Classifying the secondary component of the binary star W Aquilae

0.35 MB·English

by  T. Danilovich

Astronomy&Astrophysicsmanuscriptno.WAql˙companion-spec (cid:13)c ESO2015
January6,2015
Classifying the secondary component of the binary star W Aquilae
(Research Note)
T.Danilovich1,G.Olofsson2,J.H.Black1,K.Justtanont1,andH.Olofsson1
1 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala,
Sweden
2 DepartmentofAstronomy,StockholmUniversity,AlbaNovaUniversityCenter,SE-10691Stockholm,Sweden
e-mail:[email protected]
Received19February2014;accepted23December2014
5 ABSTRACT
1
0 Aims.TheobjectWAqlisanasymptoticgiantbranch(AGB)starwithafaintcompanion.Bydeterminingmorecarefullytheprop-
2 ertiesofthecompanion,wehopetobetterconstrainthepropertiesoftheAGBstar.
Methods.WepresentnewspectralobservationsofthebinarystarWAqlatminimumandmaximumbrightnessandnewphotometric
n
observationsofWAqlatminimumbrightness.
a
Results.Thecompositespectrumnearminimumlightispredominantlyfromthecompanionatwavelengthsλ<6000Å.Thisspec-
J
trumcanbeclassifiedasF8toG0,andthebrightnessofthecompanionisthatofadwarfstar.Therefore,itcanbeconcludedthatthe
5
companionisamainsequencestar.Fromthis,weareabletoconstrainthemassoftheAGBcomponentto1.04–3M andthemass
(cid:12)

oftheWAqlsystemto2.1–4.1M(cid:12).Ourphotometricresultsarebroadlyconsistentwiththisclassificationandsuggestthatthemain
] sequencecomponentsuffersfromapproximately2magofextinctionintheVbandprimarilyduetothedustsurroundingtheAGB
R
component.
S
. Keywords.Stars:AGBandpost-AGB–(Stars:)binaries:spectroscopic–(Stars:)binaries:visual–Stars:individual:WAql
h
p
-
o 1. Introduction tryonlargescales(10arcseconds).Herschel/PACSphotometry
r presented in Mayer et al. (2013) shows a large (∼ 100 arcsec-
t IthaslongbeenknownthattheS-typeasymptoticgiantbranch
s ond)dustenvelopeonthesamegeneralshapeasRamstedtetal.
a (AGB) star WAql has a faint companion. Herbig (1965) noted
(2011)foundintheopticalbutwithanbrightpatchofdusttothe
[ thatatminimumbrightnessWAqltookonspectralfeaturessim-
eastofthestarinboththe70µmand160µmimages.
ilar to an F5 or F8 star, thus first identifying it as a spectro-
1 Having a more precise classification of the main sequence
v scopic binary. High resolution Hubble Space Telescope (HST) companion allows us to better constrain several properties of
3 images have since shown that the binary pair is separated by the WAql system. The mass of the main sequence component,
6 0(cid:48).(cid:48)46 (Ramstedt et al., 2011). The object W Aql is a Mira vari-
whichscaleswithspectraltype,putsalowerlimitontheinitial
8 able with a period of 490 days and an amplitude of approxi-
mass of the AGB component; as the AGB component is more
0 mately7mag,rangingfrom7.3to14.3magintheVband.Due
evolved,itmusthavehadalargerinitialmass.Further,themain
0 to its variable nature, many different distance calculations have
sequencecomponentimpartsadditionalenergytotheCSEofthe
.
1 beenmadeforWAql(Danchietal.,1994;Groenewegen,1998; AGBcomponent,whichcouldhaveaneffectonthechemistryof
0 Tatebeetal.,2006;Ramstedtetal.,2009).Atadistanceof400
theCSE,forexample,throughadditionalheating.
