The post-common-envelope, binary central star of the planetary nebula Hen 2-11

0.62 MB·

by  David Jones

Astronomy&Astrophysicsmanuscriptno.Hen2-11 (cid:13)c ESO2014
January8,2014
The post-common-envelope, binary central star of the planetary
nebula Hen 2-11
D.Jones1,H.M.J.Boffin1,B.Miszalski2,3,R.Wesson1,R.L.M.Corradi4,5,andA.A.Tyndall1,6
1 EuropeanSouthernObservatory,AlonsodeCo´rdova3107,Casilla19001,Santiago,Chile
e-mail:[email protected], [email protected]
2 SouthAfricanAstronomicalObservatory,P.O.Box9,Observatory,7935CapeTown,SouthAfrica
3 SouthernAfricanLargeTelescopeFoundation,P.O.Box9,Observatory,7935CapeTown,SouthAfrica
4 InstitutodeAstrof´ısicadeCanarias,E-38200LaLaguna,Tenerife,Spain
5 DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E-38206LaLaguna,Tenerife,Spain
4
6 JodrellBankCentreforAstrophysics,SchoolofPhysicsandAstronomy,UniversityofManchester,Manchester,M139PL,UK
1
0 Received4November2013/Accepted2January2014
2

n ABSTRACT
a
J WepresentadetailedphotometricstudyofthecentralstarsystemoftheplanetarynebulaHen2-11,selectedforstudybecauseof
itslow-ionisationfilamentsandbipolarmorphology–traitswhichhavebeenstronglylinkedwithcentralstarbinarity.Photometric
7
monitoringwithNTT-EFOSC2revealsahighlyirradiated,double-eclipsing,post-common-envelope systemwithaperiodof0.609

d.Modellingofthelightcurveindicatesthatthenebularprogenitorisextremelyhot,whilethesecondaryinthesystemisprobably
]
R a K-type main sequence star. The chemical composition of the nebula is analysed, showing Hen 2-11 to be a medium-excitation
non-Type inebula. A simplephotoionisation model isconstructed determining abundance ratiosof C/O andN/O whichwould be
S
consistentwiththecommon-envelopecuttingshorttheAGBevolutionofthenebularprogenitor.
.
h Thedetectionofapost-common-envelopebinarysystemattheheartofHen2-11furtherstrengthensthelinkbetweenbinaryprogeny
p and the formation of axisymmetric planetary nebulae with patterns of low-ionisation filaments, clearly demonstrating their use as
- morphologicalindicatorsofcentralstarbinarity.
o
r Keywords.planetarynebulae:individual:Hen2-11-Stars:binaries:close-Stars:binaries:eclipsing-Stars:circumstellarmatter-
st Stars:AGBandpost-AGBISM:abundances
a
[

1 1. Introduction ery of an eclipsing post-CE central star displaying a strong ir-
v radiationeffectthatisreminiscentofAbell63(Pollacco&Bell
8 Itisnowclearthatcommon-envelope(CE)evolutionrepresents 1993) and Abell 46 (Pollacco&Bell 1994). The discovery of
5 an important channel for the formation of planetary nebulae rareeclipsingcentralstarsisakeysteptowardsmeasuringaccu-
3 (PNe), constituting a significant fraction of the total PN pop- rate, model-independentcentral star masses for a large sample
1 ulation (Miszalskietal. 2009a). However, in-depth analysis of ofPNe(Miszalskietal.2008).
1. thispopulation,aswellastheirinfluenceonthehostnebulae,is The paper is structured as follows. Sect. 2 outlines the ob-
0 stillhamperedbythesmallsampleandlackofdetailedstudyfor servations and data reduction,Sect. 3 describes the light curve
4 individualobjects.Itis,therefore,criticaltothefieldthatfurther analysisandmodelling,andtheanalysisofthestellarandnebu-
1 binarycentralstarsarediscoveredandcharacterised. larspectroscopy.Finally,Sect.4discussestheresults.
