On the kinematic evolution of young local associations and the Sco-Cen complex

0.76 MB·English

by  D. Fernandez

Astronomy&Astrophysicsmanuscriptno.paper (cid:13)c ESO2008
February2,2008
On the kinematic evolution of young local associations and the
Sco-Cen complex
D.Ferna´ndez1,2,⋆,F.Figueras1,andJ.Torra1
1 Departamentd’AstronomiaiMeteorologia,IEEC-UniversitatdeBarcelona,Av.Diagonal647,08028Barcelona,Spain
2 ObservatoriAstrono`micdelMontsec,ConsorcidelMontsec,Plac¸aMajor1,25691A`ger(Lleida),Spain
8
0 Received26April2007/Accepted13December2007
0
2 ABSTRACT

n
a Context.Over thelast decade, several groups of young (mainlylow-mass) starshavebeen discovered in thesolar neighbourhood
J (closer than ∼100 pc), thanks to cross-correlation between X-ray, optical spectroscopy and kinematic data. These young local as-
sociations–includinganimportantfractionwhosemembersareHipparcosstars–offerinsightsintothestarformationprocessin
3
low-densityenvironments,shedlightonthesubstellardomain,andcouldhaveplayedanimportantroleintherecenthistoryofthe

localinterstellarmedium.
]
h Aims.Tostudythekinematicevolutionofyounglocalassociationsandtheirrelationtootheryoungstellargroupsandstructuresin
p thelocalinterstellarmedium,thuscastingnewlightonrecentstarformationprocessesinthesolarneighbourhood.
- Methods.Wecompiledthedatapublishedintheliteratureforyounglocalassociations.UsingarealisticGalacticpotentialweinte-
o gratedtheorbitsfortheseassociationsandtheSco-Cencomplexbackintime.
r Results.Combiningthesedatawiththespatialstructureof theLocalBubbleand thespiral structureoftheGalaxy, wepropose a
t
s recenthistoryofstarformationinthesolarneighbourhood.WesuggestthatboththeSco-Cencomplexandyounglocalassociations
a originatedasaresultoftheimpactoftheinnerspiralarmshockwaveagainstagiantmolecularcloud.Thecoreofthegiantmolecular
[
cloudformedtheSco-Cencomplex,andsomesmallcloudletsinahaloaroundthegiantmolecularcloudformedyounglocalassoci-

ationsseveralmillionyearslater.Wealsoproposeasupernovainyounglocalassociationsafewmillionyearsagoasthemostlikely
1
candidatetohavereheatedtheLocalBubbletoitspresenttemperature.
v
5 Keywords.Galaxy:kinematicsanddynamics–Galaxy:solarneighbourhood –Galaxy:openclustersandassociations:general–
0 Stars:kinematics–Stars:formation–ISM:individualobjects:LocalBubble
6
0
.
1 1. Introduction (ISM)filledbyanX-rayemittingplasma,whichbecameknown
0
astheLocalBubble(LB).ThelaunchoftheROSATsatellitein
8 Thefirstmovinggroupsofstarswerediscoveredearlyoninthe
1990allowedtheLBtobestudiedinmoredetail.Theopeningof
0 history of Galactic dynamics. As early as 1869, R.A. Proctor
theX-raywindowintheskyandobservationsofstarsbelonging
:
v publishedaworkinwhichheidentifiedagroupofstarsaround to clusters with known ages also led to the realisation that X-
i the Hyades open cluster that was moving together through the rayemissionpersistsinyoungstarsforaperiodoftheorderof
X
Galaxy. He also found another 5 comoving stars in the Ursa
100Myr.Animportantfractionoftheseyoung,X-rayemitting
ar Majorconstellation.Inthe1960s,O.J.Eggensuggestedtheex- stars are T Tauri stars. These very young (<∼10 Myr) stars also
istence of a Local Association (see for example Eggen 1961,
exhibitexcessIRemission(duetothepresenceofnearbyheated
1965a, 1965b) formed by a group of young stars with approx-
dustparticlesformingdisksorenvelopes)aswellasUVlineand
imately the same spatial velocity (also referredas the Pleiades
continuumemissionproducedbytheaccretionofthesurround-
moving group). Eggen’s Local Association included the near- inggasanddust.Whenthestarsreachtheageof∼10Myr,both
est brightB-type stars, stars in the Pleiades, the α Perseus and
IRemissionsandopticalactivitydeclineconsiderably.X-rayac-
IC2602 clusters, and the stars belonging to the Sco-Cen com-
tivitythenbecomesthebasisforascertainingyouthfulness.
plex. However, it was difficult to defend a unitary view of the
groupduetothewiderangeofages(<∼100Myr)andwidespread The ROSAT All-Sky Survey (RASS) detected more than
spatialdistribution(∼300pc)ofitsconstituentstars. 100,000 X-ray sources at keV energies (Voges et al. 2000).
Usingakinematicapproachandsearchingforgroupsofcomov-
At the same time as Eggen was studying his Local
ingstarsallowsustodeterminewhichofthemareTTauristars.
Association, the first X-ray detectors were installed in rockets
Thisgreatlyreducesthenumberofcandidatestarsthatmustbe
andlaunchedintospace.Thisledtothediscoveryofthediffuse
observedspectroscopicallytoconfirmtheiryouthfulness.
soft X-ray background (SXRB). The anticorrelation observed
between the SXRB and the HI column density was rapidly in- In this way, several young stellar associations have been
terpretedasevidenceofalocalcavityintheinterstellarmedium discovered within 100 pc of Earth during the last decade (see
Jayawardhana2000, and Zuckerman& Song 2004for a recent
Sendoffprintrequeststo:D.Ferna´ndez, review). The stars belonging to these young local associations
e-mail:[email protected] (YLA) haveagesrangingfroma few million to severaltensof
⋆ At present at Institut d’Estudis Espacials de Catalunya (IEEC), millions of years. Due to their proximity to us, these stars are
c/.GranCapita`2-4,08034Barcelona,Spain. spreadoveralargeareaofthesky(uptoseveralhundredsquare
2 D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex
degrees)makingitdifficulttoidentifytheassociationsassingle
entities. Furthermore,they are far away frommolecular clouds
orstarformingregions(SFR).Thisiswhytheyremainedunno-
ticeduntilrecently.
Theseyoungstarsofferinsightsintothestarformationpro-
cess in low-density environments, which is different from the
dominantmechanismobservedindenserSFR.Thediscoveryof
theseYLAhasalsocontributedtosubstellarastrophysics,since
dozens of dwarfs have been identified in them. For instance,
it has been confirmed that isolated brown dwarfs can form in
these low-density environments (see for example Webb et al.
