from the Galaxy and add spectrometric in- Cygnusregion(includingtheupwardrevisionofthestellarpop-
formation to the COMPTEL results. Both the 26Al and 60Fe ulationofCygOB2byKno¨dlseder2000).Plu¨schkeetal.(2000)
decay signals have now been unambiguously detected by SPI show that the discrepancy could be alleviated if one includes
ontheGalacticscale (Diehletal. 2006;Wangetal.2007).The the enhanced yields from massive close binary systems in the
stronger26Alemissionmadeitpossibletoanalyseitsmorphol- calculation. Alternatively, Plu¨schkeetal. (2002) argue that the
ogyinmoredetailandappearedparticularlyprominentinsome problem may well come from an underestimate of the stellar
specific star-forming regions of the Galaxy, in agreement with content of the Cygnus OB associations due to a strong inter-
the COMPTEL observations. In a previous work (Martinetal. stellarextinction,aswasshowntobethecaseforCygOB2.In
2009,hereafterPaperI),weshowedthattheCygnusregionex- Kno¨dlsederetal.(2002),theauthorsspeculatethatfuturestellar
hibitsarelativelystrongsignalof26Aldecay,soweinvestigated models, especially those that include the effects of stellar rota-
furtherthegamma-raylineemissionofboth26Aland60Fefrom tion,couldsolvethedilemma.
Cygnus. InPaperI,weusedtheINTEGRAL/SPIobservationstore-
The Cygnus region and its peculiarities were presented in assess the 26Al and 60Fe decay emission from the Cygnus re-
detailin PaperI and we onlyrepeatthe main characteristicsof gion.In particular,we disentangledin a coherentway the con-
the region here and the principal arguments for studying it in tribution of the nearby clustered OB population of the Cygnus
nuclear gamma-ray lines. The Cygnus region, which we con- complex from the contribution of the more diffuse and spa-
sider to be that part of the Galactic disk located between lon- tiallyextendednon-clusteredpopulation,whichislikelytocon-
gitudes 70◦ and 90◦, harbours a very impressive concentration tribute as well to the observed emission. We now compare our
of massive stars at only ∼ 1-2kpc from us. About 170 O stars INTEGRAL/SPI observations of the gamma-ray line emission
have been recorded (Kno¨dlsederetal. 2002), distributed in 6 oftheCygnuscomplexwiththeoreticalpredictionsbasedonthe
OB associations and a dozen open clusters, among which the latest models of stellar nucleosynthesis.In recent years, stellar
spectacular Cyg OB2 cluster that alone comprises 100-120 O modelshavebeenimprovedwithmodifications,suchasthein-
stars(Kno¨dlseder2000;Comero´netal.2002).Inthefollowing, clusion of stellar rotation or the revision of the mass loss rates
wecollectivelyrefertotheseOBassociationsandopenclusters andnuclearcross-sections.Theseupgradeshavealreadyproven
(the details of which are presented later) as the “Cygnus com- to bring simulations closer to observations. For instance, mod-
plex”. With this term, we aim at gathering all these rich and els includingrotationcan accountforchangesin surfaceabun-
nearby stellar groups that are relatively correlated in terms of dancesoccurringduringthemainsequencephases(Hunteretal.
distance(between1and2kpc)andthuscontributetothespeci- 2008;Maederetal.2009)andfortheobservedvariationsofthe
ficityoftheCygnusregion.This“Cygnuscomplex”appearsto numberofWolf-RayettoO-typestarsasafunctionofmetallic-
be quite young, as shown by the lack of radio supernova rem- ityinconstantstarformationregions(Meynet&Maeder2005).
nants (SNRs) or pulsars. The Green catalogue1 gives about 10 Inthefollowing,weshowthatthenewstellarmodels,combined
radio SNRs between 70◦ and 90◦, but according to a compila- withtheworkdoneonobservationsinPaperI,considerablyal-
tion of distances by Kaplanetal. (2004), none of these objects leviate the long standing discrepancy between theoretical and
can be related to the Cygnus complex. Wendkeretal. (1991) observeddecayfluxesfromCygnus.
concluded from the sensitivity of their radio survey of the re-
gion thatno other radioSNR exists up to a distance of 10kpc.
2. ObservationalconstraintsfromINTEGRAL/SPI
Additionalevidencefortherelativeyouthofthecomplexcomes
from the absence of radio pulsars towardsCygnus up to 4kpc, In PaperI, we presented the analysis of about 4 years of
as indicated by the ATNF catalogue2 (Manchesteretal. 2005), INTEGRAL/SPIobservationsamountingtoatotaleffectiveex-
althoughrecentobservationswiththeLATtelescopeaboardthe posuretimeof63.2Ms.Mostofthisexposureisconfinedtothe
FermiGamma-RaySpaceTelescopehaveledtothediscoveryof Galactic plane, with 10.8Ms specifically covering the Cygnus
theyoungenergeticpulsarPSRJ2032+4127(Abdoetal.2009) region.Inthissection,wereviewthemainresultsthatwereob-
thatislikelyassociatedwithCygOB2(Camiloetal.2009).For tainedonthe26Aland60Fedecayemission.
thosereasons,theCygnusregionappearswell-suitedtostudying
massivestarsintheirhydrostaticphases.
2.1.The1809keVemissionfrom26Al
The Cygnus population of massive stars is close and rich
enough to ensure sufficient gamma-ray line fluxes and there- The 26Al isotope is produced predominantly by massive stars
fore constitutes a good opportunity to test our understand- at various stages of their existence and it is released in the
ing of the structure and evolution of massive stars, as re- ISM by both the stellar winds (of Wolf-Rayet stars espe-
vealed by their nucleosynthetic activity. A number of stud- cially) and the SNe. Nucleosynthesis of 26Al follows from the
25Mg(p,γ)26Alreaction,whichmostly occursduringH central-
1 GreenD.A.,2006,”ACatalogueofGalacticSupernovaRemnants burning(with25Mgcomingfrominitialmetallicity),C/Neshell-
(2006 April version)”, Astrophysics Group, Cavendish Laboratory, burning (with 25Mg coming from the CNO initial metallic-
Cambridge,UK(http://www.mrao.cam.ac.uk/surveys/snrs/). ity through the 14N(α,γ)18F(e−)18O(α,γ)22Ne(α,n)25Mg chain)
2 http://www.atnf.csiro.au/research/pulsar/psrcat/ and explosive burning of the C-shell (with 25Mg coming from
PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex 3
24Mg(n,γ)25Mg,24MgbeingaproductofC/Neburning). wellbethatitsstellarpopulationhassofarnotproducedasingle
The mean 26Al lifetime of about 1Myr allows it to diffuse SN, which would meanno release of 60Fe into the ISM, hence
inthelocalISMbeforedecayinginto26Mg.Asaconsequence, no1173/1332keVsignalfromtheCygnuscomplexatall.Asa
the corresponding 1809keV emission is of diffuse nature with matteroffact,wedidnotobserveany1173/1332keVemission
an overall distribution that follows that of massive stars in our fromthe Cygnusdirectionandderiveda 2σupperlimiton the
Galaxy, as also traced for instance by the microwave free-free 60Fe decay flux of 1.6 × 10−5phcm−2s−1 (consistent with the
emissionfromHIIregionsortheinfraredemissionfromheated valueobtainedbyWangetal.2007).
dust (Kno¨dlsederetal. 1999). Most of the Galactic 1809keV
flux is thus confined to the Galactic plane and inside the inner
regions,exceptforanotableemissionfromtheCygnusandSco- 3. Populationsynthesis
Cenregions(seePaperI).
