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Yale University Department of Astronomy New Haven, Connecticut 06520

1. PERSONNEL Astronomy 110a Birth, Life, and Death of Stars. Charles Bailyn FACULTY AND RESEARCH STAFF, 1994-1995 Astronomy 111b Frontier and Controversies Professors: Pierre Demarque, Richard B. Larson, Augustus in Astrophysics C. Bailyn Oemler, Jr., Sabatino Sofia, William F. van Altena, and Astronomy 120b Galaxies and the Universe. R. Zinn Robert J. Zinn Astronomy 130b Life in the Universe. Professor Emeritus: Boris Garfinkel Assistant Professors: Sabatino Sofia Charles Bailyn, Paolo Coppi, Jeffrey Kenney, and Bradley Astronomy 220b Galaxies and Cosmology. Schaefer ͑primary appointment in Physics͒ A. Oemler Senior Research Scientist ͑Emeritus͒: E. Dorrit Hoffleit Astronomy 250b Observational Astronomy. Research Scientist: Terrence Girard William van Altena Associate Research Scientist: Imants Platais Astronomy 310b Galactic & Extragalactic Department of Energy, Distinguished Postdoctoral Fellow: Astronomy. Richard Larson T.J. Lydon Astronomy 350a Stellar Astrophysics. Research Affilate: M. W. Schaefer Pierre Demarque Hubble Fellow: C. Deliyannis Postdoctoral Fellows: X. Guo, Astronomy 418b Stellar Dynamics. Jeffery Kenney Elliot Horch, Y.-C. Kim, and R. Me´ndez Astronomy 450b Cosmology. Augustus Oemler Associate in Research: John T. Lee and Vera Kozhurina- Astronomy 490a Independent Projects Platais in Astronomy. Faculty Visiting Fellows: A. Upgren and Hai-Wa Wu Research As- Astronomy 491b Independent Projects sistant: Y. Li in Astronomy. Faculty Members of the Yale Southern Observatory scientific staff in Graduate Teaching San Juan, Argentina included: M. R. Cesco, R. A. Gil Hutton, H. S. Le´pez, J. A. Torres, J. G. Sanguin, J. A. I. The Following graduate courses were Vicentela and C. E. Lo´pez; in July 1994 M. A. De Lau- offered during the year: rentis joined the group. C. E. Lo´pez continued as the YSO Astronomy 518b Stellar Dynamics. Coordinator in Argentina. Jeffery Kenney Astronomy 550a Stellar Astrophysics. ADMINISTRATIVE ASSIGNMENTS Pierre Demarque Chairman: Sabatino Sofia Astronomy 570a High Energy Astrophysics. Director of Graduate Studies: Robert J. Zinn P. Coppi Director of Undergraduate Studies: Charles Bailyn Astronomy 580a Research. Faculty Oemler and Zinn served as Yale’s astronomer members on Astronomy 580b Research. Faculty the board of the WIYN Corporation, where Oemler chaired Astronomy 600b Cosmology. Augustus Oemler the WIYN Scientific Advisory Committee. There were 17 graduate students in the department during the year. Two were awarded the PhD degree: Rene´ Me´ndez SPECIAL HONORS for the thesis ‘‘Semi-Empirical Model for the Distribution if In November 1994, C. E. Lo´pez received the Argentine Star-Counts, Colors and Kinematical Properties of Stars in National Award ‘‘Dr. Eduardo Braun Menendez’’ for his ar- the Milky Way’’ and Xinjian Guo for the thesis ‘‘Galactic ticle ‘‘Star Tales: A Journey Through the Solar System.’’ Structure, Kinematics and Chemical Abundance from UBV On March 1 Hoffleit was invited to deliver the prestigious Photometry and Absolute Proper Motions to B ϭ 22.5 To- Glover Memorial Lecture for 1995 at Dickinson College, wards the SGP.’’ Carlysle, PA. She delivered two talks, the first open to the public, ‘‘A Century of Women in Astronomy,’’ the second for the Physis Department, ‘‘The Development of Spectroscopy 3. PROGRESS AND PROBLEMS for Discovering the Physical Properties of Stars.’’ For these This year significant events have taken place in the De- she was presented the Glover Medal. partment. First, there was the dedication of the WIYN tele- scope in October 1994, followed by extensive testing and fine tuning, with regular scientific operations beginning in early Summer. It has been widely judged that the WIYN is 2. TEACHING the ‘‘finest ground-based imaging telescope’’ currently in ex- Undergraduate Teaching istence. The scheduled observing program entails the partici- The Following undergraduate courses were taught during pation of a large fraction of the Faculty and graduate students the year: in our Department. 736 ANNUAL REPORT

A second significant development in our Department has tion in plate-overlap methods in the region of the Orion been the joint implementation with the Physics Department Nebula Cluster, and the ongoing Van Vleck parallax pro- of a teaching and research program in High Energy Astro- gram. physics. Besides carrying out the work in X-ray and Gamma- Kozhurina-Platais and Girard have continued to use the ray astronomy originally planned, the observing program has PDS in playback mode to generate coronograph masks. Two been greatly expanded by the initiation of an international sets of masks were provided to C. Ftaclas ͑Hughes Danbury collaboration to carry out a deep survey of astronomical ob- Optical Systems͒ and collaborators who successfully used jects near the equator. The collaboration membership in- these in coronographic observations from the Infra-Red Tele- cludes Astronomers and Physicists at Yale and Indiana, plus scope Facility in Hawaii. Along with other targets, the diffi- scientists and engineers at the University of the Andes and cult binary Procyon A and B was successfully observed. The the Venezuelan National Observatory, both located in resulting separation measure will be combined with new par- Merida, Venezuela. Although the survey will provide photo- allax and orbital element determinations described elsewhere metric and spectroscopic information of all objects in that for the purpose of lowering the uncertainty in the mass esti- region of the sky, the primary interest of the Yale group is to mate of Procyon A. A set of masks was also produced for a detect gravitational lenses from quasar observations. It is ex- second team of coronograph operators represented by A. pected that within two years the project will have increased Crotts of Columbia University. the total number of known quasars ͑and identified gravita- A. H. Lee, in collaboration with van Altena and Girard tional lenses͒ by approximately a factor of 100. This infor- designed an optical system for the PDS to illuminate and mation will allow us to address some of the most important relay the image of a star from the plate to a small CCD astrophysical questions of our time, such as the values of the camera. The goal of this project is to increase the throughput Hubble and of the Cosmological constants that are central to of the PDS for low signal-to-noise images where the scat- modern Cosmology. tered light in the direct imaging approach will not signifi- In the study of the Sun, this past year also saw the culmi- cantly degrade the derived image position in comparison nation of lengthy pursuits. From observations carried out by with the usual single channel scan system. Work is continu- the balloon-borne Solar Disk Sextant experiment in Septem- ing on improving the uniformity of the illumination system. ber 1992 and in Sepember 1994, we could establish that during this time interval the diameter of the Sun increased by 8 km. This value, far too small to be observed by any com- 4.3 The Solar Disk Sextant peting technique, explains the apparently contradictory re- The SDS is an instrument whose purpose is to make very sults obtained to date in the sense that all of the competing precise measurement of the solar radius at different orienta- techniques were measuring noise. The SDS also measured tions, and consequently the solar shape ͑oblateness, limb os- the solar oblateness with unprecedented accuracy, and in the cillations, etc.͒. The experiment is a joint effort between Yale process it disproved the recent claim that this quantity varied University and the Goddard Space Flight Center, and cur- as a function of the phase of the activity cycle. rently it is flown every year on stratospheric balloons to avoid the turbulence of the Earth’s atmosphere. It is contem- 4. FACILITIES plated that eventually the SDS will be flown on a spacecraft. The precision of the solar radius measurements made on the 4.1 The WIYN Telescope balloon-borne SDS have an rms distribution of less than 13 The WIYN telescope went into routine operation at the milli-arcseconds, with the noise primarily due to unevenness end of June 1995. This facility is a 3.5 meter alt-az telescope of the solar surface. During the period of this report, the SDS constructed on Kitt Peak by a consortium of Yale, the Uni- had one flight from Fort Sumner, NM, on September 26, versity of Wisconsin, Indiana University, and the National 1994. Optical Astronomy Observatories. Although some instrument development remains to be completed, it is now in use every night, with most time devoted to the two facility instruments: 5. RESEARCH a CCD camera, and the multiobject spectrograph, fed by the 5.1 Astrometry Hydra fiber positioner. Performance to date has been very satisfactory. Time lost to remaining problems has been mini- Guo, van Altena and T. Ingerson ͑CTIO͒ finalized the mal, and the average image quality is excellent, with median modeling on the OFAD of the new Atmosphere Dispersion seeing of about 0.75 arc seconds. Correction ͑ADC͒ on the CTIO 4-m telescope. Plate scale and distortion coefficients were obtained for the positioning 4.2 The PDS Microdensitometer of the fibers of the multi-fiber-fed Argus spectrograph, and for other imaging applications at the prime focus. The majority of the available PDS scanning time was de- Guo, T. Girard and van Altena also modeled the OFAD of voted to the Southern Proper Motion project and other, the 3.5-m WIYN telescope at the Nasmyth focus and ob- smaller, Yale astronomy department projects. The PDS has tained preliminary solutions for the positioning of the optical also been used by visiting researchers whose investigations fibers in the multi-fiber-fed Hydra spectrograph. have included scans of objective prism plates for galactic van Altena, R. Meyer and M. Yoshizawa ͑Tokyo͒ began structure studies, a study of the Galactic bulge, an investiga- an investigation into the use of high time resolution multi- YALE UNIVERSITY 737 color astrometry as a method for compensating for atmo- the possibility of systematic effects on the derived magni- spheric turbulence as the principal limit to the accuracy of tudes. Me´ndez has found that it is possible to calibrate the Meridian Circle astrometry. photometry to about 0.07 mag., however, at both the faintest In preparation for the construction of a new ICCD and brightest magnitude regimes large systematic effects ap- Speckle Interferometer camera for the 76-cm Cesco Obser- pear that are quite difficult to model. To solve that problem, vatory and CASLEO 2.15-m telescopes at El Leoncito, Ar- Girard devised a new instrumental magnitude estimate, one gentina, van Altena developed an optical model for the cur- which is linear with magnitude. This step has simplified and, rent Stanford University Speckle Interferometer that is now hopefully, improved the photometric calibration of the SPM in use at El Leoncito for the measurement of Southern Hemi- plates. Unfortunately, the photometric calibration still re- sphere double stars. The model is based on the BEAM4 soft- mains somewhat uncertain due to an inadequate number of ware and has been used to study the variation of plate scale calibration standards, a problem that is presently being ad- and optical field angle distortion at the MAMA detector. dressed by a program to obtain at least one CCD calibration Guo, Girard, Sofia and van Altena began an optical ray frame ͑in B and in V͒ within each SPM field. tracing investigation of the OFAD effect on the measure- In addition to the 31 SPM fields in the SGP region, 18 ments of the solar disk diameter of the Solar Disk Sextant other fields have been measured by Platais and Kozhurina- balloon borne tesescope using the BEAM4 software. Platais to support the Hipparcos proper-motion link to the extragalactic reference frame program. The density of mea- 5.2 Southern Proper Motion Program sured objects is maintained at around 2,500 per field in order The Yale/San Juan Southern Proper Motion