Orbital solutions of eight close sdB binaries and constraints on the nature of the unseen companions

0.42 MB·

by  S. Geier

Astronomy&Astrophysicsmanuscriptno.muchfuss˙backup˙new (cid:13)c ESO2014
January3,2014
Orbital solutions of eight close sdB binaries and constraints on the
nature of the unseen companions
S.Geier1,R.H.Østensen2,U.Heber3,T.Kupfer4,P.F.L.Maxted5,B.N.Barlow6,M.Vucˇkovic´7,A.Tillich3,
S.Mu¨ller3,H.Edelmann3,L.Classen3,andA.F.McLeod2,4
1 EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748Garching,Germany
2 InstituteofAstronomy,KULeuven,Celestijnenlaan200D,B-3001Heverlee,Belgium
3 Dr.KarlRemeis-Observatory&ECAP,AstronomicalInstitute,Friedrich-AlexanderUniversityErlangen-Nuremberg,Sternwartstr.
7,D96049Bamberg,Germany
4 4 DepartmentofAstrophysics/IMAPP,RadboudUniversityNijmegen,P.O.Box9010,6500GLNijmegen,TheNetherlands
1 5 AstrophysicsGroup,KeeleUniversity,Staffordshire,ST55BG,UK
0 6 DepartmentofPhysics,HighPointUniversity,833MontlieuAvenue,HighPoint,NC27262,USA
2 7 EuropeanSouthernObservatory,AlonsodeCordova3107,Vitacura,Casilla19001,Santiago,Chile

n
Received Accepted
a
J
ABSTRACT
2
TheprojectMassiveUnseenCompanionstoHotFaintUnderluminousStarsfromSDSS(MUCHFUSS)aimsatfindinghotsubdwarf

] stars(sdBs)withmassivecompactcompanionssuchaswhitedwarfs,neutronstars,orstellar-massblackholes.Inasupplementary
R
programmeweobtainedtime-resolvedspectroscopyofknownhotsubdwarfbinarycandidates.Herewepresentorbitalsolutionsof
S eightclosesdBbinarieswithorbitalperiodsrangingfrom∼0.1dto10d,whichallowustoderivelowerlimitsonthemassesoftheir
. companions.Additionally,adedicatedphotometricfollow-upcampaignwasconductedtoobtainlightcurvesofthereflection-effect
h
binary HS2043+0615. We are able to constrain the most likely nature of the companions in all cases but one, making use of
p
informationderivedfromphotometryandspectroscopy.FoursdBshavewhitedwarfcompanions,whileanotherthreeareorbitedby
-
o low-massmainsequencestarsofspectraltypeM.
r
t
s Keywords.binaries:spectroscopic–stars:subdwarfs
a
[

1
1. Introduction The nature of the close companionsto sdB stars is hard to
v
constrain in general, since most of those binaries are single-
6
SubluminousB stars or hot subdwarfs(sdBs) are core helium- linedwiththehotsubdwarfbeingtheonlystardetectableinthe
4
4 burningstars with thin hydrogenenvelopesand masses around spectrum.MeasuringtheDopplerreflexmotionofthisstarfrom
0 0.5M⊙(Heber1986,seeHeber2009forareview).Alargepro- time-resolvedspectra,theradialvelocity(RV)curvecanbede-
. portion of the sdB stars (40% to 80%) are members of short- rived and a lower limit can be given for the mass of the com-
1
period binaries (Maxted et al. 2001; Napiwotzki et al. 2004a). panionfromthebinarymassfunction.Theselowerlimitsarein
0
Several studies aimed at determining the orbital parameters of generalcompatiblewithmainsequencestarsofspectraltypeM
4
1 short-period subdwarf binaries and have found periods rang- orcompactobjectssuchaswhitedwarfs.
ing from 0.05d to more than 10d with a peak around 0.5 to
v: 1.0d (e.g. Morales-Rueda et al. 2003; Edelmann et al. 2005; SubdwarfbinarieswithmassiveWDcompanionsarecandi-
i dates for supernova type Ia (SN Ia) progenitors because these
X Copperwheat et al. 2010). For these close binary sdBs, com-
systemslose angularmomentumthroughthe emissionof grav-
monenvelope(CE)ejectionistheonlyfeasibleformationchan-
r itational waves and start mass transfer. This mass transfer, ei-
a nel.Atfirst,twomainsequencestarsevolveinabinarysystem.
