Spatially extended emission around the Cepheid RS Puppis in near-infrared hydrogen lines. Adaptive optics imaging with VLT/NACO

1.5 MB·English

by  Alexandre Gallenne

Astronomy&Astrophysicsmanuscriptno.2010˙RSPUP˙NaCo˙r3v0 (cid:13)c ESO2011
January5,2011
Spatially extended emission around the Cepheid RS Puppis in
near-infrared hydrogen lines
⋆
Adaptive optics imaging with VLT/NACO
A.Gallenne1,2,A.Me´rand1,P.Kervella2,andJ.H.V.Girard1
1 EuropeanSouthernObservatory,AlonsodeCo´rdova3107,Casilla19001,Santiago19,Chile
2 LESIA,ObservatoiredeParis,CNRSUMR8109,UPMC,Universite´ParisDiderot,5PlaceJulesJanssen,F-92195Meudon,France
1 Preprintonlineversion:January5,2011
1
0 ABSTRACT
2
n Context.IthasbeenrecentlydiscoveredthatCepheidsharborcircumstellarenvelopes(CSEs).RSPupistheCepheidthatpresents
themostprominentcircumstellarenvelopeknown,theoriginofwhichisnotyetunderstood.
a
J Aims.Our purpose istoestimatethefluxcontribution of theCSEaround RS Pupat theone arcsecond scale(∼ 2000AU) andto
investigateitsgeometry,especiallyregardingasymmetries,toconstrainitsphysicalproperties.
4
Methods.Weobtainednear-infraredimagesintwonarrowbandfilterscenteredonλ = 1.644and2.180µm(NB 1.64andIB 2.18,

respectively) thatcomprisetworecombination linesof hydrogen: the12–4 and7–4(Brackettγ)transitions,respectively. Weused
]
R NACO’scube mode observations inorder toimprove theangular resolution withtheshift-and-add technique, and toqualitatively
studythesymmetryofthespatiallyextendedemissionfromtheCSEwithastatisticalstudyofthespecklenoise.
S
Results.Weprobably detect ata2σ levelanextended emission witharelativeflux(compared withthestarinthesamefilter)of
.
h 38 ± 17%intheNB 1.64filterand24 ± 11%intheIB 2.18filter.ThisemissioniscenteredonRSPupanddoesnotpresentany
p detectableasymmetry.Weattributethedetectedemissiontothelikelypresenceofanhydrogenenvelopesurroundingthestar.
-
o Key words. Stars: circumstellar matter; Stars: variables: Cepheids; Stars: individual: RS Pup; Instrumentation: adaptive optics;
r Infrared:stars;Atmosphericeffects
t
s
a
[
1. Introduction sisofthespecklenoisetoenablethedetectionoftheCSEinthe

1 specklecloudsurroundingthestar’simage.
Foralmosta century,Cepheidstars havebeenused asdistance
v ThispaperisfocusedonRSPup’senvelope,whichhasbeen
indicators thanks to the well-known period-luminosity relation
6 known for many years (Westerlund 1961; Havlen 1972) for it
4 (P-L),alsocalledLeavitt’slaw.A goodcalibrationofthisrela- is very extended: it has a spatial extension of around 2′ in V
7 tionisnecessarytoobtainanunbiasedestimateofthedistance. band, which corresponds to about one parsec at RS Pup’s dis-
0 After the detection of circumstellar envelopes (CSEs) around
tance(about2kiloparsecs;Kervellaetal.2008).Itsoriginisnot
. many Cepheids in the near- and mid-infrared (Kervellaetal.
1 precisely known; it is especially unclear whether it is made of
2006; Me´randetal. 2006, 2007; Kervellaetal. 2009a), it was
0 materialejectedbythestarorofamaterialthatpredatesthefor-
shownthattheseCSEscouldbiastheangulardiameterdetermi-
1 mation of the star. Its actual shape has been widely discussed:
1 nation using near-infrared interferometry (Me´randetal. 2006). discrete nebular knots have been invoked by Kervellaetal.
: A circumstellar envelopecould have an impacton the distance
v (2008)toestimatethedistanceusinglightechoes,whereasFeast
estimate if we do not take into account its flux contribution. It
i (2008) argued that a disk-like CSE will bias this kind of es-
X couldleadtoabiasbothininterferometricandphotometricmea-
timate. Alternately, a bipolar nebula has been considered by
surementsandthereforeinfluencetheaccuracyoftheestimated
r Bond&Sparks (2009), making a ratio between two images of
a distance.BecauseCepheidsareusedasstandardcandlestoesti-
differentepochs.WewereabletospatiallyresolvetheCSEclose
matedistancesintheuniverse,itisimportanttounderstandthe
tothestar.ThemorphologyoftheCSEisdiscussedinSect.4.3.
propertiesoftheCSEsaroundthiskindofstars.
WereportnewobservationswithVLT/NACOintheIB 2.18
andNB 1.64filtersusingafastimagingmodeinordertochar- 2. Observationsanddatareduction
acterizetheextendedemissionaroundRSPuppis(HD68860)at
sub-arcsecondangularresolution(≈0.01pcatRSPupdistance). RSPupwasobservedwiththeNACOinstrumentinstalledatthe
Data and reduction methods are described in Sect. 2. In Sects. NasmythBfocusofUT4ofESOVLT.NACOisanadaptiveop-
3 and 4 we present two different data processing analyses: the tics system (NAOS, Roussetetal. 2003) and a high-resolution
shift-and-addmethod,andamethodbasedonastatisticalanaly- nearIRcamera(CONICA, Lenzenetal.2003),workingasim-
ager or as spectrographin the range 1–5µm. We used the S13
Sendoffprintrequeststo:A.Gallenne camerafor our RS Pup observations(FoV of 13.5′′×13.5′′ and
⋆ BasedonobservationsmadewithESOtelescopesatParanalobser- 13.26±0.03 mas/pixel,Masciadrietal. 2003) with the narrow
vatoryunderprogramID382.D-0065(A) bandfilterNB 1.64(λ =1.644µmwithawidth∆λ=0.018µm)
0
Correspondenceto:[email protected] and the intermediatebandfilter IB 2.18(λ = 2.180µm with a
0
1
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
width∆λ=0.060µm).Wechosethecubemodeinordertoapply (2010) showed for the RS Pup’s reference star at 2.18µm that
the shift-and-addtechnique.We chose windows with 512×514 byusingthismethodwehaveanabsolutegainof9%inStrehl
pixels and 256×258 pixels respectively for the narrow and the ratio.
