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Pennsylvania State University and University Park, Pennsylvania 16802-6305

This report covers the period from September 1, 1994 to presented the 1994 Marker Lecture Series in September. Sir August 31, 1995. Martin Rees from Cambridge University, England, presented the 1995 Marker Lecture Series in April.

1. PERSONNEL 1.1 Faculty 2. ACADEMIC PROGRAM

The regular members of the faculty during the academic 2.1 Graduate and Undergraduate Majors 1994-1995 were Professors Peter Me´sza´ros ͑Department Twenty-nine graduate and sixty-three undergraduate as- Head͒, France Co´rdova ͑on leave as NASA Chief Scientist͒, tronomy majors were enrolled during the academic year Eric Feigelson, Gordon Garmire ͑Evan Pugh Professor͒, Lawrence Ramsey, Douglas Sampson, Peter Usher, Daniel 1994-95. During that time, seven B.S. degrees and three Ph.D. degrees were awarded in Astronomy and Astrophysics. Weedman ͑on leave as NASA Director of Astrophysics Di- Doctoral recipients were Windsor Morgan, Ralph Kraft, and vision͒ and Aleksander Wolszczan; Visiting Professor George Pavlov; Associate Professors Donald Schneider and Zhiyu Guo. Richard Wade; Assistant Professors Matthew Bershady, Jane Charlton, Robin Ciardullo, Pablo Laguna and Louis Winkler; and Senior Research Associate/Associate Professors David 2.2 Educational Initiative Burrows and John Nousek. The Department continues as a Site for NSF’s Research Research Associates in the program were Stuart Ander- Experiences for Undergraduate program Chester, PI and son, Margaret Chester, Robin Corbet, Audrey Garmire, Scott ͑ Ramsey, Co-PI . The Site grant provides stipend and travel Horner, Ron Kollgaard, Chi-Wai Lee, David Lumb, James ͒ support for undergraduates to participate in research with Neff, Mark Skinner, Guy Stringfellow, Leisa Townsley, faculty members. Six undergraduates from other colleges and Robin Tuluie, Kim Weaver, and Alan Welty. Chester was universities were fully supported by the Site Grant, and three appointed Visiting Assistant Professor of at Bucknell Penn State students received partial support. These nine stu- University for the 1994-5 academic year. Neff was appointed dents and additional Penn State students supported under to the Graduate Faculty. Joining the department as Research ͑ individual faculty research grants͒ participated in a variety of Associates are Marcos Diaz ͑formerly of the University of group activities. These included a colloquium series given by Sao Paulo, Brazil͒, Philippos Papadopoulos ͑formerly of Ar- faculty and visiting scientists, a workshop on Modern Detec- istotle University, Greece͒, George Chartas ͑formerly of Har- tors and Observational Techniques, an observing trip to the vard Smithsonian Center for Astrophysics͒, Oleg Goldshmidt NRAO at Green Bank, and an introduction to optical observ- ͑formerly of Tel Aviv University, Israel͒ and Zhiyu Guo ing at Penn State’s Black Moshannon Observatory. ͑former graduate student at Penn State͒. Adjunct Associate Professors include Richard Griffiths at the Space Telescope Science Institute and Hans Kraus at the Technischen Universitat, Munchen, Germany. 2.3 Astronomy Club The Astronomy Club completed 22 of service to the 1.2 Visitors to the Department University and the local community. It continued to hold Open Houses about once a month during each semester, and Visitors to the department included Ute Kraus, postdoc- during the summer semester. These events attract hundreds toral fellow from the University of Tuebingen, Germany, of visitors to the roof of Davey Laboratory, where the won- Istva´n Horva´th from the Central Research Institute for Phys- ders of the night sky are shown. The Club also organized ics, Budapest, Hungary, and T. Padmanabhan from the Inter- various activities for members, such as parties, observing University Center for Astronomy and Astrophysics, Pune, trips, and picnics. Their newsletter, The Nittany Observer, India, all working with Peter Me´sza´ros; Honglin Zhang from continued to carry informative and well-written articles and Ohio State University, working with Douglas Sampson; items of news. Members also participate in outreach pro- Randy McKee from Ithaca College, as a summer instructor; grams for school children, making use of the Department’s Katsuji Koyama from Kyoto University, working with Eric planetarium. Officers for 1994-1995 were: President, Maggie Feigelson; Ronald Polidan from NASA/Goddard, working Masetti; Vice President, Melissa Pantaleo; Secretary/ Trea- with Richard Wade; and Richard Kron from the University of surer, Chris Burns; Newsletter Editor, Doug Schwer; Fund Chicago, Fermi Lab, and Dario Trevese from the Universita Raiser/Sales, Eric Hilbert; and Student Advisors, Doug degli Studi, La Sapienza, Rome, both working with Matthew O’Neal and Darren Williams. Usher is the Club’s faculty Bershady. Vera Rubin from Carnegie Institute of Washington advisor. PENNSYLVANIA STATE UNIVERSITY 551

3. RESEARCH ACTIVITIES While some detailed design work has started on these instru- ments, none is fully funded. The high resolution spec- 3.1 Instrumentation for Observing trograph is furthest along with initial procurement of long 3.1.1 Optical lead items in progress at HET partner UT Austin. Detailed design work on the medium resolution spectrograph has be- The Hobby-Eberly Telescope: The year since we last re- gun at Penn State. ported has seen dramatic progress in the construction of the The design and construction of the telescope is managed Hobby-Eberly Telescope HET . The initial construction ͑ ͒ by Thomas A. Sebring, HET project manager. For more in- phase of the building has been completed with only interior formation on the HET, its science programs or partnership finishing remaining. In addition, all major structural elements contact L. Ramsey, HET project scientist, at of the telescope have been completed, delivered and installed [email protected]. at the McDonald Observatory site near Fort Davis, Texas. Black Moshannon Observatory: Ramsey and Matus con- This includes the telescope structure, mirror support truss tinued work on the implementation of a near-IR array system and dome. July 1995 was a particularly busy month with the during this last year. Matus completed the electronic and telescope structure azimuth rotation system undergoing its control software for the system and carried out extensive initial tests, the dome being lifted into place and the first two testing and characterization during spring of 1995. Ramsey primary mirror segments, out of 91, being accepted from designed a simple fiber spectrograph which was imple- Kodak. From this time the external appearance of the facility mented by Matus and Ramsey. May 1995 saw first light of will hardly change. However substantial effort remains on this system with initial observations in the H band of Arctu- the installation of the myriad electrical and electronic control rus at a resolution of ␭/⌬␭ϳ5000. This coming year we systems. hope to conduct initial observations testing the utility of this The final procurements for the tracker, mirror supports system and spectral region for precision mea- and actuators are well underway. Delivery of the tracker late surements. in the first quarter of 1996 will allow us to accurately project first light, now expected in summer of 1996. This will take place with a simple diamond-turned surrogate spherical ab- 3.1.2 X-ray erration corrector system. This surrogate will allow the vital CCD Imaging Spectrometer on AXAF: The AXAF CCD test and de-bug of the novel HET tracking system without Imaging Spectrometer ͑ACIS͒ passed its critical design re- simultaneously having to test the optically complex final view in June. The flight CCDs are being delivered to MIT spherical aberration corrector system. The final aberration and will be calibrated with X-rays during the winter of 1995/ corrector system along with the acquisition and guide sys- 96. A number of CCDs, both frontside and backside illumi- tems and instrument interface is currently in final design. At nated have been calibrated at BESSY in Berlin by our col- this time we see no threat to our plan to implement regular laborators at the MIT Center for Space Research. These science operations in mid-1997. CCDs form the standard by which the flight CCDs are com- With the construction of the telescope well along, the em- pared for absolute quantum efficiency. Backside illuminated phasis is turning to the science operations phase. The Hobby- CCDs have been made at Lincoln Laboratory based on a Eberly Telescope Consortium, which consists of University process developed for the High Energy Transient Explorer of Texas at Austin, Penn State, Stanford University and the (HETE). These CCDs have proven to be very stable under Universities of Goettingen and Munich in Germany, held a conditions expected in the AXAF and on the ground science workshop at Penn State in June of 1995. Several prior to launch. The quantum efficiency is high below 0.5 representatives from other private and national projects were keV, but the energy resolution is substantially worse than the also present. The focus of this workshop was to coordinate frontside illuminated CCDs yet sufficient to separate orders and optimize those scientific programs that the HET partners from the objective grating spectrometers. Low energy events will likely execute within the first year of science operation show considerable splitting of charge between pixels. with particular attention paid to seeking efficiencies in obser- Garmire suggested that this could be used to obtain sub-pixel vations, minimizing target redundancies, and identifying position resolution. Townsley, using a model of the CCD those targets or programs that would benefit most from col- developed by Lumb and Nousek, found that this is indeed the laboration. We also paid close attention to considering how case. Comparing with MIT data on the splitting, good agree- best to carry out synoptic and time-critical observations with ment between the model and the data was found. The posi- the HET, and identifying synoptic and time-critical programs tion resolution could be improved to about 0.1 pixel using that should be executed within the first year of science op- this technique. eration. Up to one half of the spectroscopic array will use back- A major concern remains completion of the initial instru- side illuminated CCDs. The production of these CCDs has ments for the HET near the beginning of science operations. been completed and they await packaging in the flight con- This initial complement of instruments will be a low resolu- figuration. A decision on how many of these devices will be tion (Rϭ␭/⌬␭ϳ500Ϫ2000) focal plane spectrograph, a employed in the spectroscopic array will be made this winter fiber-coupled medium resolution (4000ϽRϽ20,000) spec- after testing of all available backside illuminated CCDs is trograph with multi-object capability and a high resolution completed. (30,000ϽRϽ120,000) spectrograph, also fiber coupled. All Engineering grade optical blocking filters have been these instruments have well-developed concept designs. tested at the University of Wisconsin Synchrotron Radiation 552 ANNUAL REPORT