5 pc (Whitelock et al., 2008), this separation corresponds to 190
With conflicting results for the classification of the W Aql
1 AU.
companion (F versus K spectral types), we investigate the bi-
: Morerecently,Mayeretal.(2013)usedaperturephotometry
v nary system to find a more precise classification. We present
i toclassifythemainsequencecomponentofWAql.Theyfound new spectroscopic and photometric observations made during
X astablemagnitudeofV ≈14.8,whichimpliesanabsolutemag-
the minimum phase of the AGB component and new spectro-
r nitudeof M = 7.1foranunreddenedstarattheiradopteddis-
a V scopic observations taken during the maximum phase of the
tance of 340 pc, which would be consistent with a K4 dwarf.
AGB star for comparison. We present our observations in Sec.
ThisisverydifferenttoHerbig’s(1965)initialclassification.
2,performthespectralanalysisinSec.3,analysethephotome-
Previousstudieshaveshownanextendeddustycircumstellar
tryinSec.4,anddiscusstheresultsinSec.5.
envelope(CSE)aroundWAql.InRamstedtetal.’s(2011)obser-
vationsofpolarisedvisiblelight,theCSEextendsmorethan10(cid:48)(cid:48)
aroundtheprimary,whichiswellbeyondtheoffsetbetweenthe 2. Observationsanddatareduction
two binary components. This would certainly cause extinction
depending on the alignment of the system. Tatebe et al. (2006) ObservationswerecarriedoutusingtheAndaluciaFaintObject
usedtheUCBerkeleyInfraredSpatialInterferometertoresolve Spectrograph and Camera (ALFOSC) on the Nordic Optical
thedistributionofemittingdustat11.15µmandfoundanasym- Telescope (NOT) on two nights in May 2012 and March 2013.
metryinthedustdistributionaroundtheprimaryonsmallscales The spectroscopy was done with a grism with 600 rules/mm, a
(hundreds of mas), while Ramstedt et al. (2011) find asymme- dispersionof1.5Å,andthewavelengthrange3850–6850Å.The
1
T.Danilovichetal.:ClassifyingthesecondarycomponentofthebinarystarWAquilae(RN)
Table1.ObservationsofWAql. The Balmer series is the most prominent feature with absorp-
tion lines of Hβ, Hγ, and Hδ present, which are absent in the
Nightof Type Configuration Phase maximum brightness spectrum. There are also strong Ca II K
2012-05-11 spectroscopy 1(cid:48).(cid:48)0slit,Grism#7 max andHabsorptionfeatureswiththeCaIIHlinepossiblyblended
2013-03-29 photometry U Bes362 60 min withHε.Thestrengthsandshapesoftheselinessuggestthatthe
2013-03-29 photometry B Bes440 100 min companionstarisanFstar.TheabsenceofH8andH9Balmer
2013-03-29 photometry V Bes530 80 min absorptionlinesbluewardsoftheCaIIpairsuggestsalaterclas-
2013-03-29 spectroscopy 1(cid:48).(cid:48)0slit,Grism#7 min
sificationthanF5suchasF8 (Grayetal.,2009).
Severallessprominentmetallinesarealsopresent.Theweak
Notes.Thenumericalcodesforthephotometricfiltersgivethecentral
wavelengthsfollowedbythefullwidthathalfmaximum(FWHM)in Ca I 4226 Å line suggests an early to mid-F classification, as
nm.Maxandmininthephasecolumnrefertotheperiodsofmaximum doestheabsenceoftheFeI4046Åline.TheMgItripletaround
orminimumbrightnessofWAql. 5170ÅisnotstrongerthanthenearbyHβline,whichsuggests
anearly-Fclassification.TheslightlymoreprominentFeI4383
Å line suggests a mid-F classification. The weakness of these
photometric observations were taken using UBV filters of the
metallinescouldbeduetothestarbeingrelativelymetal-poor.
