Astronomy&Astrophysicsmanuscriptno.11228 (cid:13)c ESO2009 January23,2009 Cold gas as an ice diagnostic toward low mass protostars KarinI.O¨berg1,SandrineBottinelli1,andEwineF.vanDishoeck1,2 9 0 0 1 LeidenObservatory,LeidenUniversity,P.O.Box9513,NL2300RALeiden,TheNetherlands 2 2 Max-Planck-Institut fu¨r extraterrestrische Physik (MPE), Giessenbachstraat 1, 85748 n a Garching,Germany J 8 ] ABSTRACT R S Context.Upto90%ofthechemicalreactionsduringstarformationoccursonicesurfaces,proba- . h blyincludingtheformationofcomplexorganics.Onlythemostabundanticespeciesarehowever p observeddirectlybyinfraredspectroscopy. - o Aims.Thisstudyaimstodevelopanindirectobservationalmethodoficesbasedonnon-thermal r t icedesorptioninthecolderpartofprotostellarenvelopes. s a Methods.TheIRAM30mtelescopewasemployedtoobservetwomoleculesthatcanbedetected [ 1 bothinthegasandtheice,CH3OHandHNCO,toward4lowmassembeddedprotostars.Their v respectivegas-phasecolumndensitiesaredeterminedusingrotationaldiagrams.Therelationship 9 betweeniceandgasphaseabundancesissubsequentlydetermined. 1 0 Results.The observed gas and ice abundances span several orders of magnitude. Most of the 1 CH OHand HNCO gasalong thelinesof sight isinferredtobequiescent fromthemeasured . 3 1 linewidthsandthederivedexcitationtemperatures,andhencenotaffectedbythermaldesorption 0 9 closetotheprotostarorinoutflowshocks. Themeasuredgastoiceratioof∼10−4 agreeswell 0 withmodelpredictionsfornon-thermaldesorptionunder coldenvelopeconditionsandthereis : v a tentative correlation between ice and gas phase abundances. This indicates that non-thermal i X desorptionproductscanserveasasignatureoftheicecomposition.Alargersampleishowever r necessarytoprovideaconclusiveproofofconcept. a Key words. Astrochemistry, Molecular processes, Molecular data, ISM: molecules, Circumstellarmatter,Radiolines:ISM 1. Introduction In cold pre-stellar cores, more than 90% of all molecules, except for H , are found in ices 2 (Casellietal. 1999; Berginetal. 2002). These ices build up through accretion of atoms and molecules onto cold (sub)micron-sized silicate particles and subsequent hydrogenation to form e.g.H OfromO(Le´geretal.1985;Boogert&Ehrenfreund2004).ObservationsshowthatH Ois 2 2 themainiceconstituentinmostlinesofsight,withatypicalabundanceof1×10−4withrespectto H ,followedbyCO,CO andCH OH(Gibbetal.2004;Pontoppidanetal.2004). 2 2 3 During star formation, these ices may be modified by interactions with cosmic rays, UV ir- radiation,and heating to form complex organicspecies (Garrodetal. 2008). Gas phase complex 2 KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars specieshavebeenobservedtowardseveralhighandlowmassprotostars,so-calledhotcoresand corinos (Bisschopetal. 2007; Bottinellietal. 2007). Whether these molecules are formed in the iceandsubsequentlyevaporated,orformedinthehotgasphasefromdesorbedsimplericessuch asCH OHisstilldebated.Thisisnoteasilyresolvedbecausetheabundancesofthesolidcomplex 3 moleculesaretoolowtobedetectedwithinfraredobservationsoficeseveniftheyarepresentin the ice. Therefore,observinggas-phaseabundancesin the coldenvelopemay be the mostrobust constraintoncomplexiceprocessesavailable. Experimentalinvestigationshaveconcludedthatnon-thermaldesorptionisefficientforseveral commonicemolecules,suchasCO,CO ,andH O,withphotodesorptionyieldsof∼10−3perinci- 2 2 dentphoton(Westleyetal.1995;O¨bergetal.2008,2009).Photodesorptionispossibleinsidecold darkcloudcoresandprotostellarenvelopesbecauseofconstantUVfieldsgeneratedfromcosmic ray interactions with H (Shenetal. 2004). Thus a small, but significant, part of the molecules 2 formedintheiceshouldalwaysbepresentinthegasphase.Thisexplainsobservedabundancesof gasphaseCH OHintranslucentclouds,darkcloudcoresandprotostellarenvelopes(Turner1998; 3 Maretetal.2005;Requena-Torresetal. 