Ammonia in Infrared Dark Clouds

0.61 MB·English

by  Thushara Pillai

Astronomy&Astrophysicsmanuscriptno.tpillai c ESO2008
(cid:13)
February4,2008
Ammonia in Infrared Dark Clouds
T.Pillai1,F.Wyrowski,1 S.J.Carey2 andK.M.Menten1
6
0 1 Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,D-53121Bonn,Germany
0 e-mail:thushara, wyrowski, [email protected]
2 2 SpitzerScienceCenter,CaliforniaInstituteofTechnology,MC314-6,1200EastCaliforniaBoulevard,Pasadena,CA91125

n e-mail:[email protected]
a
J
Preprintonlineversion:February4,2008

4

ABSTRACT
1
v
Context.Whilelow mass clouds have been relatively well studied, our picture of high-mass star formation remains unclear. Infrared Dark
8
Cloudsappeartobethelongsoughtpopulationofcoldanddenseaggregationswiththepotentialofharbouringtheearlieststagesofmassive
7
starformation.Uptonowtherehasbeennosystematicstudyonthetemperaturedistribution,velocityfields,chemicalandphysicalstatetoward
0
1 thisnewcloudpopulation.
0 Aims.Knowingthesepropertiesiscrucialforunderstandingthepresence,absenceandtheverypotentialofstarformation.Thepresentpaper
6 aimsataddressingthesequestions.Weanalysetemperaturestructuresandvelocityfieldsandgaininformationontheirchemicalevolution.
0 Methods.Wemappedthe(J,K)=(1,1)and(2,2)inversiontransitionsofammoniain9infrareddarkclouds.Ourobservationsallowthemost
/ reliabledeterminationofgastemperaturesinIRDCstodate.
h
Results.Thegasemissionisremarkablycoextensivewiththeextinctionseenatinfraredwavelengthsandwiththesubmillimeterdustemission.
p
- OurresultsshowthatIRDCsareonaveragecold(T <20K)andhavevariationsamongthedifferentcores.IRDCcoresareinvirialequilibrium,
o aremassive(M >100M ),highlyturbulent(1–3kms 1)andexhibitsignificantvelocitystructure(variationsaround1–2kms 1overthe
− −
r cloud). ⊙
t
s Conclusions.WefindanincreasingtrendintemperaturefromIRDCswithhighammoniacolumndensitytohighmassprotostellarobjectsand
a hotcore/UltracompactHregionsstagesofearlywarmhigh-massstarformation.ThelinewidthsofIRDCsaresmallerthanthoseobservedin
:
v highmassprotostellarobjectsandhotcore/UltracompactHregions.Onbasisofthissample,andbycomparisonoftheammoniagasproperties
i withinacloudandbetweendifferentclouds,weinferthatwhileactivestarformationisnotyetpervasiveinmostIRDCs,localcondensations
X
mightcollapseinthefutureorhavealreadybegunformingstars.
r
a
Keywords.infrared:ISM:Clouds,ISM:Molecules,ISM:Structure,MolecularData,RadioLines:ISM
1. Introduction lite’s other bands. Recently, we have reviewed the observa-
tionalstudiesonIRDCs(Menten,Pillai,&Wyrowski2005).
Infrareddarkclouds(IRDCs)arecold,densemolecularclouds
While low mass clouds have been relatively well studied,
seeninsilhouetteagainstthebrightdiffusemid-infrared(MIR)
our picture of high-mass star formation remains unclear (see
emission of the Galactic plane. They were discovered dur-
Evansetal. 2002). IRDCs appear to be the long sought pop-
ing mid-infrared imaging surveys with the Infrared Space
ulation of cold and dense aggregations with the potential of
Observatory (ISO, Peraultetal. 1996) and the Mid-course
harbouring the earliest stages of massive star formation. It is
SpaceExperiment(MSX,Eganetal.1998).
likely that some of the stars formingin them are massive (lu-
In an initial census of a 180 long strip of the Galactic
∼ ◦ minosities of submm condensations range up to 104 L ). A
plane(between269 < l < 91 ,b = 0.5 ),Eganetal.(1998)
◦ ◦ ± ◦ recent study by Ormeletal. (2005) on an IRDC toward⊙s the
found 2000compactobjectsseeninabsorptionagainstbright
∼ W51 Giant Molecular Cloud (GMC) suggests that sources of
mid-infraredemissionfromtheGalacticplane.Examinationof
300 M are embedded within the cores, most likely proto-
2MASS, MSX and IRAS imagesof these objectsrevealsthat ∼ ⊙
stars.Recently,we(Pillaietal.2005)reportedadetailedstudy
theyappearasshadowsatallthesewavelengths,althoughthey
ofthestrongestsubmmpeakintheIRDCG11.11 0.12,where
are bestidentifiedin the 8.3µmMSX band,because,first, the −
wefindclearevidenceofaheavilyembeddedprotostar.
