Velocity field and star formation in the Horsehead nebula

0.48 MB·English

by  Pierre Hily-Blant

Astronomy&Astrophysicsmanuscriptno.hh˙vel˙jun05 February5,2008
(DOI:willbeinsertedbyhandlater)
Velocity field and star formation in the Horsehead nebula
P.Hily-Blant1,2,D.Teyssier3,4,S.Philipp5,andR.Gu¨sten5
1 LRA-LERMA,E´colenormalesupe´rieureetObservatoiredeParis,24rueLhomond,75231Pariscedex05,France
2 InstitutdeRadioAstronomieMillime´trique,300RuedelaPiscine,F-38406SaintMartind’He`res,France
3 SpaceResearchOrganizationNetherlands,P.O.Box800,9700AVGroningen,TheNetherlands
6
4 DepartamentodeAstrofisicaMoleculareInfrarroja,InstitutodeEstructuradelaMateria,CSIC,Serrano121,28006,Madrid,
0
Spain
0
5 Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,D-53121Bonn,Germany
2

n
Received/Accepted
a
J
Abstract.UsinglargescalemapsinC18O(2−1)andinthecontinuumat1.2mmobtainedattheIRAM-30mantennawiththe
6
2 HeterodyneReceiverArray(HERA)andMAMBO2,weinvestigatedthemorphologyandthevelocityfieldprobedintheinner
layersof theHorsehead nebula. Thedatareveal anon–self-gravitating (m/mvir ≈ 0.3) filamentof dust and gas(the“neck”,
1 ∅=0.15−0.30pc)connectingtheHorseheadwesternridge,aPhoton-DominatedRegionilluminatedbyσOri,toitsparental
v cloudL1630.Severaldensecoresareembeddedintheridgeandtheneck.Oneofthesecoresappearsparticularlypeakedinthe
5 1.2mmcontinuummapandcorrespondstoafeatureseeninabsorptiononISOmapsaround7µm.ItsC18Oemissiondropsat
0 thecontinuumpeak,suggestiveofmoleculardepletionontocoldgrains.ThechannelmapsoftheHorseheadexhibitanoverall
6 north-eastvelocitygradientwhoseorientationswivelseast-west,showingasomewhatmorecomplexstructurethanwasrecently
1
reportedbyPoundetal.(2003)usingBIMACO(1−0)mapping.Inboththeneckandthewesternridge,thematerialisrotating
0
around anaxisextending fromthePDRtoL1630 (angular velocity= 1.5−4.0kms−1).Moreover, velocitygradients along
6
thefilamentappeartochangesignregularly(3kms−1pc−1,period=0.30pc)atthelocationsofembeddedintegratedintensity
0
peaks.Thenodesofthisoscillationareatthesamevelocity.Similartransversecutsacrossthefilamentshowasharpvariation
/
h oftheangularvelocityintheareaofthemaindensecore.Thedataalsosuggestthatdifferentialrotationisoccurringinpartsof
p
thefilament.Wepresentanewscenariofortheformationandevolutionofthenebulaanddiscussdensecoreformationinside
-
o thefilament.
r
st Keywords.ISM:clouds–kinematicsanddynamics–individualobjects(Horseheadnebula)–Stars:Formation–Radiolines:
a ISM
:
v
i
X
1. Introduction non–self-gravitatingfilamentconnectedtothelow-massdense
r core L1512. Theoretical models with and without magnetic
a Densemolecularcoresarethebasicunitsofisolatedlow-mass
fields suggest power-lawswith exponentsrangingfrom ≈ −2
star formation. It is now common that molecular line map-
(Fiege&Pudritz 2000) to −4 (Ostriker1964; Nakamuraetal.
pingfromdarkcloudsrevealsclumpsembeddedinfilamentary
1993;Stodo´lkiewicz1963).
structures(e.g.Onishietal. 1998,1999;Obayashietal. 1998;
The importance of the velocity field in the formation of
Loren1989a;Chinietal.1997).Thesefilamentscanbeeither
clumps in filamentary clouds has already been noted several
self-gravitating or not. The clumps are mostly gravitationally
yearsagobyLoren(1989b),whodidasystematicstudyofthe
bound, and in many cases, star formation is known to have
velocity pattern of well-known filaments in ρOph. The longi-
already started. Therefore,molecular filaments are thoughtto
tudinalvelocitywas shownto exhibitgradientsnear the loca-
playacrucialroleinlow-massstarformation.
tions of the main clumpsharbouredin the filament. From the
However, little is known about the general physical prop-
theoretical and numerical points of view, recent studies also
erties of these filaments. For example, the density distribu-
stress the role of the velocity field. Dealing with self-similar
tion is very likely not uniform, but instead varies accord-
rotating magnetized cylinders, Hennebelle (2003) shows that
ing to a power-law. Stepniketal. (2003) have investigated
the velocityfield strongly dependson the relative intensity of
this observationally in a filament of the Taurus molecular
the toroidal componentof the magnetic fields with respect to
cloud, and concluded that a r−2 profile was compatible with
the poloidal one and to the gravitational force: the rotation
the observations. Steeper density profiles can however be ob-
can be mainly that of a rigid body or instead be differential.
