Ionizing spectra of stars that lose their envelope through interaction with a binary companion: role of metallicity

8.7 MB·

by  Y. Gotberg

Astronomy&Astrophysicsmanuscriptno.Gotbergetal_Z (cid:13)cESO2017
January27,2017
Ionizing spectra of stars that lose their envelope through
interaction with a binary companion: role of metallicity
Y.Götberg1,S.E.deMink1,andJ.H.Groh2
1 AntonPannekoekInstituteforAstronomy,UniversityofAmsterdam,1090GEAmsterdam,TheNetherlands
e-mail:[email protected], [email protected]
2 SchoolofPhysics,TrinityCollegeDublin,TheUniversityofDublin,Dublin2,Ireland
e-mail:[email protected]
Received......;accepted......
7
1 ABSTRACT
0
2 Understandingionizingfluxesofstellarpopulationsiscrucialforvariousastrophysicalproblemsincludingtheepochofreionization.
Massiveshort-livedWolf-Rayetstarsaregenerallyconsideredasthemainionizingsources.Weexaminetheroleoflessmassivestars
n
thatlosetheirenvelopethroughinteractionwithacompanion.WeusetheevolutionarycodeMESAandtheradiativetransfercode
a
J CMFGENtoinvestigatestrippedstarsasafunctionofmetallicity(Z).
5 Wioneizshinogwpthhoattotnysp,iceamlpitrtoinggen6i0to–r8s5,%initainaldly3102–6M0(cid:12)%,porfodthuecierheonteargnydcaosmHpIacatnsdtaHrseI(∼io4niMzi(cid:12)n,g60ra–d8i0atkioKn,,∼f1orRZ(cid:12))t=h0a.t0c0o0p1i–o0u.s0ly2.pTrohdisucies
2
comparabletoatypicalmassiveWolf-Rayetstar,whenaccountingfortheirlongerlifetimesandthefavoringinitialmassfunction.

Strippedstarsfurtheremittheirionizingphotonswithadelay(∼20Myrsafterstarformationforthisprogenitor),allowingtimefor
]
R stellarfeedbacktodispersethebirthclouds.Thisincreasesthefractionthatcanescapeandcontributetoionizationoftheintergalactic
medium.
S
We further find that Roche stripping fails to fully remove the H-rich envelope at low Z. This questions the common treatment of
h. strippedstarsinrapidpopulationsynthesissimulationsaspureheliumstars.WeexpectimplicationsfortherateoftypeIb/csupernova
p at low Z and for the advanced evolutionary channels for type Ia supernova and gravitational wave sources. We discuss how the
- characteristic spectral features of stripped stars can be used to increase the observed sample, which is urgently needed to test the
o models.
r
t Keywords. Binaries:close–Ultraviolet:general–Stars:atmospheres–subdwarfs–Wolf-Rayet–Stars:massive–Stars:mass-loss
s
a
[

1 1. Introduction ionized(Haiman&Loeb1997;Bouwensetal.2012;McQuinn
v 2016).
9Massive stars have played many important roles since the ear- Massivestarsarefrequentlyfoundinbinaryormultiplesys-
3liest epochs of star formation. They shape, heat and stir their tems (e.g. Garmany et al. 1980; Mason et al. 2009; Sana et al.
4surroundings, and they play a key role in driving the evolution 2014).Recentstudieshaveshownthat,inthemajorityofcases,
7of their host galaxies (e.g. Hopkins et al. 2014). Over cosmic massive stars have a companion that is so close that severe in-
0
time, subsequent generations of massive stars chemically en- teraction between the two stars at some point during their lives
.
1riched the Universe with elements synthesized by nuclear fu- is inevitable (e.g. Kobulnicky & Fryer 2007; Sana et al. 2012).
0sion(e.g.Woosley&Heger2007),slowlyincreasingtheaverage Similarconclusionshavebeenreachedbyvariousgroupsusing
7metallicity(i.e.massfractionofelementsheavierthanHandHe) differentdatasets(Chinietal.2012;Sanaetal.2013;Kobulnicky
1ofsubsequentstellarpopulations. etal.2014;Dunstalletal.2015;Almeidaetal.2016).
:
v During their short lives, massive stars copiously produce Binary evolution can lead to many complex evolutionary
Xi(far) ultraviolet (UV) photons. Of particular interest are their paths involving one or more phases of mass exchange between
photons with wavelengths shorter than the Lyman limit (λ < thetwostars(e.g.Paczyn´ski1966;Kippenhahn&Weigert1967)
r
a912 Å, i.e. with energies exceeding the ionization potential of and possibly a merger of the two stars. This has drastic conse-
hydrogen, hν > 13.6 eV). In the local Universe, massive stars quencesfortheobservablepropertiesofbothstars,theirremain-
areobservedtoionizetheirimmediatesurroundings,givingrise inglifetimeandfinalfate.Thisraisesthequestionabouttheim-
toluminousHIIregions(Contietal.2012).Atlargerdistances, plications for the integrated spectra of stellar populations, and
they dominate the rest frame UV part of the integrated spectra theirionizingfluxesinparticular.
ofstarforminggalaxiesandgiverisetovariousstrongemission The most widely used spectral population synthesis codes,
lines (e.g. Kewley et al. 2001). Going back further in distance Starburst99 (Leitherer et al. 1999, 2014), GALAXEV (Bruzual
andtime,theearlygenerationsofmassivestarsweretheprime &Charlot2003)andFSPS(Conroyetal.2009),donotaccount
sourcesofionizingphotonsemittedbythefirstgalaxies(Bromm forthepossibleeffectsofinteractingbinarystarsandtheirprod-
& Yoshida 2011). These galaxies are held responsible for the ucts.Inthesesimulationstheionizingfluxcomesprimarilyfrom
large-scale phase transition, known as the Epoch of Reioniza- themostmassiveandluminousstars,whichareshort-lived.Al-
tion,duringwhichtheintergalacticneutralhydrogengasbecame readyatbirththesestarsemitasignificantfractionoftheirlight
Articlenumber,page1of21
A&Aproofs:manuscriptno.Gotbergetal_Z
at wavelengths shorter than the Lyman limit. The most mas- cence).Strippedstarsarealsotypicallylong-lived.Theyarenor-
siveandluminoussinglestarslosetheirhydrogen-richenvelope mallypoweredbycentralheliumburning,anevolutionaryphase
through stellar winds and eruptions. After a few million years that accounts for roughly 10% of the lifetimes of stars. Their
theybecomeWolf-Rayet(WR)stars,whicharegenerallybeing high temperatures and long lifetime along with the expectation
consideredasthemostefficientstellarsourcesofionizingradia- that they are common, makes the stripped stars of interest as
tion. potentialstellarsourceofionizingradiation.Assessingtheirpo-
Pioneering efforts to account for the effects of binaries on tential as ionizing sources requires reliable models of their at-
thespectraofstellarpopulationshavebeenundertakenbythree mospheres. At present, no suitable grids of atmosphere models
major groups using the Brussels PNS code (Van Bever & Van- areavailablethatcoverthehigheffectivetemperaturesandhigh
beveren2003;Vanbeverenetal.2007),theYunnansimulations effectivegravitiesthataretypicalforthesestars.