therfromaccretionofheliumontotheWDduringthesdBphase
The more massive one will then enter the red-giant phase and
(seeWangetal.2013andreferencestherein),orthesubsequent
eventuallyfill its Roche lobe. Triggered by dynamicallyunsta-
merger of the system (Tutukov & Yungelson 1981; Webbink
blemasstransfer,acommonenvelopeisformed.Owingtofric-
1984), may cause the companion to explode as SN Ia. Two of
tionthetwostellarcoresloseorbitalenergy,whichisdeposited
thebestknowncandidatesystemsforSNIaaresdB+WDbina-
within the envelope and leads to a shrinking of the binary or-
ries (Maxtedetal. 2000; Geier etal. 2007; Venneset al. 2012;
bit.Eventually,thecommonenvelopeisejectedandaclosebi-
Geieretal.2013).Morecandidates,someofwhichmighteven
nary system is formed, which contains a core helium-burning
have more massive compact companions (i.e. neutron stars or
sdB and a main sequencecompanion.A close sdB binarywith
blackholes),havebeenfoundaswell(Geieretal.2008,2010a,
white dwarf (WD) companion is formed after two consecutive
2010b).Suchsystemsarealsopredictedbybinaryevolutionthe-
phasesofmass-transfer(Hanetal.2002,2003).
ory(Podsiadlowskiet al. 2002;Pfahletal. 2003; Yungelson&
Tutukov2005;Nelemans2010).
Sendoffprintrequeststo:S.Geier, The project Massive Unseen Companions to Hot Faint
e-mail:[email protected] UnderluminousStarsfromSDSS(MUCHFUSS)aimsatfinding
1
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
sdBswith suchmassivecompactcompanions.We selectedand 3300−6600Å)mounted at the ESO-VLT in the course of the
classifiedhotsubdwarfstarsfromtheSloanDigitalSkySurvey ESO Supernova Ia Progenitor Survey (SPY, Napiwotzki et al.
(SDSS,DataRelease 7,Abazajianetal.2009)bycolourselec- 2003). Data taken for studies of sdB binaries at high resolu-
tionandvisualinspectionoftheirspectra.Radialvelocityvari- tion (Edelmann et al. 2005; Classen et al. 2011) both with the
ablesubdwarfswithhighshiftswere selectedascandidatesfor FEROSspectrograph(R≃48000,λ=3800−9200Å)mounted
time-resolvedspectroscopytoderivetheirorbitalparametersand at the ESO/MPG-2.2mtelescope andwith the Cross-Dispersed
follow-upphotometryto search forfeaturessuch aseclipses in EchelleSpectrograph(R≃60000,λ=3700−10000Å)mounted
theirlightcurves. attheMcDonaldobservatory2.7mtelescopewereusedaswell.
Target selection and follow-up strategy were presented in ReductionwasmadeeitherwiththeMIDAS,IRAForPAMELAand
Geier et al. (2011a, 2012). In a spin-off project the kinematics MOLLY1packages.
of fast-moving sdBs in the halo have been studied (Tillich et
al. 2011). The spectroscopic and photometric follow-up cam-
3.2.PhotometryofHS2043+0615
paigns of the binary candidates are described in Geier et al.
(2011b), Kupfer et al. (2013), and Schaffenroth et al. (2013b). HS2043+0615 was extensively observed with the MEROPE
Wediscoveredthreeeclipsingbinarysystems,twoofthemwith camera at the Mercator telescope during the 2007 observing
brown dwarf companions (Geier at al. 2011c; Schaffenroth et season. In total we used R -band photometry from 16 differ-
C
al. 2013a), and one hybrid sdB pulsator with reflection effect ent nights, the first from April 22 and the last from November
(Østensen et al. 2013). Here we report on our supplementary 15.Theexposuretimefortheseobservationswas300s,andwe
programmethatinvestigatesknownhotsubdwarfbinaries. collected in total 516 useful observations. Most of these runs
spannedonlya fractionofan orbit,andthephotometricreduc-
tionwascomplicatedbecausedifferentsetupsandwindowswere
2. MUCHFUSSsupplementaryprogramme
used during the different runs, forcing us to use different ref-
erence stars for different runs. No standardswere observed for
In addition to the priority objects, the MUCHFUSS project
theserunseither,sotocalibratethephotometrywemadeacata-
targeted known sdB binaries of special importance, whenever
logueof22starsinthefield,startingbyassigningR-bandmagni-
scheduling constraints or weather conditions were unsuitable
tudesfromtheNOMADsurveytoeachofthestars.Thesewere
to execute the main programme.In particular, objects were in-
theniterativelycorrecteduntilconsistentvalueswereachieved.
cluded for which light variations either caused by eclipses, by
thereflectioneffect,orbystellaroscillationshavebeenreported Thecorrectedmagnitudeswerethenusedtocalibratethediffer-
entialphotometryofHS2043+0615toacommonscale.
in the literature, because this complementary informationis of
Recently, HS2043+0615 was reobserved with the three-
greatvalueforunderstandingtheirnatureandevolutionaryhis-
channelMAIAcameraontheMercatortelescope(Raskinetal.
tory.
