Radio Monitoring of Protoplanetary Discs

0.66 MB·

by  C. Ubach

MNRAS000,1–10(2015) Preprint10January2017 CompiledusingMNRASLATEXstylefilev3.0
Radio Monitoring of Protoplanetary Discs
C. Ubach1,2⋆, S. T. Maddison2, C. M. Wright3, D. J. Wilner4, D. J. P. Lommen5,
6
B. Koribalski
1NationalRadioAstronomyObservatory,520EdgemontRoad,CharlottesvilleVA22903-4608,USA
2CentreforAstrophysics&Supercomputing,SwinburneUniversityofTechnology,H30,P.O.Box218,Hawthorn,VIC3122,Australia
3SchoolofPhysical,EnvironmentalandMathematicalSciences,UNSW@ADFA,CanberraACT2600,Australia
4Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,02138Cambridge,MA,USA
7 5HwaChongInstitution,661BukitTimahRoad,Singapore269734
1 6CSIROAstronomy&SpaceScience,AustraliaTelescopeNationalFacility(ATNF),POBox76,EppingNSW1710,Australia
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a 10January2017
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ABSTRACT
]
P
Protoplanetarydisc systemsobservedat radiowavelengthsoften show excessemission
E
abovethatexpectedfroma simple extrapolationof thermaldustemission observedat short
.
h millimetre wavelengths. Monitoring the emission at radio wavelengths can be used to help
p disentanglethephysicalmechanismsresponsibleforthisexcess,includingfree-freeemission
- froma wind or jet, and chromosphericemission associated with stellar activity. We present
o
r new results from a radio monitoring survey conducted with Australia Telescope Compact
t Arrayoverthecourseofseveralyearswithobservationintervalsspanningdays,monthsand
s
a years,wherethefluxvariabilityof11TTauristarsintheChamaeleonandLupusstarforming
[ regionswasmeasuredat7and15mmand3and6cm.Resultsshowthatformostsources are

variabletosomedegreeat7mm,indicatingthepresenceofemissionmechanismsotherthan
1
thermal dust in some sources. Additionally, evidence of grain growth to cm-sized pebbles
v
9 wasfoundforsomesourcesthatalsohavesignsofvariablefluxat7mm.We concludethat
4 multiple processes contributing to the emission are common in T Tauri stars at 7 mm and
0 beyond, and that a detection at a single epoch at radio wavelengths should not be used to
2 determineallprocessescontributingtotheemission.
0
. Key words: protoplanetarydiscs, star: variables: T Tauri, radio continuum:planetary sys-
1
tems,radiationmechanisms:thermal,non-thermal
0
7
1
:
v
1 INTRODUCTION wavelengths (Rodmannetal. 2006; Lommenetal. 2009, 2010;
i
X Ubachetal. 2012). This “break" has been attributed to emission
Signatures of cm-sized pebbles have been detected in protoplan-
r mechanismsrelatedtoionizedplasma,addingtotheemissionfrom
a etary discs by extending the relationship between the dust opac- thermaldustinthesediscs.Thisexcessemissionatlongmillimetre
ity index, β, and the 1 to 3 mm spectral slope, α, to longer cen-
wavelengthscouldbecausedbyeitherthermalfree-freeemission
timetre wavelengths (e.g. Testietal. 2003; Rodmannetal. 2006;
from an ionised wind or by non-thermal processes such as chro-
Wilneretal.2005;Lommenetal.2009).Theresultsfromthisre-
mosphericemissionfromtheyoungstar,oracombinationofboth.
lationship(β ∼α−2)canindicategraingrowthupto3λ(where
At7mm,boththermaldustemissionandthermalfree-freeemis-
λistheobservedwavelength)whenβ <1,andlimitedtonograin
sionfromanionisedwindhavebeendetectedinyoungstellarob-
growth(ISMsizedgrains)forβ>1,aslongasthegrainsize≪λ
jects(e.g.Rodmannetal.2006;Lommenetal.2009),whilevari-
(Draine2006).
ableemissionat3and6cmhasbeenusedtodemonstrateacombi-
Thisapproachassumesthatthelongwavelengthemissionfol-
nationofallthreeemissionmechanisms(e.g.Lommenetal.2009).
lowsthe1to3mmemission, whichisdominatedbythermaldust
Temporal monitoring of the disc emission has been used to
emissionandisconstantovertime,whichisnotalwaysthecaseat
disentanglethesethreeemissionmechanisms(Lommenetal.2009;
longerwavelengths(e.g.Lommenetal.2010;Ubachetal.2012).
