We investigate the gravitational wake due to, and dynamical friction on, a perturber moving on a circular orbit in a uniform gaseous medium using a semianalytic method. This work is a straightforward extension of Ostriker (1999) who studied the case of a straight-line trajectory. The circular orbit causes the bending of the wake in the background medium along the orbit, forming a long trailing tail. The wake distribution is thus asymmetric, giving rise to the drag forces in both opposite (azimuthal) and lateral (radial) directions to the motion of the perturber, although the latter does not contribute to orbital decay much. For subsonic motion, the density wake with a weak tail is simply a curved version of that in Ostriker and does not exhibit the front-back symmetry. The resulting drag force in the opposite direction is remarkably similar to the finitetime, linear-trajectory counterpart. On the other hand, a supersonic perturber is able to overtake its own wake, possibly multiple times, and develops a very pronounced tail. The supersonic tail surrounds the perturber in a trailing spiral fashion, enhancing the perturbed density at the back as well as far front of the perturber. We provide the fitting formulae for the drag forces as functions of the Mach number, whose azimuthal part is surprisingly in good agreement with the Ostriker's formula, provided V p t = 2R p , where V p and R p are the velocity and orbital radius of the perturber, respectively.

four anonymous reviewers, and seminar participants at Chicago GSB, MIT-Sloan, Wharton, and Kellogg for their very helpful comments and suggestions. 2The model of institutionalization of power, while influenced in part by institutional sociology, also draws on several other theories, including resource dependence, escalation of commitment, and strategic contingencies theory (Salancik and Pfeffer, 1977;Pfeffer, 1981). Both cultural and political theories are part of the model. Perhaps a more descriptive and less confusing term for it would be the model of entrenchment of power. In this paper we will continue to use the original term used by Pfeffer (1981) but want to explicitly state that any discussion or test of the model of the institutionalization of power in this paper is not intended as pertaining to or to be a test of institutional theory or the new institutionalism. 3Corporate elites are also subject to contests for control with external factions, as in the case of hostile takeovers, which became common during the 1980s. External contests for control, while important mechanisms for elite circulation, are beyond the scope of this paper. Circulation of Controlto the study of stability and change in CEOs' functional backgrounds.We analyze the contests for control among elite members of functional groups within organizations (Perrow, 1970; Zald and Berger, 1978;Fligstein, 1987;Chaves, 1993). Our study complements recent research that highlights political struggles between the CEO and the board of directors (Wade, O'Reilly, and Chandratat, 1990; Westphal and Zajac, 1995) but focuses instead on the conflicts within top management (Hambrick, 1994). We develop a conceptual model of changes in corporate control-the circulation of corporate control-that highlights the obsolescence and contestation of executive power, as corporate elites vie for status, position, and control of the organizational hierarchy (Ocasio, 1994). We contrast our model with the model of the institutionalization of power (Salancik and Pfeffer, 1977;Pfeffer, 1981). While both models rely on political, cultural, and social psychological mechanisms, they vary in how they explain stability and change in power in organizations and the rise and fall of alternative conceptions of control (Fligstein, 1990). POLITICAL DYNAMICS IN ORGANIZATIONSInstitutionalization of power. The institutionalization of power (Salancik and Pfeffer, 1977; Pfeffer, 1981; Boeker, 1989) is the dominant model of political dynamics in organization theory.2 It highlights the ability of powerful individuals and groups to entrench themselves in the formal positions of authority and to increase their control of the corporation over time. The institutionalization of power is shaped by symbolic as well as material forces that operate both within the organization and at the level of the organizational field. According to this theory, conceptions of control (Fligstein, 1990) serve to institutionalize the power of dominant groups, as the goals and orientation of the governing subunit be...

Using high-resolution, two-dimensional hydrodynamic simulations, we investigate nonlinear gravitational responses of gas to, and the resulting drag force on, a very massive perturber M p moving at velocity V p through a uniform gaseous medium of adiabatic sound speed a ∞ . We model the perturber as a Plummer potential with softening radius r s , and run various models with differing A = GM p /(a 2 ∞ r s ) and M = V p /a ∞ by imposing cylindrical symmetry with respect to the line of perturber motion. For supersonic cases, a massive perturber quickly develops nonlinear flows that produce a detached bow shock and a vortex ring, which is unlike in the linear cases where Mach cones are bounded by low-amplitude Mach waves. The flows behind the shock are initially non-steady, displaying quasi-periodic, overstable oscillations of the vortex ring and the shock. The vortex ring is eventually shed downstream and the flows evolve toward a quasi-steady state where the density wake near the perturber is in near hydrostatic equilibrium. We find that the detached shock distance δ and the nonlinear drag force F depend solely on η = A/(M 2 − 1) such that δ/r s = η and F/F lin = (η/2) −0.45 for η > 2, where F lin is the linear drag force of Ostriker (1999). The reduction of F compared with F lin is caused by front-back symmetry in the nonlinear density wakes. In subsonic cases, the flows without involving a shock do not readily reach a steady state. Nevertheless, the subsonic density wake near a perturber is close to being hydrostatic, resulting in the drag force similar to the linear case. Our results suggest that dynamical friction of a very massive object as in a merger of black holes near a galaxy center will take considerably longer than the linear prediction.

The CS-35 antibody is widely used in the characterization of glycans containing D-arabinofuranose residues, in particular polysaccharides present in the mycobacterial cell wall. A detailed understanding of the combining site of this antibody and the measurement of its binding to different ligands is of interest as this knowledge will have implications in the characterization of arabinofuranose-containing glycoconjugates that are increasingly recognized as important biological molecules. Of even greater significance is that an in-depth study of this carbohydrate-protein interaction will provide insights into the mechanisms by which oligosaccharides containing furanose rings are bound by proteins, an area that has, to date, received little attention. This system has been refractory to X-ray crystallography, and thus we report here a study of the interaction of CS-35 with its ligands using a combination of chemical synthesis, mass spectrometry, titration microcalorimetry, and NMR spectroscopy. Through these investigations we have established that the binding pocket recognizes, as a minimum epitope, a linear tetrasaccharide motif and that the residues at the reducing and non-reducing end of the oligosaccharide are essential for tight binding. The residue at the non-reducing end appears to be bound in an aliphatic pocket, whereas the rest of the tetrasaccharide interacts more strongly with aromatic amino acids.

In many astrophysical situations, as in the coalescence of supermassive black hole pairs at gas rich galactic nuclei, the dynamical friction experienced by an object is a combination of its own wake as well as the wakes of its companions. Using a semi-analytic approach, we investigate the composite wake due to, and the resulting drag forces on, double perturbers that are placed at the opposite sides of the orbital center and move on a circular orbit in a uniform gaseous medium. The circular orbit makes the wake of each perturber asymmetric, creating an overdense tail at the trailing side. The tail not only drags the perturber backward but it also exerts a positive torque on the companion. For equal-mass perturbers, the positive torque created by the companion wake is, on average, a fraction ~40-50% of the negative torque created by its own wake, but this fraction may be even larger for perturbers moving subsonically. This suggests that the orbital decay of a perturber in a double system, especially in the subsonic regime, can take considerably longer than in isolation. We provide the fitting formulae for the forces due to the companion wake and discuss our results in light of recent numerical simulations for mergers of binary black holes.Comment: 4 pages, 3 figures, accepted for publication in ApJ

Transition metal oxides used as electrode materials for flexible supercapacitors have attracted huge attention due to their high specific capacitance and surface-to-volume ratio, specifically for cobalt oxide (Co3O4) nanoparticles.

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