served, as is shown by Hily-Blant & Falgarone(in prep) in a
The importanceof the velocityfield has been furtherstressed
Sendoffprintrequeststo:P.Hily-Blante-mail:[email protected] by Tilley&Pudritz (2003) who show how it could help in
2 P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula
distinguishingbetween collapsingand equilibriumcylindrical (HERA,Schusteretal.2003,2004)operatedattheIRAM-30m
distributions and between magnetized and unmagnetized fila- telescope. The data were obtained in the On-The-Fly mode
ments.Fiege&Pudritz(2000)performednumericalstudiesof with an array inclined on the sky by 18.5◦ in the equatorial
the fragmentation of pressure truncated isothermal filaments frame,providingadirectsamplingof8′′ inDeclination,while
threadedbyhelicalmagneticfieldsandfoundthatthetoroidal a derotatorlocated in the receiver cabin was used to keep the
velocity could change sign periodically when magnetic field HERApixelpatternstationaryintheNasmythfocalplane.The
dominatesovergravity.More recently,Sugimotoetal. (2004) half-power beam-width is 12′′. The data were obtained under
numericallystudiedthedecayofAlfve´nwavesin filamentary good-to-average weather conditions and system temperatures
clouds. They show that the propagating circularly polarized were on the orderof 400K, providingan averagenoise r.m.s.
wavesgeneratelongitudinalsub-wavesthatsteepenintoshocks intheregriddedmapof0.2K(T∗ scale)within0.1kms−1 ve-
A
wheretheinitialenergyisbeingdissipated.Whilepropagating, locity channel. The final data are scaled in T , a temper-
mb,c
thecircularlypolarizedAlfve´nwavesmakethefilamentrotate. ature scale which takes into account the emission peaked-up
Insomecases,thefilamentcanfragmentintoregularlyspaced by the 30-m error beam at 1.2mm (see Abergeletal. 2003,
clumps. and references therein for details). Figure 1 shows the cor-
Instabilities(gravitational,magnetic,orboth)areoftenin- responding integrated intensity map in the velocity interval
voked to explain the formation of dense cores in filaments. [9.1:11.8]kms−1.
Nonetheless, molecularobservationsof filamentary structures As a complement, we also use continuum emission maps
at high spatial and spectral resolution are still lacking, which at 1.2mm obtained with MAMBO2, the MPIfR 117-channel
wouldallowboththedensityandthevelocityfieldstobecon- bolometerarrayoperatedatPico Veleta(Kreysa1992).These
strained. The scales of interest are typically of the order on data are partially shown in a companionpaper (Teyssieretal.
0.1pc and velocity gradients of the order on 1kms−1pc−1. 2004),butarepresentedhereinfullforthefirsttime.Wereferto
In this paper, we investigate the velocity field and its link thisarticleforfurtherdetailsabouttheobservations.Alsoused
to the density distribution in the Horsehead nebula, a close- in this analysis is the ISOCAM map obtained on this source
by (400pc) dark protrusion emerging from its parental cloud by Abergel et al. (2002, 2003) in the LW2 filter (5.-8.5µm).
L1630 in the Orion B molecular complex. This condensation The correspondingmaps are displayed in Fig. 1, in combina-
isilluminatedbytheO9.5Vstar σOri(distance0.5◦ fromthe tionwiththeHαcoverageobtainedbyReipurth&Bally(pri-
cloud) and presents a Photon-Dominated Region (PDR) on vatecommunication)attheKPNO0.9mtelescope.
its western side seen perfectly edge-on (Abergeletal. 2003).
Until some years ago, molecular observations of this source
3. Morphology
werestillscarceandlimitedtolowspatialresolution(2′ reso-
lution, Krameretal.1996).Newinsightsathigherresolution
3.1.Spatialdistribution
(10–20′′) haverecentlybeenrevealedbyAbergeletal.(2003)
and Poundetal. (2003), who observed the Horsehead in var- Observation of an optically thin species provides a detailed
ious millimetre transitions and isotopes of CO. In particular, picture of the inner molecular layers of this complex object
Poundetal.(hereafterPRB03)analysethevelocityfieldofthis for the first time. The spatial distribution correlates remark-
source, using interferometric CO(1−0) data (10′′×8′′ resolu- ably well with the visible dust absorption (well represented
tion),andreportedageneralNE-SWvelocitygradientoforder here by the Hα image) and the optically thin dust continuum
5kms−1pc−1acrossthenebula.Theyalsonotethatstarforma- emissionat1.2mm,revealingasomewhatdifferentshapethan
tionmayhavealreadystartedinthecloudanddiscusspossible theHorseheadsilhouetteastracede.g.bytheBIMACO(1−0)
scenariosthatcouldhavedriventheformationofsuchastruc- maps of PRB03 (10′′×8′′resolution).Also, the C18O map pre-
ture. The use of an optically thick probe may however, have sentedhereextendsfurthereastthanthe12COmapoftheseau-
hinderedthemfromobtaininga thoroughpictureofthestruc- thors.Inparticular,thewesternridgeformingthePDRappears
tureintheinnermostlayersofthisobject.Inthepresentstudy, connectedtotheparentalcloudthroughathin(1.5′,or0.2pcat
wemakeuseoftherarerisotopomerC18O,aswellastheemis- adistanceof400pc)dustandgaseast-westfilament(hereafter
sionofdustatmillimeterwavelengths. calledthe“neck”)exhibitingthreenoticeableintegratedinten-
Thepaperisorganizedasfollows.Afterabriefdescription sity peaks. This filament penetrates further inside the L1630
oftheobservationsinSect.2,weanalystinSect.3thespatial cloudfollowingaSW-NEorientation.Inthisstudy,wedistin-
distributionoftheC18O(2−1)emissionandofsupportdataob- guish between these two sections of the neck and refer to the
tainedinthecontinuumatvariouswavelengthstoderiverepre- “inner”(eastmost)and“outer”(westmost)necks,respectively.