Ubachetal. 2012). Flux originating from thermal dust emission
A dozen protoplanetary discs show a “break" in the spec-
is generally assumed to be constant over time, whereas the flux
tral slope near 7 mm, with a shallower spectral slope at longer
from thermal free-free and non-thermal emission may be tem-
porally variable. Thermal free-free emission can vary by a fac-
⋆ E-mail:[email protected] tor of ∼ 20–40 per cent over a timescale measured in years
(cid:13)c 2015TheAuthors
2 Ubachet al.
(Skinner&Brown 1994; González&Cantó 2002; Smithetal. 3 RESULTSANDDISCUSSION
2003; Lommenetal. 2009), while non-thermal emission mecha-
A summary of the continuum fluxes for the combined frequency
nisms may vary by a factor of 2 or more on timescales of hours
pairsarepresentedinTable2,complementedwithfluxesobtained
todays(Kuijpers&vanderHulst1985;Phillipsetal.1991,1993;
fromliterature.Thedetectionratesforthesourcesobservedinthis
Smithetal.2003;Skinner&Brown1994;Chiangetal.1996).
studyareasfollows: 9/11sourcesweredetectedat7mm,3/5at
HerewepresentcontinuumfluxmonitoringofTTauristarsat
15mm4,and0/3sourcesatboth3cmand6cm.A3σupperlimit
7and15mmand3and6cmovertimescalesofdays,monthsand
isprovidedfornon-detections.
years. We analyse the data to determine the various contributing
Wepresent inFigure1boththespectrum, orradiofluxasa
emission mechanisms present in the discs. We further determine
functionofwavelength(left),andthe7mmfluxversustime(right)
whether it is common for protoplanetary discs to show evidence
forthe11sourcesinoursurvey. Detectionsaredenotedbyastar
bothofgraingrowthuptocm-sizedpebblesandofmultipleemis-
symbolandupperlimitsbyanarrow.Errorbarsincludetheuncer-
sionmechanisms.
taintyinthefluxfit,aswellastheprimaryfluxcalibrationuncer-
taintiesof20percentfor1.2mm,30percentfor3mm,and10per
centfor7mm,15mm,3cmand6cm.Inthespectrumplots,the
redsolidlinerepresentstheestimatedemissionduetothermaldust,
2 OBSERVATIONSANDDATAREDUCTION
which was fit using the least squares method between the 1 mm
Thissurvey targeted11T TauristarsintheChamaeleon and Lu- and3mmfluxesandextrapolatingthisspectralslope,α,tolonger
pusstarformingregions–seeTable1.Thesourceswereselected wavelengths. Fluxesat1and3mmweretaken fromUbachetal.
to overlap with the Lommenetal. (2009, 2010) and Ubachetal. (2012) and references therein. The estimated 7 mm thermal dust
(2012)samples,maximisingthenumberofsourcesandincreasing component, F7mm(therm), is denoted by a green diamond. The
themonitoringbaselinetoyears–seeTablesA1andA2. greyshaded arearepresentstheregionofuncertaintyinthespec-
ContinuumobservationswereconductedwithAustraliaTele- tralslopeusingstandardquadraticerrorpropagation. Ifwetakea
scope Compact Array (ATCA) during the winter 2012 season. ‘slice’throughtheradiospectrumat7mm,andploteachobserved
Three compact hybrid array configurations were used, providing 44GHzfluxagainsttime(aspresentedinTable2),thisgivesusthe
synthesizedbeamsbetween5to15arcsecat7mm,corresponding temporalmonitoringplotsontheright.Herethegreyregionisthe
to∼800−2400auassumingadistanceof160pctothesources. sameasinthespectrumplot,andrepresentstheuncertaintyofthe
ThetwoCABB1 intermediatefrequency bands weredivided into extrapolation of the 7 mm thermal flux estimate, F7mm(therm),
2048channelsof1MHzwidth. whichisnowrepresentedbyagreenline.Wewillusebothofthese
Alltargetswereobservedwithfrequencypairscentredat43+ plotstohelpunderstandtheemissionprocessesineachsystem.
45GHz(7mmband),fivetargets2at17+19GHz(15mmband)
andthreetargetsat5.5+9.9GHz(6and3cmbands,respectively).
Themmfrequencybandssetupprovideda4GHzwidebandwith 3.1 Temporalmonitoring
an expected RMS of 0.1 and 0.03 mJy for 30 minutes on source
for the 7 and 15 mm bands, respectively. The 3 and 6 cm bands The aim of our temporal monitoring campaign is to try and dis-
were2GHzwidewithanexpectedRMSof0.027and0.034mJy entanglethedifferentphysical processescontributingtotheradio
for30minutesonsource,respectively.Intotalthissurveycovered emissioninthissample of TTauri stars. Whileanumber of mil-
frequenciesrangingfrom4.5to46GHz. limetreandcentimetresurveysofYSOshavebeenconducted(e.g.