Besselphotometricsystem.Theobservationsaresummarisedin
Thiswouldexplaintheapparentdiscrepancybetweenthemetal
Table1.Theseeingonthenightofourphotometricobservations
lines and the Balmer series. The strength of the G band due to
rangedbetween0(cid:48).(cid:48)5–1(cid:48).(cid:48)1,sowewereunabletoresolvethetwo
the CH molecule, which is comparable to Hγ, also supports a
componentsofWAql.
laterclassification (Grayetal.,2009).
DatareductionwascarriedoutusingIRAF(v2.16).Forthe
Overall, the spectral features suggest an F8 or F9 classifi-
spectroscopy,weusedaHeNelampforthewavelengthcalibra-
cation for the companion star and do not conclusively rule out
tion.
G0.
ThestandardreferencestarPG1525-071,andtwootherstars
(labelledAandC)inthesamefield1(Landolt(1992))wereused
forourphotometriccalibration.Tocompensateforthedifferent 3.2. Featurespresentonlyatmaximumbrightness
spectral response functions between the standard UBV system
In stark contrast to the minimum brightness spectrum, the
andtheALFOSCsystem,weappliedthecorrectionsasadvised
Balmer series lines Hβ and Hγ are strongly in emission, indi-
in the ALFOSC manual2. The integration times were 3×200s,
cating shocks in the AGB star. None of the absorption features
20×15s, and 10×1s in U, B, and V, respectively. The standard
discussedinSec.3.1arepresent.
stars were observed at air mass ≈1.24 and WAql at air mass
TherearestrongZrObandsthroughoutthespectrumbutonly
≈1.66,andweusedthestandardextinction0.46,0.22,and0.12
magnitudes/airmassinU,B,andV,respectively,tocompensate a few generally weaker TiO bands, which suggests a relatively
forthedifferentialatmosphericextinction.TheUfilterhassmall high C/O ratio for the S star (∼ 0.98, see Sec. 5.3). There are
alsopossibleblendingsofZrOwithYOandLaO(seeTable2).
spectralleaksintheredspectralregionbeyond7000Å3,andfor
extremelyredsources,likeWAql,thereisariskforcontamina-
tion.However,apossibleredghostimagewouldbedisplacedby 3.3. Featurespresentduringbothphases
2(cid:48).(cid:48)5atthezenithdistanceoftheobservations,≈53◦,andwecan
Mostlyfeaturesattheredendofthespectrumarevisibleduring
excludearedghostthatisbrighterthan0.5%ofthebrightness
bothphases.
intheUband.
Thesodiumdoublet(NaD)at5892and5898Åispresentfor
both phases but with a significant difference in shape. At mini-
3. Spectralanalysis mumbrightness,theabsorptionisnarrower,andthetwocompo-
nentsofthedoubletareclearlydistinguishedwithasmallpeak
When the AGB star is at maximum brightness, it completely
betweenthem.Atmaximumbrightness,theNaDabsorptionis
dominatesthespectrumatalmostallwavelengths.Theonlyas-
bordered by ZrO and YO and is much stronger with broader
pect of the spectrum that might possibly show a contribution
wings.Althoughthetroughisslightlyasymmetric,itisnotpos-
fromthecompanionisaslightriseinthecontinuumintheblue
sible to distinguish the two components. The wings of the Na
fromaround4200Åandbluewards. D lines are apparent nearly up to the neighbouring YO band in
WhentheAGBstarisatminimumbrightness,however,sev- the maximum brightness spectrum, while there is a clear gap
eralfeatures,whichareclearlyfromthecompanionstar,become between Na D and the same, albeit weaker, YO band in the
visible in the blue end of the spectrum. The companion star minimumbrightnessspectrum.ItislikelythattheNaDfeature
dominates the spectrum up to around 5800–6000 Å, redwards comes from the AGB star in both cases, although it is possible
of which features that are characteristic of the AGB star again there is some contribution from the F star at minimum bright-
become apparent. Table 2 summarises all the spectral features ness.
andthespectrumitselfisshowninFig.1. As for the above-discussed YO band at 5972 Å, some red-
derZrObandsinthe6300to6500Årangeappearduringboth
phases.Thebandsaremuchweakerintheminimumbrightness
3.1. Featurespresentonlyatminimumbrightness
spectrum,whichistobeexpected.