sentativephysicalparameters.Theanalysisofthevelocityfield Similarly,werefertothenorthernpartofthePDRasthe“nose”
isthenexplainedinSect.4,anditsconsequencesonthegene- and to its southernpart as the “mane”,followingsome of the
sisandevolutionoftheobjectarethendiscussedinSect.5.We naming already used by PRB03. The material voids present
finallysummarizeourconclusionsinSect.6. northandsouthoftheouterneckwillberespectivelycalledthe
jawandtheopticalhole.TheseareasareindicatedinFig.5.We
notethatourC18O(2−1)integratedintensitymapismuchless
2. Observations
clumpythanthatin12CO(1−0)fromPRB03,particularlyinthe
The C18O(2 − 1) data presented in this paper have been ob- outerneck.Since12CO(1−0)tracesthelowerdensitymaterial,
servedinMay2003usingthenewHeterodyneReceiverArray thissuggeststhattheexternalenvelopeisclumpierthanthein-
P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula 3
Fig.1.(Upperleft):Hαlineemissionmapobtainedatthe0.9mKPNOtelescope(courtesyofReipurth&Bally).(Upperright):
Integratedemissionmap( T dv,Kkms−1,v∈[9.1:11.8]kms−1)ofC18O(2−1).Offsetsareinarcsecandcontoursare1.2
mb,c
to7.2by1.2Kkms−1.TheRhalf-powerbeamwidth(HPBW)isplottedatthetoprightcorner.(Bottomleft):continuumemission
around7µm(mostlyaromaticemission)coveredbyISO.(Bottomright):Continuumemissionat1.2mmobservedbyMAMBO2.
Contoursare0.15MJy/srto2.65MJy/srby0.5MJy/sr.
nerpartsevidencedbytheC18O(2−1).Indeedsome12CO(1−0) ofhighdensitiesandcoldtemperatures.Inthe12CO(1−0)map
peaksofintegratedintensity(e.g.inthemane)domatchopti- from PRB03, an elongated maximum is seen north of Peak2
calfeatures,withnocounterpartinC18O(2−1).However,itis position,butitdoesnotcoincidewitheitherofourC18O(2−1)
notclearwhetherthisisduetoexcitationorphoto-dissociation or continuum maxima in this area. A detailed study of this
selectiveeffects,ortoacombinationofboth. condensationis, however,beyondthescopeofthepresentpa-
A noticeable emission peak is observed at the base per and will be addressed in a forthcoming work (Teyssier
of the pillar formed by the outer neck (hereafter Peak2, & Hily-Blant, in preparation). Three other peaks are seen in
α =05h41m06s, δ = −2◦27′30′′), which coincides with the neck: Peak1 (α =05h40m59s, δ = −02◦27′18.7′′),
2000 2000 2000 2000
an embedded structure seen in absorption at 7 and 15µm Peak3 ((α =05h41m12.2s, δ = −02◦25′59′′), and Peak4
2000 2000
(Abergeletal. 2002, see also Fig. 1) and a strong emis- (α =05h41m15.3s, δ = −02◦25′39.5′′, see also Fig. 5).
2000 2000
sion feature in the continuum at 1.2mm, also prominent in We notethatPeak1hascounterpartsinthe 1.2mmcontinuum
other submm continuum maps obtained by SCUBA at the map;however,itdoesnotstandoutatallintheinterferometric
JCMT (850 and 450µm, Sandell private communication)and mapofPRB03. On theotherhand,Peak3andPeak4areonly
SHARC-II at the CSO (350µm, Lis private communication). visible in the molecular data, and in limited velocity ranges,
It is interesting to note that the position of the peak associ- [9.9:10.1]and[10.3:10.6]kms−1respectively(seeFig.2).
atedwiththisclumpdiffersinthe1.2mmcontinuumandinthe
C18O(2−1)maps,aphenomenonoftenassociatedwithmolecu-
lardepletionontograins(e.g.Casellietal.1999)andindicative
4 P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula
Fig.2.ChannelmapsoftheC18O(2−1)emissionbetween9.6and11.2kms−1.Contoursare1.1Kto7.4Kbystepsof0.9K(T∗
A
scale,rms=0.2K).
3.2.Physicalconditions
This section is not meant to study the variation of the phys-
ical parameters at small scale in detail but rather to give an
overviewofthetypicalconditionsassociatedtotheareasintro-
ducedabove.Thisanalysishasalreadybeen conductedin the
PDR by Abergeletal. (2003) and Teyssieretal. (2004), who
reportpeakvisualextinctionsintherange12-25magandden-
sitiesontheorderof2×104cm−3.However,basedonhighres-
olutionH NIRdata,Habartetal.(2003,seealsoHabartetal.
2
2004) have shown that this last parameter could be higher by
one order of magnitude at the PDR border. At the scales of
interest, we used the 1.2mm continuum emission in order to
compute the column densities. We applied the formula pre-
sented in Motteetal. (1998), assuming dust temperatures of
35KinthePDRarea(Teyssieretal.2004)andof10Kforall
areas eastward of the ridge. The dust mass opacity at 1.2mm
isknowntovarydependingonthetemperatureand/ordensity
(e.g. Ossenkopf&Henning 1994; Stepniketal. 2003). To ac-
Fig.3.MapoftheC18O(2−1)velocitycentroids.Indicatedby countforthisuncertainty,weconsideredvaluesofthisparam-
starsarethreeIRASsourcesidentifiedbySandelletal.(1986). eter in the range 3–6×10−3gcm−2. Excluding the 3 peaks in-
Thearrowsindicatethevelocitygradientorientationsatdiffer- troducedabove,the computedcolumndensities varybetween
entpositionsacrosstheneckandridge. some 1021cm−2 in the mostdiffuse parts(distributedoverthe
innerandouternecks)and1.0–2.5×1022cm−2(e.g.inthemane
orthe PDR). While Peak1andPeak3exhibitquantitiesinthe
rangeofthoseinthePDR,thePeak2columndensityiscalcu-
latedtobe3.5±1.0×1022cm−2.
