A search for new hot subdwarf stars by means of Virtual Observatory tools II

0.87 MB·

by  Esther Pérez-Fernández

Mon.Not.R.Astron.Soc.000,1–12(2016) Printed22January2016 (MNLATEXstylefilev2.2)
A search for new hot subdwarf stars by means of Virtual
Observatory tools II
6
1 E. Pe´rez-Ferna´ndez 1,2⋆, A. Ulla2, E. Solano3,4, R. Oreiro5 and C. Rodrigo3,4
0 1IESdeBeade,Conseller´ıadeEducacio´neO.U.,Camin˜odeOuteiro10,36312Vigo,Spain
2 2DepartamentodeF´ısicaAplicada,UniversidadedeVigo,CampusLagoas-Marcosende,36310Vigo,Spain
n 3DepartamentodeAstrof´ısica,CentrodeAstrobiolog´ıa(INTA-CSIC),POBox78,E-28691VillanuevadelaCan˜ada(Madrid)
a 4SpanishVirtualObservatory
J 5InstitutodeAstrof´ısicadeAndaluc´ıa(IAA-CSIC),GlorietadelaAstronom´ıa,s/n,18008Granada.

1
2
Accepted2016January21;Received2016January20;inoriginalform2015July6

]
R
S ABSTRACT
. Recentmassiveskysurveysindifferentbandwidthsareprovidingnewopportunitiestomod-
h
p ernastronomy.TheVirtualObservatory(VO)representstheadequateframeworktohandlethe
- hugeamountofinformationavailableandfilteroutdataaccordingtospecificrequirements.
o
In this work, we applied a selection strategy to find new, uncatalogued hot subdwarfs
r
t making use of VO tools. We used large area catalogues (GALEX, SDSS, SuperCosmos,
s 2MASS)toretrievephotometricandastrometricinformationofstellarobjects.Tothese ob-
a
[ jects,weappliedcolourandpropermotionfilters,togetherwithaneffectivetemperaturecut-
off, aimed at separating hot subdwarfs from other blue objects such as white dwarfs, cata-

2 clysmicvariablesormainsequenceOBstars.Asaresult,weobtained437new,uncatalogued
v
hotsubdwarfcandidates.Basedonpreviousresults,weexpectourproceduretohaveanover-
8
allefficiencyofatleast80percent.Visualinspectionofthe68candidateswithSDSSspec-
3
trumshowedthat65canbeclassifiedashotsubdwarfs:5sdOs,25sdOBsand35sdBs.This
3
successrateabove95percentprovestherobustnessandefficiencyofourmethodology.
3
0 The spectral energy distribution of 45 per cent of the subdwarf candidates showed in-
. frared excesses, a signature of their probable binary nature. The stellar companions of the
1
binarysystemssodetectedareexpectedtobelate-typemainsequencestars.Adetaileddeter-
0
minationoftemperaturesandspectralclassificationofthecoolcompanionswillbepresented
6
inaforthcomingwork.
1
:
v Key words: stars:early type – hot subdwarfs – Virtual Observatory tools – astronomical
i databases:miscellaneous
X
r
a
1 INTRODUCTION binaryinteraction(Mengeletal.1976).SeeHeber(2009)forare-
viewonobservationalandtheoreticalaspectsofhotsubdwarfs,or
Hotsubdwarf(hotsd)starsarecore-heliumburningstarsattheend
Geier(2013)formorerecentdiscoveries.
of thehorizontal branch or even beyond that stage. Theoriginof
these faint, blue objects is still a matter of controversy. With ef- Hotsubdwarfsarefoundinthefield,bothinthediskandhalo,
fectivetemperaturesexceeding19000Kandlogg≥5,hotsdsare butalsopopulatingthemostExtremepartoftheHorizontalBranch
objectsthathavelostmostoftheirHenvelopeinpreviousevolu- (EHB)of someGalacticclusters.Basedonthisobservational ev-
tionarystages,leadingtoa∼0.5M⊙star.Theyareunabletofollow idence, they have been proposed as an explanation for the UV-
canonical evolutionthrough theAsymptoticGiantBranch(AGB) upturnphenomenonshowninsomeellipticalgalaxies(Brownetal.
proceeding,instead,directlytowardsthewhitedwarfcoolingtrack. 1997).
Circumstances that lead to the removal of all but a tiny fraction Hotsdsaredividedintwomainclasses, sdBsandsdOs,ac-
of thehydrogen envelope, at about thesametimeasthecorehas cordingtocomposition.SdBspectraaredominatedbytheBalmer
achieved themassrequired for theHeflash(∼ 0.5M⊙ ),arestill series,whilesdOsarehotter objects caracterizedby thepresence
amatterofdebate.Theoreticalevolutionscenariosproposedsofar ofHeii4686Å andthePickeringseries.Additionally,avarietyof
include enhancement of the mass loss efficiency near the red gi- Heilinesmayappearinbothclasses,andsomesdOsshowmetal-
antbranch(RGB)tip(D’Cruzetal.1996)ormasstransferthrough licCor N lines. More complex classificationschemes havebeen
proposedinGreen,Schmidt&Liebert(1986)ormorerecentlyin
Drillingetal.(2013).
⋆ E-mail:[email protected] Thesubdwarfdatabase(Østensen2006)catalogues1600sdBs
2 E. Pe´rez-Ferna´ndez,A.Ulla,E. Solanoetal.
and 500 sdOs spectroscopically confirmed hot subdwarfs. A sig-
nificant number of new hot sdshavebeendiscovered inmorere-
cent studies like Vennesetal. (2011), Geieretal. (2011, 2015),
Ne´methetal.(2012),Kleinmanetal.(2013),Kupferetal.(2015)
or Kepleretal. (2016). Increasing the number of hot sds is im-
portant for a robust statistical confrontation with theoretical evo-
lutionary scenarios. It may also lead to the discovery of interest-
ing objects that are still scarce, such as pulsating sdBs or sdOs
(Kilkenny2002),eclipsingorreflectinghotsdbinaries(Foretal.
2010;Derekasetal.2015),andhotsdsascentralstarsofplanetary
nebulae(Alleretal.2015).Allofthemareparticularlyinteresting
for studying the stellar interiors, the mass transfer mechanism at
work, as well as the evolutionary formation channels, and would
contributetobetterunderstandtheseevolvedobjects.
Hot subdwarfs were first found analyzing faint blue stars,
startingwiththeHumason&Zwicky(1947)surveyorthePalomar-
Green(PG)catalogue(Greenetal.1986).Atpresentwehaveatour
disposaldeeperandmoreextensivesurveys,coveringlargeregions
of the sky and wide spectral ranges. Besides, with online access
toolsliketheVirtualObservatory(VO)1 wecanaccessdatafrom
mostofthesesurveysinaveryefficientway,crossmatchinginfor-
mationtoselectobjectswithparticularcharacteristics.
