Variation of the extinction law in the Trifid nebula

0.43 MB·English

by  L. Cambrésy

Astronomy&Astrophysicsmanuscriptno.cambresy (cid:13)c ESO2011
January7,2011
Variation of the extinction law in the Trifid nebula
L.Cambre´sy1,J.Rho2,4,D.J.Marshall3,andW.T.Reach2,4
1 ObservatoireastronomiquedeStrasbourg,Universite´deStrasbourg,CNRS,UMR7550,11ruedel’Universite´,F-67000Strasbourg,
France
e-mail:[email protected]
2 InfraredProcessingandAnalysisCenter,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
3 Universite´ de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France e-mail:
[email protected]
4 StratosphericObservatoryforInfraredAstronomy,UniversitiesSpaceResearchAssociation,MailStop211-3,MoffettField,CA
1 94035,USAe-mail:[email protected], [email protected]
1
0 Received;accepted
2
ABSTRACT
n
a Context. Inthepastfewyears,theextinctionlawhasbeenmeasuredintheinfraredwavelengthsforvariousmolecularcloudsand
J
differentlawshavebeenobtained.

5 Aims. Inthispaper weseekvariationsoftheextinctionlawwithintheTrifidnebularegion.Suchvariationswoulddemonstratea
localdustevolutionlinkedtovariationoftheenvironmentparameterssuchasthedensityortheinterstellarradiationfield.

] Methods. Theextinctionvalues,A /A ,areobtainedusingthe2MASS,UKIDSSandSpitzer/GLIMPSEsurveys.Thetechniqueis
A λ V
tointer-calibratecolor-excessmapsfromdifferentwavelengths toderivetheextinctionlawandtomaptheextinctionintheTrifid
G region.
Results. Wemeasuredtheextinctionlawat3.6,4.5,and5.8µmandwefoundatransitionatA ≈ 20mag.Belowthisthreshold
. V
h the extinction law is as expected from models for R = 5.5 whereas above 20 mag of visual extinction, it is flatter.Using these
V
p resultsthecolor-excessmapsareconvertedintoacompositeextinctionmapoftheTrifidnebulaataspatialresolutionof1arcmin.A
- tridimensionalanalysisalongtheline-of-sightallowedustoestimateadistanceof2.7±0.5kpcfortheTrifid.Thecomparisonofthe
o
extinctionwiththe1.25mmemissionsuggeststhemillimeteremissivityisenhancedinthedensecondensationsofthecloud.
r
Conclusions.Ourresultssuggestadusttransitionatlargeextinctionwhichhasnotbeenreportedsofaranddustemissivityvariations.
t
s
a Keywords.ISM:dust,extinction–ISM:individualobject:Trifidnebula–Infrared:ISM
[

1
v 1. Introduction of progresses has been made in the past few years through the
9 study of the extinction law. Dense clouds can be probed on
8 During the last three decades, dust evolution in the interstel- largescalesandforvariousastrophysicalenvironments.Recent
0 lar medium has been studied mostly through the grain emis- studieswithSpitzerdata(Indebetouwetal.2005;Flahertyetal.
1 sioninthefar-infraredandthenthesub-millimeterwavelengths. 2007;Roma´n-Zu´n˜igaetal.2007;Chapmanetal.2009)focused
.
1 The analysis of the surface brightnesses at 60 and 100 µm ob- on different clouds and obtained different extinction laws. We
0 served by IRAS lead to distinguish a small and a big grain propose in this work to look for possible variations of the ex-
1 componentwhere the small grains are the major contributorat tinction curve within a single cloud. We choose an active star-
1 60µm (Boulangeretal. 1988; Laureijsetal. 1991). Thetransi- formingregionwhichincludesthefamousHIIregionM20,the
v: tionfromsmalltobiggrainsoccursatatypicalvisualextinction Trifidnebula.ItslocationintheGalacticplaneatonly7degrees
i of1mag.Morerecentlyanewtransitionwasdiscoveredinthe from the Galactic center offers a high stellar density which is
X dustcompositionwiththeformationoffluffygrains.Theanaly- ideal to probe the extinction distribution. The distance of this
r sis ofPRONAOS sub-millimeterobservationsbyBernardetal. youngnebula(105years)isstilldebated.Theobservationofthe
a
(1999) yielded to the discovery of a population of enhanced- centralO7starresponsibleforthephotoionization,HD164492,
emissivity dust grains which have a lower equilibrium temper- yields a distance from 1.67 kpc (Lyndsetal. 1985) to 2.8 kpc
ature.Thisemissivityenhancementinthefar-infraredbyafac- (Kohouteketal. 1999). This active region has its first genera-
tor of about 3 (Cambre´syetal. 2001) requires the existence of tion of massive stars interacting with the interstellar medium
compositegrainsformedby the coagulationof small grains.A andis verylikelytriggeringa secondgenerationofstar forma-
large scale study on the whole Galactic anticenter hemisphere tion (Cernicharoetal. 1998). The state-of-the-artfor the Trifid
showedthistransitionalsooccursatlowextinction,A ≈1mag star-formingregionispresentedinareviewpaperbyRhoetal.
V
(Cambre´syetal.2005).Itmightbecloselyrelatedtotherecent (2008).
discovery made by Molinarietal. (2010a) with Herschel data
The extinction law is generally estimated in molecular
where this same density thresholdseems to indicate the transi-
clouds as a whole and the discrepancies between the different
tionwherediffusecloudsstarttocollapseintofilaments.
results are interpreted as variations of the dust optical prop-
With thewealthofavailabledeepdatainthenearandmid- erties from cloud-to-cloud due to the environment. Obviously
infrared it is now possible to investigate the interstellar grain variations should also be observed within molecular clouds.
propertiesat shorter wavelengths. In this spectral domain a lot As the density increases from the envelope to the core of a
2 L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula
cloud, the dust properties, hence the extinction law, should Table1.Completenesslimitmagnitudes.ForUKIDSSthelumi-
be affected. An attempt to detect such a variation was pro- nosityfunctionslopesarenotwelldefined,sothepeakvaluesof
posed by Roma´n-Zu´n˜igaetal. (2007) in their study of a dark thefunctionforsourceswithanerrorsmallerthan0.15magare
cloud core, B59, up to A ≈ 59 mag but they did not suc- displayed.
V
ceed. Olofsson&Olofsson (2010) used five different methods
to derive the extinction law in B335. The scatter in their re- 2MASS UKIDSS UKIDSS(peak) GLIMPSE
sultwasconsistentwiththeuncertaintiessotheycouldnotcon- J 14.4 16.2 17.8
clude on any evolution of the extinction law within the cloud. H 13.5 15.0 16.7
Chapmanetal. (2009) managed to find variations by averag- Ks 12.8 14.2 16.0
ing their result from several clouds and they showed evidence [3.6] 13.5
[4.5] 13.4
for grain growth at A > 1 mag (A > 9). A global galac-
Ks V [5.8] 12.2
tocentric variation of the extinctionlaw has also been reported
[8.0] 11.4
by Zasowskietal. (2009) in a large scale study that excluded
molecularclouds.