QSOB1057-79andQSOB1600-44wereusedasgaincalibra- Henningetal. 1993; Andrews&Williams2007), few monitoring
surveyshavebeencarriedoutduetotheirintenseresourcesrequire-
torsforChamaeleonandLupus,respectively.Foralltheobserva-
tionstheabsolutefluxcalibrationwasperformedusingPKS1934- ment. Given that YSOs are inherently variable (e.g. Herbstetal.
638. Weather conditions were good throughout the observations. 1994),fluxmonitoringontimescalesofaday,weeksandyearsare
requiredtounderstandthecontributingprocessestotheiremission.
Due to poor u-v coverage at the longer baselines when using the
compact hybrid arrays, and given that fluxes rather than images Herewecomparefluxesobtainedfromafewdaysapartto100sof
weretherequireddataproducts,antenna6wasnotincludedinthe daysapart,aspresentedinTable2.
dataprocessingandanalysis. Fromourpreviousobservations(Ubachetal.2012),oursam-
Thedatacalibration followedthestandard CABBprocedure ple could be divided into two distinct groups: (i) those with a
described in theATCAuser guide3 for all wavelengths using the constant spectral slope from 1-7 mm (CS Cha, Glass I, SZ Cha,
softwarepackageMIRIADversion1.5(Saultetal.1995).Inorder WWCha,MYLup,Sz111,GQLup),and(ii)thosewitha“break”
in the mm spectral slope at 7 mm (CR Cha, DI Cha, T Cha and
todeterminethesourcefluxes,weusedapointsourcefit.Toensure
Sz 32). This “break” is an indication that other forms of emis-
thatthiswasappropriate,wecheckedthevisibilityamplitudeswere
constantoveru-vdistanceforall11sourcesateachepoch. sionat7mmarepresent inadditiontothermaldust emission. In
Ubachetal. (2012) a least squares fit was used to determine the
For this analysis, the data from each day at each frequency
wascalibratedseparatelyandthenthefrequencypairswerecom- spectralslopebetween1and3mm,α,andextrapolatedto7mm
bined.Theflux(fromthepointsourcefit)andRMSvaluesforall to estimate the thermal dust component F7mm(therm). Table 3
presents the estimated thermal dust emission at 7 mm, the maxi-
observationswerethenextractedfromthevisibilitiesusingUVFIT
andUVRMS. mumobserved 7mmflux,F7mm(max),andthefractional7mm
excess,givenby(F7mm(max)−F7mm(therm))/F7mm(therm),
forthissampleof11protoplanetarydiscs.
1 CompactArrayBroadbandBackend(Wilsonetal.2011).
2 Onlytwoofthesourceshaveprevious15mmobservations.
3 http://www.narrabri.atnf.csiro.au/observing/users_guide/ 4 SeeTableB1forresultsofsourcesobservedat15mmwithnopreviously
html/atug.htmlForspecificdatareductionprocedureseeUbach(2014). reported15mmfluxes.
MNRAS000,1–10(2015)
RadioMonitoringofProtoplanetaryDiscs 3
Table1.Listofthe11sourcesobservedwithATCAinthissurvey.
Source RA DEC Cloud Distances Teff Wavelengths
(J2000) (J2000) (pc) (K) (mm)
Chamaeleon
CRCha 105906.9 -770139.7 ChaI 160 4900 7
CSChaΦ 110224.9 -773335.9 ChaI 160 4205 7,15
DIChaΦ 110721.6 -773812.0 ChaI 160 5860 7
TCha 115713.6 -792131.7 Isolated 100 5600 7,30,60
GlassIΦ 110815.1 -773359.0 ChaI 160 5630 7
SZCha 105816.7 -771717.1 ChaI 160 5250 7,15
Sz32 110953.4 -763425.5 ChaI 160 4350 7,30,60
WWCha 111000.1 -763457.9 ChaI 160 4350 7,30,60
Lupus
Sz111 160854.7 -393743.1 Lupus4 200 3573 7,15
MYLup 160044.6 -415529.6 Lupus3 165 5248 7,15
GQLup 154912.1 -353903.9 Lupus1 156 4300 7,15
Distancesandeffectivetemperaturestakenfrom:(1)Whittetetal.(1997),(2)vandenAnckeretal.(1998),(3)Luhman(2004),(4)Hughesetal.(1994),
(5)Comerón(2008),(6)Neuhäuseretal.(2008),(7)Donatietal.(2012).
ΦBinaryseparationsof61′′,4.6′′,2.5′′forCSCha,DIChaandGlassI,respectively(Guentheretal.2007;Ghezetal.1997;Feigelson&Kriss1989).
