VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries

0.36 MB·English

by  K. Werner

Astronomy&Astrophysicsmanuscriptno.aa (cid:13)c ESO2008
February5,2008
VLT spectroscopy and non-LTE modeling of the C/O-dominated
⋆
accretion disks in two ultracompact X-ray binaries
6
0 K.Werner1,T.Nagel1,T.Rauch1,N.J.Hammer2,1,andS.Dreizler3
0
2
n 1 Institutfu¨rAstronomieundAstrophysik,Universita¨tTu¨bingen,Sand1,72076Tu¨bingen,Germany
a 2 Max-Planck-Institutfu¨rAstrophysik,Karl-Schwarzschild-Straße1,85741Garching,Germany
J 3 Institutfu¨rAstrophysik,Universita¨tGo¨ttingen,Friedrich-Hund-Platz1,37077Go¨ttingen,Germany

4
2 Receivedxxx/Acceptedxxx

1
ABSTRACT
v
6
Aims.Wepresentnewmedium-resolutionhigh-S/Nopticalspectraoftheultracompactlow-massX-raybinaries4U0614+091and4U1626-67,
4
takenwiththeESOVeryLargeTelescope.TheyarepureemissionlinespectraandthelinesareidentifiedasduetoC-andO-.
5
Methods. Line identification is corroborated by first results from modeling the disk spectra with detailed non-LTE radiation transfer
1
0 calculations.Hydrogenandheliumlinesarelackingintheobservedspectra.
6 Results.OurmodelsconfirmthedeficiencyofHandHeinthedisks.ThelackofneonlinessuggestsanNeabundanceoflessthanabout10
0 percent(bymass),however,thisresultisuncertainduetopossibleshortcomingsinthemodelatom.Thesefindingssuggestthatthedonorstars
/ areerodedcoresofC/Owhitedwarfswithnoexcessiveneonoverabundance.ThiswouldcontradictearlierclaimsofNeenrichmentconcluded
h
fromX-rayobservationsofcircumbinarymaterial,whichwasexplainedbycrystallizationandfractionationofthewhitedwarfcore.
p
-
o
r Keywords. Accretion,accretiondisks–Binaries:close–X-rays:binaries–Stars:individual:4U0614+091,4U1626-67
t
s
a
:
v 1. Introduction tionforstudyingthesesystemsisthatthestrippeddonorstars
i
X offerthepossibilitytoprobetheinteriorcompositionofwhite
Low-mass X-ray binaries (LMXBs) consist of a neutron star
dwarfs,whichdependsontheinterplayofgravitationalsettling
r
a or black-hole accretor and a low-mass donor star (M<∼1M⊙). andcrystallizationofchemicalelements.
Of particular interest are those systems with orbital periods
Three of these five Ne-rich systems belong to the above-
P <80min, which is the minimum period for LMXBs with
orb∼ mentioned eight UCBs with measured orbital periods, while
hydrogen-rich main sequence donors. In these ultracompact
two of themare believedto beUCBs becauseof theirsimilar
binaries (UCBs) the mass donor must be a non-degenerate
opticalandX-rayproperties.TheclassofNe-richUCBscon-
hydrogen-deficientstar or a white dwarf (e.g. Verbunt & van
sistsof:
den Heuvel 1995). Currently eight such systems with mea-
sured orbital periods (Porb=11-50min) are known (Ritter & (1)4U1626-67 (Porb=41min)
Kolb2003). (2)4U0614+091
In the recent past, the existence of a group of five ultra- (3)2S0918-549
compactsystemswithneon-richwhitedwarfdonorshasbeen (4)4U1543-624 (P =18min,Wang&Chakrabarty2004)
orb
claimedbasedonX-rayspectralproperties(Schulzetal.2001, (5)4U1850-087 (P =20min)
orb
Juett etal. 2001, Juett & Chakrabarty 2003). The donors are
thenC/OorevenO/Ne/Mgwhitedwarfsthathavetransferred And4U1626-67mayberegardedastheprototypeofthisclass.
a significantfractionoftheir mass to the neutronstar, in con- Thedonor’sNe-richC/O-WDnatureisderivedfromX-rayand
trasttotheusualwisdomthatthedonorsaretheremainsofHe ultraviolet spectra that exhibit double-peaked emission lines
whitedwarfs.Thishascausednewexplorationsoftheforma- thatobviouslystemfromtheaccretiondisk(Schulzetal.2001,
tionofthesesystems(e.g.Yungelsonetal.2002).Ourmotiva- Homeretal.2002).
