Astronomy&Astrophysicsmanuscriptno. Letter-futau-Arxiv (cid:13)cESO2013 January21,2013 LettertotheEditor A molecular outflow driven by the brown dwarf binary FU Tau (cid:63) J.-L.Monin1,E.T.Whelan2,B.Lefloch1,C.Dougados1,andC.AlvesdeOliveira3 1 UJF-Grenoble/CNRS-INSU,InstitutdePlanétologieetd’AstrophysiquedeGrenoble(IPAG)UMR5274,Grenoble,38041,France 2 InstitutfürAstronomieundAstrophysik,KeplerCenterforAstroandParticlePhysics,Sand1,EberhardKarlsUniversität,72076 Tübingen,Germany, 3 EuropeanSpaceAstronomyCentre(ESA),P.O.Box78,28691VillanuevadelaCañada,Madrid,Spain ABSTRACT 3 We report the detection of a molecular outflow driven by the brown dwarf binary FU Tau. Using the IRAM 30 m telescope we 1 observedthe12CO(2-1)(CO)emissioninthevicinityofFUTauanddetectedabipolaroutflowbyexaminingthewingsoftheCO(2- 0 1)lineaswemovedawayfromthesourceposition.Anintegratedmapofthewingemissionbetween3kms−1and5kms−1revealsa 2 blue-shiftedlobeatapositionof∼20(cid:48)(cid:48)fromtheFUTausystemandatapositionangleof∼20◦.Thebeamsizeoftheobservationsis 11(cid:48)(cid:48)henceitisnotpossibletodistinguishbetweenthetwocomponentsoftheFUTaubinary.Howeverasopticalforbiddenemission, n astrongtraceroftheshockscausedbyoutflowactivity,hasbeendetectedinthespectrumofFUTauAweassumethiscomponentto a J bethedrivingsourceofthemolecularoutflow. Weestimatethemassandmassoutflowrateoftheoutflowat4×10−6M and6×10−10M /yrrespectively.Theseresultsagreewell 8 (cid:12) (cid:12) withpreviousestimatesforBDmolecularoutflows. FUTauAisnowthethirdBDfoundtobeassociatedwithmolecularoutflow 1 activityandthisdiscoveryaddstothealreadyextensivelistoftheinterestingpropertiesofFUTau. ] Keywords. radiolines:ISM–stars:winds,outflows–(stars:)browndwarfs–stars:pre-mainsequence–stars:formation R S . h1. Introduction in its disk (Ricci et al. 2012). Its optical jet was discovered by p Whelanetal.(2005)throughthespectro-astrometricanalysisof -Young brown dwarfs (BDs) occupy the mass regime between the [O]λ6300 emission line. Forbidden emission lines (FELs) ostars and planetsand are therefore significantto any theory de- like [OI]λ6300 are important coolants in shocks and therefore rscribingactivityinstarformingregions. Thustheyhavebecome t goodtracersofjets.TraditionallyjetsfromclassicalTTauristars sthesubjectsofincreasedscrutinyinrecentyears(Luhman2012). a (CTTSs)areinvestigatedbystudyingtheirFELregions. Phan- Their formation mechanism is at present much debated and in- [ Baoetal.(2008)alsodetectedaCOmolecularoutflowdrivenby deedithasbeenpostulatedthattheymayformbymorethanone ISO-Oph102. Theorientationoftheblueandredlobesagreed 1mechanism (Whitworth et al. 2006). The simplest idea is that withtheopticalobservations. vtheyforminthesamemanneraslowmassstarsi.e. throughthe 7gravitationalcollapseofsubstellarmasscores(Padoan&Nord- ThequestionofoutflowactivityinBDsisanimportantone, 8 as a sufficiently efficient outflow activity could provide an ex- lund2004). Thesecoresoccurdirectlybytheprocessofturbu- 3 planationastowhythecentralobjectmassdoesnotreachtheH lent fragmentation. In this scenario, BDs are just scaled-down 4 burninglimit(Bacciottietal.2011;Whelanetal.2009;Machida versions of low mass stars. Detailed studies of the circumstel- . et al. 2009). Molecular outflows are an important large-scale 1lar environments of young BDs provide critical constraints to 0different formation mechanisms and are needed to identify the expression of jet launching. Indeed, molecular outflows were 3 one of the first observational manifestations of this process to dominantmechanism. Inparticular, ifBDsformlikelowmass 1stars we expect their accretion/outflow properties to be analo- be studied (Reipurth & Bally 2001; Bachiller 1996). While gi- : antHerbig-Haro(HH)flowsareopticallyvisibleandcomposed vgous. Asalowmassstarformsitdisplaysaseriesofubiquitous