Center in an effort led by Townsley, who joined Penn State sented a full suite of behavior, including warm absorbers, last summer. Better than one percent accuracy as a function reflection and a strong Fe K line. of energy has been achieved over the complete filter area on ASCA and GINGA data were combined in studies of scales corresponding to thirty arcseconds in the focal plane X-ray binaries by Corbet and graduate student Brian Tho- of AXAF. This is the size of the dither of the spacecraft mas. Thomas has used these data to model the accretion disk during one observation. Preparations are in progress for the in EXO 0748-676 based on the behavior of the light curve calibration of a second set of engineering filters and flight with orbital phase for his Ph.D. thesis, expected in January filters during the winter of 1995/96. Chartas, who joined the 1996. Penn State ACIS team this summer, will measure the detailed G. Garmire and A. Garmire continue analysis of the bright energy dependence of the transmission of the optical block- M band enhancement south of the Galactic Center, searching ing filters this winter at the National Synchrotron Light the ASCA data for evidence to resolve the ambiguity of Source in Brookhaven. This facility has an X-ray spectrom- whether this emission originates as locally as the North Polar eter with much better energy resolution than the Wisconsin Spur, or as far away as the Galactic Center. facility. CUBIC: The CUBIC ͑Cosmic Unresolved Background Nousek has developed a calibration plan with priorities Instrument using CCDs͒ experiment was fabricated in the for the calibration of the ACIS at the Marshall Space Flight Penn State High Energy Astrophysics Laboratory for flight Center X-ray Calibration Facility. This calibration will take on the US/Argentine SACϪB minisat in February 1996. place in the winter of 1996/97. There has been some sched- CUBIC is designed to make measurements of the spectrum ule erosion in the construction of the ACIS, but ACIS will be of the diffuse X-ray background from 0.2 to 10 keV with ready for the Marshall calibration if the current schedule can energy resolution comparable to the ASCA SIS instrument. be met. A two-CCD camera is being prepared by MSFC and Intrument fabrication was completed in autumn 1994, in MIT as a backup plan in case there is a major delay. Such a parallel with the preliminary calibration of the EEV-supplied CCDs. The completed payload was calibrated in the soft camera is not ideal, but will provide the highest priority mea- X-ray calibration chamber in Osmond Laboratory. In Febru- surements that are necessary to maintain calibration integrity ary 1995, Skinner and Research Aide Mark Catalano hand- of the observatory. The CCDs for ACIS will be calibrated at carried CUBIC to INVAP ͑the spacecraft contractor͒, in Bar- MIT before they are assembled into the camera. Penn State iloche, Argentina. Skinner, Burrows, Catalano, and Cawley will provide personnel to support this activity at MIT. electrically and mechanically integrated CUBIC into the Graduate student Laura Cawley, Nousek and Garmire SACϪB payload. In April 1995 the payload was transported compared bias computation algorithms proposed by Ford and to the Brazilian Integration and Testing Laboratory ͑INPE- Somigliana ͑MIT͒ to determine which are most efficient for LIT͒, outside of Sao Paulo, for enviromental testing. Skinner, in-flight use by the ACIS on-board computer. graduate student Diego Janches, and Project Associate Le- ASCA (Astro-D): The ASCA Advanced Satellite for Cos- ͑ land Engel participated in the sucessful vibrational and ther- mology and Astrophysics was launched from the Ka- ͒ mal vacuum testing of CUBIC and SACϪB. goshima Space Center on February 20, 1993. In recognition All testing was completed in July, and SACϪB awaits of the contributions made during ASCA development, four transport to the NASA Wallops Flight Facility, for integration Penn State faculty ͑Corbet, Garmire, Nousek and Weaver͒ into the Orbital Sciences Corporation’s XL launch shared in the analysis of all ASCA data collected during the vehicle. The launch of SACϪB is contracted for 2/1/96. PV ͑Performance Verification͒ phase, and they, with String- Sounding Rocket Payloads: Our successful program of fellow and Feigelson, also received Guest Investigator time sounding rocket astronomy is continuing with a series of from competitive proposals. flights of our CCD cameras utilized as non-imaging, non- Four broad topics were studied using ASCA data. Stellar dispersive X-ray spectrometers. The first sounding rocket X-ray sources revealed strong X-ray line emission with mul- flight in this series occurred in May 1995. It will act as a tiple spectral components in the cataclysmic variable system, pathfinder for the CUBIC data analysis efforts, since the data SS Cyg ͑reported by Nousek and graduate student Chris are very similar to the data we expect to obtain from CUBIC. Baluta at the High Energy Astrophysics Division Meeting in The scientific objective of this flight was a measurement Napa, CA in 1995͒, and quiescent X-ray emission in the M of the spectrum of the diffuse background from 0.2 to 10 dwarf flare star system, AD Leo ͑Nousek and graduate stu- keV with moderate energy resolution ͑30 eV – 150 eV͒ over dent Kaori Nishikida͒. IUE data simultaneous to the ASCA a 1/3 sr field of view centered on the North Polar Spur. observations of SS Cyg revealed ultraviolet line variability in Because of an error in the Attitude Control System program, the source ͑Baluta and Nousek͒. we obtained a spectrum of a region of the sky in the constel- X-ray emission from AGN were studied in several papers lation Draco instead. The North Polar Spur flight will be involving Weaver and Nousek. ASCA observations of MCG- rescheduled for next year. 2-58-12 gave a nearly unique opportunity to measure the true Superconducting Tunnelling Junction ͑STJ͒ detectors of- X-ray continuum slope of a prototype Seyfert 1 nucleus fer an exciting alternative to bolometers for highly efficient without the complicating effects of complex absorption and detectors with energy resolution on order of 10 eV. In addi- Compton reflection. NGC2992 ASCA observations showed tion to a higher operating temperature, STJs have the advan- nearly the opposite behavior, revealing clear evidence of a tage of being able to provide position resolution within the molecular torus in this Seyfert 1.9 galaxy. IC4329A pre- detector. A collaboration with Dr. Hans Kraus of the Techni- PENNSYLVANIA STATE UNIVERSITY 553 cal University of Munich has been established, in which it is AstroϪE Nousek was one of five American scientists proposed to fly a detector developed in his laboratory on a named to serve as Experiment Advisers to the Japanese sounding rocket. We will provide a foil mirror telescope for AstroϪE mission. AstroϪE will carry US X-ray tele- this rocket flight to take advantage of the broad energy re- scopes and an X-ray calorimeter from the Goddard Space sponse of the STJ detector. The first quadrant of this tele- Flight Center, CCD cameras from Japan and MIT, and a hard scope has been built and is being tested. X-ray detector from the University of Tokyo and ISAS. CCD Detector Physics: As part of his thesis research, Launch is tentatively set for 2000, with annual trips by the Kraft has developed a new technique for performing absolute Experiment Advisers to Japan for science working group quantum efficiency measurements on CCDs. The data were meetings through that time. collected with a Loral CCD developed by the X-ray As- tronomy group in collaboration with JPL. The CCD was ex- 3.1.3 Radio posed to the broad-band synchrotron beam generated by the University of Wisconsin Synchrotron Radiation Center’s Al- Millisecond Pulsar Receiver: Graduate student Brian ladin light source using a very low beam current ͑approxi- Cadwell, R.S. Foster ͑NRL͒, Wolszczan and Anderson have concluded the fifth period of their search for millisecond pul- mately 5–10 electrons͒ to avoid saturating the detector. The inherent energy resolution of the CCD is then used to pro- sars with the 305m Arecibo radio telescope. Two new milli- second pulsars and several slow pulsars have been discov- duce a spectrum of the beam. A model of the CCD detector ered. Followup observations of all these objects await response can then be fitted to the measured data, adjusting completion of the Arecibo upgrade. Since June 1994, all ob- parameters of the model such as dead-layer thickness and servations have been made with the Penn State Pulsar Ma- depletion depth until an acceptable fit is obtained. Kraft chine ͑PSPM͒ which is a 2 ϫ 128 ϫ 60 kHz portable filter- showed that this technique provides an efficient means of bank equipped with a digital interface and a data acquisition/ measuring the quantum efficiency of the detectors and of analysis computer. It has been built at Penn State to make the obtaining information on the device physics, including mod- survey sensitive to hypothetical sub-millisecond pulsars ͑80 eling of charge splitting. ␮s sampling͒ and to increase precision of the followup tim- XMM EPIC and UV/Optical Monitor Telescope: XM M is ing observations ͑12 ␮s resolution͒. The analysis of search a major component of the European Space Agency’s Horizon data collected with the PSPM, using the IBM SP2 parallel 2000 program of scientific space missions. The Digital Pro- processor at Cornell Theory Center, is in progress. cessing Unit for the XMM UV/Optical Monitor (XMMOM͒ is the responsibility of the partners in a US consortium, led by Penn State. In October 1994, Lumb left the project to take up 3.2 Observational Research a position at the European Space Agency Technical Centre ͑ESTEC͒. Horner was promoted to US Project Manager, 3.2.1 Stellar Astronomy XMM with the responsibility of managing the entire US OM Proto-Planetary Systems: Neff, in collaboration with consortium and acting as liaison between the US consortium K.