:
v RecentworkhasshownthatPNewithpost-CE,binarycen-
i tral stars show a penchantfor low-ionisation filaments and ax-
X
isymmetricalstructures(Miszalskietal.2009b,2011b),assuch 2. Observationsanddatareduction
r PNe displaying these traits should offer the greatest chance of
a 2.1.Photometry
binarydiscovery.Whilespectroscopicmonitoringforradialve-
locity variables has shown promising results (e.g. Boffinetal. The central star of Hen 2-11 was monitored photometrically
2012b), the most efficient methodology remains photometric with EFOSC2 on the 3.6-m ESO-NTT (Buzzonietal. 1984;
monitoring(Miszalskietal.2009a,2011a). Snodgrassetal. 2008) on the nights of 27-29 February, 1-2
Hen 2-11 (PN G259.1+00.9; α = 08h37m08.3s, δ = March2012,and14,15and17January2013.Theobservations
−39◦25′08′′) is a southern PN first detected by Henize (1967). employedthe i#705(Gunn i) filter and E2V CCD with a pixel
ThenebulawasshownbyGo´rnyetal.(1999)todisplayanelon- scaleof0.24′′×0.24′′pixel−1.Atotalof200I-bandobservations
gated morphology with low-ionisation filaments. Based on its weretakenwith60-sexposuretime.Furtherindividualobserva-
similar appearance in this image to two other post-CE PNe, tions of 240-s were acquired with narrowband [Oiii] λ5007Å
NGC 6326 and NGC 6778 (Miszalskietal. 2011b), we se- (#687), Hα+[N ii] (#692) and [Sii] λ6716+6731Å (#700) fil-
lected it as a promising candidate for photometric monitoring ters, two exposuresof 20secondswith a Bessel B filter (#639)
aspartofongoingeffortsbyourteamtodiscoverbinarycentral andfiveof180-sinHβ-continuum(#743).Athree-colourcom-
stars(Corradietal.2011;Miszalskietal.2011a,b;Tyndalletal. positeimageofthenebulaispresentedinFig.1,alongwiththe
2013; Miszalskietal. 2013b). Here, we report on the discov- individualimagestakeninthenarrowbandemissionlinefilters.
1
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
(a) (b)
(c) (d)
Fig.1.NTT-EFOSC2imagesofHen2-11.(a)Colourcompositemadefrom[Sii]λ6716+6731Å(red),Hα+[Nii](green)and[Oiii]
λ5007Å(blue),(b)[Sii]λ6716+6731Åwiththecentralstarmarked,(c)Hα+[Nii]and(d)[Oiii]λ5007Å.Northisup,Easttothe
leftandeachimagemeasures2′×3′.
Alldataweredebiasedandflat-fieldedusingstandardstar- magofthecentralstarmeasuredagainstnon-variablefieldstars.
linkroutines1.Photometrywasextractedusingsextractorwith An absolute scale, with an approximate precision of 0.05 mag
anaperturetailoredtoadiameterof3×theseeingoneachframe (derived from the dispersion in detector zero points calculated
(Bertin&Arnouts1996;Jones2011),andthedifferentialI-band for each field star), was applied to the data via the methodol-
1 http://starlink.jach.hawaii.edu/
2
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
ogydescribedinBoffinetal.(2012a)usingcataloguephotome- 3.2.Lightcurve
tryfromDENIS(Epchteinetal.1999).
A Lomb-Scargleanalysis was performedin orderto determine
the periodicity of the variability displayed by the central star
2.2.Spectroscopy of Hen 2-11using the period packageof the starlink software
suite (Dhillonetal. 2001). Fig. 2 shows the data folded on the
Table 1 outlines the spectroscopic observations of Hen 2-11 ephemerisdeterminedbytheanalysis,
made with the Robert Stobie Spectrograph (RSS; Burghetal.
2003; Kobulnickyetal. 2003) of the Southern African Large min I =2455988.1617(±0.0005)+0.6093(±0.0003)E.
Telescope (SALT; Buckleyetal. 2006; O’Donoghueetal.
The lightcurve shows smooth sinusoidal variation of peak-
2006). The position angle (PA) of the slit was selected to be
71.5◦,toincludethebrighterstartotheSouthwestofthecentral to-peak amplitude ∼1.1 mag, with eclipses at phase 0 and 0.5.
star.BasicreductionswereappliedusingthePySALT2 package Theprimaryeclipse,atphase0,isapproximately1.5magdeep,
indicating that even in the I-band much of the flux originates
(Crawfordetal. 2010). Contemporaneous spectroscopic flat-
from the hot nebular progenitor. As such, the sinusoidal vari-
fields were taken with each PG900 spectrum and were applied
abilitycanbeattributedtotheprimaryirradiatingthenearface
tothescienceexposuresaftereachflatwasdividedbya50pixel
of the secondary,and the changingprojectionof this face with
medianofitselftoreducetheamplitudeoffringingonthedetec-
the binary orbit (often referred to as a reflection or irradiation
tors and to correctbad pixels. Cosmic ray eventswere cleaned
using the lacosmic package (vanDokkum 2001). Wavelength effect).Thestrengthofthisreflectioneffect,aswellasthepro-
nounced secondary eclipse, indicate that the inclination of the
calibration of arc lamp exposures was performed using stan-
dard iraf3 tasks identify, reidentify, fitcoords and transform systemmustalsobecloseto90◦.
Thesystemwasmodelledusingthenightfallcode4.Allpa-
byidentfyingthearclinesineachrowandapplyingageometric
rameters were varied over a wide range of physical solutions,
transformationtothedataframes.Onedimensionalspectrawere
with the final model being selected for having the lowest χ2
extracted for the central star and the nebula, keeping the same
fit. A “third light” component (of 9.5% at mean mag in the I-
apertureintheseparateexposures.Spectrophotometricstandard
band) was incorporatedinto the model. This value is based on
starswereusedtofluxcalibratethespectraintheusualfashion.
anestimationoftheremainingnebularcontaminationfollowing
Due to the moving pupil design of SALT only a relative (not
thebackgroundfittingandsubtractionperformedbysextractor.
absolute)spectrophotometricsolutioncanbeobtained.