1999;Lowranceetal.1999;Chauvinetal.2003).YLAalsopro-
videimportantcluesregardingrecentstarformationinthesolar
neighbourhood(since they contain the youngest stars near the
Sun)anditseffectonthelocalISM.
In Sect. 2 of this paperwe reviewcurrentunderstandingof
theLBandtheSco-Cencomplexandpresentpublisheddatafor
thenearly300starsbelongingtoYLA.InSect.3wepresentour
methodfororbitintegration,whichmakesuseofaGalacticpo- Fig.1. 3D volumic density of the Local Bubble from the iso
tentialthatincludesthe generalaxisymmetricpotentialand the equivalent width contours for W = 20 mÅ and 50 mÅ (from
perturbations due to the Galaxy’s spiral structure and the cen- Lallementetal.2003).
tral bar. This leads us to study, in Sect. 4, the origin and evo-
lution of the local structures and, in Sect. 5, to propose a sce-
narioforrecentstarformationinthesolarneighbourhood.This ofabout70-90pc,theequivalentwidthoftheNaID-linedoublet
scenario explainsthe origin of the Sco-Cen complexand YLA increasesfrom<5mÅ to>30mÅ.Theyinterpretthissuddenin-
throughthe collision of a parentgiant molecularcloud (GMC) creaseasa neutralgasboundaryofthelocalcavity,i.e.theLB
with the Sagittarius-Carina spiral arm. Finally, the conclusions limit. This boundary (see Fig. 1) has some interstellar tunnels
ofourworkaresummarisedinSect.6. thatconnecttheLBwithothercavities,suchasLoopIandthe
supershellGSH238+00+09.Inthemeridianplane,thestructure
is clearly observedto tilt by about20◦(see Fig. 5 in Lallement
2. Thenearestsolarneighbourhood et al. 2003). This tilt is probablythe result of squeezing of the
LB by the surroundingstructures belongingto the Gould Belt,
Inthissectionweconsiderthreeimportantstructures:theLocal which form a local chimney perpendicular to the plane of the
Bubble(itsspatialstructureispresented);theSco-Cen(-Lupus- Belt.(Sfeiretal.1999;Welshetal.1999).
Crux) complex (spatial structure, kinematics and age are pre- SomeLBformationmodelshavebeenproposed(seeforex-
sented); and the recently discovered YLA (for which we have ample Cox 1998), most of which involve several recent super-
produceda complete compendiumof their membersand mean novae(SNe).TheSco-Cencomplexisthemostlikelycandidate
properties). tohaveshelteredtheseSNe(Ma´ız-Apella´niz2001;Fuchsetal.
2006).
2.1.TheLocalBubble
2.2.TheSco-Cencomplex
Locally,withinthenearest100pc,theISMisdominatedbythe
LB. The displacement model (Snowden et al. 1990, 1998) for The Sco-Cen complex dominates the fourth Galactic quadrant
thisstructureassumesthatthe irregularlocalHIcavityis filled (deZeeuwetal.1999;hereafterZ99).Itisaregionofrecentstar
by an X-ray-emittingplasma, with an emission temperature of formation and contains an importantfraction of the most mas-
∼106Kandadensity,n ,of∼0.005cm−3.Usingthedistanceto sivestarsinthesolarneighbourhood.Inthe1960sthecomplex
e
theMBM12molecularcloudasadistancescale,Snowdenetal. wassplitintothreecomponents(Blaauw1960):UpperScorpius
(1998)derivedanextensionfortheLBof40to130pc,itbeing (US),UpperCentaurusLupus(UCL)andLowerCentaurusCrux
largerathigherGalacticlatitudesandsmallernearertheequator. (LCC).TheISMrelatedtothiscomplexwasstudiedbydeGeus
Nevertheless,thereisnoagreementintheliteratureconcerning (1992), who found that there is not much interstellar material
thedistancetoMBM12;valuesrangefrom60to300pc(seefor associated with UCL and LCC, whereasfilamentousstructures
exampleHeartyetal. 2000a, 2000b;Luhman2001;Andersson connect US with the cloud complex in Ophiuchus. The most
et al. 2002). This could explain why the extension of the LB widely accepted ages for the componentsof the complex were
calculated from SXRB observations does not agree (by a few derived by de Geus et al. (1989) from isochrones in the HR
tens of parsecs) with the extension of the local cavity derived diagram: ∼5-6 Myr for US, ∼14-15 Myr for UCL and ∼11-12
fromHIobservations(Paresce1984;Welshetal.1994). MyrforLCC.Blaauw(1964,1991)suggestedthatthisagepro-
Sfeir at al. (1999) and Lallementet al. (2003) obtained the gressionwastheresultofasequenceofstarformationeventsin
contoursoftheLBfromNaIabsorptionmeasurementstakento- theGMCthatformedalloftheSco-Cenregion.Theseclassical
wardsaselectedsetofstellartargetswithHipparcosparallaxes ages have recently been questionedby studies of the low-mass
of up to 350 pc from the Sun. These observations allowed the componentofthecomplex.Theagesobtainedfromsuchstudies
authorstodrawmapsoftheneutralgasdistributioninthelocal are in the range of 8-10 Myr for US and 16-20 Myr for UCL
ISMand,inparticular,totracethecontoursandextensionofthe and LCC (Sartori et al. 2001; Mamajek et al. 2002; Sartori et
LBwithanestimatedprecisionof≈ ±20pcinmostdirections. al. 2003, hereafterS03). However,Preibisch et al. (2002) once
Lallementetal.(2003)concludedthatforheliocentricdistances againobtainedanageof∼5MyrforUSfromtheHRdiagramof
D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex 3
thestarsbelongingtothisassociationwith massesbetween0.1 tainingallthedata(inASCIIformat)areavailableonawebpage
and20M⊙. wehavepublished1.
