Intro 1st draft

The interstellar medium (ISM) is a dynamic and ever changing environment. It exists in various thermal states, appearing to cycle through them continuously - from a hot, sparse phase to a cold, dense phase and back again. The evolution of the gas through these phases paints a picture of star formation, one in which the molecular clouds from which stars form are not long lived steady state structures as once believed, but rather the transient consequence of various instabilities that arise in the ISM (cf. Heitsch et al 2005 for a quick discussion of the various instabilities).

The picture of transient molecular clouds (born out of the cold, dense phase of the ISM) is the foundation of today’s theory of star formation, and is supported by many well known lines of evidence, such as: the vast majority of (if not all) molecular clouds are in the process of forming stars, these stars are young (less than 5 Myr), and molecular clouds contain a wealth of hierarchical structure that would not last long due to star-star scattering or tidal interactions (Elmegreen 2000, Hartmann 2001, Ballesteros-Paredes and Hartmann 2007, and references therein). Indeed, this idea of short cloud lifetimes and its connection to fast star formation has its roots in work that now dates back at least many decades (Hunter 1979, Larson 1981, Hunter et al 1986). As computational methods have become increasingly more powerful, these theories have been tested in simulations designed to follow the highly dynamical evolution of molecular cloud formation.

One such model that has gained widespread recognition is the ‘colliding flows’ model. This model provides a mechanism for generating clouds with the above mentioned characteristics. Importantly, the collision between 2 supersonic streams of gas has been shown to: produce non-linear density structures with ease, the density of which can grow large enough, for long enough time to allow for H2 formation, after which the molecular gas undergoes localized gravitational collapse (i.e. numerical star/cluster formation), and disperses within roughly a dynamical time (Heitsch et al. 2006, and others).

While these models are highly idealized, they are not without motivation. Coherent large-scale streams of gas are plausible in many situations in the ISM: expanding bubbles driven outward from energetic OB associations and/or supernovae, turbulent motions, density waves in the spiral arms of galaxies, and cloud-cloud collisions have been proposed (Hartmann, Ballesteros, Bergin (2001), PPVI chapter 1, Inutsuka 2015). Further, observational evidence is beginning to provide support for such mechanisms - Looney 2006 has demonstrated cloud-cloud collisions, atomic inflows surrounding molecular gas has been observed in Taurus (Ballesteros-Paredes, Hartmann, & Vazquez-Semadeni 1999) and other molecular clouds (Brunt 2003), and molecular clouds have been found at the edges of supershells (Dawson 2011, 2013).

With the large body of data supporting large scale magnetic fields in the ISM (cite), the addition of magnetic fields to these models was necessary (cite sims). Such models have identified additional instabilities formed in the flows (cite), and an overall reduction of SF is found (cite). As a natural next step, models are beginning to study how oblique shocks at the collision interface affect cloud and star formation (cite).

As the flows themselves are not likely to be exactly parallel, the question of oblique shocks is an important one to address. Such oblique flows can arise in X. As flow enters the oblique shocks, a shear velocity field is formed giving rise to the question - how does added shear affect cloud and star formation? Recently In Ostriker et al. …. In the present paper we wish to explore this parameter regime, i.e. of a magnetized oblique colliding flows.

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