Inthisregard,theaimoftheworkherepresentedistoobtaina
Figure1. SDSSDR7imagingcoverageinGalacticcoordinates,whichis
numberofnewhotsdcandidatesaslargeaspossible.Weapplythe
the region covered by this study. Source: http://classic.sdss.org/
selectionprocessdevelopedinOreiroetal.(2011)(hereafterpaper
dr7/coverage/
I),thatcombinesphotometricandpropermotioninformationfrom
differentsurveys,makinguseofVOtools,withtheintentionofdis-
criminatinghotsdsfromothertypesofobjectsofsimilarcolours,
GalacticplaneandreachingthesouthernGalactichemisphere.Fig.
mainlywhitedwarfs(WD),cataclysmicvariables(CV)andmain-
1showsthefootprintinGalacticcoordinatesoftheregionsstudied.
sequence O and B stars, considered as contamination sources in
We remind the reader that the principal aim of this work is
thiswork.
to apply a reliable selection procedure to find a large number of
Section2describesthemethodologyemployed,Section3the
newhotsubdwarfcandidates,andnottoperformadeepanalysisof
resultsobtainedtogetherwiththeiranalysis,Section4asummary
eachstar physical parameters. Inthisregard, wecomputed effec-
ofourmainachievementsonspectralclassificationofthenewhot
tivetemperatureestimatesusinganautomatedprocedureprovided
sdcandidatesdiscovered,andSection5endswithageneralsum-
by VOSA (Bayoetal. 2008), a very useful online facility of the
maryandconclusions.
SpanishVirtualObservatory4.
Fortheeaseofthereader,weoutlineheretheselectionproce-
dure.
2 METHODOLOGY
• Hotsubdwarfselectionfilters:Firststepwastocrossmatch
Themethodology described inpaperIisreproduced here.Inthat
the photometric and proper motion surveys and apply to the re-
work,ahotsdselectionprocedurewasdefinedandtestedbymeans
tainedsourcesthecutsaimedatselectinghotsubdwarfs.
of athorough retrieval, with theaid of VO tools, of multi-colour The surveys employed were SDSS DR7, GALEX GR6/GR75
photometryandastrometricinformationfromstellarcatalogues.A (Bianchi&GALEXTeam2000),2MASSPointSourceCatalogue6
filteringproceduretodistinguishamongdifferenttypesofobjects (Skrutskieetal. 2006) and SuperCosmos7 (Hamblyetal. 2001).
wasdesignedtoobtainahotsubdwarfsamplewithalowcontam-
WerequiredtheGALEXsourcestohavemeasuredmagnitudesin
inationfactor.Themethodwastestedontwoskyregions:theKe-
bothfilters(FUV >0,NUV >0)andtobebrighterthan5σofthe
pler FoV2 and a region of 300 deg2 around (α:225, δ:5 deg) ob-
magnitudelimit(FUV < 19.9, NUV < 20.8).Sourcesmustalso
taining a high rate of success (above 80 per cent) infinding new beclassifiedaspointobjectsbySDSS(cl=6).Weretainedsources
uncatalogued hot subdwarfs. Temperatures were provided by fit-
withcounterpartsinallthesurveyswithinamaximumdistanceof
tingtheirspectralenergydistribution(SED),andconsideringtwo-
5arcsec.Totheselectedsourcesweappliedthefollowingcuts,as
atmospherefitsforthoseobjectswithaclearinfraredexcess,asig-
discussedinpaperI:
natureofthepossiblepresenceofacoolcompanion.
Oncetestedthevalidityoftheproposedstrategy,inthiswork −4<(FUV0−Ks0)<0.5 (1)
we apply it to a wider sky region. The extension of this region −2<(FUV −NUV )<0.2 (2)
0 0
is about 11663deg2, limited by the SDSS3 DR7 survey coverage
19<H(NUV )<27 (3)
(Abazajianetal.2009).MostoftheGalacticnortherncapdownto 0
b=+30iscovered,aswellassomestrip-shapedareascrossingthe
4 http://svo.cab.inta-csic.es
1 http://www.ivoa.net/ 5 http://galex.stsci.edu/
2 http://kepler.nasa.gov/science/about/targetFieldOfView/ 6 http://www.ipac.caltech.edu/2mass/
3 http://www.sdss.org/ 7 http://surveys.roe.ac.uk/ssa/
New hotsubdwarfstarsbymeans ofVO toolsII 3
wherethe0subscriptstandsforGalacticextinctioncorrectedmag- inturnhavebeen taken fromthe Schlegel,Finkbeiner&Davis
nitudes,andH(NUV )forthereducedpropermotionoftheNUV (1998)extinctionmaps.
0
filter. – Model comparison: The flux-dereddened observa-
For bright stars, severe calibration problems in the GALEX tional SEDs were then compared to the TLUSTY OS-
photometryhavebeenpointedoutbyCamarota&Holberg(2014) TAR2002+BSTAR2006 NTLE models for O and B stars
who,usingawellstudiedsampleofWDswithUVspectra,derived (Hubeny&Lanz 1995; Lanz&Hubeny 2003, 2007) im-
empiricalcorrectionstotheGALEXmagnitudesinthenon-linear plemented at VOSA to derive effective temperatures. We
range.Thecorrectionsarevalidwithinthe9.321 < NUV < 17.5 consideredthewholemodelgrid,withT rangingfrom15000
eff
or10.509<FUV <17.5ranges.Wehavethusidentifiedthestars to 55000K. In the SEDs fitting procedure both surface gravity
in our sample lying within those limits, and applied to them the and metallicity were simply left as free parameters, as their
correctionfactorsestablishedinthatpaper. impact on the effective temperature determination can be
• Discriminating new from already classified objects: We consideredasnegligible.Therefore,wewarnthereaderthatthe
crossmatched our list with published and well-established cata- gravity and metallicity values obtained from the SED fitting
logues of spectroscopically confirmed subdwarfs, white dwarfs, cannot be considered as the real physical parameters of the
cataclysmicvariablesandOBstars.Theseinclude: objectslistedinTables1-4below.
Heberetal.(2000)haveshownthattheuseofLTEvsNLTE
– Thesubdwarfdatabaseforhotsds(Østensen2006).
model atmospheres yields almost identical T ’s and only sys-
eff
– AselectionofhotsubluminousstarsintheGalexsurvey
tematic logg differences, at least when fitting hot sd spectral
(Vennesetal.2011;Ne´methetal.2012)
lines. Wedo not expect other result inour procedure of fitting
– Thephotometricandspectroscopiccatalogueforluminous
SEDs.
stars(Reed2005).
We have performed, anyway, a comparison between the ef-
– The catalogue of Cataclysmic Variables, version 2006
fectivetemperaturescalculatedusingtheTLUSTYandKurucz
(Downesetal.2001).
(Castellietal.1997)gridsofatmosphericmodels.Onlyobjects
– TheSDSSDR7whitedwarf catalogue (Kleinmanetal.
with a good SED fit flag (5XX) and a TLUSTY temperature
2013).
valuelowerthan35000K(toavoidboundaryproblemswiththe
– ACatalogueofSpectroscopicallyIdentifiedWhiteDwarfs,
maximum temperature of the grids) were considered. We ob-
version2008(McCook&Sion1999) tained adifference ineffective temperatures below 10 per cent
for90percentoftheobjects(or82percentofobjectsforanup
Sources already available in these catalogues were discarded.
The remaining objects were searched in SIMBAD8, VIZIER.9 to5percentdifference),indicatingthat,asexpected,theNLTE
effectsonT determinationcanbeneglected.
and any catalogue available through online VO tools. Very re- eff
WealsoattemptedtoleaveA asafreeparameterintheSED
cent catalogues like Geieretal. (2015), Kupferetal. (2015) and v
fittingprocess.Nevertheless,duetotheA −T degeneracy,this
GentileFusilloetal.(2015) wereconsidered intheViziersearch. v eff
exercise rendered multiple solutions and we finally decided to
Othercataloguescontainingspectroscopicallyconfirmedhotsubd-
includeextinctionasafixedparameter.
warfsbutnotincludedinVizier(Vennesetal.2011;Ne´methetal.
