Density Power Spectrum in Turbulent Thermally Bi-stable Flows

In this paper we numerically study the behavior of the density power spectrum in turbulent thermally bistable flows. We analyze a set of five three-dimensional simulations where turbulence is randomly driven in Fourier space at a fixed wave-number and with different Mach numbers M (with respect to the warm medium) ranging from 0.2 to 4.5. The density power spectrum becomes shallower as M increases and the same is true for the column density power spectrum. This trend is interpreted as a consequence of the simultaneous turbulent compressions, thermal instability generated density fluctuations, and the weakening of thermal pressure force in diffuse gas. This behavior is consistent with the fact that observationally determined spectra exhibit different slopes in different regions. The values of the spectral indexes resulting from our simulations are consistent with observational values. We do also explore the behavior of the velocity power spectrum, which becomes steeper as M increases. The spectral index goes from a value much shallower than the Kolmogorov one for M=0.2 to a value steeper than the Kolmogorov one for M=4.5.Comment: 16 pages, 11 figures. Accepted for publication in Ap

Three Essays on Strategic Human Capital, Managers and Competitive Advantage

In this dissertation, I investigate the interplay between strategic human capital and the role of managers in an organization. In Essay 1, using a natural experiment setting with a dataset on change of interdependence that an organization requires, and unexpected employee exit in a professional sports league for the period 1992 to 2010, I examine the consequence of losing strategically important human resources (HR) and shows that how specific organizational recovering techniques for dealing with HR can help the firm’s strategic renewal process. The data present that the consequential impact of losing employees is depending upon the type of interdependence that organization relies on, which are pooled interdependence and reciprocal interdependence. Furthermore, the results indicate that: (1) during the individual-focused period (pooled interdependence), loss of star employees harms organizational performance, but this harm can be mitigated by strong resource-picking skill, and (2) during the collaboration-focused period (reciprocal interdependence), loss of non-star employees harms organizational performance, but this harm can be mitigated by strong capability-building skill. In Essay 2, I try to answer following question: when promoted to management, do former star performers become superior managers? If so, why? Using performance data from a professional sports league, this study finds that organizational performance is greater under star-performers-turned-managers (SPTM’s) than other managers. Organizational performance is driven by the visibility of the manager’s prior career to employees for SPTM’s only, but driven by managerial competence for other managers only, suggesting a substitution effect between skill and inspirational role modeling. Consistent with social-comparison and self-enhancement theories, this inspirational role-modeling effect of SPTM’s on performance is contingent upon the need for self-enhancement by subordinates, and situational salience of the manager’s stardom. The results are consistent across robustness checks that control for potential selection issues, endogeneity concerns, and outliers. In Essay 3, I assess the causal impact of stakeholder orientation on the impact of corporate social responsibility and CEOs’ wealth and prominence. To obtain exogenous variation in stakeholder orientation, I exploit the enactment of state-level constituency statutes, which allow corporate executives and directors to consider non-shareholders’ interests when making business decisions. Using a cross-section of Texan firms during 2002-2012, I have found that the enactment of constituency statutes leads to significant increases in the quality of a firm’s corporate social responsibility (CSR); however, the effect of CSR does not necessarily lead to superior firm performance or value. I further argue and provide evidence suggesting that the obligated stakeholder orientation decreases the impact of CSR on CEOs’ compensation but increases the impact of CSR on CEOs’ media exposure. Finally, I posit that the impact of non-shareholder orientation on CEOs’ wealth and prominence is salient in non-consumer-focused industries

Three-Dimensional Simulations of the Parker Instability in a Uniformly-rotating Disk

We investigate the nonlinear effects of uniform rotation on the Parker instability in an exponentially-stratified disk through high-resolution simulations. During the linear stage, the speed of gas motion is subsonic and the evolution with the rotation is not much different from that without the rotation. This is because the Coriolis force is small. During the nonlinear stage, oppositely-directed supersonic flows near a magnetic valley are under the influence of the Coriolis force with different directions, resulting in twisted magnetic field lines near the valley. Sheet-like structures, which are tilted with respect to the initial field direction, are formed with an 1.5 enhancement of column density with respect to its initial value. Even though uniform rotation doesn't give much impact on density enhancement, it generates helically twisted field lines, which may become an additional support mechanism of clouds.Comment: 3 pages, uses rmaa.cls, to appear in Proc. of the Conference on "Astrophysical Plasmas: Codes, Models and Observations", Eds. J. Franco, J. Arthur, N. Brickhouse, Rev.Mex.AA Conf. Serie

Three-Dimensional Numerical Simulations of Thermal-Gravitational Instability in Protogalactic Halo Environment

We study thermal-gravitational instability in simplified models for protogalactic halos using three-dimensional hydrodynamic simulations. The simulations followed the evolution of gas with radiative cooling down to T = 10^4 K, background heating, and self-gravity. Then cooled and condensed clouds were identified and their physical properties were examined in detail. During early stage clouds start to form around initial density peaks by thermal instability. Small clouds appear first and they are pressure-bound. Subsequently, the clouds grow through compression by the background pressure as well as gravitational infall. During late stage cloud-cloud collisions become important, and clouds grow mostly through gravitational merging. Gravitationally bound clouds with mass M_c > ~6 X 10^6 Msun are found in the late stage. They are approximately in virial equilibrium and have radius R_c = \~150 - 200 pc. Those clouds have gained angular momentum through tidal torque as well as merging, so they have large angular momentum with the spin parameter ~ 0.3. The clouds formed in a denser background tend to have smaller spin parameters. We discuss briefly the implications of our results on the formation of protoglobular cluster clouds in protogalactic halos. (abridged)Comment: To appear in ApJ 20 September 2005, v631 1 issue. Pdf with full resolution figures can be downloaded from http://canopus.cnu.ac.kr/ryu/baeketal.pd

Effects of Rotation on Thermal-Gravitational Instability in the Protogalactic Disk Environment

Thermal-gravitational instability (TGI) is studied in the protogalactic environment. We extend our previous work, where we found that dense clumps first form out of hot background gas by thermal instability and later a small fraction of them grow to virialized clouds of mass M_c >~ 6X10^6 M_sun by gravitational infall and merging. But these clouds have large angular momentum, so they would be difficult, if not impossible, to further evolve into globular clusters. In this paper, through three-dimensional hydrodynamic simulations in a uniformly rotating frame, we explore if the Coriolis force due to rotation in protogalactic disk regions can hinder binary merging and reduce angular momentum of the clouds formed. With rotation comparable to the Galactic rotation at the Solar circle, the Coriolis force is smaller than the pressure force during the early thermal instability stage. So the properties of clumps formed by thermal instability are not affected noticeably by rotation, except increased angular momentum. However, during later stage the Coriolis force becomes dominant over the gravity, and hence the further growth to gravitationally bound clouds by gravitational infall and merging is prohibited. Our results show that the Coriolis force effectively destroys the picture of cloud formation via TGI, rather than alleviate the problem of large angular momentum.Comment: To appear in ApJ Lett. (June 1, 2006, v643n2). Pdf with full resolution figures can be downloaded from http://canopus.cnu.ac.kr/ryu/baeketal.pd