The Life and Death of Interstellar Turbulence: The Intriguing Case of Ophiuchus

Abstract

In this thesis, I present the identification and analyses of a population of coherent structures in nearby molecular clouds, L1688 in Ophiuchus and B18 in Taurus. The newly discovered coherent structures have a typical size of 0.04 pc and a typical mass of 0.4 M⊙. In a virial analysis, the coherent structures appear to be gravitationally unbound and predominantly bound by ambient gas pressure. Owing to their small sizes and masses, as well as their potentially transient nature of pressure confinement, these newly identified structures are termed the “droplets,” and they may provide hints to a revised picture of structure and star formation in molecular clouds. To better understand the context within which the droplets are formed, I look into a potential turbulence driving mechanism—an embedded B-star wind—in Ophiuchus, in Chapter 1. A bubble with a warm dense outer “shell” and a hot, possibly ionized interior is identified in Ophiuchus. The center of the bubble is coincident with a group of five B-type stars and roughly 40 X-ray bright sources observed by Pillitteri et al. (2016). By comparing the energy budget between the stars, the shell, and the molecular cloud, I find that the stars are energetic enough to drive the expansion of the shell structure. The embedded stellar wind is found to have as much a potential to drive turbulence in the cloud as one of the more massive stellar winds surveyed by Arce et al. (2011) in Perseus. However, more embedded stellar winds, the impact from sources outside the molecular cloud, and/or other turbulence driving mechanisms are needed to fully explain the turbulence observed in the Ophiuchus cloud. In Chapter 2, I dissect a tool long believed to be tracing the turbulence and gravitational collapse in molecular clouds—the column density probability distribution function (N-PDF). With the help of dendrogram, I find that the power-law component in the high column density regime of an N-PDF can be a summation of N-PDFs of the substructures. While directly tracing turbulence with the lognormal component in the low column density regime is difficult due to observational uncertainties (Lombardi et al. 2015), I find that a method introduced by Burkhart et al. (2016) to use the “transition point” between the lognormal and the power-law components produces results consistent with observations. In Chapter 3, I present the identification of the droplets and a full analysis of their physical properties. The droplets are identified in a fashion similar to that used to define a coherent core (e.g. B5; Pineda et al. 2010), i.e. with a change in linewidth from supersonic to subsonic values across the boundary, and with a centrally concentrated distribution of NH3 emission, tracing the cold, dense gas. Similar to larger-scale coherent cores, the interiors of the droplets are found to be subsonic and nearly uniform. We find that the droplets follow a power-law mass-size relation similar to that found for larger-scale coherent cores. However, unlike the larger-scale cores, the droplets appear gravitationally unbound in a virial analysis and predominantly confined by the pressure exerted on the droplets by the ambient gas motions. I the present a comparison with a Bonnor-Ebert sphere and an analysis of the distribution of systematic velocities. The results are consistent with a formation mechanism likely related to the pressure distribution in the dense gas component of the cloud. In Chapter 4, I analyze the velocity gradients found within the droplets. Following Goodman et al. (1993), a 2D linear fit to the observed distribution of velocity centroids is used to find the velocity gradient. I find a gradient-size relation consistent with that found for larger-scale cores. With an assumption of solid-body rotation and a uniform density, the results extend the angular momentum-size relation to a size scale (down to 0.04 pc) potentially more relevant to the disk formation. The thesis is concluded with an outlook on the implication of the existence of structures like droplets—pressure-dominated coherent structures. I explore three projects that can potentially provide hints to the relation between droplets and other structures in molecular clouds and to the formation mechanism of the droplets. The results from these studies will help investigate a revised picture of structure and star formation, where structures across different size scales are created as a result of interaction between various physical processes, and where the star formation is understood as one of the many likely outcomes of hierarchical structure formation in molecular clouds.