The close relation of the other four objects (2)–(5) to
4U1626-67wasbasedonextraordinaryhighNe/Oabundance
Sendoffprintrequeststo:K.Werner
⋆ BasedonobservationsmadewithESOTelescopesattheParanal ratios (when compared to the ISM value) measured from
ObservatoryunderprogrammeID72.D-0013(A). ASCA spectra of (3)–(5)and a Chandraspectrumof (2). The
Correspondenceto:[email protected] spectraexhibitphotoelectricabsorptionedgesofneutralOand
2 K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67
OIII
OIII
CIII CIII
CII
1.5
i.s.
CIII
x
u i.s. DIB
e fl DIB 4U0614+091
v
ati
el
r
OII OII OII
1.0
CII
4U1626-67
OII OII
OII
OII OII
3600 3800 4000 4200 4400 4600 4800 5000 5200 5400
o
wavelength / A
Fig.1. Blue spectra of 4U0614+091and 4U1626-67.H and He lines are lacking. The emission lines are identified as O-
andC-(seealsoTable2).Allabsorptionfeaturesareofinterstellar(i.s.)origin:Ca3934/3968Å,the4300ÅCHband,and
twodiffuseinterstellarbands(DIB).Plottedoverthetwoobservedspectraisasyntheticaccretiondiskspectrumreddenedwith
E(B-V)=0.55andE(B-V)=0.40,respectively.
Ne in the interstellar medium (ISM) along the line-of-sight, In conclusion, the only way to confirm that the four sys-
which is suspected to originate from expelled material close tems(2)–(5)–besidestheprototype(1)–indeedcontainC/O-
to the binary systems. New X-ray spectroscopic observations rich donors,perhapsenriched with Ne, is by UV and/oropti-
of(3)and(4)obtainedwithChandraandXMM-Newtoncon- calspectroscopy.Nelemansetal. (2004)show thatthe optical
firmed the ASCA results, although different values for the spectra of (2)–(4) are devoid of hydrogen and helium emis-
Ne/O enrichment have been obtained from a Chandra and a sion lines, which are usually seen in H- or He-rich accreting
XMM-Newton spectrum of (4) (Juett & Chakrabarty 2003). systems. Their spectra exhibit low-ionisation C and O emis-
In contrast to the ASCA measurement of (5), recent XMM- sion lines, most prominent in the brightest of these objects,
NewtonandChandraobservationsfoundnoevidenceofanun- 4U0614+091.Inthispaperwe presentnewopticalspectraof
usualNe/Oratio(Sidolietal.2005,Juett&Chakrabarty2005). thissystem thatcovera largerwavelengthinterval.Theycon-
These apparently contradictory results can be attributed to a firm the earlier conclusion by Nelemans etal. (2004) that the
variableNe/Oratioduetochangesintheionisationstructure emission lines arise from the C/O-dominated material that is
inthemeasuredabsorptioncolumnsthat,however,arenotun- probably located in the accretion disk and not in the X-ray
derstood (Juett & Chakrabarty 2005). Hence, this means that heated face of the white-dwarf donor. We also present a first
the measured Ne/O ratio does not reflect the donor composi- detailed opticalspectrumof the prototype4U1626-67,which
tion. Although the orbital periods of these systems are below mainly shows weak oxygenemission lines and provesthe H-
80min(oratleastbelievedtobe sosmall), thisdoesnotnec- and He- deficiency in this system, too. Nelemans & Jonker
essarily meanthat theycontain C/O donors.For example,the (2005) also performed VLT observations of this system (also
X-rayburstpropertiesofthe4U1820-30(P =11min)suggest in spring 2004) with a similar setup. They show a section of
orb
anHe-WDdonorinthatultracompactsystem(e.g.Strohmayer theirspectrumandemphasizethesimilaritywith4U0614+91.
&Brown2002).AnotherexamplearetherelativelyshortX-ray
Inadditionwepresentresultsfromfirstattemptstomodel
bursts observed from (3) that even suggest H-burning of ma-
theobservedspectrawithnon-LTEaccretion-diskmodels.We
terialaccretedonto the neutronstar (NS) (Jonkeretal. 2001),
derive upper limits for the H and He abundances and inves-
whileopticalspectrasuggestaC/OWDdonor(Nelemansetal.
tigate the formation of neon lines. Detailed C and O line-
2004and thiswork).A possible explanationis that the heavy
formationcalculationscanalreadyqualitativelyexplaintheob-
elements(C,O,Ne)undergospallationduringaccretionleav-
served emission lines. The ultimate goal is to derive detailed
ing H and He nuclei for thermonuclear burning on the NS
abundancesand other disk parametersfrom the observedline
(Bildstenetal.1992).Ina recentpaperIn’tZandetal.(2005)
profiles.
