Correlation between lithium abundances and ages of solar twin stars

0.23 MB·

by  Marilia Carlos

Astronomy&Astrophysicsmanuscriptno.aanda (cid:13)cESO2016
January25,2016
Correlation between lithium abundances and ages of
⋆
solar twin stars
MaríliaCarlos1,PoulE.Nissen2 andJorgeMeléndez1
1 UniversidadedeSãoPaulo,IAG,DepartamentodeAstronomia,RuadoMatão1226,CidadeUniversitária,05508-900SãoPaulo,
SP,Brazil
e-mail:[email protected]
2 AarhusUniversity,StellarAstrophysicsCentre,DepartmentofPhysicsandAstronomy,NyMunkegade120,DK–8000AarhusC,
6 Denmark
1
0
2 ReceivedSeptember30,2015;acceptedDecember18,2015

n
a ABSTRACT
J
1 Aims.Wewanttodeterminethelithiumabundancesofsolartwinstarsasafunctionofstellaragetoprovideconstraintsforstellar
2 evolutionsmodelsandtoinvestigatewhetherthereisaconnectionbetweenlowLiabundanceandtheoccurrenceofplanets.
Methods.Forasampleof21solartwinsobservedwiththeHARPSspectrographathighspectralresolution(R≃115.000)andvery
] highsignal-to-noiseratio(600≤S/N≤2400),preciselithiumabundanceswereobtainedbyspectralsynthesisoftheLii6707.8Åline
R andcomparedtostellarages,masses,andmetallicitiesdeterminedfromaspectroscopicanalysisofthesamesetofHARPSspectra.
S Results.Weshowthatforthelargemajorityofthesolartwinsthereisastrongcorrelationbetweenlithiumabundanceandstellarage.
. Astheageincreasesfrom1to9Gyr,theLiabundancedecreasesbyafactorof∼50.Therelationagreesfairlywellwithpredictions
h fromnon-standardstellarevolutionmodelsofLidestructionatthebottomoftheupperconvectionzone.Twostarsdeviatefromthe
p relationbyhavingLiabundancesenhancedbyafactorof∼10,whichmaybeduetoplanetengulfment.Ontheotherhand,wefind
- noindicationofalinkbetweenplanethostingstarsandenhancedlithiumdepletion.
o
r Keywords. stars:abundances–stars:evolution–stars:planetarysystems
t
s
a
[
1. Introduction lar age, in the sense that older stars have lower Li abun-

2 dancesthanyoungeronesashasbeenshownbyBaumannetal.
vLithiumisdestroyedintheinnerlayersofstarsviaprotoncap- (2010), Monroeetal. (2013), Meléndezetal. (2014), and
4ture (7Li(p,α)α) at temperatures near to 2.5x106 K. Because
TucciMaiaetal. (2015). This is supported by several mod-
5Li burning happens when the element is transported to the in-
els such as Andrássy&Spruit (2015), Charbonnel&Talon
0nermost and hotter regions through convective motions in the
(2005), Denissenkov (2010), DoNascimentoetal. (2009), and
5star, Li abundancestudies offer an excellentopportunityto un-
Xiong&Deng (2009). The second interpretation is that en-
0
derstand the extent of the mixing processes within and below
. hanced Li depletion is due to the presence of planets
1thestellarconvectivezoneand,therefore,areimportantforcon-
(Israelianetal. 2009; Sousaetal. 2010; DelgadoMenaetal.
0strainingtransportmechanismsinstars.
2014; Figueiraetal. 2014; Gonzalez 2015) and that the degree
6 Since the amount of lithium burning depends on the con-
oflithiumdepletiondoesnotdependonageforstarsolderthan
1vectivezonethickness,whichdependsonmassandmetallicity,
∼2Gyr(Sousaetal.2010;DelgadoMenaetal.2014).
:
vwe can improve our understanding of the structure and evolu- Inthispaperwerevisitthediscussiononlithiumabundance
ition of a star by analyzing the lithium abundance dependence depletioninsolartwinstarsanditscorrelationswithstellarage,
X
of these variables. It is important to note that the lithium con-
metallicity,mass,andplanetoccurrencebasedonnew,verypre-
rtentcouldalso haveotherdependenciessuchasplanetsaround
a cise Li abundances derived from HARPS spectra of 21 solar
astar;forinstance,planetaryformationcouldchangetheinitial
twinswithagesbetween0.7and8.8Gyr.
angularmomentumofthestar,which,accordingtoTakedaetal.