Usingacombinationofthespectrumandthetemporalmoni- periodareconsistent,thoughthereisvariabilityofafactorof2.4
toringplotsinFigure1,alongwiththedatainTable3,wecanstart injustunder3months.GQLupwasalsoobservedat15mmovera
tounderstandthedegreeandpotentialsourceofvariabilityineach periodofalmost1.3years,andallthreefluxesareconsistentwithin
system5.Ifwestartwiththe(crude)fractional7mmexcess,wecan thefluxfiterrors–seeFigure2.Whilenotexactlysimultaneous,
placethe11sourcesintothreegroups:thosewithhightoextreme thetwo7mmand15mmobservationsthatweremadeabout470
fractionalexcessesabove1.5;thosewithamoderatefractionalex- daysapartgiveconsistent(non-varying)fluxesinbothbands.
cessbetween0.5–1.0;andthosewithasmallfractionalexcessless There have been many observations of WW Cha – see Ta-
than0.5. bleC1 for asummary. In Table 2we present just the44, 18, 9.9
Of the high to extreme 7 mm fractional excess sources, we
and 5.5 GHz observations for the sources in our survey,while as
can seethat CR Cha and DI Chahave excesses of order 2above
it can be seen in Table C1, WW Cha has been observed at vari-
expected thermal dust, while T Cha and Sz 32 have much larger ouswavelengthsinthe7mm,15mmand3&6cmbands.Inthe
excesses of order 4–5. Looking at the radio spectrum of the two
spectrumofWWChainFigure1wepresentallthedatafromTa-
most extreme sources, we can clearly see a break in the spectral
bleC1,withtheredinvertedtrianglesrepresentingdetectionsand
slopeat7mm,excessemissionandfluxvariabilityinTChaand upperlimitsobtainedbyLommenetal.(2009).The7mmmonitor-
Sz32.Their7mmtemporalmonitoringplotsshowfluxeswellin ingdatashowsonlythe44GHzdataforconsistency.Thecm-band
excess of that estimated for thermal dust, i.e.theobserved fluxes
dataclearlyshowsalotofvariability,byfactorsof2–4,andthetwo
areoutsideofthegreyregionsoftheplots.Overthe3yearperiod 7mmfluxestaken3.2yearsapartvarybyafactorof2.3.Withthe
oftheobservations,the7mmfluxesofTChaandSz32variedby radiomonitoringdatatodate,itappearsthatSz111andGQLup
afactorof∼ 0.4and1.4respectively.The3and6cmfluxmon- aredominatedbythermaldustemission,thoughthereisvariability
itoringofbothsourcesalsosuggestsvariablyoverthecourseofa at 7 mm in GQ Lup. WW Cha likely has a mix of thermal dust,
year by 30–50 per cent, and detailed analysis of the 15 mm and thermalfree-freeemissionandnon-thermalprocessescontributing
3+6cmintra-epochfluxesbyUbachetal.(2012)showvariability totheradioemission.
ofafactorof2onthetimescaleoftensofminutes.Thereisalsoa
The sources with low fractional excess include CS Cha and
breakinthespectralslopeat7mmseeninCRChaandDICha,and
GlassI. Once again, none show a break in their spectral slope at
overthe3yearperiodof7mmobservations,bothshowvariablyof
7mm,andtheir7mmfluxesallliewithintheregionofuncertainty
afactor of 0.3and3respectively. Theyhaveonlyasingleobser-
oftheextrapolated7mmthermaldustestimate.GlassIvarybya
vationat15mmandintra-epochanalysisbyUbachetal.(2012)of
factorof2.3and2over3.2years.CSChavariesbyaboutafactor
CSChashows15mmvariabilityofafactorof2ontimescalesof
of3over3months.ThismagnitudeofvariabilityforGlassIand
tensofminutes. Forallfoursourcesitseemslikelythatamixof
CSChasuggeststhatthenon-thermalemissionaswellasthermal
thermaldust,thermalfree-freeemissionandnon-thermalprocesses
dustcontributetotheoverallemission.
contributetotheradioemission.
The sources with moderate fractional excess include Finally,wefindthatjustthreesourcesinoursample,SZCha,
WWCha,andGQLup.Noneofthesesourceshaveabreakintheir Sz111andMYLup,havenoobservedvariabilityat7mmovera
spectralslopeat7mm,andtheir7mmfluxesallliewithinthere- timescaleof100sofdays,suggestingthatthedetectedcontinuum
gionofuncertaintyoftheextrapolated7mmthermaldustemission. flux emission is due to thermal dust emission, whereas all other
Threeofthefour7mmfluxesrecordedforGQLupovera2.2year sourcesexhibitfluxvariabilityonthetimescaleofmonthstoyears
ofatleast30percentanduptoafactorof3at7mmisobserved,
whichprovidesevidenceofthepresenceofexcessemissionfrom
5 Notethatthefractional7mmexcesspresentedinTable3doesnottake mechanismsotherthanthermaldustemission.However,the7mm
intoconsiderationtheerrorsintheestimated7mmthermaldustemission, fluxes for these three sources are also lower than expected from
andsocanonlybeconsideredaverycrudemeasureofanyemissionexcess spectral fit.Thisperhaps isindicativethat thecalculatedαistoo
abovethermaldust. shallow.Thisappearstobesupportedbythelower15mmfluxesin
MNRAS000,1–10(2015)
4 Ubachet al.
Table2.Summaryofthetemporalmonitoringresultsat7and15mmand3and6cmforoursampleof11TTauristars.(1)Sourcename.(2)Juliandateofthe
observation.(3)Timefrompreviousobservationindays.(4)Combinedfrequency.(5)Pointflux(3σupperlimitfornon-detections).(6)RMS.(7)Reference.
