Investigation of Rotational Splitting in the Pulsating White Dwarf GD 154

0.02 MB·English

by  Zsofia Bognar

Investigation of Rotational Splitting in the
Pulsating White Dwarf GD 154
Zs.Bogna´randM.Paparo´
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1 AbstractWeobservedtheZZCetistarGD154overawholeseasonatthemoun-

tainstationofKonkolyObservatory.Ourlongtimebaseallowedtodetectthesign
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R of rotational triplets around the independent modes. To check whether these can
S be real detections we made a test on our data set. We searched for characteristic
. spacingvaluesperformingFourieranalysisofnumerouspeaksdeterminedaround
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five frequencies in the main pulsation region. The analysis revealed regular peak
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- spacingswithseparationsaround3.7and2.6m Hz.Thesevaluesareinaccordance
o with the ones determined by consecutive prewhitening of Whole Earth Telescope
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t observations.
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1 1 The Method and the TestObject
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8 Handleretal.(1997)presentedamethodtoinvestigatetheregularitiesinthedistri-
9 butionofthepulsationfrequenciesofa d Scutistar. Theyassignedunitamplitude
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toallofthefrequenciesandcalculatedtheirFouriertransform(FT).Intheresulting
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1 window function, they detected a characteristic spacing and determined its value.
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Thismethodwasappliedlaterinthecaseofotherstars,too.Theseresultsgaveus
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theinspirationtotesthowthismethodisapplicableforrotationallysplitfrequencies.
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: GD 154 has been chosen as ourtest object. It’s brightnessvariationswere dis-
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coveredin1977(Robinsonetal.1978).In1991,GD154wasthetargetofaWhole
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X EarthTelescopecampaign.UsingthisdatasetPfeifferetal.(1996)determinedthree
r independentmodesandtheyhavealsofoundfrequenciessupposedtoberotational
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split components. The frequency separations at the dominant peak were 2.8 and
2.2m Hz,respectively.Theyhavealso founda peaknearto thesecondlargestam-
plitude mode with a separation of 3.9m Hz. Using a two-site campaign’s data set,
Zso´fiaBogna´r
Konkoly Observatory of theHungarian Academy ofSciences, H-1525 Budapest, P.O.Box 67.,
e-mail:[email protected]
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2 Zs.Bogna´randM.Paparo´
Hu¨rkalet al. (2005)havealso foundthe signsof rotational splittingat two modes
with3.1,3.4and3.3m Hzfrequencyseparations,respectively.
2 New Observations andOurResults
WeobservedGD154onatimebaseofsixmonthsatPiszke´steto˝ mountainstation
ofKonkolyObservatory.Wedetectedthesignofrotationaltripletsarounddifferent
modes. To check whether we found real triplet components, we performed a test
and searchedforthe characteristic frequencysplitting value(s)adoptingHandler’s
method.
Wepre-whitenedtheFourierspectrumofthewholedatasetwithfivesignificant
frequenciesinthemainpulsationregion.Thenwesearchedforpeaksintheresidual
spectrumin the ±10m Hz vicinity of these modes,and determinedthe two largest
amplitudeonesineverysinglefrequencyrangeapplyingthe usualsuccessivepre-
whitening method. The frequencylist obtained finally consisted of 3×5 elements
–thefivepulsationmodesandthetwoadditionalfrequenciesperdomains.Weas-
signedunitamplitudetoallofthefrequenciesandgeneratedtheirFT.Tocheckthe
stabilityofourresults,weperformedthedescribedanalysisbytheomissionofone
ortwofrequencydomains,too.
Using all of the 15 peaks, a frequency at 3.7m Hz and its harmonic have the
largestamplitudesintheirFT.Inthecaseofthethree-orfour-domainsolutionsthe
peakat3.7m Hzremainsdetectableandadditionalonesappearwithrelativelyhigh
amplitudesat2.5–2.7m Hzand3.9–4.1m Hz.
Wecheckedthelightcurve’swindowfunctionanditdoesnotshowanyhigham-
plitudepeaksaround3.7–4.1or2.5–2.7m Hz,comparingtothemainpeaksaround
zeroorthe1-dayalias.Thismeansthattechnicalpeaksdonotdisturbourspacing
determinations.
Based on this investigation, characteristic spacings with values around 2.6 and
3.7m Hz may presentin ourdata set. These values are in accordance with the fre-
quencyseparationsdeterminedbypreviousobservationsandconsideredasthe re-
sultsofrotationalsplitting.Thissuggeststhatwiththisindirectmethodwemaybe
abletodetectthesignoffrequencysplittingscausedbytherotationofastar.
References
1. HandlerG.etal.:NewWholeEarthTelescopeobservations ofCD-247599:stepstowards
deltaScutistarseismology.MNRAS286,303–314(1997)
2. Hu¨rkalD.O¨.,HandlerG.,SteiningerB.A.,ReedM.D.:AsteroseismicResultsforGD154.
ASPC334,577–580(2005)
3. PfeifferB.etal.:WholeEarthTelescopeobservationsandseismologicalanalysisofthecool
ZZCetistarGD154.A&A314,182–190(1996)
4. RobinsonE.L.,StoverR.J.,NatherR.E.,McGrawJ.T.:Anewpulsatingwhitedwarf–GD
154.ApJ220,614–618(1978)