In this section we present the most important aspects of the
FromtheINTEGRAL/SPIdata,wehavefoundthatthedif-
populationsynthesissimulationswe performedto compareour
fuse 1809keV emission from the Cygnus region is well repre-
INTEGRAL/SPI observations with. In this effort, we took ad-
sented by a 3◦ × 3◦ 2D Gaussian intensity distribution (which
vantageoftherecentprogressbytworesearchgroupsinthefield
implies a characteristic angular size of 9-10◦) centred approxi-
ofmassivestarsnucleosynthesis.Inthefollowing,weintroduce
matelyonthepositionoftheCygOB2cluster.Thisconfirmsex-
thespecificitiesofeachgridofstellarmodelsandprovidesome
pectationsthatthelatterdominatestheenergeticsandnucleosyn-
detailabouttheirimplementationinourpredictivetool.
thetic activity in this area. It should be emphasised, however,
thatweonlyhaveweakconstraintsonthemaximalextentofthe
emissionduetointrinsiclimitationsofthecodedmaskimaging 3.1.Thestellarmodels
systemoftheSPIinstrumentandyet,thesizewefoundisconsis-
Once the massive stars had been recognised as the dom-
tentwiththeresultsobtainedfromtheCGRO/COMPTELobser-
inant source of the Galactic 26Al (Prantzos&Diehl 1996;
vations(Plu¨schkeetal.2001).Thetotalfluxfromtheextended
sourceintheCygnusdirectionis(6.0±1.0)×10−5phcm−2s−1, Kno¨dlsederetal.1999),thequestionofitsoriginshiftedtoiden-
where(3.9±1.1)×10−5phcm−2s−1isattributedtotheCygnus tifyingthesubsetofmassivestarsand/orevolutionaryphasesre-
sponsible for its ejection. For historical reasons, mostly linked
complexandthe remainingflux stemsfromGalactic diskfore-
totheevolutionofthestellarmodels,two“competing”sources
groundandbackgroundemissions.
were discussed: stars less massive than about 35M through
ThankstothehighspectralresolutionoftheSPIinstrument, ⊙
their SNII explosions, and stars more massive than 35M
we were able to study the profile of the 1809keV line. We ob- ⊙
through their WR winds (see the review by Limongi&Chieffi
served a line position corresponding to a radial velocity of 33
±66kms−1,whichagreeswiththeoverallradialmotionsinthe 2006, hereafter LC06). The debate then settled on the relative
contributionofSNIIandWRtotheGalactic26Albudgetand,in
direction and at the distance of the Cygnus complex, as indi-
this situation, the 60Fe appeared as a promising discriminating
catedforinstancebyspectrometricobservationsoftheCO line
agentsinceitisreleasedonlybySNexplosions.
(Dameetal. 2001). We also measured a slight line broadening
that,withinthestatisticaluncertainty,isconsistentwith26Albe- TheworkofLC06hasbroadenedtheissuethankstoanew
gridofstellarmodels.Theauthorscomputedthenucleosynthe-
ingstillorinamediumwithturbulentorexpansionvelocitiesof
upto100-200kms−1. sisyieldsforawiderangeofnon-rotatingsolarmetallicitymod-
els, with initial stellar masses from 11 to 120M . The models
⊙
were followed over their entire life, including both the hydro-
2.2.The1173and1332keVemissionfrom60Fe static phases and the explosive burning episodes of the super-
novaexplosion.ThemainoutcomesoftheLC06studyare:
Themassivestaroriginofthe60Feisotopeisfarlessestablished
than for 26Al. The nucleosynthesis of 60Fe by massive stars 1. Overthemassintervalconsidered,mostoftheejected26Al
wasaddressedbyTimmesetal.(1995)andextensivelyreviewed has explosive origins. For the particular case of the most
by(Limongi&Chieffi 2006). Fromthese works, we knowthat massivestars, typicallytheWR progenitors,the production
massivestarsareasourceof60Fethroughthe59Fe(n,γ)60Fere- of26AlduetoexplosiveburningoftheC/Neshellsexceeds
actionthatoccursduringHeshell-burning,Cshell-burning,and the quantity synthesised by central H-burning and carried
explosive burning of the C-shell. In these processes, the seed awaybythestellarwinds.
amounts of Fe come from the initial metallicity and so do the 2. The productionof 60Fe (by C/Ne convectiveshells burning
neutrons,throughthe 22Ne(α,n)25Mg reaction (with 22Ne com- mostly)of themostmassivestarsthatexplodeasSNIb/cis
ing from the initial CNO, see 2.1). The 60Fe thus producedby up to a factor 10 higher than the yields associated with the
massive stars is then released in the ISM only through SN ex- lowermassstarsthatexplodeasSNII.
plosions and no wind contribution is expected. Other objects
such as AGB stars (Karakas&Lattanzio 2007) or SNe of type IthasalwaysbeenclearthatadichotomybetweenWRandSNII
Ia (Iwamotoetal. 1999) are also potential producers of 60Fe. asthesourcesof26Alisinadequateandthatthewholecontribu-
Although the relative contribution of each class of objects re- tionofeachmassivestarshouldbeaccountedfortounderstand
mains entirely unconstrained on the observational level, the theoriginofboththe26Aland60Fe.Moreover,theinitialmetal-
available models for the various potential sources indicate that licity mayhavea strongimpactontherelativecontributionsof
core-collapseSNeshouldprovidemostoftheGalactic60Fe. WRwindsandSNexplosionstotheproductionof26Al,aswill
The observed intensity of the Galactic decay emission at beillustratedlater.
1173 and 1332keV is a factor of 7-8 lower than the 1809keV In parallel to the work of LC06, grids of stellar models
emission from 26Al (Harrisetal. 2005; Wangetal. 2007, and including stellar rotation have revealed that the latter strongly
PaperI), so the detection of any 60Fe signal from Cygnusmay affects the evolution and nucleosynthesis of massive stars.
atfirstsightbedifficultgiventhesensitivityofINTEGRAL/SPI. Meynet&Maeder(2003,2005)havecomputedtheevolutionof
Inaddition,theCygnuscomplexseemsquiteyoung,anditmay rotating modelsof stars with initial masses from20 to 120M
⊙
4 PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex
3.2.Thegridofyields
The LC06 grid is complete and homogeneousbut does not in-
clude stellar rotationand is onlyavailable for solar metallicity,
while the MMP05 grid includes the effects of stellar rotation
and metallicity, but only covers the longest hydrostatic phases
of those stars initially more massive than 20M . To make the
⊙
most realistic predictionsof the nucleosynthesisactivity of the
Cygnuscomplex,wethereforebuiltourowngridofyieldsfrom
theworksofLC06andMMP05.
Before entering the description of our grid, we emphasise
theterminologythatwillbeusedthereafter.Wedividedthenu-
cleosynthesiscontributionsinto“windyields”and“SNyields”,
whichis notequivalentto a separationinto hydrostaticandex-
plosive yields. Wind yields pertain to 26Al alone and are obvi-
ouslyofhydrostaticorigin,whereasSNyieldsconcernbothiso-
topes and have both hydrostatic and explosive origins. For in-
Fig.1.Supernovayieldsof26Aland60FeasafunctionoftheCO stance,the60FereleasedinaSNexplosioncomesfromthehy-
drostaticburningoftheHeandCconvectiveshellsbutalsofrom
coremass(fromthemodelsofLC06).Thesecurvesareusedto
theexplosiveburningoftheCshell.Wealsonotethatthewind
associate the purelyhydrostaticmodelsof MMP05with super-
yieldisaprogressive,time-dependentoutflowof26Al,whereas
novayields.
theSNyieldisconsideredaninstantaneousreleaseof26Aland
60Fe.