ofmanyHHobjects,eachgrouprepresentingdifferentepisodes iobservationalproperties, forexampleaccretiondisks, outflows, X of mass ejection, molecular outflows begin when the powerful excessemissioninthenear-infraredandvisualabsorption. r bipolar jets accelerate and drive outwards the molecular gas in The observational evidences gathered to date in various a the vicinity of their parent star. Although it is accepted that wavelength domains indicate that young BDs show accretion theyarepoweredbytheprimaryjetfromtheprotostar,theexact and ejection behavior similar to low mass stars. For example wayinwhichthejetinteractswiththemolecularmaterialisstill they demonstrate T Tauri-like accretion (Jayawardhana et al. uncertain (Cabrit et al. 1997; Downes & Ray 1999; Downes & 2003;Nattaetal.2004;Moninetal.2010;Rigliacoetal.2011) Cabrit 2007). Molecular outflows are primarily detected in the and both optical and molecular outflows, driven by BDs, have CO molecule and thus millimeter observations have dominated beendetected. ISO-Oph102isagoodexample. Itisanaccretor the search for them. These outflows are mainly detected from withanobservedaccretiondisk(Nattaetal.2002,2004)andre- Class 0 and I low mass stars which are still embedded in their cent ALMA observations have detected millimetre sized grains natalmaterial. Observationsofmolecularoutflowsdrivenbythe moreevolvedClassIICTTSsaremuchrarer(Cabritetal.2011). (cid:63) BasedonobservationscarriedoutwiththeIRAM30mTelescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and Asoftoday,onlytwodetectionsofmolecularoutflowsfrom IGN(Spain). optically visible young BDs have been made so far (Phan-Bao Articlenumber,page1of4 etal.2008,2011),althoughitispostulatedthatduetothecolder the frequency of the CO(2-1) line, the main-beam efficiency of environmentofBDs,molecularoutflowsmaybemorecommon thetelescopeis0.59andthehalf-powerbeamwidthis11(cid:48)(cid:48). The thaninCTTS.WehaveconductedasurveyofyoungBDswith intensities of the measurements are expressed in units of main- theIRAM30mtelescopeinthe12CO(1-0)and12CO(2-1)totest beambrightnesstemperatureT . mb this hypothesis (Whelan et al. 2013, in prep.). Their approach ThedatawerereducedusingtheContinuumandLineAnal- istotargetBDsknowntobeaccretingandwhichalsoshowev- ysis Single dish Software (CLASS, a GILDAS software1). In idenceofoutflowactivityprimarilyinformofFELs, inamass some of the sources, the CO mesospheric emission line was rangeof0.02M to0.13M includingafewverylowmassstars detected close to the cloud emission, which peaks at v (cid:39) (cid:12) (cid:12) lsr (VLMSs). Inthisletterwereportthedetectionofaremarkable +6km.s−1, on the red side of the spectrum. For all our obser- molecularoutflowinFUTau,aspartofourIRAMsurvey. vationswehaveadjustedagaussiantotheCOmesosphericline FU Tau (04h23m35s4, +25◦03(cid:48)03(cid:48)(cid:48)05) is a BD-BD binary profile and subtracted it out. The CO mesopheric line profile with a projected angular separation of 5(cid:48).(cid:48)7 or 800 AU at the is typically a few K bright, with a linewith of about 1km.s−1 distance to Taurus and a position angle (PA) of ∼ 145◦ (Luh- (CHECK),muchlessthanthevelocityrangeofthecloudemis- manetal.2009). ItsmembershipoftheTaurusmolecularcloud sion and the outflow wing emission. When the outflow feature has been known for some time (Jones & Herbig 1979) and it ison theblueshifted partof theline spectrum, the mesospheric is situated in a relatively isolated region of the cloud. Luhman COisabsolutelyharmless. Whentheobservedoutflowwingis etal.