J/MNRAS/475/5659ivo://CDS.VizieR/J/MNRAS/475/5659doi:10.26093/cds/vizier.74755659CDSWard J.L.Kruijssen J.M.D.2024-08-18T20:18:17Z2023-09-21T15:24:24ZCDS support team

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

cds-question@unistra.frstellar-associationsob-starsproper-motionsvisible-astronomytrigonometric-parallaxIt is often stated that star clusters are the fundamental units of star formation and that most (if not all) stars form in dense stellar clusters. In this monolithic formation scenario, low-density OB associations are formed from the expansion of gravitationally bound clusters following gas expulsion due to stellar feedback. N-body simulations of this process show that OB associations formed this way retain signs of expansion and elevated radial anisotropy over tens of Myr. However, recent theoretical and observational studies suggest that star formation is a hierarchical process, following the fractal nature of natal molecular clouds and allowing the formation of large-scale associations in situ. We distinguish between these two scenarios by characterizing the kinematics of OB associations using the Tycho-Gaia Astrometric Solution catalogue. To this end, we quantify four key kinematic diagnostics: the number ratio of stars with positive radial velocities to those with negative radial velocities, the median radial velocity, the median radial velocity normalized by the tangential velocity, and the radial anisotropy parameter. Each quantity presents a useful diagnostic of whether the association was more compact in the past. We compare these diagnostics to models representing random motion and the expanding products of monolithic cluster formation. None of these diagnostics show evidence of expansion, either from a single cluster or multiple clusters, and the observed kinematics are better represented by a random velocity distribution. This result favours the hierarchical star formation model in which a minority of stars forms in bound clusters and large-scale, hierarchically structured associations are formed in situ.2018MNRAS.475.5659Whttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/475/5659CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toI/259 : The Tycho-2 Catalogue (Hog+ 2000)I/337 : Gaia DR1 (Gaia Collaboration, 2016)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/475/5659https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/475/5659http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/475/5659https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/475/5659https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/475/5659http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/475/5659GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/475/5659/tableb1 (All OB association members)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/475/5659/tableb1?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/475/5659/tableb1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/475/5659/tableb1?GETtext/xml+votable180.050000true50.7829962559.42442833330.0055555555555555564/915 1818 5/434 750-751 762-763 766 784 793 798 820-821 823 866 872 3561 3603 3609 3657 3659 5359 5700-5701 5703 5712 5714-5715 5720 7170 7176-7177 7224 7229 7266 7277-7278 7280 7364-7365 7367 9795 9800-9801 9839 9848 9924 10685 10721 11029 11079-11080 11088-11089 11098 12031 6/1643 1646 1650-1651 1657 1662 1684 1691 1702-1703 1707 1713 1715-1716 1723 1725 1727 1730 1735 1740 1743 1746-1747 1749 1755 1759 1762-1764 1767-1768 1770-1772 1776 1781 1784 1787 1931 2389 2989 2991 2995 2998-2999 3041-3043 3046-3047 3058-3059 3062-3063 3068-3070 3081 3087-3088 3091-3093 3095 3100-3101 3107 3109-3111 3119-3120 3125 3127 3130 3140-3142 3144-3145 3147 3150 3160 3162 3169 3171 3181 3183-3184 3186-3187 3196 3198 3220-3221 3269 3289 3317 3458 3492 3752-3754 