Thegridweused inourpopulationsynthesiscodeisbased
onthefollowingassumptions:
uptotheendofcentralHe-burning.Theserotatingmodelswere
computedforvelocitiescompatiblewiththemeanobservedve- 1. Thelowermassstars,from11upto20M ,arerepresented
⊙
locitiesforOBstars,andtheimpactofstellarrotationonthepro- by the models of LC06 for both their wind and SN contri-
ductionof26AlhasbeenexaminedbyPalaciosetal.(2005).The butions3. It has also been assumed that these LC06 models
mainoutcomesoftheseworks(hereaftercollectivelyreferredto arevalidatmetallicitiesotherthansolar.Wewilldiscussthis
asMMP05)are: assumptionlateron.
2. The higher mass stars, from 20 to 120M (typically those
⊙
1. Rotation increases the convection and the circulation of
thatbecomeWRstars),arerepresentedfortheirwindcontri-
chemicalspeciesinsidethestar.Thisimpliesalargerinitial
butionbytherotatingmodelsofMMP05forthefourmetal-
reservoir of 25Mg available for proton capture (in the case
licities considered by these authors. Each of these stellar
ofnon-zeroinitialmetallicity)andanearlierejectionofthe
tracks is then associated to one of the SN yields of LC06
26AlproducedbycentralH-burning.
throughitsCOcoremass(asexplainedbelow).
2. Rotationincreasesthemasslossbyalteringthesurfacegrav-
ity,theeffectivetemperature,andtheopacityoftheenvelope. Inthecompositionofourgrid,weassumedthattheSNyieldis
This,combinedwiththemoreefficientdiffusionoftheburn- strongly linked to the final CO core mass of the model (a hy-
ingproductstothestellarsurface,resultsinanearliertransi- pothesisalsousedbyCervin˜oetal.2000).Thiscanfirstbejus-
tiontotheWRphaseandadecreaseintheminimuminitial tified by the fact that, at the moment of core-collapse, most of
mass to becomeWR. As a result, the 26Al containedin the the26Aland60FesynthesisedsofarislockedintheCOcoreas
Hecore(followingcentralHburning)isreleasedsoonerand aresultoftheHe/C/Neshell-burningepisodes.Second,itseems
bymorestars. reasonable to consider that the amounts of 26Al and 60Fe pro-
ducedexplosively(throughexplosiveburningoftheC-shell)de-
In summary,stellar rotation alters the 26Al yields in two ways: pendonthesizeoftheCOcore(evenifotherparameters,such
thequantitiesejectedintheWRwindsareincreasedandthetime asthechemicalprofileandthemass-radiusrelation,maybeim-
profile of the injection into the ISM is differentas well. While portant). From the LC06 grid of models, it is possible to draw
thedetailsofthe 26Alreleasewith timeare notimportantfora relations between the SN yields and the CO core masses (see
large,stationarysystem such asthe Galaxy,forwhichonlythe Fig.1).Theserelationsarethenusedtoassociatethepurelyhy-
totalyieldsperstararerelevant,theCygnuscomplexofmassive drostatic modelsof MMP05 with SN yields throughtheir final
starsisayoung,evolvingregionandthecomparisonofobserva- COcoremassthat,forsameinitialstellarmass,differsfromthat
tionswithpredictionsthereforeincludesatemporaldimension. obtainedby LC06. Indeed,forstars belowabout60M , stellar
⊙
Therefore,areliablepopulationsynthesismodelshouldnotonly rotationincreasesthefinalCOcoremassbecauseofanenhanced
be able to reproduce the observed 1809keV flux from Cygnus convection;above60M , rotationcauses a decrease in the CO
⊙
butalso reach an agreementwithin a time rangethat is consis- coremassbecauseofanenhancedmassloss(Meynet&Maeder
tentwiththeageestimatesofthespecificstellarclusters. 2003,2005).
Besidestheaboveimprovements,bothgridsofstellarmod- Thisapproachtakesintoaccounthowstellarrotationcanaf-
elsarealsobasedonrevisedmasslossratestakingtheeffectof fecttheSNyieldsthroughitsimpactonthestellarevolution.It
clumping in the winds into account and on revised nuclear re- should still be emphasised that the SN yields used in our grid
action rates. MMP05 also explored the influence of the initial
metallicityZ bycomputingfourgridsatZ = 0.004,0.008,0.02, 3 In practice, however, the 26Al wind contribution of these stars is
and 0.04. The grid of LC06 was only computed for a Z=0.02 negligible because they experience little mass loss compared to the
solarmetallicity. highermassstars.
PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex 5
Fig.2.26AlwindyieldsfromLC06,MMP05,andWH07,com- Fig.3.26AlsupernovayieldsfromLC06andWH07,compared
paredtotheolderresultsfromMM97. totheolderresultsfromWW95.Alsoplottedaretheyieldsfor
MM97 and MMP05 extrapolated through CO core mass from
WLW95andLC06respectively.
come only from non-rotatingmodels. The impactof stellar ro-
tation on the nucleosynthesis of the latest evolutionary stages
remainsunexploreduptonow.LC06pointedouthowimportant lar winds. The WH07 wind yields are all above the yields of
the shell convection efficiency is to the synthesis of 26Al and LC06(uptoafactorof5insomecases)butalwaysremainbe-
60Fe, but how stellar rotationaffects the nucleosynthesisof the low the MMP05 yields. This fact very likely comes from the
He/C/Ne shell-burning episodes is still an open question. Our absenceofrotationintheWH07models,butthedifferencebe-
limitedknowledgeonthatpointthereforeconstitutesapotential tweenWH07andLC06couldseemsurprising.Indeed,alower
sourceoferrorforourSNyields. initialmetallicity like Z=0.016meansthatall otherparameters
In Figs. 2 and 3, we show the 26Al yields for LC06 and being unchanged, a reduced initial reservoir of 25Mg available
MMP05 (where the SN yields for MMP05 were extrapolated forprotoncapture,hencelower26Alwindyields,butathorough
through CO core mass, as explained above), compared to the comparisonoftheWH07andLC06modelsisbeyondthescope
previous generation of yields from Meynetetal. (1997, here- of this paper. The same rates were used for the main reactions
after MM97),Woosley&Weaver (1995, hereafterWW95) and governing26Alproduction,butthehighermassmodelsofWH07
Woosleyetal. (1995, hereafter WLW95). This previous gener- werefoundtoexperiencemoremassloss(asdiscussedinmore
ation of yields was used in a population synthesis analysis by detailbelow).Thismayexplainthe situationabove40M . For
⊙
Cervin˜oetal. (2000) and Kno¨dlsederetal. (2002), who con- the lower-mass models, however, the higher 26Al wind yields
cluded that they could not account for the 1809keV flux from obtainedwithalowerinitialmetallicitymaystemfromthedif-
CygnusobservedbyCGRO/COMPTEL(seealsoPlu¨schkeetal. ferenttreatmentoftheconvectivephysicsand/ortothedifferent
2001).InFig.4,weshowthe60FeyieldsforLC06andMMP05 mass loss rate prescriptionduring main sequence or red super-
(where again the SN yields for MMP05 were extrapolated giantstages.
