The dynamics of star formation in the local Milky Way as traced by OB associations

Quintana Isasi, Alexis Laurent Pierre-Henri

Abstract

OB associations are gravitationally unbound, low-density groups of young stars delineated by bright OB stars. At the intersection of star formation and Galactic structure, they are important across many areas of astronomy. However, most historical OB associations had their membership defined several decades ago. Recent data shows a lack of kinematic coherence amongst them, alongside many contaminants, which encourages us to revisit them.
I have developed a code that fits a model spectral energy distribution (SED), based on stellar atmosphere and evolutionary models and an extinction map, to an observed SED, based on photometry from several surveys and Gaia parallaxes. This process derives stellar parameters that are used to identify OB stars. I used this to identify OB stars across Cygnus, Auriga and Cassiopeia, identifying thousands of OB stars. I applied clustering algorithms to identify kinematically-coherent OB associations, which I characterized physically and kinematically.
In Cygnus, the OB associations form two groups exhibiting a large-scale expansion pattern. A kinematic traceback revealed that they reached their most compact state ∼8 Myr ago. This expansion could either be attributed to feedback or turbulence within their primordial molecular cloud. In Auriga, the OB associations are connected to the surrounding open clusters and star-forming regions. They follow an age gradient, up to ∼20 Myr ago, coinciding with the motion of the Perseus spiral arm. In Cassiopeia, the kinematics of OB stars suggest an interesting large-scale pattern similar to that seen in Cygnus, though further investigating suggests it may be due to Galactic rotation. Despite this, the OB stars in Cassiopeia can be traced back to a compact configuration ∼17 Myr ago.
This thesis highlights the importance of identifying OB associations with recent data and modern techniques. Not only are they tracers of early stellar evolution, helping to grasp the physical condition driving the expansion of stellar groups, but they can also be used to reconstruct the motion of the spiral arms in the recent past.

Citation

Quintana Isasi, A. L. P. (2023). The dynamics of star formation in the local Milky Way as traced by OB associations. (Thesis). Keele University

• Thesis Type: Thesis
• Deposit Date: Oct 12, 2023
• Publicly Available Date: Oct 12, 2023
• Award Date: 2023-10