2007).TheamountofCH OH gasobservedinthese en- 3 vironmentssuggeststhatcomplexmolecules(e.g.methylformate)thatforminthe iceshouldbe observable in the gas phase due to ice photodesorption,if their abundance ratios with respect to CH OHintheicearethesameasobservedinhotcoresandcorinos. 3 Forthefirsttime,wecombineinfrarediceobservationsandmillimetergasobservationsforthe same lines of sight to investigate the connection between ice and quiescent gas abundances. We focusontheonlycommonlyobservedice componentsthathaverotationaltransitionsin themil- limeterspectralrange–CH OHandHNCO.TheCH OHiceabundancesinlowmassprotostellar 3 3 envelopesvarybetween1–30%withrespecttoH Oice(Boogertetal.2008).Itisalsooneofthe 2 mostcommonhotcorinogasphasemoleculeswithtypicalabundancesof10−7−10−6withrespect toH .HNCOgasisalsocommonlydetectedinhotcores.SolidHNCO(intheformofOCN−)is 2 onlydetectedtowardafewlowmassprotostars,butstrictupperlimitsexistformore,resultingin anabundancespanofanorderofmagnitude(vanBroekhuizenetal.2005).Theselargevariations iniceabundancesimplythatCH OHandHNCOareappealingtestcasesforourtheorythatqui- 3 escent complex gas abundancesreflect the composition of the co-existingice mantles, underthe assumptionthatOCN− isprotonatedduringdesorption. We haveobservedgasphase CH OH andHNCO with the IRAM 30m towardfourlow mass 3 protostarsforwhichCH OHandHNCOicedetectionsorupperlimitsalreadyexist.Twoofthese 3 sourcesalsohaveOCN− iceupperlimits.Thesesourcesarecomplementedwithliteraturevalues toconstrainfurthertherelationshipbetweeniceandgasphaseabundances. 2. Sourceselection ThefoursourcesIRAS03254+3050,B1-b,L1489IRSandSVS4-5werechosenfromthe‘coresto disks’(c2d)sampleoflowmassprotostarswithicedetections(Boogertetal.2008)tospanCH OH 3 abundancesof4−25%withrespecttoH O.Thec2dsamplepartlyoverlapswithanearlierground 2 based surveyusing the VLT, for which the OCN− abundancesand upperlimits were determined (vanBroekhuizenetal.2005)andtwoofthesourceshaveOCN− iceupperlimits(Table1). KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars 3 Table1.Targetswithpointingpositionsandicedata. Source RA Dec V α cloud H Ocol.dens. CH OH HNCO LSR 2 3 kms−1 2–24µm 1018cm−2 %H O %H O 2 2 IRAS03254+3050 03:26:37.45 +30:51:27.9 5.1 0.90 Perseus 3.66 <4.6 – B1-b 03:33:20.34 +31:07:21.4 6.5 0.68 Perseus 17.67 11.2 – L1489IRS 04:04:43.37 +26:18:56.4 7.1 1.10 Taurus 4.26 4.9 <0.06 SVS4-5 18:29:57.59 01:13:00.6 7.8 1.26 Serpens 5.65 25.2 <0.27 Table2.Observedfrequenciesandtargetedmoleculesatthefourdifferentsettings. Frequency Frequency Transitiona E (K) rms(mK) Integratedintensity[uncertainty](Kkms−1) u range(GHz) (GHz) IRAS03254 B1-b L1489IRS SVS4-5 CH OH 3 1)96.739–96.756 96.739(E−) 2 –1 20.0 11–15 0.11[0.03] 1.65[0.22] 0.10[0.03] 1.47[0.33] 12 11 96.741(A+) 2 –1 14.4 0.16[0.04] 2.18[0.21] 0.17[0.05] 2.72[0.58] 02 01 96.745(E+) 2 –1 27.5 0.017[0.017] 0.37[0.20] 0.024[0.024] 0.64[0.39] 02 01 96.756(E+) 2 –1 35.4 <0.01 0.09[0.26] <0.024 0.074[0.030] 11 10 2)251.360–251.811 251.738(A±) 6 –6 98.6 28–40 <0.033 – <0.079 <0.26 33 24 HNCO 3)109.872–109.938 109.906 5 –4 15.4 15–23 0.038[0.030] – <0.027 0.22[0.07] 05 04 4)131.885–131.886 131.886 6 –5 18.6 19–29 <0.021 0.43[0.06] <0.037 0.22[0.07] 06 05 a The quantum numbers for the pure rotational transitions of CH OH and HNCO are J and J , 3 KaKc K−1K+1 respectively. According to the classification scheme of Lada & Wilking (1984),all sources are embedded class 0/I sourceswith spectralenergydistribution(SED) slopes in the mid-infrared2–24µm be- tween 0.68 and 1.26. Their envelopes are of similar mass, as traced by the H O ice abundance, 2 with the possible exception of B1-b, which has a factor of 3 higher column density. Except for SVS4-5thesourcesareisolatedonthescaleoftheIRAM30mbeam.BothB1-bandL1489IRS have however moderate outflows associated with them that may contribute to the detected lines (Jørgensenetal.2006;Girartetal.2002). SVS 4 