program to ensure high accuracy of the Hipparcos link. In total, 49 ͑SPM͒ has continued throughout this period, highlighted by SPM fields ͑or 196 plates͒ have been measured thus far. significant progress in understanding the magnitude- Outside the SGP region the stars and galaxies are ex- dependent systematics affecting the plate material. Using the tracted from the COSMOS/UKST database kindly furnished pilot region of 800 square degrees ͑31 fields͒ around the by D. Yentis ͑Naval Research Labs͒. The database provides South Galactic Pole described in last year’s report, proce- positions and Bj-magnitudes for objects from the individual dures have been developed which allow field-and- UK Schmidt plates, therefore software was written ͑Yentis & magnitude-dependent position corrections to be derived for Platais͒ to emulate the features of the previously used both the long and short exposures on each SPM plate. The COSMOS/UKST Object Catalog of the Southern Sky which corrections are essentially bootstrapped from the differences is not available outside the SGP. in positions derived from the various diffraction-grating im- With the current software and disk space it is complicated ages of those stars for which multiple grating orders can be to prepare an input catalog for the low galactic latitude or the measured. The appropriate corrections are applied to the LMC/SMC fields: the total number of objects down to Bj measured positions of all images of all stars. By correcting ϭ19.0 is reaching half a million or more per field. This cir- the individual positions, not only are systematic errors re- cumstance requires preliminary random sampling of the moved from the final proper motions, but also from the de- source catalog. Due to considerable crowding of faint images rived stellar coordinates. This is important in the context of the overlap detection algorithm must be abandoned in these the Faint Secondary Reference system which is part of the fields. Also, for these fields, the galaxy/star classifier in the overall SPM project. Substantial improvement is seen by COSMOS/UKST data is no longer credible, hence a visual comparison with Hipparcos data, although there are still in- inspection of the entire SPM plate is required. dications of a residual effect in some of the SPM fields. Platais, Girard, Kozhurina-Platais, van Altena and Lick An added benefit of the magnitude-equation procedure Observatory astronomers B. Jones, R. Hanson and A. Klem- just described is an adjustment and calibration of the PDS instrumental pseudomagnitudes utilizing differences between ola continued to work on the Hipparcos proper motion link the different grating order images, in this case differences in to the extragalactic frame. The results were reported at the pseudomagnitude. Me´ndez has derived the basic equations Hipparcos reference frame working group meetings in relating true magnitudes ͑log of flux͒ and the observed vol- Grasse and Paris ͑France͒. The number of link stars from the ume under the density profile for two grating images of the SPM program has reached nearly 2,000. Owing to the high same object as outlined by Schaefer ͑PASP 93, 253, 1981͒ number of link stars, the rotation solutions show very small taking into account the Gaussian core and exponential wings formal error ͑0.1-0.2 mas/yr per axis͒, however it is obvious Gyldenkaerne ͑Ann. d’ Ap 13, 97, 1950͒, King ͑PASP 83, that the solutions are corrupted by systematic errors, prima- 199, 1971͒, and also considering the possibility of saturation rily magnitude equation. As indicated by the Hipparcos data and magnitude equation. These equations have a number of ͑37-month solution͒, in extreme cases the systematic errors free parameters that depend upon the characteristic curve of in the uncorrected SPM proper motions of bright stars can the plate, and the conditions of the exposure ͑seeing, guiding reach over 10 mas/yr ͑!͒, although on average they tend to errors, change of the point-spread-function across the plate, cluster around 4 mas/yr. If overlooked, such a systematic etc͒. By solving for these terms using all the grating pairs error would translate into a velocity offset of 20 km/sec at a available on the plate, usually between 1,000 and 2,000 distance of 1 kpc. As mentioned earlier in this section, the pairs, it is possible to determine not only the overall value of correction procedures for eliminating the magnitude equation these terms, but also their field dependence, therefore reduc- have now been developed and we are able to remove this ing the scatter of the calibrated magnitudes and minimizing effect from the SPM absolute proper motions. 738 ANNUAL REPORT

5.3 The Solar System to Fe3ϩ. In addition, photochemical oxidants may have en- abled a significant proportion of the oxidation of the soil. The overall objective of M. W. Schaefer is research is to obtain a better understanding of the processes that have 5.4 The Sun modified the Martian surface and atmosphere through time, specifically how the atmosphere, hydrosphere, and regolith Sofia, together with W. Heaps ͑NASA/GSFC͒ and L. of early Mars interacted. Such an understanding is valuable Twigg ͑ARC, Inc͒ completed the data reduction and analysis for its relevance to the interpretation of present-day Martian for the Fall 1994 balloon flight of the Solar Disk Sextant. geomorphology, and therefore the determination of the geo- This was the second time in which the optically contacted logic history of Mars. She has approached these problems by wedge assembly, fabricated by Zygo Corp., was used, and means of a geochemical cycle model. consequently it became possible to determine solar diameter This year M. Schaefer began extending the model to rep- and oblateness changes with unprecedented sensitivity. The resent more realistically an early Martian ocean by adding results show that from 1992 to 1994, the solar diameter in- iron oxides and sulfides; iron carbonate; and calcium, mag- creased by 8 km, with a level of uncertainty below 1 km. nesium, and iron sulfates. Iron- and sulfur-bearing species in This type of change could not be detected with any compet- solution are also being included. The addition of both iron ing technique. We also determined that the solar oblateness and sulfur to the system has made quite complicated the task remained constant to a very high level of confidence. By of determining which species and reactions are important. A contrast, the change of oblateness expected from recently set of 50 equations has been determined ͑so far͒, which de- proposed activity-related variability was so large that it was scribe the reactions of the various species with each other, positively disproved by our measurements. The implications and which can be used to calculate the stability ranges ͑in pH of the oblateness measurements as a test for General Rela- and Eh͒ of the species. Some of these reactions may be tivity have been explored by T.J. Lydon and Sofia; the im- found to be unimportant to the system, and there are others plications relating to the rotation of the solar interior have that have not yet been considered. been worked out by L. Paterno and M. Di Mauro ͑University As a part of this research, M. Schaefer is also studying of Catania͒ with Sofia, and the inferences in term of the terrestrial banded-iron formations ͑BIFs͒. Some of the oldest distribution mechanisms operating in the solar interior have surviving sedimentary features on Earth are the banded iron- been worked out by M. Pinsonneault ͑Ohio State͒ and Sofia. formations, which contain alternating layers of silica-rich In particular, the extremely flat rotation curve consistent with and iron-rich bands, with diverse assemblages of iron: ox- the low quadrupole moment observed, which is also in ides, carbonates, silicates, and sulfides. The mechanism of agreement with helioseimological results, requires the opera- formation of BIFs is not well understood, and both organic tion of an angular momentum distribution mechanism which and inorganic mechanisms have been proposed. One interest- is effective even for very low gradients of angular momen- ing model ͑Eugster and Chou, 1973͒ is an inorganic model in tum, and for low rotation velocities. None of the known hy- which the BIFs are deposited in a playa-lake complex. In this drodynamic mechanisms possesses such a quality. Lydon and model, precipitation of carbonates occurs at the margins of a Sofia formulated the equations of stellar structure and evolu- largely spring-fed lake, or barred lagoon, with silicate gel tion including the effects of internal magnetic fields, and precipitation occurring in the deeper parts of the lagoon. Oc- wrote the corresponding numerical code. Numerical experi- casional floods are invoked to wash the carbonates into the ments with this code confirmed that a variable magnetic field lake to produce the banded iron-formation. This physical affected all global solar parameters, but the relative value of model has relevance to the ancient Martian environment, as the changes depended on the magnitude, shape and location well as to the Archaean terrestrial environment. M. Schaefer of the field. The code was used to model simultaneously the is using this model to constrain the relevent chemical reac- observed changes of diameter, luminosity and p-mode oscil- tions included in the Martain geochemical cycle model. lations of the Sun. A particularly puzzling result is that the Another topic of research this year has been on Martian changes could only be modeled by a variation of the mag- paleosols. The surface of Mars may be thought of as a netic field of 400 gauss at a depth of about 300 km, just paleosol—a fossil soil—and as such it reflects the weather- below the superadiabatic layer. It is not known what mecha- ing processes that have occurred on Mars throughout its long nism could stabilize such a magnetic field at that particular history. The chemical composition of the Martian paleosols, depth, as opposed, say, to the base of the convection zone. as measured by the Viking landers, shows the effects of Demarque has continued to work on models of the solar aqueous alteration under slightly oxidizing conditions. Mod- interior primarily in the context of helioseismology, in col- elling Viking surface materials as paleosols, developed from laboration with D.B. Guenther ͑Saint Mary’s University, rocks similar in composition to the SNC meteorites, pro- Halifax, Nova Scotia͒ and postdoctoral researcher Y.