P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula 5
Fig.4. Map of C18O(2− 1) spectra averaged over areas of 40′′×40′′. The vertical dotted line indicates the systemic velocity
(10.33kms−1).Thechannelwidthis0.1kms−1.Offsetsarein(′′).
In order to compare these estimates with quantities de- Table1.PhysicalconditionsintheHorsehead(seeSect.3.2).
rivedfromthemoleculartracers,we usedcomplementaryob- (a) corrected from molecular depletion, (b) from Abergeletal.
servationsofCO(2−1)(Gu¨sten,privatecommunication)and (2003).
C18O(1 − 0) (Teyssier, private communication) at positions
of interest to run LVG simulations. Assuming optically thick
diffuseparts Peak2(a) PDR(b)
emission from CO(1−0), we inferred lower limits to the ki-
netic temperatureto be appliedto C18O. Tkin are foundin the nH2 (cm−3) 3–5×103 4×104 2×104
range20-30K,translatingintovolumedensitiesof104cm−3in
N (cm−2) ≈1021 3.5±1.0×1022 1.2–2.5×1022
mostoftheridgeandouterneck.Somewhatlowervalues(fac- H2
tor2-3below)arefoundintheinnerneck,whilePeak2exhibits
a higher density of n =4×104cm−3. The C18O column den-
H2
sitywassimultaneouslyconstrainedandtranslatedintoH col-
2
umndensitiesusingthecalibrationdescribedinTeyssieretal.
(2004):N =(2.5±0.6×106) × N(C18O)+1.4×1021cm−2. 3.3.Massestimates
H2
N is found to be in very good agreement with the esti-
H2 Withthesenumbers,weestimatedtheoverallmassofthema-
mates based on the dust emission, except at the position of
terial traced by C18O(2 − 1). The analysis presented above
Peak2 where a value of 1.4×1022cm−2 is measured, sugges-
shows that H column density estimates based on either our
tiveagainofmoleculardepletioninthecoldcondensation.We 2
1.2mm dust continuum or the C18O(2 − 1) integrated inten-
estimatefromthisthatthequantitiesderivedinthiscorefrom
sity maps are in good agreement provided molecular deple-
C18O(2−1)areunder-estimatedbyafactorofatmost3.Table1
tion onto grains is not a concern. This assumption is fairly
compilesthesevaluesforvariousareasofinterest.
justified here, with the exception of the dense core asso-
ciated to Peak2. Assuming an overall conversion factor of
WefinallycomparedthevaluesofN andn toinferthe N /W(C18O(2−1)) = 2.5× 1021cm−2/(Kkms−1), the total
H2 H2 H2
clouddepthalongthelineofsight.Theirratioindicatedepths massderivedfromtheC18O(2−1)integratedintensityisfound
intherange0.15–0.30pc,verysimilartothefilamentwidthon to be 19M , of which 0.6M are located in the cold core.
⊙ ⊙
thesky.Withintheeffectsofprojection,thisisconsistentwith Assuming a depletion factor on the order of 3 (see previous
amoreorlesscylindricalshapeofthefilament. section)inthePeak2area,thetotalcorrectedmassamountsto
6 P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula
20M ,showingthatdepletioneffectsarenegligibleasregards
⊙
thetotalareaconsidered.Thisislowerby25%thanthevalue
reportedbyPRB03,andverylikelydoesnotaccountforallthe
materialpresentinthenebula,sincehereweareprobingonly
thegasintheinnerlayers.
4. Studyofthevelocityfield
In this section, we present the detailed analysis of the veloc-
ityfieldbasedontheC18O(2−1)data.Thesystemicvelocity
asdeducedfromtheintegratedspectra overthewhole fieldis
v =10.33kms−1.TheshapeisverynearlyGaussianwithato-
0
tal FWHM of 0.8kms−1. This value falls in between typical Fig.6. C18O(2−1) position-velocitycut performedalong the
linewidth in dark clouds (nearly thermal, 0.5kms−1) and that
westernridgeandthemane(fromnorthtosouth,Fig.5).The
of the more diffuse molecular phase (1kms−1). This in fact
dashed horizontalline indicates the systemic velocity v . The
0
reflects the wide range of velocities in the nebula where the
threeverticallinesdelineatethefoursegmentsformingthecut.
overalllinewidthisbroadenedbyvelocitygradients.Individual Alarge(+2.6kms−1pc−1)north–southvelocitygradientisob-
spectra indeed have FWHM=0.66±0.11kms−1, as illustrated
serveduntilthemane,wherethegradientissuddenlyinverted
in Fig. 4 where averagedspectra oversmall areasare plotted.
to finally end up at the systemic velocity with a slope of –
Whilethecentroidvelocitychangeswithinthenebula,theline- 4.3kms−1pc−1.Contoursstartat3σ(0.6K),instepsof3σ.
shaperemainsGaussian.