Differentnomenclatureisusedbytheauthorstodefineeach
Z99,searchingintheHipparcoscatalogue,found120mem-
YLA.ThenatureoftheηChaclusterisclear:thestarsarecom-
bers of US, 221 of UCL and 180 of LCC, with mean trigono-
pactedinaregionaveryfewpcindiameter.Fortheothergroups,
metric distances (corrected for systematic effects) of 145 ± 2,
authorsusedthetermsassociation,movinggrouporevensystem
140±2and118±2pc,respectively(seeTable1).Withregard
(forHD141569,withonly5knownmembers).Inthispaperwe
tokinematics,allthreeassociationshavelargevelocitycompo-
have respected the denominationfor each YLA used in the lit-
nentsinthedirectionawayfromtheSun,classicallyassociated
erature.However,the differencebetween the termsassociation
with the expansion motion of the Gould Belt (see for example
and moving group applied to the different entities is not clear,
Po¨ppel1997,butalsoseeTorraetal.2000).Z99providemean
since their spatial dispersions and kinematics are very similar
propermotionsfromthe Hipparcoscatalogueandradialveloc-
(andifslightdifferencesareapparent,theydonotcorrespondto
itiesfromthe HipparcosInputcatalogueforthe starsidentified
theclassificationsassociationormovinggroup;seeTable1).
as membersof US, UCL and LCC. Madsen et al. (2002; here-
Except for the case of AB Dor, all YLA studied here are
afterM02)obtainastrometricradialvelocitiesforthemembers
youngerthan∼30Myr(evenconsideringthelargeuncertainties
oftheSco-CenassociationsidentifiedbyZ99,assumingthatall
in the age estimations published in the literature; see Table 2).
memberssharethesamevelocityvector.S03compileradialve-
The Tuc-Hor/GAYA association has an estimated age of about
locitiesfortheSco-Cenmembersandderivefromthemthemean
20Myr,whereastheotherYLA(TWHya,βPic-Cap,ǫ Cha,η
spatialmotion,usingHipparcosparallaxesandpropermotions.
Cha,HD141569andExt.RCrA)areyoungerthan∼15Myrand
The mean spatial motions and standard deviations obtained by
of similar ages to (or younger than) the Sco-Cen associations.
these authorsare shown in Table 1. We can see thatthe results
(TheABDormovinggroupisclearlyolderandwillnotbeused
arequitesimilar,exceptforthecaseofUS,wherethereisaclear
in our analysis; see, however, Lo´pez-Santiago et al. 2006 and
discrepancyintheU value.AsstatedbyM02,USisclosetothe
Makarov2007.)
limit of their method(larger distance, smaller angular size and
Atpresent,allYLAmembersinourcompendiumarespread
smaller numberof memberstars, in comparisonwith LCC and
overaregionofabout120x130x140pc,mainlyconcentrated
UCL);thus,significantbiasesmaybeintroducedintothesolu-
in the fourth Galactic quadrant,and mostly in the southern ce-
tion they obtained for this association. Therefore, hereafter in
lestial hemisphere (see Fig. 2). The spatial distributionswithin
thispaperthe Sco-CenkinematicsfromS03 havebeenused to
eachassociationreachafewtensofpcintheGalacticplane,and
compute the back-tracingmotion of the three associations. We
lessthan20pcintheperpendiculardirection(seeTable1).
haveonlyusedMadsen’sdatainTable5.
Theheliocentricvelocitycomponentsofalltheassociations
are very similar, (U,V,W) ∼ [−(9-12), −(16-21), −(3-10)] km
s−1 (exceptfortheextendedRCrAassociationand,maybe,the
2.3.Younglocalassociations
HD 141569 system). To obtain these mean values we used all
the stars for which complete kinematic data are available. We
A decade ago, very few PMS stars had been identified less
checkedthatrejectingthosestarswiththelargerresidualsdoes
than 100 pc from the Sun. Nearly all the youngest stars (<∼30 notmodifythemeanvaluesbymorethan1.0kms−1,thoughthe
Myr) studied then were located more than 140 pc away in the
standarddeviationsareclearlyreduced(inmostcaseslessthan
molecular clouds of Taurus, Chamaeleon, Lupus, Sco-Cen and
2.0kms−1forallthecomponents).Theonlyexceptionistheex-
RCrA(allofthemregionsofrecentstarformation).Thecross-
tendedRCrAassociation.Itcontainsonly5starswithcomplete
correlation of the Hipparcos and ROSAT catalogues suddenly
kinematic data and there are large uncertainties in their radial
changed this; a few stars were identified as very young (from
velocities.Thisresultsinlargestandarddeviationsintheveloc-
their X-ray emission and lithium content) but closer than 100
itycomponents,especiallyinU,andpreventsusfromusingthis
pc, where there are no molecularclouds with SFR (see for ex-
associationinouranalysis.
ample Neuha¨user & Brandner 1998). Two explanationsfor the
TheresultingYLAvelocitycomponentsareverysimilar to
existence of these young stars far away from SFR were pro-
those of the associations in the Sco-Cen complex (see Table
posed. Sterzik & Durisen (1995) suggested that the stars were
1). Takingintoaccountthe similar kinematicsobservedamong
formed in molecular clouds and later ejected as high-velocity
YLA, and the observation that most of them are nearly coeval
starsduringthedecayofyoungmultiplestarsystems.Feigelson
(agesof∼5-15Myr),itisinterestingtoconsiderwhetherwecan
(1996)suggestedthatthestarswereformedinsidesmallmolec-
reallyspeakofdistinctentities.Figure3showsthevelocitydis-
ularclouds(orcloudlets),whichlaterdispersedamongtheISM
tribution of YLA member stars for which complete kinematic
andthereforecannolongerbedetected.
dataareavailable.Usingakernelestimation,severaloftheasso-
These youngnearbystars were groupedinto clusters, asso- ciationsare clearly distinguishedkinematically:we can clearly
ciations and moving groups, each with a few dozen members. identify peaks corresponding to Tuc-Hor/GAYA, β Pic-Cap, ǫ
DifferentapproacheswereusedineachYLAdiscovery,butmost ChaandηCha.TheHD141569systemisnotidentifiedinFig.
of them made use of Hipparcos proper motions, X-ray emis- 3 due to the small number of members whose complete kine-
sion (as a youth indicator), infrared emission (youth indicator) maticdataareavailable(only2stars).
and ground-based spectroscopy (Hα lines: youth indicator, Li Initially,it is surprising that the TW Hya association is not
lines: age estimation, radial velocities) and photometry (ages). evident in Fig. 3, and even a local minimum in density can be
We have compiledall the published YLA data in AppendixA. observednearitsexpectedpositions([U,V,W]∼[−10,−17,−5]
Table 1 shows the mean spatial and kinematic properties, ages km s−1; see Table 1). This may be due to a combination of
andnumberofmembersforeachYLA.Theadoptedageshown three factors: its position in the (UVW) planes very near other
inTable1isthatassignedforback-tracingtheassociationorbits YLA, its relatively high total velocity dispersion σ (computed
inthenextsection(seeTable2foranexhaustivelistofestimated
valuesfoundintheliterature).Moreinformationandtablescon- 1 http://www.am.ub.es/∼dfernand/YLA
4 D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex
Table1.MeanspatialcoordinatesandheliocentricvelocitycomponentsoftheyounglocalassociationsandtheSco-Cencomplex
(inthelattercase,datafromZ99,M02andS03).Inbrackets,thestandarddeviationofthesamplingdistribution.N isthenumber
ofknownmembersineachassociation(N withdistancedeterminationandN withcompletekinematicdata).