(2010)andGonzalezetal.(2010),increasesthelithiumburning.
AnotheralternativetoananomalousLiabundanceisplaneten- 2. Spectraandstellarparameters
gulfment,whichmayleadtoanincreaseintheLiabundanceof
The solar twin stars included in this paper have been applied
astar(Montalbán&Rebolo2002;Sandquistetal.2002).
byNissen(2015)tostudytrendsofabundanceratiosasafunc-
Although several factors control the level of lithium burn-
tionofstellarageandelementalcondensationtemperature.The
ing, there is a debate regarding the lithium depletion in so-
stars were selected from Sousaetal. (2008) to have effective
lar twin stars. There are two main interpretations. One is that
temperatures (T ), surface gravities (logg), and metallicities
the degree of lithium depletion primarily depends on stel- eff
([Fe/H]) close to the solar values and to have HARPS spectra
⋆ Based on data products from observations made with ESO Tele- (Mayoretal. 2003) with a signal-to-noise ratio S/N ≥ 600 af-
scopesattheLaSillaParanalObservatory,(observingprograms072.C- tercombinationofspectraavailableintheESOScienceArchive.
0488,183.C-0972,188.C-0265,and0.88.C-0323). Furthermore,asolarfluxHARPSspectrumobservedinreflected
Articlenumber,page1of6
A&Aproofs:manuscriptno.aanda
sunlightfromtheasteroidVestaisavailable.Thisspectrumhas
S/N ∼ 1200andwasusedina differentialanalysisofthestars
relativetotheSun.
DetailsabouthowtheHARPSspectrawerenormalizedand
usedtodeterminestellarparametersandabundancesbyamodel
atmosphere analysis of equivalent widths are given in Nissen
(2015). Here we just mention that T and logg were deter-
eff
mined by requesting that the iron abundances derived have no
systematicdependenceonexcitationandionizationpotentialof
the lines applied. The estimated errors of the parameters are
σ(T ) = ±6K and σ(logg) = ±0.012dex, and abundances
eff
weredeterminedwithatypicalprecisionof±0.01dex.Suchex-
tremelyhighprecisionisobtainablewhenhigh-resolutionspec-
traofsolartwinstarswithS/N > 500areanalyzedstrictlydif-
ferentially (line-by-line) relative to a spectrum of sunlight re-
flectedfromanasteroidobservedwiththesamespectrographas
thestars(Bedelletal.2014).
AsseenfromTable 1,thesolartwinstarsrangefrom5690to
5870KinT ,4.25to4.50dexinlogg,and−0.11to+0.11dex
eff
in[Fe/H].Hence,theylieinaregionoftheT -loggdiagram,
eff
where isochrones are not well separated (see Fig. 5 in Nissen
2015). Still, the small errors of T and logg make it possible
eff
to derive precise ages by comparing with the Yonsei-Yale set
of isochrones (Yietal. 2001; Kimetal. 2002). These ages are
Fig.1. Observed spectra for four starswithdifferent ages. Noticethe
giveninTable 1togetherwithstellarmassesestimatedfromthe
weakeningoftheLifeatureforincreasingages.
evolutionarytracksofYietal.(2003).A precisionof±0.01M
⊙
isestimatedforthemasses.
AsseenfromTable 1,theerrorsoftheagesrangefrom0.4
to 0.8Gyr. A comparison with ages derived by Ramírezetal.
(2014) from independent high S/N spectra of 14 of the stars
shows good agreement within these estimated errors (see Fig.
6 in Nissen 2015). Furthermore, the abundance ratio [Y/Mg]
shows a tight correlation with stellar age in a way that can be
explained from the nucleosynthesis of these elements, i.e., de-
layedproductionofyttriuminlow-massAGBstarsandprompt
formationofmagnesiuminhighmass,corecollapsesupernovae
(Nissen 2015). Thiscorrelationsupportsthat the estimated age
errorsare realistic in a relative sense; the absolute ages for the
youngestandtheoldeststarscouldbemoreuncertain.
