NATIONAL OPTICAL ASTRONOMY OBSERVATORIES NATIONAL OPTICAL ASTRONOMY OBSERVATORIES QUARTERLY REPORT JULY - SEPTEMBER 1993 November 11,1993 TABLE OF CONTENTS I. INTRODUCTION 1 II. SCIENTIFIC HIGHLIGHTS 1 A. Cerro Tololo Inter-American Observatory 1 1. The Self-Enrichment History of Omega Centauri 1 2. The Optical and X-ray Eclipse of the LMC Binary CAL 87 2 3. The Warped Disk of Centaurus A in the Near-Infrared 2 B. Kitt Peak National Observatory 3 1. What Determines the Rate at Which Young Stars Rotate? 3 2. Dark Matter in Elliptical Galaxies 4 3. Butcher-Oemler Effect Found in the Coma Ouster 5 C. National Solar Observatory 6 1. Flows in Quiescent Prominences 6 2. Spatial Scales of Solar Magnetic Structures 6 3. GONG Site Survey Network Observations of Mt. Pinatubo Dust Qoud 7 D. US Gemini Project Office 7 III. PERSONNEL AND BUDGET STATISTICS, NOAO 8 A. Visiting Scientists 8 B. New Hires 8 C. Terminations 9 D. Summer Research Assistants 9 E. Chilean Economic Statistics 9 F. NSF Foreign Travel Fund 9 Appendices Appendix A: Telescope Usage Statistics Appendix B: Observational Programs I. INTRODUCTION This document covers scientific highlights and personnel changes for the period 1 July - 30 September 1993. Highlights emphasize concluded projects rather than work in progress. The NOAO Newsletter Number 36 (December 1993) contains information on major projects, new instrumentation, and operations. The appendices to this report summarize telescope usage statistics and observational programs. II. SCIENTIFIC HIGHLIGHTS A. Cerro Tololo Inter-American Observatory 1. The Self-Enrichment History of Omega Centauri Globular clusters were among the first objects to form in our Galaxy, and as such, they hold valuable clues to the formation and early evolution of the Galaxy's halo. In particular, the abundance ratios of the different elements observed today in the globular cluster stars reflect the types of stars responsible for the pre-enrichment of the clusters. For example, Type II supernovae (SN II) produce a different ratio of O to Fe than Type I supernovae. The heavy element (e.g., Fe) abundance of most globular clusters is single- valued, whereas some star-to-star variations of the lighter elements (e.g., N, Al, Na) are seen within many clusters. The globular cluster Omega Centauri is unique in this respect. Its stars span a range of nearly 2 orders of magnitude in Fe abundance. Presumably, Omega Centauri managed to retain and self-enrich its primordial gas, unlike the other clusters. Thus, Omega Centauri stands as a point of comparison between the other globular clusters, the Galactic halo field stars, and even the dwarf elliptical galaxies. In an effort to shed light on this matter, J. Brown and G. Wallerstein (U. of Washington) have obtained very high resolution spectra of six red giant stars (spanning a range in Fe abundance) in the globular cluster Omega Centauri using the Echelle Spectrograph on the CTIO 4-m telescope. They have supplemented their data with information on other stars taken from the literature and have found that the abundances of O and the alpha-elements (Mg, Si, Ca, and Ti) appear to increase at a constant rate with the Fe abundance, as expected if all were produced by SN II. In contrast, the C+N abundance increases more rapidly than Fe, as if another source, in addition to SN II, was producing C and N. The authors note that winds from massive Wolf-Rayet stars or M supergiants could have provided this enrichment. Finally, the authors observed an excess of Na and Al relative to Fe along with a real star-to-star scatter in the abundances of these elements of a factor of 10. The authors note that both Na and Al can be produced above the hydrogen burning shell in massive evolving stars, such that it can later be released into the interstellar medium via stellar winds in the Wolf-Rayet or M supergiant evolutionary phases. The relatively low-speed winds from these stars might result in localized enrichments of N, Na and Al within the proto- cluster cloud, which manifest themselves today as the large star-to-star scatter in the abundances of these elements. Also, these slower winds would be more easily captured and incorporated into a subsequent generation of stars (or lower-mass proto stars) in the dense cluster environments, whereas the gas could more easily escape from star-forming regions in the field. The authors offer this as the explanation as to why N, Na and Al are enhanced in globular cluster stars relative to field stars of the same Fe abundance. Finally, they note that the story does not end here. The abundance ratios of the most metal-rich stars in Omega Centauri, yet to be determined because of the stars' fainter luminosities, will tell whether the period of cluster formation lasted long enough for Type I SN to begin altering the O/Fe ratio or for Asymptotic Giant Branch stars to begin altering the relative abundances of s-process elements. 