High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675

0.45 MB·English

by  Anna M. M. Scaife

Mon.Not.R.Astron.Soc.000,1–5(2009) Printed8January2010 (MNLATEXstylefilev2.2)
High resolution AMI Large Array imaging of spinning dust sources:
m
spatially correlated 8 m emission and evidence of a stellar wind in
L675⋆
0
1
0 AMI Consortium: Anna M. M. Scaife1,2†, David A. Green1, Guy G. Pooley1,
2
Matthew L. Davies1, Thomas M. O. Franzen1, Keith J. B. Grainge1,3,
n
a Michael P. Hobson1, Natasha Hurley-Walker1, Anthony N. Lasenby1,3,
J

8 Malak Olamaie1, John S. Richer1,3, Carmen Rodr´ıguez-Gonza´lvez1,

] Richard D. E. Saunders1,3, Paul F. Scott1, Timothy W. Shimwell1,
A
G David J. Titterington1, Elizabeth M. Waldram1 & Jonathan T. L. Zwart4
. 1AstrophysicsGroup,CavendishLaboratory,JJThomsonAvenue,CambridgeCB30HE
h 2DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland
p 3KavliInstituteforCosmologyCambridge,MadingleyRoad,Cambridge,CB30HA
- 4ColumbiaAstrophysicsLaboratory,ColumbiaUniversity,550West120thStreet,NewYork10027,USA
o
r
t
s
a Accepted—;received—;inoriginalform8January2010
[

2
ABSTRACT
v
We present 25′′ resolution radio images of five Lynds Dark Nebulae (L675, L944, L1103,
1
1 L1111 and L1246) at 16GHz made with the Arcminute Microkelvin Imager (AMI) Large
0 Array.TheseobjectswerepreviouslyobservedwiththeAMISmallArraytohaveanexcess
4 ofemissionatmicrowavefrequenciesrelativetolowerfrequencyradiodata.InL675wefind
. a flat spectrum compact radio counterpart to the 850m m emission seen with SCUBA and
0
suggestthatitiscm-waveemissionfromapreviouslyunknowndeeplyembeddedyoungpro-
1
9 tostar.InthecaseofL1246thecm-waveemissionisspatiallycorrelatedwith8m memission
0 seenwithSpitzer.SincetheMIRemissionispresentonlyinSpitzer band4wesuggestthat
: itarisesfromapopulationofPAH molecules,whichalsogiverisetothecm-waveemission
v
throughspinningdustemission.
i
X
Keywords: Radiationmechanisms:general–ISM:general–ISM:clouds–stars:formation
r
a
1 INTRODUCTION cificcaseofdarkcloudstherecentAMIsample(AMIConsortium:
Scaifeetal.2009;hereinafterPaperI)offourteenLyndsDarkNeb-
Thecomplete characterization of microwave emission fromspin-
ulaefoundanexcessinonlyfive.
ningdustgrainsisakeyquestioninbothastrophysicsandcosmol-
It has been suggested that cm-wave emission from spinning
ogy. It probes a region of the electromagnetic spectrum where a
dust is emitted by a population of ultra-small grains (Draine &
number ofdifferent astrophysical disciplinesoverlap. Itisimpor-
Lazarian1998).Theseultra-smallgrainsarethoughttoexistmainly
tant for CMB observations in order to correctly characterise the
in the form of single polycyclic aromatic hydrocarbon (PAH)
contaminatingforegroundemission;forstarandplanetaryforma-
molecules.PAHmoleculesaregenerallydetectedthroughtheirnar-
tionitisimportantbecauseitpotentiallyprobesaregimeofgrain
rowlineemissionfeaturesintheMIR.Fortheseemissionfeatures
sizesthatisnototherwiseeasilyobservable.
to be observed the PAH molecules must be exposed to a strong
Althoughanumberofobjectshavenowbeenfoundtoexhibit
source of UV flux. Sincethisflux isgenerally absent in the case
anomalous microwave emission, attributed to spinning dust, it is
ofdarkclouds,themicrowaveemissionfromtherotationofPAH
stillunclearwhatdifferentiatesthoseobjectsfromthemanyother
moleculesmaybetheonlywaytostudytheverysmallgrainpop-
seeminglysimilartargetsthatdonot showtheexcess.Inthespe-
ulationintheseobjects.