3. Analysis
An inspectionofthe observedspectra (Fig. 1) showsthatthere
isaclearweakeningoftheLifeaturewithincreasingages.
In order to obtain the lithium abundances, first we had to
estimate the macroturbulence and rotational velocity broaden-
ing.Themacroturbulencevelocitywasdeterminedbytheequa-
tion v = v + (T − 5777)/486kms−1 as given
macro,⋆ macro,⊙ eff
in TucciMaiaetal. (2015). From our analysis of the HARPS
solar spectrum we estimated v = 3.2 kms−1, adopting
macro,⊙
vsini = 1.9kms−1 .Theprojectedrotationalvelocitywascal- Fig.2.Syntheticsolarspectrumconsideringdifferentspeciesinthere-
⊙
culated by analyzing the line profiles of the Fe I 6027.050 Å, gionofthe6707.75ÅLiiline.
6093.644 Å, 6151.618 Å, 6165.360 Å, 6705.102 Å, and Ni I
6767.772Å lines. Notice that the instrumentalbroadeningwas
HD45184isshowninFig.3,whereitcanbeseenthattheLiline
alsoincluded.
profilechangesverysignificantlyforanabundancedifferenceof
The lithium abundances were estimated by spectral syn-
0.1dex.
thesis in the region of the 6707.75 Å Lii line using the July TheabundanceswerederivedassumingLTEandthenNLTE
2014 version of the 1D LTE code MOOG (Sneden 1973) results were obtained with the aid of the INSPECT database1,
and the Kurucz new grid of ATLAS9 model atmospheres based on NLTE calculations by Lindetal. (2009). The errors
(Castelli&Kurucz2004).ThelinelistadoptedfortheLiregion wereobtainedtakingintoaccountuncertaintiesinthecontinuum
isfromMeléndezetal.(2012),andincludesblendsfromatomic setting,thermsdeviationoftheobservedlineprofilerelativeto
andmolecular(CNandC )lines;thedependenceofthedifferent
2
speciesisshowninFig.2.Anexampleofspectralsynthesisfor 1 www.inspect-stars.com(version1.0).
Articlenumber,page2of6
MaríliaCarlos,PoulE.Nissen andJorgeMeléndez: Correlationbetweenlithiumabundancesandagesofsolartwinstars
Fig. 3. Open circles represent the observed spectra for HD 45184 in Fig.4.LTELiabundancesforoursamplecomparedwithLTEdatafrom
comparisonwiththreedifferentsyntheticspectra. theworkofDelgadoMenaetal.(2014).
the synthetic profile, and the stellar parameters. Both LTE and
stars (HD45289 and HD210918) have thick-disk kinematics,
NLTEvaluesofA(Li)=log(N /N )+12withtheirrespective
Li H while the third one (HD220507) has thin-disk kinematics like
errorsareshowninTable 1.
therestofthestars(Nissen2015),butthislatterstarhasachem-
It is important to note that our results in LTE com-
ical abundance pattern consistent with the other two thick disk
pare well with the large study by DelgadoMenaetal. (2014),
stars.
as shown in Fig. 4. Compared to previous works on so-
Considering all stars in our sample for which a Li abun-
lar twins (Monroeetal. (2013) for 18 Sco and HIP 102152,
dancehasbeenderived(i.e.,excludingstarswithonlyanupper
Meléndezetal. (2012) for HIP 56948, Meléndezetal. (2014)
limit), the Li-age correlation has about a 10σ significance; we
for HIP 1143282 and Ramírezetal. (2011) for 16 Cyg A and
performed a Spearman test and found a Spearman rank coeffi-
16 Cyg B), our results, now in NLTE, follow the same lithium
cientr = −0.89andaprobabilityof10−8 ofourresultsarising
agetrend,asillustratedinFig. 5.Theeffectivetemperaturesand s
bypurechance.Therefore,evenwhenthetwodeviantstarsare
surfacegravitiesofthestarsinthesepapersweredeterminedin
consideredthecorrelationbetweenLiandageisverystrong.