(Hanetal.2010;Chen&Han2010;Zhangetal.2012;Lietal. Thispaperisintendedasfirstinaseriesthatwillsystemat-
2012; Zhang et al. 2015) and, gaining popularity most quickly, ically explore the structural and spectral properties of stripped
BPASS (Eldridge & Stanway 2009, 2012). These studies have stars to evaluate their impact on the spectra of stellar popula-
shown that binary interaction can significantly impact the de- tions.Inthisfirstpaperwepresentacasestudyoftheeffectof
rivedagesandmassesofstarclusters(e.g.Woffordetal.2016), metallicity on a very typical, initially 12M(cid:12), star that loses its
mayhelptoexplainthespectralfeaturesobservedinWolf-Rayet envelopethroughinteractionwithacompanionaftercompleting
galaxies(Brinchmannetal.2008),affectstarformationratein- its main sequence evolution and before completing central he-
dicators(e.g.Lietal.2012;Eldridge2012)andcansignificantly liumburning.Thistypeofmasstransfer,caseBmasstransfer,is
boosttheionizingflux(seeforexampleStanwayetal.2016;Ma themostcommoncaseofbinaryinteraction.Thespecificchoice
etal.2016). ofinitialmassisempiricallymotivatedbytheobservedstripped
Oneofthechallengesforsimulationsthataccountformas- starinthebinarysystemHD45166.Thisallowsustobuildupon
sivebinariesisthescarcityofadequateatmosphericmodelsfor the work by Groh et al. (2008) who extensively analyzed and
binary products. Spectral synthesis codes rely on precomputed modeled this system. We focus on the long-lived helium burn-
gridsof(1)stellarevolutionarymodels,whichprovideinforma- ingphase,whichismostrelevantfortheintegratedspectrumof
tion about the key properties such as evolution of the surface stellarpopulations.
temperature, gravityand composition asa functionof time and An additional objective of this study is improve our under-
(2) corresponding atmosphere models, which provide informa- standing of the expected observable characteristics of stripped
tion of the emerging stellar spectrum. For single stars, exten- stars,thatcanbeusedtoobservationallyidentifythem.Stripped
sive grids of atmospheric models have been produced to cover starsareexpectedtobecommon,butveryfewhavebeenobser-
the various evolutionary phases (Kurucz 1992; Gräfener et al. vationally identified. The paucity of directly observed stripped
2002; Lejeune et al. 1997). In contrast, the coverage of atmo- stars – the apparent paradox of missing stripped stars – can be
sphere models for stellar objects that are exclusively produced understood as the result of various selection effects that hinder
throughbinaryinteractionisverysparseorevenabsent. theirdetection(deMinketal.2014,Schootemeijeretal.2017,
The studiesthat accountfor interactingbinaries sofar have inprep.).Forexample,strippedstarsareexpectedtoresidenear
adopted a variety of efficient approximations to treat the atmo- their main sequence companion, which will typically outshine
spheres of binary products. These include the usage of atmo- them in the optical and near UV. Furthermore, they typically
sphere models originally intended for the evolutionary phases havemassesthataretoolowandorbitsthataretoowidetocause
of single stars, possibly after rescaling them. Other approaches observableDopplervariationthatcanbeidentifiedeasilyinthe
areextrapolationbeyondtheexistingboundariesoftheavailable spectraoftheircompanion.Wewilldiscussspectralfeaturesthat
spectralmodelgridsorthemoststraightforwardapproach,sim- canbeusedtoovercomethesebiases.Theycanbeusedtoguide
pleestimatesbasedonblackbodyapproximations. targeted observing campaigns, that will increase the number of
In this work we focus on stars that lose most of their observedcounterpartsinthelocalUniverse.Thesewillprovide
hydrogen-rich envelope through interaction with a companion. crucialtestcasesforthespectralmodelpredictionsandsimulta-
Thisproducesveryhot,compactheliumstars(e.g.Kippenhahn neouslyprovideconstraintsontheoutcomeofthefirstphaseof
1969;Podsiadlowskietal.1992).Theyareexclusivelyproduced binarymasstransfer.
inbinarysystems.Theycanbeconsideredaslow-masscounter- This paper is organized as follows. We describe the evolu-
parts to hydrogen-deficient WR stars, i.e. helium-burning stars tionary and spectral models in Sect. 2. In Sect. 3 we use our
with current masses (cid:38) 8M(cid:12), showing strong broad emission reference model to illustrate the physical processes that deter-
lines indicative of their strong stellar winds. These stars result minethestructuralandspectralpropertiesofstrippedstars.We
from the evolution of very massive single stars that lose their continue by discussing the impact of metallicity on the evolu-
envelope through strong winds or eruptive mass loss episodes. tion, the stripping process and the structure of the stripped star
Incontrast,theprogenitorsofthelessmassivestrippedstarswe in Sect. 4. In Sect. 5 we discuss the effect of metallicity on the
are interested in do not have winds that are strong enough to emergingspectraandthespectralfeatures.InSect.6wediscuss
removetheenvelope.Theycanonlylosetheirenvelopeasare- thebroaderimplications.Wefinishwithasummaryandconclu-
sultofinteractionwithabinarycompanion.Theyarerelatedto sioninSect.7.
thelowmasssubdwarfOandB(sdO/B)stars,whichhavetypi-
calassumedcurrentmassesofabout0.5-1M(cid:12) (forareviewsee
2. Modeling
Heber2016).Thestrippedstarsoffocusinthisstudy,closethe
sequenceinmassbetweenmassivehydrogen-deficientWRstars
2.1. StellarevolutionwithMESA
andthelowmasssubdwarfs.
Strippedstarsrepresentacommonandlong-livedevolution- Wemodeltheevolutionofinteractingbinarystarsusingthe1D
ary phase for interacting binaries. Practically every interacting stellarevolutioncodeMESA(version7624,Paxtonetal.2011,
binary produces a hot stripped star, directly after the first mass 2013,2015).Wefocusonstarsthatlosetheirhydrogen-richen-
transfer phase ceases (if the two stars avoid immediate coales- velopethroughRoche-lobeoverflowaftercompletingtheirmain
Articlenumber,page2of21
Götberg,DeMink&Groh:Ionizingspectraofstarsthatlosetheirenvelopethroughinteractionwithabinarycompanion
Table1.Physicalparametersofourmodelsofstrippedstarsatdifferentmetallicities(M1,init=12M(cid:12)).