2013). MAIA splits the incoming light into three beams with
ProvidingsufficientRV informationis thereforerewarding.
dichroics to produce simultaneous photometry in a red, green,
We also keep a list of targets, that have insufficient RV cov-
and UV channel, using three cameras equipped with large-
erage, mostly from the SPY survey (Lisker et al. 2005) and
format frame-transfer CCDs. We used an observing mode in
Copperwheat et al. (2010). The highlight of our supplemen-
whichtheRandGchannelswerereadoutevery120sandtheU
tary programme so far was the discovery of the ultracompact
sdB+WD binary CD−30◦11223. It is not only the shortest- channelonlyeverysecondcycletoincreasethe signal-to-noise
period(P≃0.049d)hotsubdwarfbinaryknown,butalsoanex- ratio.Arunof7hduration,almostacompleteorbitalcycle,was
obtainedonthenightofSeptember3,2013.
cellentprogenitorcandidateforanunderluminousSNIa(Geier
etal.2013).
Herewepresentorbitalsolutionsofeightclosehotsubdwarf
4. Orbitalandatmosphericparameters
binaries, which allow us to derive lower limits on the masses
of their companions.Furthermore,we are able to constrain the Theradialvelocitiesweremeasuredbyfittingasetofmathemat-
mostlikelynatureofthecompanionsinallcasesbutone,mak- icalfunctionstoallsuitablehydrogenBalmeraswellashelium
inguse ofadditionalinformationderivedfromphotometryand lines simultaneously, using χ2-minimization and the RV shift
spectroscopy. with respect to the rest wavelengths was measured (FITSB2,
Napiwotzkietal.2004b).Gaussianswereusedtomatchtheline
cores,Lorentzianstomatch theline wings, andpolynomialsto
3. Observationsanddatareduction matchthecontinua.TheRVsandformal1σ-errorsaregivenin
theappendix.
3.1.Spectroscopicobservations
For four binaries of our sample (HE1415−0309,
Follow-up medium resolution spectra were taken during de- HS2359+1942, LB1516, and BPSCS22879−149) the
dicatedMUCHFUSSfollow-upruns(Geieretal.2011a,2011b; orbital parameters T0, period P, system velocity γ, and RV-
Kupfer et al. 2013) with the EFOSC2 spectrograph (R ≃ semiamplitude K as well as their uncertainties and associated
2200,λ = 4450−5110Å)mounted at the ESO-NTT, the ISIS false-alarm probabilities (pfalse[1%], pfalse[10%]) were deter-
spectrograph (R ≃ 4000,λ = 3440− 5270Å) mounted at the mined as described in Geier et al. (2011b). To estimate the
significance of the orbital solutions and the contributions of
WHT,theTWINspectrographmountedattheCAHA-3.5mtele-
systematic effects to the error budget, we normalised the χ2
scope (R ≃ 4000,λ = 3460 − 5630Å), and the Goodman
of the most probable solution by adding systematic errors
spectrograph mounted at the SOAR telescope (R ≃ 2500,λ =
e in quadrature until the reduced χ2 reached ≃ 1.0. The
3500−6160Å). norm
phasedRV curvesforthebestsolutionsaregiveninFig.1,the
In addition to this we used spectra taken with the EMMI
instrument (R ≃ 3400,λ = 3900 − 4400Å) mounted at the 1 http://www2.warwick.ac.uk/fac/sci/physics/research/astro/people
ESO-NTT and the UVES spectrograph (R ≃ 20000,λ = /marsh/software
2
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Fig.1.Radialvelocityplottedagainstorbitalphase.TheRVdatawerephasefoldedwiththemostlikelyorbitalperiods.Theresid-
uals are plotted below. The RVs were measured from spectra obtained with CAHA-3.5m/TWIN (upward triangles), WHT/ISIS
(diamonds),ESO-NTT/EMMI(downwardtriangles),ESO-VLT/UVES(trianglesturnedtothe left),ESO-MPG2.2m/FEROS(tri-
anglesturnedtotheright),ESO-NTT/EFOSC2(circles)andSOAR/Goodman(hexagons).RVsofLB1516takenfromCopperwheat
etal.(2010)aremarkedwithrectangles.TheRVdataofBPSCS22879−149wasfoldedtotheperiodaliasat0.478d.
χ2-valuesplottedagainstorbitalperiodinFig.2.Theminimum ments.Thealiasclosesttotheestimatefromthelightcurvewas
in χ2 corresponds to the most likely solution. The adopted identifiedassolution.Asimilarapproachwaschosenforthere-
systematic errors and false-alarm probabilities are given in flectioneffectbinaryHS2043+0615,forwhich themostlikely
Table 1. The probabilities that the adopted orbital periods are periodofthe lightcurvewas comparedwith thecorresponding
correct to within 10% range from 90% to more than 99.99%. aliasperiodsoftheradialvelocitycurve.Thephasedradialve-
ForBPSCS22879−149,nouniquesolutionwasfound.Thetwo locitycurvesofthosebinariesareshowninFig.3.
possiblesolutionsarediscussedinSect.5.4.