throughCOcoremass). In Fig. 3, the 26Al SN yields of LC06 are plotted together
In all three figures, we also compare the set of yields ob- with the SN yields extrapolated through the CO core mass for
tained by Woosley&Heger (2007, hereafter WH07). These theMMP05rotatingWRmodels.Oneclearlyseestheeffectof
yields come from a large grid of stellar models (from 13 to rotationonthesizeoftheCOcore,henceontheextrapolatedSN
120M ) that basically includes revised mass loss rates and re- yields, asdiscussed above.An oldergrid ofyieldsis also plot-
⊙
vised nuclear reaction rates, but no rotation. The authors used ted,whichconsistedoftheWW95SNIIyieldsupto25M and
⊙
an initial metallicity of Z=0.016, corresponding to the revised the SNIb/c yields associated with the MM97 non-rotating WR
solarabundancesofLodders(2003),andcomputedbothhydro- models.FortheseSNIb/cyields,theyieldversusCOcoremass
static and explosive phases. In its general features, the WH07 relationwas derivedfromthe pureHe star modelsofWLW95.
grid is very similar to the LC06 grid, except for the different Regardingthe26AlSNyieldsofWH07,theyareverysimilarto
initialmetallicity.Inthedetails,however,manyaspectsofstel- thoseofLC06upto40M (variationsremainwithin±50%)ex-
⊙
larphysicswereimplementeddifferently(suchasthemassloss ceptforthestarsaround14-15M ,forwhichLC06foundadra-
⊙
prescriptions,seebelow).Asseeninthenextparagraphs,thedif- maticincreaseofthe26AlSNyieldnotseeninWH07.Beyond
ferencesbetween the WH07 yields and the LC06 and MMP05 40M ,theyieldsofWH07arebelowthoseofLC06byafactor
⊙
yields illustrate the extent to which the implementation of the of up to 10 for the 120M model. This is very likely an effect
⊙
stellarphysicscanaffectthenucleosynthesisresults. ofthestrongermasslossexperiencedbytheWH07highermass
In Fig. 2, we see that the MMP05 yields obtainedfrom ro- models.
tatingmodelsaremoreenhancedthantheyieldsofMM97,and Again, no detailed comparison was performed but we
thisdespitethatthemasslossratesusedintheMMP05models notedthattheWellstein&Langer(1999)masslossprescription
arelowerthanintheMM97models.Thisillustratestheimpor- (scaled by 1/3 to account for clumping) used by WH07 gives
tanceoftheeffectsofrotationthat,asexplainedin3.1,favours masslossratesinthelateWRstages(typicallyWNEandWCO
the production of 26Al and its subsequent ejection by the stel- stars)thatareafactorof2-3abovetheNugis&Lamers(2000)
6 PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex
Cygnus region have fostered a great number of studies that
togetherprovideuswithaprettygoodknowledgeofthearea.In
particular,recentworkshaveleadtoasubstantialrevisionofthe
stellar contentof the Cyg OB2 cluster and thusallowfor more
accuratepredictions.
From the stellar census of Kno¨dlseder (2000),
Kno¨dlsederetal. (2002), and LeDuigou&Kno¨dlseder(2002),
weselectedthemostrelevantOBassociationsandopenclusters
of the Cygnus region. We restricted ourselves to a longitude
range of 70-90◦ and among the stellar groups falling in that
interval, we excluded the ones with ages above 20Myr (since
their decay fluxes are negligible after that time, see Fig. 6).
The selected objects are listed in Table 1, together with their
maincharacteristics.Kno¨dlsederetal.(2002)indicatethatmost
open clusters are potentially physically related to some of the
OB associations. In total, our selection amountsto a total of ∼
170 O stars, most of which are in the Cyg OB2 cluster. This
Fig.4.60FesupernovayieldsfromLC06andWH07.Alsoplot- ensemble is what we have called so far the Cygnus complex.
ted are the yields for MMP05 extrapolated through CO core Schneideretal.(2006)demonstratethatCygOB1,2,and9very
massfromLC06. likely originated from the same giant molecular cloud, which
givesaphysicalrealitytothisgrouping.TheconnectionofCyg
OB3,7,and8tothiscomplexis,however,stilluncertain.
Whether all these clusters share a common origin does not
prescriptionusedbyLC06.Thisisobviousfromthefinalpresu- matteratallforourpurpose.Ouraimhereissimplytogatherthe
pernovamassesofthe40-120M⊙ models:whilethoseobtained nearbyclusteredstellarpopulationthatisverylikelyresponsible
by LC06 are in the 12-20M⊙ range, those obtained by WH07 formostofthestrong1809keVemissionfromCygnus.Insome
are between 6-10M⊙. It is interesting to note that LC06 tested previous studies (delRioetal. 1996; Plu¨schkeetal. 2002), the
the impact of increased mass loss during the WNE and WCO total 1809keV flux from the Cygnus region was compared to
phasesbyusingtheprescriptionofLanger(1989),whichgives theoreticalpredictionsbased on the clustered stellar contentof
ratesafactorof2-3abovetheratesofNugis&Lamers(2000). the Cygnus region supplemented by a few isolated WR stars
Inthiscase,their40-120M⊙modelsendedupwithfinalmasses and SNRs. In our study, we focus both observationally and
and 26Al SN yields that are very similar to those of WH07. A theoretically on the clustered stellar contentalone, because the
stronger mass loss actually reduces the size of the He and CO clustered population is likely to be better inventoried than the
coresandleadstolower26AlSNyields.Thedifferencebetween isolated or non-clustered population. In PaperI, we used the
the WH07 and LC06 26Al SN yields is all the more important INTEGRAL/SPI data to disentangle the 1809keV flux of the
becausein the WH07 yieldsthere is a contributionfromthe ν- Cygnus complex from the more spatially extended emission
processthatisabsentintheLC06yields(see4.1.3).Thealterna- likely due to unclustered objects like non-member WR and
tivegridofWH07thusillustrateshowimportantmasslosscan OB stars (runaway stars or stars from previous star formation
befornucleosynthesis. episodes, see Comero´n&Pasquali (2007) and Comero´netal.
InFig.4,the60FeyieldsofLC06andWH07areplottedwith (2008)) or isolated SNRs. In the present paper, we compare
the SN yields extrapolated through the CO core mass for the this flux to population synthesis predictions based only on the
MMP05rotatingWR models.Asfor26AlSN yields,the effect Cygnuscomplexpopulation.
ofrotationonthesizeoftheCOcore,henceontheextrapolated
yields, is manifest. The WH07 yields are apparently quite dif-
3.4.Thecode
ferentfromthoseofLC06. Between10-20M theyareuptoa
⊙
factorof20abovethoseofLC06,thenbetween20-50M⊙ they Ourpopulationsynthesiscodeisverysimilartomanyothersuch
arestillabovethoseofLC06bymoremoderatefactorsofupto tools (like Leithereretal. 1992; Cervin˜oetal. 2000; Vossetal.