Another picture of this effect is shown in the line cen-
troid map displayed in Fig. 3. The computation was done in
such a way that the spectral window used to deduce the cen-
4.1.Longitudinalcuts
troidisoptimizedforeachspectrum.Thismethodisdescribed
in Pety&Falgarone (2003), and consists in maximizing the We first consider cuts performed along the filamentary struc-
signal-to-noiseratioontheintegratedareaintheactualspectral tures introduced in Sect. 3.1. The positions of these two cuts
window.Thecentroidisthencomputedintheusualway,i.e.as (respectively along the neck and the western ridge) are dis-
the velocity weighted by the temperature. As was already re- played in Fig. 5. Although there is some subjectivity in the
ported by PRB03, the map in Fig. 3 reveals an overall North choice of these segments, we followed, as much as possible,
(blue-shifted emission)–South (red-shifted emission) velocity the morphologytracedbythe filamentarystructuresinthe in-
gradientofabout4-5kms−1pc−1.However,thisgradienthasa tegratedintensitymap.
somewhatmorecomplexmorphology,asitsorientationseems
Figure 6 displays the first of these cuts computed along
orthogonaltoaparabolic-likecurvestarting(atleastwithinour
the western ridge, showing a large north-south velocity gra-
mapboundaries)fromtheinnerneckbasisandslowlyfolding
dient extending over more than 0.4pc. The relatively linear
towardstheHorse’snose.Thegradientorientationisthusswiv- slope of this gradient (on the order of +2.6kms−1pc−1) sug-
elling east-west, as indicated by the arrows drawn on Fig. 3.
gests that the material is rotating. The PDR and nose thus
The NE-SW gradientmentioned by PRB03 thus only applies
seem to be wrapped around an axis coincidentwith the outer
to the western part of the dark protrusion. The data also in-
neck.Furtheralongthecut,thevelocitiesassociatedtothegas
dicate thatboth red andblue velocitycomponentsseem to be
traced in the mane experience a fast drop with a gradient of
wrapped around an axis coincident with the neck shape. It is −4.3kms−1pc−1. The mane thus appears to be braked back
interestingtonotethecoincidencebetweenthepeaksinveloc-
to the systemic velocity found in the filament. We discuss in
itycentroids(around10.8kms−1)andthelocationsofthreeof
Sect.5aninterpretationofthispeculiarbehaviour.
the IRAS stars identified in the field (Fig. 3). A similar phe-
Therespectivel−vmapintheneckisshowninFig.7.In
nomenoncouldalreadybeseenonthemapofPRB03.We do
the gas associated to the outer neck, departuresfrom the sys-
not,however,seeanyobviousexplanationforthiseffect.
temicvelocityarelesspronouncedthanintheridge.However
The channelmap displayed in Fig. 2 shows the structures a close look at the inner neck indicates that the velocity gra-
associatedtotheC18O(2−1)velocityfield.Theridgeandthe dient appears to change sign regularly (amplitude of approx-
neckappearclumpy,withtheintensitypeaksmovingwhengo- imately 2.5kms−1pc−1, rough period of 2.5′, or 0.3pc). The
ingfromonevelocitychanneltotheother.Thisisindicativeof nodes of this modulation are found at a common velocity of
arichvelocityfieldinsidethestructure.Inordertoidentifythe 10.2kms−1(seeFig.7).ItisinterestingtonotethatPeaks3–4
mechanismsassociated to thisgasmotion,we studyposition- arelocatedwherethevelocitygradientchangessign.Thisgra-
velocity maps performed along various dedicated cuts in the dientthenexperiencesanotherrapidflipasonepenetratesthe
followingsections. L1630cloud(newvalueof–4.8kms−1pc−1)further.
P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula 7
Fig.5.Positionoftheposition-velocitycutsdrawnovertheintegratedemissionmapofC18O(2−1).Thefulllinescorrespond
tothecutsacrosstheneckandridge,respectively,whilethethinblacklinesillustratethepositionoftheradialcuts,spacedby
22′′(≈ 2HPBW),withsomenumbersreferringtothecutsshowninFig.9.Thecrossesindicatethepositionofthefourpeaks
introducedinSect.3.1.Alsoindicatedaretheareasdiscussedthroughoutthispaper.
4.2.Radialcuts on the centroid itself, and finally fitted by a linear slope. The
result of this computation is illustrated in Fig. 9, where gra-
Radial cuts across the neck allow a probe of the gas motion
dientsare plottedagainstthe distance alongthe neck.The re-
around the axis defined by the filament projected shape. To
sulting plot reveals a striking shape: while the outer neck ap-
this aim, we have constructedl−v maps on a series of small
pears in rigid body rotation at 1.5kms−1pc−1 (corresponding
slits perpendicular to the neck longitudinal cut analyzed pre-
to a rotation period of 4Myrs, roughly equal to the survival
viously.Their positionsand associated numbersare indicated
timeoftheHorsehead,5Myrs,estimatedbyPRB03),partsof
inFig.5.Thecorrespondingcollectionofmapsisgatheredin
the inner neck experience a sharp increase in velocity gradi-
Fig. 8. In most cases, the velocity scales almost linearly with
entwhenmovingfromeasttowest(from1.5to4kms−1pc−1
the radialposition,againindicativeofrigidbodyrotationand
inaboutthan0.1pc).Thisrapidjumpisfollowedbyasimilar
confirmingthattheneckitselfisspinningarounditsownaxis.
decreasebacktothe1.5kms−1pc−1velocitygradient.Beyond
However,somecuts(e.g.#6,#7,#11,#12)departfromthatlin-
thispoint,theradialcutsassociatedtothewestmostpartofthe
ear behaviour, suggesting that differential rotation may affect
innerneckare foundto have a 0 velocitygradient(withinthe
the transverse gas motion in these areas. Such velocitygradi-
slope fit uncertainty). It is to be noted that all three positions
entsacrosstheneckcouldalsoresultfromavelocityshearper-
of Peaks 1, 2, and 4 are foundat the commonvelocity gradi-
pendiculartotheneck.Thisinterpretationwillbecomparedto
entof1.5kms−1pc−1 (itislesstrueforPeak1,though),while
rotationinSect.5.2.
