Publications of the Astronomical Society of the Pacific

Dobias, Jan J.; Plavec, Mirek J.

doi: 10.1086/131972pmid: N/A

The authors have obtained IUE low-dispersion spectra and optical ITS scans of the semidetached Algol-type binary system RW Per. They have determined the spectral type of the primary component to be B9.6e, with good accuracy. The spectral type of the cooler component has been determined with less accuracy as K2 III - IV. The system is heavily reddened, its color excess being E(B-V) = 0.40±0.05 mag. D. S. Hall reported a rapid secular decrease in the duration of the totality of the primary eclipse. The only plausible explanation seems to be that the hotter component is actually an optically thick accretion disk. The ultraviolet spectrum observed near mideclipse does not display the expected "W Serpentis-type" emission lines. In the optical region, the system is a Be star. The secondary is probably a star of very low mass. The exciting fact about RW Per is the possibility that we are really observing an optically thick accretion disk.

Hodge, P. W.; Mateo, M.; Lee, M. G.; Geisler, Doug

doi: 10.1086/131973pmid: N/A

Integrated UBVR photometry is given for a selection of young star clusters and compact OB associations in M31. Most are very young and, except for those in the outer parts of the galaxy, most show significant reddening. The brightest have absolute magnitudes of Mv = -8.0 and very blue colors. Clusters in this sample also include objects as faint as Mv = -3.7. The oldest disk clusters in the sample have ages, estimated from their colors, of approximately 4×108years, and the youngest are only a few million years old. Comparison of the cluster luminosity function with that of the LMC suggests that present sampling of M31 disk clusters is very incomplete; there probably are at least 10,000 such clusters still undetected.

Walker, Merle F.

doi: 10.1086/131974pmid: N/A

Photoelectric UBV measurements of faint stars in the Magellanic Cloud clusters NGC 339, NGC 458, NGC 2004, and NGC 2164, obtained with the 1.5-m Cerro Tololo Inter-American Observatory in 1972 and 1973, are presented. The present as-yet unpublished sample of stars range in V brightness from 15.2 to 18.0, and in (B-V) color from -0.20 to + 1.82. The data provide zero-point standards for use both in these clusters and in other clusters in the Magellanic Clouds.

Fernie, J. D.

doi: 10.1086/131975pmid: N/A

It has been discovered accidentally that the bright G-giant, δ CrB, shows roughly sinusoidal light variations of amplitude 0.06 mag and period about 45 days. Since the star is not a spectroscopic binary, it is suggested that the variation is caused by the star having an active chromosphere, the rotation (of period 45 days) revealing starspots. The star would thus be similar to λ And. An IUE spectrum supports the suggestion of an active chromosphere.

Herczeg, T. J.

doi: 10.1086/131976pmid: N/A

Cassini, in his observations of Nova Vul in 1671, repeatedly used β Lyr for brightness comparison, apparently without even suspecting that it was a variable star. An attempt was made to correlate Cassini's "indirect" observations of β Lyr with modern ephemeris formulae in order to find out whether he could have overlooked a primary eclipse. None of the modern ephemeris formulae would allow a reliable extrapolation of minimum epochs back to 1671, but the coverage of the light curve by the unintended observations suggests perhaps a modest chance that the 17th-century observers missed the variable near its primary minimum.

Stetson, Peter B.

doi: 10.1086/131977pmid: N/A

The tasks of the DAOPHOT program, developed to exploit the capability of photometrically linear image detectors to perform stellar photometry in crowded fields, are discussed. Raw CCD images are prepared prior to analysis, and following the obtaining of an initial star list with the FIND program, synthetic aperture photometry is performed on the detected objects with the PHOT routine. A local sky brightness and a magnitude are computed for each star in each of the specified stellar apertures, and for crowded fields, the empirical point-spread function must then be obtained for each data frame. The GROUP routine divides the star list for a given frame into optimum subgroups, and then the NSTAR routine is used to obtain photometry for all the stars in the frame by means of least- squares profile fits. The process is illustrated with images of stars in a crowded field, and shortcomings and possible improvements of the program are considered.

Shara, Michael M.; Doxsey, Rodger; Wells, Eddie N.; McAlister, Harold A.

doi: 10.1086/131978pmid: N/A

It is observationally determined that a significant number of guide stars which will be used by the Hubble Space Telescope are close binaries which can prevent a Fine-Guidance-Sensor fine lock. This is because the Fine Guidance Sensors (FGSs) operate as white-light interferometers, and the fringe patterns of binaries will result in a reduction in the loop gain in the FGS control system. Consequently, multiple guide stars must be attempted to ensure a successful acquisition. The limited HST on-board computer memory may restrict the number of guide stars which can be attempted, resulting in a possible loss of several hundred hours per year of observing time. The FGSs of the next-generation (and presumably larger) space-telescopes would be even more susceptible to the binary problem. Other types of guidance systems (such as quadrant detectors) deserve serious consideration for future space observatories.

Robinson, Lloyd; Osborne, Jack

doi: 10.1086/131979pmid: N/A

Using a virtual-phase thermoelectrically cooled CCD, an inexpensive and sensitive slow-scan camera has been built for field acquisition and guiding with the 1-m and 3-m Lick Observatory telescopes. Its large dynamic range, geometrical stability, low-voltage operation, and fast recovery from overexposure make it preferable to the intensified SEC cameras used previously for telescope guiding. On nights of good seeing the CCD cameras have reached 19th-20th magnitude stars at the 1-m telescope and as deep as 22nd magnitude at the 3-m telescope with a 15-30 sec integration time.

Garmany, Catharine D.; Walborn, Nolan R.

doi: 10.1086/131980pmid: N/A

Detailed spectral classifications and remarks are given for seven recently discovered, very early O stars of high luminosity in the Large Magellanic Cloud. Three of them are definitely of type O3 III(f*), with weak N IV emission and N V absorption, bringing to five the number of objects in that category, all of them in the Magellanic Clouds; an illustration of their spectra is presented. One definite and two possible O3 objects are located together with several later O-type stars in the remarkable cluster NGC 2122, for which an identification chart is given.