intermediate band filters. Each cube (10 for RS Pup and 4 for Ourprocessingstepsareasfollows:weselectthe10%best
the reference star) contains 460 images at 1.64µm and 2 000 framesaccordingtothebrightestpixel(asatraceroftheStrehl
at 2.18µm. Each frame has the minimum integration time and ratio) in our 10 data cubes (4 for the calibrator). We then spa-
readouttypeforthismode,i.e109msand39msrespectivelyfor tially resamplethembya factor4 usingacubicspline interpo-
the1.64and2.18µmcubes.WiththeseDITsandfiltersweare lationandco-alignthemusinga Gaussianfittingonthecentral
inaquasi-monochromaticregimewithaverygoodspecklecon- core,ata precisionlevelof a few milliarcseconds(thismethod
trast (Roddier1981). The large numberof exposuresallows us isdescribedindetailinKervellaetal.2009a).Eachcubeisthen
toselecttheframeswiththebrightestimagequality. averaged to obtain ten final average images (4 for the calibra-
Datawereobtainedon2009January7,withtheatmospheric tor).We finally computethe meanof the 10 averageimagesof
conditions presented in Table 1. The PSF (point spread func- RSPup(orthe4forthecalibrator).
tion) calibrator stars were observedjust before or immediately TheresultofthismethodisshowninFig.1forthetwofilters.
afterthescientifictargetswiththesameinstrumentalconfigura- ThetwoupperframesareHD74417andRSPupasseenthrough
tions. Each raw image was processed in a standard way using theIB 2.18filter,whilethetwolowerareintheNB 1.64filter.
the Yorick1 language: bias subtraction, flat-field and bad pixel Thespatialscaleinallimagesis3.3mas/pixandwechosealog-
corrections.Thenegligibleskybackgroundwasnotsubtracted. arithmicintensitycontrastwithadjustedlowerandhighercut-off
To compare the AO system behavior and artefacts on to show details in the halo. On the 1.64µm image of RS Pup
RS Pup, we used the star Achernar and its PSF reference star comparedwith the HD 74417image we can see that there is a
observedthesamenight.Thesedatawerealsoacquiredincube resolved circumstellar emission up to ∼1′′ from the Cepheid.
modewiththesamecameraintheNB 1.64andNB 2.17(λ = This feature is fainter in the 2.18µm filter. In both filters, be-
0
2.166µm with a width ∆λ = 0.023µm) filters. Only one cube cause of this emission, diffraction rings are less visible around
was acquiredper filter, containing20 000 framesfor Achernar RSPupthanaroundHD74417.
and8000forthecalibrator.Thewindowdetectorsizeis64×66 Foreachfinalco-addedframe(i.etheimageshowninFig.1)
pixels in both filters with an integration time of 7.2ms. The wecomputedaringmedianandnormalizedtounity.Theresult-
slightly different second filter and the shorter exposure time ingcurvesarepresentedinFig.2fortwofilters.Theerrorbars
compared with RS Pup is not critical for our analysis because arethestandarddeviationofvaluesinsideringsandareplotted
we are interested in flux ratios. The smaller window size cuts onlyforfew radiiforclarity.We notice a significantdifference
theAOPSFwings,butwewillshowinSect.3thatthisdoesnot betweenthetwocurves,morepronouncedintheNB 1.64filter.
affectourconclusionsignificantly.
Oncethe pre-processingwas over,wecarriedoutaprecen-
3.2.Achernar
teringaswellasasortingaccordingtothemaximumintensityof
thecentralpeaktorejecttheimagesforwhichthecorrectionwas We carried outthe same processing for Achernar and its refer-
theleastefficient.Ontheonehand,weappliedtheshift-and-add encestarδPhe(HD9362).Thesetwostarsareunresolvedbythe
techniquetoobtainthebestangularresolutionpossible.Onthe UTs(angulardiametersareapproximately2mas).Withthiswin-
other hand, we applied a statistical study aimed at extractinga dow size (64×66pixels)the field of view is fourtimes smaller
possible diffuse (i.e. completelyresolved)componentinside of thanin the RS Pup’sframes,and evenif we can also see some
the speckle cloud of the central core (the star). These methods Airyringsinalltheimages,thebackgroundisdominatedbythe
areexplainedinSects.3and4. AOhalo.
We plot in Fig. 2 the radial profiles of the stacked images,
normalizedto10(forclarityinthegraph).
3. Shift-and-addprocess
In this section we use the shift-and-addmethod and then com-
3.3.Halofitting
paretheRSPupimageswithanotherstarobservedonthesame
nightforwhichwedonotexpectanenvelope. Wenowinvestigatethepresenceofacircumstellarenvelopeby
studying the radial profile of the stacked images we obtained
forbothRS PupanditsPSFcalibratorandcomparethemwith
3.1.RSPup
the observations of Achernar. The main idea behind this study
Both stars, RS Pup and HD 74417,are unresolvedby the tele- istocomparethevariationsofencircledenergy(theratioofthe
scope:diffractionlimitsforan8.2mUTare55masand41mas core’sfluxtothetotalflux)totheexpectedvariationscausedby
in filters IB 2.18 and NB 1.64 respectively, while angular di- atmosphericconditionchanges.IfRSPuphasindeedacircum-
ametersareapproximatelyonemilli-arcsecond(1mas)forboth stellarenvelope,itsencircledenergywillbesignificantlylower
stars. thanthatofourreferencestars.
