X-ray emission from MP Muscae: an old classical T Tauri star PDF
Preview X-ray emission from MP Muscae: an old classical T Tauri star
Astronomy&Astrophysicsmanuscriptno.mpmus (cid:13)c ESO2008 February5,2008 LE X-ray emission from MP Muscae: an old classical T Tauri star C.Argiroffi1,A.Maggio2,andG.Peres1 1 DipartimentodiScienzeFisicheedAstronomiche,SezionediAstronomia,Universita`diPalermo,PiazzadelParlamento1,90134 Palermo,Italy,e-mail:[email protected], [email protected] 2 INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,Italy,e-mail:[email protected] 7 Received22December2006/Accepted25January2007 0 0 ABSTRACT 2 n Aims.WestudythepropertiesofX-rayemittingplasmaofMPMus,anoldclassicalTTauristar.Weaimatcheckingwhetheran a accretionprocessproducestheobservedX-rayemissionandatderivingtheaccretionparametersandthecharacteristicsoftheshock- J heatedplasma.WecomparethepropertiesofMPMuswiththoseofyoungerclassicalTTauristarstotestwhetherageisrelatedto 6 thepropertiesoftheX-rayemissionplasma. 2 Methods.XMM-NewtonX-rayspectraallowsustomeasureplasmatemperatures,abundances,andelectrondensity.Inparticularthe densityofcoolplasmaprobeswhetherX-rayemissionisproducedbyplasmaheatedintheaccretionprocess. 1 Results.X-rayemissionfromMPMusoriginatesfromhighdensitycoolplasmabutahotflaringcomponentisalsopresent,suggesting v that both coronal magnetic activity and accretion contribute to the observed X-ray emission. We find a Ne/O ratio similar to that 5 observedinthemuchyoungerclassicalTTauristarBPTau.FromthesoftpartoftheX-rayemission,mostlyproducedbyplasma 6 heatedintheaccretionshock,wederiveamassaccretionrateof5×10−11M⊙yr−1. 7 Keywords.stars:abundances–stars:circumstellarmatter–stars:coronae–stars:individual:MPMuscae–stars:pre-mainsequence 1 –X-rays:stars 0 7 0 / 1. Introduction (Gu¨deletal.2007)andtheHerbigstarABAur(Telleschietal. h 2007). p Low mass stars are sources of strong X-ray radiation since In the hypothesis of X-ray emission originated in shocks - o their early evolutionary phases. Coronal plasma, responsi- greatattention has been focused also on plasma elementabun- r ble for the X-ray emission, is confined and probably heated dances.Infact,theyprobethe chemicalcompositionoftheac- st by magnetic fields which emerge from the stellar surface creting stream, and hence provide insightfulindications on the a (Feigelson&Montmerle1999;Preibischetal.2005).Thecoro- physicalandchemicalprocessesatworkintheinnercircumstel- v: nalplasmaobservedinessentiallyalllate-typestarscanbechar- lardisk(Stelzer&Schmitt2004;Drakeetal.2005). acterized by a large variety of average temperatures (from few i In this letter we present the XMM-Newton observation of X MKtotensofMK)andmetallicities(fromonetenthtofewtimes MPMuscae,oneoftheoldestknownCTTSs,aimedatstudying r the solar photosphericvalue),but a commonfeature is the low thepropertiesoftheX-rayemittingplasmaandtheroleoftheac- a density measured at the temperature of formation of O He- cretionprocess.MPMusisaK1IVestaroftheLowerCentaurus like triplets (Ne ≈ 1010−1011cm−3 atT ∼ 2MK) (Testaetal. Crux(LCC)association.Gregorio-Hetemetal.(1992)firstiden- 2004;Nessetal.2004). tified it as a classical T Tauri star by measuring enhanced Hα In very young stars, however, accretion may cause X-ray emission (EW = −47Å) and Li absorption (EW = 0.37Å). emission in addition to magnetically-confined coronal plasma. Excesses in infrared bands revealed an optically thick circum- InclassicalTTauristars(CTTSs)gasfallsfromthecircumstel- stellar disk (Mamajeketal. 2002; Silverstoneetal. 