The present study focuses on the variations of the extinc-
tion law within the molecular cloud associated with the Trifid WecorrectedtheUKIDSSphotometrytomatchthe2MASSsys-
nebula. Our analysis relies on the 2MASS (Skrutskieetal. tem.TheUKIDSSastrometryhasalsobeencorrectedtomatch
2006), UKIDSS (Lawrenceetal. 2007) and GLIMPSE II the GLIMPSE positions by adding 0.12 and 0.07 arcsec to the
(Churchwelletal. 2009) surveys. Special attention is given to right ascension and declination, respectively. In the following,
the data filtering to limit the possible biases as explained in onlymagnitudemeasurementswithanuncertaintysmallerthan
Sect.2.Ouroriginalmethodtoderivetheextinctionlawfrom3.6 0.15magwill beconsidered.Ityieldsabout2×105 sourcesat
to5.8µmisbasedonthecolormappingratherthantheclassical Ks,6×105 at3.6µmintherangeR.A. ∈ [269.50,271.29]and
analysisofthe sourcelist distributionin acolor-colordiagram. Dec.∈[−23.6,−22.2].
Thedetailsandtheadvantagesofthistechniquearepresentedin
Sect.3 whileSect.4 describesthe finalextinctionmapmaking
2.2.Sourceselectioncriteria
and a tridimensional analysis of the Trifid line-of-sight which
allowsustodisentanglethecomplexityofthisdirection.Inad- The analysis of the stellar color excess to investigatethe cloud
ditionwestudiedthedustemissivityat1.25mmbycomparing propertiesrequiresallstarstobebackgroundstars.Thecontam-
the masses derived from the dust continuum emission and the inationbyforegroundobjectscannotbeneglectedfortheTrifid
dustabsorption.WeconcludeinSect.5. nebulalocatedbetween1.7and2.8kpcprobablyintheScutum
Arm,behindtheSagittariusArm.Theproceduretoremovefore-
groundstarsis statisticalfortwo reasons:1)the numberofde-
2. Datapreparation tected sources is close to 6 × 105, and 2) there is a degener-
acybetweenintrinsicallyredforegroundobjectsandblueback-
2.1.Datasets
groundsources. A color criteria would therefore be inefficient.
Ourstudycoversalargefieldof1.6◦×1.4◦.The2MASSnear- Itwouldactuallyleadtoasourcespatialdistributioncorrelated
infrareddataprovidesJHK fluxesforthewholesky.Thehigh with the dust distribution which is notconsistent with the sup-
s
stellardensityintheTrifiddirectioninducesalossofsensitivity posedlyforegroundnatureofthestars.Theindividualidentifica-
as2MASSsourceswith lessthan5′′ separationare affectedby tionisgenerallyimpossible,exceptinthehighextinctionregion
confusion.Thedegradationreachesalmost1.5magineachfilter, where the degeneracycan be raised. Keeping these constraints
with completeness level of only 12.8 mag for the K band for in mind, we choose a two steps strategy, first purely statistical
s
instance. andthenindividualandmanual.
Deeper JHK fluxes are obtained from the UKIDSS 6th The statistical step consists in sampling the source catalog
s
data release (UKIDSSDR6plus) of the Galactic Plane sur- into a data cube with two spatial dimensions and a third di-
vey. UKIDSS uses the UKIRT Wide Field Camera (WFCAM; mension being a color axis. Only one star per box is allowed.
Casalietal. 2007). The photometric system is described in Basically,starsaresampledbya4arcmingridstepaccordingto
Hewettetal. (2006), and the calibration is described in theircelestialcoordinatesandthethirdaxisallowstocolor-sort
Hodgkinetal. (2009). The pipeline processing and science the stars within a same 16 arcmin2 pixelso thatthere is only a
archivearedescribedinHamblyetal.(2008). single star per 3D-box. The first slice of the cube represents a
Longer wavelengths from Spitzer/IRAC are publicly avail- mapwhereeachpixelcontainsthecolorofthebluestobjectof
able throughthe GLIMPSE II legacysurvey.We used the high the stack. The followingslices contain redderand redder color
reliabilitypointsourceCatalog(v2.1)whichincludesdatafrom values. The completeness limit of UKIDSS being not well de-
epochs1and2. finedweuseonly2MASSandGLIMPSEforthisstatisticalpro-
The merged catalog between 2MASS, UKIDSS and cedure.Thevariationofthestellardensitymakesthenumberof
GLIMPSE tables results from a 1′′ cone search. Completeness starsperpixel,andthereforethesizeofthestack,rangefrom85
limitmagnitudesforallfiltersarepresentedinTable1.Forbright to387.Fora16arcmin2pixel,itmeansthesourcedensityvaries
stars,the2MASSphotometryischosenovertheUKIDSSpho- from19×103to87×103deg−2.Oncethecubeisbuilt,eachplane
tometrywhichisaffectedbysaturation.Althoughthe UKIDSS fromthebluesttothereddestiscomparedtoarawcolormapof
photometryisdirectlycalibratedon2MASSwefoundasystem- thefield,i.e.anon-calibratedextinctionmap.Acorrelationwith
aticdifferencebetweenthetwosystems: the color map starts at the 38th slice, meaning the distribution
of the ∼38 bluest stars in all the 16 arcmin2 pixels is not cor-
J(2MASS) = J(UKIDSS) +0.084 related with the dust distribution. They must be considered as
H(2MASS) = H(UKIDSS) −0.074 foregroundobjectsandthefirst38starsofeachpixel,i.e.8600
K (2MASS) = K (UKIDSS) −0.003 starsdeg−2,arethusremovedfromtheoriginalcatalog.Itrepre-
s s
L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula 3
sentsfrom10%to 45%of the sourcesdetectedat K and[3.6]
s
dependingontheposition,i.e.thelocalsourcedensity.
At this point, only a fraction of the foreground stars is re-
moved.A secondstep is requiredto filter the catalogin there-
gionswherethecontaminationiscriticalbecausetheforeground
objects may even be the dominant population. Fortunately, in
these regions the color degeneracy is raised by the strong red-
dening from the dust grains. To define these regions we build
a [3.6]-[4.5] color map following the adaptive method de-
scribed in Sect. 3.1.1. Then we select the pixels where [3.6]-
[4.5]>0.35mag(A ≈ 30mag)whichyieldsa25arcmin2 sur-
V
face area. Such a high threshold guaranty the degeneracy with
the intrinsic star color is broken.For this manualoperationthe
restrictiononUKIDSSdatacanbewaivedand∼103sourcesare
detected in the 25 arcmin area at both K and 3.6 µm with er-
s
rorsmallerthan0.15mag.Thespectralenergydistributionfrom
1.25µm to 8 µm is examinedforthese sources. For each fore-
groundstarcandidate,acolor-compositeJHK imageisbuiltto
s
compare the color of the candidate itself with the color of the
surroundingobjects. This visual inspection allows to make the
finalclassificationofthesourcesinceaforegroundstarappears
bluerthanthesurroundingobjects.Amongthe∼103sourcesde-
tectedatK and3.6µm,20%areclassifiedasforegroundstars.It
s
correspondstoatotalforegroundstardensityof∼37×103deg−2.
ItworthnotingthatforsourcestogetherdetectedatJ,HandK
s
instead,thecontaminationlevelinthe25arcmin2reaches55%.
Thisisbecausemanybackgroundstarsare lostwhenthe J de-
tectionisrequired.
