Discovery of a 21 Myr old stellar population in the Orion complex. (arXiv:1811.11762v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kos_J/0/1/0/all/0/1">Janez Kos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bland_Hawthorn_J/0/1/0/all/0/1">Joss Bland-Hawthorn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Asplund_M/0/1/0/all/0/1">Martin Asplund</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buder_S/0/1/0/all/0/1">Sven Buder</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lewis_G/0/1/0/all/0/1">Geraint F. Lewis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lin_J/0/1/0/all/0/1">Jane Lin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Martell_S/0/1/0/all/0/1">Sarah L. Martell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ness_M/0/1/0/all/0/1">Melissa K. Ness</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sharma_S/0/1/0/all/0/1">Sanjib Sharma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Silva_G/0/1/0/all/0/1">Gayandhi M. De Silva</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Simpson_J/0/1/0/all/0/1">Jeffrey D. Simpson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zucker_D/0/1/0/all/0/1">Daniel B. Zucker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zwitter_T/0/1/0/all/0/1">Toma&#x17e; Zwitter</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cotar_K/0/1/0/all/0/1">Klemen &#x10c;otar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Spina_L/0/1/0/all/0/1">Lorenzo Spina</a>

Context: 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 13 Myr old. Aims: 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. Methods: 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% or better
precision. Results: While four of the clusters have ages between 11 and 13 Myr,
the ASCC 20 cluster stands out at an age of 21$pm$2 Myr. 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. Conclusions: 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 21 Myr old population on
the younger parts of the complex, including their formation.

Context: 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 13 Myr old. Aims: 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. Methods: 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% or better
precision. Results: While four of the clusters have ages between 11 and 13 Myr,
the ASCC 20 cluster stands out at an age of 21$pm$2 Myr. 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. Conclusions: 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 21 Myr old population on
the younger parts of the complex, including their formation.

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