Theestimationwascarriedoutbymeasuringtheflux,onaselec-
tionofbackgroundsubtractedimagesproducedbysextractor,
in an annulus surrounding the central star and comparing that
to the flux in an aperture centred on the sky backgroundaway
3. Analysis
fromthe nebula,andthenscalingthisfluxtothatwhichwould
3.1.Morphology contaminateanapertureofradius1.5×themedianseeingofthe
observations. Given the obviously large reflection effect in the
The narrowband images presented in Fig. 1 show Hen 2-11 to system,detailedreflectionwasemployedinthemodelling(with
comprisea brightcentralregionroughly1′ indiameterwithan 5iterations)inordertoproperlytreattheirradiationofthesec-
irregularpattenoflow-ionisationfilamentarystructuresextend- ondary by the primary. A model atmosphere was used for the
ing out to the Northwest and Southeast. As commented on in lowertemperature(secondary)componentwithsolarmetallicity
Sect. 1, the nebula bears a striking resemblance to two other and log g of 4.5 (Kurucz 1993). The final model lightcurve is
PNe previously shown to host binary nuclei, NGC 6326 and shown,alongwiththeresidualstothebinneddata,inFig.2and
NGC 6778 (Miszalskietal. 2011b). The central region of the themodelparametersoutlinedinTab.2.
nebula is bright and relatively uniform in [Oiii] λ5007Å (Fig. In general, the model fits well, with residuals of less than
1(d)), while in the light of [Sii] λ6716+6731Å and Hα+[N 0.05magatmostphases(reasonableconsideringtheerrorsplot-
ii] the central structure appears far more filamentary (low- ted are solely from the photometry and do not account for the
ionisation filaments like these are found in both NGC 6326 variablenebularcontamination).Theworstfitisattainedaround
and NGC 6778; Miszalskietal. 2011b). Away from the cen- primaryeclipse,wherethepercentagecontributionfromtheneb-
tral region, there is little emission in [Oiii] λ5007Å, but the ulaisatamaximumandthedataisoflowestsignal-to-noise,due
[Sii] λ6716+6731Å and Hα+[N ii] images reveal bipolar ex- totheintrinsicfaintnessofthesecondary.Theadditionalscatter
tensions upto 80′′ from the central star to both the Northwest inthedataaroundthisphasecouldalsobeduetostarspots,how-
everthisisimpossibletoascertaingiventhesignal-to-noiseand
and Southeast. A particularly bright filament is found at the
timecoverageofourdata.Inordertofullyassessthevalidityof
tip of the Southeastern lobe, running perpendicular to the ma-
the model, it is important to compare the model parameters to
jor axis of the nebula, while the Northwestern projection has
the observations(aswellas theory,wherepossible)and ensure
a more floccular appearance. These structures may perhaps be
thetwoareconsistent.
high velocity outflows/jets similar to those seen in NGC 6778
The model parameters (namely temperature and radius)
(Guerrero&Miranda2012),featureswhicharehypothesisedto
for the secondary indicate that it has an absolute bolomet-
beformedbymasstransferin a binarysystem (Miszalskietal.
ric mag, M , equal to 6.7 and is roughly of spectral type
2013a;Tocknelletal.2013). bol
K5V (Boyajianetal. 2012). Adopting a bolometric correction
of −0.66 (Kaler 1989), a (V − I) colour of 1.54 (Ducatietal.
0
2 http://pysalt.salt.ac.za 2001) and an extinction, c(Hβ), of 2.41 (determined from our
3 irafisdistributedbytheNationalOpticalAstronomyObservatory, SALT spectroscopy,see Sec.3.3), andusingthe reddeninglaw
which is operated by the Association of Universities for Research in of Howarth (1983), we calculate the distance to be roughly
Astronomy (AURA) under cooperative agreement with the National
ScienceFoundation. 4 http://www.hs.uni-hamburg.de/DE/Ins/Per/Wichmann/Nightfall.html
3
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
Table1.LogoftheSALTRSSobservations.
Date JD φ Grating Exptime λ ∆λ Dispersion
(DD/MM/YY) (s) (′′) (FWHM,Å) (Åpix−1)
22/12/12 2456282.412835 0.93 PG900 2350 3907-7004 6.0 0.98
21/03/13 2456373.421354 0.30 PG900 2350 6300–9290 5.5 0.95
13/04/13 2456396.353935 0.94 PG1300 2500 4082–6178 4.3 0.66
Notes.Theslit-widthforallobservationswas1.50Å.