For OGLEBUL−SC16335 and V1405Ori the orbital pe- The atmosphericparameterseffectivetemperatureT , sur-
eff
riod was independentlydeterminedfrom the variationsseen in facegravityloggandheliumabundancelogyofPG0941+280,
their light curves. These periods were kept fixed, but the other V1405OriandOGLEBUL−SC16335were determinedasde-
orbitalparametersweredeterminedinthewaydescribedabove. scribed in Geier et al. (2011a) by fitting model atmospheres
The likely period of the eclipsing sdB+WD PG0941+280was with localthermodynamicequilibriumandsupersolarmetallic-
estimated from a light curveplotted in Green et al. (2004) and ity(O’Toole&Heber2006)tothehydrogenandheliumlinesof
comparedwiththeperiodaliasesderivedfromtheRVmeasure- acoaddedspectrum.ForPG0941+280andV1405Oriweused
3
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Fig.2.χ2plottedagainstorbitalperiod.Thelowestpeakcorrespondstothemostlikelysolution.
Table1.Derivedorbitalsolutions.
Object T P γ K e logp [10%] logp [1%]
0 norm false false
[HJD−2450000] [d] [kms−1] [kms−1] [kms−1]
HE1415−0309 5240.909±0.002 0.192±0.004 104.7±9.5 152.4±11.2 18.6 −1.0 −0.4
HS2359+1942 6279.221±0.007 0.93261±0.00005 −96.1±6.0 107.4±6.8 8.3 −1.2 −1.1
LB1516 5495.73±0.05 10.3598±0.0005 14.3±1.1 48.6±1.4 4.4 <−4.0 <−4.0
BPSCS22879−149 5413.102 0.478 21.9±2.5 63.5±2.8 5.4 ... ...
5412.448 0.964 −25.5±5.3 121.7±6.3 6.2 ... ...
OGLEBUL−SC16335 4758.614 0.122 36.4±19.6 92.5±26.2 25.1 ... ...
HS2043+0615 4254.610±0.003 0.3015±0.0003 −43.5±3.4 73.7±4.3 8.3 ... ...
PG0941+280 4476.185 0.311 73.0±4.9 141.7±6.3 19.4 ... ...
V1405Ori 4477.362 0.398 −33.6±5.5 85.1±8.6 15.2 ... ...
Notes.Thesystematicerroradoptedtonormalisethereducedχ2 (e )isgivenforeachcase.Theprobabilitiesfortheorbitalperiodtodeviate
norm
fromourbestsolutionbymorethan10%(p [10%])or1%(p [1%])aregiveninthelastcolumns.Thelastfourlinesshowthebinaries,where
false false
theorbitalperiodhasbeendeterminedfromphotometry.
aTWINspectrum,whereasanEFOSC2spectrumwasusedfor 5. Natureoftheunseencompanions
OGLEBUL−SC16335.
5.1.Methodstoconstrainthenatureofthecompanion
All spectroscopic binaries in our sample are single-lined and
theirbinarymassfunctionscanbedeterminedfrom
M3 sin3i PK3
comp
f = = (1)
m (M +M )2 2πG
comp sdB
4
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Fig.3.Phasedradialvelocitycurvesplottedtwiceforvisualisation.TherectanglesmarkRVsmeasuredfromspectraobtainedwith
McDonald-2.7m/Coude.TheothersymbolsarethesameasinFig1.
The RV semi-amplitude and the orbital period can be de- panion by the hot subdwarf primary. The projected area of the
rived from the RV curve, but the sdB mass M , the compan- companion’sheatedhemispherechangeswhileitorbitsthepri-
sdB
ion mass M and the inclination angle i remain free param- mary.Thedetectionofthisso-calledreflectioneffectindicatesa
comp
eters. Adopting the canonical sdB mass M = 0.47M (see coolcompanionwithasizesimilartothehotsubdwarfprimary,
sdB ⊙
discussion in Fontaine et al. 2012) and i < 90◦, we derive a either a low-mass main sequence star of spectral type M or a
lower limit for the companion masses (see Table2). For mini- substellar objectsuch as a browndwarf. If eclipses are present
mum companionmasses lower than ∼ 0.45M the companion as well, the inclinationangle can be measuredand the mass of
⊙
maybealate-typemainsequencestaroracompactobjectsuch thecompanioncanbeconstrained.SucheclipsingsdBbinaries
as a WD. Main sequence stars in this mass range are outshone withreflectioneffectarealsoknownasHWVir-typebinaries.
by the sdBs and are not visible in optical spectra (Lisker et al.
Thelackof variationsin the lightcurve,on theotherhand,
2005). If on the other hand the minimum companionmass ex-
can be used to excludea coolcompanion,when the orbitalpe-
ceeds0.45M ,spectralfeaturesofamainsequencecompanion
⊙ riod of the binary is sufficiently short. In this case a reflection
becomevisibleintheoptical.Thenon-detectionofsuchfeatures
effect would be easily detectable and a non-detection implies
thereforeallowsustoexcludeamainsequencestar.
that the companion must be a compact object. The detection
Indicative features in the light curves constrain the nature of the very shallow eclipses from a compact WD companion
ofthecompanionsfurtherinsomecases.Asinusoidalvariation also allows us to constrain its mass. Smaller variations indica-
withorbitalperiodoriginatesfromtheirradiationofacoolcom- tiveofamassivecompactcompanion,whicharecausedbythe
5
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Table 2.Derivedmassesandmostprobablenatureofthecom-
panions.
