Chemical abundances of blue straggler stars in Galactic Globular Clusters

0.17 MB·

by  L. Lovisi

Mem.S.A.It.Vol.75,282
(cid:13)c SAIt 2008 Memoriedella
Chemical abundances of blue straggler stars in
Galactic Globular Clusters
L. Lovisi1
4
1
0
DipartimentodiFisicaeAstronomia,Universita`degliStudidiBologna,VialeBertiPichat
2
6/2,I–40127Bologna,Italy
n e-mail:[email protected]
a
J

6 Abstract. Byusingthehighresolutionspectrograph FLAMES@VLTweperformedthe
1 firstsystematiccampaigndevotedtomeasurechemicalabundancesofbluestragglerstars
(BSSs).Thesestars,whoseexistenceisnotpredictedbythecanonicalstellarevolutionary
] theory,arelikelytheproductoftheinteractionsbetweenstarsinthedenseenvironmentof
A
GlobularClusters.TwomainscenariosforBSSformation(masstransferinbinarysystems
G andstellarcollisions)havebeenproposedandhydrodynamicalsimulationspredictdifferent
. chemicalpatternsinthetwocases,inparticularCandOdepletionformasstransferBSSs.
h In this contribution, the main results for BSS samples in 6 Globular Clusters and their
p interpretationintermsofBSSformationprocessesarediscussed.Forthefirsttime,evidence
- ofradiativelevitationintheshallowenvelopesofBSSshotterthan∼8000Khasbeenfound.
o
CandOdepletionforsomeBSSshasbeendetectedin47Tucanae,M30andωCentauri
r
t thussuggestingamasstransferoriginfortheseBSSs.ThelowpercentageofCO-depleted
s
BSSssuggeststhatdepletionmightbeatransientphenomenon.
a
[
Key words. blue stragglers; Globular Clusters; stars: abundances; stars: evolution; tech-

