J/MNRAS/546/G305ivo://CDS.VizieR/J/MNRAS/546/G305CDSKuhn M.A.Benjamin R.A.Singh S.S.2026-08-12T15:11:30Z2026-08-12T16:10:06ZCDS support team

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

cds-question@unistra.frstellar-associationspre-main-sequence-starsvisible-astronomyThe Serpens OB2 association (l~18.5deg, b~1.9deg, d=1950+/-30pc) is a large star-forming complex ~65 pc above the Galactic midplane, with a clumpy, elongated structure extending ~50 pc parallel to the plane. We analyse probable association members, including OB stars and low-to-intermediate-mass young stellar objects (YSOs) from the SPICY catalogue. We use ^13^CO MWISP data to trace the molecular clouds. The OB stars are concentrated toward the centre of the association, coincident with a gap in the molecular clouds, and toward the side nearest the Galactic plane. The YSOs are distributed throughout the association, but cluster around molecular-cloud clumps. Using Gaia DR3 proper motions to probe the association's internal kinematics, we find aligned stellar velocities on length scales <~2pc, two-point statistics that show increasing velocity differences and predominantly divergent motions at larger separations, and distinct velocities for star clusters within the association. Finally, the association exhibits gradual but statistically significant global expansion perpendicular to the Galactic plane, with a spatial gradient of 0.10+/-0.02km/s/pc^. The clumpy stellar distribution, correlated velocities on small scales, and increasingly divergent motions on larger scales are consistent with an initial velocity field inherited from a turbulent molecular cloud modified by stellar feedback. The global vertical expansion may arise from large-scale turbulence or feedback-driven shell expansion, with the HII region Sh 2-54 preferentially pushing the molecular gas away from the Galactic plane. Ser OB2 demonstrates that the multi-scale expansion of an OB association can begin even while star formation is still ongoing throughout the complex.2026MNRAS.546g.305Khttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/546/G305CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toI/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)J/ApJS/254/33 : Gal. midplane Spitzer/IRAC candidate YSOs (SPICY) (Kuhn+, 2021)https://irsa.ipac.caltech.edu/data/SPITZER/SPICY/overview.html :https://als.cab.inta-csic.es :https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/546/G305https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/546/G305http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/546/G305https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/546/G305https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/546/G305http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/546/G305GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/546/G305/table1 (Probable members of Serpens OB2)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/546/G305/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/546/G305/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/546/G305/table1?GETtext/xml+votable180.050000true273.189989-11.8690770.0055555555555555565/7378 6/29471 29506 29516 29518 29537https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/546/G305?format=asciiOpticaldefaultJ/MNRAS/546/G305/table1Probable members of Serpens OB2recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameSource designation (SPICY identifier, OB-star catalogue name, or other common designation)meta.id;meta.maincharRAICRSRight ascension (ICRS/J2000)degpos.eq.ra;meta.mainDEICRSDeclination (ICRS/J2000)degpos.eq.dec;meta.mainGaiaEDR3Gaia EDR3 source designation, Gaia EDR3 NNNNNNNNNNNNNNNNNNN (1)meta.idcharGroupSubcluster / subregion label (see paper)meta.idchar