Model Z M(cid:63) MHecore log10L T(cid:63) Teff log10geff R(cid:63) Reff log10M˙ Mtot,H τstripped XH,s XHe,s
[M(cid:12)] [M(cid:12)] [L(cid:12)] [kK] [kK] [cm/s2] [R(cid:12)] [R(cid:12)] [M(cid:12)/yr] [M(cid:12)] [Myr]
M_Z02 0.02 3.61 3.51 4.21 80.0 79.9 5.35 0.66 0.66 -6.58 0.01 1.51 0.17 0.81
M_Z0166 0.0166 3.67 3.55 4.23 79.7 79.6 5.33 0.68 0.68 -6.60 0.01 1.48 0.18 0.80
M_Z0142 0.0142 3.71 3.57 4.24 79.3 79.1 5.31 0.70 0.70 -6.63 0.02 1.46 0.19 0.79
M_Z0134 0.0134 3.73 3.58 4.25 79.1 78.9 5.30 0.71 0.71 -6.64 0.02 1.45 0.20 0.79
M_Z01 0.01 3.81 3.63 4.27 78.4 78.3 5.28 0.74 0.74 -6.68 0.03 1.42 0.21 0.78
M_Z005 0.005 4.01 3.75 4.33 76.3 76.2 5.19 0.84 0.84 -6.80 0.05 1.36 0.26 0.74
M_Z0047 0.0047 4.02 3.76 4.33 76.0 75.8 5.18 0.85 0.85 -6.81 0.05 1.35 0.26 0.73
M_Z0021 0.0021 4.23 3.86 4.40 72.1 71.9 5.05 1.01 1.02 -6.95 0.08 1.31 0.31 0.69
M_Z001 0.001 4.41 3.92 4.45 67.2 67.0 4.89 1.24 1.24 -7.09 0.12 1.29 0.36 0.64
M_Z0005 0.0005 4.60 3.97 4.50 60.8 60.7 4.69 1.59 1.60 -7.21 0.18 1.27 0.40 0.60
M_Z0002 0.0002 4.77 3.95 4.52 54.5 54.3 4.49 2.04 2.06 -8.26 0.25 1.27 0.44 0.56
M_Z0001 0.0001 4.89 3.92 4.53 50.0 49.7 4.34 2.46 2.48 -8.40 0.31 1.26 0.48 0.52
Notes.Theparametersdisplayedherearetheinitialmetallicity,Z,themassofthestrippedstar,M ,themassoftheheliumcoreofthestripped
(cid:63)
star,MHecore,theluminosity,L,thestellartemperature,T(cid:63)(atanopticaldepthτ=100),theeffectivetemperature,Teff(atτ=2/3),theeffective
surfacegravity,log10geff,thestellarradius,R(cid:63)(τ=100),theeffectiveradius,Reff(τ=2/3),thewindmasslossrate,M˙,thetotalhydrogenmassof
thestrippedstar,M ,thetimeduringwhichthestarisstripped,τ (theremaininglifetimeafterdetachmentfromfirstRochelobeoverflow
tot,H stripped
phase),thesurfacehydrogenmassfraction,X andthesurfaceheliummassfraction,X .Unlessstatedotherwiseweusetheparametervalues
H,s He,s
halfwaythroughcoreheliumburningwhichwedefineas X = 0.5.Theheliumcoreisdefinedasthemostexteriormasscoordinatewhere
He,c
XH<0.01andXHe>0.1.WenotethatbyconstructionT(cid:63)≈Teff,mesa,thesurfacetemperatureresultingfromourMESAmodels.Weusethevalues
fromourstandardwindmasslossmodelsandhighlightourreferencemodelinlightgray.
sequenceandswelltobecomeredgiants(alsocalledcaseBmass HD45166(Grohetal.2008)isingoodagreementwiththerate
transfer,seeKippenhahn&Weigert1967).Thisisthemostcom- estimatedbyextrapolatingtheNugis&Lamers(2000)WRalgo-
moncaseofmasstransfer.Aboutathirdofallmassivestarsare rithm.Wenotethatitisalmosttwoordersofmagnitudehigher
estimatedtoundergothistypeofevolution(Sanaetal.2012). thanvaluesonewouldobtainbyextrapolatingtherecentresults
Westarttheevolutionattheonsetofcorehydrogenburning byKrticˇkaetal.(2016)forsdOstars.Wefindthattheirmassloss
using the nuclear network approx21, which contains reactions algorithmwouldgiveamasslossratethatistoolowtobecon-
relevantforhydrogenburningthroughtheCNOcycleandnon- sistentwiththeobservedspectralcharacteristicsforHD45166.
explosive helium burning (see Paxton et al. 2011). We follow WethereforechosetoadoptNugis&Lamers(2000).Through-
the evolution through all long-lived phases until central carbon out this work we consider variations in the adopted wind mass
depletion (defined as X < 0.02). These evolutionary stages lossrateforstrippedstarstounderstandtheeffectsofwindmass
C,c
togetheraccountfor99.9%ofthestellarlifetime. lossrate.
We account for convective mixing using the mixing length Weinitiateourmodelsatthezero-agemainsequencewith-
approach (Böhm-Vitense 1958) adopting a mixing length pa- out rotation. The donor star never reaches high rotation rates
rameterα = 2.Weallowforovershootingaboveeverycon- throughoutitsevolution(cf.Yoonetal.2010).Althoughwedo
MLT
vectiveburningregionupto0.335pressurescaleheightsabove notdiscusstheevolutionoftheaccretingstarhere,westillfol-
the convective region, following the calibration by Brott et al. low its evolution. During mass transfer the accreting star spins
(2011).Thesemiconvectionparameterissettoα =1(Langer up. We assume the accretion mechanism described in Petrovic
sem
1991). We also account for thermohaline mixing (Kippenhahn et al. (2005) where, when the secondary star rapidly reaches
etal.1980)androtationalmixing(Paxtonetal.2013),butnei- critical rotation during the mass transfer stops accreting signif-
ther of these play a significant role in the models of the mass icant amount of material. Non-conservative mass transfer fol-
losingprimarystarpresentedhere. lows, where we assume that the mass not accreted by the sec-
We use the Vink et al. (2001) wind mass loss algorithm for ondaryislostfromthesystemwiththespecificangularmomen-
effective temperatures estimated by MESA, Teff,mesa > 104 K tumoftheorbitofthesecondary.Theefficiencyofmasstrans-
and surface hydrogen mass fraction, X > 0.4. This prescrip- fer is uncertain (de Mink et al. 2007), but the properties of the
tion scales with metallicity as M˙ ∝ ZH0,.s85. In the models pre- primary star are not significantly affected by the details of the
sented here, these conditions are met throughout the main se- treatmentofthesecondarystarandtheaccretionefficiency.
quence evolution as well as the post-main sequence evolution We use the parameters derived for the observed binary sys-
beforetheonsetofmasstransfer. tem HD 45166 as a motivation for the chosen starting point
After stripping, most of our stars have a surface hydrogen of our parameter space exploration. HD 45166 consists of a
mass fraction XH,s < 0.4 and then we switch to the empiri- 4.2 ±0.7M(cid:12) quasi-WR (qWR) star that is orbiting a compan-
cally determined WR mass loss algorithm of Nugis & Lamers ionofspectraltypeB7V.Thespectraltypeofthecompanionis
(2000).Thisprescriptionscaleswithluminosity,surfacehelium consistentwithamainsequencestarofabout4.8M(cid:12)(Steiner&
abundance and metallicity, the relation with metallicity being Oliveira2005).TheqWRstarisbelievedtobethestrippedrem-
M˙ ∝ Z0.5 (cf. Kudritzki et al. 1987). Lacking an appropriate nant of an initially more massive star. We find that the param-
wind mass loss algorithm for the winds of stripped stars, we eters of this system can be approximately reproduced adopting
extrapolate this algorithm and apply it to the lower luminosi- M1,init = 12M(cid:12) and M2,init = 5M(cid:12).Theinitialmassofthesec-
ties of stripped stars compared to WR stars. The observation- ondary is not as well constrained as the primary. It depends on
allyderivedwindmasslossrateoftheobservedstrippedstarin the efficiency of mass transfer. However, here we are primarily
Articlenumber,page3of21
A&Aproofs:manuscriptno.Gotbergetal_Z
5 TheobservedstrippedstarinHD45166ismeasuredtohave
3m] 0 3g/cm] --87..00 tlhoeciwtyinbdetsapleaewdowfitvh∞β==3510(G±r4o0hkemt asl−.12w00h8e)n. Tushiinsgwainwdinsdpeveed-
[g/c −5 EnhancedM˙ g[ρ10-1-09..00 disesnucreporifsianlgaltyitulodwinfaol-rdaepsetanrdwenitthwTineffd(cid:39)in5H0D004051K6.6T(hGerroehiseetvail-.