-P. Cheng ͑Cal State, Fullerton͒, obtained very high- XMM and the other OM consortia in the UK and Belgium, and resolution, high S/N spectra of nearby A using the NSO the European Space Agency. Welty joined the project as McMath-Pierce stellar spectrograph, the McDonald Observa- Project Scientist in April. Horner and Welty have been work- tory’s 2.7m coude, and the CTIO 1.5m echelle. They are ing with colleagues at Sandia National Laboratories, Albu- using the narrow Ca II K and Na I D circumstellar absorption querque, and Los Alamos National Laboratory to develop, features as probes of the dynamics of the proto-planetary test, and document flight, simulation, and analysis software. environment. The entire sample of northern-hemisphere stars Activities this year centered on the continuation of tests of has been observed at NSO, and the southern survey has be- the breadboard model of the XMMOM Digital Processing gun at CTIO. They have discovered circumstellar gas around Unit and continued software development of the flight and twelve of the northern candidate proto-planetary systems. ground support software. Using extremely high-resolution ͑RϾ200,000) spectra ob- The XMMOM team participated in an Experiment Interme- tained at the McDonald Observatory, they are monitoring diate Design Review in April at ESTEC, where the instru- these twelve sources for variability. ment design was reviewed prior to the final designs being Pre-Main Sequence Stars: In collaboration with David completed. The Optical Monitor has been de-scoped as a Huenemoerder ͑MIT͒ and Warrick Lawson ͑ADFA͒, Feigel- consequence of the withdrawal of the Italian consortium son has conducted spectroscopic and photometric study of from the project. This de-scoping does not impact several dozen X-ray luminous stars in the nearby Chamae- XMMOM’s ability to perform its primary scientific goals, but leon I star forming cloud. They were discovered in pointed impacts the secondary scientific goal of performing studies observations with the ROSAT satellite. Most of these stars of stellar variability using XMMOM. have strong Li 6707 and H␣ emission lines, confirming that Horner and Antonello presented a poster paper at IAU they are weak-lined stars. This effort is leading to Colloquium 155, Astrophysical Applications of Stellar Pul- doubling of the cloud members placed on the Hertzsprung- sations and Horner and Welty presented a poster paper de- Russell diagram, and hence to new insights into the popula- scribing the XMMOM instrument at the 186th meeting of the tion and evolution of low mass pre-main sequence ͑PMS͒ American Astronomical Society ͑AAS͒, in Pittsburgh in June stars. In addition, two papers discussed in previous reports 1995. on X-ray properties of T Tauri stars appeared in press. 554 ANNUAL REPORT

Graduate student Lee Carkner led a collaboration with tions. Lightcurve and spectral analysis of several low-mass Feigelson, K. Koyama ͑Kyoto͒, T. Montmerle ͑Saclay͒ and M-dwarf stars during these states have been performed. The N. Reid ͑CalTech͒ in a study of X-ray emitting T Tauri stars dM3.5e star AD Leo experienced a flare in a 1.5 ksec con- in the L1551 cloud. Both ROSAT, with a wide field and tiguous ͑good-time͒ interval ͑CI͒ during a 15 ksec observa- high spatial resolution, and ASCA, with high spectral reso- tion. The preceding CI orbit, separated by a 71 minute gap, lution and hard X-ray sensitivity, were used. Thirty-eight and the next observation which followed 4 hours after the X-ray sources were found, including several likely new flare both indicated AD Leo to be in its quiescent state with weak-lined T Tauri stars. The extremely young active classi- a corresponding mean count rate of ϳ3.3 sϪ1. This flare had cal T Tauri star XZ Tau exhibits high amplitude variability a rise time of about 20 sec, reached a peak count rate of and a hard X-ray spectrum. A powerful X-ray flare was seen ϳ11 sϪ1 which it maintained only for ϳ2 minutes, decayed on the weak-lined star V826 Tau. However, the luminous to half this count rate in 8 minutes, and had settled nearly infrared embedded protostars, L1551-IRS5 and L1551NE, back to its quiescent level by halfway into the CI. Significant were not detected. variability is seen in the flare lightcurve over that of a Welty continued his investigations of PMS stars. He par- ‘smooth’ rise and decay. The Kolmogorov-Smirnov statisti- ticipated in a multiwavelength campaign to study the weak- cal test has been computed for each CI to determine whether lined T Tauri star ͑WTTS͒ HDE 283447 headed by Feigel- the null hypothesis of a constant source is consistent with the son. Welty and Ramsey completed a study of stellar activity data; 12 of 15 CIs indicate variability at Ͼ 99% confidence on the WTTS V410 Tau, including a strong emission line levels. Existence of such rapid and persistent variability se- flare. Similar work on several other rapidly rotating WTTS is verely limits the precision to which spectral model fits can be in progress. Welty also completed work for determining the performed, and alters the permitted physical parameter orbital parameters for HDE 283447, which he showed to be space. Spectral analysis of the flares requires a reduction in a double-lined spectroscopic binary. Welty and Wade com- the from solar values to obtain adequate fits to the pleted a study of the orbit and shell of 17 Lep, which may be high S/N data. Similar analysis has been performed on the a PMS system. Welty and Neff contributed to the 1992 MU- dM7e star VB8; almost half of an 8.8 ksec observation is in SICOS campaign on the Herbig Ae star AB Aur, results of flare, with a factor 7.5 increase in the count rate over the which have been completed recently. During 1995 REU stu- quiescent level ͑0.034 sϪ1). These results will be presented at dent Jennifer Kozak and Welty examined a series of spectra the Cool Star Workshop in Florence, Italy during October of several classical T Tauri stars ͑CTTS͒. They found stellar 1995. radial velocity variations suggestive of mass asymmetries in In another study, Stringfellow and Nousek obtained the inner parts of accretion disks of two CTTS and evidence ASCA observations of the dM6 star CN Leo, with the pri- of magnetospheric accretion in one of those. mary intent of investigating the nature of the quiescent co- Late-Type Active Stars: In collaboration with S.H. Saar rona. CN Leo was also observed simultaneously with ͑CfA͒, graduate student Doug O’Neal and Neff are directly EUVE ͑PI: Ambruster - Villanova͒ as a target-of- measuring the mean temperature and total area of starspots opportunity. These data are undergoing reduction and analy- on late-type stars. They have used the stellar spectrograph at sis. the NSO McMath-Pierce telescope, at Penn State’s Black Interacting Binary Stars: Wade, Diaz, and I. Hubeny Moshannon Observatory ͑BMO͒, and using the Penn State ͑NASA/GSFC͒ carried out a study of the ultraviolet spectra fiber optic echelle at KPNO. They have completed their ini- of accretion disks in cataclysmic variables, with emphasis on tial analysis on the most heavily spotted star known ͑II Pe- limb darkening. Limb darkening has a very direct effect on gasi͒ and their initial survey of active giant stars. The analy- the spectra of flat objects such as accretion disks, since no sis of the echelle data promises to provide more detailed averaging over emergent angles occurs for disks ͑unlike constraints over a larger temperature range. They have re- stars͒. As with stars, limb darkening in disks is especially cently begun combining the constraints derived from mo- important in the ultraviolet, and at a given wavelength it is lecular band observations with Doppler imaging of the more important for low temperature atmospheres. The team atomic lines ͑in collaboration with N. Piskunov at JILA͒. studied the size of the limb darkening effect in the ultraviolet In collaboration with R. Dempsey ͑STScI͒, Neff used the ͑925 Å to 1750 Å͒ spectra of accretion disks in cataclysmic Hubble Space Telescope to obtain moderate-resolution spec- variables, using a grid of models and spectra of disk atmo- tra of density-sensitive lines as well as the C IV and Mg II spheres. Radial emissivity ͑surface brightness͒ distributions lines at several rotational phases. Simultaneous observations in the ultraviolet show the expected temperature and wave- were obtained with IUE and EUVE and at several ground- length dependences. At a fixed and based sites including the VLA and BMO. wavelength, the behavior of limb darkening is similar in stars In 1994 Welty and Ramsey obtained an extensive new and in the disks studied ͑which are optically thick͒. Limb spectral time series to investigate in greater detail short term darkening correction factors for the integrated disk bright- variations of stellar and circumstellar activity, and non-radial ness at 1448 Å were derived for the grid, a large representa- pulsations they discovered in data obtained in 1989. REU tive set of disk parameters and orbital inclinations. student Peter Ratzlaff worked with Welty on those data. Sig- The other major dependence of the spectrum on the view- natures of the pulsations are not present in the new data. ing angle is the line profiles, which in disks show very strong Stringfellow has been investigating the flare and quiescent effects due to kinematic ͑Doppler͒ broadening by the rotating states of M-dwarf coronae using ROSAT PSPC observa- disk. Wade, Diaz, and Hubeny computed these effects both PENNSYLVANIA STATE UNIVERSITY 555 for full disks and for disks where the inner part of the disk persoft X-rays rather than the EUV. We also find that V1974 ͑close to the white dwarf͒ is artificially removed. In the latter Cyg was detected with the EUVE Deep Survey detector at case, there are noticeable changes in the shapes of lines from 549 days after visual maximum. This observation is compat- highly ionized species and in the ratio of lines formed at high ible with the above properties, indicating that the mechanism and low temperature, so high quality line profiles obtained in responsible for the soft X-ray emission, connected with the the ultraviolet may serve as a diagnostic of the structure of underlying white dwarf, had not yet entirely turned off. Sta- the disk. This would be a valuable check on the suggestion tistical tests for variability show all of the EUVE and most that in some cataclysmic variables the inner disk is in fact ROSAT observations of V1974 Cyg to be highly variable missing. over various time scales. The EUVE survey data show one In a talk given at IAU Colloquium 158, ‘‘Cataclysmic day variations, the EUVE DS data show ϳ30 minute fluc- Variables and Related Objects,’’ Wade summarized these tuations, while the ROSAT data vary rapidly on time scales studies, along with related work done by himself, Diaz, and of seconds. The EUVE data show no periodic variability on graduate students Catherine Grant and Darren Williams. any time scale. While there are many difficulties and traps remaining in the Supernovae and Massive Stars: Stringfellow continues to interpretation of accretion disk spectra, the art of modeling investigate the nature of the likely misclassified supernova the structure of disks and the radiative transfer within them SN 1961V with colleagues Filippenko ͑UC Berkeley͒, Goo- has advanced to the stage where reliance on blackbodies and drich ͑STScI͒ and others. In 1989 we proposed that SN stellar atmospheres can in many cases be abandoned. 