16
16.5
e
d
nitu 17
g
a
m
d 17.5
n
a
b
ntal I- 18
e
m
u
str 18.5
n
I
-0.2
0
0.2
-1 -0.5 0 0.5 1
Phase
Fig.2. Folded NTT-EFOSC2 I-band photometryof the central star of Hen 2-11 with the nightfall model overlaid in red (upper
panel).Binnedresidualsbetweentheobservedphotometryandmodelareshowninthelowerpanel.Notethatthesizeofthepoints
representsthephotometricuncertaintyandcontainnoestimateoftheuncertaintyduetovariablenebularcontamination(whichcan
besignificant,seetextfordetails).
Table2.Modelledandobservedbinaryparameters
Primary Secondary
T (K) 140000±30000 4500±500
eff
Radius(R ) 0.13±0.02 0.68±0.03
⊙
Logg – 4.5a
Inclination 90◦±0.5◦
M1/M2 0.91−+00..1141
Period(d) 0.6093±0.0003
Peak-to-peakI-magamplitudeofirradiationeffect 1.12±0.07
Min.I-magofsinusoidalregionoflightcurve 16.80±0.05b
EclipseI-mag 18.40±0.05c
Extinction-correctedmin.V-magofsinusoidalregion 13.82±0.08d
Extinction-correctedeclipseV-mag 17.36±0.08e
Notes. (a) A fixed parameter in the modelling (b) Measured value, not accounting for nebular contamination of roughly 10% at this phase
(c) Measuredvalue,notaccountingfornebularcontaminationofroughly25%atthisphase(d) Correctedusingthemeasuredc(Hβ)=2.42±0.04,
andarepresentative(V −I) = −0.4forthenebularprogenitor (Ciardulloetal.1999)(e) Correctedusingthemeasuredc(Hβ)=2.42±0.04,and
0
assumingthatthemodelledtemperatureisrepresentativeofthespectraltypeofthesecondary(seetextfordetails).
4
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
44..55
44
))olol 33..55
LLss
L (L (
g g
oo 33
LL
22..55
22
55..55 55..2255 55 44..7755 44..55 44..2255 44
LLoogg TT ((KK))
eeffff
Fig.3. Helium-burning evolutionary track for M = 2.5M ,
i ⊙
M = 0.67M from Vassiliadis&Wood (1994), with the loca-
f ⊙
tionofthemodelcentralstarofHen2-11marked(corresponding
toanageofapproximately7000years).
Fig.4. Low-resolution SALT spectrum of the central star of
700pc (assuming the I-mag at eclipse is that of the secondary Hen 2-11taken ata phase ofφ = 0.93.The zoomdisplaysthe
and accounting for the nebular contamination at this phase of regionaroundtheHeiiλ5412Åabsorptionfeature.
∼0.3 mag). While this distance is a little on the low side, it is
still within the error margins of the statistical distance calcu-
lated by Stanghellinietal. (2008, of 890 pc) and the distance minethelogarithmicextinctionatHβ,c(Hβ),tobe2.41±0.01.
by trigonometric parallax of ∼780pc (Gutie´rrez-Morenoetal. Thisis almost0.2dexgreaterthanthatfoundby Shaw&Kaler
1999).WerecalculatedthedistanceusingtheStanghellinietal. (1989), howevertheir value is derivedfrom emission-linepho-
(2008) method, our extinction value, a radius of 32′′and log tometrywith appreciablecontaminationfrom[N ii] λ6583Åin
F(Hβ)=−12.14 (Ackeretal. 1992). This yields 817pc, more in theirmeasurementoftheHαflux.Hence,weconcludethatour
linewiththeproper-motionbaseddistanceandofferingabetter measurement(derivedusingentirelynon-blendedspectrallines)
agreementwithourdistance. is a more reliable determination. Furthermore,the galactic lat-
Thespectraltypeofthesecondaryandthemodelledmassra- itude of Hen 2-11 (+0.9degrees) meansthat high extinctionis
tioimplyamassfortheprimaryofroughly0.67M .However, entirely consistent with purely foregroundreddening, although
⊙
determinationsofspectraltypebasedsolelyonmodellingofthe somecomponentmaybeinternaltotheobjectaswell.
lightcurvearenotoriouslyunreliableinthesesystems(seeSect. The nebular spectrum taken with the higher resolution
4). Itisinterestingto notethatin spiteofthesignificantuncer- PG1300gratingwasusedtomeasuretheheliocentricradialve-
tainties, the modelled effective temperature and luminosity lie locityofthenebula,whichwedeterminetobe22.1±2.6kms−1
on the evolutionarytrack for a 0.67 M remnant(formedfrom withtheemsaoprogram(Kurtz&Mink1998).