ρ0 10 StandardM˙ lo 0.68 0.69 2008). The spectral lines are consistent with the presence of a
log1−15 ReducedM˙ R[R(cid:12)] fwaistthp(ovl∞ar(cid:39)w3in0d0k(vm∞s(cid:39)−11),3w0h0ickhmcso−u1l)darnedcoanscliolewoebqsueravtoatriioanlswainndd
− theoretical expectations of hot-star winds. The derived clump-
9
ingfactoris0.5(Grohetal.2008).Weusethesemeasurements
8 ) 5.5 of wind parameters in the qWR in HD 45166 for our spectral
K
(/)TgK1067 (/Tlog10 45..92 0.68 0.69 mdweeoraidAvkeeelddsrooploiftnpisnettisgrciaiphflpigtedhdheepesrtmthatraeossr.fsmtlihonesaslwrwainitneddiassnpkdeeepadtlsscoownyosiuetallddnt.binrFcooraerdaeasrecaotnhnde-
lo 5 R[R ] stantmasslossrate,lowerclumpingfactorswouldincreasethe
(cid:12)
4 equivalentwidthofrecombinationlinessuchasHeIIλ4686.We
0.1 1 10 100 discuss the effects of the wind terminal velocity and clumping
R[R ] factorsonthespectrumofstrippedstarsinAppendixB.
(cid:12)
Fig.1.Density(top)andtemperature(bottom)structureoftheMESA
2.3. Connectingatmospherestostructuremodels
(grey)andCMFGEN(colored)modelsofastrippedstaranditsatmo-
sphere. We show our reference model (Z = 0.0166). The CMFGEN We employ the method described in Groh et al. (2014) to con-
modelsarematchedtotheMESAmodelataRosselandopticaldepth necttheMESAstellarstructurewithCMFGEN.Herewebriefly
ofτ = 20.ThedifferentcolorsoftheCMFGENmodelscorrespondto
describe the approach and refer to the paper above for further
thestandard,reducedandenhancedwindmasslossrates.
details(seealsoSchaereretal.1995).
We take a structure model from the evolutionary sequence
interestedintheeffectontheprimarystar.Theprecisechoiceof computedwithMESA,whenthestrippedstarishalfwaythrough
thecompanionmassdoesnothavelargeeffectsontheoutcome coreheliumburning,whichwedefineasthemomentwhenthe
oftheprimarystarafterstripping. centralheliummassfractiondropsto XHe,c = 0.5.Asinputfor
In this work we investigate the effect of metallicity. In a theCMFGENmodelsatRosselandopticaldepthτ=20,weuse
subsequent paper we will discuss the dependence on the fur- theeffectivetemperature(Teff,mesa),surfacegravity(log10gmesa),
ther system parameters. We compute a grid of binary systems stellarradius(Rmesa)andsurfaceabundancesextractedfromour
MESAmodelfollowingtheapproachextensivelytestedbyGroh
with these initial stellar masses and adopt an initial orbital pe-
riod of 20 days. We vary the metallicity between Z = 10−4 up etal.(2014).FromtheCMFGENmodelsweextractthetrueef-
to Z = 0.02. This corresponds to values of the extremely low fectivetemperature,Teff (τ = 2/3),andthesurfacetemperature
ofthestar,T ,definedatτ = 100.Ourstandardwindmassloss
metallicity dwarf galaxy IZw18 (Izotov & Thuan 1998; Szécsi (cid:63)
modelshavethewindmasslossratefromtheNugis&Lamers
etal.2015;Kehrigetal.2015)uptosuper-solarvalues.Therel-
evantmetallicityforHD45166isZ =0.0166(Grohetal.2008; (2000) WR algorithm, with the exception of the Z (cid:54) 0.0002
models which are slightly cooler and have the wind mass loss
Asplundetal.2009).
FortheinitialheliummassfractionweassumeY =0.24+2Z rate of Vink et al. (2001). Given the uncertainties in mass loss
rate,wealsoconsiderathreetimeshigherandthreetimeslower
following Pols et al. (1998), which approximately spans the
mass loss rate in the CMFGEN models, which we label "en-
rangebetweenanapproximatelyprimordialchemistryandanear
solarabundance.Forhydrogenweassume X = 1−Z −Y.The hanced"and"reduced"inadditiontoour"standard"assumption.
Figure1displaystheconnectionbetweentheMESAmodel
abundancesoftheheavierelementsareassumedtoscaletoso-
and corresponding three CMFGEN models for our reference
larandmeteoricabundanceratiosasdeterminedby(Grevesse&
model(Z =0.0166).Intheinsetsweshowthedifferenceinstel-
Sauval1998).Table1providesanoverviewofthemainparam-
etersoftheevolutionarymodelspresentedinthiswork. larradiuscalculatedinMESAandCMFGEN,whichis0.002R(cid:12)
inthereferencemodel.Thisradiusdifferenceisnegligibleasit
translates to effective temperature differences of about 100 K –
2.2. StellaratmosphereswithCMFGEN a value is less than 1% of the surface temperature which range
between50000and80000K(seeTable1).
We use the radiative transfer code CMFGEN (Hillier & Miller
1998) to model the spectra of stripped stars tailoring them to
the MESA stellar evolution models. CMFGEN takes into ac-
3. Evolutionandspectralfeaturesofstrippedstars
count gas out of local thermodynamic equilibrium (non-LTE)
andisthereforesuitableformodellingopticallythickstellarat- The consequences of mass loss due to interaction with a com-
mospheres.Thepresenceofanextendedatmosphereandanop- panioninabinarysystemhavebeenthetopicofseveralclassic
tically thick wind changes the optical depth scale, affecting the papers(e.g.Morton1960;Smak1962;Paczyn´ski1966;Kippen-
effectivetemperatureandeffectivesurfacegravity(botharede- hahn1969;Yungel’Son1973;vanderLinden1987).Wediscuss
finedataradiuswheretheRosselandopticaldepthisτ = 2/3). the results obtained with modern higher resolution simulations
EachCMFGENmodelprovidesthespectralenergydistribution usingupdatedinputphysics,butwenotethatseveralofthephys-
and a normalized spectrum which we compute between 50 and ical arguments concerning the evolution can already be found
50000Å.WeincludetheelementsH,He,C,N,O,SiandFe. in these early papers. More recent works on this topic includes
Articlenumber,page4of21
Götberg,DeMink&Groh:Ionizingspectraofstarsthatlosetheirenvelopethroughinteractionwithabinarycompanion
Yoon et al. (2010), Claeys et al. (2011), Eldridge et al. (2013), 3.2. Characteristicsandevolutionofthestrippedstar
Tauris et al. (2015), McClelland & Eldridge (2016) and Yoon
Our main phase of interest is the helium burning phase of the
etal.(2017).
stripped star (F). This is the longest lasting evolutionary phase
We use a 12 M(cid:12) model as an example of the primary in a afterthemainsequence,accountingforalmost10%ofthetotal
typicalmassivebinarysystem.Thecompletesetofadoptedpa-
lifetime in our model. Given the high fraction of young stars
rameters are listed and motivated in Sect. 2.1. We assume an
inclosebinariesthatwillundergosimilarevolution,onewould
initial separation such that the system evolves through case B
expectonaverageafewpercentofallstarstoresideinthisphase
masstransfer.Thisconcernssystemswheretheprimarystarfills
atanygiventime.
itsRochelobeshortlyafterleavingthemainsequence,typically
beforetheonsetofheliumburning.CaseBmasstransferisthe Despitehavinglostovertwothirdsofitsmass,from12 M(cid:12)
mostcommontypeofmasstransfer.Moreover,itproduceslong- initially down to 3.6 M(cid:12) after stripping, the bolometric lumi-
lived post-interaction products, which have not yet completed nosity,log10(L/L(cid:12)) ∼ 4.2,isstillcomparabletotheluminosity
ithadduringitsmainsequenceevolution.Foranoverviewofthe
theirheliumburningphase.Thismakesthereferencemodeldis-
cussed here useful as a starting point for our exploration. We properties,seeTable1.Thestarisverycompact∼0.7 R(cid:12),hasa
have adopted a metallicity that is comparable to that found in veryhighsurfacegravity,log10geff ∼5.3andisveryhot.
