%R 1994PASP..106..817W %J-821 %T Spectroscopy and Photometry for Low-Mass Stars in Praesepe %A SCOTT D. WILLIAMS, JOHN R. STAUFFER, CHARLES F. PROSSER, and TERRY HERTER %B We have obtained spectral types, H-alpha equivalent widths and optical photometry for a small sample of late K and M dwarf candidate members of the Praesepe open cluster. At least for the small sample of stars we have observed, all of the Praesepe members later than M2 have H-alpha in emission. The chromospheric activity of the Praesepe stars is essentially the same as that for Hyades members of the same mass, as expected since the two clusters are thought to be the same age. %R 1994PASP..106..822M %J-827 %T A Search for Chromospherically Active Stars from the ROSAT EUV Source List %A CHRISTOPHER L. MULLISS and BERNARD W. BOPP %B Optical spectroscopic observations have been made of ten G-, R-, and M-type stars which were detected as EUV sources by the ROSAT Wide Field Camera (WFC). The optical spectra reveal the signatures of enhanced chromospheric activity in nine of the ten stars. H-alpha appears as a partially filled absorption line in seven of nine stars observed in that region and is strongly in emission in two, HD 160934 and Wolf 1225. The strongest line of the Ca II infrared triplet (8542A) appears as a partially filled absorption line in five of six stars observed in that region. Two stars with filled H-alpha and Ca II 8542A lines (BD+48 3686 and HD 171488) were also observed to have a strong Li absorption line at 6707A, suggesting an age less than the age of the Pleiades (7x10^7 years). Two stars were discovered to be double-lined spectroscopic binaries (BD+27 3245 and HD 144110). This work has also identified a previously unreported chromospherically active star with a very strong level of activity, HD 160934. %R 1994PASP..106..828W %J-834 %T BVRI Photometry of the omega Centauri Hubble Space Telescope Calibration Field %A ALISTAIR R. WALKER %B BVRI photometry is given for stars in the omega Centauri (=NGC 5139, = C1323-472) calibration field for the Wide Field/Planetary Camera of the Hubble Space Telescope. %R 1994PASP..106..835V %J-842 %T Abundances in Three Heavy-Element Stars in omega Centauri %A ANDREW D. VANTURE, GEORGE WALLERTEIN, and JEFFERY A. BROWN %B The abundances of the iron-peak elements Fe, Ni and Ti, the light metals Mg, Al and K and the s-process elements Rb, Y and Zr are determined for the heavy-element stars ROA 371, ROA 5293 and ROA 3812 in the globular cluster omega Centauri. ROA 3812 and ROA 5293 are classified as S stars while ROA 371 is classified as a K5 barium star. The metallicities of ROA 3812, 5293 and 371 are [Fe/H] = -0.7, -0.8 and -1.0 respectively. Thus, ROA 371 has a metallicity slightly higher than the red giants analyzed by Brown et al., 1991 and ROA 3812 and 5293 have metallicities near the upper end of the range for the cluster. All three stars show an excess of Al, which is common in red giants of omega Cen, and a mean excess of the s-process elements Rb, Y, and Zr of 1.4 dex. In addition, all three stars are too faint to by asymptotic giant branch (AGB) stars according to current theory. Hence, they were either formed with their present composition, as suggested by Lloyd Evans (1983a) or are binaries that have had their atmospheres polluted by a now defunct companion. \par Based upon the dependence of [s/Fe] on [Fe/H] for various s-process elements over a range from [Fe/H] = -2.0 to [Fe/H] = -0.7, we show that the Lloyd Evans hypothesis is probably correct. This shows that mass-loss from AGB stars was contributing s-process elements to the intracluster gas for as much as 10^9 years before Type Ia SNe swept the cluster clear of gas and terminated star-formation. %R 1994PASP..106..843S %J-857 %T LMC Stellar X-Ray Sources Observed with ROSAT: I. X-Ray Data and Search for Optical Counterparts %A P. C. SCHMIDTKE, A. P. COWLEY, L. M. FRATTARE, T. K. MCGRATH, J. B. HUTCHINGS, and D. CRAMPTON %B Observations of Einstein LMC X-Ray point sources have been made with ROSAT's High Resolution Imager to obtain accurate positions from which to search for optical counterparts. This paper is the first in a series reporting results of the ROSAT observations and subsequent optical observations. It includes the X-ray positions and fluxes, information about variability, optical finding charts for each source, a list of identified counterparts, and information about candidates which have been spectroscopically observed in each of the fields. Sixteen point sources were measured at a a>3-sigma level, which 15 other sources