concludethattheNe/OratioandtheX-rayburstpropertiesare
Thepaperisorganisedasfollows.Wedescribeourobserva-
allbestexplainedwithanHe-richdonor.
tionsinthefollowingsection.Wethenpresentourlineidenti-
K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67 3
CIII OIII
OIII
CII CIV CII
OII CII CII
1.0
x
DIB CII
x
u
e fl 4U0614+091
v i.s.
ati
el x
r
0.5
OII
4U1626-67
CII
CIII OII
CIII
5600 5800 6000 6200 6400 6600 6800 7000 7200 7400
o
wavelength / A
Fig.2. Red spectra; 4U0614+091 shows emission lines from C- and O-, while the 4U1626-67 spectrum is virtually
featureless.Allabsorptionfeaturesareeithertelluric(markedbyhorizontalbars)orofinterstellarorigin(Na5890/5896Åand
aDIBat5780Å).Thefeaturesat5575Åareartifacts(“x”)duetoanOskyemissionline.Overplottedisthesyntheticaccretion
diskspectrumlikeinFig.1.
ficationsinSect.3.Section4containsadescriptionofourdisk completeUV/opticalwavelengthrange(1150–10000Å).The
modelcalculationsandwesummariseandconcludeinSect.5. FUV spectrum is described in detail by Homer etal. (2002)
and a first comparison with synthetic accretion disk spectra
was presented by Werner etal. (2004). In Fig.3 we compare
2. Observations
the opticalHST spectrumtakenfromthe MAST archivewith
We obtained optical medium-resolution long-slit spectra of ourVLTspectrumonanabsolutefluxscale.Theblueandred
4U1626-67and4U0614+091.TheV magnitudeofbothsys- sections of the VLT spectrum were scaled by a factorof 1.65
temsis18.5(Ritter&Kolb2003).WeusedtheFORS1spectro- and 1.45, respectively, to normalise them to the HST flux at
graphattachedto UT1ofESO’s VeryLargeTelescope(VLT) 5600Å.ItisobviousthattheVLTspectrumisnotonlyweaker
onParanalinChile.Theslitwidthwas1′′.Weusedtwogrisms but also flatter than the HST spectrum.This can be attributed
(600Band 600R, the latter in combinationwith ordersepara- eithertofluxcalibrationproblemsortosourcevariability.
tion filter GG435) and obtained spectra covering the regions
3600–6000Åand5400–7500Åwithameandispersionof1.20
3. Lineidentification
and 1.07 Å pix−1, respectively. Observations were performed
in servicemodebetweenNov.2003and Mar. 2004.Eachtar- Emissionlinesfromhydrogenandheliumarecompletelylack-
get was observed at least twice, see Table 1 for details. For ing in the VLT spectra of both systems. We detect neither
4U1626-67 the exposure time covers almost one orbital pe- BalmerlinesnorlinesfromHe(e.g.4471or5876Å)orHe
riod so that any respective spectral variation is smeared out. (e.g.4686Å), whichare usuallyseen in opticalspectraof in-
The data were processed throughthe standard ESO reduction teractingbinaries.Inthefollowingtwosectionswediscussthe
pipeline.Inadditionweperformedafluxcalibrationusingob- linespectraofbothsystemsindetail.Lineidentificationisper-
servations of the DA white dwarf EG274 with the same in- formed“byeye”withoutanequivalent-widthestimation.
strumental setup. Since this flux standard was observed only
once per wavelengthrange and since the science targetswere
3.1.4U0614+091
mostly observedat differentdates, this providesonly a rough
absolutefluxcalibration.Foreachobjectthespectrawere co- Thespectrumof4U0614+091showsnumerousemissionfea-
added to obtain one final spectrum. The blue and red spectra tures that can be assigned to ionized carbon and oxygen,
aredisplayedinFigs.1and2,respectively. namely C- and O-. The detected emission features are
Archival spectra of 4U1626-67 taken with the Hubble listed in Table2 together with possible line identifications.
Space Telescope (HST) and the STIS instrument cover the Mostfeaturesareblendsoflinesfromatleasttwoions,making
4 K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67
Table 2.Emissionfeaturesobservedinthe spectraof4U1626-67and4U0614+091andsuggestedline identifications.A plus
signinparenthesesdenotesanuncertaindetection.