SeeTableB1fornew15mmobservationsofsourceswithnopreviouslyreported15mmfluxes.
(1) (2) (3) (4) (5) (6) (7)
Source Julian Timefrom Freq. Flux RMS Reference
date prev.obs.(days) (GHz) (mJy) (mJy/Beam)
CRCha 2454980 — 44 1.7±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 1.3±0.2 0.1 Thiswork
CSCha 2454583 — 44 1.00±0.28 0.129 Lommenetal.(2009)
2454677 94 44 <0.71 0.238 Lommenetal.(2009)
2454678 1 44 1.38±0.26 0.218 Lommenetal.(2009)
2454980 302 44 1.5±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 1.5±0.2 0.1 Thiswork
2456223 81 44 0.6±0.1 0.3 Thiswork
2455753 — 18 <0.3 0.1 Ubachetal.(2012)
2456131 378 18 0.20±0.02 0.03 Thiswork
DICha 2454980 — 44 0.9±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 <0.2 0.1 Thiswork
TCha 2454980 — 44 3.0±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 4.1±0.2 0.1 Thiswork
2455761 — 9.9 <0.3 0.1 Ubachetal.(2012)
2456123 363 9.9 <0.15 0.05 Thiswork
2455761 — 5.5 0.3±0.1 0.1 Ubachetal.(2012)
2456123 363 5.5 <0.2 0.08 Thiswork
GlassI 2454980 — 44 0.6±0.2 0.1 Ubachetal.(2012)
2456143 1163 44 <0.3 0.1 Thiswork
SZCha 2454980 — 44 0.7±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 0.6±0.1 0.1 Thiswork
2456223 81 44 <0.3 0.1 Thiswork
Sz32 2454558 — 44 0.8±0.1 0.2 Lommenetal.(2009)
2454980 422 44 1.2±0.1 0.2 Ubachetal.(2012)
2456143 1163 44 0.5±0.2 0.1 Thiswork
2455761 — 9.9 <0.3 0.1 Ubachetal.(2012)
2456123 363 9.9 <0.15 0.05 Thiswork
2455761 — 5.5 <0.3 0.1 Ubachetal.(2012)
2456123 363 5.5 <0.1 0.04 Thiswork
WWCha 2454980 — 44 9.1±0.35 0.2 Ubachetal.(2012)
2456143 1163 44 3.9±0.2 0.1 Thiswork
2455761 — 9.9 <0.3 0.1 Ubachetal.(2012)
2456123 363 9.9 <0.15 0.05 Thiswork
2455761 — 5.5 <0.45 0.1 Ubachetal.(2012)
2456123 363 5.5 <0.12 0.04 Thiswork
Sz111 2454680 — 44 <0.6 0.2 Lommenetal.(2010)
2454980 300 44 0.5±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 0.6±0.2 0.1 Thiswork
2456223 81 44 0.4±0.1 0.03 Thiswork
MYLup 2454680 — 45 1.3 0.1 Lommenetal.(2010)
2454980 300 44 1.1±0.1 0.1 Ubachetal.(2012)
2456143 1163 44 1.0±0.2 0.1 Thiswork
2456223 81 44 1.2±0.2 0.03 Thiswork
GQLup 2455430 — 44 0.6±0.1 0.04 Ubachetal.(2012)
2455753 323 44 0.6±0.3 0.07 Ubachetal.(2012)
2456144 391 44 1.2±0.1 0.05 Thiswork
2456223 79 44 0.5±0.1 0.04 Thiswork
2455751 — 18 0.07±0.03 0.01 Ubach(2014)
2455761 10 18 0.08±0.04 0.02 Ubach(2014)
2456223 462 18 <0.09 0.03 Thiswork
Sz111andMYLup.Thissurveyhighlightsthesignificanceofflux 3.2 Graingrowth
monitoringovermultipletimescales,foronefluxdetectionwillnot
alwaysbeagoodindicationofapurethermaldustemissionsource Forthissampleof11protoplanetarydiscs,thetwosourceswithno
at7mmandbeyond. detectedfluxvariabilityat7mm,MYLupandSz111,alsohave
α < 3. The four sources with high to extreme 7 mm fractional
excess all have α > 3, while all sources with moderate to low
fractional excess—with the exception of Glass I— have α < 3.