2012; Kawkaetal. 2015;Kepleretal.2016)werealsoinspected. • Sourceimagechecking:Finally,wevisuallyinspectedusing
Anysourcealreadyspectroscopicallyclassifiedinthesecatalogues Aladin13theSDSSimagesandcataloguedataofourpre-candidate
wasdiscarded. listoftargetstodiscardinstrumentalfeatures,badcrossmatchesor
• Spectraldistributionfit:Foreachobjectinthepre-candidate contaminationfromnearby,brightsources.
list,weusedVOSAtoaccomplishthefollowingsteps: In fact, we found some cases with a clear mismatch between
GALEX,SDSSand2MASSsources.Thesepathologicalcasesare
– Gathering of additional photometry: GALEX-SDSS-
mostlydue tothedifferent spectral coverage and limitingmagni-
2MASSphotometrywascomplementedwithadditionalphotom-
tude of the surveys. We kept these objects without infrared pho-
etryfromUKIDSS10LASDR9(Lawrenceetal.2007),Tycho-2
tometry ina separate list,as they appear to be very hot and blue
(Høgetal.2000)andWISE11(Wrightetal.2010).Somecandi-
objects,andthusinterestingfromourpointofview(Table4).
dateshadsaturatedorbadSDSSphotometry.Inthesecaseswe
replaced SDSSby UCAC4 (Zachariasetal. 2013) photometry,
ifavailable.
– Magnitude-to-fluxtransformation:VOSAusedthegathered 3 RESULTS
photometricinformationtocalculatetheabsolutefluxesandtheir
Aftercrossmatchingthephotometricsurveysandapplyingthese-
associatederrorstakingadvantageoftheFilterProfileService12
lectionfiltersinequations(1)-(3),weendedupwithalistof1242
(FPS),aservicedeveloped bytheSpanishVirtualObservatory
pre-candidates. 638 of them were already classified in the litera-
to provide VO access and representation of many of the most
ture,withthefollowingpercentages:83percenthotsubdwarfs,12
commonphotometricsystemsinastrophysics.Fluxeswerethen
percentWD,2percentCV,2percent Bstars,andlessthan0.5
dereddened using the extinction law by Fitzpatrick(1999) and
percentothermainsequencestars.Thesenumbersagreewiththose
the E(B−V)valuesavailableintheGALEXcatalogue, which
obtainedinpaperI,demonstratingtherobustnessofourselection
procedure. Theremaining604 pre-candidates wherenot found in
theliterature.
8 http://simbad.u-strasbg.fr/simbad/ In Fig. 2 the classified and unclassified objects selected by
9 http://vizier.u-strasbg.fr/viz-bin/VizieR the photometric and proper motion cuts are pictured in Galactic
10 http://surveys.roe.ac.uk/wsa/
11 http://wise.ssl.berkeley.edu
12 http://svo2.cab.inta-csic.es/theory/fps3/ 13 http://aladin.u-strasbg.fr/
4 E. Pe´rez-Ferna´ndez,A.Ulla,E. Solanoetal.
90
threedigitsqualityflag(seeTables1-4).Thefirstdigitrangesfrom
onetofive:‘5’representsgoodSEDfitting,‘4’standsforexcess
intheredpartoftheSED(IRexcess),‘3’forbothIRandUVex-
60 cesses,‘2’forUVexcessonly,and‘1’forabadfittingofanyother
sort. Excesses inthe infrared or ultraviolet part of the SED were
definedwhenevertherelativedifferencebetweenthemodelandob-
30 served(dereddened) valueswasabove20percent,andthediffer-
e enceincreasedwithdecreasing/increasingwavelength,forUVand
d
Latitu 0 IvRis,uraelspinescptievcetliyo.nWoeftfhoeunSdEtDhisfitcsr.itDeriiffoenremnactecshewditqhuoiutetawcellelawripthata-
ctic  tern,inthemiddleoranypartoftheSED,wereconsideredbadfits
a oftype‘1’.
al
G SecondandthirddigitsrefertothequalityoftheGALEXdata:
−30
a‘1’inthesecondpositionrepresentsaproblematicGALEXarti-
fact,anda‘1’inthethirdpositionstandsforabadflaginthepho-
CV tometryextraction14.Inbothcases,‘0’standsforagoodGALEX
−60 Sd
WD flag.2MASSqualityflagswerealsoconsidered:Photometricval-
not classified ueswithanU flag(U standingforupperlimitinmagnitude)were
nottakenintoaccounttoperformtheSEDfitting.
−90 ExamplesofthedifferentqualityfitscanbeseeninFig.3.Red
0 60 120 180 240 300 360
points (grey in the grey-scale version of the figure) represent the
Galactic Longitude
derredened magnitudes of thegiven object and theconected blue
points(darkgrey)thesyntheticmagnitudesthatbestfit.Undereach
Figure2.Galacticcoordinatesofthepre-candidatesafterthephotometric
graphwerepresenttheresidualsoftheobserveddataandmodels.
andpropermotioncuts.Thosenotyetclassifiedintheliteraturearemarked
InTables1-4wepresentasampleofthehotsubdwarfcandi-
withblackcircles.
datesfoundbyourselectionmethod.Table1includesgoodfitted
objects with photometric data ranging from the ultraviolet to the
coordinates.Noticethatalargefractionoftheunclassifiedobjects infrared,andthusrepresentsclearsinglecandidates(192objects).
lay in the bands near the Galactic plane, as these tend to be less Table2showssourceswithexcessintheredpartoftheSED,the
studiedregions. mostclearbinarycandidates(110objects).InTable3weincluded
therestofthebadfittedobjects(115).Finally,Table4includeshot
objectswithnoinfraredphotometryavailable(20).
3.1 Effectivetemperatures
T estimatesforcandidateswithflags4XX,3XX,2XXand
eff
Effectivetemperatureswereobtainedfromthecomparisonbetween 1XX must betreated withcaution, which iswarned by means of
theobservationalSEDsandtheTLUSTYmodels.Afterthefitting, oneofthesebadfitflags.Weremindthereaderthat,althoughthe
wekeptcandidateswithTeff > 19000K, providedthatthefitwas estimatedTeff arebelow19000K,theseobjectsarekeptinthecan-
good.SourceswithT <19000KandabadSEDfittingwerealso didatelistbecausethecombinationofbadfitandlowtemperature
eff
kept,asthiscouldbeasignalofabinarycandidate.167outofthe isusedasindicatorforthepresenceofbinarysystems.
604unclassifiedsourcesdidnotpassthecut,leavinguswithalist In all tables, FUV and NUV were taken from the GALEX
of437finalsubdwarfcandidates. archive,andcorrectedasexplainedabove,ifnecessary;u,g,rare
Thebadfitsareofmainlythreedifferentsorts:excessinthe from SDSS Data Release 7 and J, H, K from the 2MASS Point
redpartofthespectrum(IR),ultraviolet(UV)excess,andbothIR Source Catalogue. Weincluded a column with the 2MASS qual-
and UV excesses in the same source. IR excesses are probably a ityflagsof thesource. Teff isobtained fromthebestSEDfitper-
signature of a binary system. UV excess could also indicate the formedbyVOSA.Asexplainedabove, anintervalinthetemper-
presenceof averyhotcompanion, butuncertaintiesassociatedto aturecolumnisgivenwhenevertheBayesanalysisgavethemost
theultravioletextinctioncorrectioncannotbediscarded.E(B−V) likelyTeff valuewithaprobabilitybelow80percent.Thefitflag
values have been taken from Schlegeletal. (1998), who seem to columnshowsournotationforthedifferentqualitiesintheVOSA
overestimatethereddening tolinesofsight where AV ≥ 0.5mag Teff fit.InTables2and3wealsoincludedthefilterwheretheex-
(Arce&Goodman1999). cessbeginsandtheexpectedspectraltypeofthestellarcompanion,
Asafurther check, wealsoperformed inVOSAaBayesian accordingtothecriteriumexplainedinthenextsection.