(2009)givethespectraltypeofFUTauAatM7.25,corre- ontheredshiftedside,wecheckedthattheCOmesosphericline spondingtoamassof50M andthespectraltypeandmassof is much narrower than the wing velocity range, hence does not Jup thecompanionatM9.5and15M respectively. Thewidena- hamperthedetectionofthelatter. Jup tureoftheFUTaubinarychallengesmodelswhichsuggestthat BDs form when their accretion is halted due to ejection from theirnatalclouds,sincethesystemappearstohaveformedirre- spectivelyofdynamicalinteractionwithnearbystars. Afurther intriguing property of FU Tau A is its over-luminosity with re- specttoothermembersoftheTaurusstar-formingregionofthe samespectraltype(Luhmanetal.2009;Scholzetal.2011).The spectral energy distributions (SEDs) of both components show excess emission indicating the presence of circumstellar disks andtheirdisksareclassifiedasbeingClassIIbyLuhmanetal. (2009). Furthermore,opticalspectraclearlyshowthataccretion ison-goinginFUTauA.Stelzeretal.(2010)estimatedthemass accretionratefromboththeHαandHeI(λ5876)lineswithM˙ Hα = 3.5 × 10−10 M yr−1 and M˙ = 7.5 × 10−10 M yr−1. Evi- (cid:12) HeI (cid:12) dence of outflow activity comes from the detection of the [O I]λ5577and[OI]λ6300forbiddenlinesinanopticalspectrum of FU Tau A (Stelzer et al. 2010). All of these facts combined showthattheFUTausystemisprobablyararityamongstBDs and thus it is of considerable interest to test models describing the formation and evolution of BDs. For this reason we chose topublishthediscoveryofitsmolecularoutflowseparatelyfrom theglobalpresentationofourIRAMsurvey. 2. ObservationsandDataReduction ObservationsoftheCO(2-1)lineemissionwerecarriedoutatthe IRAM 30m telescope on July 16th-18th 2011 using the EMIR Fig.1. AMapshowingthepositionsofthe11spectraobtainedfor FUTau. TheCO(2-1)lineisshownhere. Thescaleofthegridis0-15 receiversat1.3mm. kms−1 and-0.6-6Kinxandyrespectively. Forallspectraexceptthe Inafirststep,deepintegrationswereperformedtowardsthe (0,-20) and (+20,+20) positions, the rms is less than 40 mK. For the protostar and at a reference position located 20(cid:48)(cid:48) away. In a (0,-20)and(+20,+20)positionsthenoiseis∼80mK. second step, in case of significant variations of the CO emis- sionbetweenbothpositions, i.e. beyondthe3σintensitylevel, more extended mapping at 12(cid:48)(cid:48) sampling was performed. The COemissionmapdetectedtowardFUTauisdisplayedinFig1. 3. ResultsandDiscussion ObservationswerecarriedoutinFrequencySwitchmodeus- ingathrowof14.3MHzat1.3mm,withaphasetimeof0.2sec- 3.1. Outflowsignature ond. Anautocorrelatorprovidinguswithaspectralresolutionof In Fig. 2 a magnified view of the wings of the CO(2-1) emis- 40kHzwasusedasspectrometer. Theweatherconditionswere sion line at each point in the map of FU Tau (Fig. 1) is shown. rather good and stable, with system temperatures T varying sys The central (0,0) position spectrum is repeated as a dark solid between200and400K.Eachpositionwasobservedsotoreach lineinalltheplots,superimposedonthecolorlinesobservedat afinalrmsofabout40mKunlessexceptionpervelocityinterval theotherpositions. Asthebeamis≈ 11(cid:48)(cid:48) wideitencompasses of0.1k˙m.s−1,afteraveragingbothpolarizations. bothFUTauAandFUTauB.ThebulkoftheCO(2-1)emission Pointingwascheckedevery1.5to2hoursandwasfoundto comes from the cloud and is centered on the cloud velocity at beverystable,withpointingoffsetscorrectionslessthan3(cid:48)(cid:48).The telescopeparametersareadoptedfromtheIRAMwebpage. At 1 http://www.iram.fr/IRAMFR/GILDAS/ Articlenumber,page2of4 Moninetal.:FUTauoutflow PA i N(CO) M(H ) M˙(H ) 2 2 (o) (o) (cm−2) (M ) (M /yr) (cid:12) (cid:12) 20 60 3.61016 4±110−6 5±110−10 Table1.FUTauoutflowparameters tion. (cid:90) 16.5 (CO) =1.061013T exp( ) T (2−1)dv (1) (cm−2) mb T R mb Adopting a gas excitation temperature T ≈ 15K, with an mb H /CO ratio of 104 and the results of Figure 3, we compute a 2 massinthebluelobeoftheflowofM (H )=4±0.810−6M . B 2 (cid:12) If we suppose that the momentum of the underlying jet has entirelybeentransferredtothemolecularcomponentthatweob- servetoday,wecanwrite: M (H )<V >= M˙ <V >τ (2) B 2 max jet jet dyn Wemeasure<V >= 3kms−1, andwetakeacanonicalvalue max <V >= 100kms−1 ; together