r k
Association ξ′ η′ ζ′ r U V W Age N N N
r k
(pc) (pc) (pc) (pc) (kms−1) (kms−1) (kms−1) (Myr)
TWHya −21 −53 21 63 −9.7 −17.1 −4.8 8 39 19 17
(22) (23) ( 7) (30) (4.1) (3.1) (3.7)
Tuc-Hor/GAYA −12 −24 −34 49 −10.1 −20.7 −2.5 20 52 50 44
(22) (11) ( 8) ( 8) (2.4) (2.3) (3.8)
βPic-Cap −9 −5 −15 35 −10.8 −15.9 −9.8 12 33 24 24
(27) (14) (10) (11) (3.4) (1.2) (2.5)
ǫCha −47 −80 −25 96 −8.6 −18.6 −9.3 10 16 6 5
( 8) (14) ( 5) (17) (3.6) (0.8) (1.7)
ηCha −33 −80 −34 93 −12.2 −18.1 −10.1 10 18 3 2
( 2) ( 5) ( 2) ( 6) (0.0) (0.9) (0.5)
HD141569 −77 10 64 101 −5.4 −15.6 −4.4 5 5 3 2
( 3) ( 8) ( 8) ( 8) (1.5) (2.6) (0.8)
Ext.RCrA −97 −1 −30 102 −0.1 −14.8 −10.1 13 59 7 4
(44) ( 4) (16) (47) (6.4) (1.4) (3.3)
ABDor 6 4 −12 32 −7.4 −27.4 −12.9 30-150 40 36 35
(21) (17) (16) (13) (3.2) (3.2) (6.4)
US Z99 −141 −22 50 145 5-6 120
(34) (11) (16) ( 2)
M021 −138 −22 49 149 −0.9 −16.9 −5.2 120
(27) (10) (12) (28)
S03 −6.7 −16.0 −8.0 8-10 155
(5.9) (3.5) (2.7)
UCL Z99 −122 −69 32 140 14-15 221
(30) (26) (16) ( 2)
M021 −121 −68 32 145 −7.9 −19.0 −5.7 218
(26) (21) (15) (24)
S03 −6.8 −19.3 −5.7 16-20 262
(4.6) (4.7) (2.5)
LCC Z99 −62 −102 14 118 11-12 180
(18) (24) (16) ( 2)
M021 −61 −100 14 120 −11.8 −15.0 −6.7 179
(14) (15) (15) (18)
S03 −8.2 −18.6 −6.4 16-20 192
(5.1) (7.3) (2.6)
1M02derivedaninternalvelocitydispersionamongindividualstarsof1.33kms−1forUS,1.23kms−1forUCLand1.13kms−1forLCC.
Fig.2. Locations of the young local associations projected onto the Galactic (left) and meridian (right) planes. The size of the
ellipsesrepresentstheprojecteddimensionoftheassociations(morethan90%ofthestarsinsidetheplottedarea).ξ′ ispointingto
theGalacticanti-centre,η′isthedirectionofGalacticrotationandζ′ ispointingtothenorthGalacticpole.
as σ = σ2 +σ2 +σ2 and comparedwith its neighboursin inTable1,(U,V,W)forthesetwoassociations.However,inthe
q U V W case of TW Hya, no central peak is found at the expected po-
these planes), and the relatively low number of TW Hya stars
sitionontheplanesforthemeanvalues.InFig.5weshowthe
withcompletekinematicdata(N inTable1)comparedtoTuc-
k positions in the (U,V) plane for the stars belongingto the TW
Hor/GAYAand β Pic-Cap associations(resultingin a low con-
Hya association,and we comparethem with the meanvelocity
tributiontotheprobabilitydensityfunctionshowninFig.3).
components for the YLA. Whereas the other YLA have their
To study thiscase in more detail, Fig. 4 showsthe velocity own place in this plane (with low superposition of their error
distribution[(U,V)and(U,W)planes]ofthestarsbelongingto bars), the TW Hya association seems to fill a regionshared by
the three associations with the most complete kinematic data: other YLA. We have computed the distance in the (UVW) ve-
TWHya,Tuc-Hor/GAYAandβPic-Cap.Atfirstsightitisclear locityspacefromeachTWHyastartothepositionofthemean
thatthetwoentitiesTuc-Hor/GAYAandβPic-Caparemuchbet- velocity of the YLA. Only 4 out of 17 stars are nearer to the
terdefinedintheseplanesthantheTWHyaassociationis.The position of the TW Hya association in the (UVW) space than
mainkernelestimatorpeaksagreewiththemeanvaluesshown to other YLA. In Table 3 we have done the exercise of recom-
D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex 5
Table2.EstimatedagesfortheYLApublishedintheliterature.
Association Estimatedage Method Reference
TWHya 10+10Myr Spectroscopy+HRdiagram(BVI) Soderblometal.(1998)
−5
∼8Myr HRdiagram(JHK) Webbetal.(1999)
5-15Myr HDdiagram(IR) Weintraubetal.(2000)
8.3Myr Expansionage Makarov&Fabricius(2001)
4.7±0.6Myr Expansionage Makarovetal.(2005)
20+25Myr Expansionage Mamajek(2005)
−7
8.3±0.8Myr Dynamicalage delaRezaetal.(2006)
10+10Myr Spec.+HRdiagram(VIJK)+Li+Hα BarradoyNavascue´s(2006)
−7
Tuc-Hor/GAYA ∼40Myr Hαemission Zuckerman&Webb(2000)
10-30Myr X-rayemission Stelzer&Neu¨hauser(2000)
∼30Myr Kinematic+HRdiagram(BV) Torresetal.(2000)
10-40Myr HRdiagram(VRI) Zuckermanetal.(2001b)
∼20Myr Kinematicage Torresetal.(2001)
27Myr Dynamicalage Makarov(2007)
βPic-Cap 20±10Myr HRdiagram(BVI) BarradoyNavascue´s(1999)
12+8Myr HRdiagram(VI)+Li Zuckermanetal.(2001a)
−4
11.2±0.3Myr Dynamicalage Ortegaetal.(2002)
12Myr Dynamicalage Songetal.(2003)
22±12Myr Dynamicalage Makarov(2007)
ǫCha 5-15Myr HRdiagram(BV) Terranegraetal.(1999)
3-5Myr HRdiagram(VI) Feigelsonetal.(2003)
7Myr Dynamicalage Jilinskietal.(2005)
ηCha 2-18Myr HRdiagram(R)+Li Mamajeketal.(1999)
10-15Myr Dynamicalage Mamajeketal.(2000)
5±4Myr HRdiagram(VRI) Lawsonetal.(2001)
6+2Myr HRdiagram(JHK) Luhman&Steeghs(2004)
−1
7Myr Dynamicalage Jilinskietal.(2005)
HD141569 5±3Myr X-ray+Li+HRdiagram(JHK) Weinbergeretal.(2000)
4.7±0.3Myr Spectroscopy+Kurucz’s(1993)models Mer´ınetal.(2004)
Ext.RCrA 10-15Myr HRdiagram(BVRI-JHK) Neuhau¨seretal.(2000)
∼15Myr Dynamicalage Quastetal.(2001)
ABDor ∼50Myr Hαemission Zuckermanetal.(2004)
75-150Myr HRdiagram(VK) Luhmanetal.(2005)
30-50Myrand80-120Myr(2subgroups) HRdiagram(VI)+Li Lo´pez-Santiagoetal.(2006)
38Myr Dynamicalage Makarov(2007)
118±20Myr Dynamicalage Ortegaetal.(2007)
Fig.3. Distribution in the (U-V) [left], (U-W) [middle] and (V-W) [right] planes of heliocentric velocity components for YLA
memberstarswhosecompletekinematicdataareavailable.Akernelestimatorwasusedtoindicatetheisocontours.