Besidetheforegroundstars,thereisacontaminationbythe
YoungStellarObjects(YSOs)thatareformedinthemolecular
cloudassociatedwiththeTrifidnebula.Severalstudies,atvari-
ouswavelengths,provideuswithasourcelistofYSOsthatwe
removed from our catalog. The most important contribution is
from Rhoetal. (2006) who analyzed the infrared colors from
SpitzerIRACandMIPSdata.Ityieldedtheidentificationof161
YSOs(37classI/O,13hotexcess,111classII).Previousstudies
of the youngstar populationin the Trifid nebulaused 2MASS,
ASCA,ROSAT(Rhoetal.2001)andChandra(Rhoetal.2004). Fig.1.(a)Color-colordiagramforstarswithuncertaintysmaller
Finally,anotherpossiblecontaminationwouldbeextragalac- than0.15maginthe3bands.(b)Color-colordiagramobtained
tic sources. This effect is negligible based on the location of usingcolormapsinsteadofthesourcelist.Theexactsamestars
theTrifidnebulaintheGalacticplaneandclosetotheGalactic areusedforbothplots.Thegrayscalerepresentsthedensityof
Centerdirection. thedatapointsintheplotandthedotsshowtheindividualdata
pointsforlowdensity.
3. Extinctionlaw
The general idea of the method to derive the extinction law is specifiedotherwisethe overlaidsymbolssimplyhelptovisual-
trivial.Itrequirestheknowledgeoftherelativeextinctionintwo izethelowestdensitypixelsthatarehardlyvisible.
wavelengths,A /A A /A ,toevaluatetheextinctionatathird
λ1 V λ2 V
wavelengthbymeasuringtheslopeofthestarcolordistribution
withinacolor-colorplotm −m /m −m .Thecolor-colordi- 3.1.Colormapmaking
λ1 λ2 λ2 λ3
agramslopebeingequaltothecolor-excessratioE /E
λ1−λ2 λ2−λ3 3.1.1. Method
thevalueofA /A isinferred.Practically,thisprocedureisnot
λ3 V
necessarilythatsimple becauseofthescatter in thecolor-color Although a slope could be measured in Fig. 1a, the scat-
diagram. The selection of a sub-sample as clean as possible is ter is large. The Trifid line-of-sight crosses various interstellar
strongly recommended.Fig. 1a is restricted to sources with an medium environmentsand all kind of stellar populations. This
uncertaintysmallerthan0.15magatK ,3.6and4.5µmalthough diversity, the high stellar density, and the relatively large dis-
s
alargescatterisstillpresentintheplot.Inparticular,thevertical tance of M20 make our goals more difficult to reach than for
elongationfor[3.6]−[4.5] ≈ 0and K −[3.6] > 2isnotreal. nearby molecular clouds such as Chamaeleon, Taurus or Rho
S
It is dueto a severeunderestimationof the UKIDSS K uncer- Ophiuchus.
s
taintyforsomesources.Fortunately,themethodpresentedinthe Weproposeamethodbasedonthecolormapsthatspectac-
nextsectionmakestheircontributionnegligiblebecauseoftheir ularly improvesthe accuracy of the analysis. Fig. 1b was built
randomspatialdistribution. using the exact same sources as Fig. 1a and both figures have
Fig.1andseveralothersinthispaperrepresentplotsasden- the same axis, same scale. While each point in Fig. 1a corre-
sity maps for clarity reason. Dark means high density. Unless sponds to a single source, points in Fig. 1b actually represent
4 L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula
Fig.2. [3.6] − [4.5] color difference from sources de-
tected simultaneously in two different sets of 3 filters, Fig.3. Extinction from H − K color excess versus extinction
s
Ks(2MASS)/[3.6]/[4.5]and[3.6]/[4.5]/[5.8],versuscolor. from K − [3.6] color excess. By adjusting K − [3.6] to ob-
s s
tainaone-to-oneregressionline(solidline)wefindA /A =
[3.6] Ks
0.611.Thewhiteiso-densitycontourdelineatestheregionused
individualpixelstakenfromtwocolormaps,oneforeachaxis.
tocomputederegressionline.Thedashedlineistheregression
UsingthecolorallowsustokeepthesameaxisasinFig.1abut
linefor A > 20mag,itindicatesthedeviationatlargeextinc-
itisobviouslyequivalenttoanextinctionmapassuminganex- V
tion.CaseAandBrefertoTable2.
tinctionlaw.Theimpressivedifferencebetweenthetwofigures
highlightsthesuperiorityofusingmapsratherthanasourcelist
directly.Usingmapspermitsto averagethe colorofseveralin-
toryto keep sourcesdetected in all 3 bandsto plotFig. 1a, the
dividualnearbysources,andthustoreducethescatter.Different
Fig. 1b needs two color maps that can be built independently
methods are available to build an extinction map. Star count
withonlya2banddetectioncriteriaforeachofthem.Fig.2il-
methods are sometimes used on large scale and for moderate
lustratesthecolorbiasfor[3.6]−[4.5]whenitisobtainedfrom
extinction(Cambre´sy1999;Dobashietal.2005).Thisapproach
two different sets of 3 band detection criteria, K /[3.6]/[4.5]
isdefinitelynotadaptedtoahighlyobscureddistantcloudinthe s
or [3.6]/[4.5]/[5.8].To exaggerate the effect K is taken from
Galactic plane.Too sensitive to the foregroundstar contamina- s
2MASSwhichisnotasdeepasUKIDSS.The[3.6]−[4.5]aver-
tionthemapwouldbenoisyandseriouslybiased.Color-excess
agecolorisaffectedbythe K detectioncriterionwhichmakes
methodsare better adapted. The first statistical method to map s
thecolorbluer,andbythe[5.8]detectioncriterionwhichtends
the extinction from color excess, called NICE, was proposed
toselectredderobjects.ItisnotasimpleoffsetasshowninFig.2
by Ladaetal. (1994). Improvement were proposed and called
because this effectis more importantfor reddercolor. In terms
NICER(Lombardi&Alves2001)andthenNICEST(Lombardi
ofextinction,takingsourcesdetectedinthe3bluerwavelengths
2009). A variant technique from Cambre´syetal. (2002) pro-
yieldsanunderestimationoftheextinction.UsingdeeperK data
posestoreplacethearithmeticmeancolorbythemediancolor s
would obviouslyhelp since more sources detected at [3.6] and
incellsandtofixthenumberofsourceswithinacellratherthan
[4.5] would also be seen at K . However the effect cannot be
itssize.Thelatterstepmakesthemethodadaptivesincethespa- s
avoidedatlargeextinctionbecausetheK bandsufferfromtwice
tialresolutionisalwaysoptimal,adjustedtothelocaldensity.A s
theextinctionthanat[3.6].
final convolution can be applied if a fixed spatial resolution is
It is therefore critical to compare only maps that are built
wished. Thisconvolutionminimizesa bias thatwould resultin
withtheexactsamesampleofstars.Itimpliesthe[3.6]−[4.5]
an underestimationof the extinction,comparedto the usage of
color map needs to be built from a source list which contains
largercellsalreadyatthefinalresolutionsize.Moreovertheme-
starsdetectedinthe3bandsK ,[3.6]and[4.5]ifitiscompared
dianisapowerfultooltolimittheeffectoftheresidualcontam- s
to a K −[3.6] map, or from a source list which containsstars
inationbyforegroundstarsandyoungstellarobjectsasdetailed s
detectedat[3.6],[4.5]and[5.8]ifitiscomparedtoa[4.5]−[5.8]
inCambre´syetal.(2002).
map.
