Smoothed particle hydrodynamics simulations of colliding molecular clouds

Anathpindika, Sumedh V. 2008. Smoothed particle hydrodynamics simulations of colliding molecular clouds. PhD Thesis, Cardiff University.

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Abstract

The galactic disk is largely composed of hot, rarefied gas also called the inter cloud medium (ICM). The cooler regions of the ICM are dominated by molecular species and dust. Immersed in this neutral medium are dense agglomerations of primarily H2, called giant molecular clouds (GMCs). The GMCs have a velocity dispersion of order a few km s_1, superimposed on their orbital motion. A GMC, over a single period of rotation of the galaxy, may undergo a few tens of collisions. In the present work, we investigate this rather violent phenomenon and examine the prospects of star formation in the post collision composite gas body. The star formation code, DRAGON, employed for the present work is ill equipped to study the effects of cloud collision on the chemical composition of the ICM. We draw a distinction between the regime of high velocity (precollision Mach numbers in excess of ten) and low velocity (precollision Mach numbers of order unity) cloud collisions, on the basis of the evolution of the gas slab produced in either cases. While the former leads to the formation of a dense shock compressed gas slab, the latter results in a dense pressure compressed gas slab. We observe that strong internal shear in a shock compressed slab suppresses gravitational instability in it. In particular, we observe evidence for the non-linear thin shell instability (NTSI) in the shocked slab formed in a head-on cloud collision. The slab thus dissipates thermal energy and upon the loss of thermal support, collapses to form a thin, long filament along the collision axis. Star formation proceeds in this filament. There is however, no evidence of the NTSI in the oblique shocked slab resulting from off centre cloud collisions, although it is dominated by internal shearing motion. On the other hand, the pressure compressed slab is dominated by gravitational instability and fragments, when the fastest growing mode dominates. The slab develops a number of floccules, which merge to form larger clumps and filamentary structures. The densest regions in these large scale structures then collapse gravitationally. We suggest this as a possible mechanism for the formation of star clusters. YSOs forming in filamentary structures are fed with material streaming along the axis of respective filaments. This material also transfers angular momentum to the accreting protostellar core and the attendant accretion disk is orthogonal to the angular momentum vector of this inflowing material. In the filaments resulting from the collapse of the post-collision shocked slab in a head-on cloud collision, we observe that the accretion disks circumscribing the sinks, are orthogonal to the filament. However, the gas slab resulting from a low velocity, off centre cloud collision is wrapped around by angular momentum and gravitationally fragments to form filaments. This slab tumbles in the plane of the collision (and therefore the axis about which it tumbles, comes out of this plane), the filaments in the slab also tumble with it. In the process they become offset relative to each other and feed angular momentum to the candidate protostellar core along the direction normal to the angular momentum axis. Thus, any attendant accretion disk is expected to be parallel to the filament (also the angular momentum) axis (Whitworth et al., 1995). To test this hypothesis, we collated data for YSOs located in filamentary star forming regions, and outflows originating from them. The scope of our work was limited and restricted to only five filamentary star forming regions in the local universe. Outflows from YSOs generally have small opening angles and are approximately normal to the circumstellar disk. Under this premise, we can get an idea of the orientation of the circumstellar disks relative to their natal filaments. We concluded that 72% outflows were distributed within 45 of being orthogonal to their natal filaments and 28% were distributed within 45 of being parallel to their natal filaments. It is difficult to make a strong claim simply on the basis of this work, which therefore needs to be extended. None the same, it tends to support the mechanism elucidated by Whitworth et al. (1995).

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Physics and Astronomy
Subjects:	Q Science > QB Astronomy
ISBN:	9781303213977
Funders:	State Government Of Maharashtra (India), George Education Trust
Date of First Compliant Deposit:	30 March 2016
Last Modified:	09 Jan 2018 19:23
URI:	https://orca.cardiff.ac.uk/id/eprint/54779

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