7.7and8.6µm PAH featuresassociatedwith star-formingre-
gions contribute to a brighter background emission and, sec- The salient results on IRDCs are summarised below. The
ond, the MSX 8.3µm band is more sensitive than the satel- IRDCsobservedso farhavesizesof1-10pcandhavemostly
a filamentary morphology. On the basis of LVG calculations
Sendoffprintrequeststo:T.Pillai of mm H2CO observations Careyetal. (1998) find that typi-
2 T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds
cal IRDCs have gas densities of n > 106 cm 3 and temper- stagesofhighmassstarformation.Wedoasimilarcomparison
−
atures of T < 20 K. Kinematic distances determined from with localdark cloudswell-studied in NH . Finally, we spec-
3
H CO observations using a standard Galactic rotation curve ulate on a possible formationmechanismof IRDCs involving
2
thatrangesbetween2.2and4.8kpc(Careyetal.1998)indicate supernovaremnants.
thatthe cloudsare notlocal. All observedIRDCs in the sam-
ple of Careyetal. (2000) contain 1 – 4 bright sub-millimeter
2. Observations
(submm)dustcontinuumemissionpeaks(>1Jy/14 beamat
′′
850µm)surroundedbyanenvelopeofemissionwhichmatches We mapped the IRDCs listed in Table. 1 with the
the morphology of the IRDC in mid-infrared extinction. The Effelsberg 100m telescope of the Max-Planck-Institut fu¨r
corescorrespondingtothebrightestsubmmpeakshavemasses Radioastronomie in October 1999. The frontend was the fa-
100and1200M ,exceptfortwocloudsintheCygnusregion cility1.3cmmaserreceivertunedtoafrequencyof23.7GHz
⊙
thathavemassesaround40M . centeredbetweentheNH (1,1)and(2,2)transitions.Thespec-
3
⊙
Hennebelleetal. (2001) in a systematic analysis of the trometerwas a 8192channelauto-correlatorused with 2 sub-
ISOGAL images extracted about 450 IRDCs, for which they unitsof20 MHzbandwidtheach.The resultingspectralreso-
derive15µmopticaldepthsof1to4.Teyssieretal.(2002)re- lution was 0.2 kms 1 after smoothing the data to increase
−
≈
portedthatLargeVelocityGradient(LVG)modelcalculations the signal-to-noiseratio.Thebeamwidthatthefrequenciesof
ofHC3N,13CO,andC18Oyielddensitieslargerthan105cm−3 the NH3 lines is 40′′ (FWHM). The observations were con-
inthedensestparts.Theauthorsclaimtofindkinetictempera- ducted in frequency switching mode with a frequency throw
turesbetween8and25KbasedonCH3CCHobservations;the of 7.5 MHz. The maps towardall sourcescoveredthe extinc-
highervaluesbeingfoundtowardembeddedobjects,however tion seenin the MSX imagesandweremadewith half power
adetailedanalysisishithertounpublished. beamwidth(HPBW)spacing(40 ).Alternatescanswereinter-
′′
Observations of molecules in IRDCs so far have concen- spacedathalfthefullbeamwidthresultingin 20 √(2)spac-
≈ ×
tratedonjustafewspecies.Uptonowtherehasbeennosys- ing.Pointingwascheckedatroughlyhourlyintervalsbymeans
tematic study on the temperature distribution, velocity fields, ofcontinuumdriftscansonnearbypointingsources.Wefound
chemicalandphysicalstatetowardthisnewcloudpopulation. thepointingtobeaccuratetowithin12 .
′′
Knowingthesepropertiesiscrucialforunderstandingthepres- Absolutecalibrationisnotanissueindeterminingtherota-
ence, absence and the very potential of star formation. The tionaltemperature,becauseitissolelygovernedbytheratioof
present paper aims at addressing these questions. We have theNH (1,1)hyperfinelinesandthe(1,1)and(2,2)brightness
3
started with a survey of a sample of 9 IRDCs in (1,1) and temperatures.However,inordertoestimatetheexcitationtem-
(2,2)cmrotationaltransitionsofammonia(NH3).Thissample peratureandthecolumndensitythedataneedstobecalibrated.
hasbeenstudiedbeforebyCareyetal.(1998)andCareyetal. The calibration procedure is documented on our webpage 1.
(2000) and was selected on the basis of the large extent and Theimportantstepsarementionedbelow.The100mdata(nor-
highcontrastoftheIRDCsagainsttheMIRbackground. mally in CLASS format) is in arbitrary noise tube units (T )
N
Ammonia has proven to be an important tool in measur- and has to be convertedto main beam brightnesstemperature
ingthephysicalconditionsinmolecularclouds(Ho&Townes units.WeobserveastandardfluxcalibratorwithknownfluxS
λ
1983). Since only the lowest NH3 energy levels are expected at wavelength λ and thus estimate the conversionfactor from
to be populated in cool dark clouds (T < 20 K), their physi- T scaletoJy.