were either extended or less significant detections. About 50% of the sources are serendipitous detections (not found in previous surveys). More than half of the X-ray sources are variable. Sixteen of the sources have been optically identified or confirmed: 6 with foreground cool stars, 4 with Seyfert galaxies, 2 with SNR in the LMC, and 4 with peculiar hot LMC stars. Presumably the latter are all binaries, although only one (CAL 83) has been previously studied in detail. %R 1994PASP..106..858M %J-868 %T HL Canis Majoris in Preoutburst and SS Cygni -- The Interoutburst Disk Instability %A C. S. MANSPERGER, R. H. KAITCHUCK, P. M. GARNAVICH, N. DINSHAW, and E. ZAMKOFF %B SS Cygni and HL Canis Majoris were observed by IUE for three consecutive nights in November of 1992. During the first two nights, simultaneous photometric ground-based observations of SS Cyg were made at the Ball State University Observatory. Observations of SS Cyg and HL CMa were also obtained simultaneously with the 90-inch telescope at the Steward Observatory on the last two nights of the IUE run. These spectroscopic observations covered the wavelength range from 4100 A to 5000 A, while the spectra taken with the short wavelength camera on IUE resulted in wavelength coverage from 1150 A to 1980 A. SS Cyg is a U Gem type dwarf nova with an orbital period of 6.6 hours. Good simultaneous UV and optical orbital coverage was obtained for this system. HL CMa is a Z Cam type dwarf nova with a mean outburst interval of 15 days. The AAVSO reports that this system was in outburst 4 days after the observing run. Therefore, HL CMa may have been in a preoutburst state during these observations. Optical spectra of HL CMa indicate a warm front passed through the outer disk four days before outburst, but no changes were seen in the UV spectra. Signs of a preoutburst state were observed to develop in SS Cyg, but no outburst occurred for another 30 days. %R 1994PASP..106..869G %J-875 %T Three Type I Planetary Nebulae %A A. GUTIERREZ-MORENO, H. MORENO, and G. CORTES %B Observations of three planetary nebulae (PNe) with [N II] lambda-6583 > H-alpha are presented. They are He 2-111, He 2-145, and He 2-152. He 2-111 has already been classified by several authors as a type I PN. It is concluded that He 2-145 and He 2-152 are both type I PNe. Physical parameters of the nebulae and the central stars are given. It is found that He 2-152 has an extremely high He contents. The luminosities of the central stars run from 340 L_* for He 2-152 to 1090 L_* for He 2-111. When using distance independent parameters for the determination of the luminosity, much higher values are obtained, running from 5200 L_* for He 2-145 to 19800 L_* for He 2-111. %R 1994PASP..106..876C %J-878 %T The Changing Spectrum of the LMC Planetary Nebula N66 %A A. P. COWLEY, D. CRAMPTON, P. C. SCHMIDTKE, T. K. MCGRATH, and J. B. HUTCHINGS %B Recent spectroscopy and photometry of the planetary nebula N66 (SMP83) in the Large Magellanic Cloud show a continuing evolution, with a central WR spectrum becoming more visible. The planetary nebula shell and WR star have velocities with differ by ~240 km s^-1. Properties of this interesting object are reviewed, and we discuss its possible X-ray detection by ROSAT. %R 1994PASP..106..879W %J-889 %T Comments on Cross-Correlation Methodology in Variability Studies of Active Galactic Nuclei %A RUSSEL J. WHITE and BRADLEY M. PETERSON %B We discuss two separate cross-correlation methdologies, the interpolation method of Gaskell and Sparkle and the discrete correlation function of Edelson and Krolik, that are commonly used to quantify the lags between continuum and emission-line flux variations in active galactic nuclei (AGNs). We show that if similar assumptions are made to normalize the cross-correlation functions, the two methods are in good agreement for well-sampled AGN light curves. We also investigate the performance of cross-correlation methodology for less well-sampled data sets through Monte Carlo simulations that employ realistic models of the continuum behavior (based on well-observed Seyfert galaxies) and typical emission-line response times. We find that the interpolation method fairly accurately recovers the emission-line lags as the sampling is degraded (i.e., as the number of observed points is reduced). We find that for the cases investigated, the emission-line lags can be determined with reasonable accuracy even with mean sampling intervals as large as around two weeks. %R 1994PASP..106..890B %J-908 %T The Goddard High Resolution Spectrograph: Instrument, Goals, and