All the colormapsbuiltfor thiswork use the mediancolor
A total of 6 color maps are built for our study: H − K ,
and follow the adaptive cell scheme with a fixed number of 3 s
K −[3.6],[3.6]−[4.5],[4.5]−[5.8].Eachmapisobtainedwith
starspercell.Thevariablecellspatialresolutionbeingr,acon- s
i sources detected in 3 bands at the same time. There are 2 ver-
volution by a Gaussian kernel of 1−r2 arcmin is applied to sionsforthe K −[3.6]mapandthe [3.6]−[4.5]map because
i s
q
reachafinalresolutionof1arcminoverthewholemap. thethirdbandcaneitherbethebluerortheredderone.
3.1.2. Selectionbias 3.2.Results
Once a clean star sample is produced, i.e. filtered from fore- Althoughtheextinctionlawhasbeenforlongconsideredasuni-
ground stars, YSOs and noisy photometry sources, the color versal in the infrared it is now admitted there are variations at
maps are still subject to a selection bias. While it is manda- wavelengthlongerthan2µm.Thespectralrangethatispseudo-
L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula 5
universal is actually restricted to 1-2 µm where the extinction
is independentor very little dependentof the environment,i.e.
dust temperature and density. Consequently, A /A = 0.175
H V
andA /A =0.112(Rieke&Lebofsky1985)areassumedcon-
stant.KIts coVrrespondstoβ = 1.7intheexpressionA ∝ λ−β that
λ
isoftenusedtodescribethenear-infraredextinctionlaw.Values
from1.6to1.8beingreasonable,theeffectofsuchasystematic
uncertaintyisdiscussedfurtherinthissection.TheH−K map
s
canbeconvertedintoanextinctionmapusing
(H−K )−(H−K )
A = s s 0 =15.87E
V A /A −A /A H−Ks
H V Ks V
The extinction law is normalized to A mainly for historical
V
reason but it is more convenient to normalize to K . The re-
s
lation between the extinction and the color excess becomes
A =1.78E .Thereferencecolor(H−K ) =0.54magis
Ks H−Ks s 0
estimatedfromtheareawiththesmallestvalueswithinthemap,
about50arcminsouth-westfromthedensestcoreofthecloud.
This is much redder than the intrinsic stellar color because it
includes the diffuse extinction along the line-of-sight. The ref- Fig.5.Extinctionlaw.Thetworedlinescorrespondstothecases
describedinTable2forA >20mag.Theshadedareaindicates
erencecolorisrelevanttoprovideanabsoluteextinctionforthe V
thesystematicerror,forcaseBonly,resultingfromA /A un-
TrifidaloneanditisdiscussedintheSect.4.2.Ithasabsolutely H Ks
noeffectontheextinctionlawestimationnorontherelativeex- certaintywhichcanrangefrom1.52(β=1.6)to1.60(β=1.8).
tinctioncomparisonfromdifferentpartsofthefield. At longer wavelengths than Ks, β = 1.6 gives larger Aλ/AKs.
Fig.3representsA fromH−K versusA fromK −[3.6]. Diamonds are for AV < 15 mag. The Weingartner&Draine
V s V s
Since A = A /0.112 is assumed, and by definition A = (2001) modelisplottedin graywith dashedandplainlines for
E /V(1−AKs ),theonlyfreeparametertoadjusttoKsob- RV = 3.1andRV = 5.5,respectively.Theaxisscaleislinearfor
Ks−[3.6] [3.6]/AKs xandlogarithmicfory.
tainaslopeof1isthevalueofA /A .Aftertheadjustment,
[3.6] Ks
ityieldsthevalueofA /A = 0.611.Fig.3ispresentedasa
[3.6] Ks
densitymapforclarityandalsobecausethedensityinformation
– case A: A /A = 0.611 is independentfrom the extinc-
istakenintoaccountinthefittingprocesstoexcludethelowden- [3.6] Ks
tion, the value remains identical than at lower extinction
sitypixels.Basicallytheminimumdensitylevelisfixedto20%
(solidlineinFig.3).
of the maximum density in the map. The color-coding used in
– case B: A /A = 0.678 which is the value actually ob-
thedensityplotsisinlog-scaleso the20%alwayscorresponds [3.6] Ks
tainedfromtheslopeofthedashedlineinFig.3.
tothedarkregion.Thewhiteiso-densitycontourdelineatesthe
regionselectedtocomputetheregressionlineinFig.3.Anyout- The statistical errors on the slope estimations are small and
lieristhereforefullyeliminatedwiththistechnique. so are the errors on the computed A /A . The systematic er-
λ Ks
OnceA /A isdetermined,A /A isevaluatedwitha ror on A /A is the major source of uncertainty. We assume
[3.6] Ks [4.5] Ks H Ks
plotofA fromK −[3.6]versusA from[3.6]−[4.5].Athird a value of 1.563 which corresponds to a power-law index of
V s V
similarplotwillthenprovideA /A (seeFig.4).Theresults β = 1.7 for the extinction law. Our results from Table 2 are
[5.8] Ks
areA /A =0.500andA /A =0.398. plottedinFig.5.Theshadedareadelineatesthesystematicun-
[4.5] Ks [5.8] Ks
Sincethepixelsfallinginthelowdensityregionsofthe A certainty for β varying from 1.6 to 1.8. Using β = 1.6, such
V
vs.A plotareignoredthefittingisperformedonregionswhere asinChapmanetal.(2009),increasesA /A totheupperbor-
V λ Ks
the visual extinction is roughly smaller than 15 mag. At large deroftheshadedarea.Nishiyamaetal.(2006)reportedahigher
extinctionastrikingdeviationoftheslopesisobservedandout- valueofβ=1.99whichisnotcompatiblewithourobservations.
linedbythedashedlinesinFigs.3and4.Thechangesofslope The Weingartner&Draine (2001) (hereafter WD01) model is
occur at A ≈ 20 mag (A ≈ 2.2) and suggests a transition overlaid for comparison. The variations in the extinction law
V Ks
phaseinthedustgrainpopulation.Atsuchahighcolumndensity found in this work are rather small compared to the dust evo-
thisresultis unexpected.Contraryto studiesin the far-infrared lutionfromR = 3.1toR =5.5.Thevariationbeingobserved
V V
wavelengthswhichrelyonthedustemission,thereisnotemper- at visual extinction larger than 20 mag is likely the main rea-
aturedegeneracyissuesinceitisbasedonstellarcolormapping. son for this dust transition to be unknown. In case A the ex-
Adrawbackofthistechniqueisitslimitationtothenear-infrared tinction parameters at 4.5 and 5.8 µm are smaller at large ex-
spectralrange.Atwavelengthslongerthan∼5µmthenumberof tinction than at small extinction which is not realistic accord-
detectedstarsistoolowtohaveanacceptablespatialsampling ing the WD01 model. This case teaches us the small deviation
ofthemolecularcloudswhileshorterwavelengthsaresosensi- observed in Fig. 3 cannot be ignored as it would make longer
tivetotheextinctionthattheclouddenseregionsbecometotally wavelength results inconsistent. Case B is more likely even if
opaque. A direct consequence on this limitation is that no ev- ityieldsa largervaluethanreportedin the literatureuntilnow.
idence for a slope change can be seen in a J − H vs. H − K The largest value prior to this work was from Chapmanetal.
s
diagram because the visual extinction cannot go deeper than (2009)withA /A = 0.64.Howevertheirnumbersareover-
[3.6] Ks
∼20magwithJ−H.Thatpreventsanyconclusionontheextinc- estimatedcomparedtooursortoFlahertyetal.(2007)because
tion law variation at shorter wavelengths than 3.6 µm. Table 2 theychose a smaller β. Their resultscomefroman averagefor
summarizestheresultsobtainedfromtheslopemeasuredinboth Ophiuchus, Perseus and the Serpens molecular clouds which
Figs. 3 and 4. We consider two differentcases for A /A at all form low-mass stars whereas the cloud associated with the
[3.6] Ks
A >20mag: TrifidisclearlymoremassiveandformsOBstars.Flahertyetal.