5,andbeyond50M⊙ theydropbelowLC06byquitelargefac- 2009,forexample).Forthecaseweareinterestedin,theparam-
tors.Forthehighermassmodels,theloweryieldsofWH07very etersofasimulationarethecharacteristicsofthestellarpopula-
likelycomefromthestrongermasslossofWRstars,asalready tionto be modelled:slope andboundsof the IMF(initialmass
discussedabove.Hereagain,thesameeffectwasnotedbyLC06 function, assumed here to be a homogeneouspower-law func-
whentheyusedtheincreasedmasslossratesofLanger(1989). tion),stellarcontent(givenasthenumberofstarsobservedina
For the lower masses, however,the differencesare not so easy certainmassinterval),andmeandistancetothecluster(tocom-
toexplain.Thelowerinitialmetallicityshouldhaveresulted,all putethedecayfluxes).Weassumethatthestarformationoccurs
otherparametersbeingunchanged,inloweryieldssince60Feisa asaninstantaneousburst,whichmeansthatallstarsformatthe
puresecondaryproduct.Themajorreactionratesfortheproduc- sametime(discussedlater).
tionanddestructionof60FewerefoundtobeidenticalinWH07 Oncethepopulationtobesynthesisedhasbeendefined,we
and LC06, so the difference may come from another aspect of create a cluster realisation by randomly sampling the IMF un-
stellarphysicssuchasconvectionormixing. til the observationalcriterion (the numberof observedstars) is
met.Then,allstarsareevolvedintimestepstypicallyof104yrs.
For each run, we compute the time-dependent release of 26Al
3.3.ThestellarcontentoftheCygnuscomplex
and60Fe,thedecayfluxesat1809and1173/1332keVandafew
The last ingredient of our simulation is the definition of the otheroutputssuchasthemechanicalluminosity.Toevaluatethe
stellar population to be modelled. The peculiarities of the effectsoffinitesampling,agivenstellarpopulationismodelled
PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex 7
Table1.CharacteristicsoftheCygnusOBassociationsandstel- the13or16M models.Asaconsequence,itshouldberemem-
⊙
larclusterscollectivelyreferredtoastheCygnuscomplex(from beredthatthe finite grid ofyieldsand the associated interpola-
Kno¨dlsederetal.2002). tionschemearepotentialsourcesoferrors.
Name Observedpopulation Distance(pc) Age(Myr)
4. Predictionsversusobservations
CygOB1 23∈[15,40M ] 1905 4
⊙
BasedonthecodeandinputspresentedinSect.3,wehavesim-
CygOB2 120∈[20,120M⊙] 1584 2.5 ulatedthehistoryoftheejectionof26Aland60FeintheCygnus
complexandcomputedthecorrespondingdecayfluxes.Wefirst
CygOB3 14∈[25,60M ] 2187 3.5
⊙
focused on the purely photometric aspect, trying to reproduce
CygOB7 10∈[7,25M⊙] 832 3.5 theobservedfluxesbysimplyaddingthecontributionsfromall
the stellar groupslisted in Table 1. In the second part, we also
CygOB8 6∈[20,40M ] 2399 7.5
⊙ attemptedtoreproducethespatialextentofthisemission.
CygOB9 8∈[20,40M ] 1259 3.5
⊙
Ber86 11∈[7,25M ] 1660 4 4.1.Thedecayemissionfluxes
⊙
Ber87 24∈[7,25M ] 1905 4.5 All the stellar groups listed in Table 1 have been modelled in-
⊙
dependently,assumingacoevalstarformationforeachofthem.
NGC6871 13∈[7,25M ] 2399 5.5
⊙ Then, the lightcurves at 1809 and 1173/1332keV were added,
NGC6913 13∈[7,40M ] 1820 3.5 with the appropriate time shifts to take the differences in age
⊙
of the different stellar populations into account. The resulting
NGC6910 7∈[7,25M⊙] 1820 4.5 lightcurves for the whole Cygnus complex are given with t=0
correspondingtothepresentdayandthusallowedexploringthe
NGC6883 2∈[12,15M ] 1820 15
⊙
pastandfutureevolutionofthedecayfluxes.
IC4996 6∈[6,25M⊙] 1660 5.5 Asafirststep,wefocusedontheresultsobtainedforanini-
tialsolarmetallicity,firstlybecausemostofthepastpopulation
synthesisstudieswerebasedonnucleosynthesisyieldsfromso-
larmetallicitystellarmodelssoacomparisoncouldbemade,but
severaltimes(typically100-200)untilthemeanstellarmassand alsobecausetheCygnuscomplexliesonthesolarcircleandis
numberofstarsobtainedoveralltrialsarewithin1%ofthethe- thusexpectedtohaveanearlysolarmetallicity.
oreticalvaluesforthespecifiedIMF. Foreachquantity,thisal-
lows computing the mean value and the statistical variance at
eachtimestepinthelifeofthecluster. 4.1.1. ThecaseforsolarmetallicityZ=0.02
Althoughasyntheticstar cantakeanyinitialmassbetween
The results obtained for an initial solar metallicity are pre-
11 and 120M (the boundsof our grid of yields), we have the
⊙ sentedinFig.5(top),togetherwiththeobservationalconstraints
nucleosynthesis yields for only a finite number of initial stel-
we derived from the INTEGRAL/SPI observations. The pre-
lar masses, typically 11, 12, 13, 14, 15, 16, 17, 20, 40, 60, 80,
dicted decay fluxes at 1809 and 1173/1332keV are there to-
120M (the MMP05 grids of the higher mass models differ
⊙ gether with their typical variances due to the finite IMF sam-
for the various metallicities). We therefore need an interpola-
pling. From the open clusters and OB associations ages deter-
tionschemetocomputetheyieldsofthesyntheticstarsfromthe
mined by Kno¨dlsederetal. (2002), our current position along
onesatourdisposal.
the time axis is indicated by the zero point. Yet, these clus-
The characteristicsof a synthetic star with a randominitial
ter ages were determined by fitting isochrones to Hertzprung-
mass are determined from the closest lower and higher mass
Russeldiagrams,andtheisochronesusedbytheseauthorswere
models (for instance, the characteristics of a 53.6M star are
⊙ builtfromthenon-rotatingstellarmodelsavailableatthattime.
determinedfromthe40and60M models).Afirstcubic-spline
⊙ Rotationisnowknowntoincreasethelifetimeofthemodelsby
interpolationisperformedtodeterminethedurationsofthemain
15-25%, mainly through augmenting the main-sequence dura-
evolutionary sequences of the star: central H-burning, central
tion(Meynet&Maeder2003).Therefore,theclusterageshave
He-burningandfinalstages(whenavailable).Then,foreachof
probablybeenunderestimatedbythesameamount,andourcur-
thesethreesequences,the26Aland60Fequantitiesejectedovera
rent position on the time axis of Fig. 5 is likely shifted to the
giventimefractionofthesequenceareobtainedbylinearinter-
rightofthezero-point,asindicatedbythegreyshadedarea.
polationofthequantitiesejectedbytheclosestlowerandhigher
Insidethistimerange,thereisclearagreementbetweenthe
mass models over the same time fraction. When the synthetic
predicted1809keVfluxfrom26Alandtheobservedvalue,while
starexplodes,its26Aland60Feyieldsaredeterminedbyalinear
the theoretical level of the 60Fe decay emission is consistent
interpolationbasedontheCOcoremassandreleasedinthecur-
withourupperlimit.Ourpopulationsynthesisindicatesthatwe
renttimestep.