Feature Observedin Ion Transition Wavelength
(Å) 4U0614+091 4U1626-67 (Å)
3720 + + O 3p3P–3d3Do 3704–3732
O 3s5P–3p5Do 3695–3735
O 3s4P–3p4So 3713–3749
3815 + O 3p2Po–4s2P 3794–3830
O 3p1D–3d1Po 3817
O 3s3Po–3p3D 3755–3811
3920 (+) C 3p2Po–4s2S 3919,3921
4075 + + O 3p4Do–3d4F 4070–4111
C 4f3Fo–5g3G 4068–4070
O 3s3P–3p3Do 4073–4119
4180 + C 4f1Fo–5g1G 4187
C 2p3p3D–2s5f3Fo 4153–4163
4345 + + O 3s4P–3p4Po 4317–4367
O 3s’2D–3p’2Do 4347–4351
4410 + (+) O 3s2P–3p2Do 4415–4452
4650 + + C 3s3S–3p3Po 4647–4651
C 3s3Po–3p3P 4651–4674
O 3s4P–3p4Do 4639–4696
4710 + + O 3p’2Po–3d’2P 4691–4702
O 3p2Do–3d2F 4699–4742
O 3p’2Do–3d’2F 4698–4703
O 3p2Do–3d4D 4710–4753
4745 (+) C 2s2p22P–2s23p2Po 4735–4747
4940 + (+) O 3p2Po–3d2D 4941–4956
O 3p4So–3d4P 4891–4925
5590 + (+) O 3s1Po–3p1P 5592
5650 + C 3s4Po–3p4S 5641–5662
5695 + C 3p1Po–3d1D 5696
5810 + O 2p42P–3p’2Do 5754–5810
C 3s2S–3p2Po 5801,5812
C 2s4d1D–2p3d1Fo 5826
6100 + C 3p2P–3d2Do 6095–6103
O 3p2Po–3s2S 6081,6103
O 4s4P–3s4So 6113,6153
6150 + C 2s4d3D–2p3d3Do 6148–6164
C 4d2D–6f2Fo 6151
O 4s4P–3s4So 6113,6153,6214
6460 + C 4f2Fo–6g2G 6462
6580 + C 3s2S–3p2Po 6578,6583
O 3d2F–4p2Do 6496,6565,6571
6730 + C 4d2D–6p2Po 6723
C 3p4D–3d4Do 6725–6755
C 3s3Po–2p3D 6727–6773
O 3s2P–3p2So 6641,6721
O 3d2P–4p2Po 6667,6678,6718
O 4d3Do–3s3D 6707–6768
6790 + C 3s4Po–3p4D 6780–6812
C 2p3d3Po–2s6s3S 6775–6793
7235 + C 3p2Po–3d2D 7231–7237
K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67 5
HST
3
x
1
o-A

1
-s

2
-m2
c
g
r x
e

6
1
-0
1
ux / VLT x
fl1
4000 4500 5000 5500 6000 6500 7000
o
wavelength / A
Fig.3.CompleteVLTspectrumof4U1626-67comparedtoaspectrumtakenwithHST.TheVLTspectrum,scaledtotheHST
spectrumat5600Å,isflatterpossiblyduetoproblemsinabsolutefluxcalibration.TheHST spectrumhasa poorerresolution
andS/N-ratio,althoughsomeemissionfeaturesatλ< 5000Åcanberecognizedinbothdatasets.Artificialemissionspikesare
markedby“x”.Overplottedisthesyntheticaccretiondiskspectrum,likeinpreviousfigures,butnowreddenedwithE(B-V)=0.20
tofitthecontinuumshapeoftheHSTspectrum.
CII CII OIII
1.0
x CII
e flu 4U0614+091 CII
v
ati
el
r
0.7
OII
CIII
6400 6500 6600 6700 6800 6900 7000 7100 7200 7300
o
wavelength / A
Fig.4.Detailfromthreesinglespectraof4U0614+091.Thickline:SpectrumtakenonNov.03,2003.Twothinlines:Spectra
taken consecutivelyon Dec. 15, 2003(see Table 1). The strength of the two strongestemission features, the C multiplets at
6580Åand7235Å,showanincreaseoveratimeintervalofseveralweeks.
interpretationquitedifficult,butsomeofthemareprobablyun- anditispossiblethattheemissionstrengthofthestrongestC
blended.Thesearethefeaturesat5590Å(O),5650Å(C), lines is variable on a time scale of weeks. In order to assess
5695Å(C),and7235Å(C).Theemissionfeaturesat4710 the significance ofthis variability,we simulatedseveraltimes
and 4940 Å are perhaps exclusively due to O, but they are with different noise the strongest emission features observed
blends of four and two multiplets from this ion, respectively. onNov.3andDec.15,applyingtheS/NratiooftheVLTspec-
Nelemansetal.(2004)presentopticalspectraofthisstartaken tra. We find a 2σ probability that the variability is real. The
withVLT+FORS2withslightlydifferentwavelengthcoverage linewidthscorrespondtoaprojectedrotationalvelocityofthe
(4620–8620Å). We essentially confirm the detection of their order1200kms−1.
