arXiv subjects

R. W. O'Connell

Publications and source records attributed to R. W. O'Connell.

44 records · Page 3Linked to original sources

arXiv1998

Far-Ultraviolet Color Gradients in Early-Type Galaxies

We discuss far-UV (1500 A) surface photometry and FUV-B color profiles for 8 E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope, primarily during the Astro-2 mission. In three cases, the FUV radial profiles are more consistent with an exponential than a de Vaucouleurs function, but there is no other evidence for the presence of a disk or of young, massive stars. In all cases except M32 the FUV-B color becomes redder at larger radii. There is a wide range of internal radial FUV-B color gradients. However, we find no correlation between the FUV-B color gradients and internal metallicity gradients based on Mg absorption features. We conclude that metallicity is not the sole parameter controlling the "UV upturn component" in old populations.

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arXiv1998

UV Spectra, Line Indices of M31 Globular Clusters and E Galaxy Cores

Observations in integrated light (2200-4800 A, 8 to 11 A resolution, S/N > 20) of 4 M31 globular clusters and of the cores of six gE galaxies made with the HST/FOS are presented and analyzed. These data are supplemented by IUE observations of M32, Galactic globulars and stars. From these data we measure UV colors and absorption line strength (ALS) indices, including the key NH feature at 3360 A. We find that our data divide the M31 globular clusters, Galactic globular clusters/Galactic stars, and gE galaxies into three different stellar populations. The old stellar populations of M31 globular clusters and elliptical galaxies are more similar to each other than each is to the Galactic stellar population. Of special note is that the NH feature is so strong in the spectra of M31 globulars that a large (>20) overabundance in nitrogen in these stars, relative to that in Galactic stars, is likely required. Other ALS indices (of Mg, Al) also show significant differences.

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arXiv1998

The Hot Stars of Old Open Clusters: M67, NGC 188 and NGC6791

We analyze ultraviolet (~1500 A) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -- white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stragglers dominate the integrated spectrum at wavelengths shorter than 2600 A. The number of white dwarfs in M67 is roughly consistent with the number expected from white dwarf cooling models. Eight candidate sdB/sdO stars are detected in NGC 6791, and two are detected in NGC 188. The luminosity range 1.10 < log L/Lsun < 1.27, derived from the ultraviolet photometry of the six sdB candidates, is consistent with theoretical models of metal-rich hot horizontal branch (HB) stars. The fraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%, implying that the integrated spectra of both clusters should show a UV turnup at least as strong as that observed in any elliptical galaxy.

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arXiv1996

Ultraviolet Imaging Observations of the cD Galaxy in Abell 1795: Further Evidence for Massive Star Formation in a Cooling Flow

We present images from the Ultraviolet Imaging Telescope of the Abell 1795 cluster of galaxies. We compare the cD galaxy morphology and photometry of these data with those from existing archival and published data. The addition of a far--UV color helps us to construct and test star formation model scenarios for the sources of UV emission. Models of star formation with rates in the range $\sim5-20M_{\sun}$yr$^{-1}$ indicate that the best fitting models are those with continuous star formation or a recent ($\sim4$ Myr old) burst superimposed on an old population. The presence of dust in the galaxy, dramatically revealed by HST images complicates the interpretation of UV data. However, we find that the broad--band UV/optical colors of this cD galaxy can be reasonably matched by models using a Galactic form for the extinction law with $E_{B-V}=0.14$. We also briefly discuss other objects in the large UIT field of view.

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arXiv1996

Ultraviolet Imaging Telescope Observations of OB Stars in the N 11 Region of the LMC

We present an analysis of far-ultraviolet (FUV: 130-180 nm) and optical (U, B, and V) data of the stellar and nebular content of the OB associations LH 9, 10, and 13 in the Large Magellanic Cloud region N 11. The FUV images from The Ultraviolet Imaging Telescope strongly select the hot O and B stars; over 1900 stars were detected in the FUV to a limiting magnitude of m(FUV) = 17 mag. The resulting FUV photometry combined with optical ground-based data indicate there are approximately 88 confirmed or candidate O stars in the LH 9, 10, and 13 fields alone (in an area of ~ 41 arcmin$^2$), and possibly as many as 170 to 240 O-type stars within the entire 40~arcmin-diameter field of view.