are currentlyin a periodofsteep increase of the1809keV flux
The work of LC06 clearly shows that the yields are not
(overMyrtimescales)thatfollowsfromasubstantialreleaseof
smooth functions of the initial stellar mass4. For instance, the
26Al through the stellar winds of the most massive stars, sup-
lackofaCconvectiveshellintheir14and15M modelstrans-
⊙ plementedbythefirstsupernovaexplosionsthatoccurredinthe
latesintoadramaticincreaseinthe26Alproductioncomparedto
oldest stellar groups, typically those above 3.5Myr (the short-
4 Onthecontrary,thedurationsofthemainburningstagesareaquite est lifetime in our grid of models, corresponding to an initial
smoothfunctionoftheinitialmass,closetoapower-law,andthatiswhy 120M⊙ star).Atabout1Myronthetime axisofFig. 5(which
inourworktheyaredeterminedbyacubic-splineinterpolationinstead may be our current position), most of the wind 26Al has been
ofasimplelinearinterpolation. injectedintheISM;thesupernovaethentakeoverandnowre-
8 PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex
lease both 26Al and 60Fe. From there on, the 1809keV flux is showthatsomefractionoftheCOmassobservedtowardCygnus
expectedto rise again,less rapidlythough,andto reachwithin isactuallylocatedbehindtheOBassociations,whichmeansthat
∼ 1Myr a maximum mean value of ∼ 4.2 × 10−5phcm−2s−1, theappliedcorrectionwasincorrectinsomecases.
which remains in our measured flux interval. Regarding the Our work indicates that the past dilemma resulted from an
1173/1332keV emission, the most massive stars of the oldest overestimateofthe1809keVfluxoftheCygnuscomplex,com-
stellar groupsexplodedand consequentlystartedinjecting60Fe binedwithanunderestimateofthenucleosynthesisyields.First
intheISM.Thenumberofeventsuptonow,however,istoolow of all, in PaperI, a careful analysis of the INTEGRAL/SPI
tohavebuiltupadetectableamountof60Fe.Moreover,ourpre- 1809keV observations showed that the decay flux attributable
dictionshowsthateventhemaximum1173/1332keVflux,tobe to the Cygnuscomplexis only ∼ 65%of the total flux coming
reachedin∼2-3Myrfromnow,remainsbelowourpresentup- fromtheCygnusregion.Thisresultwasobtainedbyseparating
perlimit. the emission due to the Cygnus complex from the foreground
Our simulation predicts that a certain number of SNe, typ- andbackgroundmeanGalactic contribution.Then,stellar rota-
ically 10-20 over the last Myr, exploded in the Cygnus com- tion, as discussed in Sect. 3.1, turned out to be a key factor in
plex,whichcontradictstheapparentabsenceofradioSNRsand nucleosynthesissinceitenhancestheproductionof26Albutalso
the detection of only a single pulsar in its various OB associ- affectstherateofitsreleaseintheISM.Inaddition,thecompu-
ations and open clusters. The absence of SNRs could be ex- tation in a coherentway of the SNIb/c contribution (by LC06)
plainedbythefactthatSNRsexpandinginsidethehottenuous showed that the SN yields of the most massive stars cannotbe
interiorsofsuperbubblesexhibitveryweak characteristicradio neglectedorsimplyextrapolatedfrompureHestarscalculations.
and optical signatures (Chu 1997). With their high stellar con-
tent, the OB associations of the Cygnus complex are expected
4.1.2. ThecaseformetallicityZ=0.01
to blow large superbubblesinside of which the SNR are likely
to be invisible. Moreover, the typical radio lifetime of a SNR Mostofthepaststudieswerebasedonthenucleosynthesisyields
is short compared to the lifetime of 26Al (104 versus 106yrs, ofsolarmetallicitystellarmodels,sowepresentedtheabovere-
Frailetal. 1994) andall SNRs may wellbe completelydiluted sultforcomparison;yet,thereareindicationsthatthestellarpop-
in the ISM by now. In that case, we would expectto see some ulationoftheCygnusregionmaybesubsolarwithametallicity
manifestationsoftheircompactremnants.Uptonow,onlyasin- ofaboutZ=0.01(Daflonetal.2001).Althoughwedonothavea
gle pulsar has beenproposedassociated with the Cygnuscom- fullyhomogeneousgridofstellarmodelsofvariousmetallicities
plex, more precisely with Cyg OB2 (Camiloetal. 2009), de- coveringhydrostaticandexplosivephasesatourdisposal,weex-
spite the dedicated deep surveys that have been undertaken at ploitedasmuchaspossiblethe worksofLC06andMMP05to
radiofrequencies(Janssenetal.2009).Thedifficultyoffinding estimate the 26Al and 60Fe yields of a non-solar population of
pulsars in the Cygnus complex could come from an excessive massive stars. We recall here that our grid of yields was built
scintillation or dispersion of the radio signals in the strongly frompurelyhydrostaticstellartracksofvariousinitialmetallic-
ionised environment of such a concentration of massive stars. itiescombinedwithfullcalculations(hydrostaticandexplosive)
Or it may be that the narrow cones of their radio emission do onlyatsolarmetallicity,theconnectionbeingmade(whennec-
not intersect our line of sight. In the latter case, these pulsars, essary)throughtheCOcoremass(see3.2).
if they exist, may become “visible” to the Fermi/LAT instru- ThedecaylightcurvesforZ=0.01(obtainedbylinearinter-
ment through their gamma-ray emission, which is unabsorbed polation between Z=0.008 and Z=0.02) are presented in Fig.
and less-focused compared to the radio emission. The discov- 5 (bottom). Compared to the Z=0.02 case, the peak 1809keV
ery by Fermi/LAT of the gamma-ray pulsar PSR J2032+4127 flux is somewhat reduced and occurs at a later time, while the
that is likely located in Cyg OB2 is promising in this respect 1173/1332keV flux is increased by almost a factor of 3. Both
(Abdoetal.2009). differences come from the effect of lower metallicity on stel-
Theaboveresultconstitutesastepforwardcomparedtothe larevolution.Indeed,alowerinitialmetallicityimpliesreduced
previousstudiesofthenucleosynthesisactivityoftheCygnusre- masslossforstarsinitiallymoremassivethan20M ,sincethe
⊙
gion.Indeed,thelatestevaluationsofthesituationconcludethat, winds of the OB and WR phases are mostly driven by radia-
from the established stellar content(includingthe revised 100- tivepressureonmetalsthroughtheirnumerousabsorptionlines.