-C. Kim. duces self-consistent results. These results include a loss of The first study completed this year dealt with the implemen- several mobile cations during weathering, which implies the tation of recent improvements in the equation of state in the presence of flowing water with which to leach and remove solar interior model and the evaluation of the sensitivity of the cations. This loss includes a small, but definite, loss of the calculated p-mode frequencies to these improvements. iron during weathering, implying that this weathering took Both the OPAL ͑Rogers, Swenson & Iglesias 1995͒ and the place in conditions that were either slightly oxidizing, or so-called MHD ͑Mihalas, Dappen and Hummer 1988; Dap- hardly oxidizing at all, but that flow through the soil was pen 1995͒ equations of state were considered. Frequencies faster than the time constant over which Fe2ϩ is converted for lϭ 0-100 were calculated in each case. It was found that, YALE UNIVERSITY 739 except for some minor differences near the surface, both the The aim is to make possible new observational tests of stellar OPAL and MHD equations of state give similar results. They theory previously impossible with semi-empirical models, improve the agreement of the p-mode frequencies with ob- particularly in the study of stellar activity and in research servation dramatically, and in nearly identical ways. At this related to angular momentum transfer in stellar interiors dur- point, the remaining sources of uncertainties are believed to ing the course of stellar evolution. be primarily near the surface, in the atmosphere and in the Graduate student V. Bromm ͑working with Demarque͒ subphotospheric layers which are convectively unstable and compared the effects on the evolutionary track of a 3 solar highly superadiabatic. mass star of different assumptions about convective over- In another ongoing study, Demarque, Kim and Guenther shoot near the main sequence at the edge of the convective are exploring the sensitivity of the p-mode frequencies to the core. At least in this instance, it would be difficult to design characteristics of the superadiabatic peak referred to above. observable tests to distinguish between convective overshoot Helioseismology offers an opportunity to explore the struc- ͑overmixing at the core edge into the radiative region͒,or ture of the superadiabatic layer in a sensitive way. In models convective penetration ͑extension of the adiabatic tempera- constructed with the conventional mixing length approxima- ture gradient beyond the edge of the convective core͒, al- tion, the contribution from the radiative energy transport though the two processes affect the evolutionary timescale at relative to convection is underestimated in the outer layers. the ten per cent level. Penetration by a fraction of the core The numerical simulations of convection described above radius radius is similarly difficult to distinguish in the 3 solar suggest that the superadiabatic peak in the Sun is more mass star. peaked than predicted by the constant mixing length models, V. Kozhurina-Platais has been constructing theoretical which underestimates the effects of radiation near the top of isochrones for open clusters of intermediate ages. The objec- the convection zone. Some of the implications for the tive of this work is to concentrate on star clusters for which p-modes of a simple parameterization of the Kim et al. nu- good astrometric membership data has recently become merical simulations described above are being systematically available, such as NGC 3680 and NGC 752, both to test the explored. It is hoped that these experiments will eventually validity of stellar evolutionary tracks and to derive more re- serve as a guide in developing a parameterization of convec- liable ages for these clusters ͑work done with Demarque and tion suitable for stellar evolution calculations, that is physi- I. Platais͒. cally more realistic than the mixing length approximation, Kim and B. Chaboyer ͑CITA͒ have studied the effects of and free of the usual arbitrariness in choosing the mixing the new OPAL equation of state tables ͑described above in length. the context of solar models͒, on the evolution of low metal- The natural next step in this research is to develop a licity stars, and constructed isochrones for globular clusters. physically more realistic model of convection in the shallow They find that the isochrones are similar to isochrones con- outer layers, where radiation dominates over convection ͑be- structed with an analytical Debye-Huckel correction at high cause radiative transfer is treated by the diffusion approxi- temperatures. The absolute magnitude of the main sequence mation in the Kim et al. models, they become invalid in the turnoff with the OPAL or Debye-Huckel isochrones is about outer, optically thin layers͒. Work on this problem is now 0.06 magnitude fainter, at a given age, than derived from under way, by Kim, and K.L. Chan, at the Hong Kong Uni- isochrones which do not include the Debye-Huckel correc- versity of Science & Technology, and in collaboration with tion. As a consequence, globular cluster ages derived with P.A. Fox ͑HAO/NCAR͒, making use of an open boundary at the OPAl equation of state isochrones are reduced by 6-7 the top, and the Eddington approximation for the radiative using the standard isochrones. transport. This more realistic simulation should ultimately Graduate student S. Yi ͑with Demarque͒ has constructed a provide a prescription for modeling the highly superadiabatic grid of post-horizontal branch evolutionary tracks for metal- layers in stars. licities ranging from Zϭ0.0001 to Zϭ0.1, using the OPAL opacity tables and up-to-date physics. These evolutionary 5.5 Stellar Structure and Evolution tracks were constructed for the purpose of constructing syn- Graduate student S. Barnes with Sofia studied the origin thetic population models and, with the help of the model of the ultrafast rotating stars in young star clusters. They atmospheres ͑e.g. the models of Kurucz͒ or empirical spec- found that such objects require a magnetic braking of the tra, for calculting the integrated spectral energy distribution pre-main sequence stars whose strength ͑usually propertional of stellar populations. to rotation rate͒ saturates at a mass-dependent value so that, Demarque and Guenther ͑Saint Mary’s University, Hali- above this value, losses do not depend on velocity. fax, Nova Scotia͒ have continued their research on stellar Kim and Demarque have completed a calculation of turn- seismology. Stimulated by the announcement of likely detec- over times for stars of solar metallicity in the pre-main se- tion of p-mode oscillations in the subgiant Eta Bootis ͑Kjeld- quence and early post-main sequence phases of evolution. sen et al.1995,AJ,109,1313͒, they constructed a grid of evo- The models were constructed with up-to-date physical input, lutionary sequences that satisfied the conventional and included rotation during the evolution, thus making it astronomical ͑parallax, magnitude͒ and astrophysical ͑spec- possible to calculate the Rossby number of each model along troscopic abundance͒ constraints for this star. The observed its evolutionary track, based on its calculated rotation rate p-mode frequencies were then shown to constrain the mass, and its local convective turnover time near the base of the parallax, helium abundance and age of Eta Bootis much convection zone. Global turnover times were also computed. more strictly than the conventional constraints. 740 ANNUAL REPORT

Deliyannis with R. Malaney ͑CITA͒ studied the possibil- bined with tidal ͑Zahn & Bouchet 1989, A&A, 223, 112͒ ity that flares might produce the 6Li isotope in halo dwarfs considerations : young cool disk and old intermediate metal- near turnoff. Motivated by the recent report of a 6Li detec- licity dwarfs, young and old Li peak dwarfs, and old cool tion in the atmosphere of HD 84937, they coupled stellar intermediate metallicity and halo subgiants. Deliyannis and evolution calculations with light isotope production via stel- Ryan concluded that some synchronized binaries with peri- lar flares. They found that as a consequence of their small ods less than about 6 days, may be better able to preserve convective envelope mass near the turnoff, low metallicity their Li abundances, in line with Yale rotational model pre- dwarfs and subgiants may possess observable amounts of dictions. Suitable short period binaries in the Teff region of 6Li arising from such flare activity. An observational test the halo Li plateau still await identification and observation. was pointed out which could discriminate between flare pro- Deliyannis with J. King ͑University of Texas͒, A. Boes- duced 6Li and protostellar 6Li. In the Teff range 6000 - 6600 gaard ͑University of Hawaii͒, and S. Ryan ͑Anglo Australian K, the 7Li/6Li ratio on the subgiant branch should increase Observatory͒ observed lithium in a short period binary of the as a function of Teff if flare production is important, whereas old open cluster M67, which is in the ‘‘Li Peak’’ region ͑late the same ratio should be constant if a protostellar origin is F- early G stars͒. This region is especially important as it the source of the observed lithium. The absence of a flare contains the best ͑though not perfect͒ Li preserving stars, produced variation in the 7Li/6Li ratio would allow for a between the Boesgaard gap at higher Teff and the solar type more reliable inference of the - cosmologically important - and cooler dwarf Li depleters at lower Teff . From the Yale atmospheric depletion mechanisms in stars. models’ point of view, the Li peak is completely analogous Deliyannis with S. Ryan ͑Anglo Australian Observatory͒ to the halo Li plateau. The Li abundance is observed to be studied Li abundances in short period binaries to test internal lower in open clusters of older ages, and still slightly lower stellar mixing. Stellar evolution theory with rotation, as de- in the halo dwarfs. Does this reflect Li depletion with age ͑as veloped at Yale in recent years, makes some rather dramatic indicated by the Yale rotational models͒ or Galactic Li en- predictions about the evolution of the light element tracers. richment ͑as required by standard big bang nucleosynthesis͒? This includes the prediction that primordial lithium (Li) has The M67 binary was observed to specifically address this been significantly depleted to the abundance observed today issue. At least one of the components, and possibly both, was in halo dwarfs, with implications for cosmology. More gen- ͑were͒ observed to have higher than average Li abundance. erally, the Yale models predict that rotational mixing ͑related This provides strong evidence that stellar Li depletion, and to angular momentum loss and redistribution͒ is a cause of not Galactic Li enrichment, is chiefly responsible for the Li destruction in normal, single late-type dwarfs. In this decline of the Li peak with age ͑though the errors do allow study the models were tested through the complementary and for some enrichment͒. While this is suggestive, however, it striking prediction that certain short period binaries should does not provide direct evidence that the primordial Li abun- have preserved more Li than the mean trend of normal, dance is high, as might come in the future from suitable halo single stars. 7 short period halo and disk binaries were ob- short period binaries. served at various telescopes to discover the conditions under Deliyannis found that a comparison between recently which Li rich binaries may be found. A halo dwarf and a published halo Li subgiant data and his previously published halo subgiant with periods of 12.4 and 7.6 days have Li stellar evolutionary models yields quite favorable agreement abundances on the mean halo Li abundance trend ; these down to about 5000 K. This supports the predicted deepen- orbits probably synchronized too late to prevent mixing- ing of the surface convection zone with evolution past the induced Li depletion. A 5.6 day period halo subgiant, how- main sequence turnoff, and the establishment of a maximum ever, does have a higher Li abundance than otherwise similar depth of penetration ͑by mass͒ after which the convection stars, as also previously reported. An intermediate metallicity zone is pushed outward by the advancement of the hydrogen 1.8 day subgiant is found with a measurable Li abundance at burning shell. Below 5000 K, both the Li abundances and a Teff where most stars yield only low upper limits. The the 12C/13C ratio plummet, and the C ratio attains the CN remaining 3 disk stars, with 3.3 to 6.6 day periods, yielded equilibrium value of 3.5. These observations can be under- only upper limits to their Li and suitable single stars against stood in terms of rotational mixing on the giant branch. which to compare them were lacking. Previously observed Then, 3He would in fact be destroyed on the giant branch, short period binaries were also examined, not all of which not preserved as has been previously supposed, and this have been appreciated as such. High Li abundances were would move the usual Dϩ3He constraints on BBN to lower found for an intermediate metallicity cool dwarf, and also for baryonic density. Consistency in standard BBN might then three Hyades cool dwarfs ͑where the Hyades is old enough be possible with ‘‘high’’ primordial D ͑as recently inferred for its single stars to be depleted͒, but not in the younger from the line-of-sight to a quasar͒, the revised Dϩ3He con- Pleiades, all as predicted. The Yale models further predict the straint, ‘‘high’’ primordial 7Li and the usual ‘‘low’’ primor- gradual depletion of the Li peak region in open clusters, in dial 4He estimates. In this case, the implied baryonic densi- analogy to the intermediate metallicity and halo Li plateaus. ties would be just above those suggested by luminous matter Two short period binaries were found to lie above the Hy- ͑the Hubble constant could also be constained in this sce- ades Li peak, and one above the much older M67Us Li peak, nario to values below ϳ 90͒ and non-baryonic dark matter supporting the predicted depletion of the Li peak. High Li would definitively be needed in galactic halos and clusters. short period binaries have thus been observed in a variety of Another possibility might be the need for additional physics contexts, consistent with the Yale rotational models com- in BBN, such as different neutrino physics or inhomogene- YALE UNIVERSITY 741 ities. In fact, inhomogeneous BBN can easily accomodate covery. Even if none are found the single maxima are low 4He, high Lip, and the D and 3He constraints ; how- insufficiently observed to reveal a typical MACHO light ever, implications for dark matter are not clearcut. Still other curve. scenarios are possible. Among the variety of special catalogs included in the Deliyannis with S. Ryan ͑Anglo Australian Observatory͒ SPM program are the blue metal-poor main sequence ͑BMP͒ continued their analysis of Li in cool ͑Teff Ͻ 5500 K͒ halo stars found by Preston, Beers and Shectman. In collaboration dwarfs, observed at the CTIO 4m telescope. The morphology with T. Beers ͑Michigan St. U.