5%
Object f(M) M Companion
2
[M ] [M ]
⊙ ⊙
x
PHGE01944115+−2083009 00..00792 >0.402.3±70.03 WWDD ve flu
HS2359+1942 0.12 >0.47 WD elati
R
LB1516 0.12 >0.48 WD
OGLEBUL−SC16335 0.01 0.16±0.05 MS
HS2043+0615 0.013 0.18−0.34 MS
V1405Ori 0.034 >0.26 MS
BPSCS22879−149 0.013 >0.18 MS/WD
0.17 >0.57 WD
0.40 0.45 0.50 0.55 0.60 0.65
Time [HJD−2456539]
Fig.5. Multiband light curve of HS2043+0615taken with the
MAIAcamera(R,G,U-bandsfromtoptobottom).
ellipsoidaldeformationandthe relativisticDopplerbeamingof
the sdB primary, can only be detected from the ground in the
mostextremecases(e.g.Geieretal.2007,2013).However,us- 1000K,logg=5.58±0.15,logy=−3.0)ofSafferetal.(1994),
inghigh-precisionspace-basedphotometry,thesevariationscan whousedpurehydrogenmodels.
be detected and used to constrainthe binary parameters(Geier Green et al. (2004) detected shallow eclipses of an earth-
etal.2008;Bloemenetal.2011;Teltingetal.2012). sized WD companionin the light curve. The orbital period es-
timatedfromtheseeclipsesisaround0.3d.Weadoptedthepe-
riod alias of our RV analysis closest to this result as the most
5.2.Whitedwarfcompanions
likelyorbitalperiod.Thederivedmassofthecompanionassum-
HE1415−0309hasbeenidentifiedasasingle-linedsdBstarin ingsini=1is0.42±0.03M⊙.
the course of the SPY project (Lisker et al. 2005). A signifi-
cant shift in radial velocity (∼ 130kms−1), indicating a close
5.3.M-dwarfcompanions
binary,hasbeenmeasuredfromtwoUVESspectra(Napiwotzki
priv.comm.).The minimummass of the companionis too low OGLEBUL−SC16335wasidentifiedasanHWVirsystemby
(0.37M⊙) to exclude a main sequence star. However, a light Polubeketal.(2007).Sincethisanalysiswasbasedonphotom-
curve of this star (∼ 1hr) was taken with the Nordic Optical etry alone, the sdB nature of the primary could not be proven
TelescopeonLaPalmatosearchforpulsationsandnovariations unambiguously.We constrained the atmospheric parametersof
havebeenreported(Østensenetal.2010b).Duetotheshortor- OGLEBUL−SC16335byfittingmodelspectra.Duetothelim-
bital period of only 4.6hr, a reflection effect would have been itedwavelengthrangewewereonlyabletouseH andthetwo
β
easily detectable. We therefore conclude that the unseen com- Heilinesat4472Åand4922Å.However,withintheuncertain-
panionofHE1415−0309mustbeacompactobject,mostlikely ties the resulting effective temperatureT = 31500±1800K,
eff
aWD. surfacegravitylogg=5.7±0.2,andheliumabundancelogy =
HS2359+1942 (PG2359+197) was drawn from the SPY −1.8±0.1areperfectlyconsistentwithansdBprimary.
sampleandanalysedbyLiskeretal.(2005).WedetectedanRV Adoptingthe orbitalperiodderivedfromthelightcurveby
shiftofanEMMIspectrumwithrespecttothesurveyspectrum Polubeketal.(2007),wedeterminedtheRVsemiamplitudeand
taken with UVES. The minimum companion mass is 0.47M⊙, themassofthecompanion(0.16±0.05M⊙).Asubstellarcom-
similar to the adoptedmass of the sdB itself. Since no spectral panion can be excluded and the companion is a low-mass M-
featuresofacoolMScompanionhavebeenfound,weconclude dwarf.
thatthecompanionmustbeacompactobject,presumablyaWD. HS2043+0615wasagaindrawnfromthe SPY sample and
LB1516(EC22590−4819)hasbeendiscoveredtobeansdB analysed by Lisker et al. (2005). A shift in radial velocity
binarywithaperiodofafewdaysbyEdelmannetal.(2005),but (∼ 135kms−1) has been measured from two UVES spectra
nounambiguoussolutionswasfound.Koenetal.(2010)identi- (Napiwotzki priv. comm.). As mentioned by Østensen et al.
fiedthesdBtobeag-modepulsator.Subsequently,Copperwheat (2010b),itwasobservedontwoconsecutivenightsinJune2005
et al. (2010) obtained an orbital solution of this system (P = withtheNOT,andfoundtohaveastrongvariabilitywithape-
10.3592, K = 46.8± 1.8kms−1). We combined the RV mea- riodof severalhours,presumablydueto a reflectioneffect. An
surementsfromEdelmannetal.(2005) andCopperwheatetal. extensive photometricfollow-uphas then been conductedwith
(2010)withadditionalRVsmeasuredfromFEROSspectraand theMercatortelescope.