Science Results %A J. C. BRANDT, S. R. HEAP, E. A. BEAVER, A. BOGGESS, K. G. CARPENTER, D. C. EBBETS, J. B. HUTCHINGS, M. JURA, D. S. LECKRONE, J. L. LINSKY, S. P. MARAN, B. D. SAVAGE, A. M. SMITH, L. M. TRAFTON, F. M. WALTER, R. J. WEYMANN, T. B. AKE, F. BRUHWEILER, J. A. CARDELLI, D. J. LINDLER, E. MALAMUTH, C. E. RANDALL, R. ROBINSON, S. N. SHORE, and G. WAHLGREN %B The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range of 1150-3200A with spectral resolutions (lambda/delta-lambda) of approximately 2 X 10^3, 2 X 10^4, and 1 X 10^5. This paper describes the instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future. %R 1994PASP..106..909N %J-914 %T The Montreal Near-Infrared Camera %A DANIEL NADEAU, DAVID C. MURPHY, RENE DOYON, and NEIL ROWLANDS %B A near-infrard (10 microns-2.5 microns) camera in use at the Canada-France-Hawaii Telescope and at the Observatoire du Mont Megantic 1.6 m telescope is described. The camera is based on a NICMOS3 256X256 HgCdTe array detector. While it is used for broad-band imaging and polarimetry through the standard J, H, and K filters, a distinguishing feature of its design is the use of circular variable filters for spectral line imaging in regions of strong continuum emission. The control system was designed to minimize overhead and be simple to use and to maintain. The performance of the instrument is described and illustrated with images of the H2 emission from the Cepheus A star formation region and a K-band image of the environment of the quasar 3C 345. %R 1994PASP..106..915B %J-915 %T Taking the Twinkle Out of the Stars: An Adaptive Wavefront Tilt Correction Servo and Preliminary Seeing Study for SUSI %A THEO TEN BRUMMELAAR %B The Sydney University Stellar Interferometer (SUSI) at Narrabri N.S.W., Australia, operates at optical wavelengths and has a maximum baseline of 640m. In order to form fringes with this instrument the distortion introduced into the wavefront by the atmosphere must be minimized. In SUSI this is done by restricting the aperture to r_0 or less, thereby sampling a basically flat but tilted wavefront. A pair of tip/tilt servos are then used to adjust for angle of arrival. \par The thesis contains a review of atmospheric turbulence theory including a new approximation for the temporal power spectra for any Zernike coefficient due to Kolmogorov turbulence. The temporal power spectra is calculated using the Taylor hypothesis of frozen turbulence. Unlike previous work of this kind the analysis is performed in the Fourier domain resulting in a solution that does not require numerical integration. A description of the tip/tilt servo follows along with a review of digital servo theory and an analysis of the servo performance. The final section of the thesis deals with atmospheric turbulence measurements made using the system. \par The tilt correction servo consists of `pyramid' quadrant detectors and piezo-electrically controlled tip/tilt mirrors. With a sample frequency of 1000 Hz the system measures image position and re-centers the stellar image. Based on measurements of many stellar objects over several nights the system was found to hold the two beams of the interferometer parallel with a standard deviation of 0.164 +- 0.025 arcseconds. With an aperture size of 0.06 meters this implies less than a 2% loss in the visibility measurements made by SUSI. The system has been used on stars as faint as magnitude 6.5 and is predicted to have a limiting magnitude of 7.5 and possibly as high as 8.5. \par By logging the mirror positions required to center the image a direct measurement of the wavefront phase gradient is obtained. Using this data it is possible to investigate atmospheric turbulence theory and, in particular, the wavefront tilt temporal power spectra. It is shown that at low frequencies the wavefront tilt power spectrum follows the -2/3 power law predicted by the theoretical analysis. At high frequencies the power law becomes -11/3, while the theory predicts -17/3. Since only a small part of the energy of the spectrum is contained in the high frequency domain this is of little practical importance. The difference is due to the fact that the servo measures average phase gradient which is not identical to the Zernike polynomials representing tilt. The system therefore not only corrects for the tilt introduced by the atmosphere but will supply a good estimate of seeing conditions using the same optical path as that used by the visibility measurement system itself. \par A preliminary investigation of seeing at the Narrabri site was undertaken, resulting