2006) with lar envelope,funnelledby the magnetic field, and hits the stel- anestimateddustmassof∼5×10−5M⊙(Carpenteretal.2005). lar photosphere.In the resulting shock the accreted material is Batalhaetal. (1998) derived a rotational period of 5.75d from heated to temperatures of few MK (Calvet&Gullbring 1998). variabilityintheB,V,R,andIbands,althoughtheamplitudeof Withtypicalmassaccretionrates, theshock-heatedplasmacan variabilityissurprisinglylowforaCTTS. reachX-rayluminositiesashighas1031ergs−1. Mamajeketal. (2002) derived three different ages for The few CTTSs for which high-resolution X-ray spec- MP Mus, 7, 14, and 17Myr, depending on the adopted the- troscopy was performed up to date show, in most cases, cool oretical evolutionary tracks. The LCC association, to which plasma components (2 − 4MK) with large electron densi- MPMusbelongs,istheoldestportionoftheScorpius-Centaurus ties (1011 − 1013cm−3) which have been interpreted as evi- OBassociation,anditsestimatedageisbetween16and23Myr dence for X-ray emission due to an accretion shock. The best (Mamajeketal.2002;Sartorietal.2003). known examples of this behavior are the CTTSs TW Hya, BPTau,andV4046Sgr(Kastneretal.2002;Schmittetal.2005; Gu¨ntheretal.2006).NoticeableexceptionsaretheCTTSTTau 2. Observationanddataanalysis MP Mus was observed with XMM-Newton for a duration of Sendoffprintrequeststo:C.Argiroffi,e-mail:[email protected] ∼110kson2006August19–20.Weprocessedthedatausingthe 2 C.Argiroffietal.:X-rayemissionfromtheoldCTTSMPMus Fig.1. Background-subtractedlightcurveofMP Musobtained Fig.2. RGS1 spectrum in the wavelength region of the O byaddingthethreeEPICinstruments. triplet (gray) with the best fit lorentzian line profile (black). Wavelengthbinscorrespondingtobadcolumnpixelsareplotted Table1.MPMusbest-fitparamenters. withouterrorbarsandwerenotconsideredinthefittingproce- dure. Par. best-fitvalue Ta 2.7+0.1 7.2+0.4 36+18 −0.2 −0.5 −11 EMb 9.6+5.2 17.9+4.0 2.9+1.5 −2.5 −3.0 −0.8 Nc 4.6+1.8 H −1.7 Ab.d O=0.25+0.08 Ne=0.76+0.23 S=0.28±0.20 Fe=0.09+0.04 −0.07 −0.15 −0.02 aTemperature(MK).bEmissionMeasure(1052cm−3).cHydrogencol- umn density (1020cm−2). d Abundances referred to the solar photo- sphericvaluesofAsplundetal.(2005).Alltheuncertaintiescorrespond tothe68%confidencelevel. SASV6.5standardtasks.Afterhavingdiscardedtimesegments Fig.3. RGS2 spectrum around the Ne triplet (gray) with the affectedbyhighbackgroundcountrates,weobtainedgoodtime predictedspectraobtainedfromthe3-T EPICmodelandassum- intervalssummingupto∼100ks.TheX-raylightcurve(Fig.1) ingdifferentelectrondensities. showsclearhintsofflaringactivity,typicalofmagneticallyac- tivecoronalsources. Toincreasethesignal-to-noiseratiowerebinnedthePNand triplet, has a density logN > 11, and possibly as high as that e MOS spectra to obtain at least 30 counts in each bin, and the estimatedfromtheO. RGS wavelength bins were joined two by two. Spectral anal- ysis was performed using the Astrophysical Plasma Emission Database (APED V1.3, Smithetal. 2001). We adopted the 3. Discussion Asplundetal.(2005)solarphotosphericcompositionastheref- erencesetofelementabundances. FromtheanalysisoftheOtripletwefindthatthecoolplasma We derivedthecharacteristicsoftheX-rayemittingplasma componentin oursourcehasa density significantlylargerthan of MP Mus by fitting PN and MOS spectra with XSPEC typicalcoronalvalues.Itsuggeststhatshock-heatedplasmacon- V11.3.0(Arnaud1996),adoptinganoptically-thinplasmaemis- tributessignificantlytotheobservedX-rayemission.Inthisre- sion modelwith three isothermalcomponents.The best-fit 3-T spect, MP Mus is the fourth CTTS discoveredso far with evi- model(Table1)alsoprovidedindividualO,Ne,Fe,andSabun- denceofX-rayemissionproducedbycoolhighdensityplasma, dances,whiletheabundancesoftheotherelementsweretiedto likely resultingfroman accretionprocess. Previouscases were