N
calconditionscanbeprobedusingthe(1,1)and(2,2)inversion ThemainbeambrightnesstemperatureT ofthecalibra-
MB
transitionsin the metastable (J,K) rotationallevels of ammo- tor for a given beamwidth θ and wave length λ is then given
nia. Radiative transitionsbetween different K-laddersare for- by
bidden,thereforethelowestlevelsarepopulatedonlyviacolli-
sions.Theopticaldepthcanbedeterminedfromtheratioofthe (T /K)= (Sλ/Jy)×(λ/cm)2. (1)
hyperfine satellites. Thus, the population of the different lev- MB (θ/arcmin)2 2.65
×
elscan beestimatedandhencethe temperatureof thegasde-
Theprimaryfluxcalibrationisbaseduponcontinuumscans
termined.Inaddition,recentchemicalmodelsrevealthatNH
3 of NGC7027 assuming a flux density of 5.6 Jy (Ottetal.
(and also N2H+), does not deplete from the gas phase for the 1994).TheUltracompactH(UCH)regio≈nsG10.62-0.38and
densities observed in IRDCs (< 106 cm 3) (Bergin&Langer
− G34.26+0.15 and the quasar J1743-038 were used as sec-
1997).ThusNH isanexcellenttracerofthedensegaswhere
3 ondarycalibratorswhichspantheentireelevationrangeandan
manyothermoleculeswouldhaveheavilydepleted.
elevation-dependantcalibration factor was derived. The max-
In 2, we give details of our observations with the
§ imum rmsuncertaintyin calibrationforelevationslower than
Effelsberg 100m telescope. In 3, we discuss the data reduc-
tionandpresentthecorrelation§betweengasemissionandMIR 19◦is15%.
absorption.In 4,wederivetherotationaltemperature,gaski-
§
netic temperature and NH column density. Furthermore, we 3. DataReductionandAnalysis
3
analyse the velocitystructure, estimate dust mass, virial mass
The spectra were reduced using the CLASS package
andNH abundance.In 5,wecomparethecoregasproperties
3 § (Forveille,Guilloteau,&Lucas1989).Fora givensource,the
(temperature,NH linewidthsandcolumndensity)withthatof
3
otherpopulationsofobjectsthatarethoughttotracetheearly 1 www.mpifr-bonn.mpg.de/staff/tpillai/eff calib/index.html.
T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds 3
spectrawereaveragedandapolynomialbaselineoforder3–5
subtracted.
The maps were generated by convolvingthe originaldata
usingaGaussianfunctionwithaHPBWof40 .Table1reports
′′
thecoordinatesofthecentralpositionforeachmap.
MSX8µmimagesofthesecloudswithsuperimposedcon-
toursoftheNH (1,1)integratedintensityareshowninFig.1.
3
Thevelocityrangeusedforintegrationiswithin 25 kms 1of
−
±
the ’Local Standard of Rest Velocity’, V , hence including
LSR
thesatellitelines.TheNH mapsofG79.27+0.38andG33.71-
3
0.01haveclumpystructures.Someoftheclumpsmaybearti-
ficialandcausedbythelowsignal-to-noiseratio.
Thereduced(1,1)and(2,2)spectrafortheNH peakposi-
3
tions of the observed sources are shown in Fig. 2. NH (1,1)
3
observations were reduced using “METHOD NH3(1,1)” in
CLASS to fit the hyperfine structure and derive optical
depths and linewidths. The standard procedure to analyse the
NH (1,1) and (2,2) lines have been described in detail by
3
Bachilleretal.(1987).
The hyperfinestructure of the (2,2)line is too weak to be
observed; therefore, the optical depth could be obtained only
for NH (1,1). The NH (1,1) and (2,2) main beam brightness
3 3
temperatures are obtained by fitting the main line with a sin-
gleGaussian.The(1,1)and(2,2)linewidthisobtainedbythe
hyperfinefittingwhichaccountsforthelinebroadeningdueto
opticaldeptheffectsexcludingself-absorption.Theparameters Fig.3.SCUBA 850µmimage(Careyetal. 2000) of thecloud
obtainedfrom the fits to the (1,1)hyperfinelines are listed in G11.11-0.12 with NH (1,1) integrated intensity as contours.
3
Table2. The contour levels are multiples of 2σ. SCUBA image has a
Thebasicphysicalparameters,namelytheexcitationtem- resolutionof14 .