͒ the absolute proper motions of the Li-Teff relation and its dependence on metallicity can for 768 BMP stars have been derived in the SGP region. potentially distinguish between standard and rotational mod- Once their radial velocities and photometric parallaxes have els ͑and teach us a few other things about stellar interiors͒. been determined ͑now in progress at Michigan St. U.͒ it will Preliminary assessment of the data suggests that the Li-Teff be possible to derive their space velocities. The BMP stars slope is shallower than previously thought. If true, no pub- exhibit kinematics which do not indicate membership in ei- lished models would be consistent with this, although rota- ther of the known Galaxy populations and as suggested by tional mixing might be favored qualitatively. Preston, Beers and Shectman the BMPs might be young Deliyannis with J. King ͑University of Texas͒ and A. metal-deficient stars shorn from collisions between dwarf Boesgaard and A. Stephens ͑University of Hawaii͒ continued galaxies and the Milky Way. obtaining ͑at the CFHT 3.6m and the Keck 10m telescopes͒ Wu has completed the PDS measures begun by under- and analyzing Li and Be data in F stars. The relevant prob- graduate S. Dyson of the 300 plates of Procyon gathered lem is the origin of the Boesgaard Li gap: a region of only a from the collections of numerous observatories. Wu, Lee and few hundred degrees where the Li abundance drops precipi- Girard have modified the Yale parallax reduction software to tously by orders of magnitude during the main sequence, in allow the parallax and orbit solutions to be performed in blatant contradiction to the strictures of standard stellar mod- parallel and, using an iterative approach, arrive at the best els. Numerous solutions have been proposed, including mass determination of each. Preliminary reductions have indicated loss, miscroscopic diffusion ͑which could also lower globu- systematic differences between the various observatory plate lar cluster ages͒, slow mixing by gravity waves, and various series, perhaps due to magnitude equation. Two of the forms of rotationally induced mixing ͑which could raise es- anomalous plate series contain grating images, providing a timates of Li͑p͒͒. The Li/Be pattern observed in the star 110 means of correcting these plates for magnitude equation. Her can potentially discriminate among these possibilities: van Altena, J. Lee, and Hoffleit finalized the astrometric 110 Her is depleted in Be ͑by a factor of5-10͒yet retains portion of the new edition of the General Catalogue of Trigo- some Li ͑which is depleted by a factor of ϳ 100͒. This nometric Parallaxes ͑The Yale Parallax Catalog͒. Currently, surprising depletion pattern, if representative, would elimi- final checks are being made for spurious errors or omissions nate mass loss and diffusion, and strongly support slow mix- in the cross- identifications and auxiliary data. The new edi- ing ͑in fact, the Yale rotational models’ quantitative predic- tion of the YPC contains 15,993 determinations of parallaxes tions agree very well with 110 Her͒. The project was thus for 8,110 stars. Auxiliary data including cross-identifications, untertaken to establish whether or not 110 Her is representa- positions, photometry, spectral types and luminosity classes, tive, and therefore holds the key to the solution. So far, hun- 7 and data on the duplicity and variability of the stars are also dreds of stars have been surveyed ͑at R ϭ 80,000͒ for Li, given. In addition, a reference to the publication for each and many have been observed for Be. Although analysis is parallax determination is listed so that the user can examine ongoing, it is clear that many examples of 110 Her type the original paper to extract further information depletion can be found, and further that Li and Be depletion J. Lee and A. Upgren are continuing the proper-motion occurs concomitantly with more Li depleted than Be; this study in the central region of the Hyades cluster referred to would strongly support slow mixing, as indicated above. in last year’s Report using the Van Vleck 0.5-m refractor. J. Lee, A. H. Lee and A. Upgren have measured and are 5.6 Stars now reducing Van Vleck parallax series of four Hyades white Hoffleit divided her time among historical surveys, further dwarf members, vA292, vA490, vA673 and vA722, and one thoughts on checking for MACHOS among bright stars Hyades member star, vA587. needing further observations, updating the Bright Star Cata- Upgren, A. Morales, J. Herrero, J. W. Griese, J. M. Vin- logue ͑on which Wayne Warren of NASA is contributing the cent ͑all at Wesleyan University͒, and J. T. Lee have studied lion’s share͒, helping van Altena and John Lee on sleuthing the feasibility of using a CCD to replace photographic plates for errors and omissions among the stars in the up-coming with the Van Vleck refractor and concluded that a CCD of Yale Catalogue of Trigonometric Parallaxes, and checking 2048 by 2048 pixels would provide a sufficient reference references and data requested by observers, especially on frame for almost any star field. Stars to visual magnitude puzzling variable stars. She presented a summary of her 14.5 could be reached without distortion since the focal search for MACHOS in the Bright Star Catalogue at the curve of the refractor is fairly flat throughout the visual and annual meeting of the AAVSO. As expected, no definite can- red spectral regions. didates were found but there still remain early discoveries by Guo and van Altena, with the collaboration with the HST AlSufi and by Wilhelm Struve to be checked on early pho- astrometry team, continued to work on collecting FGS data tographs to verify whether or not subsequent maxima oc- for measuring the parallaxes and proper motions of selected curred after the times of the original announcements of dis- Hyades cluster stars. The third and fourth epoch observations 742 ANNUAL REPORT were successfully obtained in August 1994 and March 1995, have worked at determining a reliable calibration of the scale respectively. The new proposals for the fifth and sixth epoch and alignment of the Speckle Interferometer on the two tele- observations in September 1995 and March 1996 were sub- scopes based both on drift scans and calibration mask obser- mitted and scheduled as requested. The complete series con- vations. As mentioned in the Instrumentation part of this re- sists of six epochs of observations and are expected to be port, van Altena has been developing a ray tracing model of finished by the mid-1996. Astrometric pipelines, different the instrument as an aid to understanding its sensitivity to the field drift models, and other relevant precorrections were potential misalignment of the various components. tested and applied to the FGS POS mode data. Preliminary In order to simplify the observations and increase the pro- results for the parallaxes and proper motions of the Hyades ductivity, a new Speckle Interferometer has been designed cluster were presented at the Space Telescope Astrometry specifically for the above two telescopes. A proposal was Team meetings. submitted to the NSF for support of its construction and a Graduate student J. Zhang, van Altena and Girard began grant was received from the University of San Juan to begin an astrometric search for new very faint Hyades Cluster work on the design and preliminary construction during the members using HST WFPC-2 parallel data that was taken in latter part of the calendar year 1995. conjunction with GTO FGS parallax observations of several Graduate Student R. Meyer has started a Ph.D. disserta- Hyades cluster members. The long exposure times ͑700 to tion in which he intends to accurately measure the magni- 1000 sec.͒ used for the frames will enable them to search tudes of the components of nearby binary star systems to several Hyades center regions to approximately V ϭ 24. The place better constraints on the low-mass end of the mass- proper motions are derived from a time baseline of roughly luminosity relationship. Short- period binary systems gener- one year with two exposures at each epoch ͑F606W and ally provide the most precise mass estimates, but unfortu- F814W͒. After deleting cosmic rays around or near galaxies nately this property usually requires that the two components and stars and correcting for the optical field angle distortion, be too close to each other on the plane of the sky to be they used DAOPHOT to obtain the image centroids and ap- resolved by traditional techniques. Hence one must resort to, erture photometry. They found that the WFPC-2 system was for example, Speckle Interferometry, to resolve the compo- stable enough during one year’s orbit that they were able to nents of the binary and a current problem with that technique derive a mosaic solution for the three WF CCD chips. The is the inability to determine the magnitude difference be- standard error of a single position in one coordinate of the tween the two components to a reasonable precision. This is CCD mosaic is 9 mas for an average apparent magnitude of mostly due to inherent difficulties with analyzing the data 23 mag in the wide-V F606W filter. There is some evidence observed with the speckle cameras most commonly in use, that the precision of the relative positions will be improved which implement ICCD detectors. These difficulties are not after allowing for small relative motions of the three chips present with the ‘‘MAMA’’ ͑Multi- Anode Microchannel Ar- over the one year period. They are now improving the astro- ray͒ detector currently on loan to Yale from Stanford. Nev- metric solutions for the 147 objects in the four fields and will ertheless, a lengthy ͑although straightforward͒ calibration then search for candidate Hyades members based on the re- procedure is necessary for reliable, reasonably accurate mag- sulting proper motion distributions. nitude differences to be measured with this MAMA-based speckle camera. Meyer has begun this calibration process, 5.7 Binary Stars working on a study of the effect of channel saturation of the MAMA detector. This study is meant to extend the work in Horch, in collaboration with Girard, van Altena, Franz Horch’s Ph. D. dissertation and will hopefully fully define Lowell Obs. ,C.Lo´pez, J. Torres, H. Le´pez and C. Francile, ͑ ͒ the capabilities of the Stanford system for astrometry and the later four of the University of San Juan, Argentina, and differential photometry of double stars. As a part of this ef- G. Timothy Univ. of New Brunswick continued with their ͑ ͒ fort, Meyer has also developed an orbit calculation program program of Speckle Interferometry of Southern Hemisphere that treats the errors of observation in what he hopes will