1961V was not a supernova, but instead the superoutburst of Graduate student Craig Robinson and Co´rdova continued a massive luminous blue , very much like that their work on magnetic cataclysmic variables. One of the observed for ␩ Car. We predicted the progenitor star was still objects under study, TT Ari, exhibits X-ray modulations at present but enshrouded in an optically thick dust shell in the 836 sec, likely produced by the rotation of the white dwarf, range Vϭ25-27 or so. Analysis of our Cycle 1 HST WFC resolving a long-standing question about the basic structure observations have been completed. We find a cluster of faint of this system. It belongs to a class of objects known as ͑VϷ 24-25 mag͒, blue ͓(VϪI)o ϭϪ0.4 to 0.5 mag͔ stars intermediate polars. The origin of other variability, on time around the general location of the supernova. We also see a scales as short as 8.3 sec, has yet to be resolved. Robinson very faint (Vϭ25.6Ϯ0.3 mag͒, much redder ͓(VϪI)o ϭ 1.8 presented results on this and other cataclysmic variables at mag͔ star barely resolved from one of the blue stars. Its meetings in Cape Town, South Africa, and Cambridge, En- brightness and colors are consistent with those of a highly gland. reddened O-type star, as we predicted earlier ͑Goodrich et al. Welty and Ramsey have presented results from their 1989, ApJ, 342, 908͒. Moreover, it is the star formally closest ROSAT PSPC observations of the RS CVn binaries RT Lac to the known radio position of SN 1961V. Thus, we tenta- and SS Boo. Those are part of a study of the role of mass tively identify this as the post-eruption star of SN 1961V. transfer in enhancing X-rays and visible activity indicators in Further observations are planned to confirm our hypothesis, RS CVn systems. and differentiate between this candidate and the adjacent Novae: Stringfellow and collaborators continue to observe blue star. These results will appear in the November 1995 and model multiwavelength observations of novae during Astronomical Journal. early and late times. With T. Harrison ͑NMSU͒, graduate Pulsars: Wolszczan has completed a timing analysis of student Sally Laurent-Muehleisen, Lee, Welty, and G. Al- four years of pulse arrival time data from the planets pulsar, bright ͑U Virgina͒, Stringfellow conducted observations at PSR B1257ϩ12. The currently best timing model for this CTIO, KPNO, McDonald, APO, and BMO during 1993- pulsar includes three planets, gravitational perturbations be- 1995 for several recent classical and symbiotic novae. These tween planets B and C and a significant second derivative of data are presently undergoing analysis. the pulsar spin period. A need to include this parameter in the Stringfellow and Bowyer ͑UC Berkeley͒ presented addi- model is probably related to the presence of yet another, tional analysis and new results from EUVE observations of more distant planet in the system, but it may also be caused Nova Cygni 1992 ͑V1974 Cygni͒ at the IAU Colloquium by a timing noise due to the pulsar’s low-level rotational 152 Extreme Ultraviolet Astronomy during March 1995. An instability. extensive analysis of the observability of Classical Novae Anderson, Wolszczan, S.R. Kulkarni ͑Caltech͒ and T.A. with the EUVE Lex/B and Al/Ti/C detectors show EUVE to Prince ͑Caltech͒ have obtained reliable timing models for be quite sensitive to both the EUV and soft X-ray emission two extremely faint millisecond pulsars discovered by this emitted by the underlying hot white dwarf during novae out- group in the globular cluster M5 ͑NGC 5904͒ in 1989. Based bursts, except at the coolest white dwarf surface tempera- on the pulse arrival time data systematically collected over tures with very high intervening hydrogen column density. the last five years, it was possible to conclude that one of These results are used to interpret the emission detected dur- these object is a solitary 5.5-ms pulsar, whereas the other, ing the all-sky survey of V1974 Cyg, 279-290 days after 7.9-ms pulsar is a member of a 7-day binary system with a visual maximum. We find the best fit to the observed emis- significant of 0.14. The measured preces- sion from V1974 Cyg arises from a hot ONeMg white dwarf sion of the orbit, if due to relativistic effects, constrains a with surface temperature ϳ4ϫ105 K and a mass of ϳ1.2 companion mass to be 0.45–5.5 solar masses. This opens a M ᭪ , and derive an interstellar hydrogen column density of possibility that the companion may be a black hole. Further ϳ3ϫ1021 cmϪ2. Virtually all this emission arises from su- long term timing observations of the pulsar are necessary to 556 ANNUAL REPORT produce better constraints of the dynamics of this intriguing manque´ scenarios͒. It also suggests that the combination of binary system. absorption line spectroscopy, UV photometry, and PN mea- X. and collaborators ͑MPIEP Garching͒, Anderson surements can be a powerful probe of elliptical galaxy stellar and Wolszczan have detected X-ray emission from a 147-ms populations. pulsar in the supernova remnant S147 ͑G180.0-1.7͒ recently Graduate student Russ McMillan and Ciardullo have discovered by the Penn State/NRL group. The emission has started a program of constraining the ages of supernova pro- been found in the ROSAT All Sky Survey data. Preliminary genitors by analyzing the galactic positions of over 200 Type analysis constrains the star’s blackbody temperature to about II and Type Ia supernovae ͑SNe͒ that have gone off in spiral 0.1 keV which is consistent with the pulsar spindown age of galaxies. By considering factors such as disk scale height, 6ϫ105 years. Due to a short data span it was not possible to disk/bulge membership, and position in relation to spiral conduct timing analysis of the detected emission. arms, McMillan and Ciardullo hope to determine a statistical Pavlov, Stringfellow & Co´rdova analyzed results of age for the progenitors of Type Ia SNe relative to Type II HST Cycle 4 observations of nearby radio pulsars with the SNe. The first part of this analysis is completed and has HST Faint Object Camera. Each of the deep images of the produced the surprising result that Type Ia SNe are as closely fields around the pulsars PSR B0656ϩ14, B1929ϩ10, and correlated with spiral arms as Type II SNe. Type II SNe are B0950ϩ08 obtained with a broad band UV filter contains known to be young, but some lines of reasoning imply that only one point source whose position is compatible with the Type Ia SNe should be considerably older. The correlation of radio position of the pulsar. A comparison with ROSAT data Type Ia SNe with spiral arms means that the bulk of these and theoretical models shows that the radiation detected events occur in populations with ages of only a few hundred from the middle-age PSR B0656ϩ14 is mainly of a nonther- million years. mal origin. The UV-optical radiation from the old pulsars Distance Scale: Ciardullo, Feldmeier, Jocoby have contin- PSR B1929ϩ10 and B0950ϩ08 is likely to be emitted from ued work on using PN to measure extragalactic distances. 5 the neutron star surfaces, with temperatures 1Ϫ2ϫ10 and Until now, such studies have been mostly limited to elliptical 4 7Ϯ1ϫ10 K, respectively. and S0 galaxies, where the problems presented by internal Globular Clusters: Schneider, P. Guhathakruta ͑UCSC͒, extinction and interloping H II regions are minimal. How- B. Yanny ͑FNAL͒, and J. Bahcall ͑IAS͒ have completed their ever, reliable PN identifications in spirals are possible if the analysis of the central region of the Galactic globular cluster seeing is excellent and if H␣ and deep continuum images are M15. Images of M15 taken with HST’s Planetary camera also obtained. In the past year, Ciardullo et al. successfully show that the surface density of stars continues to rise as a derived a distance to the HST Distance Scale Key Project power law down to the limit of the observations ͑a radius of galaxy M101; data on two other spiral galaxies ͑M51 and 0.3 arcsec or 0.017 pc͒; a flat core with a radius larger than 2 NGC 3368͒ are currently being analyzed. arcsec is ruled out at the 95% confidence level. The surface Dwarf Galaxies: Dwarf galaxies are not merely an exten- distribution is consistent with the expected profile if the clus- 3 sion of the population of giant galaxies. They appear to form ter harbors a relatively massive (10 M ᭪) black hole, but a separate population with lower surface brightness, and thus dynamical information is required before the black hole we must ask whether they form in the same way as giant model can be verified. A candidate for the cusp around the galaxies. With the goal of understanding the formation black hole, a star called AC 214, has been identified in the mechanism of dwarf galaxies of various types, graduate stu- HST data; it is very near the center of the cluster, and the dent Sally Hunsberger, Charlton, and Dennis Zaritsky ͑Lick/ high-resolution images show that AC 214 consists of at least UCSC͒ are conducting a survey of dwarf galaxies in compact three very closely spaced stars. groups of galaxies. With images of 42 groups of 3-5 giant galaxies, obtained with the Palomar 1.5m telescope, they will 3.2.2 Extragalactic Astronomy establish the function of dwarf galaxies in com- Stellar Populations: Ciardullo and graduate student John pact groups and examine the clustering properties of the Feldmeier, along with G. Jacoby ͑Kitt Peak National Obser- dwarfs with respect to the giants. In some sense, the compact vatory͒ continued their ͓O III͔␭5007 measurements of ex- group environment can be considered a bridge in understand- tragalactic planetary nebulae ͑PN͒. Until now, these surveys ing the differences between field galaxies and galaxy clus- have been aimed at determining distances and identifying ters. test particles for future kinematical studies. However, PN Because of their high space density of galaxies, compact observations are useful in another area of astrophysics – the groups are particularly likely sites for tidal interactions and investigation of stellar populations. Because the production mergers. In a paper recently submitted to the Astrophysical rate of ͓O III͔ bright PN in a galaxy can vary by an order of Journal, Hunsberger, Charlton, and Zaritsky describe the dis- magnitude, this number is potentially a powerful discrimi- covery of 47 clumps of material in seven compact groups nant between age and metallicity in an evolved stellar popu- with tidal features. These tidal clumps have sizes and lumi- lation. A strong inverse correlation exists between bolometric nosities consistent with dwarf galaxies, and consideration of luminosity-specific PN density ␣2.5 and UV upturn; those the numbers and timescales predicts that a significant frac- galaxies that exhibit a large excess in flux at ␭1550 Å are tion of dwarf galaxies can be formed through the interaction strongly deficient in bright PN . The correlation supports of giant parents. H␣ images and colors of the tidal dwarf models of non-traditional post-asymptotic branch evolution candidates have been obtained and are being analyzed as part for metal-rich stars ͑i.e., the post-early-AGB and AGB- of a detailed study of the tidal dwarf formation mechanism. PENNSYLVANIA STATE UNIVERSITY 557