Withaneffectivetemperature∼80000K,theradiationemit-
nearby young star clusters and OB associations in the Milky
ted by the stripped star is expected to peak in far/extreme UV.
Way. More precisely, we adopted the metallicity measured for
the HD 45166, which is Z = 0.0166 Groh et al. (2008). The In Fig. 2 we indicate the fraction of photons emitted at wave-
effectsofmetallicityarediscussedinthenextsection. lengths blue-ward of λ (cid:54) 912 Å (Lyman continuum photons)
by a blackbody with the corresponding effective temperature,
withblueverticalcontours. Ablackbodywithatemperature of
3.1. Binaryevolutionandtheformationofastrippedstar 80000K,similartheeffectivetemperatureofourstrippedstar,
emitsabout85%ofitsradiationinLymancontinuumphotons.
InFig.2weshowtheevolutionarytrackcomputedwithMESA Forcomparison,theequivalentsinglestarmodel(showningrey
of a 12 M(cid:12) single star in grey, together with the evolutionary inFig.2)hasaneffectivetemperatureofonly∼3000Kduring
track for the primary star in our binary reference model. Ini- itsheliumburningphase.Asinglestarofthismassdoesnotemit
tially,thetwostarsevolvesimilarastheymovealongthemain anysignificantnumberofionizingphotonsthroughoutitsentire
sequence (labeled A-B). The central helium abundances XHe,c life.
(plotted in color) steadily rises as the both stars fuse hydrogen
Initially,wefindthatathinshellcontainingamixtureofhy-
intoheliumintheirconvectivecores.
drogen and helium remains at the surface. It contains less than
Afterabout18.7Myrbothstarshaveexhaustedtheircentral 0.1M(cid:12) of hydrogen. This is sufficient to sustain a weak hydro-
fuel.Nuclearburningofhydrogencontinuesinashellaroundthe genburning.Theburningshellquicklymovesoutwardinmass
heliumcoreandbothstarsexpand.Thesinglestarexpandsfreely coordinateasitconvertsHintoHeascanbeseeninFig.3.At
tobecomearedsupergiant,reachingafinalradiusinexcessof the same time mass is lost from the surface by the stellar wind
1000 R(cid:12). In contrast, the primary star in our binary model fills atarateof M˙ ∼ 2.5×10−7M(cid:12)yr−1 inthismodel.Thisishigh
its Roche lobe at point C and starts to rapidly lose mass. This enough to remove the hydrogen layer by the time core helium
can be seen in the left panel of Fig. 3, where we plot how the depletionisreached.SeealsopaneleofFig.9,whichshowsthe
masschangesasafunctionoftime.Wealsoshowthemassco- evolutionofthesurfaceabundanceofhydrogenandheliumasa
ordinatesoftheregionswherenuclearburningtakesplace(blue functionoftime.
shading) and the interior regions that are mixed by convection Whencentralheliumburningceases,theentirestarcontracts
and overshooting (green diagonal lines and purple cross hatch- andheatsuptopointG,wheretheheliumshellignites.TheC/O
ing,respectively). corecontinuestocontractwiththeheliumburningshellontop.
Theprimarystarhasaradiativeenvelopeattheonsetofmass The envelope responds by expanding until the star reaches bal-
transfer.Astheouter,highestentropylayersareremoved,theen- ance at point H. During the expansion, carbon and later also
velopeinitiallyrespondsonadynamicaltimescalebyshrinking. oxygen ignite in the core. We stop the computations at central
On a longer, thermal timescale the star is still trying to expand carbon depletion. The following phases of the evolution are so
andcrosstheHertzsprunggapleadingtocontinuedstablemass fast that the luminosity and effective temperature will not have
transfer.TheRochelobelimitsthesizeofthestar. time to change significantly any more. With a helium layer of
Duringthemasstransferphasetheluminositydropsbymore about1.3M(cid:12) andalmostpureheliumonthesurface,thestaris
thananorderofmagnitude.Thisisbecausethedeeperlayersof expectedtoenditslifeasaH-deficient(typeIb)supernova.
thestarneedtoexpandastheyadjusttothequicklydecreasing
totalmass.Theenergyneededforthiscausesabrief,largedrop
3.3. Theresultingspectrumofastrippedstar
in the luminosity. This continues until the maximum mass loss
rate is reached (D). Once the mass loss rate starts to decrease Wecomputeatailor-madeatmospheremodelwithCMFGENfor
again,thereadjustmentsinthethermalstructurerequirelessen- thestrippedstarcomputedwithMESAatthemomentwhenitis
ergysoluminositycanagainincrease. halfwaythroughheliumburning,usingtheproceduredescribed
AtpointEthedonorstarhaslostmorethan8 M(cid:12).Thesur- inSect.2.3.Figure4showstheresultingspectrum(blackline).
facelayersthatarenowexposedarehelium-richlayersthatwere The wavelengths corresponding to photon energies required to
once part of the convective core. The star detaches and mass ionize neutral hydrogen (HI), singly ionize helium (HeI) and
transfer stops. The now stripped core of the primary contracts fully ionize helium (HeII) are marked (vertical dotted white
andheatsupuntilthermalequilibriumisrestoredatpointF.The lines). For comparison, we overplot a blackbody spectrum for
central regions have now reached temperatures high enough to a temperature of ∼80 000 K (grey dashed line), which corre-
igniteheliumburning,whichburnsintheconvectivecoretocar- spondstoTeff,mesa,whichisthecomputedeffectivetemperature
bonandoxygen. resultingfromtheMESAmodel.
Articlenumber,page5of21
A&Aproofs:manuscriptno.Gotbergetal_Z
% % % %
90 70 50 20 offluxisHIionizing
6 1.0
0.8
5
H
E
1
)⊙ G B C 000R 0.6
L
/L 4 F A ⊙ ,ec
( H
0 X
1
g 0.4
o
l
3
0.2
D 1
0
0
0. 0. 1 R
0 1 0
1 R 1 R ⊙
2 R R
⊙ ⊙ ⊙ ⊙ 0.0
5.5 5.0 4.5 4.0 3.5
log T [K]
10 eff,MESA
Fig.2.Hertzsprung-Russelldiagramshowingtheevolutionofastarthatlosesitshydrogen-richenvelopethroughRoche-lobeoverflowinabinary
(ourreferencemodel,metallicityZ =0.0166).Thecolorofthetrackshowsthecentralheliummassfraction,highlightingthelong-lastingphases
ofhydrogenandheliumcoreburningwithcolorchange.Thethicker,blackbackgroundlinemarkstheRoche-lobeoverflow.Lettersmarkdifferent
evolutionarystages,moreindepthdescribedinthetext.AtpointFthestrippedstar’scentralheliummassfractiondecreasesandpassesX =0.5
He,c
–thepointwhenwemodelthespectrum(seeSect.3.3andFigs.4and6).Thegraytrackisthatofacorresponding12M(cid:12) singlestar.Theblue
backgroundshadingshowsthefractionofemittedfluxthatishydrogenionizing,assumingblackbodyradiation.Ingreystraightlinesweshow
linesofconstantradius.