Itisalsoknownthatradiocontinuumemissionindarkclouds
⋆ Werequestthatanyreferencetothispapercites“AMIConsortium:Scaife mayarisefromionizedgasassociatedwithastellaroutflow.When
etal.2010” aluminousstarispresentthisariseseitherastheresultofacom-
† E-mail:[email protected] pactHIIregionoranionizedstellarwind.Inthecaseofveryyoung
2 Scaifeetal.
lowluminositystarsradiocontinuumemissionmaybealsobede-
Table1.AMILALynds DarkNebulae. Column [1]Nameofcloud, [2]
tected.Inthisinstanceitisgenerallyattributedtothepresenceof RightAscension,[3]Declination, [4]AMILAsynthesizedbeamFWHM
apartiallyionized(0.026xe60.35;Bacciotti&Eislo¨ffel1999) majoraxis,[5]AMILAsynthesizedbeamFWHNminoraxis,and[6]r.m.s.
stellar wind (Wright & Barlow 1975; Panagia & Felli 1975), or noisefluctuationsonthecombinedchannelmap.
possiblyaneutralwindwhichhasbeenshock-ionizedfurtherfrom
thecentralsource byimpactingonadenseobstacle(Curiel etal. Name RA Dec D qmaj D qmin s rms
1989). (J2000) (J2000) (arcsec) (arcsec) (mJy)
bm
In this paper we present follow-up observations of the five
L675 192352.6 110739 49.9 27.4 35
AMISmallArray(SA)spinningdustdetections(PaperI)athigher
L944 211740.8 431808 36.5 31.2 31
resolutionwiththeAMILargeArray(LA)overthesamefrequency
L1103 214210.2 564344 32.0 26.3 25
range.Allco-ordinatesinthispaperareJ2000.0.
L1111 214027.1 574810 39.4 30.8 29
L1246 232530.1 633830 31.2 26.9 25
2 OBSERVATIONS
AMI comprises two synthesis arrays, one of ten 3.7m antennas
(SA) and one of eight 13m antennas (LA), both sited at Lord’s
Bridge, Cambridge (AMI Consortium: Zwart et al. 2008). The
telescope observes in the band 13.5–17.9GHz from which eight
0.75GHzbandwidthchannelsaresynthesized.Inpractice,thetwo
lowestfrequencychannels(1&2)arenotgenerallyusedduetoa
lowerresponseinthisfrequencyrangeandinterferencefromgeo- Figure2.L675SourceA:datapointsarefluxdensitiesfromAMILAchan-
stationarysatellites. nels3–8.Thebest-fitspectralindexofa =0.10±0.36isshownasadashed
Observations of five Lynds dark nebulae selected from the line.
originalAMISAsampleweremadein2009February–Marchus-
ingtheAMILA.Theco-ordinatesofthesefieldsarelistedinTa-
3 RESULTS
ble1alongwiththesizeoftheAMILAsynthesizedbeamtowards
eachobjectandther.m.s.noisemeasuredoutsidetheprimarybeam L675: The AMI LA observations of L675 show two obvious re-
ontheCLEANedmaps.WenotethattheAMILAobservationof gionsof compact emission, seeFig.1. Thefirstof these, slightly
L1246 is towards the north–east of this cloud where anomalous offset from the pointing centre, is coincident with both the peak
emissionwasdetectedbytheAMISAanddoesnotcoverthesame of the AMI SA emission and also the compact emission seen at
areaastheoriginalSCUBAobservation. 850m mbytheSCUBAinstrument(Visseretal.2001;2002).We
Data reduction was performed using the local software tool denote this source “A” (19h23m50.s5, +11◦07′44′′). The second,
REDUCE, see Paper I for more details. Flux calibration was per- justoutsidetheLAprimarybeamFWHMtothenorth-east,isco-
formedusingshortobservationsof3C286nearthebeginningand incident withtheprobableextragalacticpoint sourceidentifiedas
end of each run. We assumed I+Q flux densities for this source “B”(19h24m02.s4,+11◦10′54′′)intheoriginalAMISAobserva-
intheAMILAchannelsconsistentwiththefrequencydependent tions(PaperI).
