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Search Results: 1 - 10 of 145233 matches for " Theodore B. Stecher "
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UV Signposts of Resonant Dynamics in Disk Galaxies
William H. Waller,Christine Winslow,Michael Fanelli,Theodore B. Stecher
Physics , 1998,
Abstract: Imaging in the restframe ultraviolet has proven to be an effective and vital means of tracing dynamical patterns of star formation in galaxies out to high redshifts. Using images from the Ultraviolet Imaging Telescope (UIT), Hubble Space Telescope (HST) and complementary groundbased telescopes, we have investigated the starburst activity and associated dynamics in nearby early-type disk galaxies. Concentrating on the starburst-ring (R)SA(r)ab galaxy M94 (NGC 4736), we find compelling evidence for bar-mediated resonances as the primary drivers of evolution at the present epoch. Similar ring-bar dynamics may prevail in the centers of early-type disk galaxies at high redshift. The gravitationally-lensed "Pretzel Galaxy" (0024+1654) at a redshift of ~1.5 provides an important precedent in this regard.
HST Observations of New Horizontal Branch Structures in the Globular Cluster omega Centauri
Noella L. D'Cruz,Robert W. O'Connell,Robert T. Rood,Jonathan H. Whitney,Ben Dorman,Wayne B. Landsman,Robert S. Hill,Theodore P. Stecher,Ralph C. Bohlin
Physics , 1999, DOI: 10.1086/308375
Abstract: The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are ``extreme'' HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a significant population of hot sub-HB stars, which we interpret as the ``blue-hook'' objects predicted by D'Cruz et al. (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. omega Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass loss efficiency which increases with metallicity.
UIT Detection of Hot Stars in the Globular Cluster NGC362
Ben Dorman,Ronak Y. Shah,Robert W. O'Connell,Wayne B. Landsman,Robert T. Rood,Ralph C. Bohlin,Susan G. Neff,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1997, DOI: 10.1086/310613
Abstract: We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2 mission to obtain a deep far-UV image of the globular cluster NGC 362, which was formerly thought to have an almost entirely red horizontal branch (HB). 84 hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in NGC 362, and at least 34 are cluster members. The number of cluster members is made uncertain by background contamination from blue stars in the Small Magellanic Cloud (SMC). There are six candidate supra-HB stars which have probably evolved from the HB. We discuss the implications of these results for the production of hot blue stars in stellar populations.
Ultraviolet Imaging of the Globular Cluster 47 Tucanae
Robert W. O'Connell,Ben Dorman,Ronak Y. Shah,Robert T. Rood,Wayne B. Landsman,Ralph C. Bohlin,Susan G. Neff,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1997, DOI: 10.1086/118619
Abstract: We have used the Ultraviolet Imaging Telescope to obtain deep far-UV (1620 Angstrom), 40' diameter images of the prototypical metal-rich globular cluster 47 Tucanae. We find a population of about 20 hot (Teff > 9000 K) objects near or above the predicted UV luminosity of the hot horizontal branch (HB) and lying within two half-light radii of the cluster center. We believe these are normal hot HB or post-HB objects rather than interacting binaries or blue stragglers. IUE spectra of two are consistent with post-HB phases. These observations, and recent HST photometry of two other metal-rich clusters, demonstrate that populations with rich, cool HB's can nonetheless produce hot HB and post-HB stars. The cluster center also contains an unusual diffuse far-UV source which is more extended than its V-band light. It is possible that this is associated with an intracluster medium, for which there was earlier infrared and X-ray evidence, and is produced by C IV emission or scattered light from grains.
Luminosities and Star Formation Rates Of Galaxies Observed With the Ultraviolet Imaging Telescope: A Comparison of Far-UV, H-alpha, and Far-IR Diagnostics
Michael N. Fanelli,Theodore P. Stecher,the UIT Science Team
Physics , 1996, DOI: 10.1063/1.52770
Abstract: During the UIT/Astro Spacelab missions, the Ultraviolet Imaging Telescope obtained spatially resolved far-UV (lambda 1500 A) imagery of ~35 galaxies exhibiting recent massive star formation. The sample includes disk systems, irregular, dwarf, and blue compact galaxies. The objects span an observed FUV luminosity range from -17 to -22 magnitudes. We estimate global star formation rates by comparing the observed FUV fluxes to the predictions of stellar population models, and compare the FUV-derived astration rates to those derived from H-alpha and far-IR photometry.