in a median r_0 value of 7.1_-1.6^+2.1 cm at a wavelength of 500nm. Based on these measurements the seeing at the site has been shown to have a median of 1.3 seconds of arc with seeing better than 1.5 seconds of arc 75% of the time. These values are consistent with similar seeing studies at other observatory sites. %R 1994PASP..106..916D %J-916 %T Radio Spectra and Star Formation Histories in H II Galaxies %A HANS-JORG DEEG %B A sample of actively star forming H II galaxies has been observed in the radio continuum and several optical bands. These galaxies are currently undergoing very active star formation and have been selected on the basis of strong radio continuum emission. Most of the galaxies are irregular and have been associated with merger or interaction events, which may have triggered the star formation and the radio emission. Radio continuum observations at 0.32 GHz, 1.5 GHz, and in the range of 8-15 GHz were obtained at the NRAO-VLA, to determine their radio spectra. Several of the spectra were found to flatten towards lower frequencies, which is unusual. Surface and aperture photometry was obtained in the B,R, and I bands and in the H-alpha emission line. \par Radio emission, absorption, and relativistic electron loss mechanisms are reviewed and their suitability to account for the observed spectral shapes is discussed. Energy equipartition calculations led to galaxy magnetic fields of 10-30 mu-G; the radiation density inside the H II regions has been derived by a new method and was found to be in the range of 2 to 15 eVJcm^-3. Mechanisms which may account for the observed radio spectra were fitted to the radio continuum data. The spectra resulting from a time variable relativistic electron injection ('synchrotron aging') have been calculated over a greater range of parameters than previously published. Fits of these spectra show variations of the injection rate with time scales of the order of a few Myrs. A fit based on free-free absorption uses the emission measure to balance both free-free absorption and the rmal emission, thereby constraining the maximum size of the thermally emitting region. This permits a direct comparison with the optical H-alpha observations which typically show H II regions with sizes between 0.5 and 1 kpc. The fits allowed the derivation of a variety of physical parameters, such as the electron density, which is typically between 10 and 60 cm-3, and the emission measure, which is of the order of a few 10^5 pc cm^-6. Fits of different mechanisms may describe the same spectrum well, thus, in some cases, a unique determination of a particular mechanism is not possible, although an estimate for its likelihood can be made. Parts of this work have been published by Deeg et al. (1993, ApJ 410, 626). \par The fraction of thermal emission in the galaxies has been determined with a higher reliability than usual by combining radio, H-alpha, and UV data. A correlation between the fraction of thermal emission and the size, as well as the total luminosity of the galaxies has been found. A simple model of relativistic electron diffusion losses--dependent on a galaxies' size--can reproduce the observed correlation well; its suitability and limits are discussed. This correlation may lead to a better understanding between the supernova rate in a galaxy and the expected non-thermal emission, a relation which is currently only very poorly known. \par Star formation rates based on thermal and nonthermal radio and FIR emission all indicate star formation which is enhanced during the last 10^6-7 years as compared to the long term (1 Gyr) rates based on B band photometry. `Synchrotron aging', optical colors and the thermal to FIR ratio were used to derive typical ages of star formation. Based on the star formation rates and the age indicators, the galaxies were sorted into an sequence of their starbursts ages. The physical picture of a region in which star formation, subsequent SN explosions, and the resulting nonthermal radio emission takes place, can be accounted for well, by comparing the different star formation estimators which are based on a variety of radiative processes and across three regions of the electromagnetic spectrum. %R 1994PASP..106..917L %J-917 %T Group Delay Tracking with teh Sydney University Stellar Interferometer %A PETER R. LAWSON %B The Sydney University Stellar Interferometer (SUSI) is a long baseline optical interferometer, located at the Paul Wild Observatory near Narrabri, in northern New South Wales, Australia. It is designed to measure stellar angular diameters