puting the mean velocity componentsand their dispersions for at all, and one may even wonder if its member stars could be
the YLA after addingthe TW Hya stars to the nearest YLA in redistributedamongtheotherYLA.However,therearetwoim-
the (UVW) space. We can see that the change in the mean ve- portant facts that do not support this hypothesis. The first one
locityvectorsoftheassociationsismuchsmallerthantheircor- concernsthe spatial distributionof the YLA atpresent. As can
responding standard deviation. A controversial hypothesis one beseeninFig.2,TWHyaisclearlyisolatedinthe(X,Z)plane
couldproposeisthattheTWHyaassociation(thefirstonedis- (notconsideringthemucholdermembersoftheABDormoving
covered among the YLA) could not actually be an association group),withaspatialextentinagreementwiththatobservedin
6 D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex
−5 3. Stellarorbits
TW Hya
Tuc−Hor/GAYA The integration back in time of the Sco-Cen and YLA orbits
β Pic−Cap
ε Cha allowsustostudytheiroriginandpossibleinfluenceontheim-
HD 141569 mediateISMoverthelastmillionyears.Tocomputethestellar
−10 (cid:127)TW Hya individual stars
orbitsbackintimeweusedthecodedevelopedbyAsiainetal.
(1999b)basedontheintegrationoftheequationsofmotionus-
ingarealisticmodeloftheGalacticgravitationalpotential.
−1) If we consider a coordinatesystem (ξ,η,ζ)2 centred on the
m s −15 SunandrotatingaroundGalacticcentrewithaconstantangular
V (k velocityΩ⊙,theequationsofmotionofastarare:
(cid:127)
∂Φ
ξ¨ = − −Ω2⊙(R⊙−ξ)−2Ω⊙η˙
∂ξ
−20
∂Φ
η¨ = − +Ω2⊙η+2Ω⊙ξ˙
∂η
∂Φ
ζ¨ = − (1)
−25 ∂ζ
−20 −15 −10 −5 0
(cid:127)U(cid:8) (km s −1) where Φ = Φ(R,θ,z;t) is the gravitational potential of the
Galaxyingalactocentriccylindricalcoordinates.
Fig.5.MeanhelicocentricvelocitycomponentsoftheYLA(in- WhenΦisaknownfunction,theseequationscanbesolved
dicatingtheir1σerrorbars;standarddeviation)andpositionin
numericallythrougha fourth-orderRunge-Kuttaintegrator.We
the (U,V) plane of those stars (with complete kinematic data) havedecomposedΦintothreecomponents:thegeneralaxisym-
belongingtotheTWHyaassociation. metricpotentialΦ (Allen&Santilla´n1991),thepotentialdue
AS
tothespiralstructureoftheGalaxyΦ ,andthatduetothecen-
Sp
tralbarΦ (seeTable4).Inthiswaywegetarealisticestimate
B
Table3.Meanheliocentricvelocitycomponents(andtheirstan-
oftheGalacticgravitationalpotential.
darddeviations)oftheyounglocalassociationsbeforeandafter
ThespiralstructurepotentialistakenfromLin’stheory(Lin
addingthosestarsbelongingtoTWHya(seetext).
&Shu1964;Linetal.1969):
Association U V W Nk ΦSp(R,θ;t)=Acos m(Ωpt−θ)+ψ(R) (2)
(kms−1) (kms−1) (kms−1) h i
Tuc-Hor/GAYA −10.1(2.4) −20.7(2.3) −2.5(3.8) 44 where:
addingstars −10.2 −20.5 −2.5 47
(2.6) (2.3) (3.7)
βPic-Cap −10.8(3.4) −15.9(1.2) −9.8(2.5) 24 A = (R⊙Ω⊙)2fr0tani
addingstars −11.2(3.3) −16.3(1.4) −9.2(1.6) 28 m
ǫCha −8.6 −18.6 −9.3 5
(3.6) (0.8) (1.7) m R
addingstars −9.5(3.0) −19.2(1.4) −8.9(1.9) 9 ψ(R) = − ln +ψ⊙ (3)
HD141569 −5.4 −15.6 −4.4 2 tani R⊙!
(1.5) (2.6) (0.8)
addingstars −5.2 −14.3 −1.9 8
(1.9) (2.5) (1.9) where A is the amplitude of the spiral potential, f the ratio
r0
betweentheradialcomponentoftheforceduetothespiralarms
andthatduetothegeneralGalacticfield,Ω theconstantangular
p
velocityofthespiralpattern,mthenumberofspiralarms,ithe
pitchangleofthearms,ψtheradialphaseofthespiralwaveand
ψ⊙itsvalueatthepositionoftheSun.Wehavetakenthevalues
theotherYLA.Thepreviouslydiscussedkinematicreassigment
obtainedbyFerna´ndezetal.(2001;seealsoFerna´ndez2005)for
of stars would enlarge, clearly in excess, the present accepted
amodelwith2spiralarms.
sizefortheotherassociations.Thesecondimportantfactisthat,
In the case of the potential due to the central bar, we have
whereasalmostallTuc-Hor/GAYAandβPic-Capmembershave
chosen the triaxialellipsoid modelof Palous˘ et al. (1993) with
Hipparcosparallaxes,70%of the TW Hyamembershave only
a rotation speed Ω = 70 km s−1 kpc−1 (Binney et al. 1991).
an estimated distance from broadbandphotometry(Song et al. B
Althoughlargeuncertaintiesarestillpresentinthebarparame-
2003).Theassumptionofarelativeerrorofabout20%forthis
ters, thisstructurehasan almostnegligibleeffectonourstellar
parameterwouldhaveacontributionofupto3-4kms−1 inthe
trajectories(asfarbackas30Myr;seeAsiain1998).