The shift-and-add method was originally proposed by We use ananalyticalfunctionto fitthe residuallightoutof
Bates&Cady (1980) as a technique for speckle imaging. thecoherentcore(i.e.thehalo).Weusedaturbulencedegraded
Applied to NACO’s cube mode, it enhances the Strehl ratio profilefromRoddier(1981)
by selecting the frames that are the least altered by the atmo-
spheric turbulence. It also reduces the halo contribution when 0.488 11 r 2 −161
I (r)= f 1+ , (1)
used with adaptive optics by selecting the best AO-corrected halo ρ2  6 ρ! 
pfroasmurees.mTohdeeeiffiscdiiesncucyssoefdtihneKcuebrveemllaodeetavls..(t2h0e0s9tabn).dGaridralrodngeteaxl-. wherethefittingparametersaretheFWHMρand f proportional
to the flux of the envelope.In this case, the parameterρ corre-
1 http://www.maumae.net/yorick/doc/index.php spondstotheequivalentseeingdiskforlongexposure.NACO’s
2
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
Date UTC(start-end) φ Filter Name seeing seeing AM Nbr r (cm) t (ms)
DIMM λ 0 0
2009-01-07 00:30:41-00:31:56 - NB 2.17 HD9362 0.72′′ 0.55′′ 1.15 1 9.2 3.1
2009-01-07 00:35:33-00:36:47 - NB 1.64 HD9362 0.77′′ 0.62′′ 1.15 1 9.0 2.9
2009-01-07 00:51:16-00:54:16 - NB 2.17 Achernar 0.84′′ 0.64′′ 1.25 1 9.1 2.3
2009-01-07 01:01:30-01:04:28 - NB 1.64 Achernar 0.83′′ 0.63′′ 1.27 1 9.1 2.3
2009-01-07 04:47:17-05:00:46 0.035 IB 2.18 RSPup 0.71±0.05′′ 0.54±0.04′′ 1.03 10 13.6±0.4 3.6±0.2
2009-01-07 05:01:16-05:13:17 0.035 NB 1.64 RSPup 0.68±0.04′′ 0.54±0.03′′ 1.03 10 13.0±0.1 3.4±0.1
2009-01-07 05:21:03-05:26:27 - IB 2.18 HD74417 0.55±0.01′′ 0.42±0.01′′ 1.05 4 19.3±0.8 5.1±0.4
2009-01-07 05:26:57-05:31:43 - NB 1.64 HD74417 0.54±0.01′′ 0.43±0.01′′ 1.04 4 20.3±0.1 5.1±0.1
Table1.LogoftheNACOobservations.seeing aretheseeingmeasurementsinV fromtheDifferentialImageMotionMonitor
DIMM
(DIMM)station,whileseeing areinterpolatedtoourwavelength.Theuncertaintiesdenotethestandarddeviationsoverthenumber
λ
ofcubes.Thestart-endUTCindicateddenotesthetimewhenthefirstandthelastcubewasrecorded.ThephaseφoftheCepheid
wascomputedusing(P,T )fromKervellaetal.(2008).AMdenotestheairmass,whileNbristhenumberofcubesrecorded.r and
0 0
t aretheFriedparameterandthecoherencetimegivenbytheAOrealtimecalculator.
0
HD 74417 2.18µm RS Pup 2.18µm
N N
0.5’’ E 0.5’’ E
HD 74417 1.64µm RS Pup 1.64µm
N N
0.5’’ E 0.5’’ E
Fig.1.Finalframesfromshift-and-addprocess.InthetwoupperpanelsthePSFcalibratorstarHD74417andRSPupareshownin
theIB 2.18filter.ThebottompanelsshowthesamestarsintheNB 1.64filter.Thescaleintensityislogarithmic.
resultingcore,evenforanunresolvedsource,isfairlycomplex Achernar(anditscalibrator).Theblacklinesareourdata(ring-
andsuffersfromresidualaberrations,thisiswhywedecidedto median),whilethegraylinesrepresenttheadjustedcurves.The
fit the halo at a large radial distance of the core. We will later verticalscaleislogarithmicandnormalizedtothetotalflux.We
define the core as the residualof the fit of the halo function to alsoevaluatedthenormalizedencircledenergy(E ),i.e.thera-
enc
ourdata. tio ofthe flux in the core(i.ewithin 1.22λ/D) to the totalflux.
We can already see from these curves that the flux in the halo
We show the resulting fits in Fig. 4. They were applied for
r > 0.26′′at 2.18µm and 2.17µm while the range was for r > isdifferentintheimagesofRSPupandAchernar.Thesediffer-
0.2′′at1.64µmforRSPup(anditscalibrator)andr > 0.26′′for encescanbelinkedtotheemissionoftheCSEaroundRSPup
3
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
10.0 10.0
2.18µm 1.64µm
1.0 1.0
x x
d flu 0.1 d flu 0.1
e e
z z
ali ali
m m
or 0.01 or 0.01
N N
0.001 δ Phe 0.001 δ Phe
Achernar Achernar
HD 74417 HD 74417
RS Pup RS Pup
0.01 0.1 1.0 0.01 0.1 1.0
Angular distance (arcsec) Angular distance (arcsec)
Fig.2.Ontheleftarepresentedtheradialprofilesat2.18µmforRSPup,Achernarandtheirreferencestars.Ontherightthesameat
1.64µm.Radialprofileswerecomputedusingthemedianvalueonone-pixelthickcircularrings.ThegraycolordenotesAchernar
andδPheprofiles,shiftedbyafactor10forclarity.Thescaleislogarithmicforbothaxes.
parametersinTable2.Ourfitgivesaseeingvariationofaround
RS Pup @ 2.18 µm HD 74417 @ 2.18 µm
9%,whiletheDIMMmeasurementsgivea29%variation,lead-
100
200 ing to the same conclusionas beforeaboutthe accuracyof the
DIMM station seeing estimates. The encircled energies mea-
mber mber sured in IB 2.18 are Eenc(RSPup) = 0.417and Eenc(HD74417)
Nu Nu =0.517,givinganencircledenergyratioε∼80%.