⊙
a2.5M progenitor)withanageof∼7000years(Fig.3).This
⊙
is a reasonableage fora planetarynebula,and wouldimplyan
3.3.1. Plasmadiagnostics
expansionvelocity,v ,ofroughly30–40kms−1(giventhedis-
exp
tancetoandangularsizeoftheobservednebula),whichisfairly Using the neat code (for full details of the analytic processes
typicalforaPNwithabinarycentralstar(e.g. Jonesetal.2010, andatomicdataemployed,seeWessonetal.2012),wemeasure
thoughgreaterthantheaverageforPNeingeneral).Collectively, an electron density of 570±90 cm−3 from the [Sii] lines, and
thisshowsthatourmodelprovidesareasonablesolutiontothe 1800±1600cm−3fromthe[Ariv]λ4711/4740Ålines.Although
systemicparameters,inspiteofthelackofradialvelocitystudy the uncertainty on the [Ar iv] density is large, the discrepancy
whichwouldberequiredinordertoprovidestrongerconstraints between these two values may be due to the presence of high
(SeeSect.4). density clumps (Zhangetal. 2004); the critical densities of the
[Sii]linesare3.625×103and1.419×103cm−3,whilethoseof
the [Ar iv]lines are1.012×105 and 1.630×104 cm−3, and so
3.3.Stellarandnebularspectra
anymaterialatdensitiesof1×105cm−3wouldemit[Ariv]but
Figure4showsthelow-resolutionPG900spectrumofthecentral not[Sii].
starwithweakabsorptionfromHeiiλ5412Ådetectedontopof The temperatures derived from [O iii] and [N ii] are in ex-
a heavily reddenedcontinuum,confirmingit to be the ionising cellentagreementat11500±250Kand11850±450K,respec-
source of the PN. The lack of emission linesis consistentwith tively. The [S iii] λ9069/6312Å line ratio implies a very high
theirradiatedzoneofthesecondaryfacingawayfromusatthe temperature, but the λ9069Å line lies in a vignetted region of
phaseobserved. thespectrumanditsfluxisprobablyunderestimated.Afactorof
The extracted nebular spectra were combined to produce a 2 increasein its measuredflux wouldbringthe [Siii] tempera-
singlespectrumwith wavelengthcoveragefrom∼4000–9000Å tureintolinewiththeotherdiagnostics.
(showninFig.5).FromtheratiosofHα,HγandHδtoHβ,and Usingthe“crossover”methodofKaler&Jacoby(1989),we
usingthe Galactic extinctionlaw of Howarth(1983), we deter- wereabletoestimatethecentralstartemperature,giventhemea-
5
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
suredHeii/Hβdereddenedfluxratio,tobe∼108000K–inrea-
sonable agreement with the temperature derived by modelling
thelightcurve,140000±30000K5.
3.3.2. Empiricalanalysis
We first carry out an empirical analysis using the neat code
(Wessonetal.2012),whichpropagatestheobservationaluncer-
tainties into the final quantities using a monte carlo technique.
Ionicabundanceswerederivedusingatemperatureof11800K
andadensityof500cm−3forsinglyionisedspecies,and11500
K and1800cm−3 formorehighlyionisedspecies.Ourspectra
donotextendasfarbluewardsastheOiilinesatλ3727/3729Å,
and the lines at λ7320/7330Å lie in a gap in our spectral cov-
erage. Without an O+/O2+ ratio, we are unable to estimate the
necessary ionisation correction factors to derive total elemen-
tal abundancesfor any heavy elements. However,the presence
of He2+ (while still only comprising a relatively small amount
ofthe totalHe 6) impliesthatO2+ willbethe dominantionisa-
tionstageofoxygen.TheabsenceofO+ linesintroducesarela-
tivelysmalluncertaintyintotheO/Habundance,whichwefind
tobe(3.3±0.3)×10−4,asO+/O2+ willbelow..TheN/Habun-
Fig.5. A combinednebular spectrumof Hen 2-11, composited
dance, on the other hand, is essentially unconstrained as only
fromthethreeexposureslistedinTable1.
N+ is observable; the fraction of N+ is very small and is im-
possible to estimate from the currentdata. We find He/H to be
0.104±0.007; this indicates that Hen 2-11 is not a Type i PN Table 4. Abundances used in the photoionisation model of
Hen2-11.
(Peimbert 1978). Table 3 gives the measured and de-reddened
line intensities, as well as the ionic abundances calculated us-
Element Abundance
ingtheneatcode(Wessonetal.2012),derivedfromthenebular
H 1.0
spectrashowninFig.5.
He 0.104
C 0.8×10−4
N 3.0×10−5
3.3.3. Photoionisationmodelling
O 5.5×10−4
Ne 7.5×10−5
Given that we are unable to estimate the ionisation correc-
S 1.1×10−6
tion factors needed to empirically derive heavy element abun-
dances,weconstructedaphotoionisationmodelusingmocassin Ar 1.8×10−6
(Ercolanoetal.2003)totryconstraintheNabundance(andfur-
therconfirmthenon-TypeinatureofHen2-11).
nebularabundancesandthecentralstartemperaturetoachievea
Fromtheimagesofthenebula(Fig.1)itisclearthattheneb-
goodmatchtoourobservedlinefluxes.
