Structure, kinematics, and ages of the young stellar populations in the Orion region

Abstract

We present a study of the three dimensional structure, kinematics, and age distribution of the Orion OB association, based on the second data release of the Gaia satellite (Gaia DR2). Our goal is to obtain a complete picture of the star formation history of the Orion complex and to relate our findings to theories of sequential and triggered star formation. We select the Orion population with simple photometric criteria, and we construct a three dimensional map in galactic Cartesian coordinates to study the physical arrangement of the stellar clusters in the Orion region. The map shows structures that extend for roughly 150 pc along the line of sight, divided in multiple sub-clusters. We separate different groups by using the density based clustering algorithm DBSCAN. We study the kinematic properties of all the groups found by DBSCAN first by inspecting their proper motion distribution, and then by applying a kinematic modelling code based on an iterative maximum likelihood approach, which we use to derive their mean velocity, velocity dispersion and isotropic expansion. By using an isochrone fitting procedure we provide ages and extinction values for all the groups. We confirm the presence of an old population (∼ 15 Myr) towards the 25 Ori region, and we find that groups with ages of 12 − 15 Myr are present also towards the Belt region. We notice the presence of a population of ∼ 10 Myr also in front of the Orion A molecular cloud. Our findings suggest that star formation in Orion does not follow a simple sequential scenario, but instead consists of multiple events, which caused kinematic and physical sub-structure. To fully explain the detailed sequence of events, specific simulations and further radial velocity data are needed.

Star Clusters Near and Far

“…Using OmegaCAM photometry they identified three populations with an age difference of 3 Myr between the oldest and the youngest sequence. These sequences cannot be described with binary or triplet systems alone leading to the conclusion that they are real, which is in agreement with the above findings by Zari et al (2019) and suggesting star formation happens sequentially possibly triggered by the luminous OB stars.…”

“…The ONC revealed the first direct imaging of protoplanetary disks (proplyds, McCaughrean and O'dell 1996;. A recent 3D kinematic study by Zari et al (2019) using Gaia DR2 data (Prusti et al 2016;Brown et al 2018) showed that the ONC is highly sub-structured, with stellar ensembles of different ages in several kinematic groups, mixed in 3D space, which are overlapping in projection. Jerabkova et al (2019) suggested that these YSCs may harbor multiple populations.…”

“…New instruments like large integral field units (IFUs), such as the Multi Object Spectrographic Explorer (MUSE, Bacon et al 2010) mounted at the VLT, the Gaia satellite (Prusti et al 2016;Brown et al 2018), and long baseline photometric observations allow us, for the first time, to study the detailed 3D dynamics of the majority of stars in these resolved star clusters, including the dynamics of the gas (e.g., Kamann et al 2013;McLeod et al 2015;Zeidler et al 2018;Lennon et al 2018;Wright and Mamajek 2018;Ward and Kruijssen 2018;Ward et al 2019;Getman et al 2019;Zari et al 2019). This provides insights into the star cluster formation modes: Do star clusters form hierarchically, following the structure of the giant molecular cloud (GMC) (e.g., Kruijssen et al 2012b;Parker et al 2014;Longmore et al 2014;Walker et al 2015Walker et al , 2016Barnes et al 2019;Ward et al 2019), or do they form in monolithic, central starburst-like events (e.g., Lada et al 1984;Bastian and Goodwin 2006;Banerjee and Kroupa 2015)?…”

Corona-Australis DANCe

“…Kraus et al (2017) and Zhang et al (2018) also report on a distributed population of young stars in the Taurus region, which is older (>10 Myr) than the classical members of the region (Luhman 2018). Another wellknown example is the Orion complex, which is made up of sev-eral groups and clusters of YSOs with different ages (see Alves & Bouy 2012;Bouy et al 2014;Kounkel et al 2018;Zari et al 2019;Chen et al 2019). Our analysis conducted in this paper shows that Corona-Australis is one more such substructured starforming region that will require further investigation to understand its star formation history.…”

The 800 pc long tidal tails of the Hyades star cluster

“…The advent of the ESA Gaia satellite for the first time allows the study of young stellar populations on large scales (>100 pc) in six-dimensional (6D) position and velocity space. For example, using Gaia Data Release 2 (DR2) (Gaia Collaboration 2018a), Kounkel et al (2018) and Zari et al (2019) demonstrated that the Orion star-forming region is composed of a variety of populations with different spatial and kinematic properties each that are all likely generated in multiple events instead of in a progressive star formation history (see also Beccari et al 2017;Jerabkova et al 2019b;Kroupa et al 2018, discussing three bursts of star formation in the Orion Nebula cluster). Zari et al (2018) described the 6D properties and age structures in which young stars are found within a region of 500 pc around the Sun, and a number of other studies focusing on individual star-forming regions have been reported.…”