Rotation of Horizontal Branch Stars in Globular Clusters

The rotation of horizontal branch stars places important constraints on angular momentum evolution in evolved stars and therefore rotational mixing on the giant branch. Prompted by new observations of rotation rates of horizontal branch stars, we calculate simple models for the angular momentum evolution of a globular cluster giant star from the base of the giant branch to the star's appearance on the horizontal branch. We include mass loss, and infer the accompanied loss of angular momentum for each of four assumptions about the internal angular momentum profile. These models are compared to observations of horizontal branch rotation rates in M13. We find that rapid rotation on the horizontal branch can be reconciled with slow solid body main sequence rotation if giant branch stars have differential rotation in their convective envelopes and a rapidly rotating core, which is then followed by a redistribution of angular momentum on the horizontal branch. We discuss the physical reasons why these very different properties relative to the solar case may exist in giants. Rapid rotation in the core of the main sequence precursors of the rapidly rotating horizontal branch star, or an angular momentum source on the giant branch is required for all cases if the rotational velocity of turnoff stars is less than 4 km s$^{-1}$. We suggest that the observed range in rotation rates on the horizontal branch is caused by internal angular momentum redistribution which occurs on a timescale comparable to the evolution of the stars on the horizontal branch. The apparent lack of rapid horizontal branch rotators hotter than 12 000 K in M13 could be a consequence of gravitational settling, which inhibits internal angular momentum transport. Alternative explanations and observational tests are discussed.Comment: 32 pages, 7 figures, submitted to the Astrophysical Journa

Exploring the role of information technology in organizational downsizing: A tale of two American cities

This study explores the role information technology (IT) plays in organizational downsizing by studying two medium-sized American cities over a period of 10 years (1985-1995). Data were collected through 73 interviews, a questionnaire, and numerous documents. Four main findings emerged from the case studies. First, IT was found to facilitate organizational downsizing, but not to cause it. New City invested heavily in state-of-the-art IT over the years and more successfully downsized the organization than Old City, which lagged behind in IT investment and made no serious attempts to use IT as a tool to support strategic actions. Second, adverse environmental conditions triggered downsizing in both cities and determined the change strategies that managers used. When environmental pressures were mild (1985-1990), managers favored a convergent change strategy that resulted in limited downsizing efforts and small personnel reductions. In contrast, when environmental pressures were strong (1990-1995), managers of both cities engaged in strategic reorientation and in downsizing efforts that led to larger personnel reductions. Third, the role IT played in organizational downsizing varied according to the change strategy. IT was used to facilitate work redesign in a convergent change strategy and to facilitate more significant structural and work redesign in strategic reorientation. Fourth, more integrated and better use of IT allowed managers of New City to downsize more rationally and efficiently.It facilitated the transfer of personnel within departments, from middle management to the operations level, and across departments, from internally oriented to customer-oriented personnel. In doing so, managers of New City minimized operating costs while maintaining the same level of services. In contrast, IT in Old City did not facilitate such an agenda and managers downsized more superficially across the board, in all departments. Differences in IT consequences in the two cities are explained using the theory of slack resources in organizations

The effects of electronic meetings on group processes and outcomes: An assessment of the empirical research

In this paper we analyze the empirical research on the impacts of electronic meetings on group processes and outcomes. We define and differentiate two broad types of electronic meeting systems: Group Decision Support Systems (GDSS) and Group Communication Support Systems (GCSS). We then present a framework and method for analyzing the impacts of such information systems on groups that we develop from the literature of organization behavior and group psychology. We review the empirical research and findings concerned with the impacts of GDSS and GCSS on groups, and we compare and contrast these findings. Finally, we conclude by discussing the implications of our analysis on the focus of attention and the design of future research. Our review of the empirical research suggests that GDSS and GCSS have similar impacts on some aspects of group processes and outcomes, but opposite impacts on other aspects. GDSS and GCSS both increase the depth of analysis of groups, increase participation, decrease domination by a few members, and increase decision quality. On the other hand, GDSS increase consensus reaching, decrease decision time, increase confidence in the decision by the group members, increase the satisfaction of group members with the process, and increase the satisfaction of the group members with the decision. GCSS decrease cooperation, increase the time to reach a decision, and decrease the confidence in decisions. © 1990