velocitydispersion components.Thisvalue, convolvedwith an
Allthe figuresin the nextsectionuse the (ξ′,η′,ζ′) coordi-
intrisic velocitydispersionof about1 kms−1 (Mamajek2005),
nate system centred on the Sun’s current position and rotating
wouldjustifythevaluesshowninTable1.Furthermore,wehave
aroundGalacticcentreataconstantvelocityΩ⊙ (seeFig.6).ξ′
confirmedthatthediscrepanciesbetweenthesephotometricdis-
pointstotheGalacticanti-centreandisdefinedasξ′ = R−R⊙.
tancesandthemovingclusterdistancesbyMamajek(2005)are
η′isalinearcoordinate,measuredalongthecircleofradiusR⊙,
about20%.Fromtheaboveargumentsitisclearthatmoreaccu-
which is positive in the direction of Galactic rotation. The co-
rateastrometricdataismandatorytoclarifyboththekinematic
ordinate system (ξ′,η′,ζ′) is convenient for our purposes as it
dispersion and membership assignment of TW Hya members.
minimisesvariationinthevariables.
Thelargedispersionobservedintheestimatedagesforthisas-
sociation (see Table 2) also contributesto the deficient charac- 2 ξpointstowardsGalacticcentre,ηinthedirectionofGalacticrota-
terisationofthisentity. tionandζtowardstheGalacticnorthpole.
D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex 7
Fig.4.Distributioninthe(U,V)[toprow]and(U,W)[bottomrow]planesofheliocentricvelocitycomponentsforthosestarswith
completekinematicdatabelongingtotheTWHyaassociation(leftcolumn),theTuc-Hor/GAYAassociation(middlecolumn)and
theβPic-Capmovinggroup(rightcolumn).
Table4.PositionandvelocityofLSR,andlocaldensity(Kerr& ξ’
Lynden-Bell1986),bulge,diskandhaloparameters(Miyamoto
η
& Nagai 1975; Allen & Santilla´n 1991), together with spiral *star
structure (Ferna´ndezet al. 2001) and central bar (Binney et al. ξ (< 0)
1991; Palous˘ et al. 1993) parameters used in the integrationof ξ’
stellarorbits. R η’
o
η’
R⊙ 8.5kpc
Θ⊙ 220kms−1
ρ⊙ 0.15M⊙ pc−3
MB 1.4·1010M⊙ Ωo
MD 8.6·1010M⊙
MH 1.1·1011M⊙
a 0.39kpc
B
aD 5.3kpc Fig.6. Heliocentric coordinates (ξ,η,ζ) and (ξ′,η′,ζ′) (from
bD 0.25kpc Asiain1998;seealsoAsiainetal.1999b).
a 12.0kpc
H
m 2
i −6◦
ψ⊙ 330◦ Wehaveintegratedtheorbitsoftheassociationsasawhole,
f 0.05 usingthemeanpositionandvelocityforeachassociationshown
r
Ω 30kms−1kpc−1 inTable1.TheresultsarepresentedinFigs.7and8,wherenot
p
aB/bB 2.381 onlydoweconsidertheorbitsintheGalacticplane(asisusual
aB/cB 3.030 intheliterature),butalsointhemeridianandrotationplanes.In
q 5kpc Fig.7weshowtheestimatederrorinthepositionoftheassoci-
B
t 5·108years ationsatbirth(greyarea).Theseerrorareashavebeenobtained
o
Ωθo 4750◦kms−1kpc−1 computingthe orbitsback in time, usingthe valuesU ±2SeU,
B V ±2Se aspresentvelocitycomponents(whereSe andSe
V U V
are the standard errors in the U,V mean velocity components
8 D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex
0000
−−−−55550000
−−−−111100000000
c)
p
ξ’ (
−−−−111155550000
TW Hya
Tuc−Hor/GAYA
β Pic−Cap
ε Cha
−−−−222200000000 η Cha
HD 141569
US
UCL
LCC
−−−−222255550000
−−−−111155550000 −−−−111100000000 −−−−55550000 0000 55550000 111100000000 111155550000 222200000000 222255550000
η’ (pc)
Fig.7. Positionsandorbitsin the Galactic plane (ξ′,η′) ofYLA andthe Sco-Cencomplexgoingback in time to their individual
ages.Theorbitsarealsoshown(smalldots)asfarbackas−20Myr.Thegreyareasshowtheexpectedpositionalerrorsatbirthdue
tokinematicobservationalerrors.Thecentreofthe(ξ′,η′)coordinatesystemiscomovingwiththeLSR.
obtainedfromthestandarddeviationsinTable1).Wehavecon- componentsof the associations do nothave a crucial influence
firmedthatuncertaintiesin theW componenthavea negligible onthepreviousresults:theerrorareashavetypicalsidelengths
effect on the orbits shown in Fig. 7 3. An error in age shall be ofabout10-30pc.
readasadisplacementoftheerrorareasalongtheplottedorbits InthecaseoftheSco-Cenassociations,bothUCLandLCC
in the figure. Unfortunately, age uncertainties are large for all beganlifeveryneartheGalacticplane;forbothofthem−19 <∼
theassociations(seeTable2).Laterinthispaperwestudyhow ζ′ <∼−12pcatbirth(theywerecloserthan∼4pctotheGalactic
theselargeuncertaintiesinagemayaffectourresults. plane).ForUS,ζ′ ∼40pcatbirth.
ThemostobvioustrendobservedinFigs.7and8isthespa-
tialconcentrationofalltheassociations(Sco-Cencomplexand
YLA) in the first Galactic quadrant in the past. All the YLA 4. Originandevolutionofthelocalstructures
(except the relatively old Tuc-Hor/GAYA) were in the region
of the Galactic plane bounded by −70 <∼ ξ′ <∼ −30 pc and Theresultsobtainedintheprevioussectionsuggestthatthefor-
35 <∼ η′ <∼ 110 pc at their time of birth. There is a very con- mation of YLA was triggered in a neighbouring region of the
spicuousspatialgroupingatthetimeofbirthofTWHya,ǫCha, first Galactic quadrantbetween 5 and 15 Myr ago. In this sec-
ηChaandHD141569(inasphere25pcinradius).Atitsbirth,β tionwepresentadescriptionofthismechanismandlinkittothe
Pic-Capwaslocatedabout50pcfromtheotherYLA.Alltheas- originandevolutionoftheSco-CencomplexandtheLB.