MNRAS000,1–10(2015)
RadioMonitoringofProtoplanetaryDiscs 5
100
CR Cha 2.5 CR Cha
10-1
10-2 2.0
Flux (Jy)111000---543 Flux (mJy)11..05
10-6
0.5
10-7
10-8 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100 3.5
CS Cha CS Cha
10-1 3.0
10-2 2.5
Flux (Jy)1100--43 Flux (mJy)21..05
10-5 1.0
10-6 0.5
10-7 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
DI Cha DI Cha
10-1
10-2 1.5
Flux (Jy)111000---543 Flux (mJy)1.0
10-6 0.5
10-7
10-8 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
T Cha 5 T Cha
10-1
10-2 4
Flux (Jy)111000---543 Flux (mJy)32
10-6
1
10-7
10-8 10-3 10-2 10-1 00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
Figure1.Radiospectrum(left)andtemporalfluxmonitoring(right)foreachsourceinthesurvey.Detectionsaredenotedbyastarandupperlimitsbyan
arrow.Errorsbarsincludethefluxfituncertaintyandprimaryfluxcalibrationuncertainties.Theredsolidlineinthespectrum(left)representsthe1–3mm
spectralslopeα,thereddashedlinetheexpectedαff = −0.6,andthegreyshadedarearepresentstheregionofuncertainlyintheestimatedthermaldust
emissionfromstandarderrorpropagationinthespectralslopecalculation.Thegreendiamond(left)andgreenline(right)correspondtotheestimatedthermal
dustemissionat7mm.Continuedonthenextpage.
MNRAS000,1–10(2015)
6 Ubachet al.
Table3.Summaryof7mmanalysisofthespectralslope,estimated7mmthermaldustemission,maximum7mmemissionobserved,andthefractional7mm
excessabovethermal.TheαvaluesarefromUbachetal.(2012),exceptforWWChawhichwasobtainedfromLommenetal.(2009).Theestimatedthermal
dustisobtainedbyextrapolatingαto44GHz.Thefractional7mmexcessabovethermalistheestimated7mmexcess,(F7mm(max)−F7mm(therm)),
dividedbytheestimatedthermaldust,F7mm(therm).
Source α F7mm(therm) F7mm(max) Fractional
(mJy) (mJy) 7mmexcess
CRCha 3.4 0.6 1.7 1.8
CSCha 2.9 1.2 1.5 0.3
DICha 3.2 0.3 0.9 2.0
TCha 3.1 0.8 4.1 4.1
GlassI 3.1 0.5 0.6 0.2
SZCha 2.9 0.8 0.7 -0.1
Sz32 3.8 0.2 1.2 5.0
WWCha 2.8 5.5 9.1 0.7
Sz111 2.5 1.1 0.6 -0.5
MYLup 2.3 1.8 1.2 -0.3
GQLup 2.2 0.7 1.2 0.7
ApplyingthesimplifiedDraine(2006)relationshipofβ ∼ α−2, hotspotscreatedbyanaccretionshock,variablecircumstellarex-
our results suggest that the majority of the sources have β < 1, tinction, or a combination of these processes (Scholz 2012). Al-
indicativeofgraingrowthuptocmsizes,andthefourmostactive thoughvariabilityabove20percentintheopticalandNIRisrare,
sourceswithclearsignsofexcessemissionabovethermaldusthave ithasbeendocumented. Additional opticalimagesintheRandI
β >1,suggestinglittlegraingrowth.Theseresultsshowthatboth bands and NIR images in the J and K bands have provided evi-
variability and evidence for grain growth can be present for the dencethatSz32isvariableatthesewavelengthsbymorethanthe
same protoplanetary disc, but that grain growthappears inhibited expected 20 per cent (Rodgers-Leeetal. 2014). The cause of the
inthemostactivesources,atleastforthissmallsample. additionalfluxvariabilityisstillunderinvestigation.