V
6 L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula
Fig.4.Eachplotrepresentstheextinctioncorrelationfromtwodifferentcolorexcesses.ThetwoparametersA /A =0.500(left
[4.5] Ks
plot)and A /A = 0.398(rightplot)areadjustedtoobtainaone-to-oneregressionline(solidline).Thedashedlinestracethe
[5.8] Ks
deviationatlargeextinction,thecorrespondingslopesintheseplotsare0.55±0.02and2.52±0.09,respectively.
Table 2. Color excess ratios and resulting extinction law parameters. The two rows for A > 20 mag represent different cases
V
for A /A : case A propagatesthe same value as for small extinction,case B takes the actual measured value. We assume the
[3.6] Ks
power-lawindexβ = 1.7(i.e.A /A = 1.563)andresultsforβ = 1.6and1.8areprovidedforcaseB.Themainresultsfromthe
H Ks
literaturefordenseinterstellarmediumdirectionsandfromadustmodelareindicatedforcomparison.
E /E E /E E /E A /A A /A A /A A /A
H−Ks Ks−[3.6] Ks−[3.6] [3.6]−[4.5] [3.6]−[4.5] [4.5]−[5.8] H Ks [3.6] Ks [4.5] Ks [5.8] Ks
A <15 1.446±0.002 3.508±0.011 1.087±0.007 1.563 0.611±0.002 0.500±0.003 0.398±0.005
V
A >20
V
caseA 1.446±0.002 1.915±0.018 1.496±0.089 1.563 0.611±0.002 0.408±0.003 0.272±0.011
caseB 1.749±0.003 2.317±0.027 1.810±0.091 1.563 0.678±0.006 0.540±0.003 0.463±0.007
caseB,β=1.6 1.520 0.703±0.005 0.573±0.003 0.500±0.007
caseB,β=1.8 1.601 0.656±0.006 0.507±0.003 0.424±0.007
Roma´n-Zu´n˜igaetal.(2007)forBarnard59 1.55 0.619±0.029 0.512±0.022 0.459±0.024
Flahertyatal.(2007)forOrion 1.55 0.636±0.003 0.540±0.003 0.504±0.003
Chapmanetal.(2009)forOphiuchus,PerseusandSerpens 1.52 0.640±0.030 0.530±0.030 0.460±0.030
Weingartner&Draine(2001)withR =3.1 0.40 0.25 0.17
V
Weingartner&Draine(2001)withR =5.5 0.60 0.49 0.40
V
(2007) investigate the high-mass star-forming region of Orion peaksoutsidethe[3.6]IRACfilter(3.2-3.9µm).Besidetheab-
andobtaina similarnumberof0.636±0.003,significantlybe- sorptionfeatures,icemantlesmakethegrainsmoresticky,help-
lowourA /A =0.678±0.006.Theydidfindsmallervalues ingthecoagulationprocess.UsingSpitzer/IRSspectraMcClure
[3.6] Ks
at all wavelengthsin low-mass star-formingregions. The pres- (2009) found evidence for a flattening of the extinction law at
enceofmassivestar,inadditiontoahighdensity,maytherefore A > 1 mag which would be due to the coagulation of ice-
K
berelevant. mantledgrains.Thissuggestsicemakesthecoagulationprocess
WD01 attribute the variation of the extinction law with R moreefficientbutitisnotthedominantparametersincetheco-
V
tograingrowthonly,althoughicemantlesontodustgrainscon- agulationstartswellafterice-mantlesappear.Theabruptchange
tribute to the opticalpropertyvariations. The main ice features in the extinctionlaw at A ≈ 20 is notunderstoodand we can
V
relevanttoourstudyoriginatefromH OandCO iceswithab- only speculate that the high density in the Trifid cloud might
2 2
sorptionat3.0µmand4.27µm,respectively.BothH OandCO allowanewstructuringofthedustgrainsorthattheshocksob-
2 2
icesaredetectedatvisualextinctionbelow4mag(Whittetetal. servedintheTrifidcloudcouldbedeterminant.Inarecentwork
2001; Berginetal. 2005) where the WD01 modelis still valid. Boogertetal. (submittedtoApJ) also reported a flatter extinc-
Thisindicatesiceisnotsufficienttosignificantlymodifytheex- tionlawthanWD01basedonaspectrumfrom3to24µmfora
tinction law in the Spitzer/IRAC filters. We can also rule out starwith25magofvisualextinction.
the possibility that ice features saturate at A ≈ 20 mag be-
V
causetheyareobservedatlargerextinctionwithoutanysatura-
tion(A.Boogert,privatecommunication).Anexampleofspec-
trum is presented in Whittetetal. (1998, Fig.1) for a star with
A = 21 mag. It worth noting the H O ice band at 3.0 µm
V 2
L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula 7
Fig.6.Left: largescaleextinctionmapoftheTrifidregion.Right:GLIMPSE8µm imageoftheTrifidnebulawithiso-extinction
contours.InbothfigurescontourscorrespondtoA = [10,15,20,40,60]mag,thedashedlinefollowstheGalacticplane.Thered
V
shadedregionsdelineatetheS > 10mJy/11”beamTrifidCondensations(TC,Cernicharoetal.1998)studiedinSect.4.3ata
1.25
1arcminresolution.
4. Fromcolortoextinction we performeda tridimensionalanalysiswhichaimstoseparate
thedifferentcomponentsalongtheline-of-sight.
4.1.Compositemap
AthreedimensionalstellarmodeloftheGalaxy(Robinetal.
The extinction law parameters derived in the previous section
2003) is usedto simulatethe stellar distributionalongtheline-
allowustoconvertthecolormapsintoextinction.Toproceedwe
of-sight towards the Trifid nebula. The simulated stars are not
followthemorerealisticcaseBforA /A whenA > 20mag
λ Ks V subjected to extinction and so they provide an estimate of
(seeTable2)andareferencecolorof H−K = 0.54mag.The
s the intrinsic color and distance of the stars along this line-of-
associatedzeropointsderivedfortheothercolorsareK −[3.6]=
s
sight. By comparing the color distribution of the unreddened,
0.497, [3.6]−[4.5] = −0.017 and [4.5]−[5.8] = 0.187 mag.
simulated, stars to the observed stellar color distribution it is
The sensitivity of each map relies on the wavelengths used to
possible to infer the extinction distribution along the line-of-
buildthecolormaps.Shorterwavelengthsaremoresensitivetoo
sight(Marshalletal.2006).Weuseobservationsfrom2MASS,
smallerextinctions.ThefinalextinctionmappresentedinFig.6
UKIDSSandGLIMPSEandusetheKs-[3.6]colortoprobethe
isacombinationdefinedasfollow:
line-of-sightextinction.Thistechniqueisrelativelyindependent
K −[3.6] if A <15mag of the stellar countsand reflects the changingstellar color as a
s V
AV from  [3.6]−[4.5] if 10<AV <80mag functionofdistance(see Marshalletal.2006formoredetails).