theFeabundancebecausethefitqualitydoesnotimproveifthey TW Hya, BP Tau, and V4046 Sgr. Moreover, MP Mus shows aretreatedasadditionalfreeparameters. also clear evidence of intense coronal activity, as indicated by Table 2 shows the fluxes of the strongest RGS lines, mea- theflares(seeFig.1)andbythehotplasmacomponent. sured with PINTofALE V2.0 (Kashyap&Drake 2000), adopt- ingalorentzianfunctiontofitthelineprofile.We checkedthat 3.1.Abundances thederived3-T EPICmodelprovidesareasonablygooddescrip- tionoftheRGSlinefluxes. The X-ray emitting plasma in MP Mus is heavily depleted of WeidentifiedtheOandNeHe-liketripletsintheRGS Fe,Si,andMg;O,andSaremoderatelydepleted,whileNedis- spectra.TheOlines(indicatedasr,i,and f inFig.2)provide playsalargerabundance.IfweassumethatsoftX-raysarepro- a density-sensitive ratio f/i = 0.28 ± 0.13, which implies an duced by plasma heated in the accretion process, the observed electrondensitylogN =11.7±0.2fortheplasmaatT ∼2MK. abundancesprobethechemicalcompositionoftheinfallingcir- e The Ne line flux measurements are affected by large uncer- cumstellar material. We compared the present abundance val- taintiesduetothestrongblendingwithFelines.Figure3shows ues of MP Mus with those obtained for the other three CTTSs theobservedNetripletwiththatpredictedonthebasisofthe showingevidenceofX-rayemissionduetoshock-heatedplasma 3-T EPICmodelassumingdifferentN values.Thiscomparison (Table3).InallcasestheX-rayspectraindicatethattheaccreted e suggeststhattheplasmaatT ∼4MK,whichproducestheNe materialhasaNeabundanceenhancedwithrespecttotheother C.Argiroffietal.:X-rayemissionfromtheoldCTTSMPMus 3 Table2.StrongestRGSlinesofMPMus. Table3.CTTSsproperties. λa λa Ion logTb (F±σF)c 12o.b1s4 12p.r1ed3 NeNeFe 6.8m0ax 22.2±3.4 Star Stellar Age Ne/O Tmbed Ne 12.30 12.28 FeFe 7.00 5.5±2.6 Associationa (Myr) (MK) (1011cm−3) 12.86 12.85 FeFeFeFe 7.00 3.4±2.3 BPTau Taurus 0.6c 0.43d 16.1d 3.2d 13.46 13.45 NeFe 6.60 21.1±3.7 TWHya TWA 8e 0.87f 4.8d 21.1d 13.55 13.52 NeFe 6.90 8.6±3.1 V4046Sgr BPMG 12g 1.04h 6.4h 3.2h 13.73 13.70 Ne 6.60 8.8±2.8 MPMus LCC 17i 0.46 8.5 5.0 14.19 14.21 FeFe 6.90 3.3±1.7 15.02 15.01 Fe 6.70 11.8±2.9 15.21 15.18 OOFeFe 6.50 7.6±2.5 a TWA = TW Hydrae association, BPMG = β Pictoris Moving 16.02 16.01 OOFeFe 6.50 22.6±6.3 Group, LCC = Lower Centaurus Curx; b Tmed is derived as 16.78 16.78 Fe 6.70 8.8±1.8 (ΣiEMiTi)/(ΣiEMi); c Gullbringetal. (1998); d Robrade&Schmitt 17.08 17.05 FeFe 6.70 17.2±3.0 (2006); e Makarov&Fabricius (2001); f Drakeetal. (2005); 18.65 18.63 O 6.30 8.7±2.6 g Zuckermanetal. (2001); h Gu¨ntheretal. (2006, T has been 18.98 18.97 OO 6.50 65.6±4.7 derivedfromthelinefluxes);iMamajeketal.(2002). med 21.60 21.60 O 6.30 30.2±4.0 21.81 21.80 O 6.30 27.9±7.9 22.10 22.10 O 6.30 8.0±2.8 24.80 24.78 NN 6.30 10.5±2.8 28.49 28.47 CC 6.20 3.8±1.3 33.74 33.73 CC 6.10 16.4±5.2 aObservedandpredicted(APEDdatabase)wavelengths(Å).bTemperature(K) ofmaximumemissivity. c Linefluxes(10−6phs−1cm−2)withuncertainties at the68%confidencelevel. elements (with respect to the solar photosphericabundancera- tios). Stelzer&Schmitt(2004)suggestedthatthenon-solarabun- dancesoftheshock-heatedplasmaofTWHyamaybeexplained byassumingthattheaccretingmaterialunderwentgraindeple- tion, a mechanism already invoked by Herczegetal. (2002) to explainthelowSiabundance.Dependingonthetemperature,the circumstellarmaterialiscomposedbygasanddustphaseswhich havedifferentchemicalcompositions.Theactualabundancera- tiosinthegasphasearedeterminedbythedifferentcondensation Fig.4. Average plasma temperature and Ne/O ratio vs age for temperatures T of the various elements (Savage&Sembach c the sample of four CTTSs with evidence of high density cool 1996). One of the proposedscenariosof disk