Sumedh V. Anathpindika

Recent observations of Galactic and extragalactic Molecular clouds suggest that the ambient environment, i.e., the magnitude of external pressure affects their physical properties, including their ability to form stars. These observations, however, are in contrast with the longstanding belief that clouds are objects in approximate Virial equilibrium, as signified by the size – linewidth scaling relation, and have roughly uniform surface density (Larson 1981, Solomon et al. 1987). More recent observations show that the coefficients in the size – linewidth scaling relation depend on the position of a cloud in the Galactic disk (Rice et al. 2016). This observation is consistent with the inference that ambient environment modulates physical properties of a cloud, drawn from more recent studies. Furthermore, clouds with a higher column density appear to have a higher size – linewidth coefficient (Heyer et al. 2009). These observations, it has been suggested, can be reconciled by including the contribution due to external pressure in the expression for the simple Virial equilibrium – the so called, Pressure modified Virial equilibrium (Field et al. 2011). By a Virialised object we mean that the energy due to self-gravity of a cloud is approximately balanced by the energy due to turbulence within a cloud and its thermal energy. By extension, by pressure modified Virial equilibrium we mean that this net energy budget of a cloud is regulated by the pressure confining it.

In the work presented in this poster we explored this hypothesis and studied the impact of external pressure on various cloud properties, including the impact on their potential star-forming ability. We observed that clouds do not exhibit a unique set of properties, but rather a bouquet of properties modulated by the magnitude of pressure confining them. Star – formation appears to be inhibited at both extremes of external pressure. In a low pressure environment, such as that found at the periphery of the Galactic disk, gas within a cloud remains largely atomic and diffuse. These conditions are not conducive to star-formation. At the other extreme when the external pressure is significantly large, such as that close to the Galactic centre – i.e., the CMZ – although the average gas density is significantly higher, very little of it is actually cycled into the dense, putative star-forming phase. We found that the non-linearly growing Thin Shell instability in clouds confined by a large pressure inhibited the process of cycling gas into the dense phase. Our simulations showed that gravitational instability leading to the formation of star-forming fragments in a cloud was more likely to grow when the cloud was confined by an intermediate magnitude of pressure, similar to that found in the Solar neighbourhood. Results from our simulations also show that the size – linewidth coefficient is better reconciled by the Pressure modified Virial coefficient implying that clouds are unlikely to be entities obeying the simple Virial equilibrium. Our results therefore have profound implications for the theory of star-formation, for it is based on the assumption of clouds being Virialised entities and that the efficiency of star-formation is modulated only by the turbulent Mach number and the Virial parameter for a cloud.