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arXiv1996

Ultraviolet Imagery of NGC 6752: A Test of Extreme Horizontal Branch Models

We present a 1620 A image of the nearby globular cluster NGC 6752 obtained with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 mission of the Space Shuttle Endeavour in 1995 March. An ultraviolet-visible color-magnitude diagram (CMD) is derived for 216 stars matched with the visible photometry of Buonanno et al. (1986). This CMD provides a nearly complete census of the hot horizontal branch (HB) population with good temperature and luminosity discrimination for comparison with theoretical tracks. The observed data show good agreement with the theoretical zero-age horizontal branch (ZAHB) of Sweigart (1996) for an assumed reddening of E(B-V) = 0.05 and a distance modulus of 13.05. The observed HB luminosity width is in excellent agreement with the theoretical models and supports the single star scenario for the origin of extreme horizontal branch (EHB) stars. However, only four stars can be identified as post-EHB stars, whereas almost three times this many are expected from the HB number counts. If this effect is not a statistical anomaly, then some non-canonical effect may be decreasing the post-EHB lifetime. The recent non-canonical models of Sweigart (1996), which have helium-enriched envelopes due to mixing along the red giant branch, cannot explain the deficit of post-EHB stars, but might be better able to explain their luminosity distribution.

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arXiv1996

UV Colors and Extinctions of HII Regions in the Whirlpool Galaxy (M51)

Far-UV (1520 ang.), U, H-alpha, and R images of the interacting Sbc spiral galaxy M51 were obtained by the Ultraviolet Imaging Telescope (UIT) and at Mt. Laguna Observatory. The mu(152)-mu(U) radial gradient of >1 mag, becoming bluer with increasing radius, is attributed primarily to a corresponding radial extinction gradient. Magnitudes in both UV bands and H-alpha fluxes are reported for 28 HII regions. Optical extinctions for the 28 corresponding UV sources are computed from the measured m(152)-U colors by fitting to the optical extinctions of Nakai and Kuno (1995). The normalized far-UV extinction A(152)/E(B-V) increases with radius or decreasing metallicity, from 5.99 to 6.54, compared with the Galactic value 8.33. The best-fit m(152)-U color for no extinction, -3.07, is the color of a model solar metallicity starburst of age ~2.5 Myr with IMF slope -1.0. HII regions show decreasing observed H-alpha fluxes with decreasing radius, relative to the H-alpha fluxes predicted from the observed f(152) for age 2.5 Myr, after correction for extinction. We attribute the increasing fraction of ``missing'' H-alpha flux with decreasing radius to increasing extinction in the Lyman continuum. Increasing extinction-corrected far-UV flux of the HII regions with decreasing radius is probably a result of the corresponding increasing column density of the interstellar gas resulting in larger mass OB associations. The estimated dust-absorbed Lyman continuum flux is ~0.6 times the far-infrared energy flux of M51 observed by IRAS.

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arXiv1994

Are Analogues of Hot Subdwarf Stars Responsible for the UVX Phenomenon in Galaxy Nuclei?

We present the case that populations of sdB/sdOB/sdO-type stars may be a common constituent of galactic stellar populations, responsible for the UV upturn (`UVX') observed in the spectra of spiral bulges and normal galaxy nuclei. Extreme Horizontal Branch stars with $\log g > 5$ and $\log \teff > 20,000K$ have emerged in the last few years as the most likely candidate for the origin of the UVX. The magnitude of this far-UV flux in some systems (e.g. NGC~1399, NGC~4649) indicates that galactic nuclear regions must contain larger numbers of these subdwarfs than does the solar neighbourhood. This paper summarizes the results of a quantitative study of the UV radiation from evolved stellar populations. We have computed a large grid of stellar models in advanced stages of evolution, as well as a set of isochrones for ages 2-20 Gyr, for a wide range in composition. We use these calculations to derive synthetic UV colour indices for stellar populations with hot components.

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