120OstarsofCygOB2)andthestellar yieldsofMeynetetal. ReducedmasslossleadstolargerCOcoremasses,hencehigher
(1997) and Woosley&Weaver (1995), the observed 1809keV SNyields,asweassumedthatthelattermostlydependsonthe
flux from Cygnus is a factor of 2-3 above the predictions (see COcoremass.Thisaccountsfortheconsiderableincreaseinthe
forinstanceKno¨dlsederetal.2002).Plu¨schkeetal.(2000)point 60Fe productionin the Z=0.01 case. For 26Al, however,the in-
out that the possible enhanced yields of massive close binary creaseintheSNyieldsiscompensatedbyadecreaseinthewind
systems could alleviate the discrepancy. While it is clear that yields;indeed,alowerinitialmetallicitymeansareducedmass
such systems may occur in the Cygnus complex and could in- loss but also a reduced initial 25Mg reservoir for proton cap-
deed have a strong impact on the nucleosynthesis yields, our ture(hencesynthesisof26Al)duringcentralH-burning.Onthe
theoretical knowledge of this scenario is still too uncertain to whole,the1809keVlightcurveisnotdramaticallyalteredbythe
allow us to take it quantitatively into consideration. Following reduced metallicity, but the stronger SN contribution explains
the significant upward revision of the Cyg OB2 stellar con- the delay in the 1809keV peak flux compared to the Z=0.02
tent(Kno¨dlseder2000),Plu¨schkeetal.(2002)suggestedthatthe case.
stellar content of the other OB associations of the Cygnus re- Within the likely present-day time window, the mean pre-
gion maybe underestimateddue to a strong interstellar extinc- dicted 1809keV flux is at best a factor of two below the ob-
tion;with correctionfactorsbased on the CO columndensities served value.Consistency is foundlater on, at ∼ 2-3Myr from
in the direction of Cygnus, the authors show that the resulting now,butthenthe1173/1332keVfluxhasrisenclosetooreven
increaseinthestellarcontentcouldsolvetheproblem,andyet, aboveourupperlimit.Itthereforeseemsthattheoverallagree-
thisrevisionofthestellar populationhasnotbeenobservation- ment obtained at Z=0.02 does no longer holds if the subsolar
ally substantiatedsince then.Moreover,Schneideretal. (2006) metallicity of Cygnus is accounted for. We may question this
PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex 9
subsolar metallicity. It is indeed somewhatsurprising that stel-
lar associationslying at aboutthe same galactocentricdistance
astheSunandbeingmuchyoungercouldbesubsolar,although
Daflonetal. (2001) mention that their abundances are consis-
tent with the expected values, as predicted by various Galactic
abundance gradients. While the C, N O, Si, Al, and Fe were
found to be underabundant relative to solar values, Mg and S
werefoundtobesolarandMghasstraightforwardrelevancefor
26Alnucleosynthesis.Theauthorsstate thatBstarsinthesolar
neighbourhoodgenerallyhavesubsolarabundances,soonemay
worry abouta generalsystematic bias in determiningchemical
abundancesinBstars.Wealsonotethatasolarabundancewas
derivedbyNajarro(2001) forthe P-Cygnistar,whichliesat ∼
2kpc,closetotheCygnuscomplex.
Fromthesearguments,wethereforeconsiderthatthemetal-
licity of the Cygnuscomplexis still uncertain,especially since
the associations investigated by Daflonetal. (2001) are Cyg
OB3 and 7, which are not those contributing the most to the
Cygnus complex. In addition to this, we discuss below several
other potential sources of error or uncertainty that can mod-
ifyand/oralleviatethecurrentsituationforthegamma-rayline
emissionoftheCygnuscomplex.
4.1.3. Sourcesoferror/uncertainty
Uptonow,wehaveonlyconsideredtheuncertaintyonthepre-
dicted fluxes due to finite IMF sampling, but the uncertainty
on the exact distances and ages of the stellar associations of
the Cygnus complex should also be taken into account. From
Kno¨dlsederetal. (2002), we estimated that around the present
date, the uncertainties on the ages and distances of the stellar
clusters translate into typicaluncertaintiesof ∼ 1.0 and 0.25 ×
10−5phcm−2s−1 on the decay line fluxes of 26Al and 60Fe, re-
spectively. In connection with the age of the stellar clusters, it
Fig.5.Predictedversusobserveddecayfluxes,forZ=0.02(so-
shouldbe notedthat ourassumption of a coevalstar formation
lar) initial metallicity (top) and Z=0.01 initial metallicity (bot-
favoursmaximumpeakfluxes.Themostmassivestars(between
tom). The red curve corresponds to the 1809keV flux and the
30and120M )haveverysimilarlifetimes,soinacoevalpopu-
⊙ bluecurvetothe1173/1332keVflux.Thedottedlinesgivethe
lationthenucleosynthesiscontributionsoftheirlaststages(WR
typicalvarianceduetothefiniteIMFsampling.Theorangeband
andSNe)arereleasedatalmostthesametimeandthereforelead
indicatestheobserved1809keVfluxandthegreenlinetheupper
to a steep rise in the decayfluxes.In contrast,if star formation
limitonthe1173/1332keVemissionfromtheCygnuscomplex.
occurs over a few Myrs, the nucleosynthesis contributions are
Theverticalshadedareaindicatesourprobablecurrentposition
spreadovera longertimeintervalandtheresultingpeakfluxes
alongthetimeaxis(seetext).
arelower.
We emphasise once again that our grid of nucleosynthesis
yields is not homogeneousand self-consistent and that the SN
yields of the higher-mass stars at all metallicities were extrap- correspondingnuclearprocessesseemtodependonlymildlyon
olatedfromnon-rotatingsolar models(whereasthe hydrostatic theinitialmetallicity(althoughthemanyinvolvedspeciesdonot
phases of these higher mass stars were computed by including allowdrawingdefinitiveconclusions).Inthiscase, ouroriginal
stellar rotation). As a first potential issue, the direct effect of assumptionthattheSNyieldspredominantlydependontheCO
metallicityontheSNyieldsofnon-solarmodelsisnottakeninto coremassappearstobemorereliablefor26Althanfor60Fe.
account.Indeed,ourgridofyieldstakesintoaccounttheeffectof AsecondpotentialissuestemsfromSNyieldscomingfrom
metallicityonstellarevolutionprocesseslikethemasslossand non-rotatingmodels. Rotation has proven to be efficient at en-
the late stagecore sizes. Butthe directinfluenceof metallicity, hancing the 26Al production during central H-burning, mostly
that is the differing chemical composition of the stellar matter thankstoanincreasedconvectionandmixing.Ifrotationwereto
fromwhichthenucleosynthesisproceeds,isnotconsidered.For modifytheyieldsoftheC/Neshell-burningepisodesinasimilar
instance, we have shown before that an initial subsolar metal- manner, the corresponding26Al and 60Fe production would be
licityleadstobiggerCOcore,hencepresumablytohigher60Fe higherthanthosecomputedbyLC06.Inaddition,increasingthe
SN yields; however,this should be mitigated by the fact that a shellconvectionefficiencywouldalsoalterthefinalmass-radius
lowermetallicityalsomeansreducedinitialquantitiesofFeand relation on which our estimate of the explosive nucleosynthe-
CNOforthesynthesisof60Fe.Since60Feisapuresecondaryel- sis depends. We should also mention that, in the grid of yields
ement,itislikelythatourgridoverestimatesthe60FeSNyields we are using, the ν-process5 has not been taken into account.
of the subsolar metallicity models. Regarding 26Al, the direct
impactofthemetallicityislessstraightforward.Indeed,mostof 5 Intheν-process,protonsliberatedbytheν-spallationofe.g.20Ne,
the 26Al released by supernovae is of explosive origin and the 16O,23Na,and24Mgcaptureon25Mgtoform26Al.
10 PierrickMartinetal.:Predictedgamma-raylineemissionfromtheCygnuscomplex
Thisproductionchannelwasestimatedtoenhancetheexplosive quantitativeanalysisoftheimpactofthenew60Felifetime,see
26Alyieldsby30-50%(Woosley&Weaver1995;Timmesetal. Vossetal.2009).