Peak3 is found at the high velocity gradient of 4kms−1pc−1.
It is also interesting to note that the velocity gradient ob-
Interestingly enough, Peak3 sits almost in the middle of the
served at each position is varying when moving from east to
roughlysymmetricgradientincreasebracketedbyPeaks2and
west. In orderto quantifythisvariation,andmotivatedbythe
4.Attheeastmostoffsets,theradialcutsexhibitanothergradi-
assumptionofrigidbodyrotation,we performeda systematic
entincrease,butthisoneisverylikelyduetotheknownlarge
linearfitofeachindividuall−vcutandinterpretedthisslope
gradientfoundalongthemaneandnose,andislessmeaningful
of as an angular velocity. In each cut, spectral lines were fit-
intermsofradialgradientacrosstheneck.
ted by Gaussians, and their central velocity was stored. This
setofcentralvelocitieswasthenweightedtakingintoaccount Itisalsointerestingtocomparetheratioofthegravitational
both the signal-to-noise ratio (SNR) of the data and the error to centrifugal forces in various areas of the Horsehead. This
8 P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula
Fig.7.SameasFig.6alongtheentireneck(fromeasttowest,
Fig.5).Thedashedlineindicatesthesystemicvelocity,while
the vertical one shows the position of Peak2 (see Sect. 3.1),
where the longitudinal cut changes direction. Positions of
peaks4to1areatoffsets80,140,250and380′′ respectively.
Recall that Peak2 is not a maximum in the C18O(2 − 1) in-
tegrated intensity map and was inferred from the continuum
data.Notethe3differentvelocitygradients(positive,negative,
positive) as one moves from 0 to 200′′(see text for details).
Contoursstartat3σ(0.6K),instepsof3σ.
ratioisgivenbyζ =πGhρi/ω2(withhρithemeanmassdensity
Fig.8.TransverseC18O(2−1)position-velocitycutsperformed
andωtheangularvelocity),translatingintoζ ≈10intheouter
acrosstheneck,correspondingtotheblackthinlinesofFig.5.
neck,whileζ ≈ 1nearPeak2.Thismeansthatgravityisable
The cuts are separated by 22′′. The x-axis offsets increase
to ensure the confinement in the outer neck, while it is only
northwise,sothatthevelocitydecreasesfromSouthtoNorth.
marginallysufficientnearPeak2.
Dashed lines indicate the results of the bi-dimensionalfits to
thel−vdiagram(seetextfordetails),showinghowtheradial
velocity gradientevolvesas one movesfrom East to West (cf
5. Discussion
Fig.9).Contoursstartat3σ(0.6K),instepsof3σ.
5.1.FormationandevolutionoftheHorseheadnebula
In the scenario proposed by Reipurth&Bouchet (1984), the
Horsehead is assimilated to an early Bok globule emerging Based on the velocity cuts described in the previous sec-
fromitsparentalcloudviatheerodingincidentradiationfield tions, the overallpicture is that of an elongatedstructure (the
emittedbyσOri.Theseauthorsalsoproposethatthejawcav- neck)spinningarounditsownaxis,andconnectedatitswest-
ity was the resultof a collimated outflow,butthis seems now ern end to a ridge (mane to nose) in rotation around this
highlyunlikely(e.g. Warren-Smithetal.1985).Basedontheir same axis. We found that this filament is roughly axisym-
CO(1−0)data,PRB03discussthepossibleoriginofthisdark metric with a projected diameter D ≈ 0.15pc and a depth
cloud, and conclude that both the work of instabilities and of L≈0.15−0.30pc.InthePRB03scenario,allthatisneededis
theionizationfrontcouldbeconsidered.Theyproposethatthe someasymmetryatthefrontendoftheneck,thereforeitisnot
initialdensityenhancementintheparentalcloudwasasymmet- inconsistentwiththepresentconclusion.
ric,atleastatthewesternendoftheneck,andthatthismaterial The reason the Horsehead followed such a peculiar route
hasbeenpushedoutoneithersideoftheneck,resultinginthe from L1630 might be linked to a pre-existing rotating veloc-
maneandthejaw.TheyalsodeducethattheHorsehead’sneck ity field in the region of the parental cloud that resisted the
is not axisymmetric but rather presents an oval cross-section. incidentradiationfieldandgavebirthtotheprotrusion,apos-
Theirconclusionsare,however,relatedtothegeneralshapeof sibility also mentionedby PRB03. As rotationproceeded,the
thecloud,so theymayhavemissed someofthe detailsinthe centrifugalforce has progressively detached the nose and the
innerlayersofthecloudduetothehighopacityoftheirtracer. manefromtheneck.Thisscenariorequiresinitialdensityinho-
ThedataobtainedinC18Othusshednewlightontheformation mogeneitieswithinthepillar:thedensestpartshaveformedthe
andevolutionoftheHorsehead. neck, mane, and nose, while the most tenuousones remained
P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula 9
as the jaw cavityand the opticalhole.We note thatthisinho-
mogeneityisalsorequiredbythescenarioreportedbyPRB03.