model of Baarset al.(1977), ≃3.3Jy at16GHz. AsBaarset al. SourceAiscompletelyunresolvedbytheAMILAandshows
measureIandAMILAmeasuresI+Q,thesefluxdensitiesinclude aflatspectrumacrosstheAMIband,a 17.9=0.10±0.36,consis-
14.3
correctionsforthepolarisationofthecalibratorsourcederivedby tentwithfree–freeemission,seeFig.2.Thisspectralindexdiffers
interpolatingfromVLAmeasurements.Acorrectionisalsomade considerablyfromthatmeasuredbyAMISA.Thisisbecausethe
for the changing intervening air mass over the observation. The LAisnotsensitivetothelargescaleemissionseenwiththeSA.In-
phasewascalibratedusinginterleavedobservations ofcalibrators deeditseemslikelythattheemissionseenbythetwoarraysarises
selectedfromtheJodrellBankVLASurvey(JVAS;Patnaiketal. fromcompletelydifferentsources.
1992).Aftercalibration,thephaseisgenerallystableto5◦forchan- L944: The original AMI SA observations of L944 revealed
nels4–7,and10◦forchannels3and8.TheFWHMoftheprimary acompactregionofemissiontothenorthofthecloud,coincident
beamoftheAMILAis≈6′at16GHz. withonesideoftheprotostellaroutflow.AMILAobservations,see
Reduced data were imaged using the AIPS data package. Fig.1,revealthisemissionarisesnotfromapoint-likeobjectbut
CLEANdeconvolutionwasperformedusingthetaskIMAGRwhich ratherfromadiffuseregionofemission,thepeakofwhichoccurs
appliesadifferentialprimarybeamcorrectiontotheindividualfre- at21h17m42.s6,+43◦20′08′′.Weestimatethefluxspectrumbyin-
quency channels to produce the combined frequency image. De- tegratingthefluxdensityfromtheprimarybeamcorrectedchannel
convolved maps weremade fromboth thecombined channel set, mapswithinatwoarcminuteradiusoftheLApointingcentre.This
seeFig.1,andforindividual channels. Thebroadspectralcover- showsasteeplyrisingspectrumwitha 17.9=−2.1±0.5.Thisis
14.3
ageofAMIallowsarepresentationofthespectrumbetween14.3 consistentwiththatfoundfromtheAMISAdata,howeverthiscor-
and 17.9GHz to be made independently of other telescopes and respondenceisnotmeaningfulasthelowsignaltonoiseintheSA
inwhat followsweusetheconvention: S(cid:181) n −a ,whereSisflux dataprecludesapreciseestimate.Thefluxdensityfoundtowards
density, n isfrequency anda isthespectralindex. Allerrorsare thisregionintheLAmapisonlymarginallylowerthanthatfound
quotedto1s . from the comparatively coarser resolution SA map. This implies
AMILA spinningdustobservations 3
L675 L944
L1103 L1111
L1246
Figure1.AMILAcombinedchanneldataisshownasgreyscaleinunitsofmJy/beam,greyconrtoursat−6,−3,±2,±1s andblackcontoursat3,6,12,24s
etc.SCUBA850m mdataisshownasredcontourswithlevelsasinVisseretal.(2001;2002)fortheallcloudsexceptL1246.TheAMILAobservationof
L1246doesnotcovertheregionobservedbySCUBA.AMISAdataisshownasbluecontourswithlevelsasinPaperI.TheAMILAprimarybeamFWHM
isshownasacircleandthesynthesizedbeamasafilledellipseinthebottomleftcorner.
4 Scaifeetal.
Table2.IntegratedfluxdensitiesinmJyforAMILAobservationsofL675,L944andL1246.Errorsarecalculatedass =p(0.05S)2+s r2ms,wheres rmsis
ther.m.s.noiseintheindividualchannelmap.