100 50 Weevaluatedtheuncertaintiesaccordingtothevariationsof
the encircledenergyin the cubes. Because we alreadyselected
the10%bestframes,wekeptthesesamplesandplottedthehis-
togramoftheencircledenergytocheckthedistributionfunction
0 0
0.35 0.40 0.45 0.40 0.45 0.50 0.55 (Fig.3).Atthefirstorder,the E distributioncanbeapproxi-
Eenc Eenc enc
matedbyaGaussiandistribution.Wethencomputedtherelative
RS Pup @ 1.64 µm 20 HD 74417 @ 2.18 µm
60 standarddeviationσ(E )/E forbothstars. We assessed the
enc enc
uncertaintyoftheencircledenergyratioεwith
15
Number 40 Number 10 σ(εε) = vt"σ(EEenc)#2 +"σ(EEenc)#2 , (2)
20 enc RSPup enc HD74417
5
We assume that the relative standard deviation of the
0 0.25 0.30 0.35 0 0.35 0.40 0.45 encircled energy is a good approximation of the see-
Eenc Eenc ing variations. We measured a relative standard deviation
Fig.3. Histogram of the encircled energy computed for 10% [σ(E )/E ] ∼ 8% at 2.18µm and ∼10% at 1.64µm
enc enc RSPup
bestframes.ThesolidlinesrepresentaGaussianfit.
while [σ(E )/E ] ∼4% and ∼6% respectively. We
enc enc HD74417
also evaluated the statistical uncertainties with a bootstrap
method that gave σ ∼ 0.5% for both stars. These statistical
or a modification of the seeing between the two observations. uncertaintiesdonottakeintoaccounttheseeingvariations.We
Thispointwillbediscussedbellowwithastatisticalstudyofthe therefore chose Eq. 2 as a reliable estimate of the final uncer-
encircledenergy. tainty. This gives σ(ε)/ε ∼ 9% and ∼ 12% respectively in
For RS Pup in the NB 1.64 filter we found from our fit IB 2.18andNB 1.64.
comparing the two FWHMs, i.e (ρ −ρ )/ρ , a For Achernar, the fit gives a seeing variation ∼ 3% both
RSPup HD74417 HD74417
changeofhalosizeof11%andencircledenergiesE (RSPup) in NB 1.64 and IB 2.17. The encircled energies measured in
enc
= 0.327and E (HD74417)= 0.452(see Table2). Thisyields IB 2.18 are E (Achernar) = 0.495 and E (δPhe) = 0.505,
enc enc enc
an encircled energy ratio ε ∼ 72%. The local DIMM station giving an encircled energy ratio ε ∼ 98% (see Table 2). In
registered a seeing variation of around 26%. Different values NB 1.64 we have 0.374 and 0.376 respectively, which gives
can be explained by the station location, which is not inside ε ∼ 99%. The encircled energies are nearly identical, giving
the telescope dome, but a few tens of meters below, rendering ratios close to 100%. Using Eq. 2 as previously, we found a
itmoresusceptibletogroundlayereffectsthatareknowntoaf- relative standard deviation ∼ 15% in IB 2.17 and ∼ 11% in
fect the seeing (Sarazinetal. 2008). As a result the UT seeing NB 1.64.BecauseAchernardoesnothaveanextendedcircum-
isconsistentlybetterthantheonemeasuredbytheDIMM.For stellar component in NB 1.64 and IB 2.17, we attribute these
IB 2.18weshowthefittingcurvesinFig.4andtheresultingfit variationstotheStrehlvariationsandtheAOcorrectionquality.
4
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
100.0
100.0 1.64µm
1.64 µm
Achernar
RS Pup δ Phe
HD 74417
10.0
d flux ed flux 10.0
alize 1.0 maliz
orm Nor
N
1.0
0.1
100.00.0 0.5 2.18 µm 1.0 100.00.0 0.1 0.2 0.3 2.170µ.m4
10.0
malized flux 1.0 malized flux 10.0
Nor Nor
0.1 1.0
0.0 0.5 1.0 0.0 0.1 0.2 0.3 0.4
r (arcsec) r (arcsec)
Fig.4.Halofitforallfilterswithlogarithmicordinates.Thefluxesarenormalizedtothetotalflux.Leftpanel:halofitfromtheradial
profileofimagesinthetwofiltersforRSPup.Rightpanel:halofitfromtheradialprofileofimagesinthetwofiltersforAchernar.
The fit in IB 2.18was forr > 0.26′′ andin NB 1.64forr > 0.2′′ for RS Pup andfor r > 0.26′′ for Achernar.Thedashed lines
denotethereferencestars,whilethethicklinesdenotetheCepheidandAchernar.Thefittedcurvesareplottedingray.