be binary stars. During the past report year they were scheduled a realistic manner. on the Cesco Observatory 76-cm telescope for 39 nights and the CASLEO 2.15- m telescope for 12 nights, yielding 577 5.8 Open Clusters observations of 421 double stars, of which approximately 95% are located south of the equator. Given their experience One of the SPM fields in the SGP region contains the during this year, they expect to have an output of 1000 to bright open cluster Blanco 1 which is important in several 2000 observations per year. The reductions of the data have respects. The combination of its high galactic latitude ͑bϭ- been carried out by Horch with the assistance of Dinescu and 79.3 deg͒, large z-distance (ϳ 250 pc͒, young age ͑lg tϭ7.5͒ a paper reporting the first results is nearly complete. Horch and possibly unusual luminosity function makes Blanco 1 developed fitting software for the autocorrelation functions unique. The presence of this cluster on the SPM plates makes based on a chi-squared minimization technique and they are it possible them to calibrate the magnitude equation indepen- using this software to obtain precise instrumental position dently over a wide range of magnitudes, 7- 17 mag. and angles and separations. He has also developed a routine compare the calibration with ones derived through the use of which computes directed vector autocorrelations from the the grating images referred to earlier in this report. In addi- MAMA data, so that the position angles can be determined tion to the standard set of four SPM plates, four more unabiguously without recourse to the computer intensive intermediate-epoch plates as well as sets of 5‘‘ x 7’’ plates process of full image reconstructions. Both Horch and Girard taken in 1967 and 1995 with the same telescope have been YALE UNIVERSITY 743 measured. Due to the large number of images available for V for V Ͻ 17.5 and 0.009 in ͑B-V͒. For the faint stars with the proper motion solution, the accuracy of the individual V Ͼ 17.5 the standard error is two times larger. The main proper motions for BϽ14 mag is as good as 1 mas/yr. An sequence and red-giant clump are clearly seen, however, as excellent agreement of preliminary proper motion cluster before the lower part of the main sequence ͑V Ͼ 15.0͒ is still membership with respect to a radial-velocity survey con- difficult to identify with confidence. The new color- ducted by J.-C. Mermilliod ͑Switzerland͒, has been found. magnitude diagram has been used to re-determine the age of I. Platais, Vansevicius ͑Lithuania͒, Paupers, Abolins NGC 3680 ͑see Stellar evolution section͒ as 1.6 Gyr. ͑Latvia͒ have completed a photometric study of the open cluster NGC 7209 in the Vilnius system. In total 96 probable 5.9 Globular Star Clusters cluster members, selected from the proper-motion study, Zinn is the P.I. of a HST GO project to measure the ages have been measured. The photometric data provide an inde- of 4 metal-rich and 3 metal-poor globular clusters belonging pendent estimate of spectral types, absolute magnitudes, in- to the disk and halo systems respectively ͑Co-I’s are P. De- terstellar reddening and metallicity. The cluster’s distance is marque,; B. Carney and L. Fullton, UNC; G. Da Costa, Mt. almost exactly 1 kpc but the data do not support the exist- Stromlo; J. Heasley, U. of Hawaii; K. Janes, B.U.; C. Chris- ence of a second cluster at a distance of 760 pc as reported tian, Berkeley; E. Olszewski; Steward Obs.; P. Seitzer, by Pena and Peniche ͑Mexico͒. Michigan͒. Observations of the metal-rich cluster NGC 6352 I. Platais, van Leeuwen ͑UK͒ and Mermilliod ͑Switzer- were obtained before the HST repair mission, and the aber- land͒ prepared a review on high- accuracy astrometry’s im- rated images of these data were reduced using an iterative/ pact on the research of star clusters and associations pre- recursive deconvolution algorithm that was developed for sented at the Cambridge workshop ‘‘Future Possibilities for this purpose at the University of North Carolina. The color- Astrometry in Space.’’ The Workshop was organized by the magnitude diagram ͑CMD͒ that was obtained from these Royal Greenwich Observatory and the European Space data reached fainter than the main-sequence turnoff and Agency to explore the scientific and technological aspects of yielded a reasonably precise estimate of the cluster’s age, the future astrometric space mission ͑GAIA͒ aimed at 10 which to within the errors is identical to the well-studied micro-arc-second accuracy in positions, proper motions and cluster 47 Tuc ͑Fullton et al. 1995, A.J. 110, 652͒. The other parallaxes for 50 million stars down to Vϭ15 mag. clusters have now been observed after the repair mission, and Dinescu, Girard, van Altena, Yang ͑Shaanxi, China͒ and from these much improved data sets, exquisite CMD’s are Y.-W. Lee ͑Korea͒ have completed a long term determination being obtained. Preliminary results for NGC 5927, which is of the relative proper motions for 1127 stars in the field of significantly more metal rich than 47 Tuc and NGC 6352, the open cluster NGC 188, from plates taken with the 76-cm indicate that it is slightly younger. Thaw refractor of the Allegheny Observatory. The plates The horizontal branch ͑HB͒ morphologies of globular span a 60-year interval and the study is complete to magni- clusters have long been mysterious because of the fine struc- tude B ϭ 16.2. Special attention has been paid to the correc- ture in some of them ͑e.g., the presence or absence of ‘‘gaps’’ tions for systematic errors as a function of magnitude and and ‘‘blue tails’’͒ and because the overall morphology ͑e.g., color. The final relative proper motions have an accuracy of whether the majority of stars are bluer or redder than the 0.3 mas/yr for well-exposed stars. Given the large proper- instability strip͒ is related to some ‘‘second parameter’’ in motion distribution of this cluster, a more appropriate tech- addition to Fe abundance. The second parameter effect is nique was used to determine the membership probabilities, related galactic structure for there is well-determined gradi- which involved the use of individual proper-motion errors in ent in its strength with galactocentric distance. Zinn has be- estimating the probabilities, rather than the overall proper- gun an investigation of previously poorly studied globular motion distribution. These probabilities were used to better clusters in order to increase the sample with well-measured define the color-magnitude diagram and to study dynamical HB morphologies. With graduate student Sydney Barnes, he effects such as mass segregation in this cluster. They identi- has constructed CMD’s of NGC 6139 and NGC 6426 from fied nine highly probable candidate blue stragglers in addi- observations obtained with the CTIO 0.9m telescope. Al- tion to the sample proposed by Eggen and Sandage in 1969. though the CMD of NGC 6139 is affected by substantial In addition, they found that the population of possible blue differential reddening across the field, it shows that the clus- stragglers was not more centrally concentrated than the red ter has a blue HB. The cluster is therefore similar in HB giants thus suggesting binary collisions ͑Leonard and Linnell morphology to other clusters of its metal abundance and ga- 1992͒ as a possible mechanism to produce a not-yet-relaxed lactocentric distance. The CMD of NGC 6426 confirms that population of blue stragglers in this old open cluster. it is one of the most metal-poor clusters known. Its HB is Kozhurina-Platais, in collaboration with graduate students predominantly blue, but unlike M15, which it resembles in J. Orosz and S. Barnes have processed CCD observations of ͓Fe/H͔, there is no evidence of a long blue tail. the open cluster NGC 3680 taken in February 1993 with the Dinescu continued here Ph.D. thesis research, which con- CTIO 0.9 m telescope in B- and V-bandpass. An area of 26 sists of determining absolute proper motions with respect to arcmin square centered on the cluster was covered by a mo- faint galaxies for a sample of globular clusters located in the saic of four 2048x2048-chip snapshots. All reductions of the southern sky using SPM plate material. The study of the first CCD photometry were performed using the standard fixed- two SPM fields containing NGC 288, led to a better under- aperture routine from the IRAF software package. The final standing of the errors involved in working with this plate star-list of 2176 objects has a standard error of 0.009 mag. in material as well as new techniques to minimize them. The 744 ANNUAL REPORT most important errors affecting both the correction to abso- intercloud medium; at the same time, cloud debris that is lute proper motion ͑or the zero point͒ and the relative motion dispersed in neutral shells or fragments accounts for much of of the cluster with respect to faint stars, are the magnitude the cloud structure of the ISM. Both the clouds and the in- equation and the modelling error involved in the plate trans- tercloud gas are created with velocities of the same order as formation. Making use of some of the techniques recently the observed velocities of 10 km/s, which is the sound speed developed at Yale for the study of proper motions in clusters, in the ionized gas. The pressure of the atomic clouds and the she can correct for magnitude equation given a well- intercloud gas are regulated to remain near the maximum populated cluster. In order to overcome the errors due to pressure that the intercloud medium can sustain without re- plate modeling she has developed a method which uses local, condensing back into clouds, in agreement with observed independent reference systems for each target object and ap- pressures. Massive molecular clouds have much higher pres- plied these techniques to NGC 288, NGC 1851 and NGC sures than this, which are generated by the collisonal ag- 6752 and obtained very good results for the zero point ͑ glomeration of smaller atomic clouds in spiral arm regions. agreement to within 0.7 mas/yr from blue and yellow pairs͒. The resulting typical pressure in star-forming clouds deter- The accuracy of the relative motion of the cluster ͑from one mines a characteristic Jeans mass for fragmentation which is plate pair͒ is limited by the number of cluster stars and it of the order of one solar mass, as observed. The highest ranges from 0.3 to 1.0 mas/yr. The next clusters ready to be pressures are found in the dense and massive cloud cores measured are NGC 6584 and NGC 6362 for which the sec- where dense clusters of stars form, and these regions may be ond epoch plates have already been taken. Eight additional compressed to exceptionally high densities by the energetic clusters will be measured in the coming years. effects of prior episodes of star formation in the same vicin- Girard, Li, and van Altena, along with J. Nu´n˜ez and A. ity. Prades of the University of Barcelona, continued to explore techniques for obtaining optimal astrometric precision from 5.11 Galactic Structure HST Planetary Camera 1 images of several globular clusters. ´ The goal is to use existing PC 1 images and future PC 2 Mendez completed his Ph.D. Thesis in which he devel- images to determine relative proper motions precise enough oped a new kinematic model of the Galaxy. In his thesis he to derive the tangential velocity distribution within these compared the preliminary results from the SPM towards the clusters. Much effort has been devoted to the use of image SGP and showed that there was excellent agreement between reconstruction of the PC 1 images, including reconstructions the observations and the model proper-motion distribution in performed on a ‘‘super-resolution’’ grid, i.e., a finer grid than the U component ͑U positive toward the Galactic center͒.In the actual PC pixel spacing. The validity of the astrometry the V component ͑V positive toward Galactic rotation͒ there based on the reconstructed images has yet to be conclusively is a systematic difference of 2 to 4 mas/yr between the ob- demonstrated. servations and the model. On the other hand, he found good agreement between the observations