emissionfeaturesandlineidentifications,however,ourspectra
extendto shorterwavelengths,downto3600Å, whichallows
ustodetectsomeadditionalemissionlines.Ontheotherhand, 3.2.4U1626-67
wedonotseefouroftheweakemissionfeaturesidentifiedin
theNelemansetal.(2004)spectra(atλλ5140,5190,5280,and The spectrum of 4U1626-67 clearly shows less features than
6070Å).Thiscouldbethe consequenceofline variability.In does that of 4U0614+091. Only five emission lines can be
Fig.4wecompareoursinglespectraof4U0614+091indetail, detected in the blue spectrum (see Table2) and they are seen
in both binaries. The red spectrum of 4U1626-67is virtually
6 K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67
Table1.Logoftheobservations. Table3.Summaryofmodelatomsusedinthediskmodelcal-
culations.ForeachionwelistthenumberofNLTElevelsand
Date,UT Grism T Airmass Seeing thenumberoflinetransitions.Inbracketswegivethenumber
exp
(sec) (arcsec) of lines in the wavelength range covered by our optical spec-
traafterfine-structuresplittingfordetailedline-profilecalcula-
4U0614+091
tions.
2003Nov.04,07:22:08 600B 1735 1.22 0.73
2003Nov.04,07:54:57 600B 1735 1.20 0.78
Element Ion NLTElevels Lines
2003Nov.03,07:40:17 600R 1735 1.21 1.00
2003Dec.15,06:25:42 600R 1735 1.28 0.52
H I 10 45(8)
2003Dec.15,07:00:31 600R 1735 1.37 0.57 II 1 –
4U1626-67 He I 29 61(16)
2004Mar.23,08:22:00 600B 1735 1.37 0.76
II 14 91(14)
2004Mar.23,09:01:42 600B 1735 1.36 0.77 III 1 –
2004Mar.21,08:48:53 600R 1735 1.36 1.58
C I 7 4(0)
2004Mar.22,08:19:40 600R 1735 1.37 0.87
II 38 160(42)
2004Mar.22,09:00:00 600R 1735 1.36 0.71
III 58 329(143)
2004Mar.23,07:41:51 600R 1735 1.40 0.57
IV 9 17(2)
EG274 V 1 –
2004Mar.23,09:48:17 600B 50 1.05 0.67
O I 1 –
2004Mar.23,09:53:25 600R 40 1.05 0.67
II 29 82(56)
III 36 42(47)
IV 11 5(0)
continuous.ThestrongCemissionsseenin4U0614+091at
V 6 4(0)
6580Åand7235Åarelackingin4U1626-67.Thatcouldpoint VI 1 –
to a higher ionisation of carbon in 4U1626-67, although the
Ne I 3 0
C 5810 Å line is lacking, too. The presence of O lines in
II 68 232(49)
thebluespectrumof4U1626-67(e.g.at4345Å)alsocontra- III 4 0(0)
dictsa higherionisation,rather,itappearsthatthedifferences IV 1 –
in the spectra of both binaries could be assigned to a differ-
entC/O ratio,beinghigherin4U0614+091.Theentiresetof
emissionlinesin4U1626-67couldbeduetooxygenalone,but
intheUVspectrumCisclearlypresent(Homeretal.2002).In regionsin whichwe believethe physicalpropertiesare repre-
addition,thelinewidthsappearbroaderthanin4U0614+091, sentativefortheformationoftheobservedopticalspectra.This
however, this is difficult to quantify because of possible line shouldatleastgivearoughideaoftherelativestrengthoflines
blendsandanuncertainidentificationofthecontinuum. fromdifferentionisationstagesand,thus,isprimarilythought
All absorption features seen in our optical spectra are ei- to put our line identifications on firm ground. At present we
therofinterstellaroriginortelluric.TheyaremarkedinFigs.1 neglect irradiation of the disk by the neutron star, because it
and2. would introduce new free parameters. We expect that the ra-
dial ionisation structure of the disk will be shifted to larger
radii when irradiation is taken into account, but we hope that
4. Exploratorydiskmodels
therelativelinestrengthsarenotaffectedtotheextentthatour
We began the construction of accretion disk modelsto calcu- identifications become completely wrong. Although the stud-
latesyntheticspectraandreporthereonthecurrentstateofour ied systemsarestrongX-raysources,we donotsee recombi-
work.WeuseouraccretiondiskcodeAcDc,whichisdescribed nation lines fromhighlyionised species in the opticalspectra
indetailbyNageletal.(2004).Inessence,itassumesaradial as might be expected.It is conceivable that the outer parts of
α-disk structure (Shakura & Sunyaev 1973). Then the disk is thedisksinwhichtheopticalspectrumarisesisshieldedfrom
dividedinto concentricannuli.For each annuluswe solve the X-rayirradiationbyaninflatedinner-diskregion.