Theencircledenergyratioseemsstablebetweenthetwostarsat filter name ρ(′′) f E
enc
the2%levelinIB 2.18(seeTable2). NB 1.64 RSPup 0.46±0.01 0.58±0.01 0.327
HD74417 0.43±0.01 0.43±0.01 0.452
We can notice that the normalized curves of Achernar and Achernar 0.49±0.01 1.00±0.01 0.374
itscalibratoralmostoverlap,andthisiswhatisexpectedforun- δPhe 0.46±0.01 0.97±0.01 0.376
resolvedstars withoutenvelopetaken insame atmosphericand IB 2.18 RSPup 0.56±0.01 0.45±0.01 0.417
instrumentalconditions.ItisnotthecaseforRSPup(seeFig.2 HD74417 0.50±0.01 0.34±0.01 0.517
and 4). The additional emission coming from the envelope of NB 2.17 Achernar 0.52±0.01 0.59±0.01 0.495
the Cepheidaddsa componentthatis clearly notnegligible.In δPhe 0.54±0.01 0.61±0.01 0.505
thesameatmosphericconditionsandAOcorrectionbetweenthe Table 2. Best-fit results for the residual halo where. ρ is the
staranditscalibrator,theencircledenergyratioshouldbeclose FWHMand f isaparameterproportionaltothefluxofthehalo
to100%.ThatiswhatweobtainfromAchernar’sdata.Scaling normalizedtothetotalflux.E representsthefluxinthecentral
enc
RS Pup’s images to the same window size (i.e. r . 0.42′′) core(i.e.within1.22λ/D)tothetotalflux.Note:ThefittedI
halo
slightly changes Eenc because we normalized to the total flux, analyticalfunctionisnotnormalizedto f,i.e Ihalordr, f.
but we still have a discrepancy of 100% for the encircled en-
R
ergy ratio (ε ∼ 79% and ε ∼ 87% respectively in NB 1.64
andIB 2.18).AsmallerwindowonRSPupmakesthetotalflux
lowerbecausewe donotintegrateallthe envelopeflux, conse-
TheencircledenergyofRSPupcanbewrittenas
quentlythe normalizedencircledenergyis largerand therefore
theE ratioaswell.
enc
F +αF
AchTehrneadr’rsopdaitnaEaenndcothfeRfSacPtuthpactotmhipsadrreodpwisithlatrhgeersttahtaisntitchseoovneer Eenc = cFoh+Fenvenv,
directlyestimatedfromtheseeing halovariations,motivatethe
followingsectionwherethisdropisanalyzedundertheassump- where F is the total flux of the star, without the extendedflux,
tionthatitiscausedbyCSEemission. Fcoh isthefluxinthecoherentcore,Fenv istheemissionofthe
environmentandαisthefractionoftheenvelopefluxthatlays
inthecore.Forsimplicity,wewillassumeα<<1,whichmeans
3.4.Encircledenergyvariation thatweassumethattheCSEissignificantlylargerthanthecore.
Accordinglywecanwrite
Inthispartweevaluatethefluxoftheenvelopeusingthefactthat
thetheoreticalencircledenergyratioshouldbe100%andthatan F /F
E ≈ coh ,
additionalcomponenttothefluxshoulddecreasethisratio. enc 1+F /F
env
5
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
Assuming that the encircled energy is stable in NACO for stackedimagetheradialintensityprofileI(r)andweestimatethe
ourobservingconditions,wecancalibratethepreviousformula termσ2 ofEq.3with
ph
toobtaintheextendedemissionflux:
I(r)
E (Cepheid) 1 σ2 (r)=γI(r)=γ ,
ε= enc ≈ ph N
E (reference) 1+F(env)/F(Cepheid)
enc
where N is the number of frames in the cube (1999 frames in
The relative emission F(env)/F(Cepheid) can be estimated IB 2.18 and 457 in NB 1.64) and γ the gain of the detector in
with the encircled energy ratio from our measured values ordertoconvertnoisesinelectronunit.Thereadoutnoiseises-
(Table 2). We can also take the uncertainty values assessed in timatedfromtheouterpartof theimages(see inFig. 5 thetop
the previous section, defined as being the relative standard de- panels).Weestimatethespecklenoisefromthetotalvarianceof
viationon10%ofbestframesinthecubes.Wefoundarelative thecubesas
extendedemissionaroundRSPupcontributingto 38±17%of
σ2(r)=σ2(r)−σ2 (r)−σ2 ,
thestar’sfluxintheNB 1.64bandand24±11%inIB 2.18. s ph RON
In the bottom panels of Fig. 5 we present the relative im-
portanceofthedifferentnoisesources.Weseethatthedifferent
4. Morphologicalanalysis
variances show a different radial behavior, and we see in par-
Wenowpresentaqualitativeanalysisoftheenvelopemorphol- ticularthatthephotonnoiseofthecoreneverdominates.More
ogy. In this analysis, based on the statistical properties of the precisely,wecandefinetworegimes:
noiseowingtospeckleboiling,wepresentedtheratioσ(F)/Fas
– σ(r) ≃ σ (r) for r . r , in this regime, the speckle noise
afunctionofthefactor(g).Thisparameterisintrinsictospeckle s 1
dominatestheothervariances(σ >3σ )
temporalandspatialstructures,isnotexpectedtodependonaz- s RON
imuth,andisproportionaltotheenvelopeflux.Wemappedthis – σ(r)≃ σ2(r)+σ2 forr&r
s RON 1
factor,andanyazimuthalasymmetryshouldbeattributedtothe q
morphologyoftheenvelope. withr ≃0.22′′inNB 1.64bandandr ≃0.24′′inIB 2.18.The
1 1
fairly weak (∝ λ−12/5) wavelength dependency of the speckle
noisemayexplainthesameorderofmagnitudebetweenthelimit
4.1.Analysismethod
radiusinIB 2.18andNB 1.64.