Active Galaxies and Schneider, M. Schmidt cating that the energy output of this AGN is near the mini- ͑Caltech͒, and J. Gunn ͑Princeton University͒ have com- mum energy level. pleted the observations of their multiyear, large-scale survey Kollgaard, Feigelson, Laurent-Muehleisen, H. Spinrad for faint, high-redshift quasars. Using high efficiency solid ͑UCB͒, A. Dey ͑UCB͒, and W. Brinkmann ͑MPIfE͒ com- state detectors ͑CCDs͒ on the 5m telescope on Palomar pleted a search of ‘optically quiet quasars’ ͑OQQs͒, which Mountain, they have searched over 60 square degrees of sky are X-ray and radio loud AGN with very weak optical con- and have identified distant quasars down to a limiting mag- tinuum emission. Multifrequency observations of three pre- nitude of ϳ21. The survey catalog, published in 1994, con- viously known OQQs found evidence of excess absorption. tains over 900 emission-line objects; 90 of these sources are However, the wide range of optical and radio properties ar- quasars with redshifts ranging from 2.7 to 4.7. The analysis gues against the absorption arising very close to the AGN, of the luminosity function of the high-redshift quasars shows perhaps instead being due to unrelated systems along the line dramatic evolution of these objects during the first few bil- of sight. In addition, multifrequency observations of 10 can- lion years after the Big Bang; the number density appears to didate OQQs from the ROSAT All-Sky Survery ͑RASS͒ did fall by a factor of nearly three for every unit increase in not uncover any clear examples of new OQQs and sources redshift at redshifts above three. with very extreme spectral energy distributions must be rare. Schneider, J. Bahcall, and S. Kirhakos ͑Institute for Ad- Laurent-Muehleisen, Kollgaard, Feigelson and under- vanced Study͒, have obtained HST images of 20 nearby, graduate student P.J. Ryan, in collaboration with W. Brink- luminous quasars. The observations, through a very broad- mann and J. Siebert ͑MPIfEP͒, continued their study of ϳ2,000 X-ray and radio bright sources discovered in the band red filter, were designed to investigate the properties of RASS. High resolution radio observations, performed at the the host galaxies of the sample. There were a number VLA, provide subarcsecond positions, enabling optical iden- of indications from ground-based work that radio-loud qua- tification of many of the previously unknown sources. The sars resided in luminous elliptical galaxies and that radio- sample consists primarily of new radio-loud active galaxies, quiet quasars had luminous spiral hosts. The HST data re- including quasars, BL Lac objects and broad line radio gal- veal that luminous quasars live in a wide variety of axies. environments: luminous ellipticals and spirals ͑but there is Kollgaard has continued work on radio observations of clearly not a one-to-one correspondence between quasar ra- active galaxies made as part of multifrequency campaigns. dio properties and host morphology , interacting galaxies, ͒ These data can be used to constrain relativistic jet models and a number of quasars who do not show any evidence of a and to search for connections between radio emitting shocks host ͑detection limits for the hosts are L* or fainter͒. and high energy emission. Multifrequency VLA observations Bershady has continued work on the study of the variabil- were made in January, 1995 as part of a campaign to monitor ity properties of faint objects. Following previous analysis of 3C 279. quasar variability, he has expanded the technique to incorpo- Usher and collaborators presented results of a study that rate objects with extended light profiles. Using a unique col- defines the interface between optically resolved and unre- lection of deep, 4m photographic plates spanning a 17 year solved active galaxies and quasars at the 186th meeting of time-baseline, he and collaborators D. Trevese ͑U. Rome͒ the AAS. A complete description of methodology and the and R. Kron ͑U. Chicago͒, have selected a sample of roughly implications for the nature of galaxies that host quasars is 50 variable galaxies to Bϭ23 in 0.23 square degrees. This given by Usher et al. in an upcoming paper. This paper is the sample is selected to be highly reliable in terms of variabil- seventh in the series on the selection and nature of faint blue ity. Three of the brightest candidates are spectroscopically objects at high galactic latitude and it gives for the first time confirmed to have broad line-emission. Most of the remain- a working definition of a quasar that is unresolved on cam- ing sample appear to be blue relative to the ‘non-variable’ eras of the 1.2m Schmidt class. This definition is semi- galaxy population. Individual light curves have been in- empirical inasmuch as its form is dictated by a simple theory spected to verify that variability is inconsistent with super- proposed by J. Kristian in 1973, while the constants of the fit novae events. Hence there is high expectation that this are determined empirically. As a result, the US survey is the sample contains a large number of low-luminosity AGN at only one that defines the limits of selection on all four intermediate redshift. Spectroscopic confirmation of the en- boundaries of the Hubble Diagram. This result enables sta- tire sample is in progress. tistical completeness of combined surveys to be achieved. Feigelson, Laurent-Muehleisen, Kollgaard and E. Foma- How this translates into an astrophysical criterion ͑e.g. a lu- lont ͑NRAO͒ completed a study on the discovery of inverse- minosity cutoff͒ is also addressed; if the Hubble constant is Compton emission from the lobes of the radio galaxy Fornax 1 small (Hoϭ50, qoϳ 2), then bright and medium-bright qua- A. The lack of Faraday depolarization in the lobes argues sars with magnitudes in the range 14 to 18.5 should be against a significant contribution from a thermal plasma, and counted only if their absolute magnitudes are less than about the X-rays are likely produced by inverse-Compton ͑IC͒ -24, unless it can be demostrated that there is no bias against scattering of background photons in the lobes. This is the resolved sources. The paper also gives new spectroscopic first clear example of IC emission from outside the central results for the narrow emission-line galaxy component of the engine of an AGN. The X-ray flux is consistent with that US survey. Evidence for the existence of ‘naked quasars’ is expected from an assumption of equipartition between the regarded as minimal. magnetic fields and relativistic electrons in the lobes, indi- As part of an ongoing project searching for rapid (Ͻ500 558 ANNUAL REPORT sec͒ X-ray variability in deep, pointed ROSAT PSPC im- emission line quasars. Kollgaard, Laurent-Muehleisen, and ages, Stringfellow, Anderson, and Co´rdova detected the AGN colleagues at Hamburger Sternwarte studied 35 new BL Lacs NGC 3227, which is a Seyfert 1 galaxy. Our search tech- discovered in the RASS using spectroscopic information nique was run on a 15 ksec PSPC image, which revealed a from the Hamburg Quasar Survey and new radio observa- Ͼ5␴ event in one OBI over time scales ranging from 30 to tions. The resulting logN – logS is generally consistent with 500 sec which we identified with the off-axis source NGC that found in previous XBL surveys, but shows indications of 3227/3226. Detailed analysis of the light curve, which dis- a higher number of X-ray faint BL Lacs than previously plays a factor of ϳ2 increase in the count rate in only ϳ20 believed. As a followup to this study, new VLA observations min, showed that this event occurred in a high-state. Analysis were made in June 1995 of faint, unidentified sources from of another pointed observation of NGC 3227/3226 resolves the Hamburg Quasar Survey, a number of which may be BL the pair, and shows NGC 3226 to be substantially fainter Lacs. than NGC 3227 in X-rays; they are separated by 2Ј.4. While Laurent-Muehleisen, Kollgaard and Feigelson continued rapid variability on time scales of hours occurs in both their spectroscopic observations of candidate BL Lacertae ROSAT observations, the hardness ratio in the high-states objects, chosen on the basis of their broadband colors. These are substantially different. Such rapid variability indicates sources are a subset of the radio-loud AGN discovered in significant changes in either the accretion rate or geometrical their survey of RASS sources with counterparts in the 1987 obscuration/unveiling on short time scales; either points to Green Bank northern hemisphere radio survey. These obser- an extremely compact source for the X-ray emission ͑e.g., a vations have produced a new large sample of supermassive black hole͒. Spectral analysis of the low-state ROSAT-selected BL Lacs which will be used to study the in the resolved image of NGC 3227 ͑excluding NGC 3226͒ physical properties, such as jet speed and orientation, of this for a simple power law model yields an acceptable fit with a class of AGN. Laurent-Muehleisen visited collaborators photon index ⌫Ϸ1. Some, but not all, high-states are also Brinkmann and Siebert at MPE in Garching for two months, modelled well by this simple model, although the parameter where she re-analyzed RASS data for the new BL Lac ob- values change: ⌫Ϸ1.6. However, extraction of the data com- jects. The soft X-ray spectra of the RASS XBLs appear flat- bining all states ͑high and low͒ requires a partial covering, ter than quasars with similar broad-band colors. 