3.3 Excessemissionatmillimetrewavelengths
4 CONCLUSIONS
Wehavethusfardeterminedthatthepresenceofemissionmecha-
nismsotherthanthermaldustemissionat7mmiscommoninpro- We present a radio flux monitoring survey of 11 T Tauri stars in
toplanetarydiscs,andthatsignaturesofgraingrowthcanbefound the Chamaeleon and Lupus Southern star forming regions over
insourceswithandwithout7mmfluxvariability.The7mmflux timescales of 100s of days. We found that the 7 mm flux varies
variabilityof∼30percentoveraperiodof100sofdaysisconsis- byatleast30percentanduptoafactorof3formostsources,in-
tentwiththecharacteristicsofthermalfree-freeemission.Thepres- dicatingthatprocessesotherthanjustthermaldustarecontributing
enceoffluxvariabilityhasalsobeendetectedatotherwavelengths, totheemission.FluxmonitoringofCSChaandGQLupat15mm
suchasintheX-ray,optical,andinthenear-infrared(NIR). over the course of a year found consistent fluxes, suggesting no
Allthesourcesinoursample(withtheexceptionofMYLup variabilityoverthesetimescalesfortheseobservations.Monitoring
andSz111)havebeendetectedintheX-raywithalog(Lx/Lbol) of T Cha, Sz 32 and WW Cha at 3 and 6 cm also suggests ex-
between −3 and −4, classifying these stars as X-ray active (e.g. cessemissionabove thermaldust. Fromour analysisof theradio
Stelzeretal. 2004; Feigelson&Lawson 2004). A few sources in spectrumandfluxmonitoring,onlytwosourcesshownovariabil-
our sample have been studied more closely in the literature. Us- ityandseemtobedominatedbythermaldustemission:Sz111and
ingtheXMM-NewtonobservationsofCRCha,Robrade&Schmitt MYLup.
(2006) detected a small flare and flux variability consistent with Additionally, we looked at the flux variability in the X-ray,
coronalactivityandthepresenceofaccretion.CSCha,TChaand optical and near-infrared wavelengths, and found further support
GQ Lup were included in Güdeletal. (2010) analyses of Spitzer fortheneedformultipleepochandmulti-wavelengthobservations
[NeII]lineandXMM-Newtonarchivedata,wheretheyclassified todeterminethecauseofthefluxvariability.
CS Cha as a jet-driving object with a transitional disc, T Cha as We found seven sources with β < 1, indicative of grain
atransitional discand GQ Lupas anopticallythick discwithout growthuptocentimetresizedgrainsandfoursourceswithaβ>1,
a known jet. Similarly, X-ray flares have also been detected for suggestinglittletonograingrowth.Theseresults,withtheexcep-
Sz32 and WWCha(Feigelson&Lawson2004). Itisinteresting tionofSz32,areconsistentwithreportedvaluesforspectralslope
to note that sources with flux variability at mm wavelengths are from1–3mm.Ofthesevensourceswithsignaturesofgraingrowth,
alsovariableintheX-ray;specificallythesourceswithhighampli- onlyCRChaandMYLuphavenoreported7mmfluxvariability.
tude variabilityintheradio(CS Cha, GQLup, Sz32, WWCha) Thus,theseresultsshow thatbothsignaturesofgraingrowthand
arealsoveryactiveintheX-ray.Thissuggeststhatthecauseofthe evidenceofotheremissionmechanismat7mmmaybepresentfor
variability could be the same at both wavelengths. Simultaneous thesameprotoplanetarydisc.
observationsatbothwavelengthswouldbeneededtoconfirmthis In the near future, the high resolution and sensitivity of At-
relationship. acama Large Millimeter/Submillimeter Array (ALMA) will play
Flux variability of up to 20 per cent is commonly found in animportantroleinansweringthesequestionsbysignificantlyre-
young stellar objects in the optical and NIR wavebands (Scholz ducing the uncertainties in the 1 mm fluxes for the Chamaeleon
2012;Rodgers-Leeetal.2014).Thisvariabilitycanbecausedby sources, and thusreducing theuncertainties inthe1–3 mmspec-
MNRAS000,1–10(2015)
RadioMonitoringofProtoplanetaryDiscs 7
100
Glass I Glass I
10-1 1.6
10-2 1.4
1.2
Flux (Jy)111000---543 Flux (mJy)01..80
0.6
10-6
0.4
10-7 0.2
10-8 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
SZ Cha SZ Cha
10-1 2.0
10-2
1.5
Flux (Jy)1100--43 Flux (mJy)1.0
10-5
0.5
10-6
10-7 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
Sz 32 Sz 32
10-1 2.0
10-2
10-3 1.5
Flux (Jy)1100--54 Flux (mJy)1.0
10-6
10-7 0.5
10-8
10-9 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
101
WW Cha WW Cha
12
100
10
10-1
Flux (Jy)1100--32 Flux (mJy) 68
10-4 4
10-5 2
10-6 10-3 10-2 10-1 00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
Figure1.Continued.
tralslopeandβvalues,andprovidingabetterunderstandingofthe ACKNOWLEDGEMENTS
signaturesofgraingrowthintheseprotoplanetarydiscs.