ournewmeasurementsto obtaina moreaccuratesolution.The To determine the ephemeris we phase-folded the seven
ratherlongperiodof10.3958dleadstoaminimumcompanion months of MEROPE photometry on different trial periods and
mass of 0.48M⊙. Since no spectral features of the companion selectedthatwiththelowestvariance.Theresultinglightcurve
aredetectable,thecompanionislikelytobeaWD. afterfoldinginto50phasebinsisshowninFig.4.Thereareno
PG0941+280 (HXLeo) has been identified as an sdB star significantcompetingaliasesintheperiodogram.Asthereareno
bySafferetal.(1994).TheeffectivetemperatureT =29400± sharpeclipsesthatcanbeusedtoaccuratelyphaseobservations
eff
500K,surfacegravitylogg=5.43±0.05andheliumabundance atdifferentepochs,theerrorontheperiodisquitelarge.Weesti-
logy=−3.0±0.1areconsistentwiththeresults(T =29000± matethatwecanphaseourdatatoaprecisionof1/10ofacycle,
eff
6
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
16.15
e 16.20
d
u
t
i
n
g
a
m
d
n
a
b 16.25
-
R
d
e
n
n
i
b
e
s
a
h 16.30
P
16.35
0.0 0.5 1.0 1.5 2.0
Phase
Fig.4.Phase-binnedlightcurveofHS2043+0615.The516MEROPEdatapointsfrom2007werephase-foldedonP=0.30156day
using50bins,andareplottedtwicetobettervisualisethedifferenceinwidthofthepeaksandthroughs.Theerrorbarsindicatethe
rmsforeachphasebin.
Table 3. Photometric amplitudes from three-channelphotome- 1.00
try. MR22
0.80
Band A B
]⊙
R 0.0660(6) 0.0119(6) R 0.60
GU 00..00431067((61)4) 00..00008441((61)4) ], R [⊙2 M2 > M1
M
M [2 0.40 psing
RL Ecli
andsinceourusefulobservationsspan206.8d≈686cycles,the 0.20 R > 2
phaseerrorwouldbe∼0.3/10/686.Wethusstatetheephemeris
asT = 2454213.70±0.03and P = 0.30156±0.00005d,per- 0.00
0 0◦ 30◦ 60◦ 90◦
fectly consistent with the corresponding alias of the RV peri-
Inclination angle
odogram.TheMAIAmultibandlightcurvesareplottedinFig.5.
We fitted the light curves with a pair of phase-locked cosine Fig.6. Mass and radius of the M-dwarf companion to
functionsasinØstensenetal.(2013),eq.1,andtheseareplot- HS2043+0615 as a function of inclination angle, as indicated
tedwithsolidlinesinFig.5.Thesemi-amplitudesfortheorbital bythemassfunctionandthemass-radiusrelationforM-dwarfs.
period,A,andforthefirstharmonic,B,aregiveninTable3. The boxlimited by the three solid lines marksthe possible pa-
From the orbital solution we derive a minimum compan- rameterrange.ThecompanioncannotfillitsRochelobeRL,the
ion mass of 0.17M consistent with an M-dwarf compan- binary is not eclipsing, and the companionmass cannotnot be
⊙
ion. Following the simple modelling approach described in higherthanthemassofthesubdwarf,becauseitwouldthenbe
Østensen et al. (2013) and adopting the atmospheric parame- visible in the optical spectrum (for details see Østensen et al.
ters of HS2043+0615 given in Lisker et al. (2005) as well as 2013).
the theoretical mass-radius relation for M-dwarfs from Baraffe
et al. (1998), we constrain the likely range of orbital inclina-
tions to 30◦ < i < 75◦ and the companion mass range to V1405Oriwasdiscoveredtobeashort-periodsdBpulsator
0.18M < M <0.34M (seeFig.6). (Koenetal.1999)withareflectioneffect(Reedetal.2010).We
⊙ comp ⊙
7
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
6. Discussion
Wederivedorbitalsolutionsofeightclosehotsubdwarfbinaries
and constrained the most likely nature of the unseen compan-
ions in all cases but one, using additional information derived
fromphotometryandspectroscopy.Thesebinariescoverthefull
parameterrangeoftheknownclosebinarysdBpopulation(see
Fig. 7). Theircompaniontypesare also consistentwith the ap-
parentsplitbetweenM-dwarfandsubstellarcompanionsonone
handandWDcompanionsontheotherhand,especiallyatshort
orbitalperiods< 0.3d.Furthermore,oursamplecontainssome
peculiarbinariesthatdeserveamoredetailedanalysisinthefu-
ture.