Ultraviolet Imaging Telescope (UIT) Observations of the SMC
Robert H. Cornett,Theodore P. Stecher,the UIT Science Team
Physics , 1996,
Abstract: A mosaic of four UIT far-UV (FUV; 1620A) images, which covers most of the SMC bar, is presented, with derived stellar and HII region photometry. The UV morphology of the SMC's Bar shows that recent star formation there has left striking features including: a) four concentrations of UV-bright stars spread from northeast to southwest at nearly equal (~30 arcmin=0.5 kpc) spacings; b) one concentration comprising a well-defined 8-arcmin diameter ring surrounded by a larger H-alpha ring, suggestive of sequential star formation. FUV PSF photometry is obtained for 11,306 stars, and FUV photometry is obtained for 42 H-alpha-selected HII regions, both for the stars and for the total emission contained in the apertures defined by Kennicutt & Hodge. The flux- weighted average ratio of total to stellar FUV flux is 2.15; the stellar FUV luminosity function indicates that most of the excess total flux is due to scattered FUV radiation, rather than faint stars. Both stellar and total emission are well correlated with H-alpha fluxes, and yield FUV/H-alpha ratios that are consistent with models of single-burst clusters with SMC metallicity, ages from 1-5 Myr, and moderate (E(B-V)=0.0-0.1 mag) internal SMC extinction.
The Ultraviolet Imaging Telescope: Instrument and Data Characteristics
Theodore P. Stecher,Robert H. Cornett,Michael R. Greason,Wayne B. Landsman,Jesse K. Hill,Robert S. Hill,Ralph C. Bohlin,Peter C. Chen,Nicholas R. Collins,Michael N. Fanelli,Joan I. Hollis,Susan G. Neff,Robert W. O'Connell,Joel D. Offenberg,Ronald A. Parise,Joel Wm. Parker,Morton S. Roberts,Andrew M. Smith,William H. Waller
Physics , 1997, DOI: 10.1086/133917
Abstract: The Ultraviolet Imaging Telescope (UIT) was flown as part of the Astro observatory on the Space Shuttle Columbia in December 1990 and again on the Space Shuttle Endeavor in March 1995. Ultraviolet (1200-3300 Angstroms) images of a variety of astronomical objects, with a 40 arcmin field of view and a resolution of about 3 arcsec, were recorded on photographic film. The data recorded during the first flight are available to the astronomical community through the National Space Science Data Center (NSSDC); the data recorded during the second flight will soon be available as well. This paper discusses in detail the design, operation, data reduction, and calibration of UIT, providing the user of the data with information for understanding and using the data. It also provides guidelines for analyzing other astronomical imagery made with image intensifiers and photographic film.
UIT Astro-2 Observations of NGC 4449
Robert S. Hill,Michael N. Fanelli,Denise A. Smith,Theodore P. Stecher,the UIT Team
Physics , 1996, DOI: 10.1063/1.52826
Abstract: The bright Magellanic irregular galaxy NGC 4449 was observed by the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission in March, 1995. Far ultraviolet (FUV) images at a spatial resolution of ~3 arcsec show bright star-forming knots that are consistent with the general optical morphology of the galaxy and are often coincident with bright H II regions. Comparison of FUV with H-alpha shows that in a few regions, sequential star formation may have occurred over the last few Myr. The bright star forming complexes in NGC 4449 are superposed on a smooth, diffuse FUV background that may be associated with the H-alpha "froth."
Orchestration of Starbirth Activity in Disk Galaxies: New Perspectives from Ultraviolet Imaging
William H. Waller,Theodore P. Stecher,the Ultraviolet Imaging Telescope,Science Team
Physics , 1996, DOI: 10.1063/1.52794
Abstract: Ultraviolet imaging of nearby disk galaxies reveals the star-forming activity in these systems with unprecedented clarity. UV images recently obtained with the Shuttle-borne Ultraviolet Imaging Telescope (UIT) reveal a remarkable variety of star-forming morphologies. The respective roles of tides, waves, and resonances in orchestrating the observed patterns of starbirth activity are discussed in terms of the extant UV data.
The UIT Survey of the Ultraviolet Sky Background
William H. Waller,Theodore P. Stecher,Ultraviolet Imaging Telescope Science Team
Physics , 1996,
Abstract: When viewed from above the Earth's atmosphere, the nighttime ultraviolet sky background is profoundly dark - up to 100 times fainter than the equivalent visible background as measured by groundbased telescopes. Because the UV background is so faint, its measured strength and spatial distribution remain controversial topics. Estimates range from a few hundred photons/(sec cm^2 sr^2 A) (~27 mag/arcsec^2) with no obvious spatial distribution to several thousand ``photon units'' with a strong gradient toward the Galactic midplane. Herein, we summarize recent results from an analysis of UV images obtained by the Ultraviolet Imaging Telescope - where relatively high nighttime UV intensities are found near the Galactic midplane which then diminish to levels of ~300 photon units or less at high Galactic latitude.
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