J/MNRAS/495/663ivo://CDS.VizieR/J/MNRAS/495/663doi:10.26093/cds/vizier.74950663CDSWard J.L.Kruijssen J.M.D.Rix H.-W.2024-08-21T20:15:05Z2020-06-24T06:37:32ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.fropen-star-clustersob-starsHistorically, it has often been asserted that most stars form in compact clusters. In this scenario, present-day gravitationally-unbound OB associations are the result of the expansion of initially gravitationally-bound star clusters. However, this paradigm is inconsistent with recent results, both theoretical and observational, that instead favour a hierarchical picture of star formation in which stars are formed across a continuous distribution of gas densities and most OB associations never were bound clusters. Instead they are formed in-situ as the low-density side of this distribution, rather than as the remnants of expanding clusters. We utilise the second Gaia data release to quantify the degree to which OB associations are undergoing expansion and, therefore, whether OB associations are the product of expanding clusters, or whether they were born in-situ, as the large-scale, globally-unbound associations that we see today. We find that the observed kinematic properties of associations are consistent with highly substructured velocity fields and additionally require some degree of localised expansion from sub-clusters within the association. While most present-day OB associations do exhibit low levels of expansion, there is no significant correlation between radial velocity and radius. Therefore, the large-scale structure of associations is not set by the expansion of clusters, rather it is a relic of the molecular gas cloud from which the association was formed. This finding is inconsistent with a monolithic model of association formation and instead favours a hierarchical model, in which OB associations form in-situ, following the fractal structure of the gas from which they form.2020MNRAS.495..663Whttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/495/663CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toI/345 : Gaia DR2 (Gaia Collaboration, 2018)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/495/663https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/495/663http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/495/663https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/495/663https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/495/663http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/495/663GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/495/663/tablea1 (Association positions and sizes)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/495/663/tablea1?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/495/663/tablea1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/495/663/tablea1?GETtext/xml+votable180.050000true54.331711.07860.0055555555555555566/93 346 1425 1436 1454 2940 3057-3060 3187-3188 3256-3257 3273 3278 3292 3307 3589 3606 3632 3635 3656 12985 12990 13732 14251-14253 14291 14302 14308-14309 14352 14368 14638 14645-14646 14732 15379 15393 15476 15557 15665 15682 20803 21646 22245 22902 23636 24325 24336 28802-28803 28829 28891 28913 29081 29089 29095 29461 29463 29513 30655 37333 37593 37861 37869 37905 37926 38017 38024 38026 39195 39254 39755 39793 39862 41654 41663 41814 41816 42150 42154 42166 42169 42205 42217 42224 42348 42395 42416 42435 42437 42456 42459-42460 43030 43034 43042https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/495/663?format=asciidefaultJ/MNRAS/495/663/tablea1Association positions and sizesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintAssocAssociation ID number (G1)meta.id;meta.mainintNstar[40/177657] Number of starsmeta.numberintNOBNumber of OB starsmeta.numberintRAJ2000Mean right ascension of star association (J2000)degpos.eq.ra;meta.mainDEJ2000Mean declination of star association (J2000)degpos.eq.dec;meta.mainDistmean line-of-sight distance to starspcpos.distancefloats_Diststandard deviation of distancepcstat.stdev;pos.distancefloatstdXstandard deviation in plane-of-sky positionpcstat.stdev;pos.distance;posfloatstdYstandard deviation in plane-of-sky positionpcstat.stdev;pos.distance;posfloatstdPMXstandard deviation in plane-of-sky velocitykm/sstat.stdev;phys.velocfloatstdPMYstandard deviation in plane-of-sky velocitykm/sstat.stdev;phys.velocfloatSimbadSimbad column added by the CDSmeta.ref.urlcharJ/MNRAS/495/663/tablea2Derived kinematic propertiesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintAssocAssociation ID number (G1)meta.id;meta.mainintNratNumber ratio, Nv_R_/Nv_t_meta.numberfloatRVmedMedian radial velocitykm/sspect.dopplerVeloc;stat.medianfloate_RVmedrms uncertainty in VRmedkm/sstat.errorfloatRV/VtmedMedian (v_R_/|v_t_|) velocity ratiophys.veloc;arith.ratio;stat.mediandoublee_RV/Vtmedrms uncertainty in VT/Vtmedstat.errordoublebetaRadial-anisotropy parameterstat.fit.paramfloate_VT/VtOBsmedrms uncertainty in VT/VtOBsmedstat.errordoublebetaOBsRadial anisotropy using OB star centrestat.fit.paramfloate_betaOBsrms uncertainty in betaOBsstat.errorfloate_betarms uncertainty in betastat.errorfloatNratOBsNumber ratio using OB star centremeta.numberfloatVROBsmedMedian (v_R_) using OB star centrekm/sspect.dopplerVeloc;stat.medianfloate_VROBsmedrms uncertainty in VROBsmedkm/sstat.errorfloatVT/VtOBsmedMedian (v_R_/|v_t_|) using OB star centrephys.veloc;arith.ratio;stat.mediandoubleJ/MNRAS/495/663/allstarsGaia DR2 designations for all stars in sampleGaiaDR2Gaia DR2 designationmeta.id;meta.mainrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintAssoc[1/110] Association ID number (G1)meta.idint