 [4.5]−[5.8] if AV >60mag Furthermorethereisno needto removeforegroundstarsas we
Thetransitionbetweentwomapsisprogressiveandlinearwithin acrluedceodm.Hpaorwinegvtehr,ecoabresenreveadtisotnosbtoeatamkeondteolwenhseurreeatlhlasttathrseamreodine-l
the overlapping extinction ranges. A maximum visual extinc- isindeedrepresentativeoftheobservations,sothesimulatedcat-
tion of ∼80 mag is reached. In complement to the extinction alogiscutatthecompletenessmagnitudeoftheobservationsin
map Fig. 6 shows a close-up on the Trifid nebula observed at the K , [3.6] and [4.5] bands. Recently, Marshalletal. (2009)
s
8µmwiththeiso-extinctioncontourssuperimposed.Theparent improved on this method using a genetic algorithm to deduce
molecularcloudisanelongatedstructurewithitsdensestregion theline-of-sightextinctionnecessarytoreproducetheobserved
lyingjustWestofthefamousopticalnebula. stellarcolordistributionalongagivenline-of-sight.Thisversion
istheonewechosetouseasitprovideshigherangularresolu-
tion (∼3 arcmin compared to 15 arcmin previously) and more
4.2.Tridimensionalanalysis,distanceestimation
sensitivitytoextinctionfeatureswithin2kpcfromtheSun.The
Theextinctionestimationreliesonbackgroundstars.Themean- resultingdistancesaresubjecttoanuncertaintyofroughly15%
ingofbackgroundisnotthattrivialinthedirectionoftheTrifid butthis maybe higherat largedistancesandthroughhighcol-
nebulasincetheline-of-sightcrossesthewholegalacticdiskat umn densities. Thisresults in a smearing outof extinctionfea-
only1kpcoftheGalacticcenter.Thecomplexityoftheline-of- turesalongthelineofsight,andcouldexplainwhy,inFig.7,a
sightandthedistanceofthestarsusedfortheanalysisarecriti- narrowextinctionpeakisfoundat6.5kpcwhentheextinctionis
calandcanchangeourinterpretationofthecolormaps.Eventhe lowbutthissamepeakisfoundtobewiderthroughthehigher
distanceofthenebulais stilluncertain.To addresstheseissues density lines-of-sightcrossing the Trifid nebula. At such a dis-
8 L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula
totalforegroundstar densityofatleast37×103 deg−2 towards
thedenseregionofthecloudandinagreementwiththedistribu-
tionof8600starsdeg−2detectedby2MASSwhichwasfoundto
beuncorrelatedwiththeextinction,i.e.foregroundtothecloud.
It worth notingthe Sect. 2.2 results are also incompatiblewith
theshortdistancehypothesis.The12COvelocityobtainedfrom
Dameetal.(2001)showsavelocitystructureconsistentwithour
extinction distribution which corroborates our estimation. The
Trifid molecular clouds LSR velocity is about 18-20 km s−1.
Assuminga SunkinematicfromMcMillan&Binney(2010), a
distance to the Galactic center of 7.8 kpc and a rotation speed
of247kms−1theTrifidvelocitycorrespondstoakinematicdis-
tance of 3.2-3.4 kpc which is even larger than our estimation
basedonthestellarpopulationmodel.
The3Danalysisestimatesthediffuseextinctioninthisdirec-
tionto∼3magkpc−1anditshowsthatnootherdensecloudsig-
nificantlycontaminatestheTrifidline-of-sight.Thestellarfrac-
tion at distance larger 8 kpc represents only 7.7%of the back-
groundstarsatlargeextinctionand2.6%atsmallextinction.In
bothcasestheirmediandistanceis∼5.8kpc.Thezeropointcho-
senfortheextinctionmap(Fig.6)isvalidatedbythe3Dstudy
as1)thereisnopeakat∼3kpcatlowextinctionand2)itcom-
pensates for the diffuse extinction on the line-of-sight and the
contaminationat6.5kpc.
4.3.Cloudmass
The cloud mass is derived from the extinction map using the
followingrelation(Dickman1978):
N
M =(αd)2µ H A (i) (1)
V
A
V Xi
Fig.7.Extinction(top)andstardensity(bottom)alongtheTrifid
line-of-sight.The black line is the average valuefor the region where α is the angular size of a pixel map, d the distance to
where A > 20 mag in Fig. 6; the gray line is for the region the cloud, µ the mean molecular weight corrected for the he-
V
where A ∈ [0,1]mag.ThedashedlinesindicatetheTrifidlo- lium abundance, and i represents a pixel map. With the dust-
V
cationatadistanceof2.7±0.5kpc. to-gas ratio proposed by Savage&Mathis (1979), NH/AV =
1.87×1021 cm−2 mag−1 (N = N + N ) and a distance of
H HI H2
2.7 kpc, we obtain a mass of 5.8×105 M . This value corre-
⊙
tancethe interstellarmaterialcanbe relatedto severalGalactic spondstotwicetheOrion(A+B)cloudsmass(Cambre´sy1999;
structureswhicharethemolecularring(about3-4kpcfromthe Wilsonetal.2005).Figure8showshowtheintegratedmassdis-
Galacticcenter),thestartoftheNormaArm,andtheextremity tribution varies with the extinction within the cloud. Regions
oftheGalacticbar. withvisualextinctiongreaterthan20magaccountforlessthan
The results of the 3D modeling is presented in Fig. 7. The 4%ofthe totalmass ofthe cloud.The indexof the cumulative
extinctionprofilefortheregionwhereA >20mag(seeFig.6) massdistribution,α, changesat A ≈ 25magbya factorof2.
V V
providesuswithadirectestimationoftheclouddistance.Fitting For M(A )beingthemassenclosedinthecontourofextinction
V
aGaussian yieldsadistanceof2.7±0.5kpc.Theextensionof A ,wehave:
V
the cloud alongthe line-of-sightis likely from50 to 100pc so
theGaussianstandarddeviationrepresentstheuncertaintyinher- M(AV)= M(0)×10−αAV (2)
entto the3D modeling,notthe cloudsize. The literatureoften α=8.39×10−2forA ∈[8,20]
with V
refersto1.7kpc(Lyndsetal.1985)fortheTrifidnebuladistance (α=3.76×10−2forAV ∈[30,60]
although a serious alternative is proposed by Kohouteketal.