analysisofthemodelfits.Wefoundthat,for356sources,theprob- Full tables, with all the photometric filters and other data,
abilityassociatedtotheT valueobtainedfromthechi-squarefit- including links to the SED fitting diagrams and the SDSS spec-
eff
tingwasover80percent.Fortherestofsources(81),weprovide trum,whenavailable,canbeaccessedusingtheSVOhotsubdwarf
an effective temperature interval covering an accumulated proba- archive(seeAppendixA).
bilityof,atleast,80percent.Withthisprocedureweobtainfairly Fig. 4 shows a histogram of the effective temperatures ob-
temperatureestimationsforthewholesample.Theonlyexception tainedforboththegoodfittedsinglecandidates(fitflag5XX)and
tothiswerethetargetswhosetemperatureestimatereachedtheup- the most clear binary candidates (fit flag 4XX). The majority of
per limit of the TLUSTY models, 55000K. This is not suprising starswithinthesinglesamplelayinthetemperaturerange20000-
as we know that some sdOs can achieve very high temperatures
(Stroeeretal.2007).Fortheseobjects,justalowerlimitineffec-
tivetemperaturesisprovided. 14 seetheGALEXdocumentationathttp://galex.stsci.edu/GR6/?
Toclassifythequalityofthefitswetaggedeachtargetwitha page=ddfaq#6
New hotsubdwarfstarsbymeans ofVO toolsII 5
TLUSTY, Teff:55000, logg:4.75, Av:0.11 TLUSTY, Teff:18000, logg:2, Av:2.18 TLUSTY, Teff:15000, logg:1.75, Av:0.05
0,01 0,001
0,001
s)
2m/
c
g/
er 0,0001
2A
(λ
F
3λ SDSS J075854.30+174817.8 SDSS J043542.04+562414.6 SDSSJ133405.51+664957.3
(fit100) (fit200) (fit300)
1e-06
1 1 1
0,8 0,8 0,8
λ
F 0,6 0,6 0,6
|/Fλ 0,4 0,4 0,4
∆
| 0,2 0,2 0,2
0 0 0
1000 10000 1e+05 10000 1000 10000 1e+05
λ(A) λ(A) λ(A)
TLUSTY, Teff:55000, logg:4, Av:0.49 TLUSTY, Teff:55000, logg:4.75, Av:0.10 TLUSTY, Teff:30000, logg:4.75, Av:0.17
0,001 0,0030
0,0025
s)
2m/ 0,0020 SDSS J005040.10-092546.3
c (fit500)
g/
er 0,0001 0,0015
2A
3λF (λ S(fDit4S0S0 J)012458.96+475640.9 00,,00001005 S(fDit5S1S1 J)023547.65+053524.3
0,0001
1e-05 0,0000
1 1 1
0,8 0,8 0,8
λ
F 0,6 0,6 0,6
|/Fλ 0,4 0,4 0,4
∆
| 0,2 0,2 0,2
0 0 0
1000 10000 1e+05 1000 10000 1e+05 10000
λ(A) λ(A) λ(A)
Figure3.ExamplesofdifferentqualitySEDfittingsperformedbyVOSA.Fromlefttorightanduptobotton:badfitting(flag100),ultravioletexcess(flag
200),UVandIRexcesses(flag300),infraredexcess(flag400),goodfitting(flag500)andgoodfittingwithbadflagsinGALEXphtometry(flag511).Red
points(greyinthegrey-scaleversionofthefigure)representthederredenedmagnitudesofthegivenobject,connectedbluepoints(darkgrey)themagnitudes
givenbythebestfitmodel.Undereachgraphwerepresenttheresidualsoftheobserveddataandmodels.
30000K whiletheeffectivetemperaturesofthebinarysampleare rather thick hydrogen-rich envelopes. The merger channel gives
shiftedtowardslowervalues. Thisisnotsurprising,aswearein- rise to single subdwarfs whose hydrogen-rich envelopes are ex-
cludinginthe4XXcategoryobjectswitheffectivetemperaturesbe- tremelythin.Thischannel isbelievedtoexplaintheformationof
low19000K(seeabove).Ontheotherhand,thepeaksat15000K helium-richhotsubdwarfs(Zhang&Jeffery2012).
and55000KaresignalingthelimitsinTeff oftheTLUSTYgridof Nevertheless, the contribution of the binary channels to the
models. formation of the different subtypes of hot subdwarf stars is still
unclear,withnewdiscoverieschallengingthestandardbinaryevo-
lutionscenarios(seeGeier2013forarecentreview).
3.2 Binarysample
Regarding sdBs, the binary fraction is estimated around 40
Animportantissueregardinghotsubdwarfsistoknowthebinary percentorhigher,dependingonthenatureofthesamplesandthe
fractionoftheseobjects,assomeoftheproposedformationchan- methodusedtodetectthestellarcompanion(seeNapiwotzkietal.
nels involve evolution in binary systems (Hanetal. 2002, 2003; 2004, or Heber 2009 for areview). InLiskeretal.(2005) thees-
Clausenetal.2012). timatedbinary fraction is32 per cent, although this value should
Thethreemainbinaryevolutionchannels,asproposedinthese be taken as a lower limit as target selection was biased against
papers,arethecommonenvelope(CE)ejectionchannel,thestable composite spectrum objects. A recent study on IR excess in
Rochelobeoverflow(RLOF)channelandthedoubleheliumwhite knownradialvelocity(RV)variablesdBbinariescanbefoundin
dwarfs (WDs) merger channel. The CE ejection channel leads to Kupferetal.(2015).
the formation of subdwarfs in short-period binaries with typical The sdO binary fraction is more controversial, with differ-
orbital periods between 0.1 and 10 days and very thin hydrogen- entstudiesreachingoppositeconclusions:Napiwotzkietal.(2004)
richenvelopes. On theother hand, thestable RLOFchannel pro- foundonlyoneoutof23RVvariablesdO,whileGreenetal.(2008)
ducesstarswithlongorbitalperiods(400to1500days)andwith andGeieretal.(2011)foundasimilardistributionofRVvariations
6 E. Pe´rez-Ferna´ndez,A.Ulla,E. Solanoetal.
Table1.AsampleofsubdwarfcandidateswithgoodSEDfit.FUVandNUVweretakenfromtheGALEXarchive(andcorrectedasexplainedinthetext,
ifnecessary);u,g,rarefromSDSSDataRelease7and J, H,K fromthe2MASSPointSourceCatalogue. Weincludedthe2MASSqualityflagsofthe
source,where‘U’standsforupperlimitinthecorrespondingphotometricvalue.Teff isobtainedfromthebestSEDfitperformedbyVOSA.Anintervalin
thetemperaturecolumnisgivenwhenevertheBayesanalysisgavethemostlikelyTeffvaluewithaprobabilitybelow80percent.Thelastcolumnrepresents
aqualityflagontheTefffit:‘5’inthefirstdigitstandsforgoodfitting;a‘1’inthesecondorthirddigitrepresentssomeproblemintheGALEXphotometry.
Thecompletetablecanbefoundathttp://svo2.cab.inta-csic.es/vocats/hsa/.
