Abstract

Contributed Talk - Splinter LowMet

Friday, 13 September 2024, 16:45   (S23)

The critical metallicity in the formation of magnetized molecular clouds driven by atomic colliding flows

Masato Kobayashi, Tsuyoshi Inoue
Astrophysik I.Physik, University of Cologne; Department of Physics, Konan University

The physics in molecular cloud formation is important to understand the star formation, because most of the stars form in molecular clouds across the Universe. In particular, gas-phase metallicity is a key parameter to control the heating and cooling efficiency in the interstellar medium (ISM) and the resultant thermal states of the ISM. It is known that the atomic phase of the ISM in the Solar neighborhood is bistable. Therefore, some dynamical mechanisms are required to compress the most abundant diffuse atomic gas (warm neutral medium: WNM) to overcome this pressure barrier and initiate the runaway cooling, so-called thermal instability, which eventually forms colder and denser atomic gas (cold neutral medium: CNM) and hydrogen molecular gas. Supersonic compressions due to the expansion of supernova remnants and HII regions are promising candidates. Many previous numerical simulations endorse this idea by performing colliding WNM flows as a zoomed view toward the edge of shock-compressed regions. Our previous simulations (Kobayashi et al. 2023) show that this supersonic compression is still viable as a cloud formation mechanism down to a 0.2 Zsun environment, where Zsun stands for the Solar metallicity. However, that is limited to the condition where colliding flows are aligned with global magnetic fields. We newly perform simulations of low-metallicity WNM colliding flows with inclined global magnetic fields with a spatial resolution of 0.02 pc to fully resolve the thermal instability growth. We found that, if the magnetic fields are inclined, the CNM formation becomes more inefficient than the dependence on the cooling efficiency expects. This regulation occurs because of the shear motion along the magnetic field lines in the pos-shock region, which suppresses the thermal instability growth. This magneto-hydrodynamical mechanism in particularly important in < 0.2 Zsun environments and naturally explains the smaller maximum cloud mass observed in lower metallicity environments. In this presentation, we will discuss how the cloud formation efficiency depends on metallicity, the inclined angle of magnetic fields, and the compression speed. We will also discuss how subsequent star formation modes change across the Universe.