2 Multiwavelength studies of the BL Lac PKS 2155-304, warm absorber model to obtain an acceptable ␹␯ fit. These results will appear in the proceedings of the Ro¨ntgenstrahl- for which Kollgaard and Feigelson obtained radio observa- ung from the Universe meeting in Wu¨rzburg, Germany in tions, were published. The evidence suggests that radio September 1995 and elsewhere. variations lag behind optical/UV variations by a week or BL Lacertae objects: Kollgaard, D.C. Gabuzda ͑Lebedev͒ longer, and the UV variations lag behind soft X-ray varia- and Feigelson obtained VLBI observations of four X-ray se- tions by 2-3 hours. lected BL Lacertae objects ͑XBLs͒. It is generally believed that BL Lacs are radio galaxies oriented towards the line of 3.2.3 High Energy Astrophysics sight, with XBLs being intermediate between galaxies and Eridanus Enhancement: Burrows and Guo continue to radio selected BL Lacs ͑RBLs͒. These new VLBI observa- work on soft X-ray emission from the Eridanus Enhance- tions find differences in the -scale jets of XBLs and ment, a large diffuse feature produced by emission from the RBLs, including the size-scale and polarization properties, interior of the Orion-Eridanus superbubble, using ROSAT that cannot be attributed only to changes in orientation. sky survey and pointed observations to obtain more detailed Other factors probably related to the intrinsic power output spectral and spatial information. of the central engine must distinguish XBLs and RBLs in ROSAT sky survey data provide a wealth of detail on addition to orientation effects. As a followup to this study, small angular scales not visible in previous X-ray maps of new VLBI observations of eight additional XBLs were made this object, including the discovery of a number of shadows in July, 1995, as part of an ongoing effort to monitor nearby cast against the X-ray enhancement, presumably by clumps XBLs from the HEAOϪ1 sample. of neutral material in its shell. Kollgaard, Feigelson, undergraduate student C. Palma, ROSAT PSPC pointed observations have been analyzed and Laurent-Muehleisen completed deep VLA imaging of to investigate spectral variations across this object, and are several XBLs, completing their study of the HEAOϪ1 also producing a number of unexpected shadow detections. sample. A comparison of the core and extended radio lumi- The latter have been used to constrain the physical location nosities with RBLs and galaxies shows good agreement with of the absorbing clouds, using 21 cm observations to deter- the expectations of unified schemes. Simple jet models in- mine the radial velocities of the absorbing clouds and optical volving discrete components are preferred over more con- absorption measurements to constrain the distances to these tinuous jets, and there is evidence that the pattern speed of clouds. This work constituted Guo’s thesis research and has superluminal components in BL Lacs is less than the speed led to improved constraints on the distance to the edge of the of the underlying fluid. Kollgaard also worked with col- Local Hot Bubble surrounding the solar system. leagues at MPE on blazars detected during the RASS. The R CrA: G. Garmire, A. Garmire and graduate student Jeff X-ray properties of XBLs and RBLs are similiar, although Mendenhall have completed the analysis of a ROSAT obser- the former show evidence for absorption in excess of that vation of R CrA, a cometary nebula 17 degrees south of the expected from galactic NH . RBLs differ in their broadband galactic center. The results were presented at the Pittsburgh spectral energy distribution from XBLs but are similiar to AAS meeting. The spectrum of the emission toward the PENNSYLVANIA STATE UNIVERSITY 559 thickest part of the cloud, which is cool compared to the hot result to the broad-band analysis: The galaxy spectral mani- foreground plasma, represents a sample of the emission from fold is largely two-dimensional, with some power in a third the hot component of the local interstellar medium. The dimension ͑i.e. some ‘thickness’ in an otherwise two- emission spectrum is fit by a two temperature Raymond and dimensional spectral space͒. Smith plasma. The emission from beyond the cloud has an Bershady and Jangren have begun work on measuring the unusual spectrum in that no power law component is re- concentration index for a large sample of galaxies at inter- quired to fit the data as would be expected from other direc- mediate redshift with ground-based multi-band images ͑see tions at high galactic latitude. This spectrum may be repre- above͒. This work builds on a study, recently in press, mea- sentative of the hot Galactic Coronae and Loop I southern suring model-independent image structure parameters for extension in this direction. A total of 18 stars or point sources galaxies at characteristic redshifts of 0.3-0.5. The latter used was detected. About five of them have been previously stud- images from the HST Wide Field Camera ͑WFC͒. The ex- ied. Several objects show rising spectra with energy, which is pectation is that image concentration will correlate strongly quite unusual. These could be extragalactic objects, but with- with galaxy spectral properties, as first noticed by Morgan out optical identifications, it is not possible to be more defi- and Mayall. The question remains whether high-resolution nite. images are needed to measure a viable concentration index X-ray Variable Sources: Stringfellow, Anderson, and Co´r- for galaxies at intermediate redshift. Using Cycle 4 WFPC2 dova continue their search for short time-scale variability images of fields with existing ground-based images, Ber- during long, pointed ROSAT observations. Statistical varia- shady and Jangren will explore this issue in the upcoming tions in the photon count rate to that of the X-ray background year. are searched over time periods of 5-500 seconds; these can The nature of blue, compact, emission-line galaxies be produced by such astrophysical events as X-ray bursters, ͑CNELGs͒ has been another continuing line of Bershady’s stellar flares, and accretion episodes in cataclysmic variables research. With collaborators Koo, Guzman and others at Lick and the nuclei of active galaxies. Dozens of transient objects and U. Chicago, work is proceeding to understand whether have been found so far, some without persistent X-ray coun- these objects represent a new class of galaxy, or an evolved terparts, and others without optical identifications. Full population at intermediate redshift. These objects were first analysis of the X-ray variable sources are proceeding. Opti- detected as quasar candidates from multi-color 4m plate sur- cal observations are being acquired in order to locate the veys. Subsequent images of a sample of these objects with optical counterparts and to ascertain the nature of the sources the HST WFC revealed that they had small sizes ͑high and the observed variability for those without optical coun- surface-brightness͒ for their luminosity, and often complex terparts. and disturbed structure. These data were augmented by high- ASCA Analysis of the Diffuse X-ray Background: G. resolution spectroscopy ͑HIRES͒ from the Keck telescope, Garmire and A. Garmire have analyzed an ASCA observa- which revealed narrow line-widths relative to their luminosi- tion of a very bright patch of the diffuse background in the ties. If the gas kinematics are assumed to trace the gravita- 3/4 keV band of the spectrum. The region shows a very low tional potential, then the small sizes and velocities and large abundance of metals in the formal fits to the spectrum, but imply very small mass-to-light ratios. This as- serious questions about the spectral models used to fit the sumption must be checked. A better characterization of the data must be answered before these conclusions can be con- stellar population is one approach. Keck spectra, HST WFC cidered credible. images, and K band photometry from KPNO are currently being gathered to enlarge the sample. 3.2.4 Observational Cosmology The nature of galaxies in deep near-infrared images from A focal point of Bershady’s research has continued to be the Keck telescope is being explored by Bershady and col- galaxy spectral classification. Two of Bershady’s studies of laborators ͑Koo and Lowenthal, Lick͒. First results show that galaxy spectral energy distributions, appearing in press this the galaxy counts in the K band continue to rise to Kϭ23.5, year, centered around a statistically representative sample of contrary to previous measurements at brighter limits. Be- galaxies at intermediate redshift. The work demonstrated that cause images were taken in superlative ͑0.55 arcsec FWHM͒ broad-band energy distributions from 0.3 to 2.2 microns can seeing, objects have been counted as a function of image be characterized by the sum of only two stellar types. Such a size. The median redshift of galaxies in these deep images is characterization provides a simple scheme for spectral clas- expected to be large (Ͼ1), and hence apparent size should sification of galaxies, which in turn is a well defined ap- be well correlated with luminosity. Counts as a function of proach for classification at non-zero redshift. A newer data image size show that the smallest objects have the steepest paper, containing photometry and redshifts for over 600 gal- count slope, and dominate the counts by Kϭ23. Investiga- axies to redshifts of 0.8, has recently been submitted, and can tion of the color, size, and surface-brightness distribution is be used to extend this analysis to higher redshift. Bershady’s in progress. most recent analysis along these lines, in collaboration with Bershady’s first results from a new survey to measure Connolly and Szalay ͑Hopkins͒, has been to show that a set rotation curves for galaxies at intermediate redshift show that of ‘eigen-spectra’ can be constructed to formally represent it is possible to construct a Tully-Fisher-like relation at sub- the manifold of galaxy spectral types. Analysis using the stantial look-back times. The hallmark of his approach is to Karhunen-Loeve transform applied to moderate-resolution select galaxies in a relatively narrow range of ͑high͒ lumi- spectroscopy from 0.22 to 1 microns yields a comparable nosity, which are also blue. This selection strategy has sev- 560 ANNUAL REPORT eral key advantages: ͑i͒ it avoids Malmquist bias; ͑ii͒ sources mum luminosity may be as large as 102, compatible with are physically large, and therefore apparently well resolved; having 90% of the observed sources with a luminosity within ͑iii͒ sources have strong line-emission and therefore are ef- one decade of each other. D. Reichart and P. Me´sza´ros inves- ficient to observe spectroscopically; ͑iv͒ sources have large tigated chi-squared fits of cosmological number counts with rotation speeds, and therefore do not require very high- a more general luminosity function shape and arbitrary resolution spectroscopy. Preliminary results imply that Ho