RA DEC NUV FUV u g r J H K 2MASS Teff Fit
(J2000) (J2000) flag (VOSA) flag
00:03:07 +24:12:12 16.028 15.916 16.105 16.148 16.533 16.628 16.085 17.106 BUU 25000 500
00:11:43 -10:40:34 14.044 13.947 14.686 14.987 15.471 15.95 15.601 16.524 ABU 32-35000 501
00:50:40 -09:25:46 12.845 12.553 13.818 14.174 14.716 15.216 15.307 15.402 AAC 55000 500
01:11:56 +15:17:53 14.866 14.499 15.154 15.206 15.627 15.74 15.948 15.615 ACD 25000 501
01:30:32 +52:33:50 16.679 16.402 16.072 16.067 16.18 15.931 15.965 15.998 ACD 37500 500
01:32:33 +51:57:57 15.281 15.309 14.935 14.996 15.167 15.202 15.186 15.291 AAB 37500 500
02:20:35 +17:04:07 14.912 14.615 15.011 14.957 15.274 15.241 15.103 14.992 AAC 24000 500
02:31:45 +22:08:30 16.749 16.597 16.545 16.392 16.646 16.319 16.277 15.914 ADD 23-24000 510
02:34:56 -06:09:13 14.884 14.167 15.604 15.963 16.475 16.888 16.566 16.862 CDU 42-55000 500
02:35:48 +05:35:24 14.725 14.269 15.407 15.518 15.999 16.806 16.327 15.609 CCU 30000 511
Table2.AsampleofhotsubdwarfcandidateswithinfraredexcessintheSEDfit.ColumnlabelsarelikeinTable1.Avalueof‘4’inthefirstdigitofthe
FitflagcolumnindicatestheexistenceofexcessintheredpartoftheSED.Thebeginningoftheredexcess(Excessfromcolumn)isobtainedfromthefirst
bandwherethedifferencebetweenthemodelandthedereddenedobservedvalueisabove20percent.Thepossiblespectraltypeofthecompanionstaris
establishedbycomparisonwiththeØstensen(2006)subdwarfdatabase(seeSec.3.2).Thecompletetablecanbefoundathttp://svo2.cab.inta-csic.
es/vocats/hsa/
RA DEC NUV FUV u g r J H K 2MASS Teff Fit Excess Binary
(J2000) (J2000) flag (VOSA) flag from class
00:14:40 +08:03:52 17.11 16.87 16.83 16.57 16.48 15.69 15.36 15.21 AAB 15000 410 z GK
01:23:41 +30:02:32 16.02 15.62 16.33 16.35 16.64 16.42 15.84 15.29 BCU 18-21000 410 z GK
01:24:59 +47:56:41 15.98 15.48 16.53 16.88 17.06 15.82 15.23 15.00 AAA 55000 400 i FGK
01:33:14 +48:57:28 13.33 13.15 14.00 14.57 15.04 12.10 12.14 12.16 AAA 18000 410 B F
02:28:23 +25:35:19 14.37 13.35 13.50 14.16 13.12 13.04 13.10 13.09 AAA 15000 400 B F
02:41:13 +21:57:43 14.14 14.28 14.38 12.92 13.06 12.67 12.68 12.72 AAA 15000 400 B F
02:44:14 +30:07:23 15.40 14.79 15.06 14.67 14.60 13.89 13.63 13.63 AAA 16000 400 i FGK
02:57:48 +37:15:35 15.42 14.93 16.07 16.41 16.87 16.74 16.28 15.71 CDU 55000 400 J GK
03:18:23 +41:55:22 14.36 13.99 14.49 14.63 14.92 14.69 14.52 14.34 AAA 40000 410 J GK
03:48:30 +16:39:46 17.89 17.58 17.41 17.22 17.07 16.15 15.57 15.79 ABD 21000 410 i FGK
between sdBs and sdOs. On the other hand, while Ulla&Thejll Inourlistof437hotsubdwarfcandidates,20ofthemhaveno
(1998) found that 6 out of 14 (43 per cent) new hot sdOs with infraredphotometricdataavailable(seeTable4).Fortheother417,
IR excesses, Stroeeretal.(2007) found 8out of 52 (18 per cent) weconsiderasbinarycandidatesthosewithqualitySEDfit4or3
sdOswithphotometricinfraredexcesses,althoughagainthisvalue (IRorbothIRandUVexcess,Tables2and3).Thereare189of
shouldbetakenasalowerlimit. them,makingatotalfractionof45percent.Thisfractioncouldbe
A combination of optical and infrared photometry is com- overestimated,assomeofthefluxexcessesmaybeapparent,due
monlyusedtofindlate-typecompanionssuchasF,GorKtypes, toinaccuratephotometricmeasuresorbadSEDfittings.
becausethehotsubdwarfwillshineintheblue,whilethecompan- The possible spectral type of the companion star was esti-
ion will have brighter red colours. Stark&Wade (2003) found a mated analyzing the excess of hot sds with main sequence com-
40percentofbinarysystemsinamagnitude-limitedsampleofhot panionscataloguedinØstensen(2006).Wehaveselectedallsub-
subdwarfsfromtheKilkennycatalogue(Kilkennyetal.1988),us- dwarfsclassifiedassds+F,sds+Gorsds+Kofthiscatalogueand
ing2MASSinfraredfiltersandJohnsonorStro¨mgrenopticalpho- identified from which band the excess is detected in VOSA. Our
tometry.InGirvenetal.(2012)theyalsocombineGALEXultravi- criterionforthecompanionspectraltypewasthefollowing:
oletphotometrywithopticalandinfraredfilterstoselectsubdwarf
• ExcessfromB,Vorgband:typeF(17objects)
candidatesindoublesystems.Theycompletethephotometricstudy
• Excessfromrband:typesF,G(6objects)
withaspectroscopicclassification,findingalargefractionofcom-
• Excessfromiband:typesF,G,K(86objects)
positesystemswithF,GorKcompanions.Ontheotherhand,de-
• ExcessfromzorJband:typesG,K(69objects)
tectionofradialvelocityvariationsinsdBstarsisusedtofindclose
• ExcessfromH,KsorW1band:typeK(11objects)
binaries with invisible companions such as white dwarfs (see for
instanceMorales-Ruedaetal.2003andCopperwheatetal.2011). Hotsdscanhavecompanionsofotherspectraltypes.Closebi-
Thephotometricdataofoursamplerangesfromtheultravio- nariesformedbycoolmainsequenceM-typeorsubstellarobjects
lettothefarinfrared(WISEcolours), whichenabledustodetect mayalterthemeasuredphotometricvalues.Thevariationisdueto
fluxexcesses fromthe Bmagnitude. Thecompanions found with a reflection effect in the light curve caused by the irradiated sur-
ourphotometricmethodologyareexpectedtobelate-typemainse- faceofthemuchcoolercompanion.Itisestimatedthatonly1/5of
quencestars,suchasF,GorK. shortperiodsdBscontainadM(Østensenetal.2013).Infact,few
New hotsubdwarfstarsbymeans ofVO toolsII 7
Table3.AsampleofhotsubdwarfcandidateswithbadSEDfits.ColumnlabelsarelikeinTable1.WeincludeherebadfitswithapparentUVexcess(‘2’as
firstdigitintheFitflagcolumn),apparentexcessinbothUVandIRbands(‘3’asfirstdigit)andbadfitsofanyothersort(‘1’asfirstdigit).Thecomplete
tablecanbefoundathttp://svo2.cab.inta-csic.es/vocats/hsa/
RA DEC NUV FUV u g r J H K 2MASS Teff Fit
(J2000) (J2000) flag (VOSA) flag
02:12:44 +68:07:08 19.267 18.996 19.312 19.065 18.758 15.598 14.956 15.017 ABC 15000 100
02:25:44 +72:49:44 18.642 17.982 16.902 16.125 15.575 14.055 13.661 13.361 AAA 29-37500 200
02:27:18 +73:36:11 17.864 17.382 16.32 15.454 14.907 13.483 12.976 12.969 AAA 19000 200
02:51:46 +75:09:04 19.360 19.049 18.504 18.023 17.711 16.597 15.767 15.942 BUU 15000 200
03:33:56 +17:56:36 17.699 17.156 17.975 18.011 18.26 16.703 15.656 16.914 BUU 18000 300
03:53:07 +16:48:49 15.646 15.113 15.036 14.675 14.598 13.577 13.11 13.046 AAA 15000 300
04:07:24 +14:44:06 18.995 18.562 17.535 16.905 16.51 15.156 14.639 14.566 AAA 15000 300
04:31:18 +55:53:08 18.696 18.598 17.365 17.054 16.889 15.899 15.531 15.277 ABB 37500 200
04:35:42 +56:24:15 19.770 19.514 18.022 17.523 17.105 15.931 15.644 15.444 ABC 18000 200
04:38:22 +19:03:06 17.079 16.814 16.619 16.34 16.283 15.546 15.14 15.003 AAB 18000 310
04:41:41 -06:11:29 15.389 15.132 15.735 15.54 15.614 15.061 14.833 14.867 AAB 15000 300
Table4.Hotsubdwarfcandidateswithoutinfraredphotometricdata.Weincludeheresourceswithgoodfit(‘5’asfirstdigitintheFitflagcolumn),apparent
UVexcess(‘2’asfirstdigit),andbadfitsofanyothersort(‘1’asfirstdigit).
