A SEARCH FOR CHROMOSPHERICALLY ACTIVE STARS FROM THE ROSAT EUV SOURCE LISTMulliss, Christopher L.; Bopp, Bernard W.
doi: 10.1086/133449pmid: N/A
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 (7x107 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.
ABUNDANCES IN THREE HEAVY-ELEMENT STARS IN OMEGA CENTAURIVanture, Andrew D.; Wallerstein, George; Brown, Jeffery A.
doi: 10.1086/133451pmid: N/A
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. 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 109 years before Type Ia SNe swept the cluster clear of gas and terminated star-formation.
LMC STELLAR X-RAY SOURCES OBSERVED WITH ROSAT: I. X-RAY DATA AND SEARCH FOR OPTICAL COUNTERPARTSSchmidtke, P. C.; Cowley, A. P.; Frattare, L. M.; McGrath, T. K.; Hutchings, J. B.; Crampton, D.
doi: 10.1086/133452pmid: N/A
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.
HL CANIS MAJORIS IN PREOUTBURST AND SS CYGNI -- THE INTEROUTBURST DISK INSTABILITYMansperger, C. S.; Kaitchuck, R. H.; Garnavich, P. M.; Dinshaw, N.; Zamkoff, E.
doi: 10.1086/133453pmid: N/A
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.
THREE TYPE I PLANETARY NEBULAEGutierrez-Moreno, A.; Moreno, H.; Cortes, G.
doi: 10.1086/133454pmid: N/A
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.
COMMENTS ON CROSS-CORRELATION METHODOLOGY IN VARIABILITY STUDIES OF ACTIVE GALACTIC NUCLEIWhite, Russell J.; Peterson, Bradley M.
doi: 10.1086/133456pmid: N/A
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.