with τ ≈ 200yr (see § 3.3, jet dyn first paragraph), we obtain a mass loss rate for the blue-shifted lobe of M˙ = 6 ± 1.310−10M /yr. This value can be modi- out (cid:12) fiedbyvariousfactors. Forinstance,wecanadoptacorrection factor to take into account the fact that the jet might have been episodic in the past. (Phan-Bao et al. 2011) use a factor of 10 for this purpose. Also, the excitation temperature is uncertain althoughtheT exp(16.5/T )factordoesnotvarymuchover mb mb Fig.2. MontageofsomeoftheCO(2-1)emissionlineprofilesob- T21 = 10−25Krange. Wecouldalsotakeintoaccountextinc- servedforFUTaushownwithazoomontheregionofthewings. The tion effects and the fact that we are only measuring half of the central (0,0) position spectrum is repeated as a dark solid line in all flowemission.Thusthisvaluemustbetakenasafirstorderesti- theplots,superimposedonthelinesobservedattheotherpositions,in mationoftheoutflowrateandmostprobablyunderestimatesthe color.Wedetectexcessemissionintheblue-wingbetween3kms−1and rate of the underlying jet. The outflow parameters are summa- 5kms−1,atthe(0,20),(10,20)and(0,30)positions. Thispointstoa rizedinTable1. molecularoutflowdrivenbytheFUTausystem. 3.3. Outflowpoweringsource 6km.s−1,andwesearchforoutflowsignaturefromvariationsin AlthoughwecannotdisentangleFUTauAfromFUTauBitis the CO(2-1) wing emission with respect to the emission on the mostlikelythattheoutflowisdrivenbytheprimaryasforbidden centralsource.Theoutflowemissionwhichisshiftedinvelocity emissionassociatedwiththeprimaryhasalreadybeendetected with respect to the cloud is much fainter than the cloud emis- (Stelzeretal.2010). Thusfortherestofthediscussionwewill sionandthereforeitwillbedetectedinthewingsoftheCO(2-1) assumethatFUTauAisthedriveroftheoutflow. Thepeakof emission line. Fig. 2 shows that a blue component develops in thebluelobeismeasuredat∼20(cid:48)(cid:48) fromthecentralsource,pro- the wing as we move towards the north, with an excess wing jectedontheplaneofthesky.Adoptingaprojectionangleof60o emission seen at the (0, 20), (10, 20) and (0, 30) positions be- (Stelzeretal.2013),thelineardistanceis≈50(cid:48)(cid:48),corresponding tween a velocity of 3 kms−1 and 5 kms−1. There is also a hint to7000AUatthedistanceofTaurus(140pc).At100kms−1,this ofblue-shiftedexcessemissionatthe(0,10)position,andared yieldsadynamicalageτ ≈ 200yrsfortheobservedoutflow dyn wing appears in the 8-10 km.s−1 range in the (0,-20) position event. spectrum. Thedetectionofthisexcessemissionstronglypoints Previous to the results presented here, ISO-Oph 102 and to a molecular outflow driven by FU Tau. In Fig. 3 we present MHO 5 with masses of 60 M and 90 M were the lowest Jup Jup anintegratedintensitymapoftheblue-shiftedwingemissionin massobjectsforwhichmolecularoutflowsweredetected(Phan- thevelocityrange3kms−1 to6kms−1. Theblacksquaresmark Baoetal.2008,2011). Theoutflowmassandmassoutflowrate the positions at which data was collected and we have super- were estimated at Mout = 1.6 × 10−4 M(cid:12); M˙out = 1.4 × 10−9 imposedanopticalimage(WFCAM/UKIRT)oftheFUTaubi- M(cid:12)/yrandMout =7.0×10−5 M(cid:12); M˙out =9.0×10−10 M(cid:12)/yrre- narytakenfromLuhmanetal.(2009). Thedetectionoftheout- spectively. Thusourestimatesofthemassandmassoutflowrate flowintheformofablue-shiftedlobetowardsthenorth-eastis oftheFUTauoutflowagreewithpreviousresultsandareinline clear. Without further data, we estimate a PA of ∼ 20◦ for the with the fact that FU Tau A has the smallest mass of the three outflowaxis. objects. ThederivedvaluesofM˙ arealsoconsistentwithM˙ out out measured for the optical components of BD outflows (Whelan etal.2009). ForISO-Oph102theoutflowrateinthemolecular 3.2. OutflowParameters component was found to be slightly higher than the the optical component. However,itisreasonablethat M˙ foramolecular out Following Bachiller et