RA DEC NUV FUV u g r Teff Fit
(J2000) (J2000) (VOSA) flag
02:51:03.80 +75:15:03.4 16.7211 16.3976 17.243 17.194 17.564 42500 200
04:55:29.84 +24:45:07.6 18.8049 18.0702 18.501 18.631 18.853 42500 201
04:59:12.49 +60:51:56.5 19.0112 18.7774 19.017 18.631 18.929 37500 200
04:59:39.94 +59:48:53.5 17.5742 17.1230 17.219 17.25 17.56 42500 100
05:02:51.04 +13:49:26.9 20.4623 19.3643 19.512 19.522 19.552 42500 200
05:11:25.05 +15:03:01.0 18.9370 18.3932 17.986 18.012 18.026 40000 500
06:11:51.35 +34:04:01.5 19.3621 19.2375 19.924 20.152 20.544 42500 200
06:25:53.27 +34:54:28.2 16.8423 16.4591 17.304 17.298 17.666 42500 210
06:40:51.07 +26:44:28.0 12.9711 13.1221 13.524 11.024 11.064 15000 100
08:00:03.16 +07:40:43.3 14.6031 13.9893 15.507 15.907 16.432 55000 501
08:06:08.31 +10:24:20.1 16.4314 16.1312 17.044 17.195 17.682 28000 500
08:13:32.86 +05:54:30.1 14.8425 14.1695 15.608 16.093 16.639 55000 500
08:23:15.22 +00:18:46.0 15.6761 15.0236 16.502 17.019 17.3858 55000 500
08:28:16.33 +22:32:26.5 14.3126 13.6464 13.754 12.389 12.777 15000 400
16:07:41.25 +25:42:20.6 16.4320 16.1035 16.825 16.971 17.423 28000 500
17:37:03.25 +50:40:41.1 15.4195 15.2508 15.982 16.041 16.539 27000 500
20:14:55.06 +08:42:13.9 18.8628 18.5533 19.143 19.03 19.424 30000 500
20:46:23.13 -06:59:26.8 16.4524 15.9148 17.081 17.455 17.89 55000 500
21:08:04.46 +05:15:28.5 16.2921 15.8421 16.777 17.042 17.462 42500 500
23:28:59.74 +52:16:24.1 18.4366 18.0371 18.742 18.888 19.255 42500 200
reflecting sdBs+dM/BD systems are known, and they show typi- 3.3 Colour-colourdiagrams
calpeak-to-peakphotometricamplitudesof∼ 0.2magorless,di-
Colour-colour or colour-magnitude diagramsallow ustoseparate
minishingtowardsbluewavelengths.Thiseffectisnotexpectedto
sourcesofdifferentnatureusingphotometriccolours.Thisiscom-
significantlyalterourprocedure.
monly used to detect ultraviolet or infrared excesses signaling a
probablebinarynatureoftheobjectsunderstudy.
Wehavefirstplottedourcandidatesinthecolour-colourplane
Lesscommon arehot subdwarfs ineclipsingbinaries, either u − FUV versus V − u , as seen in Fig. 5. The 0 subscript
0 0 0 0
with dM/BD or WD companions (see Foretal. (2010)). In these standsforderredenedmagnitudes,withtheE(B−V)valuestaken
cases, deep eclipses with ∼< 0.8 mag variations occur (although fromSchlegeletal.(1998),andthecorrespondingcorrectionfac-
seetheextraordinarysdO+dMsysteminDerekasetal.(2015)).A tors computed from the formulae in Cardelli,Clayton&Mathis
raw estimate of the number of eclipsing sdB+dM systems in our (1989).FUV anduareGALEXandSDSSfilters,respectively.As
sample leads to a negligible number of photometric disturbances wedonothavedataoftheV magnitudeforallourcandidates,we
causedbyphotometrybeingacquiredoneclipse-phase.Ofcourse, useaV valuecomputedwiththetransformationformulagivenin
wecannotruleoutthatchancesofthiseffectoccur,whichcould Jesteretal.(2005)betweentheugrizandUBVsystems:
explainanyofthebadfitsencounteredinthiswork.
V =g−0.58(g−r)−0.01 (4)
Thefigureshowstherelativedifferenceinthebluecoloursof
Amorerigorouscheckofallpossiblebinarycandidates, us- ourcandidates.Mostofthegoodfittedobjectslayontherightside
ingatwo-componentfit,ispresentlybeingaddressed,andwillbe ofthediagram,aswouldbeexpected,becausetheystandforrela-
presentedinafuturework. tivelyhotsinglecandidates.Thebluerstarslayonthebottomright
8 E. Pe´rez-Ferna´ndez,A.Ulla,E. Solanoetal.
3
binary spec.
40 binary candidates single spec.
flag 200
single candidates
flag 300
2
flag 400
flag 500
30
1
J0
20 V­0
0
10
−1
0
40 40 40 40 40 40 40 40 40 40 40 −2
1 ×1 ×1 ×1 ×1 ×1 ×1 ×1 ×1 ×1 ×1 −0.75 −0.5 −0.25 0 0.25 0.5 0.75
1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 J0­H0
T (K)
eff
Figure6.AnanalogousofthegapinGreenetal.(2008)ispresentedhere.
Figure4.Effectivetemperaturehistogramforsingle(fitflag5XX)andbi- Singlesubdwarfcandidates(flag500)concentrateinthebottom-leftofthe
nary(fitflag4XX)candidates.Thetwopeaksat15000and55000Kshow diagram,whilecompositecandidates (flag400and300)areintheupper
theboundaryeffectsoftheTLUSTYmodelslimits(see3.1formorede- rightside.Wealsorepresentthebinariesandsingleobjectsspectroscopi-
tails). callyidentifiedusingSDSSspectra(seeSect.4).