′′
perature, rotational temperature, the kinetic temperature and
ammonia column density, have been derived using the stan-
fortheIRDCG24.72 0.75andthenatureoftheMIRobjectis
dardformulationforNH3spectra(Ho&Townes1983).Table3 unknown. −
summarisestheestimatesoftheseparameterstowardthecores.
Ingeneral,thecloudgeometryisextendedandfilamentary
We give the formal errors (1σ), derived from Gaussian error
andinnocaseclosetospherical.Themeanaspectratiodeter-
propagation.
minedbyfitting2DellipsetotheentireNH emittingregionis
3
2.2andthetotalextentofthecloudsrangesfrom0.4–8.3pc.
4. Results G11.11-0.12isafilamentarycloudasrevealedbythe8µmex-
tinctionandthe850µmdustemissionasshowninFig.3.The
Oursampleconsistsofboth’Infrareddarkclouds’,withextents peaksofthesubmmemissionarestrikinglycoincidentwiththe
of 1–10pc, and ’Infrareddark cloud complexes’,which are ammonia cores. The NH map reveals at least two peaks to-
∼ 3
comprised of multiple individual clouds. IRDC G11.11-0.12 wardthe north-eastsegmentandanotherset ofpeakstowards
would thus be a complex(> 4 pc) while G24.63+0.15would the southwest extension of the filament suggesting that there
be a dark cloud. The IRDCs G79.34+0.33 and G79.27+0.38 are several unresolved sub-structures or cores within the ex-
are essentially two partsof the same extendedfilamentin the tendedfilament.
Cygnus-Xregionconnectedbyabrightpatchofdustemission ThestrongestsubmmemissionpeakP2inG28.34isinthe
asseeninSCUBAandMSXimages(Redmanetal.2003). closevicinityoftheIRASsource18402-0403.Butthepeakof
the ammoniaemission forthe northernextensionseemsto be
offsetfromthatoftheIRASsource(seeFig.4).Thiscouldbe
4.1.CloudMorphology
duetotheinteractionoftheIRASsourcewiththecloud.
There is generallya close match between the ammoniaemis-
sion and the mid-infrared extinction as shown in Fig. 1.
4.2.SourceSizes
In G24.72 0.75 and G79.34+0.33, the NH emission peak
3
−
is however correlated with bright and compact MIR emis- Duetothelargedistancestotheclouds,thecompactstructures
sion. Redmanetal. (2003) find that the MIR emission in withinthecloudaremarginallyresolvedwiththe40 beamof
′′
G79.34+0.33correspondstoaluminous’YoungStellarObject’ our NH observations. Hence, estimating the size of the core
3
(YSO)withastrongIRexcesswhichmightbeinteractingwith from NH might only deliver upper limits of the order of the
3
the foreground IRDC. No such interaction has been reported beamsize.Weusechannelmapstosolvetheproblemofiden-
4 T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds
Fig.1. MSX image of the clouds at 8.3µm (greyscale) with NH (1,1) integrated intensity as contours. The NH (1,1) maps
3 3
have a resolutionof 40 and the same spacing.The greyscalecorrespondsto intensity range shownby the wedge in the upper
′′
rightcorner(fromdarktolight).Thecontourlevelsare 2σ,1σ,2σ,4σ,6σ...ThermsnoiselevelsaregiveninTable2.The
−
approximatemapboundaryisalsoindicated.Tickmarksarecoordinateoffsets(inarcseconds)relativetothepositionsgivenin
Table1.Thebarindicatesalengthof1pcatthedistanceofeachIRDC.
tifyingclumps.Theinformationinthethirddimensionenables Gaussian fit routine in GRAPHIC. It searches for the bright-
ustodisentangleclumpsthataresufficientlywellseparatedin est pixel in each velocity channel of the channel map across
velocity.ForthosesourceswithSCUBA850µmdata,wecross- the NH (1,1) main componentand fits a 2D Gaussian to de-
3
identifythe clumpswithinhalfthe NH beam( 20 )atthe terminesourcesize.Theoutputisthencross-checkedoverthe
3 ′′
∼
higher SCUBA resolution ( 14 ) and estimate the source different channels and the fit obtained for the brightest emis-
′′
∼
sizebya2DGaussianfittotheclumpsintheSCUBAdata.In sionisused.We find1–5clumpsforeachcloud.Thesource
caseswherewedonothavedustemissionmaps,weusethe2D sizes after correcting for the beam size (after subtracting the
T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds 5
Fig.2. NH (1,1) (lower histogram in each panel) and (2,2) (upper histogram, offset from 0 K for clarity) spectra toward the
3
brightestpositionofthe9observedsources.ThebrightestpositionsarereferredtoasNH P1inTable3.