Freq.(GHz)
Name 14.3 15.0 15.7 16.4 17.2 17.9 a
L675 2.74 ±0.34 2.24 ±0.12 2.26 ±0.12 2.12 ±0.11 2.42 ±0.14 2.22 ±0.12 +0.10±0.36
L944 - 2.03 ±0.11 2.40 ±0.13 2.89 ±0.15 2.86 ±0.15 - −2.11±0.49
L1246 - 0.62 ±0.18 0.55 ±0.03 0.62 ±0.03 0.58 ±0.04 - −0.40±0.82
thattheemissioncomesnotfromonesmoothextendedregionthat
ispartiallyresolvedoutbytheLAbaselines,butfromacollection
ofsmallerfragmentsorfilaments.Thesefragmentsareunresolved
byeitherarray,althoughthegranularitybecomesmoreevidentin
thehigherfrequencychannelsoftheLA.Theamountoffluxlost
inchannels5to8relativetochannel4issignificantlysmallerthan
wouldbeexpectedfromaGaussiansourceofsimilardimensions.
L1103 and L1111: AMI LA observations of L1103 and
L1111donotshowanydistinctregionsofcompactcm-waveemis-
sion.Thediffusepatchesoflowlevelemissionpresentwithinthe
primary beam towards both sources are indicative of larger scale
structureswhichhavebeenresolvedoutbythesynthesizedbeam.
Wecanprovideanestimateofthefluxdensityseentowardsthese
objectswiththeLAbyfittingandremovingatiltedplanebaselevel
attheprimarybeamFWHM.Fromthecombinedchanneldatathis
givesS16=5.1±0.6mJyandS16=2.4±0.3mJyforL1103and
L1111,respectively.Thesevaluesindicatethatthefluxlossiscon- Figure3.L675:AMILAcombinedchanneldataisshownaswhitecontours
siderable: approximately 45% and 96%. The sensitivity of these at3,6,12s etc.SpitzerBand4dataisshownasgreyscaleinMJy/sr,and
issaturatedatbothendstoemphasisethestructurepresent.AMISAdatais
LA observations is much greater than those of the SA and it is
shownasbluecontoursasinFig.1.
possiblethatthepatchyemissioninthesefieldscorrespondstoen-
hancementsintheextendedemissionwhicharebelowthedetection
thresholdintheSAdata.
L1246: AMI SA observations towards L1246 did not show
anyexcessemissioncoincidentwiththeSCUBAidentificationof
thedarkcloud,butdidrevealaregionofemission≈2′tothenorth–
east of the cloud, in a region not covered by the SCUBA map,
whichhadnocounterpartinthelowerfrequencyobservations.AMI
LAobservationsofthisNEregionshowanarcofemission(peak:
23h25m20.s4,+63◦38′40′′),seeFig.1.Weassessthespectralbe-
haviour of this object in two ways. Firstly, we estimate the flux
densityofthearcitself.Wefitandremoveatiltedplanebaselevel
withinanirregularpolygondrawnaroundtheobjectandintegrate
the remaining flux. Secondly, we fit a tilted plane baselevel to a
circleattheprimarybeamFWHMandintegrateallthefluxabove
thisbaselevelwithinthatradius.Bothmethodsgiveconsistentre-
sults, as might be expected since the primary beam is relatively
empty otherwise. From the first method we find a spectral index
a 1157..09=−0.40±0.87,andfromtheseconda 1157..09=−0.47±0.82. Figure4.L1246:AMILAcombinedchanneldataisshownaswhitecon-
toursat1,2,4,8s etc.SpitzerBand4dataisshownasgreyscaleinMJy/sr,
saturatedatbothendsofthescaletoemphasisethestructurepresent.The
AMILAprimarybeamisshownasacircleandthesynthesizedbeamasa
4 DISCUSSIONANDCONCLUSIONS filledellipseinthebottomleftcorner.
L675andL1246havearchivalSpitzerIRACdata,whichshowsin
bothcasesasignificantamountofemissioninBand4(6.4–9.4m m) InBand4itispresentasanarc,coincidentwiththatseenat16GHz
andverylittleintheotherthree(3.2–3.9,4.0–5.0and4.9–6.4m m, intheAMILAdata.