0
differentsamplesofhotsubdwarfsandproposingalternativeways
flag 500
toseparatesinglefrombinarystarsusingcolour-colourdiagrams.
flag 400
flag 300 InStark&Wade(2003)theyplotJ−K versusV−K andfound
s s
0.5 flag 200 single stars remain inside a box limited by V − Ks ≤ +0.2 and
J−K ≤+0.05,whilethecompositestarslayoutsidethisrectangle
s
(see fig. 1 in that paper). In Greenetal. (2008) they plot V − J
versusJ−Hofasampleofconfirmedsubdwarfs,findingagapin
1 thecolourdiagram,separatingsinglefromcompositestars(fig.5of
V0 thatpaper).Finally,inGirvenetal.(2012)FUV−risplottedversus
FU r−Ks,showingalsoseparateregionsforthesingleandcomposite
u­0 subdwarfs.
1.5
SimilarlytoGreenetal.(2008),toshowthesingleorbinary
natureofourcandidatesweplottedV −J versus J −H ,theV
0 0 0 0
magnitudecomputedasinequation(4)andJandHfrom2MASS.
TheresultisshowninFig.6.
2
Not all the candidates are represented there. Many of our
sources are quite faint in the infrared, and thus do not have very
good 2MASS flags: 173 sources (41 per cent) have at least anU
(upperlimit)inoneofthe2MASSfilters;whileother 46sources
2.5
−1 −0.5 0 0.5 1 1.5 (11percent)haveatleastonefilterwithaDorEflag15.Toavoid
V0­u0 bigerrors,werestrictedourplottosourceswitherr(J)+err(H)<
0.3. For the rest of the objects, we substituted 2MASS data with
Figure5.Therelativedifferenceinbluecoloursofourcandidatesisvisible UKIDSS,ifavailable.
inthisplane.Thebluerstarslayonthebottomrightcorner. In Fig. 6, we can see that single candidates tend to concen-
tratearoundV −J ≃−0.6,andbinarycandidates,althoughmore
0 0
spread, around V − J ≃ 0.25. In spite of the source spreading,
0 0
cornerofthediagram,wherewecanseeobjectslabeledwithultra- thegapbetweencompositeandsinglesystemsisstillvisible.The
violetexcess(qualityfit200).TodiscardtheUVexcesswasonly sourcespreadingmaybeduetoavarietyoffactors.Wefirstnoted
aneffectofapossibleoverestimationintheextinctioncorrection, that the majority of binary candidates spread around thediagram
wechecked the A values of theseobjects. They turned up tobe
V
relativelyhigh (A > 1.14 mag), but not higher than many other
V
objectswithoutUVexcess(singleorbinarycandidates). 15 http://www.ipac.caltech.edu/2mass/releases/allsky/doc/
Therearevariousreferences intherecent literaturestudying sec1_6b.htmlgivesdetailedexplanationsoftheirqualityflags.
New hotsubdwarfstarsbymeans ofVO toolsII 9
arethosewheretheexcessbeginsatBorrmagnitudes,causingthe
He II + H
optical region of the SED to be untypical. Other uncertainties in 8 J060715.69+643007.9
SdO9 He3
theSDSSphotometry,duetoeitherfaintmagnitudesorbrightval-
uesnearthesaturationlimits,couldcausethestartobemisplaced. J172221.05+292431.4
7
Finally,wecannotdiscardotherphenomena, likethepresenceof SdB0 He9
planetarynebulae. J082902.62+224636.7
nst. 6 SdB0.2 He9
co J165809.14+214046.4
x+ SdB0.5 He9
4 SPECTRALCLASSIFICATION d Flu 5 J173710.22+432920.0
A first classification scheme for hot subdwarfs was proposed in ze SdB1.5 He5
Greenetal. (1986), where an eight class system was defined. In ali 4
m
Drillingetal.(2013)anevolvedMK-likesystemisdeveloped. In r J083435.40+134336.8
o
thissystemhotsubdwarfsaredividedinfoursequences:He-weak, N SdB2.5 He7
He-normal,He-strongandHe-strongC,withcarbonorothermetal- 3
J233331.06+462206.6
liclines.Thedifferencesinheliumcontentofeachobjectarealso
quantifiedmeasuring ratiosofhydrogen toheliumlinedepths. In Mg II SdB3 He7
2
each of thefour helium sequences, thespectral subclasses would
He I
rangefromsdO1tosdB9,asintheMKsystem,linedepthsandra-
tiosvaryingsmoothlywithineachsequence(seefigs.1to4inthat 4000 4200 4400 4600 4800 5000 5200 5400
paper). Stars former classified within the somehow ad-hoc sdOB Wavelenght (Å)
subclass(Moehleretal.1990)areplacednaturallyinthisscheme,
inthetransitionbetweenlatesdOandearlysdBsubclasses.Inthe
Figure7.SubdwarfsintheHe-normalspectralsequence,followingDrilling
present work we will appply the Drillingetal. (2013) system to
etal.(2013).StaridentificationsaretakenfromSDSS.
classifythecandidateswithspectrum.
Only68starsofourlistofsubdwarfcandidates(16percent)
hadSDSSspectrum.Webegunwithavisualinspectionofeachob- Calibrations made in Drillingetal. (2013) demonstrate that
ject’swholespectrum.Onespectrumwastoonoisytoallowiden- He-weakstarshavesubsolar heliumabundances,He-normalstars
tification. The rest were identified as one white dwarf, one prob- morenearly solar abundances andHe-strongobjects high helium
able cataclysmic variable and 65 subdwarfs: 5 sdOs (8 per cent), abundances. Theproblemofheliumabundancesinhotsubdwarfs
25sdOBs(38percent)and35propersdBs(54percent).Notethe has been addressed in O’Toole (2008) and Geieretal. (2013). In
effectivenessofourselectionprocedureimprovesto95.6percent Edelmannetal.(2003)acorrelationbetweeneffectivetemperature
withinthissubset. and helium abundance insdB stars wasdiscovered, showing two
Wealsoinspectedthepresenceofcharacteristiclinesofcooler sequenceswithapproximatelythesametrendwithincreasingT
eff
main sequence stars, to identify binary candidates. In particular, (fig.5inthatpaper).Theratioofobjectsinthelowersequenceto
we looked for the Mgi triplet (5172, 5183, 5167Å), the G band those in the upper one is argued to be between 1:10 and 4:10 in
(4300Å), and the Caii K line (3933Å). The Nai doublet (5889- O’Toole(2008).Ourratio14:48ofweaktonormalheliumstarsis
5895Å) can also be an indicator, although it may be overlapped consistentwiththesemargins.
with a near Heii line. We found 23 probable binary systems: 22 Drillingetal.(2013)suggesttheirspectralsequencesaretem-
binarysdBs(includingsdOBs)and1binarysdOs.Thebinaryfrac- peraturesequences,andfindlineartrendsplottingtheeffectivetem-
tionforsdBsobtainedbyvisualinspectionofthespectrumwasof perature against the spectral class (fig. 10 and 11 in that paper).