The principle of the method is to recover the flux of the enve- Indeedthereisanotherregimeintherange06r<r where
1
lopeintheshortexposures.Thedifferencebetweenasimpleun- the Strehlvariationsoccur.The PSF core is mainlyaffectedby
resolved star and a star surrounded by an envelope is that the thesevariations.Assaidpreviously,weareinterestedinthePSF
envelopewillpreventthefluxbetweenthespecklesfromreach- halotodetecttheenvelopebetweenthespeckles,whichiswhy
ingzero. weneglectthisterm.
Statistical descriptions of stellar speckles have
been proposed by many authors (Racineetal. 1999;
4.2.Specklenoiseinvariant
Fitzgerald&Graham 2006), who showed that speckle noise
is not negligible. The distribution of speckle statistics in AO We use an interesting property of the speckle noise, namely
were also studied by Canales&Cagigal (1999). For example, that its variance is proportionalto the squared flux in the cube
Racineetal. (1999) (hereafter Ra99) showed that the speckle (Ra99),F/σisthereforeaninvariant.Moreprecisely:
noise dominates other noise sources within the halo of bright
stars corrected with AO systems. We know that the total σs =g(r,λ,∆t,S,...)F⋆(λ),
variancein the cube shortexposuresafter the basic corrections
wherethefunctiongisafunctiondependingonthewavelength
(i.ecorrectedforbias,badpixels,flat-fieldandrecentered)is
λ, the integration time ∆t, the Strehl ratio S and atmospheric
σ2(r)=σ2(r)+σ2 (r)+σ2 +σ2 (r), (3) parameters.
s ph RON Strehl We checked this property by plotting in Fig. 6 the mean g
function for each star, i.e we evaluated a g function for each
whereσ2 is the varianceofthe speckle noise,σ2 the variance
s ph cubethatwethenaveraged(theerrorbarscorrespondtothestan-
ofthephotonnoise,σ2 thevarianceofthereadoutnoiseand darddeviationbetweenthecubes).We tookcaretoremovethe
RON
σ2 the noise owing to Strehl ratio variations. We neglected fluxcontributionfromthecircumstellarenvelopewiththevalues
Strehl
thevarianceoftheskyphotonnoise,whichisnegligibleinour foundinSect.3.Obviouslythereisnocleardifferencebetween
wavelengthforbrighttargets.BecausetheStrehlratiovariations the two stars except in the core. This can be explained by the
occurinthecoherentcoreandbecauseweareparticularlyinter- Strehlratiovariationsweomittedbeforeinthecore.Wecanas-
estedinthehalopart,weomitthesevariationsintherestofthis sessthesevariationsbyevaluatingtheencircledenergyforeach
section. frame of each calibrating cube (here we take 100% of frames,
We present for one calibrating cube the variance observed withoutselection)andbyusingitasanestimatoroftheStrehlra-
inthetoppanelsofFig.5forbothfilters.We plottedtheradial tioinfunctionoftime.ByfittingaGaussiandistributiontoE
enc
profilecomputedusingmedianringsonallazimuths.Thedashed for each star and both filters, we estimate the relative standard
curvedenotesthevarianceofthereadoutnoise.Wecanestimate deviationforRSPuptobeσ(E )/E ∼ 15%inIB 2.18and
enc enc
some of the componentsin Eq. 3 from our data. For example, ∼ 17%inNB 1.64.ForHD74417wefound∼ 8%and∼ 9%
fromthestackedimagethatweobtainedbysummingthecube, respectivelyat2.18µmand1.64µm.
weobtainthecoreandhaloprofile,aswedidinSect.3.Wethen The total invariance of the g function, i.e for the complete
obtain the photonnoise variance using its Poissonian property, radius,dependsontheStrehlratiovariations.Nevertheless,be-
knowingthatthevarianceisequaltothemean.Wedefineforthe causethesevariationsoccurinthecoherentcore,wecanneglect
6
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
10+8 10+8
2.18µm 1.64µm
cube variance cube variance
cube sum cube sum
RON2 RON2
10+6 10+6
e e
nc nc
a a
ari ari
V V
10+4 10+4
0.0 0.5 1.0 0.0 0.5 1.0
RON RON
σ σ
s s
σ σ
ph ph
10+6 10+6
σ2 10+4 σ2 10+4
10+2 10+2
10+0 10+0
0.0 0.5 1.0 0.0 0.5 1.0
r (arcsec) r (arcsec)
Fig.5.Leftpanel:atthetopweplotthevariance(circles)andtheaveragevalue(triangle)ofonedatacubeofHD74417at2.18µm.
Bottom:contributionofthedifferentnoisesources.Rightpanel:thesameforthe1.64µmfilter.Thescaleforallplotsislogarithmic
inordinates.Thedottedcurvesshowthe3σlimitsofthereadoutnoise,whilethegraycurvesdenotethehalofit.
2.18µm 10−2 1.64µm
10−3
g(RS Pup) g(RS Pup)
g(HD 74417) 10−3 g(HD 74417)
10−4
g g
10−4
10−5
10−5
10−6
0.40.0 0.5 1.0 10−06.0 0.5 1.0
0.2 0.2
g g
g/ 0.0 g/ 0.0
∆ ∆
−0.2 −0.2
−0.4 −0.4
0.0 0.5 1.0 0.0 0.5 1.0
r (arcsec) r (arcsec)
Fig.6. Mean g function for each star. The error bars correspond to the RMS. The bottom plot represents the absolute relative
difference.WecanseealargerdifferenceinthecoreowingtotheStrehlvariations.Fromr > r thefunctionsaresimilarata9%
1
levelinNB 1.64andat6%inIB 2.18.
them if we go far away from the central part. For r > r we AllvariablesinEq.4arefunctionsofthepixelcoordinates
1
foundameanabsoluterelativedifference(seeFig.6)of5%in (x,y),butweomitthisnotationforclarity.