stratification pre- plasma. dicts that dust grains mostly settle in the disk midplane, while the gas extends up to the disk surface, where it is easily ion- izedbythestellarradiation.Hereitisfunnelledalongthemag- neticfieldlinesandaccretesontothecentralstar.Ifthegasand that the condensation temperature of O is quite low (180K), dustphasesarespatiallyseparatedintheinnercircumstellardisk and/orthegasaccretesmoreefficientlythanthedust,theshock- thereforetheseparationbetweengasanddustmustoccuratlow temperatureto producesignificantO depletion in the accretion heated material should be depleted of those elements (like Fe) streams. whicheasilycondenseintodustgrainsandconverselyenriched ofmorevolatileelements(likenoblegases).Followingthissce- AlargeNe/OabundanceratioisobservedalsointheX-ray nario, the accreted material should have an abundance pattern spectrum of V4046 Sgr (Gu¨ntheretal. 2006), where high den- similartothatobservedintheinterstellargas,i.e.withtheabun- sity hints again at X-rays from shock-heated plasma. Instead, dancesdecreasingforincreasingT .Notethatthephenomenon both MP Mus and BP Tau have Ne/O ratios typical of stellar c ofgas/dustseparationincircumstellardisks,andthesubsequent coronae.Drakeetal.(2005)explainedtheNe/OratioofBPTau, accretionofonlythegasphase,isalso heldresponsibleforthe lowerthanthatofTWHya, onthebasis ofthedifferentevolu- peculiarabundancesofRVTauristars(seeGiridharetal.2005, tionarystagesoftheircircumstellardisks.SinceTWHyaissig- andreferencestherein). nificantly olderthan BP Tau, it is conceivablethat the dust/gas Drakeetal.(2005)showedthatTWHyadisplaysaNe/Ora- separationprocess,andthe subsequentdepletionof highTc el- tio larger by a factor ∼ 2 than the uniform value observed in ements,is notvisible in the latter case becausethese processes a vast sample of coronal X-ray sources (Drake&Testa 2005). occuronatimescalelongerthantheageofBPTau(∼0.6Myr). Moreover,Ne/Fe abundance ratios as large as that of TW Hya InTable3wereporttheagesofthefourCTTSsintroduced were observed only in very few cases of putatively coronal above.Weadoptanageof17MyrforMPMus(Mamajeketal. sources. These results support the hypothesis that soft X-ray 2002),sinceitiscompatiblewiththeageoftheLCCassociation. radiation from TW Hya is not produced by coronal plasma. Forthesubsequentdiscussion,theabsoluteageofeachCTTSis Drakeetal. suggest that the Ne/O ratio can be used as a cri- unimportant, while only the age sequence matters, whose reli- terion to identify X-ray sources where the emitting material in ability depends only on the correctness of the membership of theaccretionstreamsufferedgraindepletion.However,wenote theseCTTSstotherelevantstellarassociations. 4 C.Argiroffietal.:X-rayemissionfromtheoldCTTSMPMus Figure 4 shows the variations of plasma average tempera- 1012cm−3,respectively),findingthatatleast80%oftheOis ture and Ne/O ratio with respect to stellar age, for the sample duetohighdensity(i.e.shock-heated)plasma,andatmost20% of fourCTTSs (havingspectral types rangingfrom K1 to K7). tolowdensity(i.e.coronal)plasma. Both T and Ne/O do not have a monotonic trend with age, HencethederivedaccretionrateM˙,whichdependsonlyon med but these two plots suggest that stars with hotter plasma have the hypothesis (1) and (2), but not on N , is acceptable, but a e lowerNe/Oratios,andviceversa.ItislikelythathighT in- larger M˙ could be possible if part of the X-ray emission is ab- med dicates a large contribution from coronal plasma to the whole sorbed. X-ray emission. In this scenario of mixed accretion-drivenand ThemeasuredN ismoreuncertain:asmallcontributionof e coronalX-rayemission,themeasuredNe/Oratioisaweighted lowN coronalplasmatotheOtripletmightcauseanunder- e