Publications of the Astronomical Society of the Pacific

Bothun, G.; Impey, C.; McGaugh, S.

doi: 10.1086/133941pmid: N/A

In twenty years, low surface brightness (LSB) galaxies have evolved from being an idiosyncratic notion to being one of the major baryonic repositories in the Universe. The story of their discovery and the characterization of their properties is told here. Their recovery from the noise of the night sky background is a strong testament to the severity of surface brightness selection effects. LSB galaxies have a number of remarkable properties which distinguish them from the more familiar Hubble Sequence of spirals. The two most important are 1) they evolve at a significantly slower rate and may well experience star formation outside of the molecular cloud environment, 2) they are embedded in dark matter halos which are of lower density and more extended than the halos around high surface brightness (HSB) disk galaxies. Compared to HSB disks, LSB disks are strongly dark matter dominated at all radii and show a systematic increase in M/L with decreasing central surface brightness. In addition, the recognition that large numbers of LSB galaxies actually exist has changed the form of the galaxy luminosity function and has clearly increased the space density of galaxies at z =0. Recent CCD surveys have uncovered a population of red LSB disks that may be related to the excess of faint blue galaxies detected at moderate redshifts. LSB galaxies offer us a new window into galaxy evolution and formation which is every bit as important as those processes which have produced easy to detect galaxies. Indeed, the apparent youth of some LSB galaxies suggest that galaxy formation is a greatly extended process. While the discovery of LSB galaxies have lead to new insights, it remains unwise to presume that we now have a representative sample which encompasses all galaxy types and forms.

Wolff, S.; Simon, T.

doi: 10.1086/133942pmid: N/A

Rotational velocities are reported for intermediate-mass main sequence stars it the field. The measurements are based on new, high S/N CCD spectra from the Coudé Feed Telescope of the Kitt Peak National Observatory. We analyze these rotation rates for a dependence on both mass and age. We compare the average rotation speeds of the field stars with mean velocities for young stars in Orion, the Alpha Persei cluster, the Pleiades, and the Hyades. The average rotation speeds of stars more massive than ∼1.6 M⊙\experience little or no change during the evolutionary lifetimes of these stars on the zero age main sequence or within the main sequence band. Less massive stars in the range betwee n 1.6M⊙\ and 1.3M⊙\ also show little decline in mean rotation rate while they are on the main sequence, and at most a factor of 2 decrease in velocity as they evolve off the main sequence. The {e}-folding time for the loss of angular momentum b y the latter group of stars is at least 1--2 billion years. This inferred characteristic time scale for spindown is far longer than the established rotational braking time for solar-type stars with masses below ∼1.3 M⊙. We conclude from a comparison of the trends in rotation with trends in chromospheric and coronal activity that the overall decline in mean rotation speed along the main sequence, from ∼2 M⊙\ down to ∼1.3 M⊙, is imposed during the pre-main sequence phase of evolution, and that this pattern changes little thereafter while the star resides on the main sequence. The magnetic activity implicated in the rotational spindown of the Sun and of similar stars during their main sequence lifetimes mus t therefore play only a minor role in determining the rotation rates of the intermediate mass stars, either because a solar-like dynamo is weak or absent, or else the geometry of the magnetic field is appreciably less effective in removing angular momentu m from these stars.

King, J. R.

doi: 10.1086/133943pmid: N/A

We present high resolution, high S/N spectra in the λ6707 Li 1 region of the cool metal-poor dwarfs HD 103095 (={HR 4550}={Gmb 1830}), 134439, and 134440. Previous detections of weak Li in the first two stars are confirmed. We suggest that neither standard nor rotational stellar models we consider are able to satisfactorily account for the Li depletion of HD 103095. In contrast, both standard and rotational models are able to account for the Li abundance of HD 134439. This apparent disparity reappears when we find that the metallicity-sensitivity of the Li depletion models can not be confirmed by our or previously published Li abundances of HD 103095 and HD 134439. HD 134439 and 134440 form a common proper motion pair with virtually identical kinematics and heavy element abundances. Thus, it is likely that the Li abundances of these two field stars can be compared without undue concern for differences in their ages, compositions, or any Li enrichment history. Moreover, we do not find any significant Na abundance differences, hinted at by previous work, which would cloud the interpretation of the proper motion pair Li abundances. The {≥}0.6 dex Li abundance difference between HD 134439 and the {∼}200 K cooler HD 134440 implies that the Li-Teff morphology is a sharp function of temperature below Teff{∼}5000 K. This rather precipitous decline in Li abundance is consistent both with the standard and rotational stellar models considered here.

Maceroni, C.; Rucinski, S. M.

doi: 10.1086/133944pmid: N/A

The photometric data for a short-period (0.1984 day) eclipsing binary V38 discovered by the OGLE microlensing team in Baade's Window field BW3 have been analyzed. The light-curve synthesis solution of the I-filter light curve and de-reddened color (V-IC)0=2.3 suggest a pair of strongly-distorted M-dwarfs, with parameters between those of YY~Gem and CM~Dra, revolving on the tightest known orbit among binaries consisting of main sequence stars. The primary, more massive and hotter, component maybe filling its Roche lobe. The very small amount of angular momentum in the orbital motion makes the system particularly important for studies of angular momentum loss at the faint end of the main sequence. Spectroscopic observations of the orbital radial velocity variations as well as of activity indicators are urgently needed for a better understanding of the angular-momentum and internal-structure evolutionary state of the system.