A B-F G H 1] A B-F G H 1]
− −
12 7 s 12 7 s
1 1
Z = 0.0166 − Z = 0.0002 −
g g
10 6 g 10 6 g
r r
e e
[ [
0 0
1 1
8 5 g 8 5 g
o o
l l
] n, ] n,
⊙ o ⊙ X = 0.5 o
[mM 6 X c ,H e = 0.5 4 erati [mM 6 c,He 4 erati
n n
4 3 ge 4 3 ge
y y
g g
r r
2 2 ne 2 2 ne
E E
C O,Ne
r r
0 H7 He 6 He C,O5 4 C O,Ne3 1 uclea 0 H7 He 6 He C,O5 4 3 1 uclea
log (t t)[yrs] N log (t t)[yrs] N
10 end− 10 end−
Fig.3.Leftpanel:Kippenhahndiagramshowingtheinteriorstructureofthe12M(cid:12)primarystarofthereferencemodelovertime.Weshownuclear
burningregionsshadedinblue,convectioninhatchedgreenandovershootingincrosshatchedpurple.Thesolidblacklineshowsthesurfaceofthe
star,whichdecreasesquicklywhentheenvelopestrippingoccurs.ThelettersmarkthesameevolutionaryphaseaslabeledinFig.2.Rightpanel:
CorrespondingKippenhahndiagramforthelowermetallicity(Z = 0.0002)model.Notethelargeramountofleft-overhydrogen-richenvelope
afterRoche-lobeoverflow.
Theblackbodyapproximationstronglyoverestimatestheex- the form of recombination lines. The excess in infrared and ra-
tremeUVflux(short-wardof228Å)andunderestimatestheflux dio wavelengths compared to the blackbody is due to free-free
at peak (which occurs between 228-512Å). It also underesti- emission.Thesharpdropatλ = 228Å isduetorecombination
mates the emission at longer wavelengths (most clearly visible ofHe2+toHe+intheouterpartsofthewind,allowingforHe+to
in Fig. 4 for wavelengths longer than about 4000Å, where re- absorbasignificantfractionofthehigh-energyphotons.Wedo
processedionizingphotonsareemittedatlongerwavelengthin
Articlenumber,page6of21
Götberg,DeMink&Groh:Ionizingspectraofstarsthatlosetheirenvelopethroughinteractionwithabinarycompanion
10 5
−
y
2]− 10−9 kbod
m c
a
c Bl
1
s− 10−13
g
r
e
[
c
p 10 17
k −
1
t
a
Fλ10 21 EnhancedM˙
λ − StandardM˙
ReducedM˙
HeII HeI HI
10 25
−
102 103 104
Wavelength[A˚]
Fig.4.Spectralenergydistributionofourreferencemodelwithstandardmasslossrateinblacktogetherwiththeblackbodywithtemperature
sameasTeff,mesaindashedgrey.Inpinkandblueweshowthereducedandenhancedmasslossratemodels.Wehighlightthehighrateofionizing
fluxemittedfromthisstarinpurpleshading.
VaryingtheadoptedmasslossrateintheCMFGENmodels
0
a) H+ d) He2+ hasalargeeffectontheextremeUVfluxascanbeseeninFig.4.
-4 He+ Increasing the mass loss rate enhances the density in the wind,
-8 effectivelymakingitmoreopticallythickinthecontinuum.This
otal) -12 ReducedM˙ H0 He0 pmeorvateusreth.eInpthhoetocsapsheeorefoouurtwmaorddealnwditrhedeuncheasnctheedemffaescstilvoesst,ewme-
T
/ 0 findthatthephotosphereislocatedat0.99 R(cid:12),whilethestellar
nce -4 b) e) surface is located at 0.68 R(cid:12). The resulting effective tempera-
da ture, Teff ∼70 000 K, is almost 10 000 K lower than the tem-
bun -8 StandardM˙ peratureatthestellarsurface.Thiseffectiscommonlyobserved
A -12 forthethehighermasscounterpartstostrippedstars,WRstars,
(
0 whichtypicallyhavedense,opticallythickwinds.Forthemod-
1 0
g elsthatadoptourstandardandreducedmasslossrateswefind
o c) f)
l -4 no significant difference between the effective and the surface
temperature, indicating that in these cases the wind is optically
-8
EnhancedM˙ thininthecontinuum.Manyspectrallinesarehoweveroptically
-12
thick,aswediscussinSect.3.4.
1 10 100 1 10 100
r/R r/R
⋆ ⋆
3.4. Characteristicspectralfeatures
Fig.5.Theionizationstructureofhydrogen(left)andhelium(right)in
thethreeatmospheremodelsofourreferencemodel.Fromtoptobot- Toshowthespectralfeaturesmoreclearly,weplotthenormal-
tomweshowthereduced,standardandenhancedmasslossratemodels. ized spectra in Fig. 6 for our standard, enhanced and reduced
masslossrate.OurCMFGENmodelsshowthatmostemission
linesareopticallythickinthesemodels.
The strongest line in the optical spectra is the HeII λ4686
notobserveasimilardropatthehydrogenionizinglimit(912Å),
withanequivalentwidthof95.6Åandwithfluxatthepeakcor-
sincethehydrogeniscompletelyionizedthroughoutthewind.
responding to more than ten times that of the neighboring con-
InFig.5,weshowtheionizationstructureofhydrogenand tinuum in the standard wind mass loss model (see enlargement
helium as a function of the distance from the stellar surface. inpaneleofFig.6).
For helium we find a helium recombination front at a distance Other strong lines in the optical band are the blend of
of about 80 stellar radii (panel e). For our standard model, we HeII λ6560 and Hα (panel f) and a mix of NIV lines in the
find that hydrogen is fully ionized out to a distance of about range 7100-7140 Å (panel g). We also find weaker emission in
100R , which is the computational domain considered (panel CIV λ5801 and CIV λ5812. In the enhanced mass loss model
(cid:63)
b).Atlargerdistancesthedensitydropsfurtherandthereforewe HeI λ5875 is showing up due to the higher abundance of He+
donotexpecthydrogentorecombine.Thisassuresthatnomajor in the wind compared to the lower mass loss models. The
changes to the spectrum will be caused by the wind outflow at UV spectrum’s strongest line is HeII λ1640, but also the Lyα
largerdistancesthanconsideredinourmodel. and HeII λ1215 blend, NIV λ1487 and NIV λ1719 are strong
Articlenumber,page7of21
A&Aproofs:manuscriptno.Gotbergetal_Z
9 a) b)
6
3
0
1200 1400 1600 1800 2000 2200 2400 2600 2800
9 c) d)
6
x 3
u
fl
d 0
ze 3000 3200 3400 3600 3800 4000 4200 4400 4600
ali 9 e)
m
or 6
N
3
0
4600 4800 5000 5200 5400 5600 5800 6000 6200
9 f) g)
Enhanced
6 Standard
3 Reduced
0
6400 6600 6800 7000 7200 7400 7600 7800 8000
Wavelength[A˚]
128 a) HeII1215Lyα NV1239NV1243 20 b) NIV1487 CIV1548CIV1551 HeII1640 NIV1719
10
4
x
u
fl
d 0 0
ze 1220 1240 1500 1550 1600 1650 1700
ali 6 12 6
Norm 8 eII3203 c) 4 d) NIV3479NIV3483NIV3485 20 e) NV4604 NV4621 HeII4686 8 HeII6560Hα f) 4 g) NIV HeII7178
H
4 2 10 4 2
0 0 0 0 0
3190 3210 3480 3500 4600 4640 4680 6530 6580 7100 7150 7200
Wavelength[A˚]
Fig.6.CMFGENspectraforourreferencemodel.Weshowthestandard,enhancedandreducedmasslossratemodelsinblack,pinkandblue
respectively.Thegreyshadedregionsareshownaszoom-insinthebottompanels.