PG0941+280is one of only five sdB+WD binaries, where
the shallow eclipses of the white dwarf have been detected
in the light curves (the others are KPD0422+5421, Orosz &
Wade1999;PG2345+318,Greenetal.2004;KPD1946+4340,
Bloemenetal.2011;CD−30◦11223,Geieretal.2013).Itsmass
of0.42M issignificantlysmallerthantheaveragemassofsin-
⊙
gle CO-WDs (∼ 0.6M , Liebertet al. 2005) and very close to
⊙
thetentativeuppermasslimitforWD companionsto sdBstars
seeninthesdBbinarysample(Fig.7,seediscussioninKupferet
al.2013).Time-resolvedphotometryisneededtoobtainahigh-
quality light curve of this system, perform a detailed analysis,
Fig.7.RVsemiamplitudesofallknownsdBbinarieswithspec- andderiveobservationalconstraintsonthemass-radiusrelation
troscopicsolutionsplottedagainsttheir orbitalperiods(Kupfer oftheWD.Furthermore,sdB+WDbinariesareimportantlabo-
et al. 2013). Circles mark sdB binaries with compact compan- ratoriesforstudyingrelativisticeffectssuchasDopplerboosting
ions, squares systems with M-dwarf or substellar companions, andmicrolensing(Geieretal.2008;Bloemenetal.2011;Geier
anddiamondsbinarieswherethenatureofthe companioncan- etal.2013).
not be further constrained by photometry. The lines mark the OGLEBUL−SC16335isthefaintestHWVirsystemknown
regionstotherightwheretheminimumcompanionmassesde- (V ≃16.5mag).ItislocatedintheGalacticdiscataGalaticlat-
rivedfromthebinarymassfunction(assuming0.47M forthe itudeofonlyb = −3.5◦.StudyingsdBbinariesindifferentstel-
⊙
sdBs)exceedcertainvalues.ThebinariesfromtheMUCHFUSS lar populations is important for understanding their formation.
supplementary programme including CD−30◦11223 (Geier et Differentmetallicitiesandespeciallydifferentagesareexpected
al. 2013) are marked with filled symbols, binaries taken from to influence the properties of the progenitor binaries. Time-
theliteraturewithopensymbols. resolved spectroscopy and multicolour photometry are needed
toperformafullanalysisofOGLEBUL−SC16335,whichwas
formed in the young disc population, and compare the results
withtheHWVirsystemsfoundinthecourseoftheMUCHFUSS
projectthatoriginatefromolderpopulationslike the thick disc
orthehalo.
phasedour RVs to the orbitalperioddeterminedfromthe light
V1405Ori is one of only four short-period pulsators in a
curve(0.398d)andderivedaminimummassof0.26M forthe
⊙ reflection effect binary (the others are NYVir, Kilkenny et al.
M-dwarfcompanion.TheeffectivetemperatureT = 35100±
eff 1998; HE0230−4323, Kilkenny et al. 2010; 2M1938+4603,
800K,surfacegravitylogg=5.66±0.11,andheliumabundance
Østensenetal.2010a;andFBS0117+396,Østensenetal.2013).
logy=−2.5±0.2arequitetypicalforshort-periodsdBpulsators
Suchbinariesareimportantasobservationalcalibratorsforcur-
ofV361Hyatype(seeØstensen2010andreferencestherein).
rentasteroseismicmodelsofpulsatingsdBs(e.g.vanGrootelet
al.2013). Furthermore,thetidalinfluenceofclosecompanions
might also influence the pulsationalpropertiesof the sdBs and
5.4.Unconstrainedcompaniontype
shouldthereforebetakenintoaccountinthenextgenerationof
thesemodels.
BPSCS22879−149 was identified as an sdB star by Beers et
al.(1992)andchosenasabrightbackuptargetforthesouthern
Acknowledgements. A.T. was supported by the Deutsche
sky.Since no uniqueorbitalsolution couldbe found,the mini-
Forschungsgemeinschaft (DFG) through grants HE1356/45-1. The re-
mummassofthecompanioniseitherconstrainedto0.18M or
⊙ searchleadingtotheseresultshasreceivedfundingfromtheEuropeanResearch
to0.57M⊙.WhileinthelattercaseaWDcompanionwouldbe Council under the European Community’s Seventh Framework Programme
mostlikely,acompactobjectoflowmassispossibleaswellas (FP7/2007–2013)/ERC grant agreement No227224 (prosperity), from the
an M dwarf in the former case. Comparing the two orbital so- ResearchCouncilofKULeuvengrantagreementGOA/2008/04.Wethankthe
refereeDaveKilkennyforhisconstructivereport.
lutionswiththesampleofknownsdBbinaries,thelong-period
BasedonobservationsattheParanalObservatoryoftheEuropeanSouthern
solutionappearstobemorelikely,becausethenumberofknown Observatory for programmes number 165.H-0588(A) and 167.H-0407(A).