1995).Thesevaluesmayhavetoberevisedwithmoresophisti- We also emphasise that all the results presented so far are
catedandrealisticneutrinospectraandtransport. basedona“classical”IMFslopeof1.35(Salpeter1955).Since
More generally, the nucleosynthesis yields should be re- the universalityof thisparameteris still a matterof debate,we
garded as sensitive to most ingredients of stellar models, such showinFig.6howthedecaylightcurvesforCygOB2varywith
asconvectionandmixing,opacitiesornuclearreactionrates.In theslopeoftheIMF.Theimpactisfairlystraightforward:aflat-
3.2,wecomparedtheLC06andWH07setsofyieldsandshowed terIMFmeansmoreverymassivestars(foragiventotalnumber
thatthechoiceofmasslossprescriptionortheimplementationof ofmassivestars)andthe1809keVpeakat3-4Myrattributable
convectivephysicsdoessignificantlyaffectthe nucleosynthesis tothewindsandSNeofWRstarsisconsequentlystronger,while
of26Aland60Fe.Wedidnotobservedrasticvariationsbymany asteeperIMFmeansfewerverymassivestars,whichreducesthe
orders of magnitude in the outputs of the two grids of stellar 1809peakfluxat3-4Myrandenhancesthefluxat15-16Myr,as
models,but the typicaldifferencesare high enoughto possibly aresultofmoreSNexplosionsof13-15M stars.
⊙
modify our conclusions. Therefore,although they were not in- InconnectionwiththepotentialeffectoftheIMFslope,we
cludedinourpopulationsynthesis,wequalitativelyassessedthe want to draw attention to the possible impact of binarity. It is
impactthattheWH07yieldswouldhaveonthepredicteddecay commonlyacceptedthatforstellar clustersmoredistantthan a
fluxesfromtheCygnuscomplex. fewkpc,mostoftheobservedpoint-sourcesassumedtobemem-
Forstarsinthe20-120M massrange(whichdominatethe bersoftheclustersmayactuallybeunresolvedbinariesormul-
⊙
current nucleosynthesis activity of the Cygnus complex), the tiplesystems.Thecensusoftheirstellarcontentisconsequently
26AlwindandSNyieldsoftheWH07modelsareclearlylower biased,particularlyforpurelyphotometricapproaches,buteven
than those of MMP05 and LC06 (see Figs. 2 and 3). We esti- when relyingonspectroscopy.Kobulnicky&Fryer (2007) find
matedthattheuseoftheseyieldsassuchinourpopulationsyn- thatthebinaryfractionofCygOB2isveryhigh,andmaywell
thesiswouldlowerthe1809keVfluxbyatleastafactorof2and becloseto1;moreover,itseemsthatmassivestarspreferentially
thusleadtoamismatchbetweenpredictionsandobservationsif havemassivecompanions.We thereforewonderaboutthepos-
the Cygnus complexhas nearly solar metallicity. Nevertheless, sibilitythatasubstantialfractionofthe100-120OstarsofCyg
the WH07 modelsdo notincludestellar rotation,whichturned OB2,manyofwhichhavebeenidentifiedphotometricallyfrom
outtostronglyenhancethe26Alyields.Then,keepingthehope- the2MASSdata,mayinfactbeunresolvedmassivebinariesor
fully morerealistic yieldsof MMP05for the 26Al wind contri- multiplesystems.Ifthisweretrue,thenumberofmassivestars
bution, the lower 26Al SN yields of the WH07 models are not in our population synthesis would have to be increased, which
expectedtoseriouslyaltertherisingpartoftheearly1809keV could lead to higher predicted fluxes. To properly account for
lightcurvethatweobservetoday(seeFig.5),becausethelatter thisbinarityeffect,however,thevariousobservationsdevotedto
is dominatedby thewind contribution.Thisisclearly apparent CygOB2(andtheotherOBassociations)havetobethoroughly
inFig.6forthesimulationofCygOB2only:whenthefirststars compared to clearly identify which fraction of the objects as-
of the coevalcluster explode,at ∼3.5Myr, the flux has already sumedtodayassinglemassivestarsmayactuallyhidemultiples,
risento70-80%ofitsmaximumvalueandisalreadyconsistent butthisisbeyondthescopeofthepresentpaper.
withtheobservedvalue.Consequently,eveniftheWH07mod-
elsmayleadtoless26AlbeingejectedbySNe,thewindyields
4.2.Thespatialdiffusionof26Al
ofthepresentlyavailablerotatingmodelsalonecanaccountfor
mostoftheobserved26Aldecayflux.Regarding60Fe,theWH07
Intheprevioussections,wemadeuseofthephotometricaspects
yields are not very different from the yields we have been us- oftheINTEGRAL/SPIobservationsofCygnusalone.Herewe
ing (MMP05 models extrapolatedthrough CO core mass from
wanttoextendourinterpretationoftheSPIresultstothespatial
LC06)overthe20-120M massrange(seeFig.4).Wethusex-
⊙ andspectrometricinformationprovidedbythisinstrument.
pectthe1173/1332keVlightcurvetostillbeconsistentwithour
The numerous massive stars of the Cygnus complex are
upperlimiteventiftheWH07yieldswereused.
expected to have blown one or several superbubbles (SBs)
WethereforebelievethattheWH07setofyieldswouldnot
throughthe combinedaction of their winds andSN explosions
modifyourresultsforthesolarmetallicitycaseifwestillusethe
(Castoretal. 1975; Weaveretal. 1977; Tomisaka&Ikeuchi
enhanced26Al wind yields of the rotating models. A definitive
1986; MacLow&McCray 1988; Chuetal. 2004; Chu 2008).
conclusionwouldrequirefullyimplementingWH07inourpop-
SoftX-rayobservationsbyHEAO-3andROSAThaverevealed
ulationsynthesiscode(inorder,forinstance,totakethespecific
a large structure in the Cygnus direction that may well be the
SN yields to CO core mass relation of this grid properly into
outermostpartofagiganticSB,thecentreofwhichisconcealed
account). More generally, as alternative sets of yields become byinterveninginterstellarmaterial(Cashetal.1980).Fresh26Al
available,especiallyfromrotatingmodels,itwillbeinteresting
isexpectedto beinjectedinsidethehotandtenuousinteriorof
to see if ourconclusionsstill hold.Evenin the absenceof ma- thisSB, andits diffusiononlargescaleswillthenbe drivenby
jorimprovementsofthestellarmodels,theevolutionofsomeof
thegrowthofthatstructure.
theirinputsalone,suchasnucleardata,canalterthenucleosyn-
Undercertain simplifications,a self-similaranalyticalsolu-
thesis results. Some critical reaction rates, such as α(2α,γ)12C
tioncanbefoundforthegrowthoftheSB(Castoretal.1975):
or 12C(α,γ)16O, are known to have strong effects on the final
yields(seeforinstanceTuretal.2009).Asanillustrationofthe
R =66×L1/5n−1/5t3/5 pc (1)
uncertaintieson nucleardata, the half-lifeof 60Fe was recently s 38 1 6
revised to 2.6Myr, instead of the 1.5Myr value we have been V =40×L1/5n−1/5t−2/5 kms−1 (2)
s 38 1 6
usingthroughoutthiswork(Rugeletal.2009).Intermsofpre-
dicted1173/1332keVlightcurvesfortheCygnuscomplex,how- where R and V are the outer radius and velocity of the SB,
s s
ever,thislongerlifetimesimplyimpliesanevenloweremission respectively,givenasfunctionsofthemechanicalluminosityof
at early times, still consistent with our upper limit (for a more thecentralclusterL (in1038ergs−1),theuniformdensityofthe
38