regionis one ofthe densestnearbystar-formingregionsandSVS 4-5is located∼20” awayfromthe class0 low-massprotostarSMM 4,which hasa largeenvelopeandan associated outflow.Inaddition,the youngstellar objectsSVS4-2–12arealllocatedwithin30′′ ofSVS4-5 andtheemissionfromSVS4-5isprobablycontaminatedbyemissionfromitssurroundingswhen observedwith a beam of width as largeas 24”(as is the case here).Despite these complications ininterpretingthedata,SVS4-5isincludedinthesamplebecauseofitsunusuallyhighCH OH 3 iceabundance,whichwasdeterminedfromicemappingoftheSVS4regionbyPontoppidanetal. (2004). 4 KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars 3. Observations The observations were carried out in March 2008 with the 30-m telescope of the Institut de RadioAstronomieMillime´trique(IRAM).ThepositionsusedforpointingarelistedinTable1.The linefrequenciesaretakenfromtheJPLmoleculardatabase.Althoughtheobservationswerecen- teredontheprotostarsthemselves,thechoiceoflowexcitationlinesandrelativelylargebeamsen- suresthatthecoldouterenvelopeisalmostcompletelysampled.Specifically,wetargetedCH OH 3 transitions with E , the energy of the upper level of the transition, between 7 and 100 K. The up HNCOlineswereobservedafterthepreliminaryreductionoftheCH OHdata,andbecauseofits 3 lowexcitationtemperaturewechosetoobservetwoofthelowestlyingHNCOtransitionswithE up of 15 and 19 K. The observationswere carriedoutusing fourdifferentreceiversettings with the frequencyranges shown in Table 2. Each receiver was connected to a unit of the autocorrelator, withspectralresolutionsof80or320kHzandbandwidthsbetween80and480MHz,equivalent to a velocity resolution of 0.3, 0.4 and 0.2 km s−1 in settings 1, 2, and 3/4, respectively.Typical systemtemperatureswere100-200K,200-500K,and700-1000K,at3,2,and1mm,respectively. All observations were carried out using wobbler switching with a 110′′ throw in azimuth. Pointing and focus were regularly checked using planets or strong quasars, providinga pointing accuracyof3′′. All intensitiesreportedin thispaperareexpressedin unitsofmain-beambright- nesstemperature,whichwereconvertedfromantennatemperaturesusingmainbeamefficiencies of76,69,and50%,at3,2,and1mm.Atthesewavelengths,thebeamsizeswere24,16,and10′′, respectively. 4. Results Figure 1 shows the spectra derived for setting 1, which is the only setting in which CH OH is 3 detected.Figure2showstheobtainedspectrainsettings3and4,withalltargetsobservedinboth settingsexceptforB1-b.TheobservedlineswerefittedwithasingleGaussiantocalculatetheline widthsandintegratedintensitiesinTable2.TheGaussianfitswererestrictedtoexcludethewings observable for B1–b and SVS 4–5. The resulting line widths range from 0.4 to 4.0 km s−1, but threeof the sources(B1-b,IRAS 03254,andL1489IRS) consistentlyhaveline widthsof below 1 km s−1 (Table 3). Coupledwith the low excitationtemperatures(below),we mostlikely probe thequiescentenveloperatherthanoutflowsorhotcorinos.ThefourthsourceSVS4–5hasseveral emissioncomponents,reflectingthecomplexityoftheSVS4regionandcontainingcontributions from non-quiescent gas in for example the nearby outflow from SMM 4. The typical envelope angularsize forthe sourcedistanceis∼1’,whichislargerthanthelargestbeamsize.Hence,we assumeintheanalysisthatthereisnobeamdilution. In Figs.3 and4, weuse therotationaldiagrammethod(Goldsmith&Langer1999)to derive rotational temperatures and column densities. The relations evident in the CH OH diagrams are 3 approximatelylinear, with a possible slight deviation for the one CH OH A detection. A line is 3 fitted to all CH OH detections, with the assumption that the populationsof E and A species are 3 approximately equal. The 2σ upper limits (derived from the rms in Table 2) are overplotted to enableustoensurethattheydonotprovidefurtherconstraintsonthefittedline.ExceptforSVS4- 5,HNCOcolumndensitieswerederivedusingtheCH