sdBbinarieswithsuchorbitalparametersishigherthanthenum- Based on observations at the La Silla Observatory of the European Southern
ber of binaries with the short-period parameters (see Fig. 7). Observatory forprogrammes number073.D-0495(A),079.D-0288(A), 080.D-
However, selection effects also favour the detection of higher 0685(A),082.D-0649(A)and086.D-0714(A).Basedonobservationscollected
at the Centro Astrono´mico Hispano Alema´n (CAHA) at Calar Alto, oper-
RV-shifts.Weconcludethatthenatureofthecompanioncannot
ated jointly by the Max-Planck-Institut fu¨r Astronomie and the Instituto de
befirmlyconstrainedatthispoint. Astrof´ısica de Andaluc´ıa (CSIC). Based on observations with the William
8
S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Herschel Telescope operated by the Isaac Newton Group at the Observatorio Schaffenroth,V.,Geier,S.,Heber,U.,etal.2013a,A&A,submitted
delRoquedelosMuchachosoftheInstitutodeAstrofisicadeCanariasontheis- Schaffenroth,V.,Geier,S.,Ziegerer,E.,etal.2013b,A&A,submitted
landofLaPalma,Spain.BasedonobservationswiththeSouthernAstrophysical Telting,J.H.,Østensen,R.H.,Baran,A.S.,etal.2012,A&A,544,1
Research(SOAR)telescopeoperatedbytheU.S.NationalOpticalAstronomy Tillich,A.,Heber,U.,Geier,S.,etal.2011,A&A,527,137
Observatory (NOAO), the Ministrio da Ciencia e Tecnologia of the Federal Tutukov,A.V.,&Yungelson,L.R.1981,NauchnyeInformatsii,49,3
Republic of Brazil (MCT), the University of North Carolina at Chapel Hill vanGrootel.V.,Charpinet,S.,Brassard,P.,Fontaine,G.,&Green,E.M.2013,
(UNC), and Michigan State University (MSU). Based on observations at the A&A,553,97
McDonaldobservatoryoperatedbytheUniversityofTexasinAustin.Basedon Vennes,S.,Kawka,A.,O’Toole,S.J.,Ne´meth,P,&Burton,D.2012,ApJ,759,
observations madewiththeMercator Telescope, operated ontheislandofLa L25
PalmabytheFlemishCommunity,attheSpanishObservatoriodelRoquedelos Yungelson,L.R.,&Tutukov,A.V.2005,ARep,49,871
MuchachosoftheInstitutodeAstrofisicadeCanarias. Wang,B.,Justham,S.,&Han,Z.2013,A&A,559,94
Webbink,R.F.1984,ApJ,277,355
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S.Geieretal.:OrbitalsolutionsofeightclosesdBbinariesandconstraintsonthenatureoftheunseencompanions
Appendix:Radialvelocities HS2043+0615
BPSCS22879−149 mid−HJD RV[kms−1] Instrument
−2450000
mid−HJD RV[kms−1] Instrument
−2450000 2387.90703 27.2±5.0 UVES
2521.66074 -108.9±5.0
4252.43920 2.6±9.0 EMMI
4253.31834 -49.0±9.4 4586.73051 -19.2±11.0 ISIS
4253.40334 11.1±11.2 4586.73799 -31.0±10.9
4254.25248 -47.9±9.0
4692.49332 39.2±10.0 TWIN
4254.30548 -24.9±9.5
4693.48658 -51.9±7.0
4254.36750 31.8±4.2
4693.51664 -83.2±6.0
4756.50430 99.6±11.0 EFOSC2 4694.47290 -119.5±4.0
5146.52218 -15.2±7.1 4696.57722 -97.3±6.0
5147.51507 12.5±7.3 4696.62917 -78.9±6.0
5147.52352 3.0±11.1
4756.53557 -71.6±21.0 EFOSC2
5147.52970 -1.3±6.3
4757.52335 -121.4±13.0
5147.53589 6.5±8.3
4757.63634 27.6±16.0
5147.54742 33.6±7.0
4758.66733 -116.3±17.0
5147.55499 37.9±11.6
4758.67470 -133.9±15.0
5147.56256 42.8±11.0
5147.57460 40.4±8.1
5148.59060 79.5±8.5
5148.59678 74.4±8.5
5148.60296 82.1±7.7
5148.61428 77.9±8.0
5148.62717 86.3±8.5
5412.82527 59.0±12.7 Goodman
5412.82707 42.0±12.4
5412.82867 70.0±10.7
5412.89817 -14.0±13.9
5412.89977 1.0±12.0
5412.90137 -25.0±13.0
HE1415−0309
mid−HJD RV[kms−1] Instrument
−2450000
1740.63899 258.2±8.0 UVES
1755.48571 203.1±8.0
4253.49757 149.0±7.9 EMMI
4253.63156 -86.9±8.0
4253.70556 214.9±29.6
4254.52251 185.0±10.0
4476.85174 27.0±8.6 TWIN
4479.86504 253.0±15.0
4587.64673 -22.5±7.8 ISIS
5240.78331 252.4±16.3 Goodman
5240.79081 190.1±3.0
5240.79851 174.1±8.7
5240.80821 137.6±9.9
5240.81561 98.3±12.3
5240.82282 65.6±13.6
10