ulaisirregularandfilamentary.However,ourlongslitspectrum
does notprovidesufficient observationalconstraints to realisti- Ourbestfittingmodelhasastellarluminosityof5140L⊙and
callymodelthenebularmorphology,nordowehaveanyinfor- atemperatureof115000K,ingoodagreementwiththevalues
mation about the surface abundances of the central star which derivedinSection3.3.1althoughslightlylowerthanthetemper-
would allow us to realistically estimate the ionising spectrum. aturederivedfromthelightcurvemodelling(butstillwithinthe
Our modelis, therefore,highly simplified and is intended only uncertainty).Theemissionlinefluxespredictedbythemodelare
as a first approximation, a more rigorous modelling would re- givenalongsidetheobservedvaluesinTab.3.
quirespatiallyresolvedspectroscopyofthenebula. Themodelreproducesmostlineintensitieswell,inparticular
Wemodelledthenebulaasaspherewitharadiusof1018cm, the strengthsof He i and He ii, and the O iii nebularto auroral
line ratio. The observed [N ii] temperature diagnostic ratio is
estimated from our images and the distance of 700 pc derived
lowerthanthatpredictedbythemodel,indicatingthattheactual
inSection3.2.Ourmodelhadauniformhydrogendensity,with
N+temperatureishigher,andtheN+abundanceevenlowerthan
thecentralstarapproximatedbyablackbody.Wethenvariedthe
predictedbythemodel.Thismayindicatethepresenceofhigh-
temperaturelow-ionisationregionsof the nebula whichare not
5 Unlike the Zanstra method, this technique does not depend on capturedbyoursimplemodel(seeSec.4forfurtherdiscussion).
themagof thecentral star,themeasurement of whichcan beheavily ThetotalabundancesusedinourmodelaregiveninTab.4.
skewedforbinarysystemswhereasignificantproportionoftheoptical
Thesestrengthentheindicationfromtheempiricalanalysisthat
fluxoriginatesfromthesecondary. EvenforHen2-11,wherethepri- Hen 2-11 does not have Type i abundances. In particular, the
maryisintrinsicallymuchbrighterthanthesecondary,thecontribution
nitrogen abundance in the model is very low, with N/O∼0.05;
fromthesecondaryissignificantatmostphasesduetothestrengthof
Galactic Type i PNe have N/O>0.8 (Kingsburgh&Barlow
theirradiationeffect.This,alongwiththenebula’sopticaldepth,may
accountforthediscrepancybetweenourtemperatureandtheHydrogen 1994).Whilethiscannotbeconsideredarigorousdetermination
Zanstratemperatureof89000±11000K(Shaw&Kaler1989). ofthetruetotalabundances,andthereforethetrueN/Oratio,it
6 AgreaterfractionofHeinHe2+wouldimplythatO3+wouldbethe doesprovideastrongindicationthatthetrueN/Ovaluewillbe
dominantionisationstage. muchlowerthanforatypicalTypeiPN.
6
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
Table3.Observed,F(λ),anddereddenedI(λ),nebularemissionlinefluxesrelativetoHβ=100,andthefractionalionicabundance
inthatlinerelativetothatofHydrogen, X(line).Thefinalcolumnshowsthefluxespredictedbyourmocassinphotoionisationmodel.
H
Thelowerpartofthetablelistsaseriesofemissionlinediagnosticratiosandtheirmeasuredandmodelledvalues.
λ Ion F(λ) I(λ) X(line) I (λ)
H model
3967.46+3970.07a [Neiii]+Hi 14.63 ±1.07 47.10 ±3.46 - - Ne=29.99
4340.47 Hi 23.64 ±0.40 47.77 ±0.92 - - 4H=71.05.908
4363.21 [Oiii] 7.09 ±0.40 13.91 ±0.82 4.23×10−4 +8.89×10−5 14.08
4471.50 Hei 3.00 ±0.39 5.09 ±0.66 0.098 ±−10.1.30×1102−4 5.41
4685.68 Heii 14.32 ±4.76 13.25 +2.90 0.011 +2.42×10−3 14.24
−3.72 −3.11×10−3
4711.37 [Ariv] 4.25 ±0.22 5.23 ±0.27 1.07×10−6 +1.87×10−7 5.19
−1.92×10−7
4740.17 [Ariv] 4.37 ±0.86 4.87 +0.83 1.07×10−6 ±2.03×10−7 4.30
4861.33 Hi 100.00 ±0.70 100.01 ±−1.000.00 - - 100.00
4958.91 [Oiii] 541.81 ±4.87 474.40 ±5.30 3.51×10−4 ±4.91×10−5 472.30
5006.84 [Oiii] 1696.82 ±4.82 1391.98 ±9.67 3.46×10−4 ±4.82×10−5 1409.35
5412.73 Heii 3.24 ±0.44 1.54 ±0.21 0.013 ±0.002 1.31
5754.60 [Nii] 5.99 ±0.45 2.04 ±0.15 1.06×10−5 +9.79×10−7 1.78
−1.07×10−6
5875.66 Hei 48.16 ±0.46 14.64 ±0.16 0.094 +7.59×10−3 14.82
−4.87×10−3