respectively).InthecaseofL675thisemissionispresentonavery Spitzer Band 4 contains two of the PAH emission lines, in-
largescale,seeFig.3.Theemissionseenat16GHzwiththeAMI cluding the strongest (7.7m m). Of the three other Spitzer bands
SAappearsonasimilarscale,howeverthesmallfieldofviewofthe only Band 1 contains an emission line (3.3m m) and for ionized
Spitzer data precludes amore detailed comparison. L1246 shows PAHsthislineisexpectedtobesignificantlyweaker.Itisprobable
anarcofemissionat16GHzwhichisalsoevidentinSpitzerIRAC therefore that the MIR correlated cm-wave data seen in the AMI
Band4,seeFig.4.ThisemissionisagainnotpresentinBands1–3. maps isaconsequence of spinning dust emission froma popula-
AMILA spinningdustobservations 5
tionofionizedPAHmolecules.NeutralPAHmoleculesdonotin Towards L675 wehaveobserved flatspectrum compact cm-
generalpossessapermanentdipolemomentandarethereforenot waveemissioncoincidentwiththeSCUBA850m memissionfrom
expectedtohaverotationalemission(Tielens2008).Thisemission, the same region. These characteristics suggest that this source is
themechanismofwhichisdescribedindetailbyDraine&Lazar- associatedwithastellarwindfromadeeplyembeddedyoungpro-
ian(1998),arisesfromtheintrinsicdipolemomentsofsmalldust tostar.
grains,mostlikelytobePAHmolecules,whichemitpowerwhen We detect extended cm-wave emission to the North of the
theyrotate.Thisrotationhasavarietyofcontributingfactors,the L944SMM-1protostarwhichdisplaysspectralbehaviour consis-
relativeimportanceofwhichvarieswithgrainenvironment.How- tentwitheitherspinningdust,oralternativelyacollectionofultra-
ever,inthemajorityofcasesexcitationthroughcollisionwithions compactHIIregions.
ispredominant. L1246showsanarcofcm-waveemissionwhichiscoincident
InthecaseofL675A,wemustconsiderthepossibilitythatwe with emission seen in Spitzer Band 4. We suggest that this is an
areobserving acoincidental extragalacticradiosource. Usingthe exampleofemissionfromapopulationofPAHmolecules,seenin
extended9Csurvey15GHzsourcecounts(Waldrametal.2009), emissionlinesintheSpitzer data,andemissionasaconsequence
wheren(S)=51(S/Jy)−2.15Jy−1sr−1,theprobabilitythatasource ofrapidrotationofthemoleculesinthecm-wavedata.
withfluxdensitygreaterthan2mJylieswithintheFWHMofthe
AMILAprimarybeamis0.12,andonly0.01withintheSCUBA
field.ItislikelythereforethattheradiosourceL675Aisassociated 5 ACKNOWLEDGEMENTS
withtheSCUBAcore.
Wethank the staff of the Lord’sBridge Observatory for their in-
Afurtherquestioniswhetherthecm-waveemissionmightbe
valuable assistance in the commissioning and operation of AMI.
explainedbythermal(Planckian)dustemission.Asinglegreybody
spectrumwithadusttemperature,T ≈27K,mightbeusedtoex- AMIissupportedbyCambridgeUniversityandtheSTFC.NH-W,
d
plaintheLAfluxdensity,howeveritwouldrequireab of0.6.Such CR-G, TWS,TMOF, MO and MLD acknowledge the support of
PPARC/STFCstudentships.Thisworkisbasedinpartonarchival
avaluewould beunusual evenfor objectsknown topossess flat-
dataobtainedwiththeSpitzerSpaceTelescope,whichisoperated
teneddusttails,suchasprotoplanetary disks.Thissimplefitalso
bythe JetPropulsion Laboratory, CaliforniaInstituteof Technol-
neglectsthefluxlostbytheAMILAbaselinedistribution.SAob-
ogyunder acontractwithNASA.Supportforthisworkwaspro-
servationshavealreadyshownthissourcetopossessasignificant
videdbyanawardissuedbyJPL/Caltech.
amountofextendedemissionwhichwouldmakethisscenarioeven
moreunlikely.