sociationswereconcentratedintheregion0<∼ζ′ <∼45pcattheir
birth.Thereforetheassociationswereformedslightlyabovethe
Galactic plane (the Sun, whose position is the origin of the ζ′ 4.1.OriginoftheSco-Cencomplex
coordinate,islocatedabout16pcabovetheGalacticplane;see
The gas associated with the Sco-Cen complex has classically
Binney&Merrifield1998).Theregionwheretheseassociations
beenconsideredasapartoftheLindbladRing;atoroidalstruc-
were formed had a size of 40 x 75 x 45 pc (ξ′ x η′ x ζ′). At
ture of HI and molecular clouds in expansion (Lindblad 1967)
present,thestarsbelongingtoYLAaredistributedinaregionof
belonging to the Gould Belt. Close to Sco-Cen, the clouds of
about120 x 130 x 140 pc. The volume they occupyhas there-
LupusandρOph,andtheso-calledAquilaRift,wouldalsoform
fore increased by a factor of ∼ 16 since their birth. As can be
partoftheLindbladRing.
observedinFig.7,theerrorsassociatedwiththemeanvelocity
Thestructurethat resultsfromconsideringall these molec-
ular complexes has a length of about 120◦, and extends from
3 Avariationof±2Se inWimpliesachangeintheverticalposition
W the Vela regionto the Aquila Rift. Below, we discuss the most
(ζ′) with time, thus slightly affecting the radial and azimuthal forces
plausiblescenariosforstarformationinthisregionandwecon-
actingonthestar(seeEq.9inAsiainetal.1999bandEqs.1,3and5
inAllen&Santilla´n1991).However,wehaveconfirmedthatthiseffect cludethatthemostlikelyisthecollisionofaGMCwithaspiral
resultsinachangeinpositionoflessthan10−3 pcinthe(ξ′,η′)plane arm.WementionthereviewpublishedbyS03,whereotherless
inthepresentcase. plausiblescenarioscanbefound.
D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex 9
ξ’ (pc) the successive sources of star formation would not necessarily
−320 −280 −240 −200 −160 −120 −80 −40 0 40 80 formgroupsofstarswithclearlyidentifiedradialvelocitycom-
120
ponents.Thus,contrarytotheopinionofS03,wethinkthatthe
TW Hya
Tuc−Hor/GAYA sequentialstar-formationmechanismcouldhaveacrucialrolein
80 β Pic−Cap
ε Cha theglobalscenarioforstarformationinthisregion.
η Cha
40 HUDS 141569
UCL
c) LCC 4.1.2. TheGouldBeltmodelbasedonanexpandinggasring
p 0
ζ’ ( The kinematicsof the stars belongingto the Gould Belt seems
−40 compatiblewiththeexpansionmodelproposedbyOlano(1982):
agasringwithacentrelocatedatadistanceofabout166pcin
−80 the direction l ∼ 131◦. However, the Sco-Cen complex is pre-
ciselyoneoftheregionsoftheGouldBeltthatdoesnotfitthis
−111222000
model,since the observedpropermotionsare notorientatedas
predictedbyOlano(seealsoMorenoetal.1999).Moreover,as
8800
shownabove,asweprojecttheorbitsofthestarsinthisregion
back in time, they no longer indicate the centre of the Gould
4400
Belt.S03alsomentionsotherproblems,suchasthefactthatthe
c)c) ageofthestarsinSco-Cenisanimportantfractionoftheageof
pp 00
ζζ’ (’ ( theGouldBeltpredictedbythismodel(30Myr),inspiteofthe
largedistance betweenthe presentpositionofSco-Cenandthe
−−4400
expansioncentreoftheBelt.
−−8800 Furthermore, as shown in a previous paper (Torra et al.
2000), the Sco-Cen associations are not needed to explain the
−−112200 peculiarkinematicsoftheGouldBelt.
−−116600 −−112200 −−8800 −−4400 00 4400 8800 112200 116600 220000 224400
ηη’’ ((ppcc))
Fig.8.PresentpositionsandorbitsintheGalacticplanes(ξ′,ζ′) 4.1.3. Starformationtriggeredbyaspiralarmshockwave
(top) and (η′,ζ′) (bottom) for YLA and the Sco-Cen complex.
Theregioninquestioncouldalsohavebeenformedasaresultof
Theerrorareas(notshown)areofsimilarsizeasinFig.7.(See
theinteractionoftheparentGMCwiththeshockwaveofaspiral
commentsatFig.7.)
arm.Inthiscase,themainproblemtobefacedisthattheclassi-
calviewofthespiralstructureoftheMilkyWayplacestheSun
inaninterarmregion,atabout1kpcfromthenearestarm(see,
4.1.1. Thesequentialstar-formationmodel
forexample,Georgelin&Georgelin1976)wherethereisnospi-
ral shock wave capable of triggering star formation. However,
This model was formulated by Blaauw (1964, 1991) and sug-
in recentyearstherehasbeensome evidencethatthisclassical
geststhatstarformationbeginsatoneendofaGMCandpropa-
viewcouldbewrong(see,forinstance,Ferna´ndezetal.2001for
gatestoadjacentregionsduetostellarwindsandsupernovae.
astellarkinematicsstudyorAmoˆres&Le´pine2005foramodel
Preibisch&Zinnecker(1999)laterproposedahistoryofstar
of interstellar extinction in the Galaxy), with the Sun closer to
formationintheSco-Cencomplex,usingtheclassicalagesofde
the inner (Sagittarius-Carina) arm. This forces us to study this
Geusetal.(1989).StarformationinSco-Cenwouldhavebegun
possiblescenarioinmoredetail.
around 15 Myr ago in UCL. 12 Myr ago, when star formation
started in LCC, the most massive star in UCL would have ex- Figure9showstheorbitsofYLAandtheOBassociationsin
plodedasasupernova,formingthelargestHIshellsurrounding Sco-Cenfortimes0>t>−30Myrinagalactocentricreference
theassociation.5Myrago,theshockfrontfromthissupernova frame(X′,Y′)whichisrotatingwithanangularvelocityΩ=Ωp
wouldhavepassedthroughthecloud,whichwastobecomethe (therotationvelocityofthespiralarmsoftheGalaxy;so,inthis
parentof US, triggering star formationthere. Shortly after, the systemthespiralarmsremainfixed).WeusedthevalueΩp =30
strong stellar winds from the most massive stars in US would km s−1 kpc−1 obtained by Ferna´ndez et al. (2001). The figure
begintosweepawaythemolecularcloud,stoppingthestarfor- shows that, back in time, the orbits tend to concentrate these
mationprocess.Only1.5Myrago,themostmassivestarinUS associations in a region with a phase of the spiral structure of
wouldhaveexplodedasasupernova,completelydispersingthe 0◦<∼ψ<∼10◦,verynearthespiralpotentialminimumatψ=0◦.