IB 2.18and8%inNB 1.64.Soforr > r1 theinvarianceofthe The parameter α can be estimated with the results from
gfunctionisverifiedwithagoodaccuracy. Sect. 3. The fluxratiobetweenthe two finalimagesofRS Pup
We can measure g using our reference star. Assuming it is andHD74417isafunctionofαandF(env)/F⋆:
stable in time, we can define the observable Γ as the average
ofthefluxofagivenpixelinthecube,dividedbythestandard
F(RSPup) F +F 1+F /F
deviation.DefiningFcal = αFRSPup = αF⋆ andmakingthedif- = ⋆ env = env ⋆, (5)
ferencebetweenΓsci andΓcal (wheretheindexsciandcalrefer F(HD74417) αF⋆ α
toRSPupandHD74417respectively)wehave
Bymeasuringthetotalfluxinourfinalimages(Sect.3)and
usingF /F estimatedinSect.3.4wegetα = 1.06 ± 0.13at
env ⋆
F +F αF 1.64µmandα=0.96 ± 0.08at2.18µm.Wealsousedthetotal
ξ =Γsci−Γcal = ⋆ env − ⋆ , (4) fluxvariationsin our10(forRS Pup)and 4(forthe reference)
g2F2 +σ2 g2α2F2 +σ2 averagedimagestoassesstheuncertainties,buttheyturnoutto
⋆ ⋆ RON ⋆ ⋆ RON
q q
7
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
2.18µm 1.64µm
N N
0.5’’ E 0.5’’ E
Fig.7.ξparameter(Eq.6)proportionaltotheenvelopefluxat2.18µm(left)andat1.64µm(right).Thewhitecirclerepresentsthe
r limitofEq.7separatingthetworegimes.Thescaleintensityislogarithmic.
1
be small(< 2%)comparedto F /F uncertainties.As a first aring,whileinthesubtractedimage(0◦−90◦)wecomputean
env ⋆
approximationwecansimplifyEq.4withα∼1toobtain averagevaluein a smallwindow(seeFig. 8).Makingtheratio
ofthesetwoaverages(i.e.theaveragevalueinthesmallwindow
ξ = Fenv . (6) overtheaveragevalueinthering),weexcludeadeparturefrom
symmetrylargerthan7%inNB 1.64and6%inIB 2.18.
g2F2 +σ2
⋆ ⋆ RON
q
ξ is thus directly proportional to the flux of the CSE. We
expect this function to be null if there is no envelope.The de- 2.18µm 2.18µm
nominatorisaradialparameter,thereforeanyspatialasymmetry
inξwillbelinkedtoanasymmetryoftheCSE.
Fromourdifferentdatacubeswecomputedameanξbysub-
tractingtoeachΓ ameanΓ ,i.e:
sci cal
n m
1 1
ξ(x,y)= (Γ −Γ ) and Γ = Γ ,
n sci,j cal cal m cal,i
Xj=1 Xi=1
N N
wherenandmarethenumberofcubeforRSPupandHD74417
0.5’’ E 0.5’’ E
respectively. This process provides us with an average image
proportionaltothefluxoftheCSEineachfilter. 1.64µm 1.64µm
4.3.SymmetryoftheCSE
In Fig. 7 we show the ξ function for the NB 1.64 and IB 2.18
band.Forradiilargerthanr ,weexpectthefunctiontobenullif
1
thereisnoCSE(seeEq.6),whereasweclearlyobtainapositive
image,indicativeofalikelydetectionofaCSE.Thecentralpart
oftheimagewithr<r (whitecircleforr =r )denotesthepart
1 1
wherethepreviouslydefinedfunctiongisnotinvariantbecause N N
ofStrehlratiofluctuations.Itisdifficulttoestimateandtherefore 0.5’’ E 0.5’’ E
doesnotsupplyreliableinformationsclosetothestar.
Fig.8. Estimate of the symmetry level. Top panels: on the left
This method is useful for studying the morphology of the
is plotted the ξ parameter at 2.18µm with a ring of radius
CSE,inparticularwhetherornotithasacentralsymmetry,like
r < r < r = 0.92′′ wherewecomputedameanvalue.Onthe
one expects for a uniform shell structure or a face-on disk; or 1 2
rightwe plotthe difference of the initial (0◦) and 90◦ rotation
anasymmetricalshape,likeinaninclineddisk-likestructurefor
imageswherewecutasmallwindowtoassessameanresidual
instance.Despiteadaptiveopticsartefacts,theimagesinFig. 7
value.Theratiobetweenthemeanresidualvalueandthemean
seem to present a uniform intensity distribution in agreement
valuegiveusalevelofsymmetry.Bottompanels:sameprocess
withashelloraface-ondiskstructure.Moreprecisely,weesti-
at1.64µm with a ringof radiusr < r < r = 1.1′′. Thescale
mated a symmetrylevelusing a residualmap obtainedby sub- 1 2
intensityislogarithmic.
tracting 90 degree rotated versions of themselves from the im-
agespresentedinFig.7.Theresultofthissubtractionisshown
in Fig. 8. In the initial image we assessed the average value in
8
A.Gallenneetal.:SpatiallyextendedemissionaroundRSPuppis
5. Conclusion angularresolutionpartnershipbetweenONERA,ObservatoiredeParis,CNRS,
andUniversityDenisDiderotParis7.ThisworkmadeuseoftheSIMBADand
Wepresentastudyofthecloseenvironment(1000-2000AU)of VIZIERastrophysical database fromCDS,Strasbourg, France andthebiblio-
the Cepheid RS Pup using near-infrared AO-assisted imaging. graphicinformationsfromtheNASAAstrophysicsDataSystem.Dataprocess-
Weusedtwodifferenttechniquesto1)measurethephotometric ing for this work have been done using the Yorick language, which is freely
availableathttp://yorick.sourceforge.net/.