(1999) with a distance between 2.5 and 2.8 kpc. Both estima- The variationof α is not a side-effectof the map combina-
tionsarebasedonthephotometryofthe centralstars. Thesec- tiontechniquenorthe applicationofa correctedextinctionlaw
ond work is more complete as it uses more stars and includes at large A since the variation is also observed for a raw ex-
V
spectroscopyandaccuratephotometry.Ouranalysisofthetridi- tinctionmapobtainedonlyfromthe[3.6]-[4.5]color-excessand
mensionalstellar distributionunambiguouslysupportsthe long assumingaconstantextinctionlaw.Itishoweverdifficulttoin-
distance hypothesis which place the Trifid in the Scutum Arm terprettheindexvariationasaconsequenceofthedusttransition
(Valle´e 2008) rather than the Sagittarius Arm as generally re- responsibleof theextinctionlaw evolution.Thetwo eventsoc-
ported.Inthefollowingweassumed =2.7kpc. curringatthesamelevelmayjustbeacoincidence.TheN /A
H V
Forsuchadistance,theforegroundstarnumberisexpected value is generally assumed to be universal although some au-
to be at least 37×103 deg−2 (see Fig. 7), or 11×103 deg−2 if thorssuggestedadecreasebyafactorof3indeeplyembedded
werestrictthesampleto2MASSsources.Thisisingoodagree- regions(Winstonetal.2010).Dividingthemassestimatebyany
mentwiththeanalysispresentedinSect.2.2wherewefounda factoraboveagivenextinctionthresholdwouldonlyproducean
L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula 9
would lead to 11 to 8 K. Such low temperatures have already
been reported by De´sertetal. (2008) from submillimeter point
sources observed by the Archeops experiment. However, the
condensationswearedealingwithcoveranextendedfieldfrom
3to12arcmin2.Assumingaminimumtemperatureof12Kfor
TC3/4,an emissivity enhancementby a factor 1.9 wouldmake
themassestimatesinagreement.TC00andTC0arestarlesscon-
densations while TC3/4/5 form stars and undergo shocks. Our
analysis suggest the dust emissivity at millimeter wavelength
is enhanced for these condensations, especially for the densest
ones.Complementaryobservationsatlongerwavelengths,from
HerschelandPlanck,willpermittobetterunderstandthephysi-
calpropertiesoftheinterstellarmediumintheTrifidregion.We
plantofollowupthisstudyassoonasthedatafromtheHerschel
InfraredGalacticPlaneSurvey(Hi-GAL,Molinarietal.2010b)
areavailable.
5. Conclusion
Fig.8.Cumulativemassdistributionford=2.7kpc.Theαindex
Weperformedadetailedanalysisoftheextinctioninthemolecu-
isdefinedinEq.2.
larcloudassociatedwiththeTrifidnebula.Thedirectionandthe
distanceofthiscloudisamajordifficultycomparedtotheother
similar investigations published in the literature. To overcome
offsetinFig.8,notachangeofslope.Amorecomplexevolution thisspecificityweproposedanoriginalmethodwhichcombines
ofNH/AV withtheextinctionwouldaffectdifferentlytheFig.8 thecolor-excessmappingwiththecolor-colordiagramanalysis.
distributionbutitseemsunlikelytoonlyproducea sharpslope The gainin sensitivity with thismethodallowedus to measure
change. to extinctionlaw at 3.6, 4.5 and 5.8 µm and to unambiguously
The millimeter wavelength emission from the cold dust detecta variationat large absorptionthroughan abruptchange
continuum has been mapped by Cernicharoetal. (1998) and ofslopeinthecolor-colorplots.Weinterpretedthisresultasan
Leflochetal. (2008). They have detected several dust conden- evidence for a rapid dust evolution at high density like a new
sationsfromthe1.25mmemission.Forsimplicitywedesignate dusttransitionphaseinthedenseinterstellarmedium.
thesedustcondensationsTCNNasintheoriginalpapers.Thisis Theextinctionlawparameters,A /A ,arefoundtobelarger
λ Ks
fineistheTrifidcontextbutobviouslyambiguousinthegeneral in the very dense cores of the cloud, in agreement with dust
contextandthefullSIMBADnameisactually[CLC98]TCNN. models. Our values for A > 20 mag are however larger than
V
Forthemoreextendedcondensationswecomparethemassfrom predictedbyWD01fordenseregions(R =5.5)andlargerthan
V
the dust infraredextinctionand 1.25mm emission. The results previouslyreportedintheliteratureat3.6µm.ForA <15mag,
V
arepresentedinTable3.Themassfromextinctionisobtainedby which does still refer to dense material, our values remark-
integratingtheareadefinedinthemapat1.25mm,convolvedat ably match those predicted by the model for R = 5.5, as ex-
V
1arcminresolution,bythecontouratS =10mJy/11′′beam pected. Our study is not sensitive to the diffuse medium for
1.25
(red shaded area in Fig. 6). Because of the observationaltech- whichR =3.1withatypicalvisualextinctionaslowas1mag.
V
niquein themillimeterwavelengths,a spatialfilteringremoves Moreover,Zasowskietal. (2009)foundacorrelationoftheex-
the structure at scale larger than 4.5 arcmin in the dust contin- tinction law for the diffuse interstellar medium with the galac-
uumemission.Tocomparethismapwiththeextinctionmapwe tocentric radius. They concluded the extinction curve follows
scaleitbyaddingthemedianextinctionmeasuredinthevicinity the dust model with R = 5.5 for a galactocentric radius con-
V
ofeachTCregion.Basicallythemedianextinctionismeasured sistentwith theTrifiddistance.Itsuggeststhe so-calleddiffuse
in the area between the contours at S = 5 and 10 mJy/11” mediumalreadycontainsbiggrainsinthispartoftheGalaxy.If
1.25
aroundeachcondensation.Theresultingoffsetsaregiveninthe thediffusemediumattheTrifiddistancebehaveslikethedense
column labeled outer of Table 3. The mass from the map at medium in the solar neighborhood we can imagine the Trifid
1.25mmisderivedassuminga dusttemperatureof15Kanda dense cores are also affected by the distance with a flatter ex-
dustopacityofκ =4.08×10−1cm2gr−1followingthemodel tinctionlaw.
1.25
B for R = 5.5 by WD01. The correspondingextinction cross Using our varying extinction law we built a composite ex-
V
sectionperhydrogenatomisC =7.336×10−27cm2H−1after tinction map of the Trifid nebula region. The maximum visual
1.25
correctionfortheheliumabundanceandassumingagas-to-dust extinctionis80magat1arcminresolution.Wehavealsoinves-
ratioof125(Li&Draine2001).Leflochetal.(2008)measured tigatedthematterdistributionalongtheline-of-sightsincesev-
temperaturesfrom20to30KusingCOobservations.Thegaski- eralsmallcloudscouldhavemimickedthepresenceofasingle
netictemperatureandthedusttemperaturearehoweverdifferent giantmassivedark cloud.We foundno significantcontribution
insuchregionsandanarbitraryT =15Kismoreappropriate. ofothercloudsinthisdirectionandwewereabletoestimatethe
dust
Themasseswederivedfromthedustemissionaresystematically distanceoftheTrifidnebulaat2.7±0.5kpc.
smaller.Twoparameters,thetemperatureandtheemissivity,can The Trifid molecular cloud is about twice the mass of the
beadjustedtoreachanagreementwiththemassesfromextinc- Orion molecular cloud. The comparison of the dust extinction
tion.WhileTC00andT0onlyneedasmallincreaseofthedust and1.25mmcontinuumemissioninthedensestcoressuggests
emissivityat1.25mm,ortodecreasethetemperaturefrom15to the dust emissivity is probably enhanced by a factor of 2-3 in
13K,TC3/4/5requireamoresignificantemissivityincreaseby these regions. This cloud appears as a young and massive ver-
afactor1.7to2.7.Usingonlythetemperatureasfreeparameter sion of the Orion or Rosette regions. The Trifid represents an
10 L.Cambre´syetal.:VariationoftheextinctionlawintheTrifidnebula
Table 3. Properties of the more extended Trifid cores detected at millimeter wavelength (Leflochetal. 2008). Masses are
expressed assuming a distance of 2.7 kpc. The mass from dust continuum emission is derived using T = 15 K and
dust
κ = 4.08×10−1 cm2 gr−1 (i.e.C = 7.336×10−27 cm2 H−1). By modifyingthese parameters the mass can be adjusted
1.25 1.25
tothemassestimatedfromthedustextinction.Thetwolastcolumnsgivetheseparametervalues,providingonlyoneofthetwo
parametersismodifiedatthesametime.