and the model in the V B. Chaboyer ͑CITA͒, Demarque and A. Sarajedini component of the proper motion dispersion. In the U com- ͑NOAO͒ have combined recent Yale isochrone calculations ponent there is a tendency for the model to underestimate the with a variety of relations for Mv͑RR͒ to calibrate age as a function of the difference between the magnitude of the turn- predicted proper motion dispersion, particularly at the bright- off and the horizontal branch. There is no evidence for an est bins. Since in the model the proper-motion lag in V is age-Galactocentric distance relationship among the 43 globu- proportional to the velocity dispersion in U ͑via the asym- lar clusters considered. An age-metallicity relation exists, metric drift equation͒, an underestimation in the velocity dis- with the metal-poor clusters being the oldest. The age distri- persion in the U- component would produce a smaller than bution function reveals that there is a range of about 5 Gyr expected proper motion lag, as is indeed found. In order to fit among the bulk of the globular clusters. In addition, 10 sub- the observed proper motion dispersion in U, the model re- stantially younger, and including them in the analysis in- quires an unreasonable increase of 40 velocity dispersion in ´ creases the age range to 9 Gyr. These statements are inde- U for Disk main-sequence stars. Mendez has explored the origin of these discrepancies with the aid of the model and pendent of the Mv͑RR͒ relation. Second parameter clusters are found to be about 2-3 Gyr younger than the older clus- the detailed results of that investigation are reported in his ters. These results suggest that globular clusters were formed theses and are now being prepared for publication. over an extended period of time, with progressively more Guo completed his Ph.D. Thesis which was an investiga- metal-rich globular clusters being formed at later times. tion of Galactic structure, kinematics and chemical abun- dance distribution towards the South Galactic Pole. The proper motions were derived from CTIO 4-m prime focus 5.10 Interstellar Medium and Star Formation plates taken from 1981 to 1992 after accurately modeling the In a review of the interplay between star formation and optical field angle distortion of the prime focus relative to an the interstellar medium ͑ISM͒, Larson proposed that the vari- astrographic plate as a flat field. The magnitude and color ous feedback effects of star formation on star-forming equations were corrected using NGC 288 members as stan- clouds, including the effects of ionization, stellar winds, and dards and the zero-point of the absolute proper motion was supernovae, can account for both the structural and the ther- defined by several hundred galaxies. Photometric parallaxes, mal properties of the ISM. Ultraviolet radiation from hot stellar tangential velocities and dispersions were then derived stars ionizes and evaporates away much of the mass from and that data has enabled him to probe the stellar spatial, these clouds, and this replenishes and pressurizes the warm kinematical and chemical distributions to very large vertical YALE UNIVERSITY 745 distances. He has shown that the thick disk as an intermedi- bulge S0 within ϳ1 Gyr. Among the more unusual features ate component is necessary and distinct from the old thin of NGC 4424 is the banana-shaped stellar distribution, which disk and spheroidal halo. It has a scale height of 1 kpc, a to our knowledge has not been observed in any other galaxy. local density ratio to the thin disk of 0.03, a mean asymmet- The highly flattened remnants produced in some merger ric drift velocity of 60 km/sec, and a mean metallicity of simulations are subject to a bending instability, which can ͓Fe/H͔ϭϪ0.58Ϯ0.06. The lack of a significant rotational produce a symmetric, banana-shaped stellar distribution. In velocity gradient and the small or zero metallicity gradient the simulations, the banana-shaped morphology is not long- suggest that the thick disk is the product of some heating or lived, and the remnant subsequently forms an X-structure merger event͑s͒ that occurred in the young Galaxy. For the and contains boxy isophotes. The case of NGC 4424 sug- halo, which takes over from the thick disk as the dominant gests that minor mergers may be an important process for component at 5.5 kpc, the local density ratio to the thin disk transforming spirals galaxies into S0’s in clusters. Although is 0.1-1.66Ϯ0.09. The mean rotational velocity is Vrot ϭ mergers do not occur at the high velocities which character- 32Ϯ7 km/sec, indicating that the halo in mean is in prograde ize many of the interactions that occur in clusters, there may rather than retrograde motion. No metallicity and kinemati- be enough low velocity interactions in clusters to make cal gradients or correlations were found for the halo. Finally, merging an important process. as a by product of this investigation, the space motion of Koopmann and Kenney continued their study of massive NGC 288 was determined to be ͑u,v,w͒LSR ϭ star formation in a sample of 63 Virgo cluster and 35 isolated (31Ϯ15,Ϫ263Ϯ16,54Ϯ1) km/sec, indicating that it is mov- spirals. H␣ and broadband R images, as well as rotation ing on a retrograde orbit. curves derived from optical spectra, are being analyzed in an Among the catalogs of stars included for measurement in attempt to better understand the role of environment in gal- the SPM, there are several in which Halo giants have been axy evolution. Observations for this study were completed at identified and both distances and radial velocities deter- CTIO and KPNO in February 1995. A comparison of H␣ mined. The space velocities derived from those distances and radial distributions shows a larger scatter among Virgo Clus- the new SPM absolute proper motions are now being used by ter Sc galaxies than isolated Sc galaxies, especially in the van Altena and M. Miyamoto ͑Tokyo͒ to analyze the Gal- inner disks ͑Rр0.4R25). This indicates that star formation in axy’s kinematic and dynamical structure perpendicular to the the inner galaxy as well as the outer galaxy is affected by plane, as well as the orientation of the Halo velocity ellipsoid cluster processes. Surprisingly, the mean H␣ surface bright- as a function of distance from the galactic plane. ness of the Virgo Sc galaxies is 2 times HIGHER than the mean surface brightness of the isolated sample. Much of this 5.12 Galaxies difference is caused by the high surface brightnesses of sev- Oemler and collaborators J. Schombert ͑NASA͒, R. Pildis eral galaxies known to be tidally interacting and several HI- ͑Michigan͒, and J.A. Eder ͑Arecibo͒ have reported the exist- rich Virgo Sc galaxies located more than 6 degrees from ence of a new class of galaxies, dwarf spirals. Although they M87. Some of these HI-rich galaxies have extended, irregu- have s similar morphology to their giant counterpars, the lar HI distributions, suggesting they may have recently ac- creted gas. Of special interest are HI-deficient galaxies which dwarf spirals have faint luminosities (M BϾϪ17), small di- have truncated gas disks. We find that Virgo cluster galaxies ameters (R26Ͻ5 kpc͒ low central surface brightnesses ␮B Ͼ Ϫ2 9 deficient in HI gas have truncated star-forming disks com- 24 mag arcsec , and low HI masses M HIϽ10 M ᭪ . These objects are not abundent: they represent a small fraction of pared to HI normal Virgo cluster and isolated galaxies. The field dwarfs, and are apparently absent from rich clusters. inner star formation distributions of many of these galaxies Kenney and Koopmann, in collaboration with V. Rubin are similar to HI normal galaxies, consistent with a stripping ͑DTM͒, and J. Young ͑UMass͒ undertook an imaging and process efficient only in the outer disk. However, a few of kinematic study of the relatively obscure Virgo cluster Sa these galaxies ͑e.g., NGC 4450 and NGC 4548͒ are deficient galaxy NGC 4424, which suggests that this peculiar system in star formation across the entire disk. The most extreme HI is the product of a merger. The broadband R image reveals deficient galaxies contain only circumnuclear star formation, banana-shaped isophotes, shell-like features and other com- whose intensity in some cases is enhanced with respect to plex structure generally associated with mergers. The only other Virgo spirals. It seems that a variety of environmental H␣ emission arises from a bar-like structure containing a processes, including gas stripping, tidal interactions, and re- few bright HII complexes located within 500 pc of the cent gas accretion, are important in the Virgo cluster. nucleus and inside the bulge-dominated region. Although the The study of star formation in barred galaxies can provide main stellar body of NGC 4424 is highly elongated in pro- additional evidence for recent tidal interactions. Most of the jection, and the outer part of the galaxy has a disk-like ex- barred galaxies in both the Virgo cluster and isolated samples ponential light profile, gas velocities are remarkably low in harbor strong central H␣ emission, but little other emission the central kpc, indicating strong non-circular motions or within the bar region. However, some barred galaxies show complex geometry for the inner gas. The peculiar properties strong star formation along the bar, which is a relatively rare are consistent with an intermediate mass ratio ͑0.1-0.5͒ phenomenon. These patterns of star formation in barred gal- merger. The merger may be responsible for the creation of axies can be explained by an evolutionary sequence, based the bar, as is found in minor merger simulations. The degree on the recent simulations. Strong star formation along a bar of morphological peculiarities suggest that the merger is re- occurs only in young bars, since bars drive gas to the center cent, and we propose that the galaxy will become a small- of galaxies. Galaxies with star formation along a bar may 746 ANNUAL REPORT therefore be in a short-lived evolutionary phase. This is es- structed. The first prototype camer contains a 4ϫ4 array of pecially interesting in the study of cluster galaxies, since bars Loral 2048 square ccd’s with 15␮ pixels. This pixel size can be created in tidal interactions and mergers. A compari- corresponds to 1 arc second on the sky. The dewar and data son between the barred galaxies in our isolated and Virgo system are being constructed at Yale, and the CCD control- cluster sample reveals a trend for star formation along the bar lers are being built by J. Musser at Indiana University. In- to be more common and somewhat stronger in Virgo Cluster cluded in the camera is a new field flattener lens which is galaxies. This suggests that ongoing tidal interactions signifi- free of distortions over the entire 5 degree field of the tele- cantly influence the morphology and evolution of cluster spi- scope. In drift scan mode the telescope is fixed and the ccds, ral galaxies. aligned East - West, are clocked at the sidereal rate. In this Jogee and Kenney continue their study of molecular gas mode, the camera ϩ telescope are capable of surveying 60 and star formation in the circumnuclear ͑inner kiloparsec͒ square degrees per hour, between declination limits of plus regions of starbursting and other spiral galaxies. Circum- and minus 8 degrees. Each of the 4 rows of 4 chips can be nuclear regions are time-dependent systems whose morphol- covered by a different filter, permitting simultaneous 4-color ogy and dynamics can change significantly over less than a photometry. The second camera will be capable of surveying Hubble time, especially under the effect of gas inflows to- between plus and minus 25 degrees, with a resolution of 0.5 wards the center. The goals of our study are to understand the arc second. For the survey, direct UBV imaging and objec- panoply of CO morphologies displayed by the circumnuclear tive prism images will be taken. These images will be used to regions of starburst and non starburst galaxies, the temporal construct a catalog of QSO’s, down to a limiting magnitude evolution of the CO morphologies, and the creation of new of roughly 