radiationtransferequation(assumingplane-parallelgeometry) Since we do not compute the spectrum of the entire disk,
togetherwith the non-LTErate equationsfor the atomic level wecannotexpecttomatchtheoverallobservedcontinuumflux,
populations,plusenergy-andhydrostaticequations,inorderto and we have alreadypointedoutthat there are problemswith
calculateadetailedverticalstructure.Theintegrateddiskspec- theabsolutefluxcalibration.Weuseinterstellarreddeningasa
trumisthenobtainedbyco-addingthespecificintensitiesfrom freeparameterinordertoroughlyfitthemodeltotheobserved
theindividualannuli,accountingforinclinationandKeplerian fluxlevel.Theappliedreddeningisgiveninthefigurecaptions.
rotation. The synthetic spectra presentedhere are based on the fol-
Itisnottheaimofthispapertopresentadetailedfittothe lowingchoiceofdisk-modelparameters.Thecentralobjectisa
emissionline spectraofthetwo binaries.Thisrequiresexten- neutronstarwith10kmradiusandamassof1.4M .Themass
⊙
siveparameterstudiesthatareextremelytimeconsuming.We accretion rate is 2·10−10 M /yr. The disk extendsfrom 1000
⊙
ratherchoosetocalculatesyntheticspectrafromselecteddisk to2000stellarradii.ThecorrespondingKeplerianvelocitiesat
K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67 7
0
70
C III C IV
-2
T / kK50 Temperature -4 C V C II
Carbon
30 C I
-6
0
) -7 O IV O III
3
-m
c Density -2
g -9
ρ / -4 O II
g ( Oxygen
lo-11 O V O I
-6
2 on 0
cti Ne III
0 a
τlog Ross--42 Optical depth nization fr--42 NeoNneN eII IV Ne I
o
-6 g i-6
o
l
6.0 0
) HII
2
-s
m 5.5 -2 Hydrogen
g / c Gravity -4 HI
(
g 5.0
o
l
-6
0
2.5 He II
)
m -2 He I
k
(z / 2.0 Height above mid-plane Helium
g -4
o
l1.5 He III
-6
-5 -4 -3 -2 -1 0 1 -5 -4 -3 -2 -1 0 1
log (m / g cm-2) log (m / g cm-2)
Fig.5.VerticalstructureoftheC/O-diskmodelatadistanceof Fig.6.Verticalionisationstratificationofchemicalelementsin
20000kmfromtheneutronstar.Theemergentdiskfluxatthis the disk models at a distance of 20000 km from the neutron
locationcorrespondstoTeff=28000K.Thephysicalvariables star.Theverticallinedrawnatlogm=−0.9indicatesτRoss =1.
areplottedagainstthecolumnmassmeasuredfromthesurface
towardsthemid-plane.
Fortheopacityandemissivitycalculations,itisessentialto
solve the non-LTE rate equations with detailed model atoms.
OurmainemphasiswasputontheC-andO-ions.The
number of non-LTE levels and radiative line transitions are
theseradiiamountto4300and3000kms−1,andtheeffective
summarisedinTable3.Levelenergies,oscillatorstrengths,and
temperaturesto47000Kand28000K,respectively.Thedisk
bound-free cross-sections for photoionisation are taken from
is divided into five annulisuch that T decreases almost lin-
eff the Opacity Project (Seaton etal. 1994) TOPbase1. Electron
earlywithradiussteps.Theinclinationangleissetto10◦.The
collisional rates for (de-) excitation, ionisation, and recom-
Reynoldsnumberusedtoparametrisethediskviscositywasset
bination are computed with the usual approximate formulae.
to10000,whichcorrespondstoα≈0.5.Thechemicalcompo-
sitionisC=50%andO=50%(bymass). 1 http://vizier.u-strasbg.fr/topbase/topbase.html
8 K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67
He-, C-, O-, andNe. Theyessentiallycontainmost
of the possible C and O line identifications given in Table 2,
1.3 4U0614+091
plusmanyothersthatturnouttobetooweaktobeseeninthe
rotationallybroadenedspectra.