6300.30 [Oi] 18.31 ±10.94 1.68 +0.59 1.84×10−6 +6.66×10−7 0.76
−0.91 −1.04×10−6
6312.10 [Siii] 12.47 ±11.64 0.58 +0.27 6.65×10−7 +6.65×10−7 0.32
−0.50 −3.57×10−7
6363.77 [Oi] 7.43 ±10.59 0.07 +0.07 2.39×10−7 +2.39×10−7 0.38
−0.14 −4.73×10−7
6548.10 [Nii] 197.20 ±12.59 33.77 ±2.16 1.16×10−5 ±1.38×10−6 36.44
6562.77 Hi 1680.91 ±11.84 284.62 +0.87 - - 283.83
−0.71
6583.50 [Nii] 549.71 ±11.99 91.53 ±2.13 1.03×10−5 ±9.80×10−7 111.30
6678.16 Hei 25.82 ±10.08 2.60 +0.65 0.067 +0.016 4.14
−0.86 −0.022
6716.44 [Sii] 88.57 ±2.06 13.30 ±0.33 4.29×10−7 ±3.62×10−8 13.16
6730.82 [Sii] 85.22 ±2.07 12.66 ±0.33 4.30×10−7 ±3.63×10−8 12.64
7065.25 Hei 21.84 ±2.10 2.54 ±0.25 0.102 ±0.010 2.96
7135.80 [Ariii] 151.84 ±2.02 16.84 ±0.28 1.21×10−6 ±1.23×10−7 12.59
7281.35 Hei 6.09 ±5.95 0.13 +0.06 0.02 +8.09×10−3 0.84
−0.11 −0.02
7751.43 [Ariii] 52.08 ±2.98 3.91 ±0.23 1.17×10−6 ±1.37×10−7 3.02
9068.60 [Siii] 352.73 ±4.24 13.75 ±0.26 1.42×10−6 ±1.25×10−7 8.04
4740/4711 [Ariv] - - 0.93 +0.20 - - 0.83
6731/6717 [Sii] - - 0.95 ±−00.1.603 - - 0.96
(4959+5007)/4363 [Oiii] - - 134.05 ±7.96 - - 133.60
(6584+6548)/5754 [Nii] - - 61.94 ±4.92 - - 82.92
Notes. (a)Thevaluemeasuredinthespectrumisablendofthetwolineslisted,whilethemodelvaluesarefortheindividualcomponents
4. Discussion Forthemodelledsystem,wewouldexpecttoobserveradial
velocityvariationsofaround40 kms−1(K ≈ 20 kms−1 about
2
ThecentralstarofHen2-11hasbeenshowntobeaphotomet- our measured nebular heliocentric velocity of ≈21 km s−1).
ricvariablewithaperiodof0.61d.Modellingofthelightcurve We,therefore,stronglyencouragefurtherspectroscopicfollow-
indicatesthatthesystemisapost-CEbinarywithahotprimary, up of the system in the form of a time-resolved radial ve-
alsothenebularprogenitor,oftemperatureof110−140kK,and locity study. Given the extremely large reflection effect in the
a K-type main sequence secondary of temperature ≈ 4500 K. system, one would expect the secondary to show very strong
These values, as well as the other modelled stellar parameters Ciii andNiiiemissionlinesatalmostallphases(Corradietal.
(size/luminosity),areroughlyconsistentwithevolutionarymod- 2011;Miszalskietal.2011b),thusmakingHen2-11particularly
elsandthedistancespublishedintheliterature. apt for radial velocity study in spite of its intrinsic faintness.
Unfortunately,our two spectra coveringthese lines were taken
Itisimportanttopointoutthatgiventhelackofradialveloc-
veryclosetoprimaryeclipsesuchthattheirradiatedfaceofthe
ity measurements, we can place only very loose constraintson
secondarywasnotobserved,andthereforetheselineswerenot
themassratioandthereforeonthemassoftheprimary(particu-
detected.However,sucharadialvelocitystudywouldallowthe
larlyknowingthatthespectraltypeofthesecondary,determined
determination of key system parameters such as the mass ra-
from its modelled temperature, is often at odds with the mea-
tio and individual component masses. The component masses
sured mass in these systems; see below for further discussion)
are essential, not only to test the model, but also to compare
However, our model’s best-fitting value of 0.67 M is a fairly
⊙ thebinaryparameterstothoseofotherpost-CEcentralstars.In
typical remnant mass and fits well with a similarly typical PN
general,the secondaries,in the few systems thathave been the
age(evenaccountingforthefairlylargeuncertaintyintherela-
subjectofdetailedstudy,arefoundtodisplayinflatedradiiand
tivelyarbitrarystartingtimeoftheevolutionarytracks).Without
increased spectral temperatures with respect to those expected
furtherobservationswecannotplaceanystrongerconstraintson
fromtheirmasses(Pollacco&Bell1993;deMarcoetal.2008;
the system parameters, other than to say that all aspects of the
Afs¸ar&Ibanogˇlu2008).Assuch,itwouldbeofgreatinterestto
modelareconsistentwithcurrentobservationsandthoseofsim-
add further statistics in this small sample. If this were the case
ilarsystems.