1 niques:spectroscopic
v
6
7
1. Introduction 1997; Fiorentino et al. 2013), thus mimick-
9
3 ingarejuvenatedstellarpopulation.According
Dense Globular Clusters (GCs) are efficient
. to the canonical stellar evolutionary theory,
1 “furnaces” for generating exotic objects such
they should have already exhausted their nu-
0 as low-mass X-ray binaries, cataclysmic vari-
clear fuel and evolved in cool white dwarfs
4 ables, millisecond pulsars and blue straggler
1 long time ago. Moreover, since they are the
stars(BSSs).Mostoftheseobjectsarethought
: most massive stars in GCs (and thus they are
v to be formed by stellar interactions between
strongly affected by dynamical friction), they
i single or (more likely) binary stars. In partic-
X areidealtoolstoprobethedynamicalevolution
ular, among these exotica, BSSs are the most
ofthehostingcluster.Indeed,recentresultsby
r
numerous. Firstly discovered by Sandage in
a Ferraroetal.(2012)showthatBSSradialdis-
the GC M3 (Sandage 1953),they are brighter
tribution in GCs can be used as a dynamical
and bluer than the turnoff and they are more
clockthatprovidesadirectmeasurementofthe
massive than normal MS stars (Shara et al.
clusterdynamicalage.
In spite of their importance, BSS formation
Sendoffprintrequeststo:L.Lovisi
Lovisi:ChemicalabundancesofbluestragglerstarsinGalacticGlobularClusters 283
mechanismsandtheirmainpropertiesarevery dances for these 6 BSSs can be interpreted in
poorlyknown.Two main scenarioshave been termsoftheMTprocess.
proposedfortheirformation:BSSsmightorig-
inatethroughmasstransfer(MT)inbinarysys-
3. M4:thelargestpercentageoffast
tems or through stellar collisions (COL), that
rotatingBSSs
are expectedto be frequentinthe densecores
of GCs. MT- and COL-BSSs are expected to A different result has been obtained for M4
have different chemical patterns. In fact, ac- (Lovisi et al. 2010) where all the stud-
cording to hydrodynamical simulations, MT- ied BSSs share the same chemical composi-
BSSs should show C and O depletion (Sarna tion of MS stars and no CO-depleted BSSs
& De Greve 1996), since the material should have been found. According to the percent-
come from deep in the donor star, where the age (14%) identified in 47 Tucanae, 1-2 CO-
CNO cycle already occurred. On the con- depleted BSSs should have been expected in
trary, negligible mixing between inner cores M4.However,themostintriguingresultinthe
andouterenvelopesisexpectedforCOL-BSSs study of this sample does not concern chem-
thatshouldshownormalCandO abundances istry but rotational velocities. In fact, a large
(Lombardietal.1995). percentage (40%) of fast rotating BSSs (rota-
In order to deeply investigate the chemi- tional velocities higher than 40kms−1, up to
cal properties of these puzzling objects, in more than 100kms−1) has been found.This is
2006 we started a systematic campaign de- thefirsttimethatsuchahighfractionoffastro-
voted to secure high-resolution spectra with tatingBSSshasbeenidentified.However,due
FLAMES@VLT for BSSs in 6 GCs, namely to their high rotational velocity, for 8 out of
47Tucanae,M4,NGC6752,NGC6397,M30 20 BSSs it was not possible to derive C and
and ω Centauri. The selected clusters span a Oabundances.
widerangeofproperties,inparticularmetallic-
ity and dynamical status. Moreover, the sam-
4. NGC6752andNGC6397:the
ple includes two peculiar GCs, namely M30
andωCentauri.Indeed,M30isknowntohost occurrenceofradiativelevitation
twodistinctandwellseparatedBSSsequences
Also the study of NGC 6752 (Lovisi et al.
(Ferraro et al. 2009), whereas almost all the
2013a) reveals the lack of CO-depleted BSSs
BSSpopulationinωCentauriisthoughtohave
and their total compatibility with C and O
non-collisional origin (and thus to be formed
abundances of dwarf stars. Nevertheless, for
mainlythroughMT,Ferraroetal.2006a).
theseBSSstheirondistributionisquitespread
(∼1 dex) and shows a trend with the stel-
lar temperature, with higher iron content for
2. 47Tucanae:firstdetectionof hotter stars. The same trend was already ob-
CO-depletion servedbyLovisietal.(2012)inNGC6397.In
this cluster, BSSs show an even larger spread
Firstresultshavebeenobtainedin47Tucanae in the iron content (up to more than 2 dex)
(Ferraroetal.2006b):theanalysisof42BSSs and some BSSs have solar or super-solar val-
ledtothediscoveryof6starswithlowCandO ues. This behavior is known to occur in the
abundances. These abundances disagree with HorizontalBranchinstarshotterthan∼11000
CandOcontentoftheotherBSSsinthesam- Kthathavehighironcontentsuptosolarval-
ple and also of cluster MS stars. Moreover, ues (Behr 2003). The common interpretation
such a depletion is totally incompatible with for this feature is the occurrence of radiative
the second generation scenario, according to levitationintheatmospheresofstarswithshal-
which second generationstars (depleted in C, lowornon-convectiveenvelopes,whenthera-
O andotherlightelements)havebeenformed diativeaccelerationovercomesthegravityone.
from material reprocessedby the first genera- The main effect is that many chemical ele-
tionstars.Onthecontrary,lowCandOabun- ments are pushed toward the stellar surface,
284 Lovisi:ChemicalabundancesofbluestragglerstarsinGalacticGlobularClusters
in M30 is a quite challengingtask, since they
are relatively faint (V magnitude between 17
and 18.8) and very concentrated toward the
cluster center. Moreover, the brightest BSSs
arealsothehottestonesandthereforelikelyaf-
fectedbyradiativelevitation.Inspite of these
difficulties, we obtained 11 spectra including
8 BSSs in the red sequence and 3 in the blue
one.Asexpected,mostofthestarssufferfrom
radiativelevitationandthusthechemicalanal-
Fig.1.HeIlineat∼5876ÅforaBSSofNGC ysis has been performed only on the 5 cold-