9839 9851 9913 9917 9930 9933 9951 9957 9960 9976-9977 10120 10708 11270 11273 11277 11279 11287 11293 11296 11301 11315 11317 11319 11328 11336 11360 11396 11409 11415 11520 14223 14234 14241-14243 14248-14249 14251-14254 14256 14258 14264 14411 14424 14426 14433 14435 14441 14448 14450 14603 14605-14607 14616 14618-14619 14622 14625 14627 14633 14635 14724-14725 14736-14738 14740-14741 21433-21435 21480 21482-21483 22078-22079 22164-22165 22167 22173 22208 22210-22211 22216-22217 22219-22220 22222-22223 22244-22245 22837 22852 22854 22874 22884-22885 22887 22889 22894 22896 28674-28675 28684 28686-28687 28713 28716-28717 28720 28783 28798-28799 28855 28865 28867 28870-28872 28874-28875 28877-28879 28884 28888 28893-28895 28900-28902 28906 28908-28909 28915 28920 28925-28927 28949 28957 28971 28977 28980-28981 28983 28987-28990 28994 28998 29005 29010-29011 29013 29015 29017-29019 29027 29030-29033 29035-29036 29039-29042 29045-29046 29050-29051 29053-29054 29057 29059 29061 29063 29071 29074 29077-29079 29081-29083 29086 29104 29106-29107 29116-29118 29124 29126-29127 29130-29134 29137-29139 29144 29152-29153 29155 29158 29160 29162 29166-29167 29194 29212 29219 29223 29227 29238 29248 29251-29252 29256 29260 29262 29267-29269 29272-29273 29276-29278 29300 29312 29316 29319-29321 29344 29351-29352 29358-29360 29384 29386 29408 29440-29442 29447-29448 29452 29506 29508-29509 29512-29513 29517 29519 29530 29699-29700 29707-29708 29728 29730 29736 30721 30723 30728 30737 30741 30745 30754-30756 30759 30761 30764 30769 30827 30852 30857 39031 39037 39174-39175 39177-39179 39184 39186-39187 39192-39194 39208-39209 39216 39349 39351 39354-39355 39363 39371 39374-39375 39396 39400-39401 39684-39685 39687 39700-39702 39704-39705 42327 42333 42335 42357 42716 42736 42739 42746-42747 42749 42782 42796 42801 42805 42807-42808 42810-42811 42813 42827 42830-42831 42848-42849 42852-42853 42856-42858 42860-42861 42863-42864 42866 42868-42869 42872-42873 42876 42879 42882-42883 42888-42889 42894 42897 42899-42900 42902-42904 42906-42909 42911-42913 42915 42917 42920 42923 42925-42928 42931-42932 42934-42935 42941 42943-42944 42951 42953-42955 42957 42959-42961 42965 42968-42970 42972 42975-42978 42980-42981 42985 42987 42989-42990 42994-42995 42997 43003 43007 44113-44114 44121-44124 44133 44146 44155-44156 44244-44245 44248 44254-44255 44277 44288 44292-44293 44296 44299-44301 44306-44308 44311-44313 44325-44326 44336-44338 44341 44343 44346-44347 44360-44361 44363-44366 44368-44369 44371-44372 44376 44378 44380 44382 44385-44386 44389 44397 44399-44400 44403 44407-44409 44412 44417 44423-44424 44432-44434 44440 44447 44452-44453 44480 44484 44489 44496-44497 44502-44503 44511 44523 44525 44537 44816 44866 44887 47999 48046 48063 48106-48107 48109-48110 48113 48116 48120 48122 48768 48771 48801-48802 48805 48807 48809-48810 48813-48815https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/475/5659?format=asciiOpticaldefaultJ/MNRAS/475/5659/tableb1All OB association membersSimbadNameSimbad column added by the CDSmeta.idcharnullableplxTGAS parallaxmaspos.parallax.trigdoublee_plxTGAS parallax errormasstat.error;pos.parallaxdoublepmRATGAS proper motion along RAmas/yrpos.pm;pos.eq.radoublee_pmRATGAS proper motion along RA errormas/yrstat.error;pos.pm;pos.eq.radoublepmDETGAS proper motion along DEmas/yrpos.pm;pos.eq.decdoublee_pmDETGAS proper motion along DE errormas/yrstat.error;pos.pm;pos.eq.decdoubleOB?[0/1] indicates whether the star is listed as spectral type O or B in the Simbad data basemeta.codeintrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintOBAssOB association to which the star has been assigned membershipmeta.id.parentcharHIP? HIP numbermeta.id;meta.mainintnullableTycho2Tycho-2 designationmeta.idcharRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.main