DIB Asan examplewe presentthe verticalstructureof the ac-
cretiondisk modelat a distanceof 2000stellar radiifromthe
ux neutron star. The emergent flux at this location corresponds
ve fl1.0 Hβ HeII to Teff=28000K. Figure 5 shows the run of several quanti-
ati ties above the disk midplane on a column mass scale m. The
el Rosseland optical depth reaches unity at logm = −0.9. Here
r
the gravity amountsto logg=5.8. Mass andelectrondensities
4U1626-67 are 6·10−8 gcm−3 and5·1015 cm−3, respectively.Hencethe
physicalconditionsintheline-formingregionsarecomparable
tothoseintheatmosphereofasubluminousBstar.Theverti-
0.7
caldistributionofionicfractionsofCandOisshowninFig.6.
TherewealsoshowtheH,He,andNefractionsthatweretaken
fromthetestmodelsthatincludethesespeciesinanamountof
4800 5000 5200 5400
wavelength / Ao 10%. The dominant ionisation stages of oxygen at τRoss = 1
are O and O, giving rise to prominent emission lines of
Fig.7.Comparisonofmodelswithzeroand10%hydrogenand theseionsinthespectra.Inthecaseofcarbon,Cdominates,
helium content to the observations. From the lack of Hβ and closely followed by C and C. This explains why we see
He emission lines in the observedspectra, we concludethat lines from three C ions. The dominant neon ions in the line-
theaccretiondisksarestronglyHandHedeficient.Themodel formingregionsare Ne and Ne. Forhelium we have He
spectrawerenormalisedtothelocalcontinuumflux. dominating,followedbyHe,sowewouldexpectstronglines
fromthese ionsif neonandheliumwere abundant.Hydrogen
is ionised by about99.9%,but still, prominentemission lines
CIII
NeII arepredictedifHwereanabundantspecies(seebelow).
1.5 4U0614+091 CII 4.1.CandOlinescomparedtoobservations
DIB The synthetic spectrum is plotted together with the observed
ux spectra in Figs. 1 and 2. Let us first consider 4U0614+091.
e fl Generally, many of the observedfeatures are also seen in the
v
elati 4U1626-67 mcoorrdoebl,oarlattheosuoguhrtlhineeyiddoenntoifitcmataitocnhs.thTehsetrCengltihn.esTohfisthbeasmicoadlleyl
r
1.0 aretooweak(e.g.at6580Åand7235Å),whiletheCemis-
sionsareinsomecasestooweak(e.g.4650Å)orinothercases
too strong (4180Å). The C 5810Å line matcheswell. The
OII OII
OII oxygen lines show a similar behaviour. The O lines of the
OII modelaretooweak(e.g.at4940Å).SomelinesofOmatch
reasonably well (e.g. at 3720 Å), while others do not (e.g. at
4400 4500 4600 4700 5590Å,whichismuchtooweakinthemodel).
o
wavelength / A Themodelcomparisonwith4U1626-67confirmsourideas
from the first inspection of the spectrum. The lines in the
Fig.8.Comparisonofmodelswithzeroand10%neoncontent
blue region can be explained qualitatively by the mere pres-
to the observations. The model including neon exhibits weak
Ne lines in the region 4350 – 4450 Å, which cannot be de- ence of oxygen lines. This, and the missing C 5810 Å line
mightoriginateinarelativelylowC/Oratiowhencomparedto
tectedintheobservedspectra.
4U0614+091.
It is disappointing that our NLTE disk model obviously
gives a poorer fit to the observedline spectrum than the sim-
The neon model atom was taken from Dreizler (1993). For
pleisothermal,constant-density,LTEslabmodelpresentedby
the final spectrum synthesis, fine structure splitting of atomic
Nelemans etal. (2004). However, that slab model can at best
levels must be considered;level populationswere taken from
fit a a limited spectral range, as it is emitted from a partic-
themodelswithappropriatestatisticalweights.Levelenergies
wereobtainedfromtheNIST2 database.Theopticalsynthetic ular emission region with an assumed value for temperature
spectra finally include a total number of 377 lines from H, anddensityandwillneverbeabletosimultaneouslyfittheob-
servedspectrafromtheX-raytotheUVandopticalranges.In
2 http://physics.nist.gov/ goodagreementwiththeobservation,ourmodelexhibitslines
K.Werneretal.:VLTspectroscopyofLMXBs4U0614+091and4U1626-67 9
fromthreeionisationstagesofcarbon,indicatingthattemper- 67.ThiscouldsuggestthatthestrippingprocessoftheWDin
ature and density in the line-forming regions are reproduced 4U0614+091ismoreadvanced.