6397. est BSSs (Lovisi et al. 2013b). Unfortunately
they all belong to the red sequence, thus pre-
ventingthecomparisonwith BSSs intheblue
thus alteringthe initial chemicalcomposition. one. For these stars, only O upperlimits have
Another diffusionprocessgenerallyoccurring been obtained (whereas no information of C
simultaneously with radiative acceleration is abundances could have been inferred). O up-
the gravitational settling of He. For one BSS perlimitsarelowerthan0.3dexandincompat-
of NGC 6397, we secured also high signal- iblewithabundancesofRGBstarsinthesame
to-noise UVES spectra with wavelength cov- cluster (therefore a second generation origin
eragesuitabletoobservetheHeIlineat∼5876 forthesestarsshouldbediscarded).
Å. A comparison with synthetic spectra com- Concerning ω Centauri, it is one of the most
puted with the same atmospheric parameters, puzzling stellar systems in our Galaxy. The
metallicity and rotational velocity of the star, flat radial distribution of its BSS population
revealsthattheobservedHelinecannotbefit- (Ferraro et al. 2006a) suggests that this clus-
ted with a normal He mass fraction (Y=0.25) ter is not dynamically relaxed, since dynam-
and that a much lower He content (Y=0.001) ical friction did not have enough time to oc-
is needed, thus confirming the occurrence of cur yet. For this reason, stellar interactions in
diffusionprocesses(seeFig.1). this cluster are thought to be very rare and
almost the entire BSS population is probably
formed through MT in primordialbinary sys-
5. Twopeculiarclusters:M30and tems. Therefore, ω Centauri is the best target
ωCentauri
for investigating chemical properties of MT-
BSSs. Despite the large intrinsic spread in its
Highqualityandaccuratecolor-magnitudedi-
iron content (due probably to a non-genuine
agrams of M30, revealed the presence of two
originofωCentauri),theanalysisofthe[Fe/H]
parallel BSS sequences, similarly populated
ratio as a function of stellar temperatures re-
and very well separated. According to the in-
vealstheoccurrenceofradiativelevitationalso
terpretationby Ferraro et al. (2009),the bluer
in the hottest stars of this cluster. In order to
sequenceismadebyBSSsformedthroughcol-
avoid alteration in the chemical abundances
lisions whereas the redder one hosts mainly
duetoradiativelevitationeffects,wemeasured
BSSs formed through MT. Since the two se-
C and O abundances only for the 9 coldest
quences are so well separated, a short lived
BSSs. A comparison with results by Marino
event(insteadofacontinuousprocess)should
et al. (2012) shows that most of the BSSs
have occurred 1 or 2 Gyr ago and boosted
sharethesamechemicalcontentofdwarfstars
the BSS formation through both the MT- and
(Mucciarelli et al. 2014, in prep.). However,
COL-process. The intriguing presence of two
one BSS has low C abundances whereas an-
BSSsequencesmakesM30theidealtargetfor
otherBSSshow[O/Fe]ratioincompatiblewith
studyingthedifferenceinthechemicalproper-
thedwarfstars.Alsointhiscase,aninterpreta-
ties of MT- and COL-BSSs and searching for
CO-depletion. Nevertheless, observing BSSs
Lovisi:ChemicalabundancesofbluestragglerstarsinGalacticGlobularClusters 285
tionintermsoftheoccurrenceofaMTprocess
hasbeenprovided.
6. Conclusions
Weperformedthefirstsystematiccampaignin
ordertodeeplyinvestigatethe chemicalprop-
erties of BSSs in 6 different GCs. Our study
is based on the analysis of high-resolution
spectra obtained with FLAMES@VLT. For
the first time, we found evidence of the oc- Fig.2.[Fe/H]ratios(scaledonthemeanclus-
currence of radiative levitation in the shallow terironcontent)asafunctionofstellartemper-
convective envelopes of BSSs. This radiative aturesfortheBSSsamplesofM30,NGC6397
process is usually observed in Horizontal andNGC6752.
Branchstarshotterthan11000K.Atvariance
with Horizontal Branch stars, we found that
Ferraro, F. R., Sabbi, E., Gratton, R., et al.
radiativelevitationinBSSatmospheresoccurs
2006b,ApJ,647,L53
atlowertemperaturearound∼8000K.Thisis
Ferraro,F.R.,Beccari,G.,Dalessandro,E.,et
particularlyevidentfromFig.2,where[Fe/H]
al.2009,Nature,462,1028
ratiosfortheBSSsamplesofM30,NGC6397
Ferraro,F.R.,Lanzoni,B.,Dalessandro,E.,et
andNGC6752scaledtothemeanclusteriron
al.2012,Nature,492,393
contenthavebeenplotted.
Fiorentino,G.,Lanzoni,B.,Dalessandro,E.,et
The analysis of BSSs in the red sequence
al.2013,arXiv:1312.0388
of M30, revealed that 4 out 5 BSSs (with
Lombardi,J., C., Jr., Rasio, F. A., & Shapiro,
chemical abundances not altered by radiative
S.L.1995,ApJ,445,L117
levitation) are O depleted. Since these stars
Lovisi,L.,Mucciarelli,A.,Ferraro,F.R.,etal.
should have been formed through MT, this
2010,ApJ,719,L121
suggests that depletion might be considered
Lovisi, L., Mucciarelli, A., Lanzoni, B., et al.
a real signature of the MT process. However,
2012,ApJ,754,91
the low percentage of CO-depleted BSSs
Lovisi, L., Mucciarelli, A., Dalessandro, E.,
detected in the sample (spanning from 0% in
Ferraro, F. R., & Lanzoni, B. 2013a, ApJ,
M4 and NGC 6752 to ∼14% in 47 Tucanae)
778,64
suggests that CO-depletion might be a tran-
Lovisi, L., Mucciarelli, A., Lanzoni, B., et al.
sient phenomenon. As already suggested by
2013b,ApJ,772,148
Ferraro et al. (2006b),BSSs couldexperience
Marino,A.F.,Milone,A.P.,Sneden,C.,etal.
different evolutionary stages that might end
2012,A&A,541,A15
with mixing processes restoring the original
Sandage,A.R.1953,AJ,58,61
chemicalabundancesandthuserasingtheMT
Sarna,M.J.,&DeGreve,J.-P.1996,QJRAS,
signature.
37,11
Shara,M.M.,Saffer,R.A.,&Livio,M.1997,
Acknowledgements. This research is part of the ApJ,489,L59
project COSMIC-LAB funded by the European
ResearchCouncil(undercontractERC-2010-AdG-
267675).
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