2. The Optical and X-ray Eclipse of the LMC Binary CAL 87 CAL 87 is one of only three known low-mass X-ray binaries (LMXBs) in the Large Magellanic Cloud (LMC). Such binary systems are thought to be composed of a massive stellar remnant, such as a neutron star or black hole, in orbit with a smaller companion star which is transferring material to the more massive component. Because of the presence of an eclipse, CAL 87 presents the potential for determining many physical parameters of the system, such as orbital period and mass, to better constrain the models of LMXB systems. A. Cowley, P. Schmidtke (Arizona State U.), D. Crampton, and J. Hutchings (Dominion Astrophys. Obs.) obtained simultaneous spectroscopic and photometric observations of this eclipsing X-ray binary system with the CTIO 4-m and 1.5-m telescopes in December 1988. From the photometric observations, they found an orbital period of 10.624 hours. With the spectroscopic observations, they followed the orbital velocity of the system to determine a velocity amplitude of 80 km/s. Both the photometric and spectroscopic data indicate that the majority of the optical light from this system is coming from a luminous accretion disk. Combining the orbital period and velocity, they find that the compact remnant at the center of the accretion disk is likely to be a black hole with a mass greater than 6 M0. To probe the structure of the inner regions of the accretion disk and better understand the system, P. Schmidtke, T. McGrath, A. Cowley, and L. Frattare (Arizona State U.) obtained ROSAT PSPC data on the system together with further optical photometry using the CTIO 0.9-m. They found that the low X-ray luminosity of the system was consistent with the presence of a thick accretion disk which prevents the observer from viewing the X-ray source directly. The X-ray light curve showed a shallow dip which corresponds to the deep minimum in the optical light curve. The investigators interpreted this as an indication that the X-rays seen from the system come from an extended accretion-disk corona partially eclipsed by the orbiting companion star. Thus, from the combination of ROSAT X-ray and CTIO optical data, a picture of the binary system can be derived, consisting of a small companion star (mass approximately 1 MJ orbiting a massive compact object, most likely a black hole (mass > 6 MQ), which is encircled by a thick luminous accretion disk with a hot extended corona. 3. The Warped Disk of Centaurus A in the Near-Infrared Centaurus A (Cen A), the nearest giant radio galaxy, provides a unique opportunity to study in detail the structure and dynamics of a candidate system formed by the merger of a small gas-rich spiral galaxy with a larger elliptical galaxy. Its prominent gas disk and dust lane are strong evidence for merger activity. In addition, it possesses shell structure, which is a prediction of galaxy merger simulations. Mergers are thought to affect the evolution of many galaxies in rich clusters. Therefore, Cen A is a key object of study for understanding this important aspect of galaxy evolution. The gas kinematics of Cen A are well-modeled by a warped disk composed of connected rings of gas undergoing circular motion. Folds in the disk correspond to the edges of the dust lane observed at optical wavelengths. At near-infrared wavelengths, it is possible to view through the outer fold into the inner region of the disk. A. Quillen (California Inst, ofTech.), J. Graham (U. California, Berkeley) and J. Frogel (Ohio State U.) observed Cen A with the IR imager on the CTIO 1.5-m telescope to produce a large-scale mosaic in the J, H and K bands. Using a model of the gas distribution, they integrated the light distribution of the galaxy through an absorptive warped disk and were able to reproduce successfully the morphology and colors seen in the data. The deduced disk geometry is consistent with the kinematics from published CO and Ha data. One explanation for warped gas disks considers gas stripped from a smaller galaxy merging with a larger one. The stripped gas is assumed to accumulate on a plane that is misaligned with the larger galaxy's principal axis. Differential precession then causes a twist, or warp, in the gas surface. Eventually, the gas will settle onto a principal plane of the galaxy. The merger in Cen A, however, was too recent for this to have occurred. Moreover, Quillen, Graham and Frogel find that the disk inclination as a function of radius is inconsistent with precession from an initially planar system.