thesamewayasinNissen(2015),i.e.,byadifferentialanalysis
Excluding the anomalous stars HD 96423 and HD 38277,
of Fe lines relative to the Sun, and ages were also obtained by
the Li-age correlationfor 17 stars in our sample that have firm
interpolatingbetweenYonsei-Yaleisochrones.Hence,weexpect
detectionsoftheLilineisatthe23σsignificancelevelandhasa
thattheLiabundancesandagesareonthesamesystemasinthe
verysmallprobabilityofnotbeingreal(10−11)withaSpearman
presentpaper.
rank coefficient r = −0.96, meaning an almost perfect Spear-
s
mancorrelation.Thisisincontrasttothepoorcorrelationfound
4. Discussion by DelgadoMenaetal. (2014) (their Fig. 8). Notice, however,
thattheyincludestarswithmassessignificantlylowerandhigher
The results for the whole sample can be seen in Fig. 5, where thantheSun’sandtheyalsoincludestarsinabroadermetallicity
we notice a strong connection between lithium abundance and range.Thewiderintervalinbothmetallicityandmassapparently
stellar age. 16 Cyg A and two stars in oursample, namelyHD blursthestrongcorrelationthatwefind.
96423 and HD 38277, have higher abundances than expected
In Fig. 5 we compare our Li-age results with Li de-
in comparison to both models and the other stars. A possible
pletion models for a one solar mass star with solar
explanationforthisLienhancementcouldbeplanetengulfment
metallicity (Xiong&Deng 2009; DoNascimentoetal.
asdescribedinMontalbán&Rebolo(2002)andSandquistetal.
2009; Charbonnel&Talon 2005; Denissenkov 2010;
(2002).
Andrássy&Spruit 2015) that take into account physics
Anotherinterestingresultisthattheα-enhancedstars(repre-
not included in the standard solar model. Different transport
sentedbytealbluefilledcirclesinFig. 5)fallonthesameLi-age
mechanismsareexploredinthesemodels,includingmeridional
sequenceasthethindisksolartwins.Two ofthese α-enhanced
circulation,diffusion (gravitationalsettling and radiative accel-
eration), turbulence, gravity waves, convective overshooting,
2 AnewbetterspectrumofHIP114328hasbeenrecentlyacquiredby
and convective settling. Overall, all models have a reasonable
oneoftheauthors(J.M.).ThestellarparametersareT =5763±12K,
eff
logg=4.33±0.04dexand[Fe/H]=-0.050±0.014dex,resultinginan qualitative agreement with the data, as expected because they
ageof7.6+0.6Gyrandmassequalto1.00±0.01M .Consideringthese were calibrated using the Sun or earlier observations of solar
−0.9 ⊙
newresultsthestarfitsbetternowtheLi-agetrend. twins. With the data presented here, the models above could
Articlenumber,page3of6
A&Aproofs:manuscriptno.aanda
Table1.Liabundances,ages,massesandstellarparameters.
A(Li)LTE A(Li)NLTE Age Error Mass T logg [Fe/H]
Star eff
(dex) (dex) (Gyr) (Gyr) (M ) (K) (dex) (dex)
⊙
HD2071 1.39+0.02 1.43+0.02 3.5 0.8 0.97 5724 4.490 −0.084
−0.04 −0.04
HD8406 1.69+0.01 1.73+0.01 4.2 0.8 0.96 5730 4.479 −0.105
−0.01 −0.01
HD20782a 0.67+0.08 0.71+0.08 7.5 0.4 0.97 5776 4.345 −0.058
−0.12 −0.12
HD27063 1.67+0.02 1.71+0.02 2.6 0.6 1.04 5779 4.469 0.064
−0.01 −0.01
HD28471 0.86+0.10 0.90+0.10 7.0 0.4 0.97 5754 4.380 −0.054
−0.09 −0.09
HD38277 1.56+0.03 1.58+0.03 7.3 0.4 1.01 5860 4.270 −0.070
−0.03 −0.03
HD45184b 2.08+0.01 2.10+0.01 2.7 0.5 1.06 5871 4.445 0.047
−0.02 −0.02
HD45289∗ ≤0.57 ≤0.61 8.5 0.4 1.00 5718 4.284 −0.020
HD71334 0.58+0.14 0.62+0.14 8.1 0.4 0.94 5701 4.374 −0.075
−0.11 −0.11
HD78429 0.59+0.11 0.63+0.11 7.5 0.4 1.04 5756 4.272 0.078
−0.19 −0.19
HD88084 0.96+0.12 1.00+0.12 6.0 0.6 0.96 5768 4.424 −0.091
−0.06 −0.06
HD92719 1.88+0.01 1.90+0.01 2.7 0.6 0.99 5813 4.488 −0.112