We thank Hans Guenther and Scott Wolk for their help with the
XMM-NewtonSerendipitousSourceCatalogueandforhelpfuldis-
cussions. We also thank Leondardo Testi for useful discussions,
and the referee for their useful comments and constructive feed-
MNRAS000,1–10(2015)
8 Ubachet al.
100
Sz 111 Sz 111
10-1 2.5
10-2
2.0
Flux (Jy)1100--43 Flux (mJy)1.5
1.0
10-5
10-6 0.5
10-7 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
MY Lup 4.0 MY Lup
10-1 3.5
10-2 3.0
Flux (Jy)10-3 Flux (mJy)22..05
10-4 1.5
1.0
10-5
0.5
10-6 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
100
GQ Lup GQ Lup
2.0
10-1
10-2
1.5
Flux (Jy)1100--43 Flux (mJy)1.0
10-5
0.5
10-6
10-7 10-3 10-2 10-1 0.00 500 1000 1500 2000
Wavelength (m) Julian Date (-2454500)
Figure1.Continued.
back.TheAustraliaTelescopeCompactArrayispartoftheAus- Ghez A. M., McCarthy D. W., Patience J. L., Beck T. L., 1997, ApJ,
traliaTelescopeNationalFacilitywhichisfundedbytheCommon- 481,378
wealthofAustraliaforoperationasaNationalFacilitymanagedby GonzálezR.F.,CantóJ.,2002,ApJ,580,459
CSIRO.ThisresearchwassupportedinpartbyaSwinburneUni- GüdelM.,etal.,2010,A&A,519,A113
versityPostgraduate Research Awardand CSIROOCEPostgrad- GuentherE.W.,EspositoM.,MundtR.,CovinoE.,AlcaláJ.M.,Cusano
uate Top Up Scholarship. CMW acknowledges support from the F.,StecklumB.,2007,A&A,467,1147
AustralianResearchCouncilthroughDiscoveryGrantDP0345227 HenningT.,PfauW.,ZinneckerH.,PrustiT.,1993,A&A,276,129
HerbstW.,HerbstD.K.,GrossmanE.J.,WeinsteinD.,1994,AJ,108,1906
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HughesJ.,HartiganP.,KrautterJ.,KelemenJ.,1994,AJ,108,1071
KuijpersJ.,vanderHulstJ.M.,1985,A&A,149,343
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RadioMonitoringofProtoplanetaryDiscs 9
APPENDIXB: RESULTSAT15MM
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Fl0.2
0.1
0.0
1300 1400 1500 1600 1700
Julian Date (-2454500)
Figure2.The15mmcontinuumfluxesforGQLupplottedfromthefirst
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APPENDIXA: OBSERVINGLOGS
MNRAS000,1–10(2015)
10 Ubachet al.
TableA1.ATCAobservinglogforLupussourcesfromthisworkandliterature. (1)Sourcename.(2)Dateofobservations. (3)Frequencypair.(4)Flux
calibrator.(5)ATCAarrayconfiguration.(6)Synthesizedbeamsize.(7)Totalintegrationtimeusedforanalysis.(8)RMS.(9)Reference.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Source Date Freq. Flux Antenna(s) Beamsize Tint RMS Reference
Pair(GHz) Cal. Config. arcsec (min) (mJy/Beam)
Sz111 Aug-08 43+45 Uranus H214 4 — 0.2 Lommenetal.(2010)
May-09 43+45 Uranus H214 5.2×3.5 60 0.1 Ubachetal.(2012)
Aug-12 43+45 PKSB1934-638 H75 14.6×10.3 60 0.1 Thiswork
Oct-12 43+45 PKSB1934-638 H214 5.0×3.6 60 0.03 Thiswork
Oct-12 17+19 PKSB1934-638 H214 13.1×9.8 90 0.02 Thiswork
MYLup Aug-08 44.2+46.2 Uranus H214 4 0.1 Lommenetal.(2010)
May-09 43+45 Uranus H214 4.8×3.4 50 0.1 Ubachetal.(2012)
Aug-12 43+45 PKSB1934-638 H75 14.8×10.5 50 0.1 Thiswork
Oct-12 43+45 PKSB1934-638 H214 4.7×3.4 80 0.03 Thiswork
Oct-12 17+19 PKSB1934-638 H214 13.6×9.7 40 0.02 Thiswork
GQLup 21-Aug-10 43+45 Uranus H168 6.7×4.4 80 0.04 Ubachetal.(2012)
10-Jul-11 43+45 Uranus H214 6.8×3.6 18 0.07 Ubachetal.(2012)
4-Aug-12 43+45 PKSB1934-638 H75 22.2×9.2 53 0.05 Thiswork
22-Oct-12 43+45 PKSB1934-638 H214 5.0×3.6 30 0.04 Thiswork
8-Jul-11 17+19 PKSB1934-638 H214 18.3×9.7 24 0.01 Ubach(2014)
18-Jul-11 17+19 PKSB1934-638 H214 17.8×9.1 20 0.02 Ubach(2014)
22-Oct-12 17+19 PKSB1934-638 H214 13.2×9.4 30 0.03 Thiswork
APPENDIXC: SUMMARYOFWWCHARADIO
MONITORING
MNRAS000,1–10(2015)