The impact of technological support on groups: An assessment of the empirical research

In this paper we analyze the empirical findings on the impacts of technological support on group. We define and differentiate two broad technological support systems for group processes: Group Decision Support Systems (GDSS), and Group Communication Support Systems (GCSS). We then present a framework and method for analyzing the impacts of such information systems on groups. We develop the framework from the literature of organization behavior and group psychology and apply it to literature of MIS. We then review the empirical research and findings concerned with the impacts of GDSS and GCSS on groups, and we compare and contrast these findings. Finally, we conclude by discussing the implications of our analysis on the focus of attention and design of future research. Five Major implications stem from our analysis: (1) there is lack of research on some important "formal" factors of groups, (2) there is a paucity of research on the impacts of GDSS and GCSS on the informal dimension of groups, (3) there is a need to move away from laboratory settings to field study in organization settings, with "real" manager, (4) more research is needed on stages of group development and on how they affect the impacts of GDSS and GCSS on groups, and (5) more research is needed to understand how the structure imposed by the technological supports affect group processes. © 1989

The Lithium-Rotation Correlation in the Pleiades Revisited

The dispersion in lithium abundance at fixed effective temperature in young cool stars like the Pleiades has proved a difficult challenge for stellar evolution theory. We propose that Li abundances relative to a mean temperature trend, rather than the absolute abundances, should be used to analyze the spread in abundance. We present evidence that the dispersion in Li equivalent widths at fixed color in cool single Pleiades stars is at least partially caused by stellar atmosphere effects (most likely departures from ionization predictions of model photospheres) rather than being completely explained by genuine abundance differences. We find that effective temperature estimates from different colors yield systematically different values for active stars. There is also a strong correlation between stellar activity and Li excess, but not a one-to-one mapping between unprojected stellar rotation (from photometric periods) and Li excess. Thus, it is unlikely that rotation is the main cause for the dispersion in the Li abundances. Finally, there is a strong correlation between detrended Li excess and potassium excess but not calcium-- perhaps supporting incomplete radiative transfer calculations (and overionization effects in particular) as an important source of the Li scatter. Other mechanisms, such as very small metallicity variations and magnetic fields, which influence PMS Li burning may also play a role. Finally, we find no statistical evidence for a decrease in dispersion in the coolest Pleiades stars, contrary to some previous work

Angular Momentum Evolution of Stars in the Orion Nebula Cluster

(Abridged) We present theoretical models of stellar angular momentum evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades. We demonstrate that observations of the Pleiades and Hyades place tight constraints on the angular momentum loss rate from stellar winds. The observed periods, masses and ages of ONC stars in the range 0.2--0.5 M$_\odot$, and the loss properties inferred from the Pleiades and Hyades stars, are then used to test the initial conditions for stellar evolution models. We use these models to estimate the distribution of rotational velocities for the ONC stars at the age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades distributions of rotation rates are not consistent if only stellar winds are included. In order to reconcile the observed loss of angu lar momentum between these two clusters, an extrinsic loss mechanism such as protostar-accretion disk interaction is required. Our model, which evolves the ONC stars with a mass dependent saturation threshold normalized such that $\omega_{crit} = 5.4 \omega_\odot$ at 0.5 \m, and which includes a distribution of disk lifetimes that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This model for disk-locking lifetimes is also consistent with inferred disk lifetimes from the percentage of stars with infrared excesses observed in young clusters. Different models, using a variety of initial period distributions and different maximum disk lifetimes, are also compared to the Pleiades. For disk-locking models that use a uniform distribution of disk lifetimes over the range 0 to $\tau_{max}$, the acceptable range of the maximum lifetime is $3.5 < \tau_{max} < 8.5$ Myr.Comment: 21 pages, 7 figures, submitted to Ap