Evans, N. R.; Bohm-Vitense, E.; Carpenter, K.; Beck-Winchatz, B.; Robinson, R.

doi: 10.1086/133945pmid: N/A

Two medium resolution spectra of the hot companion of the Cepheid V350 Sgr have been obtained with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. Comparison of these spectra (in the wavelength range 1840 to 1880 Å) with the spectrum of α Lyr shows that V350 Sgr B has a projected rotation velocity which may be as high as 150 km sec-1. The velocity difference between the spectra of V350 Sgr B at two orbital phases is measured to be -23.1 ± 3.8 km sec-1. The error is dominated by the aperture centering. When combined with the orbital velocity variation of the Cepheid derived from the ground-based orbit and the mass of the companion deduced from IUE spectra, the mass of the Cepheid is found to be 5.2 ± 0.9 M\sun. The observed mass--luminosity combination of V350 Sgr A is a good match to recent evolutionary calculations which use moderate convective overshoot near the main sequence, however, the blue loops do not extend to temperatures as hot as the Cepheid.

Goswami, A.; Rao, N. K.; Lambert, D. L.; Gonzalez, G.

doi: 10.1086/133946pmid: N/A

Z Ursa Minoris was classified by Benson et al. (1994) a a R Coronae Borealis (RCB) variable star from its light variations. Hydrogen deficiency, which is a defining feature of RCB stars, was not established. To investigate this aspect we have obtained high resolution spectra in both blue (4200-4630Å) and red (5050-7950Å) regions. Lines of the CH molecule (G band) at about 4300Å, which are present in spectra of N-type carbon stars are weak or absent in the spectrum of Z UMi indicating its hydrogen deficient nature and membership of the rare class of RCB variables.

Geisler, D.; Claria, J. J.; Minniti, D.

doi: 10.1086/133947pmid: N/A

One hundred and ten stars associated with ten intermediate metallicity Galactic globular clusters have been investigated with the Washington photometric system. The data yield a luminosity class, temperature and metal abundance for each star. The luminosity classification is used to separate field stars from cluster giants. Mean metal abundances for an average of ten member giants per cluster are determined to ∼0.1 dex (internal error). These mean abundances are in good general agreement with existing metallicity scales.

Smutko, M. F.

doi: 10.1086/133948pmid: N/A

This paper details the design, construction, and performance of the deformable mirrors and their driving electronics built for the Chicago Adaptive Optics System (ChAOS). Both the mirrors and the drivers are constructed at the University of Chicago using a combination of commercially available and custom-built components. Mirrors ranging in size from 7 to 201 actuators have been constructed and several of these mirrors have been in use on the Apache Point 3.5 m telescope since April of 1995. I discuss some theoretical design issues involved in building deformable mirrors and I demonstrate the functionality of these mirrors by presenting some images obtained with ChAOS.

Roddier, F.; Roddier, C.

doi: 10.1086/133949pmid: N/A

The detection of faint light sources very close to a bright star is primarily limited by light scattered by the Earth's atmosphere. This source of scattered light can now be reduced by means of adaptive optics, or totally eliminated by using a telescope in space. Then diffraction by the telescope aperture becomes the primary source of scattered light. Whereas a classical Lyot coronograph can reduce the amount of light diffracted away from the star, it becomes inefficient very close to the star. Instead of forming the stellar image on an opaque mask, it is proposed here to use a small phase plate which produces a 180 degrees phase shift on the core of the stellar image. Light diffracted outside the core is then eliminated by destructive interference. Applied to the Hubble Space Telescope, the technique would easily allow detection of a stellar companion 0.3" away from a star and at least 8 magnitude fainter.

Gorjian, V.; Wright, E. L.; McLean, I. S.

doi: 10.1086/133950pmid: N/A

We present the first results using infrared arrays, devices which do not employ charge coupling, in a new "drift scanning" mode to significantly increase the area of the sky which can be surveyed, especially in the high background 3-5 \mm bands. Even with fast electronics, wide field imaging in the thermal IR is difficult due to the high backgrounds. Often, only a sub-array of the frame can be readout before saturation. Drift scanning allows this sub-array to be a long narrow strip which is stepped perpendicular to the long side, by allowing the telescope to track at non-sidereal rates, and then a mosaic is constructed in the computer memory in real time. The method has been implemented with the UCLA Twin-Channel Infrared Camera system which employs two different classes of arrays. One is the NICMOS 3 HgCdTe array and the other is an SBRC InSb array; both detectors have 256\xx256 pixels. Using a drift rate of 1 arcminute/minute on the Shane 3-m telescope at Lick Observatory, we are able to survey about 180 square arcminutes per hour at two wavelengths simultaneously (e.g. J,K or H,L). Limiting magnitudes are typically J=15, L= 12. Data reduction methods are described and observations of the globular cluster M3, the star forming region M17, and the Moon are used as illustration.