−0.01 −0.01
HD96116 2.19+0.01 2.21+0.01 0.7 0.7 1.05 5846 4.503 0.006
−0.01 −0.01
HD96423 1.88+0.01 1.93+0.01 6.0 0.4 1.03 5714 4.359 0.113
−0.01 −0.01
HD134664 2.08+0.02 2.11+0.02 2.3 0.5 1.07 5853 4.452 0.093
−0.01 −0.01
HD146233 1.58+0.02 1.62+0.02 3.8 0.5 1.04 5809 4.434 0.046
−0.03 −0.03
HD183658 1.24+0.04 1.28+0.04 5.0 0.5 1.03 5809 4.402 0.035
−0.07 −0.07
HD208704 1.05+0.05 1.08+0.05 6.9 0.4 0.98 5828 4.346 −0.091
−0.03 −0.03
HD210918∗ 0.67+0.06 0.71+0.06 8.5 0.4 0.96 5748 4.319 −0.095
−0.17 −0.17
HD220507∗ <0.50 <0.55 8.8 0.4 1.01 5690 4.247 0.013
HD222582c 0.89+0.08 0.93+0.08 6.5 0.4 1.01 5784 4.361 −0.004
−0.05 −0.05
Sun 1.03+0.03 1.07+0.03 4.6 – 1.00 5777 4.438 0.000
−0.02 −0.02
Notes.(∗)α-enhancedstar.(a)Detectedplanetwith1.8M (Jonesetal.2006).(b)Detectedplanetwith0.04M (Mayoretal.2011).(c)Detected
Jup Jup
planetwith7.8M (Butleretal.2006).
Jup
be improved. However, such refinements should take into Baumannetal. (2010), Fig. 5 shows a lack of connection be-
account that the initial Li abundance of stars probably have tweenlithiumdepletionandpresenceofplanets.Thereisnosig-
increased in time due to Galactic evolution, i.e., Li production nificant segregation between stars without planets (black filled
inAGBstarsandnovae(Chenetal.2001;Romanoetal.2001; symbols)andplanethostingstars(redfilledsymbols)inthisfig-
Lambert&Reddy 2004; Ramírezetal. 2012). The models ure.
assumean initialLiabundanceof A(Li)= 3.3asinmeteorites, AsshowninFig.6,wherethestarsinoursamplehavebeen
but according to the chemical evolution model of Izzoetal. divided into three metallicity groups (upper panel) and three
(2015),theinitialLiabundancehasincreasedfromaboutA(Li) massgroups(lowerpanel),thereisnoobviousindicationofade-
= 2.8 for the oldest disk stars to about A(Li) = 3.6 for the pendenceoflithiumdepletiononmetallicityandmassatagiven
youngest.Taking thisinto accountwould make the A(Li)- age age.AnunweightedlinearleastsquaresfittoA(Li)asafunction
relationpredictedbythemodelssteeperthanshowninFig.5. of age (in Gyr) for stars with a detected Li line (including the
Sun)leadsto
For models including rotational induced mixing, variations
of the initial rotational velocity cause a considerable scatter in A(Li)=2.437(±0.098)−0.224(±0.018)Age (1)
A(Li)atagivenage;accordingtoCharbonnel&Talon(2005)a
changeinV from15kms−1 to110kms−1 decreasesA(Li) Fittingtheresidualsof A(Li)relativetothisequationasafunc-
rot,init.
byabout1dexforthe oldeststars. Differencesin theinitialro- tionof[Fe/H]andmassresultsintherelation
tationalvelocitymay,therefore,contributetothescatterseenin
∆A(Li)=−0.05 −2.17(±1.30)[Fe/H]
Fig. 5, although very large differences in V are needed to
rot,init.
explaintheoutliers. +3.55(±2.25)(M/M⊙−1.0) (2)
An alternative explanation for enhanced lithium depletion Although,the coefficients have the expected sign, i.e., increas-
is the presence of planets. Israelianetal. (2009) suggest that ingLidepletionwithincreasingmetallicityanddecreasingmass
planet hosting stars have less lithium than stars without plan- (e.g., Castroetal. 2009), the dependence is hardly significant,
ets, but Baumannetal. (2010) contest this result claiming that i.e., a 1.7σ correlation with metallicity and a 1.6σ correla-
the sample of the former work is biased in metallicity and in- tion with mass. Furthermore, the mass dependence is smaller
cludesstars of differentevolutionarystages. In agreementwith than predicted from models (Xiong&Deng 2009; Castroetal.