20. Gravitationally lensed candidates will be iden- stellar components. As part of this study, we have mapped 3 tified from this set of QSO’s, and observed in more detail starburst galaxies NGC 4102, NGC 4536 and NGC 2782 in with WIYN and other telescopes. This survey will produce CO͑Jϭ1-Ͼ0͒ with the OVRO millimeter-wave interferom- many byproducts: essentially any class of stellar object eter at a resolution of 2 . For NGC 4102, the radio con- which can be identified from its UBV colors and low disper- tinuum, H␣ and CO dataЉ reveal that the star formation rate sion spectrum. It is the intention of the collaborators to pub- per unit gas mass remains high in the central 150 parsecs but lish an object catalog as quickly as possible to enable other shows a significant drop where there is a discontinuity in the investigators to take advantage of it. As of November 1995, gas kinematics. The region of strong star formation is defi- the prototype camera is nearing completion. It is hoped that nitely inside the Outer Inner Lindblad Resonance ͑OILR͒ the camera can be mounted on the telescope by early 1996 so and may extend inside the Inner Inner Lindblad Resonance that some data can be obtained in that year. ͑IILR͒. In the region of star formation, the Toomre Q param- eter is close to 1. This suggests that the simple gravitational 5.14 High Energy Astrophysics instability picture which has been successfully used to ex- plain cutoffs in star formation in the outer disks of spiral In August 1994, Bailyn and graduate students J. Orosz galaxies could be applicable to the inner circumnuclear CO and S. Jogee identified the optical counterpart of the newly ϭ disk. The outflow timescale of the central starburst wind in discovered gamma-ray transient GRO J1655-40 ( Nova NGC 4102, determined from optical spectroscopy of the io- Sco 1994͒. This is only the second galactic source to exhibit nised swept up gas is 106 years, suggesting it is at an earlier superluminal radio jets, and the first to be identified optically. evolutionary phase than M 82 and NGC 253 whose outflow Follow-up observations suggested the presence of an eclipse. timescales are significantly larger. The compact CO mor- Further observations in spring 1995 by Orosz and graduate phology of NGC 4102 is also consistent with that of a young student S. Barnes confirmed the presence of eclipses, and starburst. The CO morphology will evolve from centrally determined the orbital period to be 2.6 days. In April 1995, peaked to ring-like as star formation consumes and starburst Bailyn and Orosz, in collaboration with J. McClintock ͑CfA͒ winds expel the gas more quickly in the central few hundred and R. Remillard ͑MIT͒ observed the radial velocity curve of parsecs. The compact CO disk will turn into a compact cir- the secondary star in the system, and showed that it has a ϭ Ϯ cumnuclear stellar disk harboring a significant young popu- semi-amplitude of K 227 4 km/s. Combined with the pre- lation. Compact stellar disks are a previously unrecognized viously determined orbital period, this results in a mass func- ϭ Ϯ . Since the mass function major component of spiral galaxies, and NGC 4102 is clearly tion of fg(m) 3.16 0.15M ᭪ represents a minimum mass for the compact object, this re- in the process of forming one. sult confirms the suspicion that this binary system contains a black hole. 5.13 QSO Bailyn and Orosz also studied the X-ray nova GRO Sofia, Oemler, Bailyn, Coppi, B. Schaefer, graduate stu- J0422ϩ32 (ϭ Nova Per 1992͒. Using an innovative data dent C. Sabbey, with C. Baltay and J. Snyder ͑Yale Physics͒ reduction technique developed in collaboration with R. have begun a collaboration with astronomers from Indiana Remillard ͑MIT͒, they showed that the mass function of this University, the Venezualan Centro de Investigaciones de As- source is between 0.5 and 1.4 solar masses. Subsequent ob- tronomia ͑CIDA͒, and the Univesidad de Los Andes servations by others at the Keck telescope confirmed that ͑Merida, Venezuale͒. This colloaboration is constructing a f (m)ϭ1.2M ᭪ . While these observatons do not themselves large CCD drift scanning camera for the 1 meter CIDA confirm the presence of a black hole, reasonable assumptions Schmidt telescope, to be used for a search for gravitiationally regarding the inclination of the system and the mass of the lensed QSO’s. It is hoped that two cameras can be con- secondary star support such an interpretation. YALE UNIVERSITY 747

Bailyn continued to collaborate with J. Grindlay ͑CfA͒,A. Another important result is that the universe essentially acts Cool ͑U. Cal. at Berkeley͒, H. Cohn and P. Lugger ͑Indiana as a calorimeter for particles of energy above a few TeV. All U.͒ and P. Hertz ͑NRL͒ on studies of low luminosity X-ray emission above a few TeV is eventually reprocessed into sources in globular clusters. In the past year, HST FOS spec- radiation below a TeV. The spectrum of this reprocessed ra- tra were obtained from three previously identified candidate diation is remarkably insensitive to the details of the original optical counterparts of X-ray sources in NGC 6397. The high energy emission and its intensity is a measure of the spectra proved to be nearly identical to those of magnetic total luminosity of the universe above a few TeV. Combined cataclysmic variables, finally resolving the question of the with recent EGRET constraints on the diffuse gamma-ray nature of the X-ray sources, at least in this case. Additional background below 30 GeV, this calculation should prove candidate optical counterparts have been obtained in another useful in ruling out several speculative particle physics cluster ͑NGC 6752͒ by HST WFPC2 photometry. One of model. these candidates has been shown to be a variable star with a Coppi has also continued to collaborate with A. Konigl period of 2.5 hours. ͑U. Chicago͒ in studies of gamma-ray emission from blazars. B. Schaefer and undergraduate Samuel Dyson have stud- A radiation transfer code has been developed to study in ied 49 properties of Gamma Ray Bursts for significant cor- detail models which try to produce the observed radiation by relations between any properties. The motivation for this the acceleration of particles to very high energies near the work lies in the fact that bursts have virtually no known central black hole of the blazar. Such models can fit the correlations between any properties, and any such correla- tions could be the key to the underlying physics. Previous current, albeit somewhat sparse, multi-wavelength data on searches have looked at a few pairs of parameters, with none blazars. However, there are definite predictions ͑e.g., the ra- considering the light curve shapes. The new study systemati- tio of gamma-ray to x-ray luminosity͒ of the models that cally searched for relationships between all pairs of the 49 should begin to be tested by upcoming multi-wavelength ob- parameters ͑many of which are measures of light curve serving campaigns. If these predictions are not borne out, shape͒. The number of relations tested was carefully counted particle acceleration must occur rather far from the black and used in the setting of the criterion of significance. Un- hole, which will cause problems for theoretical modeling of fortunately, no new significant correlation was found. In ad- blazar jets. ͑The observed gamma-ray luminosity from blaz- dition, no significant new bimodality was discovered. ars appears to be a significant fraction of their inferred jet B. Schaefer led a variety of other studies related to kinetic luminosities.͒ Gamma Ray Bursts. In collaboration with T. Cline ͑NASA/ Together with collaborators from Chicago, Northwestern, GSFC͒ and K. Hurley ͑UC Berkeley͒, 16 orbits of Cycle 5 Goddard, and ISAS, Coppi has proposed for and obtained time on the Hubble Space Telescope are being used to search simultaneous ASCA and OSSE observations of LMC X-1 for ultraviolet counterparts. The first epoch data on three of and LMC X-3. Unfortunately, both sources were in unusually four bursts shows no counterpart. In collaboration with un- low gamma-ray states, and we not detected by OSSE. The dergraduate Dyson, the shape of the decaying tail of burst sources were easily detected by ASCA, however, and do light curves is being examined for ’exponentiality’. That is, show a hard power law tail. Theoretical modeling of the if the light decays closely to an exponential function, then a accretion disk coronae producing the emission is underway, case can be made that a physically significant time scale is and will become even more relevant with the impending ar- being measured. It has been found that some bursts closely rival of XTE data. Together with Andrzej Zdziarski and Greg fit exponentials for over six e-folding times. In collaboration Madejski, Coppi has increased the efficiency of a previously with M. Schaefer and the BACODINE team S. Barthelmy, ͑ developed pair plasma code by a factor of 5, and is adapting PI, NASA/GSFC͒, the bright southern burst on 24 April 1995 it to run as a model in the X-ray analysis package XSPEC. was imaged with the CTIO 0.9m to Vϭ21 mag at a time 18 This code allows models much more sophisticated than the hours after the burst, but no fading counterpart was detected. standard Sunyaev-Titarchuk Componization models to be fit Coppi, in collaboration with F. Aharonian ͑Max-Planck to the data. Institut fu¨r Kernphysic͒, E. Waxman ͑Institute of Advanced Coppi, D. Lamb and T. Bulik of the University of Chi- Study͒, and G. Sigl and S. Lee ͑U. Chicago͒, is investigating of the propagation of very energetic particles ͑1TeV-2 cago, have continued to study gamma-ray bursts and halos of EeV͒ through intergalactic space. This topic is becoming high velocity neutron stars. Prompted by the large upward very relevant now that Cerenkov telescopes are beginning to revision in the numbers of high velocity neutron stars, they work reliably at TeV energies and below. The collaboration constructed an N-body code to follow the trajectories of neu- is developing codes to predict much more accurately the tron stars that receive a high velocity kick at birth and move photon and cosmic-ray spectra expected at earth from cos- in the combined gravitational potentials of our galaxy and mological sources of energetic particles. One interesting ap- Andromeda. Despite claims to the contrary, when accurate plication is to the formation of ‘‘halos’’ of cascade radiation galactic potentials are used, both the current burst brightness about such sources. From the angular distribution and spec- and sky distributions can be reproduced for a non-negligible trum of this cascade radiation one can place constraints on parameter space using a halo of neutron stars with birth ve- the level of the local infrared/optical background, the locities greater than ϳ900 km/sec. These results were pre- strength of the intergalactic magnetic field, and potentially sented in the ‘‘Great Debate’’ in defense of galactic gamma- the physical distance to the source ͑i.e., the Hubble constant͒. ray burst models. 748 ANNUAL REPORT