OHrotationaltemperatures.Thelowerlimit 3 totheCH OHtemperatureof4KwasusedforIRAS03254toaccommodatethestrictupperlimit 3 KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars 5 Fig.1.TheobservedCH OHlinesinsetting1towardthefourlowmassprotostarsplottedversus 3 ∆V,thedeviationfromthesourceV .Thedataintheleftpanelarecenteredonarestfrequency lsr of96.756GHzandintherightpanelonarestfrequencyof96.741GHz. Fig.2.TheobservedHNCOlinesinsetting3and4plottedversus∆V,thedeviationfromthesource V . The data in the left panel are centered on a rest frequencyof 109.906GHz and in the right lsr panelon131.886GHz. 6 KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars Fig.3.CH OHrotationdiagramsincludingdetectionsandupperlimits. 3 Fig.4.HNCOrotationdiagramswherebothlinesaredetectedortheupperlimitisstrict. from setting 3 (Table 3). The resulting CH OH and HNCO temperatures vary between 4 and 9 3 KandtheCH OHandHNCO columndensitiesvarybetween1.8−27×1013 and0.095−2.4× 3 1013 cm−2,respectively.CH OHiseasilysub-thermallyexcitedattheexpecteddensitiesinouter 3 protostellarenvelopesofapproximately104 cm−3 andhencetherotationaltemperaturecannotbe directlytranslatedintoakinetictemperature(Bachilleretal.1995). In previousstudies, gas phase CH OH was observed toward three other low mass protostars 3 thatwerealsoobservedbySpitzertostudyices(Boogertetal.2008):Elias29,RCrA7A,andB (Table 3). The line widths imply thatthe observedCH OH lines toward Elias 29 trace quiescent 3 material,whilethoseobservedtowardRCrA7AandBdonot. Figure5showsthecorrelationbetweengasandiceabundancesofCH OHandHNCO,includ- 3 ingtheliteraturesources.TheabundancesarewithrespecttotheH Oicecolumndensity,which 2 wasfoundtocorrelatewellwiththecolddustcolumndensity(Whittetetal.2001).Hencethisis KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars 7 Table3.Thecalculatedtemperaturesandcolumndensities. Source Molecule Linewidth T N rot X (kms−1) (K) (×1013cm−2) IRAS03254 CH OH 0.42–0.49 6±2 1.8±1.8 3 HNCO 0.77 ∼4 ∼0.48 B1-b CH OH 0.77–0.88 7±1 25±13 3 HNCO 0.86 ∼7 ∼2.4 L1489IRS CH OH 0.75-0.97 6±2 1.8±1.3 3 HNCO – ∼6 <0.095 SVS4-5 CH OH 2.4–4.0 7±1 27±10 3 HNCO 2.6–2.9 9±6 0.38±0.38 Elias29a CH OH 1.3 ∼9 ∼0.73 3 RCrA7Ab CH OH 2.4–3.0 18±2 59±28 3 RCrA7Bb CH OH 2.1–2.6 19±1 94±28 3 aBuckle&Fuller(2002)bderivedfromScho¨ieretal.(2006)usingrotationaldiagrams. areasonablenormalizationfactorforthelinesofsightwithquiescentgas,heredefinedtobeline widths . 1 km s−1, that originates in the cold envelope. It is not a priori a good normalizer for sourceswith non-quiescentemission, butfor consistencythe same normalizationmethodis used forallsources.Figure5illustratesapossiblecorrelationbetweenthegasandiceabundancesand upperlimitsforthequiescentsources–thecorrelationisnotstatisticallysignificantduetothemany upperlimits.ThetwoopentrianglesinthefigurearetheRCrAsources,whosehigherratioofgas toicephaseabundancecanbeattributedtoanenhancedradiationfieldintheregion(vanKempen 2008).Themeasuredaveragegastosolidabundanceratiois1.2×10−4forthequiescentgas.This probablyunderestimatesthetruegastoiceratiobecausetheabsorptionandemissionobservations differ,i.e.,thegasphaseobservationsprobeonaveragelessdenseregionsthantheiceobservations. 5. Discussion Icephotodesorptionpredictsgastoiceratiosof10−4−10−3 (seeAppendix)fortypicalphotodes- orptionyieldsandenvelopeconditions.The measuredgasto ice ratio in thisstudy of1.2×10−4 agrees well with this prediction, when accounting for the fact that the measured ratio probably underestimatesthetruegastoiceratio.ThedashedlinesinFig.5furthershowthatallquiescent detectionsandupperlimitsareconsistentwithgastoiceratiosof(1−5)×10−4.Thisagreement andthe narrownessofthe line widths, supportstheinterpretationthatthe emission in these lines ofsightoriginatesinthecold,quiescentenvelope.Italsodemonstratesthatphotodesorptionalone