3
Gaussian beam size in quadrature) are listed in Table 3. The 4.3.LineProfiles
densecoreswithintheseIRDCsarethusnotresolvedwiththe
40 100m beam. We find several secondary peaks offset by
′′ Ingeneral,NH linesarebrighterthanthemmrotationalH CO
3 2
>1σinmostsources;offsetsfromthecentralpositionaregiven
transitionsobservedbyCareyetal.(1998).AsgiveninTable2,
inTable3.Table3shouldbereferredtoforthenomenclature
the (2,2) peak line intensity is on average 40% of that of the
of the clumps identified in the NH maps and/or the SCUBA
3 (1,1) line. Gaussian line profiles with extended wings have
850µmimageshereafter.
beenreportedforH COlinesinmostoftheclouds(Careyetal.
2
1998) but we do not observe any pronounced wings in NH .
3
Leurinietal.2006(submitted)findlineprofilessimilartothose
observedinH COinthemmtransitionsofCH OH.
2 3
6 T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds
4.8 kpc) has a linewidth of 2.65 km s 1. Neither of the two
−
coreshaveaMIRcounterpartwhichmightindicateanydeeply
embedded protostar influencing its immediate environments.
Averaging all the emission in G79.27+0.38 over an area of
200 200 ,weobtainalinewidthof1.5kms 1,stillsmaller
′′ −
∼ ×
than 2.65 km s 1. Therefore, the large linewidths derived ap-
−
pears to be mainly due to the velocity dispersions within the
beam.
One of the assumptionsmadeto derivethe temperatureis
thatthebeamfillingfactoristhesameforbothinversionlines;
thatis,thetwolinesareemittedbythesamevolumeofgas.The
(2,2)linewidthsareslightlylargerthanthe(1,1)linewidths(cf.
Table2)towardonlysomecores,suggestingthatthesamegas
isnotexactlytracedbybothlinesinthosesources.
Thelinewidthsexhibitedbythesesourcesaremuchlarger
thanthethermallinewidth,∆v ,whichforT 20K should
th kin
∼
be 0.22kms 1aspertherelation
−
∼
8ln(2)kT
∆v kin, (2)
th
∼ s mNH3
where k is Boltzmann’s constant and m is the mass of the
NH3
ammonia molecule. Average linewidths 2 km s 1 could be
−
∼
explainedin termsof velocitygradientsdueto rotationofthe
cloud, or turbulent cloud movements. The linewidths seems
to be especially high towards the cloud G28.34+0.06, where
Fig.4. SCUBA 850µm image of G28.34 with NH3 (1,1) in- there are several unresolved NH3 clumps, identified in the
tegratedintensityascontours.Thecontourlevelsare(2σ,4σ, SCUBAimages.IntheG79IRDCthelinewidthdecreasesfrom
6σ...). G79.34+0.33 NH P1 (1.22 0.03 kms 1) to G79.27+0.38
3 −
±
NH P3(0.82 0.03kms 1).AsshowninFig.5,wefindthatthe
3 −
±
linewidth anti-correlates with optical depth for G28.34 while
4.4.Linewidth thereisapossiblepositivecorrelationG79.27.
G79.27+0.38NH P1seemstobemorequiescentthanits
Thewidthofaspectrallineisanimportantparameterbecause 3
surroundings.IntheG79cloudcomplex,thereisconsiderable
it is a measure of the total kinetic energy of the cloud. The
difference in linewidth ( 0.5 kms 1) between the different
NH3(1,1)linewidthsforallsourcesliebetween0.8–3 kms−1. clumps along the cloud f∼rom east to−west. Thus, these clouds
The H CO linewidths are significantly larger (factor 2) than
2 might be harbouring objects at different stages of evolution
theNH linewidthsinallcasesexceptG81.50+0.14,wherethe
3 (Redmanetal.2003).
NH linewidthsaretwiceaslarge.