5.15 Cosmology which have a constant slope of 2 over the ͓Fe/H͔ range in- vestigated here. Stellar models predict that subgiants will Oemler, recent Yale PhD D. Tucker ͑now at Postdam͒,R. dilute their turnoff Be abundance by about a factor of 7. The Kirshner ͑CFA͒,H.Lin͑CITA͒, S. Shectman and S. Landy sugbiant Be upper limits indeed lie significantly below the ͑OCIW͒, and P. Schechter ͑MIT͒ have completed observa- mean dwarf Be - Fe trend, consistent with the magnitude of tions for the Las Campanas Redshift Survey. About 25000 the predicted dilution. Other relevant issues ͓͑O/Fe͔, redshifts were obtained of galaxies in 6 strips, each 6Li,Be dispersion͒ were also examined. For the continuation 1.5ϫ80 degrees, 3 in the Northern and 3 in the Southern of the program ͑in addition involving S. Ryan, Anglo Aus- hemisphere. The median redshift of the galaxies is ϷϪ.1. tralian Observatory, S. Vogt and M. Keane, both at Lick Ob- Analysis of these data are now underway. One noteworthy servatory acquisition of Be spectra in additional halo stars early result is a 2 dimensional power spectrum which shows ͒ a significant excess of power of scales of about continued at the Keck 10m telescope, and analysis is ongo- 100hϪ1Mpc, consistent with earlier claims by Broadhurst ing. Finally, in a related project to study the evolution of et al. ͑1992͒. boron in the early universe ͑in addition involving F. Primas Deliyannis with A. Boesgaard ͑University of Hawaii͒,J. and L. Hobbs, University of Chicago͒, aquisition of Cycle 3 King ͑University of Texas͒, and D. Duncan ͑University of and 4 HST data was completed, and analysis is ongoing. Chicago͒ completed the first phase of their program to study Deliyannis with A. Boesgaard ͑University of Hawaii͒ and the evolution of beryllium in the early Universe. A paper was J. King ͑University of Texas͒ observed lithium in M92: submitted ͑to AJ͒ of observations obtained with the CFH knowledge of the primordial lithium abundance Li͑p͒ tests 3.6m and CTIO 4m telescopes, with measured spectral reso- and constrains models of big bang nucleosynthesis ͑BBN͒, lution is 0.14 A and, average signal-to-noise ratio of 50. and may have implications for dark matter and the laws of Derivation of Be abundances benefitted from a detailed in- physics. An apparent small dispersion in the Li abundances vestigation of the stellar parameters Teff, log g, and ͓Fe/H͔. of field halo dwarfs had been predicted to result from differ- In addition, abundances from previously published data were ences in the Li depletion of models with rotationally-induced rederived to place them on the same consistent scale. It was mixing, and would imply a higher Li͑p͒ than what is ob- found that, in the range -2.5 у͓Fe/H͔ Ͻϭ -0.8, there is a served today in these stars. However, this dispersion could clear change of slope in the Be - Fe relation from shallow at also be explained by differential Galactic Li enrichment higher metallicity to steeper at lower metallicity. ͑The final ͑from lower Li͑p͒͒ coupled with a halo age spread and/or data set consists of 21 stars, after the following are removed incomplete mixing. To differentiate between these possibili- from consideration: the diluted subgiants, upper limits, and ties, Keck/HIRES observations at R ϭ 45,000 were ob- otherwise depleted or possibly depleted dwarfs and subgiants stained in stars of one of the oldest and most metal poor - all of which may obfuscate the trends if not removed.͒ For globular clusters, M92, where differential Li enrichment ͓Fe/H͔ Ͻϭ -1.5, the slope is about 1.5, which is steeper than within the cluster is unlikely. Some evidence was found for the slope of 0.8 derived by Gilmore et al. ͑1992, Nature, 357, differences in the Li abundances of three otherwise appar- 379͒ in a study of only 6 stars. Our data are in near perfect ently identical M92 subgiants in the Spite Li plateau. Evi- agreement with ͑and strongly support͒ the models of Ryan dence against cosmic ray, supernova, and AGB star Li pro- et al. ͑1992, ApJ, 388, 184͒ and of Prantzos et al. ͑1993, duction as causing these Li differences was provided, and it ApJ, 403, 630͒, where Be is produced as a secondary ele- was suggested that different stellar surface Li depletion his- ment by spallation ͑primarily of protons onto oxygen in the tories in these stars from a higher initial abundance is a more ISM͒. These models take into account mass outflow from the likely explanation ͑as is also the case for open clusters͒. This Galactic halo and several other characteristics that distin- higher initial abundance may have been the Li͑p͒, or a com- guish the halo from the disk. The Prantzos et al. model also bination of Li͑p͒ plus significant pre-M92 Galactic Li. Im- agrees with the few existing observations of boron, but pro- plications for BBN were examined. duces more 7Li than is observed in the Spite halo Li plateau; B. Chaboyer ͑CITA͒, P.J. Kernan and L.M. Krauss ͑both this may not be a problem for the model if 7Li has been at Case Western University͒, and Demarque have completed depleted from a higher initial abundance as suggested by a detailed numerical Monte Carlo study designed to estimate stellar evolutionary models with rotationally induced mixing. both the the absolute ages and the uncertainty in age ͑with The halo outflow chemical evolution model is the only confidence limits͒ of the oldest globular clusters. This esti- model that was found that is consistent with the Be data. mate is essential if a comparison with the Hubble age of the With the now larger available sample of metal-poor Be data, universe is to be made within the context of cosmological more thorough studies of uniqueness issues in chemical evo- models. Utilizing estimates of the uncertainty range ͑and dis- lution models should be conducted. The Be data argue tribution͒ in the input parameters of stellar evolution codes against ͑though do not absolutely rule out͒ recently proposed 1000 Monte Carlo realizations of stellar isochrones were pro- complex origins for Be that result in a linear Be - Fe relation, duced, and used to fit the 18 oldest globular clusters. Incor- including: its production as a primary element in supernovae, porating the observational uncertainties in the measured its production by spallation in the metal-rich environment of color-magnitude diagrams for these systems and the calcu- supernovae or an environment confined to globular cluster lated isochrone, a probability distribution for the mean age of sized objects, or spallation in the general ISM with the com- these systems was derived. The one-sided 95 distribution oc- plication of time-dependent cosmic ray spectra. The data curs for an age of 12.07 Gyr. The peak of the distribution is similarly argue against the simplest of traditional models, at 14.56 Gyr. Uncertainties due to varying M v͑RR͒, ͓alpha/ YALE UNIVERSITY 749

Fe͔, the mixing length parameter, the efficiency of helium B. Schaefer and F. Ringwald ͑Univ. Keele͒ have measured diffusion, the 14N reaction rate, and the primordial helium the first accurate eclipse period for the recurrent nova U abundance were evaluated independently. Scorpii. After the next eruption the orbital period will change slightly due to the mass loss, so the new accurate period allows for a later dynamical measure of the ejecta mass. In 5.16 Other Research addition, the new ephemeris has allowed for the correct phas- ing of widely scattered radial velocity measures. It is found Bailyn has completed a comprehensive review of objects that previous claims for a low white dwarf mass ͑a catastro- which are formed or deformed by dynamical processes. This phe for models of recurrent novae͒ are incorrect. review, to be published in Annual Reviews of Astronomy and B. Schaefer has examined the claims that ‘socket stars’ Astrophysics, discusses blue stragglers, color gradints, blue exist in the Orion Nebula. These were claimed to appear as subdwarfts, X-ray sources, radio pulsars, and cataclysmic roughly circular dark regions of nebulae that surround young variables. These objects are shown to be created by many stars in diffuse nebulae. Images were obtained through U, B, different dynamical and evolutionary processes. V, R, I, J, and K filters, but no evidence was found for the Bailyn and graduate student E. Rubenstein continued their existence of sockets, even around the specified ‘socket stars’. study of variability in nearby globular clusters. An extensive Examinations of the original plates showed that all claimed HST WFPC2 data set on the core of NGC 6752 has been sockets were actually caused by a variety of photographic obtained. Analysis has revealed an SX Phoenicis variable processing effects. among the blue stragglers, and a catacysmic variable. In ad- B. Schaefer and M. W. Schaefer have obtained a dense dition, a number of possible main sequence binaries have light curve for Nereid, a small and eccentric outer moon of been tentatively identified. The high precision color- Neptune. The light curve consists of 72 points stretching magnitude diagram obtained from this data set should allow from 18 March to 3 July 1995. During this time, Nereid a detailed study of the binary sequence parallel to the main underwent a smooth secular increase of brightness at the rate sequence. of 0.0042 mag/day, for a total change of 0.45 mag. Super- B. Schaefer has been studying the pre-telescopic superno- posed on this trend was a significant variation with a ϳ0.2 vae with a variety of new techniques that include heliacal mag amplitude. In previous years, a large amplitude (Ͼ 1.5 rise magnitudes and quantitative light curve template fitting. mag͒ variation has been seen with a time scale of days. No 1 ‘SN185’ is unlikely to be a supernova, since the reported ͑ ͒ periodicity has been identified. The Schaefers believe that dates of visibility force the event to be greatly too luminous the variations are evidence for chaotic rotation, and they sug- for any model. Instead, the event reported is likely to be a gest the possibility that Nereid might be a captured Kuiper concatenation of a nova and the known return of the famous Belt object. comet P/Swift-Tuttle. ͑2͒ SN1006 peaked at VϭϪ5.3Ϯ0.8 and is certainly an extremely subluminous Type Ia event. ͑3͒ SN1054 was not recorded on Byzantine gold coins of Con- PUBLICATIONS stantine IX, nor in the stories of Macbeth, nor in any of Bailyn, C. D., Orosz, J. A., Girard, T. M., Jogee, S., della various chronicles of Scotland, England, or Ireland. ͑4͒ Valle, M., Begam, M. C., Fruchter, A. S., Gonzalez, R., SN1572 reached a peak magnitude of Ϫ4.53Ϯ0.18 and had Ianna, P. A., Layden, A. C., Martins, D. H., & Smith, M. a light curve shape similar to the very fast SN1991bg. ͑5͒ 1995, ‘‘The optical counterpart of the superluminal source SN1604 reached a peak magnitude of Ϫ3.02Ϯ0.10 and had GRO J1655-40,’’ Nature, 374, 701. a similarly fast decline rate. ͑6͒ None of the historical events Bailyn, C. D., Rubenstein, E. P., Girard, T. M., Dinescu, D. can be used for Hubble Constant purposes because none can I., Rasio, F. A., & Yanny, B. 1994, ‘‘A Possible Optical be guaranteed to be a Type Ia event, the various procedural Counterpart for the Triple Pulsar System PSR B1620-26,’’ problems create large uncertainties, and the measurement er- ApJLett 433, L89. rors themselves are too large to resolve any controversy. Bailyn, C., Jogee, S., Orosz, J. 1994, ‘‘X-ray Nova in Scor- Type Ia supernova are one of the best standard candles for pius,’’ IAU Circular ##6050. measuring the Hubble Constant. What is needed is ͑1͒ a re- Bailyn, C., Jogee, S., Orosz, J. 1994, ‘‘X-ray Nova in Scor- liable calibration of peak magnitude for a known velocity, ͑2͒ pius,’’ IAU Circular ##6060. reliable distances to nearby Type Ia events, and ͑3͒ reliable Bailyn, C. D. & Orosz, J. A. 1995, ‘‘Mini-Outbursts from the peak magnitudes for these events. For the first task, the Black Hole Candidate Nova Vela 1993,’’ ApJ, 440, L73. Calan-Tololo survey has accumulated large amounts of high Bailyn, C. D. & Pinsonneault, M. H., 1995, ‘‘On the Lumi- quality data and has proven the decline-rate calibration. For nosity Function, Lifetimes, and Origin of Blue Stragglers the second task, the Hubble Space Telescope has recently in Globular Clusters,’’ ApJ, 439, 705. returned Cepheid distances to five host galaxies of Type Ia Bailyn, C. D., Orosz, J. A., Jogee, S., Girard, T. M., Della explosions. For the third task, Schaefer has been performing Valle, M., Begam, M. C., Fruchter, A. S., Gonza´lez, R., a comprehensive analysis of data for the five HST superno- Ianna, P. A., Layden, A. C., Martins, D. M. & Smith, M. vae. This includes measuring the old comparison stars on 1995, ‘‘The Optical Counterpart of GRO J1655-40,’’ Na- modern magnitude systems, remeasuring the original plates ture 374, 701. with the Yale PDS machine, ferreting out unpublished data, Bailyn, C., Orosz, J., McClintock, J., Remillard, R. 1995 and fitting quantitative light curve templates. In all cases, the ‘‘X-ray Nova 1994 in Scorpius,’’ IAU Circular ##6173. deduced Hubble Constant lies between 50 and 65 km/s/Mpc. Bailyn, C. D. — see Callanan, P. J. 750 ANNUAL REPORT

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