J/A+A/631/A166      Members of 5 cluster in Ori OB1a association    (Kos+, 2019)

Discovery of a 21 Myr old stellar population in the Orion complex. Kos J., Bland-Hawthorn J., Asplund M., Buder S., Lewis G.F., Lin J., Martell S.L., Ness M.K., Sharma S., De Silva G.M., Simpson J.D., Zucker D.B., Zwitter T., Cotar K., Spina L. <Astron. Astrophys. 631, A166 (2019)> =2019A&A...631A.166K 2019A&A...631A.166K (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Proper motions ; Parallaxes, trigonometric ; Radial velocities Keywords: surveys - parallaxes - proper motions - stars: early type - open clusters and associations: individual: Ori OB1a - Hertzsprung-Russell and C-M diagrams Abstract: The Orion complex is arguably the most studied star-forming region in the Galaxy. While stars are still being born in the Orion nebula, the oldest part was believed to be no more than 13Myr old. In order to study the full hierarchy of star formation across the Orion complex, we perform a clustering analysis of the Ori OB1a region using new stellar surveys and derive robust ages for each identified stellar aggregate. We use Gaia DR2 parameters supplemented with radial velocities from the GALAH and APOGEE surveys to perform clustering of the Ori OB1a association. Five overdensities are resolved in a six-dimensional parameter space (positions, distance, proper motions, and radial velocity). Most correspond to previously known structures (ASCC 16, 25 Orionis, ASCC 20, ASCC 21). We use Gaia DR2, Pan-STARRS1 and 2MASS photometry to fit isochrones to the colour-magnitude diagrams of the identified clusters. The ages of the clusters can thus be measured with 10% precision. While four of the clusters have ages between 11 and 13Myr, the ASCC 20 cluster stands out at an age of 21±3Myr. This is significantly greater than the age of any previously known component of the Orion complex. To some degree, all clusters overlap in at least one of the six phase-space dimensions. We argue that the formation history of the Orion complex, and its relation to the Gould belt, must be reconsidered. A significant challenge in reconstructing the history of the Ori OB1a association is to understand the impact of the newly discovered 21Myr old population on the younger parts of the complex, including their formation. Description: The Gaia DR2 parameters supplemented with radial velocities from the GALAH and APOGEE surveys are used to perform clustering of the Ori OB1a association. Table 1 contains the properties of 5 identified clusters clusters, including ages (derived using Gaia DR2, Pan-STARRS1 and 2MASS photometry to fit isochrones to their colour-magnitude diagrams). Tableb 1 - tableb 5 contain the identified members of the different clusters. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 72 5 Parameters used in the final step of the clustering algorithm tableb1.dat 87 644 Members of ASCC 16 tableb2.dat 87 254 Members of ASCC 21 tableb3.dat 87 264 Members of ASCC 21a tableb4.dat 87 218 Members of ASCC 20 tableb5.dat 87 141 Members of ASCC 18 -------------------------------------------------------------------------------- See also: I/345 : Gaia DR2 (Gaia Collaboration, 2018) Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 A8 --- Cluster Cluster name 10- 15 F6.3 deg RAdeg Right ascension (ICRS) at Ep=2015.5 17- 21 F5.3 deg DEdeg Declination (ICRS) at Ep=2015.5 23- 26 F4.2 deg r Radius enclosing 50% of the stars 28- 32 F5.2 mas/yr pmRA* Proper motion along RA, pmnRA*cosDE 34- 38 F5.2 mas/yr pmDE Proper motion along DE 40- 43 F4.2 mas Plx Absolute parallax 45- 49 F5.2 km/s RV Radial velocity 51- 54 F4.2 km/s sigma Velocity dispersion 56- 60 F5.2 Myr Age Age 62- 65 F4.2 Myr e_Age rms uncertainty on Age 67- 69 I3 --- Nstar Number of stars 71- 72 I2 --- NstarRV Number of stars with known radial velocities -------------------------------------------------------------------------------- Byte-by-byte Description of file: tableb?.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 19 I19 --- Gaia Gaia DR2 id (source_id) 21- 28 F8.5 deg RAdeg Right ascension (ICRS) at Ep=2015.5 30- 37 F8.5 deg DEdeg Declination (ICRS) at Ep=2015.5 39- 44 F6.3 mas/yr pmRA* Proper motion along RA, pmnRA*cosDE 46- 50 F5.3 mas/yr e_pmRA* rms uncertainty on pmRA* 52- 57 F6.3 mas/yr pmDE Proper motion along DE 59- 63 F5.3 mas/yr e_pmDE rms uncertainty on pmDE 65- 69 F5.3 mas Plx Absolute parallax 71- 75 F5.3 mas e_Plx rms uncertainty on Plx 77- 81 F5.2 km/s RV ? Radial velocity GALAH and APOGEE surveys 83- 87 F5.2 km/s e_RV ? rms uncertainty on RV -------------------------------------------------------------------------------- Acknowledgements: Janez Kos, janez.kos(at)fmf.uni-lj.si
(End) Patricia Vannier [CDS] 22-Oct-2019
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