US cloud. Nowadays, the wave front of this supernova would Followingthenon-lineardensitywavetheory,themotionof
betravellingthroughtheρOphmolecularcloud,triggeringstar thespiralstructureintheGalacticplanegeneratesashockwave
formationthere. just before the arm potential minimum for regions outside the
FollowingS03,thismodelwouldresultinsystematicveloc- corotationradius(Roberts1969;Bertin&Lin1996).Inthecase
ity fields with radial components directed away from the dif- of our galaxy,the phase separationbetween the position of the
ferent centres of star formation. This is not what we observe shock wave and the potential minimum is not exactly known
intheresultsobtainedinprevioussections.However,theprob- (see, for example, Gittins & Clarke 2004). To quantify this, it
lem could be a little more complicated since the velocities of iscommontodefinetheoffsetfunctionΘ(R)=m(θ −θ ),
shock min
the youngstars would not only depend on the velocity and di- where m is the numberof spiralarms of the Galaxy,and θ
shock
rectionoftheshockfrontthatcompressedtheparentcloud;the and θ are the Galactic longitude of the shock wave and the
min
kinematics of the new stars would also depend on the original spiralpotentialminimumrespectively,atgalactocentricdistance
velocity of the molecular cloud, which could be similar to, or R. Roberts(1969) derivesa smallvalue forthis function,close
evenlargerthan,thevelocityoftheshockfront.Followingthis, to 0. However,Shu et al. (1972) obtain Θ(R) = −72◦, whereas
10 D.Ferna´ndezetal.:OnthekinematicevolutionofyounglocalassociationsandtheSco-Cencomplex
400
US
UCL
LCC
200
V
i
0
c)
p
X’ ( Vf
−200
−400 Vf
−600 V
−200 0 200 400 600 800 1000 1200 1400 i
Y’ (pc)
400
TW Hya
Tuc−Hor/GAYA
β Pic−Cap
ε Cha
η Cha
200 HD 141569
US
UCL
LCC Fig.10. Variation of the velocity vector of a test particle when
ithitstheshockwave(solidline)inthevicinityofthecentreof
0
c) a spiral arm (shadow) and in a region inside (top) and outside
p
X’ ( (bottom)thecorotationradius(inareferenceframeinwhichthe
−200 armsarefixed.)
−400
wave of a spiral arm hits a molecular cloud, it is compressed,
andstarformationcanbetriggered.
−600
−200 0 200 400 600 800 1000 1200 1400 ThisistheprocessproposedbyS03fortheformationofthe
Y’ (pc)
Sco-Cencomplex.TheyassumethattheSunisplacedinaregion
Fig.9. Orbits in the Galactic plane (X′-Y′) integrated back in slightly inside the corotationradius.The gasand stars that fol-
time to t = −30 Myr for YLA and the three associations of low the Galactic rotationalmotion are then moving faster than
theSco-Cencomplex(below,onlytheorbitsforSco-Cen).The the shock wave. In this framework, with respect to a reference
thick-dashedlineshowsthepositionoftheminimumofthespi- frame that is rotating at the same velocity as the spiral arms,
ralpotential(Ferna´ndezetal.2001).Thethin-dashedlineisthe the gas is slowed when it hits the shock wave, and afterwards
positionofthephaseofthespiralstructureψ=10◦. it moves practically parallel to the spiral arm, approachingthe
Galacticcentre(seeFig.10,top).However,withrespecttoLSR,
the compressedgasand the newlybornstars have velocitiesin
the direction opposite to the Galactic rotation and away from
Yuan&Grosbøl(1981)adoptΘ(R)=−30◦.Despitetherebeing Galacticcentre.S03arguesthat,asthisisthepresentmotionof
nogoodagreementonthevalueofthisparameter,itshouldhave the Sco-Cen stars, it favours the following star formation sce-
anabsolutevalueofnomorethanafewtensofdegrees. narioforSco-Cen:ashockwithaspiralarmpositionedslightly
Recent works have studied the formationof a GMC due to insidethecorotationradius.However,S03doesnotconsiderthe
thearrivalofthespiralshockwave,eithergatheringtogetherpre- variation in the orientation of the velocity vector of these stars
existing molecular gas or inducing high densities in the shock, fromtheirbirthtonow.
converting HI into HII (see Dobbs & Bonnell 2007 and refer- FromFig.7wecanseethat,infact,attheirbirththeSco-Cen
ences herein).Accordingto these authors,GMC formationoc- associationsweremovinginGalacticantirotationandawayfrom
curs within a very few Myr (<∼ 5 Myr) and star formation be- Galactic centre.This motionis notcompatiblewith an interac-
ginsveryquickly,∼5Myrafterthecloudformation(Clarketal. tion with a spiral arm placed inside the corotation radius, and
2005).Thefirstsupernovaeeventoccursabout∼4Myrafterthe thereforecontradictsthescenarioproposedbyS03.However,it
formationoftheveryhighmassstars,haltingthestarformation is perfectlycompatiblewith the expectedvelocityif the arm is
process.ThestarsoftheOBassociationcomplexarethenborn, outside the corotation radius (see Fig. 10, bottom), as recently
about∼10-15Myrafterthearrivalofthespiralshockwave. derivedinseveralworks(see,forexample,Mishurov&Zenina
When the shock wave hits the gas, its direction of motion 1999andFerna´ndezetal. 2001). Here,thedifferencein veloc-
changes.Assumingthattheshockisstrongandsufficientlydis- itybetweentheshockwaveandtheGalacticrotationisabout2
sipative,thevelocitycomponentperpendiculartothespiralarm km s−1 kpc−1, so the difference in the velocity in the direction
is stronglyreduced(see for exampleLandau & Lifshitz 1982). of Galactic rotation is about 12-13 km s−1. Following numer-
After interaction with the shock wave, the gas moves in a di- ical simulations of the collapse of nuclei in molecular clouds
rection practically tangential to the spiral arm (see Fig. 10). (Vanhala&Cameron1998),onecouldexpectaspiralarmshock
Considering Ωp = 30 km s−1 kpc−1, in the vicinity of the Sun wave to trigger star formation with relative velocities with re-
theshockwaveismovingaround4kms−1 kpc−1fasterthanthe specttothegasofaslowas10kms−1 (thoughthemechanism
localstandardofrest(LSR;equivalentto30-35kms−1,depend- ismoreefficientwhenthevelocitiesarebetween20and45km
ing on the value adoptedfor the distance to Galactic centre) in s−1).Thisscenariowouldthenbepossibleinourcase.Moreover,
the Galactic rotation direction. In this framework, if the shock therelativevelocitybetweentheparentmolecularcloudandthe