The presence of a neutral or partially ionized wind from an
outflowsourcethathasbeenshockedthroughencounteringadense REFERENCES
obstacle (Torrelles et al. 1985; Rodr´ıguez et al. 1986) is used to
AMI Consortium: Hurley-Walker N., et al., 2009, MNRAS, 396,
understand the spectral indices seen towards exciting sources in
365
theradioregime(Curieletal.1990;Cabrit&Bertout1992).This
AMIConsortium:ScaifeA.M.M.,etal.,2009,arXiv:0908.1655
model allows a spectral index range of 0.1 (optically thin) to -2
AMIConsortium:ScaifeA.M.M.,etal.,2008,MNRAS,385,809
(optically thick), which explains results which deviate from the
AMIConsortium:ZwartJ.T.L.,etal.,2008,MNRAS,391,1545
value of a =−0.6 required by a spherically symmetric ionized
BaarsJ.W.M.,GenzelR.,Pauliny-TothI.I.K.,WitzelA.,1977,
wind(Wright &Barlow 1975; Panagia &Felli1975). Usingthis
A&A,61,99
model as described in Curiel et al. (1989; 1990) the radio emis-
BacciottiF.,Eislo¨ffelJ.,1999,A&A,342,717
sionisexpectedtobeopticallythin(t =0.1),consistentwiththe
CabritS.,BertoutC.,1992,A&A,261,274
spectralindexseenacrosstheAMIband. Assumingadistanceof
ClemensD.P.,YunJ.L.,HeyerM.H.,1991,ApJS,75,877
300pcandastellarwindwithawindspeedof200kms−1,wecan
CurielS.,RaymondJ.C.,MoranJ.M.,RodriguezL.F.,CantoJ.,
calculatethattheAMIfluxdensitiestowardsL675Aareconsistent
1990,ApJ,365,L85
withamasslossof3.5×10−7M⊙yr−1.Amasslosssuchasthis CurielS.,RodriguezL.F.,BohigasJ.,RothM.,CantoJ.,Torrelles
impliesamechanicalluminosityfromthewindofLmech≈1.1L⊙, J.M.,1989,ApL&C,27,299
comparabletothevaluesfoundbyCurieletalforL1448.
DraineB.T.,LazarianA.,1998,ApJ,508,157
ThenatureoftheemissionseentowardsL944withtheAMI
LadaC.J.,1985,ARA&A,23,267
LA is uncertain. The spectral index of this emission is consis-
PanagiaN.,FelliM.,1975,A&A,39,1
tentwithspinningdustemissionoralternativelytheopticallythick
Patnaik A. R., Browne I. W. A., Wilkinson P. N., Wrobel J. M.,
componentoffree–freespectrum.Suchafree–freespectrummight
1992,MNRAS,254,655
be exhibited at 16GHzby ultra-compact HII regions. However a TorrellesJ.M.,HoP.T.P.,RodriguezL.F.,CantoJ.,1985,ApJ,
turn-overfrequency above16GHzwouldhaveanextremelyhigh
288,595
massandshouldthereforebeobviousinsub-mmobservations.This
RodriguezL.F.,CantoJ.,TorrellesJ.M.,HoP.T.P.,1986,ApJ,
needs tobe confirmed by either higher radio frequency measure-
301,L25
mentsinordertomeasuretheopticallythinregionofthespectrum
VisserA.E.,RicherJ.S.,ChandlerC.J.,2001,MNRAS,323,257
andtheturn-over,orsub-mmmeasurementstoplaceconstraintson
VisserA.E.,RicherJ.S.,ChandlerC.J.,2002,AJ,124,2756
themassofsucharegion.
WaldramE.M.,PooleyG.G.,DaviesM.L.,GraingeK.J.B.,Scott
Inconclusion,wehaveusedtheAMILAtoobserveasample P.F.,2009,arXiv:0908.0066
offiveLyndsDarkNebulaeselectedascandidatesforspinningdust WrightA.E.,BarlowM.J.,1975,MNRAS,170,41
emissionfromtheAMISAsampleofLyndsDarkNebulae(Paper
I).Towardstwooftheseclouds(L1103andL1111)wedetectonly
patchy diffuse emission characteristic of the presence of a larger ThispaperhasbeentypesetfromaTEX/LATEXfilepreparedbythe
author.
structurewhichhasbeenmostlyresolvedout.