Mostofthefeaturesuniquetotheminimumbrightnessspectrum The final feature common to both phases is the Hα emis-
arefoundintheblueend,wherethecompanionstardominates. sionline.AswiththeotherBalmeremissionlinesdiscussedin
Sec. 3.2, it most likely originates in shocks in the AGB star. In
1 http://www.not.iac.es/instruments/stancam/photstd/pg1525.html bothcases,itislocatedonthesideofabroadabsorptiontrough,
2 http://www.not.iac.es/instruments/alfosc/zpmon/ whichislessdeepatminimumbrightness,makingitdifficultto
3 seehttp://www.not.iac.es/instruments/filters/curves/png/7.png compare with the other Balmer lines. Woodsworth (1995) per-
2
T.Danilovichetal.:ClassifyingthesecondarycomponentofthebinarystarWAquilae(RN)
Table2.Spectralfeaturesatdifferentphases. 5. Discussion
Feature λ[Å] Maximum Minimum 5.1. Orientationofsystem
Hα 6563 emission emission
TheAGBcomponentissurroundedbyadustyenvelopeandthe
Hβ 4861 emission absorption
Hγ 4340 emission absorption envelopeextentissignificantlylargerthantheapparentsepara-
Hδ 4102 ... absorption tion between the AGB star and its companion (Ramstedt et al.,
CaIIH(+Hε) 3970 ... absorption 2011; Mayer et al., 2013). The significant extinction we calcu-
CaIIK 3934 ... absorption lateof A = 2suggeststhatthecompanionstarmostlikelysits
V
CaI 4226 ... absorption withinorbehindtheAGBstar’sCSE.Althoughsomeoftheex-
FeI 4383 ... absorption tinction we see may be interstellar, we believe it is unlikely to
MgI 5167,5173,5184 ... absorption
accountforalltheextinction,particularlyasnointerstellarcon-
Gband(CH) ∼4300 ... present
tamination has been seen in WAql’s molecular emission lines
NaD 5892,5898 absorption absorption
(Danilovich et al., 2014; Ramstedt et al., 2009; Scho¨ier et al.,
ZrObands 4620,4641,5304 present ...
2013).
5562,5718,5724 present ...
5849 present ...
6136,6344,6474 present weaker
6495 present weaker 5.2. Propertiesofthefaintercompanion
YObands 5972,6132 present weak
Mayeretal.(2013)usedaluminosity-basedclassificationforthe
Notes. References for wavelengths: Kramida et al. (2013) and Gray companion.UsingHSTimages,theycalculatedaK4Vclassifi-
etal.(2009). cation,whichissignificantlylaterthanourclassificationofF8or
F9.Webelievethedifferencearisesinthelackofconsideration
Table3.Observedphotometricresultsatphaseminimum. ofdustinMayeretal.’s(2013)analysis.
Spectrally,theabsenceoftheMgHfeatureat4780Åandthe
Band Flux(mag) Uncertainty
MgH band at 5198 Å in our spectrum (see Fig. 1) counters the
U 16.01 0.03
K4classification,asdoesthepresenceoftheBalmerabsorption
B 15.26 0.01
lines,thestrengthoftheCaIIKandHabsorptionlines,andthe
V 14.12 0.01
strengthoftheGband,whichshouldbemuchshallowerbythe
U−B 0.75 0.03
B−V 1.14 0.02 mid-K.
OurresultofanF8toG0stargivesaneffectivetemperature
in the range of 6170 – 5900 K (Gray et al., 2009) and a stellar
formed a detailed analysis of Hα lines in S stars at different massof1.09–1.04M(cid:12)(Habets&Heintze,1981).
phases,butwedonothavesufficientresolutioninourspectrum
tocomparewithhisresults.