is power law slopes. These are being applied to the 3B data set. between 60 and 90 km/s/Mpc. This is a useful sanity check A. Me´sza´ros and P. Me´sza´ros presented the exact analytic that the measurements are reliable. However, the final goal is expressions for the cosmological number counts of bursting to use this technique to make differential measurements of sources, and calculated the mean redshifts and time dilations qo and evolution of M/L in galaxies. One vexing issue is the as a function of the flux range, as well as the dispersion of ability to determine inclination angles, since this leads to these quantities, for various values of the density evolution scatter in the luminosity and rotation-speed correlation. High and the luminosity function dispersion. Time dilation values resolution images via CFHT and HST are being sought to of 2.25 as reported in the literature are found to imply red- remedy this situation. Bershady is in the process of acquiring shifts which, while large, are within one standard deviation larger samples of galaxy rotation curves. of conventional redshifts associated with galaxy formation. Graduate student Hara Papathanassiou and Me´sza´ros cal- culated the gamma-ray spectra expected from internal shocks 3.3 Theoretical Studies in time-varying relativistic wind models of gamma bursters, 3.3.1 Theoretical Astrophysics as well as the spectra of the external shocks arising from Gamma Ray Burst Sources: Me´sza´ros reviewed the obser- interaction with the external medium. Graduate student Alin vational requirements and theoretical predictions of the most Panaitescu, L. Wen, Laguna and Me´sza´ros are working on a generic type of gamma-ray burst source model. This is called relativistic hydrodynamic code that calculates the time devel- the dissipative relativistic fireball model, which is a fireball opment of shocks with moderate to very large Lorentz fac- due to the high density of Ͼ MeV photons in a small vol- tors. ume, relativistic due to the need to be below the pair produc- Accretion and Rotation Powered Pulsars: T. Bulik ͑U. tion threshold when GeV photons are observed, and dissipa- Chicago͒,Me´sza´ros, Thomas and colleagues at Tokyo Uni- tive because the energetics require the reconversion, versity derived the physical characteristics of the polar caps presumably via shocks, of the kinetic energy of the baryons of the accreting pulsars Vela X-1 and 4U1538-52, by means accelerated by the fireball ͑Proc. Texas Conf. Relat. Ap.͒. of chi-squared to a detailed physical model of the accretion Me´sza´ros also discussed the use of gamma-ray burst sources column involving asymmetries and cap structure. U. Kraus as a diagnostic for the physical parameters of the interstellar and Me´sza´ros, in collaboration with Blum, Schulte and and intergalactic medium with which these fireballs interact. Ruder from the University of Tuebingen ͑Germany͒ devel- In particular, the characteristics of the bursts depend on the oped a method for deriving beam shapes of accreting X-ray average ambient medium density and the spectrum of density pulsars. The method decomposes the observed light curves fluctuations, which can affect the overall duration and vari- into their Fourier components and finds the minimum num- ability timescales ͑Proc. Elba Conf. Physics of ISM/IGM͒. ber of beam elements needed to achieve a fit to derive the P. Me´ sza´ros and A. Me´sza´ros calculated analytic expres- polar cap magnetic geometry. Garmire, Me´sza´ros and Pavlov sions in three different asymptotic regimes of the number collaborated with colleagues at NASA Marshall and Russia, counts of bursting or steady sources in relativistic cosmol- calculating the scientific return prospects of a space-based ogy, under the assumption of a 1ϩz power law density evo- X-ray polarimeter aimed at pulsars and AGN. lution and either a standard candle or a power law luminosity Radiation Processes in Compact Objects: Pavlov and function. These analytic expressions are useful for modeling Potekhin ͑A.F. Ioffe Institute for Physics and Technology, St. the data of gamma ray burst sources, in particular the number Petersburg, Russia͒ analysed absorption of polarized radia- distribution of objects per peak flux interval. An approximate tion associated with bound-bound transitions of hydrogen at- eye fit to the BATSE 2B catalogue of gamma bursts is suf- oms in atmospheres of strongly magnetized neutron stars. ficient for estimating, based on these expressions, the value Thermal motion of atoms across the magnetic field causes of the standard candle luminosity, or alternatively the slope striking changes of the binding energies and oscillator of the luminosity function, the minimum source luminosity strengths, which leads to a new type of broadening of spec- and a limit on the maximum luminosity. tral lines. This magnetic broadening exceeds the collisional Horva´th, P. Me´sza´ros and A. Me´sza´ros carried out de- and Doppler broadenings by orders of magnitude, smearing tailed chi-squared fits of relativistic cosmological number the lines over wide, overlapping frequency bands. The count distributions under the above assumptions to the 2B bound-bound transitions are expected to determine the spec- BATSE and the PVO catalogue of gamma bursts. The results tra and the light curves of the soft-X-ray/UV thermal-like indicate that for standard candles the 2B/PVO data are com- radiation from middle-age cooling neutron stars. patible with a comoving constant source density, but to Pavlov and Bulik investigated polarization properties and within one standard deviation they are also compatible with absorption coefficients of the normal modes of radiation in a density evolution steeper by up to two powers in 1ϩz or also strongly magnetized hydrogen gas. The bound-free and significantly flatter. For a nontrivial power law luminosity bound-bound absorption spectra are considerably different function slope of about 1.9, the range of maximum to mini- for the ordinary and extraordinary normal modes; they PENNSYLVANIA STATE UNIVERSITY 561 strongly depend on the angle between the magnetic field and determine if it will be possible to observe this effect for wave vector of radiation. clusters in the Milky Way. Globular Cluster Evolution: Graduate student Roger Bar- tlett and Ciardullo have begun a project to study the evolu- 3.3.3 Atomic Physics tion of systems of globular clusters via N-body simulations. The project entails modeling a population of globular clus- Sampson, Zhang and Fontes ͑LANL͒ have continued their ters over a Hubble time and tracing their luminosity function work on fully relativistic distorted wave calculations of col- from an initial power law spectrum, as is observed in the lision strengths, or cross sections, for excitation and ioniza- Antennae galaxies, to the final log-Gaussian form seen in the tion of highly charged ions by electron impact. The results Milky Way, M31, and several Virgo ellipticals. The physical for all possible transitions among the nϭ2 levels in carbon- effects under consideration are the dynamical shocking of like ions have been submitted for publication and those for globular clusters by passages through the disk or close to the the more than 4000 transitions between the nϭ2 and nϭ3 galactic bulge, evaporation of stars from clusters in a tidal levels in C-like ions have been completed and are being pre- field, and the early loss of mass from evolving stars within pared for publication. the clusters. The goal will be to describe the evolution of an Fontes, Sampson and Zhang are using the ionization pro- entire system of clusters in a galaxy, and place constraints on gram with inclusion of the generalized Breit interaction in the behavior of the globular cluster luminosity function in the calculation of ionization cross sections for a wide range different galactic environments. of conditions. A relativistic code for calculating electron capture and autoionization rates, which uses a greatly simplified expres- 3.3.2 Computational Astrophysics sion obtained by summing over the total angular momentum of the doubly excited levels, has been used in calculating Dark Matter: N-body simulations of galaxy pairs, per- dielectronic recombination rates and the resonance contribu- formed by Bartlett and Charlton, have been compared to ob- tion to excitation of He-like ions. Sampson and Zhang made served distributions of the projected separations and radial a similar approximation in the treatment of excitation to very velocity differences of pairs. This work concluded that the high singly excited levels. data are best fit by models with large dark matter halos. It is An approximate method for calcuating the large angular difficult to reconcile the very small velocity differences that momentum contribution to the cross section for optically al- have been measured for close pairs with any choice of model lowed transitions between magnetic sublevels with the same parameters. The addition of gas dissipation to models could n value produced by impact with an electron beam was de- resolve this difference, but larger velocity differences are veloped by Zhang and Sampson and applied to He-like ions predicted by models for pairs of intermediate separation. On- of interest for EBIT experiments at Livermore. going efforts include an extension of the analysis of galaxy pairs to fainter luminosities and design of N-body simula- tions that will address the effect of the mass differences of 3.3.4 Statistical Astronomy galaxy pairs. Methodology: Feigelson and G.J. Babu ͑Statistics, Penn Charlton, Bradley Whitmore ͑STScI͒, and Diane Gilmore State͒ completed a monograph entitled Astrostatistics to in- ͑STScI͒ considered the morphological trends for galaxy pairs troduce students and researchers in astronomy and statistics within clusters, finding a tendency for elliptical galaxies to to methodological problems in the field. It includes an over- preferentially appear in pairs. Undergraduate student, Mark view of each field, samplings of contemporary research prob- Ludwick, and Charlton are conducting toy model simulations lems, and discussions of research problems and possible to try to understand how this tendency might arise as the methods for their solution. The book will appear in the In- cluster is built hierarchically with ongoing mergers that ad- terdisciplinary Statistics series of Chapman & Hall ͑Lon- just the galaxy morphologies. don͒. They also began organizing a conference, ‘‘Statistical Globular Clusters: Globular clusters are affected by tidal Challenges in Modern Astronomy II,’’ to be held at Penn shocking as they pass through the changing potential of a State in June 1996, five years after the first interdisciplinary