in emission. In the infrared we find the strongest lines to be Girardi et al. 2000; Maeder & Meynet 2001; Campbell & Lat-
HeIIλ18636,λ30908andλ47622. tanzio 2008; Brott et al. 2011; Pietrinferni et al. 2013; Szécsi
Varying the mass loss rate affects the strength of the emis- etal.2015).Summarizingtheseworks,wecandistinguishthree
sionlines.Lowermasslossratesconsistentlyshowweakerlines. main effects of metallicity on (1) the opacity, most notably in
If stripped stars had mass-loss rates even lower than those pre- thesubsurfacelayers,(2)thenuclearburningrate,especiallyof
sented here, our models indicate that they would show little or hydrogen fusion through the CNO cycle, and (3) the mass loss
negligibleemission,andmostlyanabsorption-linespectrum. rates for radiatively driven winds. Here, we describe how the
differenteffectsofmetallicityinterplayincaseofthemasslos-
ing star in a binary system. We consider metallicities between
4. Effectofmetallicity(I):theformationand
Z = 0.0001andZ = 0.02.Wediscusstheconsequencesforthe
structureofstrippedstars
pre-interactionphase,theremovaloftheenvelope,theresulting
Theeffectofmetallicity,i.e.themassfractionofelementsheav- characteristics of the helium star and its emerging spectrum in
ierthanheliumthatcharacterizesthecompositionofthegasout thesectionsbelow.
of which the star is formed, on the structure and evolution of
singlestarshasbeendiscussedinnumerousstudies(e.g.Boden-
heimer et al. 1965; Copeland et al. 1970; Bertelli et al. 1986;
Maeder 1990; Schaller et al. 1992; Cassisi & Castellani 1993;
Articlenumber,page8of21
Götberg,DeMink&Groh:Ionizingspectraofstarsthatlosetheirenvelopethroughinteractionwithabinarycompanion
% % % %
90 70 50 20 of flux is HI ionizing
6 1.0
1
0
0
0
R
Z = 0.0001 ⊙ 0.8
5
Z = 0.02
)
⊙ 0.6
L
/L 4 ,ec
( H
0 1 X
g1 00 0.4
o R
l ⊙
3
0.2
1
0
1
2 R R
⊙ 0.0
⊙
5.5 5.0 4.5 4.0 3.5
log T [K]
10 eff,MESA
Fig.7.EvolutionarytracksofdonorstarswithmetallicityrangingfromZ = 0.0001uptoZ = 0.02(similartoFig.2)astheyevolvefromthe
zero-agemainsequenceuntiltheyexperiencemasslossbyRoche-lobeoverflow(blackoutlined).Theirsubsequentevolutionasstrippedstarsis
highlightedintheinsetpanel.Thelowmetallicitymodelsaremorecompactandhotterbeforeinteraction.However,aftermasstransfertheyare
coolerandmoreluminousthanthehighmetallicitystrippedstars.ForadiscussionoftheevolutionwerefertoSects.4.1,4.2and4.3.
4.1. Thepre-interactionphase leastnotdirectly:κ =0.2(1+X )cm2g−1.However,whenini-
es H
tializing our models we chose to scale the initial mass fraction
Duringthepre-interactionphasebothstarseffectivelyevolveas of hydrogen and helium with metallicity. The hydrogen abun-
single stars. Throughout the main-sequence evolution, we find dances decrease as the metallicity rises in agreement with the
thatmetal-poorstarsarehotter,morecompactandalittlemore overall trend expected for the chemical enrichment of galaxies
luminous,ascanbeseeninFig.7.Thisisconsistentwithwhat over cosmic time. This way we introduced a mild indirect de-
hasbeenfoundforsinglestarsasdescribedinthestudiesmen- pendencyofκ onthemetallicity.Ourmetal-poormodelshave
es
tionedabove. initiallya∼8%highervalueforX thanourmetal-richmodels.
H
Thisis,inpart,duetotheeffectofmetallicityonthenuclear Theeffectontheopacityintheinteriorsissmall,butofim-
reaction rates for hydrogen burning through the CNO cycle, as portance since the electron scattering plays a role in determin-
alreadypointedoutintheearliestpapersonthesubject(cf.Bo- ing the extent of the convective core, which in turn determines
denheimeretal.1965).Atlowermetallicity,thecatalystsforthe howmuchfuelthecentralburningregionscanaccess.Figure3
CNO cycle are more scarce and hydrogen burning is less effi- showstheextentoftheconvectivecoreinmasscoordinate.Our
cient.Thestarcompensatesbycontracting,whichincreasesthe metal-poor model has indeed a more massive convective core
central temperature. This, in turn, raises the nuclear reactions at the start of its evolution, due to a combination of the effects
ratestolevelthatisrequiredforthestartobeinthermalequilib- discussedabove.
rium.Theresultisahotter,moreluminousstar. We can further see the effect of the metallicity dependence
Thesedays,stellarevolutionarycalculationsadoptmorere- of mass loss by radiatively driven stellar winds. Our metal-rich
alistic opacity tables (e.g., OPAL, Iglesias & Rogers 1996), modellosesafewpercentofitsmassoverthecourseofthemain
which have a more sophisticated treatment of the many bound- sequenceevolution(labeledA-BinFig.3).Ourlowmetallicity
boundandbound-freetransitionsduetometals.Thesearemost modeldoesnotshowanysignificantmasslossbystellarwind.