Name Massa Massb A (mag) Surface κ T
V dust
[CLC98] (M ) (M ) peak median outer (arcmin2) (×) (K)
⊙ ⊙
TC00 936 911 27 22.7 20.0 3.5 1.3 13
TC0 1607 1570 25 20.3 16.9 6.9 1.2 13
TC3 3535 2783 78 40.4 27.6 7.3 2.6 8
TC4 1149 945 52 32.8 23.7 3.0 2.7 8
TC5 3543 3087 53 25.5 18.1 11.6 1.7 11
a Massfromthedustnear-infraredextinction.
b Massfromthedustcontinuummillimeteremission.
earlier evolutionary status in the star-formation process with a Lombardi,M.&Alves,J.2001,A&A,377,1023
first generation of OB stars but no significant embedded clus- Lynds,B.T.,Canzian,B.J.,&Oneil,Jr.,E.J.1985,ApJ,288,164
ter yet. Severalprotostarsin TC3/4 are likely the precursorsof Marshall,D.J.,Joncas,G.,&Jones,A.P.2009,ApJ,706,727
Marshall, D. J., Robin, A. C., Reyle´, C., Schultheis, M., & Picaud, S. 2006,
such clusters. Longer wavelengths observations from Herschel
A&A,453,635
and Planck are needed to better understand the dust properties McClure,M.2009,ApJ,693,L81
andtheinteractionbetweentheinterstellarmediumandtheOB McMillan,P.J.&Binney,J.J.2010,MNRAS,402,934
stars. Molinari,S.,Swinyard,B.,Bally,J.,etal.2010a,A&A,518,L100+
Molinari,S.,Swinyard,B.,Bally,J.,etal.2010b,PASP,122,314
Acknowledgements. We thank the anonymous referee for his comments that Nishiyama,S.,Nagata,T.,Kusakabe,N.,etal.2006,ApJ,638,839
help to clarify and improve this paper. We thank T. Dame for providing us Olofsson,S.&Olofsson,G.2010,A&A,522,A84
with the CO datacube and for his assistance in using it. We are also grate- Rho,J.,Corcoran,M.F.,Chu,Y.,&Reach,W.T.2001,ApJ,562,446
fulto A.Boogert forhis help understanding the possible ice contributions to Rho,J.,Lefloch,B.,Reach,W.T.,&Cernicharo,J.2008,M20:StarFormation
ouranalysis.ThisworkisbasedinpartonobservationsmadewiththeSpitzer inaYoungHIIRegion(ASPConferenceseries),509
SpaceTelescope,whichisoperatedbytheJetPropulsionLaboratory,California Rho,J.,Ram´ırez,S.V.,Corcoran,M.F.,Hamaguchi,K.,&Lefloch,B.2004,
InstituteofTechnologyunderacontractwithNASA.Thisworkisbasedinpart ApJ,607,904
ondataobtainedaspartoftheUKIRTInfraredDeepSkySurvey.Thispubli- Rho,J.,Reach,W.T.,Lefloch,B.,&Fazio,G.G.2006,ApJ,643,965
cationmakesuseofdataproductsfromtheTwoMicronAllSkySurvey,which Rieke,G.H.&Lebofsky,M.J.1985,ApJ,288,618
isajointprojectoftheUniversityofMassachusettsandtheInfraredProcessing Robin,A.C.,Reyle´,C.,Derrie`re,S.,&Picaud,S.2003,A&A,409,523
andAnalysisCenter/CaliforniaInstituteofTechnology,fundedbytheNational Roma´n-Zu´n˜iga,C.G.,Lada,C.J.,Muench,A.,&Alves,J.F.2007,ApJ,664,
AeronauticsandSpaceAdministrationandtheNationalScienceFoundation 357
Savage,B.D.&Mathis,J.S.1979,ARA&A,17,73
Skrutskie,M.F.,Cutri,R.M.,Stiening,R.,etal.2006,AJ,131,1163
References Valle´e,J.P.2008,AJ,135,1301
Weingartner,J.C.&Draine,B.T.2001,ApJ,548,296
Bergin, E. A., Melnick, G. J., Gerakines, P. A., Neufeld, D. A., & Whittet, Whittet,D.C.B.,Gerakines,P.A.,Hough,J.H.,&Shenoy,S.S.2001,ApJ,
D.C.B.2005,ApJ,627,L33 547,872
Bernard,J.P.,Abergel,A.,Ristorcelli,I.,etal.1999,A&A,347,640 Whittet,D.C.B.,Gerakines,P.A.,Tielens,A.G.G.M.,etal.1998,ApJ,498,
Boogertetal.submittedtoApJ L159+
Boulanger,F.,Beichman,C.,Desert,F.X.,etal.1988,ApJ,332,328 Wilson,B.A.,Dame,T.M.,Masheder,M.R.W.,&Thaddeus,P.2005,A&A,
Cambre´sy,L.1999,A&A,345,965 430,523
Cambre´sy,L.,Beichman, C.A.,Jarrett,T.H.,&Cutri, R.M.2002,AJ,123, Winston,E.,Megeath,S.T.,Wolk,S.J.,etal.2010,AJ,140,266
2559 Zasowski,G.,Majewski,S.R.,Indebetouw,R.,etal.2009,ApJ,707,510
Cambre´sy,L.,Boulanger,F.,Lagache,G.,&Stepnik,B.2001,A&A,375,999
Cambre´sy,L.,Jarrett,T.H.,&Beichman,C.A.2005,A&A,435,131
Casali,M.,Adamson,A.,AlvesdeOliveira,C.,etal.2007,A&A,467,777
Cernicharo,J.,Lefloch,B.,Cox,P.,etal.1998,Science,282,462
Chapman,N.L.,Mundy,L.G.,Lai,S.,&Evans,N.J.2009,ApJ,690,496
Churchwell,E.,Babler,B.L.,Meade,M.R.,etal.2009,PASP,121,213
Dame,T.M.,Hartmann,D.,&Thaddeus,P.2001,ApJ,547,792
De´sert,F.,Mac´ıas-Pe´rez,J.F.,Mayet,F.,etal.2008,A&A,481,411
Dickman,R.L.1978,AJ,83,363
Dobashi,K.,Uehara,H.,Kandori,R.,etal.2005,PASJ,57,1
Flaherty,K.M.,Pipher,J.L.,Megeath,S.T.,etal.2007,ApJ,663,1069
Hambly,N.C.,Collins,R.S.,Cross,N.J.G.,etal.2008,MNRAS,384,637
Hewett,P.C.,Warren,S.J.,Leggett,S.K.,&Hodgkin,S.T.2006,MNRAS,
367,454
Hodgkin,S.T.,Irwin,M.J.,Hewett,P.C.,&Warren,S.J.2009,MNRAS,394,
675
Indebetouw,R.,Mathis,J.S.,Babler,B.L.,etal.2005,ApJ,619,931
Kohoutek,L.,Mayer,P.,&Lorenz,R.1999,A&AS,134,129
Lada,C.J.,Lada,E.A.,Clemens,D.P.,&Bally,J.1994,ApJ,429,694
Laureijs,R.J.,Clark,F.O.,&Prusti,T.1991,ApJ,372,185
Lawrence,A.,Warren,S.J.,Almaini,O.,etal.2007,MNRAS,379,1599
Lefloch,B.,Cernicharo,J.,&Pardo,J.R.2008,A&A,489,157
Li,A.&Draine,B.T.2001,ApJ,554,778
Lombardi,M.2009,A&A,493,735