37percent,somehowlowerthanthephotometricfraction.Besides Foracheck,weplottedourgoodSEDfittedcandidatesintheT -
eff
thepossibleoverestimationofthephotometricfraction,asargued spectral classplane, together withDrillingetal. (2013) linear re-
above,therelativelylowsignal-to-noiseratioofsomeSDSSspec- gressions.Asaresult,andalthoughourtemperaturesappearingen-
tra may be obscuring the binary nature of some candidates, hid- eral subestimated, wefound our candidates from classessdO9 to
ingthecoldcompanion lines.Wepointoutthatallthespectrum- sdB3toroughlyfollowDrillingetal.’strendlines.Onthecontrary,
detectedbinarysystemsarealsobinarycandidatesfromthephoto- thehottersdO3-sdO7classesdonotseemtofollowthisbehavior.
metricexcesspointofview. Sucharesultisnotsurprising,duetotheeffectivetemperatureup-
ToclassifythesubdwarfswithintheDrillingetal.(2013)sys- perlimitoftheTLUSTYmodels(55000K).
temwecutandnormalizedthespectrabetween4000and5000Å
andthencomparedthemwiththestandard starsdefiningthesys-
tem.Thecompleteclassificationofourcandidatescanbefoundin
5 CONCLUSIONS
theelectronictables.AsampleisshowninFigures7-8.Sumarizing
ourresults,wehavefound: Inthisworkwehaveextendedtheselectionproceduredevelopedin
Oreiroetal.(2011)toidentifyhotsubdwarfs,takingadvantageof
• 1star(1.5percent)belongingtotheHe-strongCsequence,
VirtualObservatorytools.Theselectionprocedureincludesphoto-
• 2stars(3.1percent)intheHe-strong sequence,
metricandpropermotionsfiltersandaneffectivetemperaturecut-
• 48stars(73.9percent)intheHe-normalsequenceand
off.Wehaveidentified437newsubdwarfcandidatesfroma11663
• 14stars(21.5percent)intheHe-weaksequence.
deg2skyregion,limitedbytheSDSSDR7imagecoverage.Weex-
Notethatthesignal-to-noiseratioofsomespectramightbemask- pectaneffectivenessofatleast80percent,althoughthesubsam-
ingweakmetalliccarbonornitrogenlines,affectingtheHe-strong pleofobjectswithSDSSspectrareachedasubdwarfidentification
CandHe-strongrelativeabundances. above95percent,provingtheaccuracyofourselectionfilters.
10 E. Pe´rez-Ferna´ndez,A. Ulla,E. Solanoet al.
6 100000Keffectivetemperatures(Stroeeretal.2007).Theseobjects
arescarcebetweensubdwarfs,andsomeofthemhavebeenproved
to have planetary nebulae (Alleretal. 2013, 2015), a signal of a
He II + H
probablepostAGBorigin(Heber1991).Photometricandspectro-
5
scopicaccuratedatawouldbeneededtoreachfurtherconclusions
J030211.56+371109.5
st. SdO7 He0 abouttheoriginofthesestars.
on Finally, a detailed line spectral analysis of the hot subdwarf
c
* 2 + 4 J160741.26+254220.3 scpanhderidicatmeso,dteolsb,ewpoeurflodrmyieedldumsinogreardevlaianbcleed/vaaclcuuersatfeorNtLhTeEstaatrmeof--
ux SdB1 He0 fectivetemperatures,heliumabundancesandsurfacegravities.The
d Fl positionofourcandidatesintheTeff−loggplanewouldcontribute
ze 3 J083350.04+110103.8 to discriminate between the different origins and evolution paths
mali SdB2 He0 proposedforhotsubdwarfs.
or Regardingthesearchofnewhotsubdwarfcandidates,differ-
N
J061533.29+83011 entapproachescouldbeused.Oneofthosewouldimplyapplying
2 sdB2.5 He 3 ourselectionproceduretonewreleasesofsomeofthesurveysal-
ready considered (e.g. SDSS, GALEX) or using new catalogues,
bothintheoptical(e.g.Pan-Starrs,orJ-PASinthenearfuture)and
intheinfrared(UKIDSS,VISTA).
1 Anotherpossibilitywouldbeemployingothercataloguescon-
4000 4200 4400 4600 4800 5000 5200 5400
Wavelenght (Å) taining astrometric information. At this moment, our routine dis-
cardsanysourcewithoutproper motiondataintheSuperCosmos
survey. This fact could be modified making the routine look for
Figure8.SubdwarfsintheHe-weakspectralsequence,followingDrilling
propermotioninformationinothercatalogues,likeUCAC4orPP-
etal.(2013).StaridentificationsaretakenfromSDSS.
MXL(Roeser,Demleitner&Schilbach2010).
Itwouldalsobeinterestingtoexplorethepossibilitiesofthe
Fromour spectralenergydistributionanalysis, wehaveesti- datathatGAIA16wouldprovideregardingthispoint.GAIAwould
matedaphotometricbinaryfractionof45percent,whileidentifica- measuredistanceswithgreataccuracy,givingusinformationofstar
tionofcoolstarmetalliclinesinthespectrayieldsa37percentof luminositiesandtheirpositionintheHRdiagram.Thiswouldyield
binariesamongthissubsample.Bothnumbers,althoughroughesti- estimationsofbothstarmassesandages,animportantinformation
mates,areinagreementwithpreviousstudies.Ourmethodmainly totracktheevolutionarypathsofhotsubdwarfs.
selectsbinaries withlate-type main sequence stellar companions, Attention must be paid as well to other forthcoming mis-
likeF,GorK. sionssuchasCHEOPS(Fortieretal.2014)orPLATO(Raueretal.
The colour-colour diagrams of Fig. 5and 6show the differ- 2014),astheyareexpectedtohaveanimpactonultra-highpreci-
encein,respectively,theblueandredcoloursofoursubdwarfcan- sionphotometryandstellarastroseismologyforbrigthtargets,cov-
didates. Objects in the lower right corner of Fig. 5 show a clear ering large fractions of the sky (up to 50 per cent in the case of
ultravioletexcess(labeledwithSEDfittingflag200–seetextfor PLATO),andwidening,then,thepossibledetectionofnewpulsat-
details).InFig.6weseeaclearclusteringofthesinglecandidates inghotsds.
around V − J ≃ −0.6, and the colour gap between single and
0 0
binarysubdwarfsdiscoveredinGreenetal.(2008)isreproduced.
We have also performed a detailed spectral classification of ACKNOWLEDGEMENTS
the68candidateswithSDSSspectra,followingarecentMK-like
We thank A. Aller for helping us improving the spectra classifi-
system for hot subdwarfs developed by Drillingetal. (2013). We
cation.ThispublicationmakesuseofVOSA,developedunderthe
found our candidates perfectly suit in one of the four helium se-
Spanish Virtual Observatory project supported from the Spanish
quencesproposedinthatpaper.
MICINNthroughgrantAyA2011-24052.Ithasalsobeenpartially
Muchworkremainstobedoneaftertheselectionprocedure
supportedbygrantINCITE09312191PR (whichincludesFEDER
developed here. Regarding the list of hot subdwarf candidates, a
funds),givenbytheXuntadeGalicia,andbygrant12VI20,given
deeper analysis of the binary sample is presently under study. A
by the Universidade de Vigo. This research has made use of the
two-body SED fitting will yield temperature values of the cold
SIMBAD database, the Vizier catalogue access tool (originally
companion, which will aid to estimate its spectral class and its
published inOchsenbein,Bauer&Marcout 2000) and theAladin
distance. As argued in Clausenetal. (2012), composite systems
skyatlas,alloperatedatCDS,Strasbourg,France.
of thetypesdB +earlyFcould becrucial indetermining thebi-
nary formation channel of hot subdwarfs, depending on the peri-
odsmeasuredforthesesystems.Recently,inBarlowetal.(2013)
andVosetal.(2013),longperiodsdB+F/Gsystemshavebeenre- REFERENCES
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Both the twenty candidates without available infrared pho-
AllerA.etal.,2013,A&A,552,A25
tometryandtheseventeenobjectswithapparentultravioletexcess
are very interesting from our point of view, as good candidates
for very hot objects. Thehotter sdOsaremeasured tohave upto 16 http://sci.esa.int/gaia/