3
The NH linewidths for our sample are higher than those
3
ofNH coresreportedinJijinaetal.(1999),whicharemostly 4.5.KineticTemperature
3
low mass cores. The large linewidths might be explained as
Sincetheopticaldepthisknown,wederivetheexcitationtem-
due to clumping. Clumps with smaller linewidth but with a
peratureoftheNH (1,1)inversiontransition(Table3)viathe
higher clump-to-clumpvelocity dispersion may add up to the 3
relation
observed linewidths. The sources closest to us (G79 IRDCs)
have the smallest linewidth and belong to the Cygnus-X re- T
gion.In order to test whether the largerlinewidth we observe TEX = 1 MeBτ +2.7K, (3)
atlargerdistanceisadistanceeffect,weaveragetheG79IRDC − −
linewidthoveran areaδA (d /d )2 whered isthe dis- where T and τ represents the temperature and the optical
∝ far Cyg far MB
tance to a source farther than Cyg and with a much higher depthoftheofthe(1,1)line.Byfittingthemainandthehyper-
linewidth. On comparing the resultant scaled values, we find fine componentsof the (1,1) line and the main componentof
thatthe linewidthsare still significantly largerforthe sources the(2,2)line,weobtaintherotationaltemperature.Ananalyt-
atlargerdistancesafteraccountingforthelargerregionineach ical expression(Tafallaetal. 2004) has been used to estimate
beamforthemoredistantcores. the kinetic temperature from the rotational temperature. The
To illustrate this point, let us take the example of the kinetictemperatureisgivenbytheexpression
two extremevaluesof linewidthsfromoursample.Dark core
G79.27+0.38NH3 P2(at1kpc)hasalinewidthatthebright- T = Trot , (4)
est position of 0.83 km s−1 while G28.34+0.06 NH3 P2 (at kin 1− T4r2otln[1+1.1exp(−16/Trot)]
T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds 7
InG28.34+0.06,thegastemperaturederivedtowardsNH
3
P3, 12.7 1 K andNH P5,13.6 1 K issignificantlylower
3
± ±
than in the rest of the cloud. CH OH observations on the P1
3
position(Leurinietal.2006submitted)revealacoldandahot
component,thelatterwithasmallfillingfactor.Weobtainki-
netictemperaturesof 16KtowardsthepeaksofNH P1and
3
∼
NH P2.Higherangularresolutionobservationsareneededto
3
confirmahotcomponentofsmallextentalsoinammonia.
In G19.30+0.07, the temperature at the position of peak
NH emissionP1is17.9 1.6K,muchwarmerthantherestof
3
±
thecloud(13.1 1.0K).
±
In the G79 complex, we find that the temperature in
G79.34+0.33NH P1 is higher (14.3 0.8 K) than the other
3
±
2 coresG79.27+0.38NH P1 (12.2 0.8K) & G79.27+0.38
3
±
NH P2(11.1 1.2K)inthewesternpartofthefilament.
3
±
4.6.VelocityStructure
Theaveragevelocity-positiondiagramalonganaxiswithposi-
tionangleof+40 (toalignthepositionaxisroughlywiththe
◦
directionofthefilamentonthesky)forthecloudG11.11-0.12
is shown in Fig. 6. At every position along the position axis,
the NH (1,1)spectra were averagedalonga line perpendicu-
3
lartothepositionaxis.Thereisacleartrendforthevelocities
to decrease from the south to the P1 position and increase to
the north of the NE filament. This is also seen in the channel
Fig.5.CorrelationplotbetweentheNH3(1,1)opticaldepthand mapsshown in Fig. 7 where the differentclumpsin the north
theNH3 (1,1)linewidth.Bottompanel:G79.27+0.38NH3 P1, and south appear at distinctly separate velocities. Given that
Toppanel:G28.34+0.06NH3P2. the NH3 (1,1) linewidth at all positions is 2.5 kms−1, this
≤
velocityshift(> 5 kms 1) betweenthe peaksis certainlysig-
−
nificant.TheunresolvedclumpG11.11 0.12NH P4ismore
Thetypicaltemperaturesforthecoresrangefrom11–17K − 3
associatedwiththeG11.11 0.12NH P2clumpwhileP3fades
asdisplayedinTable3.Insomecases,wefindthatthefittothe − 3
awayatthosevelocities.
NH (1,1)lineslightlyunderestimatesthepeakintensity.
3
Recently, this filament had been studied in absorption
We findthatthekinetictemperatureissignificantlyhigher
against the diffuse 8 µm Galactic background and in emis-
thantheexcitationtemperature,asgiveninTable3.Thebeam
sion from cold dust at 850 µm (Johnstoneetal. 2003). They
fillingfactorηisameasureofthefractionofthebeamfilledby
modelthe850µmemissionbyfittinganon-magneticisother-
theobservedsource.Assumingthatthecoresareinlocalther-
malcylinderprofiletotheradialstructureoftheentirefilament
modynamic equlibrium (LTE), we may estimate this fraction
as η = T /T where T is the kinetic temperature of the (south and north segment).The velocity structure observed in
EX kin Kin
NH (1,1), however,shows several distinct clumps. This sug-
gas.Wederivefillingfactorsof 0.3–0.5foralltheclumps. 3
∼ geststhatthedensitystructureofthefilamentcannotberigor-
Theselowfillingfactorssuggesteithersub-thermalexcitation
ouslydescribedbyasimplecontinuouscylinder.