emission due to the CSE and 2) obtain an image proportional
totheCSE througha statisticalanalysisofthelargenumberof
shortexposuresinwhichspecklesarestillvisible. References
Using a simple image modeling method, based on the un-
resolved PSF calibrator, we estimated (Sect. 3) a likely pho- Bates,R.H.T.&Cady,F.M.1980,OpticsCommunications,32,365
Bond,H.E.&Sparks,W.B.2009,A&A,495,371
tometric emission of 38±17% and 24±11% of the star flux
Canales,V.F.&Cagigal,M.P.1999,AppliedOptics,Vol.38,766
in the NB 1.64 and IB 2.18 bands respectively for RS Pup’s Feast,M.W.2008,MNRAS,387,L33
CSE.Apartfromtheactualinteresttobetterunderstandthemass Fitzgerald,M.P.&Graham,J.R.2006,ApJ,637,541
loss in Cepheids, the presence of such an extended emission Girard,J.H.V.,Kasper,M.,Quanz,S.P.,etal.2010,inSocietyofPhoto-Optical
Instrumentation Engineers(SPIE)ConferenceSeries,Vol.7736,Societyof
may have an impact on the distance estimate with the Baade-
Photo-OpticalInstrumentationEngineers(SPIE)ConferenceSeries
Wesselinkmethod.
Havlen,R.J.1972,A&A,16,252
Usinganoriginalstatisticalstudyofourshortexposuredata Kervella,P.,Me´rand,A.,&Gallenne,A.2009a,A&A,498,425
cubes, we qualitatively showed (Sect. 4) that the envelope in Kervella,P.,Me´rand,A.,Perrin,G.,&Coude´DuForesto,V.2006,A&A,448,
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thetwobands(NB 1.64andIB 2.18)hasacentro-symmetrical
Kervella,P.,Me´rand,A.,Szabados,L.,etal.2008,A&A,480,167
shape (excluding a departure from symmetry larger than 7%),
Kervella,P.,Verhoelst,T.,Ridgway,S.T.,etal.2009b,A&A,504,115
suggesting that the CSE is either a face-on disk or a uniform Lenzen,R.,Hartung,M.,Brandner,W.,etal.2003,inSocietyofPhoto-Optical
shell. Instrumentation Engineers (SPIE) Conference Series, Vol. 4841, Society
Our observations were obtained in two filters isolating the of Photo-Optical Instrumentation Engineers (SPIE)Conference Series, ed.
M.Iye&A.F.M.Moorwood,944–952
Brackett 12–4 (1.641µm, NB 1.64 filter) and Brackett γ 7–4
Marengo,M.,Evans,N.R.,Barmby,P.,Bono,G.,&Welch,D.2009,inThe
(2.166µm,IB 2.18filter)recombinationlinesofhydrogen.We EvolvingISMintheMilkyWayandNearbyGalaxies
thereforeproposethatourobservationsshowthepresenceofhy- Masciadri,E.,Brandner,W.,Bouy,H.,etal.2003,A&A,411,157
drogenintheCSEofRSPup.Thestarisnaturallyrelativelyfaint Me´rand,A.,Aufdenberg,J.P.,Kervella,P.,etal.2007,ApJ,664,1093
Me´rand,A.,Kervella,P.,Coude´DuForesto,V.,etal.2006,A&A,453,155
in these spectralregions, because of deep hydrogenabsorption
Nardetto,N.,Groh,J.H.,Kraus,S.,Millour,F.,&Gillet,D.2008,A&A,489,
linesinitsspectrum.Thisappearsasanaturalexplanationtothe
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highrelativecontributionfromtheCSE comparedtothestellar Racine,R.,Walker,G.A.H.,Nadeau,D.,Doyon,R.,&Marois,C.1999,PASP,
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in hydrogen spectral lines instead of in the form of a contin-
Westerlund,B.1961,PASP,73,72
uum.RSPup’sinfraredexcessinthebroadNB 1.64andIB 2.18
bandsislimitedtoafewpercents(Kervellaetal.2009a),which
excludesacontinuumCSEemissionof40%intheNB 1.64filter
and24%intheIB 2.18filterasmeasuredinourNACOimages.
Our discovery is also remarkably consistent with the non-
pulsating hydrogen absorption componentdetected in the visi-
ble Hα line of RS Pup by Nardettoetal. (2008), together with
aphase-dependentcomponentinemission.Theextendedemis-
sionweobserveintheinfraredisthereforeprobablycausedby
thesamehydrogenenvelopeastheonethatcreatestheHαemis-
sionobservedinthevisible.Itshouldalsobementionedthatour
result is consistent with the proposalby Kervellaetal. (2009a)
thattheenvelopeofRSPupismadeoftwodistinctcomponents:
acompactgaseousenvelope,whichisprobablyattheoriginof
theobservedNACOemission,andaverylarge(≈ 2′)andcold
(≈ 45K) dusty envelope (Kervellaetal. 2008) that is mostly
transparent at near-infrared wavelengths owing to the reduced
scattering efficiency. These two envelopesappear to be largely
unrelatedbecauseoftheirverydifferentspatialscales.However,
it should be noted that Marengoetal. (2009) recently detected
largeenvelopesaroundseveralCepheidsusingSpitzer.Thepres-
enceofhydrogenclosetotheCepheidsoftheirsamplecouldbe
investigatedathighangularresolutionusingtheobservingtech-
niquepresentedhere.
Acknowledgements. Wethank Drs.Jean-Baptiste LeBouquin and Guillaume
Montagnier for helpful discussions. We also thank the ESO Paranal staff for
theirworkinvisitormodeatUT4.WereceivedthesupportofPHASE,thehigh
9