averageofthevaluesintheshock-heatedplasmaandinthecoro- estimation of N ; conversely an UV field might influence the e nal plasma. Hence any large Ne/O ratio of the accreted mate- populations of the O atomic levels by photoexcitation and rial may be partly hidden by the coronal plasma abundances. hence mimic an high density plasma. No UV excess emission, TocheckthispossibilitywefittedtheobservedEPICspectraof whichcouldoriginatefromthe accretionhotspot,hasbeenre- MPMusassumingahighNe/Oratioforthecoolestplasmacom- ported for this star. However a sufficiently high UV radiation ponent,but the modeldoes notreproducethe observedspectra density can be present only very near the accretion hot spot aswellasthemodeldescribedinSect.2.Moreover,inMPMus onthestellarsurface,andthephotoexcitationhypothesiswould the hot coronal plasma does not contribute significantly to the anywayindicatethatthecoolX-rayemittingplasmaiscloseto observedOandNelineemission(seebelow). thebaseoftheaccretionfunnel. We concludethat the relatively low Ne/O ratio in MP Mus isacharacteristicofthecoolaccretioncomponent,andthestel- lar age is likely not the only parameter which determines the 4. Conclusions Ne/O ratio observed in CTTSs with evidence of high density FromtheanalysisoftheXMM-NewtonobservationoftheCTTS coolplasma. MPMuswederivedevidencesthatplasmaheatedintheaccre- tionshockproducesthesoftpartoftheX-rayemission.Wemea- 3.2.Accretion sured a Ne/O ratio similar to that of BP Tau and reduced by a factor 2 with respect to that of TW Hya and V4046 Sgr: this For the subsequentdiscussion we first assume that the cool X- result suggests that the stellar age is not a useful parameter to rayemittingplasmaofMPMusisonlyduetotheshockaccre- predictthe amountof grain depletion suffered by the accreting tion, with no contribution from coronal plasma. Starting from material. theMamajeketal.(2002)resultsonMPMus,andbasedonthe Siessetal.(2000)stellarmodels,weadoptforMPMusamass Acknowledgements. CA,AM,andGPacknowledgepartialsupportforthiswork of1.2M⊙andaradiusof1.3R⊙. from contract ASI-INAF I/023/05/0 and from the Ministero dell’ Universita` Using the O triplet and the O Lyα lines we infer the e della Ricerca. Based on observations obtained with XMM-Newton, an ESA electrondensity(N = 5×1011cm−3),temperature(T = 3MK, science mission with instruments and contributions directly funded by ESA e MemberStatesandNASA. obtained from the O Lyα and O r lines), and emission measure(EM = 2.4×1053cm−3) of the postshockplasma. In the strong shock scenario, the relevant plasma parameters are References linkedbytherelations: Arnaud, K. A. 1996, in ASP Conf. Ser. 101: Astronomical Data Analysis SoftwareandSystemsV,ed.G.H.Jacoby&J.Barnes,17 1 3 µm N =4N , v = v , T = Hv2 (1) Asplund,M.,Grevesse,N.,&Sauval,A.J.2005,inASPConf.Ser.336:Cosmic 1 0 1 4 0 1 16 k 0 AbundancesasRecordsofStellarEvolutionandNucleosynthesis,ed.T.G. Barnes,III&F.N.Bash,25 wherethesuffixes0and1indicatethepre-shockandpost-shock Batalha,C.C.,Quast,G.R.,Torres,C.A.O.,etal.1998,A&AS,128,561 plasma, N the density,v the velocity,T the temperature,and µ Calvet,N.&Gullbring,E.1998,ApJ,509,802 Carpenter,J.M.,Wolf,S.,Schreyer,K.,Launhardt,R.,&Henning,T.2005,AJ, the mean molecular weight (in our case µ = 0.61). From the 129,1049 measured temperature T1 we infer that the pre-shock velocity Drake,J.J.&Testa,P.2005,Nature,436,525 is 470kms−1. This value correspondsto a free fall from an in- Drake,J.J.,Testa,P.,&Hartmann,L.2005,ApJ,627,L149 ner radius of the circumstellar disk of 3R , or from a larger Feigelson,E.D.&Montmerle,T.1999,ARA&A,37,363 ⋆ distance if some energy loss occurs during the fall. 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