importantinthecoolersubsurfacelayersofthestar,whereheavy Boththeextentoftheconvectivecoreandstellarwindmass
elementsareonlypartiallyionized.Themostprominentexample loss have consequences for the final mass of the helium core
istheso-calledironpeak,whichplaysaroleinthesub-surface whenthestarleavesthemainsequenceM .Wefinda
Hecore,TAMS
layersatlog10T ∼5.2(Cantielloetal.2009).Theloweropacity smalldifferenceinmassofabout6%,withthemetalpoorstar
inourmetal-poormodelsiscontributingtothefactthattheyare having the more massive core (M = 3.14 and 2.95
Hecore,TAMS
hotterandmorecompact. M(cid:12),respectively)inadditiontosubtledifferencesinthechemi-
In the deep interior, electron scattering is the dominant calprofileabovetheheliumcore.Theslightlylargercoremass
source of opacity, which does not depend on the metallicity, at andlargertotalmassforourmetal-poormodelsexplainwhyour
Articlenumber,page9of21
A&Aproofs:manuscriptno.Gotbergetal_Z
metal-poor stars are substantially brighter when they leave the Weexpectthatthereducedopacityintheouterlayersofthe
mainsequenceascanbeobservedwhencomparingthelocation metal-poormodelsisanimportantfactor.Inthesubsurfacelay-
ofthecharacteristichookfeaturethatmarkstheendofthemain erswheretheironpeakplaysarole,wefindthattheopacityis
sequenceFig.7. about three times higher in our metal-rich models. In addition,
In Fig. 8 (panel a) we compare the main sequence lifetime. also the subtle differences in the interior chemical profile that
Reducing the metallicity from 0.02 down to ∼0.0021 increases are inherited from the pre-interaction evolution play a role. A
the lifetime by about 8%, owing to the amount of nuclear fuel furthereffectthatmaycontributeisthedifferenceincentraltem-
that the star can access. When we reduce the metallicity fur- perature. The metal-poor models have more massive and more
therto0.0001,thiseffectsaturates.Thevariouseffectsthatlead compact cores, and thus higher central temperatures, which al-
to a higher luminosity start to dominate. The stars effectively lows for the ignition of central helium burning before the star
burn faster through their available fuel. We find that the main detachesfromitsRochelobe.Ourmostmetal-richmodelisstill
sequencelifetimeslightlydecreasesagain. primarilypoweredbyHshellburningwhenitdetaches.
Whenthestarleavesthemainsequenceitcontinuestoburn Afterthestarsdetachwefindthatallmodelscontract,fully
hydrogen in a shell around the core. The shell briefly drives a igniteheliumintheircenteriftheyhadnotalreadydoneso,and
convection zone, which influences the details of the shape of settle to their thermal equilibrium structure as a central helium
chemical profile above the core, but we find that it has no in- burningstar.
fluence on the stripping process for the models presented here.
For our most metal-rich model, we find that a region of about
4.3. Theresultingstrippedstar:thelong-livedheliumburning
1M(cid:12)abovethecoreispartiallyburned,whenthestarleavesthe
phaseandfurtherevolution
mainsequence.Forourmostmetal-poormodelwefindthatthe
regionextendstoabout1.5M(cid:12)abovethecore. Intheprevioussectionwedescribedthatthestrippingprocessis
inefficientatlowermetallicity:itfailstoremoveentireenvelope.
Our metal-poor models consist of a helium core surrounded by
4.2. TheonsetofRoche-lobeoverflowandtheremovalof
theenvelope aremaininglayerofenvelopematerialofmorethan1M(cid:12) (right
panel of Fig. 3). This layer consist of a mixture of hydrogen
ThestarsweconsiderherefilltheirRochelobeshortlyafterleav- andhelium,containingatotalamount0.38M(cid:12)ofpurehydrogen
ingthemainsequenceduringthehydrogenshellburningphase. right after detachment from the Roche lobe. For our metal-rich
Throughouttheirpre-interactionevolution,themetal-poormod- modeltheremainingshellcontainslessthan0.05M(cid:12) ofhydro-
elshavebeenmorecompact.Theyneedtoexpandfurtherinor- gen, which initially allows for weak burning in a shell around
dertofilltheirRochelobe.Effectively,theyfilltheirRochelobe thecore(leftpanelofFig.3).
ataslightlymoreadvancedevolutionarystage. Thishasconsequencesfortheluminosityandeffectivetem-
ThesizeoftheRochelobeisapproximatelythesameatthe perature. The metal-poor stripped stars are about 30 000 K
momenttheprimaryfillsitsRochelobeforthefirsttime,about cooler, and 0.3 dex brighter and almost four times bigger than
27R(cid:12) inalloursimulations.Thereisasmalldifferencebecause their metal-rich counterparts, see Fig. 8 and Table 1. This is
of the metallicity dependent stellar wind mass loss rates. Mass somewhat counterintuitive, since it is the opposite of what is
lossfromourmetal-richsystemsintheformoffaststellarwinds foundforsingleandpre-interactionstars.Beforeinteraction,we
leadstowideningoftheorbitandthusincreasingtheRochelobe. findmetal-poorstarstomemorecompactandhotter.Afterstrip-
Thiseffectispartiallycompensatedbythereductionofthemass pingwefindmetal-richstarstobemorecompactandhotter.
ratio,q = M /M ,whichreducestherelativesizeoftheRoche The difference in mass, luminosity and composition affects
2 1
lobe of the donor. The net result is that the metal-rich models theremaininglifetime.PanelbinFig.8showsthattheremain-
are slightly larger (2 %) at the time they fill their Roche lobe. inglifetimeincreaseswithmetallicitybyabout20%(seepanel
However, we do not expect this to play a role of significance. b). This is due to the lower mass of the stripped stars at higher
Whenconductingtestexperimentsvaryingthebinaryseparation metallicity.Thefactthatourmetal-poorstarsstillprovidepartof
andmassratiowefindvariationsinoutcomethatarenegligible. their luminosity through H burning in a shell is not prolonging
The process of mass stripping by the companion (marked theirlife.
withablackoutlineinFig.7)proceedsverysimilarinallmod- The further panels in Fig. 8 provide an overview of vari-
els, see Sect. 3.1. Initially, they respond by contracting rapidly ouspropertiesofstrippedstars,whentheyarehalfwaytheirhe-
on a dynamical timescale and they subsequently expand on the lium burning phase (defined as the moment when X = 0.5).
He,c
slower thermal timescale of the outer layers. (Note that the dy- The shaded bands spans the variation in properties during the
namical phase is not modeled explicitly, but enforced by the hot phase of their helium burning lifetime (which we define as
assumption of hydrostatic equilibrium.) The mass transfer rate 0.9> X >0.05).
He,c
peaksatslightlyhighervaluesinthemetal-poormodels.Eventu- In Fig. 9 we compare the evolution as a function of time
ally,whenmostoftheenvelopeisremovedandheliumenriched during the helium burning phase of our low metallicity model
layersareexposedtothesurface,thereisacertainpointwhere (Z = 0.0002) and our metal rich reference model. In the top
the star is no longer able to expand in response to any further rowwe comparetheevolution oftheeffectivetemperature asa
mass removal. At this point the stars detach as the donor starts functionoftime.Plussymbolsmarkduringcoreheliumburning
tocontract. thecentralheliummassfractions X = 0.9,0.5and0.05.For
He,c
For our metal-rich models, we find that a larger amount of our metal-rich model, we see the first quick initial rise of the
massremovalisneededtoreachthispoint.Ourmetal-poordonor effectivetemperature,whenthestarisstillcontractingwithinits
stars are still about 4.9M(cid:12) when they detach, more than a so- Rochelobe.Afterwards,theeffectivetemperaturesteadilyrises
larmassmorethanourmetal-richmodelswhichareonlyabout byabout0.1dexoverthecourseoftheheliumburningphase.At
3.8M(cid:12) at this moment. For our metal-rich models the helium theendoftheheliumburningphasethetemperaturerisesbriefly,
surfacemassfractionatthemomentthetwostarsdetachisabout untilheliumshellburningignitesandthestarexpands,visiblein
0.75,comparedtoabout0.55inourmetal-poormodels. the diagram as a drop in the effective temperature. In contrast,
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