This picture is moreover consistent with the gradient seen in
thelongitudinall−vcutshowninFig.6:thematerialextend-
ing from the nose down to the middle of the PDR (offsets 0
to 200′′) is rotating,with thenose blue-shiftedand partofthe
manered-shifted.Thesouthernendofthemane(offsets200to
300 ′′ in Fig 6) is, however,brakedback to the neck velocity.
This suggests that part of the mane has remained attached to
the pillar (their physical link is already obvious from various
tracers, see Fig. 1), resulting in the material loop seen e.g. in
theoptical.Asaconsequenceofthismechanism,thenoseand
themanecannotlieintheplaneoftheskyastheneckpresum-
ablydoes.Rather,thenosewouldbeintheforeground,while
part of the mane would be in the background. The fact that
thenoseappearstotallydetachedfromtheneckalsoindicates
that the material void associated to the actual jaw was more
pronouncedthanthatintheopticalhole.Obviously,thecontin-
uederodingworkofσOrihasproceededduringthisprocess.It
mustthushavealsocontributedtotheshapingofthestructure
weseeinprojectionontheplaneofthesky.Thismustbepar-
ticularlytrue inthe jaw and theopticalhole,wherethe initial Fig.9. Evolution of the transverse velocity gradient with the
density contrast within the pristine pillar may thus have been distance along the neck. The gradients and associated er-
enhanced. ror bars were computed following the method described in
Sect. 4.2. The dashed line shows the constant gradient of
Incomplementtothiseffect,thenosemayalsobepushed
+1.5kms−1pc−1, indicating the area suggestive of rigid body
towards us by the rocket effect resulting from eventual back-
rotation. Also shown are the peaks discussed in this paper.
groundionization.However,the amplitudeof this effectmust
Somecutnumbersarerecalledforthepurposeofcomparison.
benegligiblewithrespecttotheprevailingvelocityfieldsince
partofthemaneremainsred-shifted.Moreover,therocketef-
fectwouldalsoaffecttheneckwhichisseentoberotating.We
thereforediscardthepossibilityofanon-negligiblebackground
ferredfromthe columndensity calculation.The timescale for
ionizationflux.
thismotioncanberoughlyestimatedfromthatofmomentum
transfer between the ionized and the condensedand cold gas.
5.2.Rotationvsshear Assuming an average collisional cross-section for H–H+ en-
counters,huσi = 2.2×10−9cm−3s(Spitzer1978),andadopt-
Wediscussherethequestionwhetherthevelocitygradientre- ingameandensityofprotonn asfoundintheHorsehead,one
f
vealedintheneckcouldbethesignatureofavelocityshearin yields∆t ≈ 2/(n huσi) ≈ 106s, whichevenif underestimated
f
adirectionperpendiculartoboththeneckandthelineofsight, byordersofmagnitude,wouldbeshortenoughbyfartohave
rather than a rotation of the filamentary neck. We see at least disruptedthecylindershapeonestillseestoday.
threeargumentsfavouringtherotationinterpretation. Finally,ifdespitetheargumentsgivenabove,shearwould
First of all, we have proposedin the previoussection that still play a role in the dynamic at work in the neck, and the
boththe(likelycombined)effectofanabrasiveionizingfront torqueitwouldexertonthemaneandnosewouldinanycase
and that of a pre-existingvelocity field may have contributed resultinrotationinthefilament.
tothepeculiarshapingoftheHorseheadnebula.Itisveryun-
likely that the ionization front arising from σOri could have
5.3.DensecoreformationintheHorsehead
generated a shear perpendicular to the line-of-sight, so that a
velocityshearpresentintheneckshouldhaveitsorigininthe Densecoresareroutinelyobservedinfilaments;however,what
parentalcloud. A careful look at the position-velocitycuts of triggers their formation is still an open issue. Fragmentation
Krameretal.(1996)in13CO(2−1)shownovelocitygradientin ofgaseouscylindersresultingfromperiodicinstabilitiesisof-
the vicinityof the Horseheadnebula (RightAscension offsets ten invoked,but their nature is uncertain, be they gravitation-
between -4 and -12arcmin). There would thus be no shear in allyormagneticallydominated,orevenacombinationofboth.
theparentalcloud,andthesewouldlikelydiscarditsexistence Determiningthe balance between these two forcesappearsto
intheneck. beacriticalissue.
A second argument consists in considering the morpho- We haveshownthattheHorseheadneckisafilamenthar-
logicalconsequenceof such a shear. Indeed,after some time, boring (at least) four density peaks (as evidenced by both
a shear in the neck would result in a filament far from be- 1.2mm and C18O maps). The longitudinal cuts in the neck
ing cylindrical, which is in disagreement with the result in- (Fig.7) showthatthe longitudinalvelocityis notuniformbut
10 P.Hily-Blantetal:VelocityfieldandstarformationintheHorseheadnebula
insteadvariesperiodicallyin theinnerneck(period≈ 0.3pc), since m/m ≈ 0.3. This value falls in the range of ratio pre-
vir
aroundV =10.2kms−1closetothesystemicvelocity.Thisin- sented by Fiege&Pudritz (2000). This again raises the ques-
dicates that the filament is not located strictly in the plane of tion of the confinementof the filament. However, the ratio is
thesky.Thisoscillationcouldbeexplainedconsideringmatter notthatsmall,andthegravityisexpectedtoplayaroleinthe
that flows along a filament regularly bent with respect to the dynamicsofthefilament.