using light collected from a pair of siderostats, with 11 fixed siderostats giving separations between 5 and 640 m. Apertures smaller than Fried's coherence length, r_0, are used and active tilt-compensation is employed. This ensures that when the beams are combined in the pupil plane the wavefronts are parallel. Fringes are detected when the optical path-difference between the arriving wavefronts is less than tne coherence length of light used for the observation. While obsering a star it is necessary to compensate for the changes in pathlength due to the earth's rotation. It is also highly desirable to compensate for path changes due to the effects of atmospheric turbulence. Tracking the path-difference permits an accurate calibration of the fringe visibility, allows larger bandwidths to be used, and therefore improves the sensitivity of the instrument. \par I describe a fringe tracking system which I developed for SUSI, based on group delay tracking with a PAPA (Precision Analog Photon Address) detector. The method uses short exposure images of fringes, 1-10 ms, detected in the dispersed spectra of the combined starlight. The number of fringes across a fixed bandwidth of channeled spectrum is directly proportional to the path-difference between the arriving wavefronts. A Fast Fourier Transform, implemented in hardware, is used to calculate the spatial power spectrum of the fringes, thereby locating the delay. The visibility loss due to a non-constant fringe spacing on the detector is investigated, and the improvements obtained from rebinning the photon data are shown. The low light level limitations of group delay tracking are determined theoretically with emphasis on the probability of tracking error, rather than the signal-to-noise ratio. Experimental results from both laboratory studies and stellar observations are presented. These show the first closed-loop operation of a fringe tracking system based on observations of group delay with a stellar interferometer. \par The Sydney University PAPA camera, a photon counting array detector developed for use in this work, is also described. The design principles of the PAPA camera are outlined and the potential sources of image artifacts are identified. The artifacts arise from the use of optical encoding with Gray coded masks, and teh new camera is distinguished by its mask-plate, which was designed to overcome artifacts due to vignetting. Nw lens mounts are also presented which permit a simplified optical alignment without the need for tilt-plates. The performance of the camera is described. %R 1994PASP..106..918T %J-918 %T Optical and Ultraviolet Spectroscopy of VW Hydri %A A. TALAT SAYGAC %B Spectra of the SU UMa type dwarf nova VW Hydri obtained in the visual (lambda-lambda 3400-7200A) and ultraviolet (lambda-lambda 1150-3200A) spectral regions are presented. \par During the quiescent phase, visual spectra of VW Hydri present strong double-peaked Balmer emission lines. In addition, He I lambda 5876, Fe II lambda-lambda 4233, 5196 He I lambda 5015, Fe II lambda 5018, He I lambda 4922, Fe II lambda 4293, He II lambda 4686, and He I lambda-lambda 4471, 6678 appear in emission. The S-wave behavior of the hot spot is well defined. The amplitude of the radial velocity curve for H-alpha is 39 (+-11) km/s and for H-beta is 85 (+-23) km/s. From the radial velocity amplitude and the total line width we derived the primary mass in the range of 0.91 solar mass to 1.28 solar mass and the mass ratio q = M_1/M_2 = 4.4 to 6.4. We applied the method given by Schoembs and Vogt (1981), which is a modified version of the method described earlier by Warner (1973). \par We compared and discussed the fundamental physical and spectral parameters with those in the literature. \par In the ultraviolet region we obtained disk temperatures in the range of 14500-48000 K, using continuum fluxes. The continuum energy distributions were compared with optically thick accretion disk models. In the IUE SWP region the best-fit spectrum is of the form F_lambda proportional to lambda ^-2.4. In this region the effective disk radius is between 0.69R_odot and 0.75R_odot. The mass accretion rate (M) was computed with a relation given by Faulkner (1971), and is about 0^-10 M_odot yr^-1. From the P Cygni profile of the C IV doublet, terminal velocities of about 2250 km/s for a normal outburst and 2500 km/s for superoutburst were derived. Mass loss rates were derived using these velocities as about 0.8 to 6 x 10^-13 M_odot yr^-1 for normal outbursts and about 0.8 to 7 x 10^-13 M_odot yr^-1 for superoutburst.