issufficienttoreleasetheobservediceintothegas,eventhoughothernon-thermalprocessesare notexcludedsincelackofexperimentalstudiesonmostnon-thermaldesorptionpathwaysprevents usfromquantifyingtheirrelativeimportance.Gasphasereactionscanhoweverbeexcludedsince thereisnoefficientgasphasereactionpathwaytoformeitherCH OHorHNCO attheobserved 3 abundances(Garrodetal.2007,Hasselprivatecomm.). The tentative correlation between gas and ice phase abundances in this pilot study supports theideathatitispossibletodetermineicecompositionbyobservingthesmallfractionoftheice 8 KarinI.O¨bergetal.:Coldgasasanicediagnostictowardlowmassprotostars Fig.5. The correlation between ice and gas phase abundances of CH OH (diamonds and trian- 3 gles) and HNCO (squares). The filled symbols represent quiescent gas (<1.3 km s−1) and the open symbols non-quiescentgas (>2.4 km s−1). In the case of CH OH, the diamonds represent 3 measurements from this study and triangles from the literature. The dashed lines show constant CH OH(gas)/CH OH(solid)orHNCO(gas)/OCN−(solid)ratiosof1×10−4and5×10−4. 3 3 that is non-thermally released into the gas phase. To show conclusively that this method works, the size of the sample studied here must be increased and the uncertaintyin the derived column densitiesmustbereducedbyobservingmoreemissionlines.Itisalsoimportanttorememberthat untilphotodesorptiondata are available forall potentialice species therewill be at least a factor of two uncertainty in ice composition estimates using this method due to the different break-up probabilitiesofdifferentmoleculesduringphotodesorption(O¨bergetal.2008,2009).Nonetheless, themethodpresentedhererepresentsasignificantimprovementonthecurrentlackofobservational toolstostudycomplexicesinquiescentregions. Acknowledgements. WethankRuudVisserforstimulatingdiscussions.FundingisprovidedbyNOVA,theNetherlands Research School for Astronomy, the European Early Stage Training Network (‘EARA’ MEST-CT-2004-504604), a NetherlandsOrganisationforScientificResearch(NWO)SpinozagrantandtheEuropeanCommunity’ssixthFramework ProgrammeunderRadioNet(R113CT20035058187). 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UV gr Thecosmic-ray-inducedUVfluxassumesacosmicrayionizationrateof1.3×10−17s−1.Because photodesorptionisasurfaceprocess,thephotodesorptionrateofspeciesxdependsonthefractional iceabundance f ,whichisdefinedtobetheratioofthenumberdensityofspeciesxintheice,ni, x x tothetotalicenumberdensity,ni.Thefreeze-outrateofspeciesxdependsonthegastemperature T,whichissetto15K,themolecularweightm ,andthegasnumberdensityng.Foranaverage x x molecularweightof32,thisresultsinagasphaseabundanceng/n of(3−9)×10−4f /n .From x H x H this and an average total ice abundanceni/n of 10−4, the predicted gas to ice phase abundance H ratiois: ng (3−9)×10−4/n × f x ∼ H x ∼(3−9)/n (A.3) ni ni/n × f H x H x For a typical envelope density of 104 cm−3, ice photodesorption hence predicts a gas to ice ratioof10−4−10−3.Thederivationofagastoiceratiofromobservedcoldgasemissionlinesand ice absorption features in the same line of sight is complicated by the fact that different regions cancontributebyvaryingamounts.Theemissionfeaturestracegasintheenvelopeandcloudboth in front and behind the protostar, while the ice absorption features only trace envelope material directly in front of the protostar. The column is hence twice as long for the gas observations. Thisis probablymorethancompensatedfor byusing beamaveragedgascolumndensities, asis donein thisstudy,becausethelargebeamtracesonaveragelessdensematerialcomparedtothe pencilbeamoftheiceabsorptionobservations.NotealsothattheCH OHiceabundancemayvary 3 betweenlower andhigherdensityregions.To quantifya conversionfactorbetweenthe observed and true gasto ice ratio requiresdetailed modelingof each source which is outside the scope of thisstudy.Insteadwehereassumethattheobservedratioisalowerlimittothetrueratio.