(nonLTEconditions)orclumpingwithinthebeam.Fromour
estimationofthesizes(Table3),wefindmostofthecoresare The channel maps for G28.34+0.06 are shown in Fig. 8.
unresolvedwith the 40 beam. Thus clumping is more likely The two extensions of the cloud around P1 and P2 differ in
′′
toexplainthesmallfillingfactors. velocity by about 1.5 kms−1. This is the difference in veloc-
We find hints of temperaturegradients(inside-out)within ity for the two peaks at which they are brightest. From the
thecoresinthreesourcesandareversegradientinonesource. channelmapsitappearsthatthereisabridgebetweenthetwo
Howeverthelargeerrorbarsassociatedwiththerotationaltem- parts of the cloud connecting P1 and P2. At the velocity of
peraturedoesnotallowustomakeaconvincingcase.Thetem- 80.16 kms−1, the three unresolved clumps, which are identi-
perature structure will be analysed in detail in a future paper fiedindustcontinuumemissionbutnotclearlyidentifiedinthe
withobservationsathighangularresolution(Pillaietal.2006 NH3(1,1)integratedintensityemission,arerevealed.
inprep). G33.71 0.01showsanotherinterestingcaseofdistinctve-
−
Thereisalsoavariationintemperaturewithinthedifferent locityvariationsacrossacloud.Theaveragevelocityposition
coresofthe samecloud.InG11.11-0.12,witha projectedex- mapalongthedeclinationaxisisdisplayedinFig.9.Theveloc-
tentofafewpc,theaverage(overthecoresize)temperatures ityincreasesfromthesouthat 103kms 1towardsthenorth
−
∼
of thesoutherncoresare2 – 3 K (10.9 1 K) lower thanthe to 106kms 1 while showinga verywide weak component
−
± ∼
northerncores(13.4 1K). at 105 kms 1 at the (0,0) position. Its detection at (0,0) is
−
± ∼
8 T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds
Fig.7.ChannelmapsforG11.11 0.12overthemaincomponentfortheNH (1,1)transition.
3
−
onlyatthe2σlevelbutisalsoobservedinNH (2,2).Apossi- where Sbeam is the flux density in Jy/beam, λ the wavelength
3 ν
bleinteractionwiththenearbySNRG33.6+0.1isdiscussedin in µm, Z/Z is the metallicity relative to the solar metallic-
⊙
5.2. ity (we assume Z/Z = 1) and θ is the FWHM of the 100m
§ ⊙
beaminarcseconds.Hereweassumethatthedusttemperature
T is approximately equal to the gas temperature. We derive
4.7.ColumnDensity,NH AbundanceandMasses d
3 an effective H column density of the order of 6 – 20 1022
2
×
We have estimated the column densities and total masses for cm 2 towardthepeaksofthedustemission.Togetherwiththe
−
the bright, compact sources for which the SCUBA 850 µm NH column density, this is used to determine the ammonia
3
maps are available, after smoothing the data to the resolution abundance(χ = N(NH )/N(H ) ). We determinethe peak
NH3 3 2
of 100m beam (40 ). Assuming a dust opacity of κ of 1.85 effectiveHcolumndensityfromtheSCUBAmapandtheNH
′′ m 3
cm2/gat850µmforgrainswiththickicemantlesandgasden- column density for the same position for each clump and get
sity n(H) = 106cm 3(Ossenkopf&Henning 1994), the effec- an upper limit for the fractional NH abundance as listed in
− 3
tiveH columndensityis(Launhardt1996) Table3.Theaverageabundanceis 4 10 8,howeverhasa
2 ∼ × −
lowvaluetowardsG79.27+0.38NH P2(7 10 9,aslistedin
3 −
N(H )= 6.2×1016Sνbeamλ3e1.4T4d×λ104 Z⊙ (5) Table3). ×
2 κ (λ)θ2 Z
m
T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds 9
Fig.8.channelmapforG28.34+0.06overthemaincomponentfortheNH (1,1)transition.
3
Fig.9.Leftpanel:Theaveragevelocity-positionmapofNH (1,1)alongthedeclinationaxisforthecloudG33.71 0.01.Right
3
−
panel:Thespectraobtainedatthecorrespondingpositioninthemap(indicatedbythearrows).Notethelargelinewidthcompo-
nenttowardthecentrepartofthecloudandthechangeinLSRvelocityfromsouthtonorth.
10 T.Pillai,F.Wyrowski,S.J.CareyandK.M.Menten:AmmoniainInfraredDarkClouds
5. Discussion
5.1.GasTemperatureandDistribution
In G11.11 0.12, the position NH P2 is a faint MSX source
3
−
withnocounterpartinthevisibleorNIR;itisverylikelythat
this is a heavily embedded protostar (Pillaietal. 2005). The
southern clumps are colder by 2 – 3 K than the northern
∼
clumps. Thus, these southern filaments may be at an earlier
stageofevolutionthanthecoresbelongingtothenorthernfila-
ment,wherestarformationactivitymightheatupthegas.The
core to core variation in temperature within an IRDC is ob-
servedtowardseveralothersources(see 4.5).Therefore,these
§
densecoresinIRDCsareidealsitesforinvestigatingtheinitial
conditionsinformingamassivestar.