al. (1990) we compute the CO column outflow could be greater than the outflow rate in the underly- densityinthebluelobeoftheoutflowwiththefollowingequa- ingjet. Assumingthatthejetispoweringthemolecularoutflow Articlenumber,page3of4 (Downes & Cabrit 2007), the mass outflow rate of the molecu- 4. Summary larcomponentwillgrowwithtimeasthejettransfersincreasing ThediscoveryofamolecularoutflowdrivenbyFUTauAadds amounts of energy and momentum. The size of the outflow of ∼ 20 (cid:48)(cid:48) is compatible with the ISO-Oph 102 and MHO 5 flows significantly to the interesting properties of this source and its binary companion. The FU Tau binary has a large separation and with observations of BD optical outflows. The agreement comparedtootherbinarysystemsandisthoughttohaveformed between the scale of the molecular and optical components is inrelativeisolation. BothcomponentsharborClassIIaccretion important if one is to accept that the jet drives the molecular disksandFUTauAissomewhatover-luminousforitsspectral flow. FinallywecomparethemassoutflowrateoftheCOout- type. ThefactthatFUTauAisdrivinganoutflowdemonstrates flowfromFUTauwiththederivedmassaccretionrateandfind theratioofmassoutflowtomassaccretion, M˙ / M˙ =0.8to thatdespitehavingunusualcharacteristicsitstillexhibitsprop- 1.7. M˙ / M˙ measured for other BDs andoVutLMSacschas also erties which are strongly linked to the formation of low mass out acc protostars. The mass, scale and mass outflow rate we measure beenfoundtobehighcomparedtoTTauristarswhereitismea- fortheFUTauACOoutflowagreeswithpreviousobservations suredat1-10%(Bacciottietal.2011;Whelanetal.2009). We stressthatsuchahighvalueof M˙ / M˙ cannotbeusedasa ofBDmolecularoutflows. Whilethisresultisafurtherimpor- out acc tant piece of evidence linking how BDs form to low mass star cluethatthisoutflowremovesorhasremovedalargefractionof formationthederivedratioofmassoutflowtoaccretionratesis thecentralobject’mass(asintheMachidaetal.(2009)model), much higher than what is observed in low mass protostars and becausetheobservedFUTaumolecularoutflowresultsfromthe inparticulartheTTauristars. ForotherBDsthetworateshave entrainmentbyanunderlyingjet,andconcernsanejectedmass been found to be comparable thus these new results for the FU several orders of magnitude smaller than in the (Machida et al. Tau system support other studies of BD outflow activity (Bac- 2009)model. Onthesamehand,anotherexplanationforthefor M˙ /M˙ ratio ≈ 1inthecurrentknownseriesofBDsources ciotti et al. 2011; Whelan et al. 2009). M˙out/M˙acc ratio ≈ 1 out acc in the current known series of BD sources could be due to an couldbeduetoanobservationalbias,becausethefirstcurrently observationalbias,becausethefirstcurrentlyavailableobserva- availableobservationsareonlysensitivetothemostextremejets tions could be only sensitive to the most extreme jets in brown in brown dwarfs. If this is true, the current ratios should prove dwarfs. More observations are needed to solve this issue. FU much higher than the (as-yet unobserved) mean in BDs. More Tau is an excellent candidate for follow-up observations with sensitiveobservationsarethusneededtosolvethisissue. sub-millimeter interferometer such as the Plateau de Bure in- terferometry or the Sub-miillimeter Array (SMA). With higher angularresolutionobservationswewillbeabletofullyresolve the outflow, search for a red-shifted lobe and confirm whether FUTauAisthedrivingsourceoftheflow. Acknowledgements. The authors would like to acknowledge the help of the IRAM 30m team during the observations. E.T. Whelan is supported by an IRCSET-MarieCurieInternationalMobilityFellowshipinScience,Engineering andTechnologywithinthe7thEuropeanCommunityFrameworkProgramme. Wethankthereferee,S.Mohanty,forafastandthoroughreviewthathelpedto improvethequalityofthispaper. 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