galactic spheroid, or near to a supermassive black hole at the conference. Further information can be obtained from http:/ center of a galaxy. The destruction of globular clusters by /www.stat.psu.edu/astrostat. this mechanism was investigated by Charlton and Laguna Statistical Consulting Center: The Statistical Consulting using N-body simulations. For clusters that do not pass as Center for Astronomy, operated at the Penn State Department close to the center of the galaxy or for those in galaxies of Statistics under the direction of M. G. Akritas, has re- where the bulge is not very concentrated, the effect of the ceived dozens of questions, many of which have been an- tidal shocking is a distortion of the cluster shape. Under- swered. Topics discussed include time series analysis of graduate student Robert Klinger, Charlton, Laguna, and Poisson arrival times, parameter estimation and confidence Michael Warren ͑LANL͒ are investigating this effect using a intervals using chi-squared and likelihoods, robust methods, very efficient ‘oct-tree’ N-body code that allows the repre- clustering algorithms, comparing censored samples, treating sentation of the cluster by the true number of stars. The measurement errors in goodness-of-fit and multivariate prob- globular cluster is found to become more eccentric in its lems, and goodness-of-fit and confidence intervals from trun- outer layers, and the major axis of the elliptical contours cated samples. Two papers have been prepared on testing for shifts systematically with radius. One goal of this effort is to partial correlation with censored data and linear regression 562 ANNUAL REPORT for data with both measurement error and intrinsic scatter. larly useful in addressing issues of how clustering depends Questions and answers are available at http:// on the morphology and luminosity of a galaxy. Graduate www.stat.psu.edu/scca. student Jason Best, Charlton, and Gottfried Mayer-Kress ͑Il- Bershady has worked with Akritas to develop new statis- linois͒ have applied this technique to the RC3 catalog of tical techniques for fitting linear-regressions to data with in- galaxies. The distribution of the point-wise dimension is trinsic scatter. This is relevant for application to Tully-Fisher found to have a significantly smaller median for early type relations and the Tolman test discussed in Section 3.2.4. galaxies. For late type galaxies, the slopes are not very much different from those expected for a random distribution. The 3.3.5 Cosmology small median for early types arises mostly due to ellipticals in dense cluster environments. Continuing efforts include ap- QSO Absorption Lines: The goal of constructing a de- plications of the technique to the distribution of galaxy clus- tailed history of the internal workings of galaxies over much ters and to the structure and morphological tendencies within of cosmic time is now attainable through a new set of tools galaxy clusters. e.g. large telescopes, high resolution spectrometers, and the ͑ Computational Cosmology: Laguna and Tuluie have been Hubble Space Telescope applied to the study of quasar ab- ͒ numerically investigating secondary, gravitationally induced sorption line systems. Even small amounts of material in the anisotropies of the cosmic microwave background ͑CMB͒ most distant galaxies that lie between our telescopes and a radiation. The simulations involve a two-step numerical quasar produce absorption lines and allow a sensitive diag- scheme: first, dark matter model universes are evolved using nosis of the chemical compositions, temperatures, and mo- particle-mesh techniques; second, CMB photon bundles are tions within structures whose own light is absent or much too propagated through the gravitational potentials obtained faint for direct detection. Charlton, graduate student Suzanne from the matter simulation. The study shows three types of Linder, and collaborator Christopher Churchill Lick/UCSC ͑ ͒ secondary CMB anisotropies arising from time varying have developed a research program whose goal is to extract a gravitational potentials: CMB anisotropies due to ͑1͒ intrin- picture of the formation and evolution of galaxies and all sic collapse and evacuation of forming matter structures, ͑2͒ structures from the spectral signatures imprinted in quasar transverse motion of clusters or super-clusters, and ͑3͒ cos- spectra. mological decay of gravitational potential. The third effect is Charlton, Churchill, and Linder developed a battery of not present for model universes with ⍀ ϭ1. Results show tests to diagnose the shapes, sizes, and internal motions o that CMB anisotropies from these effects are ⌬T/Tр10Ϫ6. within the structures that produce Lyman-alpha absorption lines, of which there are hundreds along every QSO line of sight. The tests, described in a paper in press at Astrophysical 3.3.6 Numerical Relativity Journal, rely on observations of double line of sight ͑DLOS͒ quasars, where two quasar images are close enough together Black Hole Collisions: Papadopoulos and Laguna began a that their spectra contain absorption lines due to the same systematic investigation of the geometric and algorithmic is- structures. Applications of the DLOS tests to existing obser- sues underlying the mixed initial value formulation of the vations yielded the result that the Lyman-alpha forest struc- Einstein equations. In collaboration with members of the Bi- tures are sheet-like, not spherical. nary Black Hole Grand Challenge Alliance, a number of The absorption systems observed through MgII have projects are underway. The emphasis, so far, has been on a commonly been interpreted as being due to cloudlets within model problem of relativistic collapse, the spherically sym- spherical absorbing halos. However, Charlton and Churchill metric self gravitating scalar field evolution. A generic code have shown that it is equally likely that some large fraction for the evolution of this system has been written and is being of the MgII absorption lines are produced by galaxy disks. tested. This work used Monte Carlo simulations to explore both disk REU student Josh Bish, Laguna and Papadopoulos have and spherical cloud models and compares to observations, been working on the problem of creating initial data for a taking careful account of selection effects. Predictions are self gravitating scalar wave evolution code. An implementa- made for the expected distributions of MgII line equivalent tion of York’s conformal method has been developed and is widths as a function of the inclinations and orientations of going to be used for the creation of strong field data for the the absorbing galaxy disks, data that should very soon be evolution problem. available so that the issue of the dominant geometry respon- Radiation Tails: A natural tool for the study of a number sible for MgII absorption can be resolved. of radiation phenomena is the Cauchy-Characteristic initial Additional efforts in this area include investigation of the value formulation of evolution equations. One surprising ef- contribution of low surface brightness galaxies to Lyman- fect, which arises in the simplest system of a wave propagat- alpha absorption ͑Linder͒, further investigations into the na- ing in a curved background, is the existence of remnant ra- ture of MgII absorbers ͑Charlton and Churchill͒, and DLOS diation ͑tail͒ that is left behind after the main pulse has analysis of the results of N-body/SPH simulations ͑Charlton, passed. This behavior is in stark contrast with the behavior of and Yu Zhang, Peter Anninos, and Michael Norman ͑Illi- waves in a flat three-dimensional geometry which propagate nois͒͒. without any trailing components. Papadopoulos and Laguna Large Scale Structure: The point-wise dimension, simply have been constructing a numerical algorithm for the inves- the slope of the curve representing the number of galaxies tigation of this phenomenon in a general, spherically sym- within radius R of a galaxy, is a simple descriptor particu- metric, asymptotically flat space-time. The matching formu- PENNSYLVANIA STATE UNIVERSITY 563 lation ͑mixed initial value problem͒ seems to greatly enhance Bershady, M.A., Lowenthal, J., & Koo, D.C. 1995, ‘‘Galaxy the accuracy and versatility of the algorithm. Counts as a Function of Image Size to Jϭ25 and Kϭ24,’’ ApJ, submitted ACKNOWLEDGMENTS Berthiaume, G.D., Burrows, D.N., Garmire, G.P.,& We thank the Zaccheus Daniel Foundation, NASA Gradu- Nousek, J.A. 1993, ‘‘CCD Soft X-ray Observations of the ate Student Researchers Program, Pennsylvania Space Grant Puppis A SNR,’’ ApJ, 425, 132 Consortium, the Wheeler P. Davey Memorial Trust and Au- Best, J.S., Charlton, J.C., & Mayer-Kress, G. 1995, ‘‘Gal- gust and Ruth Homeyer for their continuing support of axy Morphology and Evolution Using the Pointwise Di- graduate research scholarships. Substantial portions of re- mension,’’ ApJ, in press search described in this report were supported by grants and Bo¨hm, T., Catala, C., Donati, J.-F., Simon, T., Welty, A., contracts from the National Science Foundation, the National Houdebine, E., Huang, L., Jiang, S., Zhai, D., Neff, J., Aeronautics and Space Administration, and the Lawrence Foing, B., Ghosh, K., Butler, J., Collier-Cameron, A., Livermore National Laboratory. The Eberly Family Chari- Baudrand, J., Czarny, J., Zhao, F., Talavera, A., Kennelly, table Trust provided gifts supporting research in the Depart- T., Walker, G., Carter, B., Rees, D., Semel, M., Cutispoto, ment. G., & Rodono`, M. 1996, ‘‘Azimuthal Structures in the Wind and Chromosphere of the Herbig Ae Star AB Aur. PUBLICATIONS Results from the MUSICOS 1992 Campaign,’’ A&J, in Akritas, M.G., Bershady, M.A., & Bird, C.M. 1995, ‘‘Linear preparation Regressions for Astronomical Data with Measurement Er- Bulik, T. & Pavlov, G.G. 1995, ‘‘Polarization Modes in a ror and Intrinsic Scatter,’’ ApJ, submitted Strongly Magnetized Hydrogen Gas,’’ ApJ, submitted Anderson, S.B., Wolszczan, A., Kulkarni, S.R., & Prince, Bulik, T., Riffert, H., Me´sza´ros, P., Makishima, K., Mihara, T.A. 1995, ‘‘Two Radio Pulsars in the Globular Cluster T. & Thomas, B. 1995, ‘‘Geometry and Pulse Profiles of NGC5904,’’ ApJ, submitted X-ray Pulsars: Asymmetric Relativistic Fits to 4U1538-52 Antunes, A., Burrows, D.N., Garmire, G.P., Lumb, D.,& and Vela X-1,’’ ApJ, in press Nousek, J.A. 1993, ‘‘A Predictive Model for Space-Based Burrows, D.N., Skinner, M.A., Catalano, M.A., Cawley, CCD Degradation,’’ Experimental Astronomy, 4, 159 L.J., Cocklin, E.J., Engel, L.G., Entingh, T.J., Bahcall, J.N., Kirhakos, S., & Schneider, D.P. 1995, ‘‘ Garmire, G.P., Green, R., Janches, D., Kelly, D.A., HST Images of Nearby Quasars II. 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