Articlenumber,page4of6
MaríliaCarlos,PoulE.Nissen andJorgeMeléndez: Correlationbetweenlithiumabundancesandagesofsolartwinstars
-0.05 < [Fe/H] < +0.05 dex
Fig.5.ConnectionbetweenstellaragesandNLTElithiumabundances Fig. 6. Connection between stellar ages and lithium abundances with
for our current sample (circles) and some previous results (triangles) different symbols for three metallicity groups (top) and three mass
referenced in the text. Teal blue circles indicate alpha-enhanced stars groups(bottom).
and red symbols refer to stars hosting planets. The models of Li de-
pletionwerenormalizedtothesolarLiabundance. Insomecases,the
lithiumabundanceerrorsaresmallerthanthepoints. dex, which we interpret as being due to gradual destruction of
lithiumnearthebottomoftheouterconvectionzone.Thus,the
new data can be used to constrain non-standard models of Li
2009), and also smaller than inferred from Li abundance mea-
depletion,andthereforetounderstandbetterthetransportmech-
surements for stars in the open cluster M67, which suggests a
anismsinsidestars.
mass coefficienton the orderof 10 for ∼1M stars (Paceetal.
⊙ TwostarsdeviatefromthemaintrendbetweenA(Li)andage
2012). Apparently, the short mass range for our sample (from
byhavingLiabundancesenhancedbyafactorof∼10.Asimilar
0.94M to 1.07M ) in combination with errors in age, mass,
⊙ ⊙ enhancementofLiispresentfor16CygArelativeto16CygB
and Li abundance make it difficult to estimate the dependence
eventhoughthetwostarshavethesameageandnearlythesame
of Li abundanceon mass based on our sample. We note, how-
massandmetallicity.WesuggestthattheseenhancementsofLi
ever, that according to our results, it is unlikely that the large
areconnectedtotheaccretionofJupitersizedplanets.
difference in A(Li) between 16 Cyg A and 16 Cyg B is due to
We findnoevidencefora linkbetweenplanethostingstars
thedifferenceinmass(0.05 M )assuggestedbyRamírezetal.
⊙ andenhancedLidestructionassuggestedinsomerecentworks.
(2011).Instead,thehighLiabundancein16CygAmaybedue
Three stars with planets of different masses in our sample, 16
to a recent accretion of a Jupiter sized planet, which is consis-
CygBwithaJupitersizedplanet(Ramírezetal.2011),andthe
tent with the fact that no such planet has been detected for 16
Sun show no systematic deviation from the A(Li)-age relation
Cyg A, whereas 16 Cyg B has a giant planet with a minimum
forstarswithoutplanets.
massof1.5M (Cochranetal.1997).Arelatedevidencesup-
Jup
porting the planet engulfment scenario could be the enhanced Acknowledgements. Therefereeisthankedforimportantcomments,thathelped
abundancesfor all elements in 16 Cyg A relative to 16 Cyg B toimprovethispaper.M.C.wouldliketoacknowledge supportfromCAPES.
P.E.N.acknowledges support from the Stellar Astrophysics Centre funded by
(Ramírezetal. 2011; TucciMaiaetal. 2014), and in particular
The Danish National Research Foundation (Grant agreement no.: DNRF106)
fortherefratoryelements(TucciMaiaetal.2014). andfromtheASTERISKproject(ASTERoseismicInvestigations withSONG
andKepler) fundedbytheEuropeanResearch Council (Grantagreement no.:
267864).J.M.wouldliketoacknowledgesupportfromFAPESP(2012/24392-
5. Conclusions 2)andCNPq(BolsadeProdutividade).
BasedonHARPSspectrawithveryhighsignal-to-noise(S/N ≥
600) we were able to determine high-precision lithium abun-
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