The gas temperatures we derive generally agree with the
Fig.6.Theaveragevelocity-positionmapofNH (1,1)forthe dust temperaturesof Careyetal. (2000) based on submm ob-
3
cloud G11.11-0.12 at a position angle of 40 . Note that servationsandanassumeddustemissivityindexβ=1.75.T
◦ gas
∼
the Y axis is the offset from the reference position, given in mightbelowerlimitsforthosesourcesthatshowupasbright
Table1.offsetsrunfromsouth-west(negativeoffsets)tonorth- and compact objects in the submm dust emission, indicating
east(positiveoffsets). thepresenceofaheavilyembeddedobject,butreasonablefor
thecoldgasenvelope.
Themassoftheeachcloudcanbedeterminedfromthedust 5.2.PhysicalProperties,Chemistry&Lifetime
continuum and from NH emission. The gas mass is derived
3
fromthedustusingtherelation(Launhardt&Henning1997) The excellent correlation between the NH3 line and submm
continuumemissionisconsistentwiththepredictionsofchem-
ical models that NH is relatively more abundant in high
M(H2)[M⊙]=1.8x10−14Sνλ3κdm2(eλ1).4T4d×λ104 ZZ⊙[M⊙] (6) duemnnsidtyenrseigtiieosntrthananslaott3ehetor emxotrleemcuelleysh(iBgehrgeixntin2c0t0io3n).vTahlueecsoolf-
55 450mag.ThediscrepanciesinNH andH COlinewidths
3 2
−
whereS is theintegratedfluxdensityin Jyandtheotherpa- reported by Careyetal. (1998) (see 4.4) indicate the differ-
ν
§
rametersarethesameasineq.(5). encesinthegasvolumetracedbythetwomolecules.Theline
Our estimates for the mass and H2 column density are wingsseeninH2CO andCH3OH(Leurinietal.2006submit-
in general agreement with the previous work of Careyetal. ted)gastowardsomesourcesismostlikelyhigh-velocityout-
(2000)forthesubmillimetercores. flow.ThecriticaldensityoftheH2COlinesstudied(106cm−3)
ismuchhigherthanthatoftheNH inversionlines(104cm 3).
The total gas mass can be derived from the NH column 3 −
3
Hence, while physical parameters derived from NH are rep-
densitymapsassuminga uniformfractionalabundanceofthe 3
resentative of the general, cool IRDC material, H CO probes
molecule,forthosesourceswithoutdustcontinuumdata.The 2
the dense gas, which might be influenced by embedded pro-
gas mass derived from SCUBA observations can be directly
tostars. The derived NH abundance of 0.7 – 10.1 10 8 to-
compared with the virial mass estimate. The virial parameter 3 −
×
getherwiththecentrallycondensedNH emissionisconsistent
(Bertoldi&McKee1992)foraclumpisdefinedas 3
with the chemical model predicted for pre-protostellar cores
(Bergin&Langer1997).ThesevaluesimplythatNH isover-
5σ2R 3
α= (7) abundant by factors of 5 – 10 relative to ”normal”(= lower
GM
density) and less turbulent dark clouds. In contrast, H CO is
2
under-abundantbyafactorof 50(Careyetal.1998).Hence
whereσisthethreedimensionalrootmeansquare(r.m.s)ve- ∼
theIRDCsappeartoexhibitacomplexchemistry.
locity dispersion and R is the radius of the clump, and M is
the gas mass. Note that σ = √3/8ln2 FWHM. The virial Wederivetheflowcrossingtime,forthecloudtodisperse
mass is definedas M = 5σ2R/G. For×the clumpsto be sta- duetoitsowninternalmotions.Perturbationswithinthecloud
vir
ble against collapse, α 1. We find an average value of 2.1 woulddisintegratethecloudunlessthepropagationwavelength
∼ ismuchsmallerthanthedistanceacrossthecloudwhichittra-
forthevirialparametertowardtheindividualcores.Astheun-
certaintyin thedustopacityaloneisa factorof2, α 1,and verses.Theestimateddynamicaltimescalesof 0.5–2Myrs,
∼ provideanupperlimittothelifetimeofthesec∼louds.
mostofthesecoresappeartobevirialised.Therefore,thecore
structureisconsistentwiththembeingsupportedbyturbulent Thevirialmassesforthedifferentclumpsspanawiderange
pressurewithoutanyevidenceofexternalboundingpressures. from50to7880 M .Astypicalforintermediatetohighmass
G28.34+0.06NH P4isanextremecase,whereα=6.53. star-formingregion⊙s,mostofthemassesareskewedtovalues
3