5.3. PropertiesoftheAGBcomponent
The most prominent spectral features of the AGB component
4. Photometricresults
are the multiple ZrO bands, some of which are still visible in
Our photometric results are summarised in Table 3, which in- the red end of the spectrum even at the AGB star’s minimum
cludestheU −BandB−V colours.Itshouldbenotedthatour brightness. On the other hand, no clear TiO bands are seen in
photometricresultsarenotexclusivelyofthefaintercompanion eitherminimumormaximumspectra.Ingeneral,TiOmolecules
and include contamination by the AGB star, as the seeing was are less resistant to temperature increases than ZrO and hence
insufficienttoresolvethetwocomponentsofWAql. TiObandsareexpectedtobeweakatmaximumbrightnessand
potentiallymoreprominentatminimumbrightness(Richardson,
Based on our observations, we can make some further de-
1933).Theirabsencecouldbeduetothecompaniondominating
ductions. If we assume that there is no extinction, which is an
the spectrum, but this could only be true at the bluer end. The
unlikelyprospectgiventhedustyAGBenvelope(seeDanilovich
absenceofTiOattheredderendofthespectrum,evenatmini-
etal.,2014,Fig.2),wefindthattheabsolutemagnitude,assum-
ing a distance of 400 pc, is M = 6.1 mag in the V band. mumbrightness,indicatesthattheAGBcomponentismoreC-
V,obs likethanM-likeintermsoftheC/Oabundanceindex.Following
ThiswouldsuggestaK1classification(Grayetal.,2009),sim-
thecriterialaidoutinKeenan&Boeshaar(1980),wefindC/O
ilar to what Mayer et al. (2013) found. However, if we instead
∼0.98withtheclassificationofS6/6e(thefirst6beingthetem-
assume that our spectral classification is accurate, we can use
peratureclassification,takenfromKeenan&Boeshaar’s(1980)
canonical values (taken from Cox, 2000; Gray et al., 2009) to
calculatetheextinction.Considering(B−V) =0.52foranF8V observationalresultsofWAqlandtheeindicatingthepresence
star, the excess E(B−V) = 0.62 is consist0ent with an extinc- of emission lines). This is the highest C/O ratio before the SC
tionof A = 1.9mag,assumingstandardinterstellarextinction classificationtakesoverandisbroadlyconsistentwithmolecu-
V
(R = 3.1, Draine, 2011). If we calculate the extinction based larabundancesdeterminedinDanilovichetal.(2014).
V
on M = 4.0 instead, we find A = M − M = 2.1 If we assume that the two components of WAql formed
V,F8V V V,obs V,F8V
mag.Thiscouldsuggestaslightdifferenceinthedustbeingpro- contemporaneously, before it left the main sequence, the AGB
ducedbytheAGBstarascomparedwithdustintheinterstellar star must have been more massive than the fainter compan-
medium(ISM). Anothersourceofuncertaintyinthederivation ion. Additionally, the presence of s-process elements and more
ofA occursbecausetheV andB−V measurementsarenotex- specifically Tc (Little-Marenin & Little, 1988) constrains the
V
clusively of the F star but are likely contaminated by the AGB AGB star’s mass to less than ∼ 3 M (Herwig, 2005). This al-
(cid:12)
star. lowsustoconstrainthetotalsystemmassto2.1–4.1M .
(cid:12)
3
T.Danilovichetal.:ClassifyingthesecondarycomponentofthebinarystarWAquilae(RN)
Fig.1. Observed spectra. The blue curve is the spectrum at maximum brightness and the red curve is the spectrum at minimum
brightness.Theverticalaxesareinarbitraryfluxunits.
6. Conclusions research.ThisresearchhasmadeuseoftheInternationalVariableStarIndex
(VSX)database,operatedatAAVSO,Cambridge,Massachusetts,USA.
Themainconclusionsofthispaperareasfollows:
– Our spectroscopic observations of WAql suggest that the
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