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Search Results: 1 - 10 of 201441 matches for " Theodore P. Stecher "
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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.
A Comparison of Ultraviolet, Optical, and X-Ray Imagery of Selected Fields in the Cygnus Loop
Charles W. Danforth,Robert H. Cornett,N. A. Levenson,William P. Blair,Theodore P. Stecher
Physics , 2000, DOI: 10.1086/301365
Abstract: During the Astro-1 and Astro-2 Space Shuttle missions in 1990 and 1995, far ultraviolet (FUV) images of five 40' diameter fields around the rim of the Cygnus Loop supernova remnant were observed with the Ultraviolet Imaging Telescope (UIT). These fields sampled a broad range of conditions including both radiative and nonradiative shocks in various geometries and physical scales. In these shocks, the UIT B5 band samples predominantly CIV 1550 and the hydrogen two-photon recombination continuum. Smaller contri- butions are made by emission lines of HeII 1640 and OIII] 1665. We present these new FUV images and compare them with optical Halpha and [OIII], and ROSAT HRI X-ray images. Comparing the UIT images with those from the other bands provides new insights into the spatial variations and locations of these different types of emission. By comparing against shock model calculations and published FUV spectroscopy at select locations, we surmise that resonance scattering in the strong FUV permitted lines is widespread in the Cygnus Loop, especially in the bright optical filaments typically selected for observation in most previous studies.
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.
A New Probe of the Molecular Gas in Galaxies: Application to M101
Denise A. Smith,Ronald J. Allen,Ralph C. Bohlin,Natalya Nicholson,Theodore P. Stecher
Physics , 2000, DOI: 10.1086/309172
Abstract: Recent studies of nearby spiral galaxies suggest that photodissociation regions (PDRs) are capable of producing much of the observed HI in galaxy disks. In that case, measurements of the HI column density and the far-ultraviolet (FUV) photon flux provide a new probe of the volume density of the local underlying H_2. We develop the method and apply it to the giant Scd spiral M101 (NGC 5457). We find that, after correction for the best-estimate gradient of metallicity in the ISM of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30-1000 cm^-3 is found near star- forming regions at all radii in the disk of M101 out to a distance of 12' (approximately 26 kpc), close to the photometric limit of R_25 = 13.5'. In this picture, the ISM is virtually all molecular in the inner parts of M101. The strong decrease of the HI column density in the inner disk of the galaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gas ratio there, resulting in an increase of the H_2 formation rate on grains and a corresponding disappearance of hydrogen in its atomic form.
Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud
Joel Wm. Parker,Dennis Zaritsky,Theodore P. Stecher,Jason Harris,Phil Massey
Physics , 2000, DOI: 10.1086/318765
Abstract: Using photometry from the Ultraviolet Imaging Telescope (UIT) and photometry and spectroscopy from three ground-based optical datasets we have analyzed the stellar content of OB associations and field areas in and around the regions N 79, N 81, N 83, and N 94 in the LMC. We compare data for the OB association Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF) may depend on different photometric reductions and calibrations. We also correct for the background contribution of field stars, showing the importance of correcting for field star contamination in determinations of the IMF of star formation regions. It is possible that even in the case of an universal IMF, the variability of the density of background stars could be the dominant factor creating the differences between calculated IMFs for OB associations. We have also combined the UIT data with the Magellanic Cloud Photometric Survey to study the distribution of the candidate O-type stars in the field. We find a significant fraction, roughly half, of the candidate O-type stars are found in field regions, far from any obvious OB associations. These stars are greater than 2 arcmin (30 pc) from the boundaries of existing OB associations in the region, which is a distance greater than most O-type stars with typical dispersion velocities will travel in their lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massive star formation) will have to be determined by further observations and analysis.
Ultraviolet Imaging of the Irregular Galaxy NGC 4449 with UIT: Photometry and Recent Star-Formation History
Robert S. Hill,Michael N. Fanelli,Denise A. Smith,Ralph C. Bohlin,Susan G. Neff,Robert W. O'Connell,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1998, DOI: 10.1086/306302
Abstract: The bright Magellanic irregular galaxy NGC 4449 was observed during the Astro-2 Space Shuttle mission by the Ultraviolet Imaging Telescope (UIT), which obtained images of a ~40 arcmin field centered on the galaxy in two broad far-ultraviolet (FUV) bands centered at 1520 A and 1620 A, with 3 arcsec - 5 arcsec spatial resolution. Together with H-alpha and H-beta fluxes from ground-based Fabry-Perot images, these data are analyzed in order to explore the recent star formation history of NGC 4449. Maps of the flux ratios H-alpha/FUV and FUV/blue continuum are presented and interpreted using evolutionary synthesis models. Photometry is presented both for 22 apertures containing large OB complexes and for 57 small apertures containing compact FUV-emitting knots. The OB complexes along the northern edge of the visible system have high H-alpha/FUV ratios, and thus appear to be more dominated by the current generation of stars than are other parts of the galaxy. However, young sources do exist elsewhere and are particularly conspicuous along the bar. The small aperture analysis shows three candidate regions for sequential star formation. Surface brightness profiles are consistent with an exponential disk in both the FUV and the optical continuum.
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.
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