7
D.Jonesetal.:Thepost-CEbinarycentralstarofPNHen2-11
forHen2-11,thesecondarymassmightbeexpectedtobemore similar methodology,knownas the energy balance method,by
like that of a late K- or early M-type main sequence star (re- Preite-Martinez&Pottasch1983).
ducing the primary mass to possibly be more in line with that In addition to detailed study of their central stars, it is
ofAbell46,anothereclipsingpost-CEcentralstardisplayinga important that these objects are subjected to detailed spatio-
largereflectioneffect,assumingasimilarmassratiotothatde- kinematical study, in order to ascertain the three-dimensional
terminedbythelightcurvemodelling;Pollacco&Bell1994). morphology and velocity structure of the nebula, which can
then be related to the evolution and parameters of the central
Littledataexistsonthenebularabundancesofpost-CEPNe,
star system (particularly the inclination of nebular symmetry
unfortunatelyourdatadonotpermitustoperformafullanaly-
axiswithrespectto thatofthebinaryorbitalplane;Jonesetal.
sisandderivetotalelementalabundances.Wehavederivedionic
2011, 2012; Tyndalletal. 2012). To that end, we strongly en-
abundances where possible, and shown that Hen 2-11 is not a
PeimbertTypeinebula.Thiscouldbetakenasanindicationof courage the acquisition of high-resolution, spatially-resolved
spectroscopy (either longslit or integral field) of the nebulae
the common-envelope cutting short the AGB evolution of the
Hen 2-11 and NGC 6326 (NGC 6778 has already been the
nebularprogenitor,butthisisdifficulttoassesswithoutaccurate
subjectofdetailedstudy;Guerrero&Miranda2012).Thisdata
C/OandN/Oratios(DeMarco2009).We,therefore,constructed
wouldalsoindicatewhetherthestructuresintheNorthwestand
aphotoionisationmodelofthenebulatoprovideanestimateof
Southeasternpartsofthenebulaareakintothejets/outflowsseen
theseratiosandtestthevalidityofthisscenario.Unfortunately,
in other binary PNe, and if so whether they were formed con-
asclearlyshownbytheimageryinFig.1,thenebulaisfarfrom
temporaneouslyto the central regionor before – a strong indi-
homogenous and this is borne out in the results of our mod-
cation of a phase of pre-CE mass transfer (Boffinetal. 2012b;
elling. These inhomogeneities are present throughout the neb-
Corradietal.2011;Miszalskietal.2013a).
ula not only in density but also in temperature and chemistry,
meaningthatthederivedestimatesforC/OandN/Ointheneb- Finally, it is worthwhile to note that Hen 2-11 has been
ulaarehighlyuncertain(N/Oparticularlyso,giventhediscrep- observed with the Chandra X-ray Observatory as part of the
Chandra Planetary Nebulae Survey (ChanPlaNS; Kastneretal.
ancybetweenobservedandmodelledtemperature)meaningthat
2012),and,assuch,representsanimportanttestcaseofaclose
thetruevaluesmayindeedbegreater.However,inspiteofthis
binarycentralstarinthelocalsample.
uncertainty,it remains clear that the nebula is both carbonand
nitrogenpoor,consistentwithabinarypartnercuttingshortthe
AGBevolutionofthenebularprogenitor. Acknowledgements. Wethanktheanonymousrefereefortheircommentswhich
helpedtoimprovetheclarityofthemanuscript.
Therecentspateofdiscoveries,ofshortperiodCSPNewith Based on observations made with ESO Telescopes at the La Silla
main sequence secondaries (this work; Miszalskietal. 2011b; Paranal Observatory under programme IDs088.D-0573 and 090.D-0693, and
the Southern African Large Telescope (SALT) under programme 2012-2-
Corradietal.2011),furtherhighlightsthediscrepancybetween
RSA-002. This work was co-funded under the Marie Curie Actions of the
the observed post-CE period distribution and that predicted by
EuropeanCommission(FP7-COFUND).ThisresearchhasmadeuseofNASA’s
population synthesis models (even when accounting for obser- AstrophysicsDataSystemBibliographic Services.Thisresearchhasmadeuse
vationalbiases, deMarcoetal. 2008; Rebassa-Mansergasetal. oftheSIMBADdatabase,operatedatCDS,Strasbourg,France.
2008;Miszalskietal.2009a).Knowledgeofallsystemparame-
ters(onlypossiblethroughradialvelocitystudy)iskeyinfurther
constrainingthedownfallsofthesepopulationsynthesismodels. References
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