planeofthesky.Moreover,itisworthmentioningthepresence To our knowledge, no estimation of the magnetic field
of line wings around Peak3 (see offsets (200′′,80′′) in Fig. 4). strength is available for the Horsehead. Polarized absorbed
However,the spatial resolutionof our data is not sufficientto starlight is reported in Warren-Smithetal. (1985), completed
concludeascenariosuchasoutfloworinflow. by the large-scale dataset of Zaritskyetal. (1987). In the
TheradialcutsshowninFig.8indicatethatthephenomena Horsehead, the polarization is probably due only to align-
atworkaremorecomplex:thelongitudinalgradientsarecom- ment of absorbing dust grains. The transmitted light is there-
bined to rotation.This rotation is mainly that of a rigid body, fore aligned with the magnetic fields projected on the plane
althoughtheangularvelocityexperiencesasharpincreasefrom of the sky.Obviously,thistechniquedoesnotallow detection
1.5 to 4kms−1pc−1 (Fig. 8 and Fig. 9). Interestingly,the lon- ofanypolarizationinthedensestpartsoftheHorsehead.The
gitudinal gradient appears to change signs at the positions of overallpictureisthatofa B-fieldorientednearlyNorth-South
Peak3 and4 (Fig. 7), positionsthatalso appearto play a par- i.e. perpendicularto the filament. Moreover,it coincideswith
ticular role in the picture of radial gradients (Fig. 9). It sug- the large-scale field. In the nose area, however, the polariza-
gests a dynamical link between the formation of the conden- tionvectorsappearperpendiculartothestructurewithanangle
sationsobservedintheneckandthematerialflowrevealedby ≈ 45◦ withrespecttothelarge-scalefield.Thenosemaythus
thecomplexvelocitystructureofthefilament.Finally,wenote have bent the field lines in its centrifugal motion. In the fil-
that another likely effect of the combination of both longitu- ament, though, we cannot yet distinguish between a toroidal
dinal and radial motions is the morphology change observed component threading the neck and pure transverse magnetic
between,e.g.,cuts#8and#14(Fig.8):cut#14ismuchmore field across the neck itself. It is worth notingthat the same is
rounded than the straight contours of cut #8. At this stage it observedinsomedarkcloudsintheTaurusandinOphiuchus:
becomesdifficult to derive an accurate picture accountingfor the magnetic fields orientation at the scale of the dark clouds
the combination of all these phenomena. We believe that de- fitsintosmooth,larger-scalefields.IntheTaurus,themagnetic
tailed interpretation of all these signatures requires dedicated fields appear perpendicular to the cloud’s long axis, while in
numericalmodelling. Ophiuchusitwouldbeparallel(Heilesetal.1992).
We can, however, already infer preliminary constraints We therefore think that both gravity and magnetic fields
about some of the principal ingredients of this modelling, are expectedto playa role in the confinementanddense core
namelygravityandmagneticfield.Incylindricalgeometry,the formationintheHorseheadnebula.
virialmassperunitlengthmaybewritten(e.g.Fiege&Pudritz
2000):
6. Conclusions
2hσ2i
m = . (1) We havestudiedthe morphologyandvelocitystructureofthe
vir
G
Horsehead nebula using new observations at high frequency
Assuminghσ2i≈σ2,thisgives and spatial resolution in the (2 − 1) transition of C18O. Our
v
conclusionscanbesummarizedasfollows:
σ 2
mvir =4.6(cid:18)0.1kmv s−1(cid:19) M⊙/pc. (2) 1. the emission of the likely optically thin tracer reveals a
filamentary morphology similar to the one seen in dust
For the velocity dispersion, we apply the Fuller&Myers millimetre continuumemission and visual extinction.The
(1992)formula: cloud appears as a curved ridge (the PDR) connected to
L1630 by a thin (0.15pc diameter) pillar hosting several
1 1
σ2 =σ2 +k T − (3) condensations.Oneofthem(Peak2)alsoappearsinabsorp-
v v,obs B kin m˜ mobs! tioninthemid-IRemissionat7µm.
2. wenotethatourthinC18O(2−1)mapismuchlessclumpy
wherem˜ isthemeanmassofmoleculargas,takenhereequalto
2.33;m isthemassofthemoleculartracernamelyC18O;and than the possibly thick 12CO(1 − 0) from PRB03, a phe-
obs
nomenonthatcouldbeduetoeitherexcitationorphotodis-
T thekinetictemperature.WeadoptT =20Kandσ =
kin kin v,obs
0.66/2.35kms−1, resulting in a virial mass m ≈ 65M /pc. sociationselectiveeffectsorthecombinationofboth.
vir ⊙
3. these cores are density peaks (n in the range 2 − 4 ×
Ontheotherhand,themassperunitlengthwecandeducefrom H2
104cm−3) embedded in a medium that is more diffuse by
thedensityandsize(seeSect.3.2)is
a factor of 2-3 on average. Kinetic temperatures in these
n R coresarefoundintherange20-30K,althoughthebrightest
m=18 (cid:18)104cHm−3(cid:19) 0.1pc! M⊙/pc. (4) ofthem(Peak2)showshintsofC18Odepletionontograins,
pointingtowardsasomewhatcolderenvironment.
Taking n = 5×103cm−3 and R ≈ 0.2pc results in mass per 4. theoverallshapeofthefilamentisfoundtobefairlycylin-
H2
unitlengthm ≈20M /pc.Thefilamentisnotself-gravitating, dricalwithadiameterofabout0.15–0.30pc.Thetotalmass
⊙