reasonably well. The fact that the strength of many emission Itishardtoestimatethesystematicerrorofthederivedup-
linesof a particularionis either over-or underestimatedmay perlimitfortheneonabundancesothatthefollowingconclu-
stemfromdrawbacksinthemodelatoms.Onereasoncouldbe sionsareatthemomentratheruncertain.FromthelackofNe
thatthemodelionsarestilltoosmallandignoretheinterlock- lineswefindthatthe Neabundancecannotexceed≈ 10%or,
ing effects of neglected energy levels. A further extension of inotherwords,theNe/Oratioisatmostoftheorder0.2.The
themodelionsishamperedbythelackofatomicdata,mostly muchhigherNe/Oratios(≈ 0.7)derivedfromthe ISMX-ray
oscillatorstrengths.Anotherreasonforthepoorlinefitscould absorptionedgesofneutralNeandO(Schulzetal.2001,Juett
beerrorsintheelectroncollisionalrates.Onlyfewareknown etal. 2001) would produce detectable Ne lines in the disk
fromexperimentsorquantummechanicalcalculations. spectra. This confirms the conclusion of Juett & Chakrabarty
These problemsalso might affectour neon line-formation (2005),thatthe determinedISM abundancesof Ne and O are
calculations.However,theemployedNemodelatomhasbeen affected by ionisation effects and, hence, do not reflect the
designedfortheanalysisofsdO starsandgavesuccessfulfits abundancesofthedonorstars.
toobservedNelines(Dreizler1993).Thereforewethinkthat For an initial solar metallicity the 22Ne abundance in an
thepredictedNeemissionlinesinthemodelaremorereliable Ne-enrichedcrystallizedandfractionatedWDcorecanbeesti-
thanthemajorityoftheCandOlines.AtomicdataforHand matedfromtheoreticalmodelsto≈0.07,butthisvalueisvery
Heareaccuratelyknownandourmodelatomshavebeenem- uncertain(Yungelsonetal.2002).Itcouldbeevenhigherbya
ployedwithsuccesstoanalysemanyclassesofstellarspectra, factorof3(Isernetal.1991).SuchanNe-richcorewouldhave
hence, we regardthe computedline strengthsof H and He as amassof≈ 0.06M⊙ (Yungelsonetal.2002).Themassofthe
muchmorereliablethanthoseofthemetallines.Anotherrea- WDdonorin4U1626-67ismuchsmaller(0.01M⊙;Yungelson
sonforthepoorlinefitsmightbethattheassumedα-diskdoes etal.2002).IfweacceptanupperlimitofNe=0.1fortheob-
notdescribethephysicsoftheemittingregionwell. servedneonabundance,thenwemaydrawthefollowingcon-
clusion.Eitherthe WD core hascrystallizedand fractionated,
then our observation favors a relatively small Ne-enrichment
4.2.Limitsonabundancesofhydrogen,helium,and
asaresultofthecrystallizationprocess.Or,ifoneacceptsthat
neon
fractionationwouldresultinahighNeabundanceof0.2(unob-
served),thentheWDcorein4U1626-67hadnotimetocrys-
The upper abundance limits we derive for H and He are re-
tallize which, depending on various details, lasts several Gyr
gardedasrealisticbecauseoftheaboveconsiderations,butthe
(e.g.Hernanzetal.1994).
limitforneonislesssecure.
Futureworkwillconcentrateonthediskmodelingforthe
In additionto the pureC/O disk modelwe also computed
prototype4U1626-67,forwhichtheobservationaldatabaseis
a modelincluding10%hydrogenanda modelincluding10%
thebestofallsuchsystems.Ultimately,thespectralproperties
heliuminordertoseeifthisallowstheHandHeabundancesto
(flux distribution and emission line strengths) over the com-
beconstrained.InFig.7wedisplaytheresultingemissionlines
plete wavelengthrangecomprisingspectral observationswith
of H and He 5412 Å. From the lack of observed emission
β
Chandra, HST, and VLT, must be explainedby a unique disk
lines,itisevidentthatHandHeareatmostpresentatthe10%
model.
level. This excludesthe possibility that the disks of 4U1626-
67and4U0614+091aredominatedbyhydrogenorheliumand
Acknowledgements. T.R. is supported by the DLR under grant
confirmsthesuspicionthattheyareinfactC/O-dominated. 50OR0201. We thank the referee for constructive criticism that
We also computed a C/O model that includes 10% neon. helpedtoimprovethepaper.
It exhibits weak Ne lines, but no such lines are detected in
theVLTspectra(Fig.8).Weconcludethattheneonabundance
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