Engineering topics

O'Connell, Robert W.

Publications and source records attributed to O'Connell, Robert W..

At least 19 records

NASA NTRS1995

Ultraviolet radiation from evolved stellar populations. 2: The ultraviolet upturn phenomenon in elliptical galaxies

We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We derive the broad-band UV colors 1500-V and 2500-V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlation. Clusters can be bluer than any galaxy in 15-V and 25-V, implying larger hot star populations, but galaxies are significantly bluer than clusters in 15-25 at a given 15-V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. These redder colors of the galaxies also imply that the UVX in galaxies is not produced by metal-poor subpopulations similar to the clusters. We devlop a simple spectral synthesis formulation for all phases of single star evolution from the zero-age main sequence (ZAMS) to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary horizontal branch (HB) morphologies in the age range 2 less than t less than 20 Gyr and for six metallicities in the range -2.26 less than (Fe/H) less than 0.58; we also consider the efect of enhanced Y in metal-rich models. The maximum lifetime UV output is produced by EHB stars with (M(sub env))(sup 0) approximately 0.02 solar mass and can be up to 30 times higher than for post-asymptotic giant branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of (M(sub env))(sup 0)P(M(sub env))(sup 0), on the ZAHB. The UV output is not very sensitive to (Fe/H) or to Y, but it can change very rapidly with (M(sub env))(sup 0). Thus it is extremely sensitive to the precise nature of giant-branch mass loss. Our models use simple descriptions of P(M(sub env))(sup 0) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. We find that models with (Fe/H) greater than or = 0 that do not contain EHB stars cannot reproduce the colors of most of the galaxies. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is approximately 70% and approximately 20% for moderate UVX and strong UVX systems, respectively. We find that 25-V, but not 15-V, is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place limits of (Fe/H) greater than -0.5 and less than 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. We discuss several implications of the observations and the models, including the question of light metal versus iron peak enhancements in galaxies, whether the UV color-Mg(sub 2) correlation is continuous or discrete, effects of helium abundnace on the UVX, and the key question of whether red giant branch mass loss can be large enough to produce the necessary EHB population in the strong UVX galaxies.

Dorman, Ben↗

NASA NTRS1995

Ultraviolet imaging telescope and optical emission-line observations of H II regions in M81

Images of the type Sab spiral galaxy M81 were obtained in far-UV and near-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission of 1990 December. Magnitudes in the two UV bands are determined for 52 H II regions from the catalog of Petit, Sivan, & Karachentsev (1988). Fluxes of the H-alpha and H-beta emission lines are determined from CCD images. Extinctions for the brightest H II regions are determined from observed Balmer decrements. Fainter H II regions are assigned the average of published radio-H-alpha extinctions for several bright H II regions. The radiative transfer models of Witt, Thronson, & Capuano (1992) are shown to predict a relationship between Balmer Decrement and H-alpha extinction consistent with observed line and radio fluxes for the brightest 7 H II regions and are used to estimate the UV extinction. Ratios of Lyman continuum with ratios predicted by model spectra computed for initial mass function (IMF) slope equal to -1.0 and stellar masses ranging from 5 to 120 solar mass. Ages and masses are estimated by comparing the H-alpha and far-UV fluxes and their ratio with the models. The total of the estimated stellar masses for the 52 H II regions is 1.4 x 10(exp 5) solar mass. The star-formation rate inferred for M81 from the observed UV and H-alpha fluxes is low for a spiral galaxy at approximately 0.13 solar mass/yr, but consistent with the low star-formation rates obtained by Kennicutt (1983) and Caldwell et al. (1991) for early-type spirals.

Hill, Jesse K.↗

NASA NTRS1994

Hubble Space Telescope photometry of the central regions of Virgo Cluster elliptical galaxies. 1: Observations, discussion, and conclusions

Using the Hubble Space Telescope we have observed at 10 pc resolution the nuclei of a luminosity-limited sample of 14 E and E/SO galaxies in the Virgo Cluster with magnitudes B(sub T) = 9.4 to 13.4. In this paper we present the images, and discuss the results of the detailed analysis of the surface photometry given in two companion papers. We find that the nuclear and near-nuclear morphologies confirm and strengthen the previously recognized dichotomy of 'E' galaxies into 'true' and 'disky' subtypes. The latter, usually classified E4 or later, often show a bright nuclear disk of radius approximately 100 pc. Essentially all early-type galaxies with -18 greater than M(sub B) greater than -20 are disky. Most true E galaxies are classified E4 or earlier. Most galaxies of both types show dust in the nuclear regions, the most remarkable example being a compact dust disk in NGC 4261. Other than dust, no anomalies were detected in the centers of the three galaxies in our sample which show clear kinematic evidence for a decoupled component.

Jaffe, Walter↗

NASA NTRS1994

Hubble Space Telescope photometry of the central regions of Virgo cluster elliptical galaxies. 2: Isophote shapes

The isophotal shapes of a magnitude limited sample of Virgo ellipticals are presented. These are derived from high resolution Hubble Space Telescope (HST) photometry. The absence of atmospheric seeing and accurate knowledge of the Point Spread Function (PSF) allows us to perform an accurate deconvolution. Model galaxies were constructed to test the deconvolution algorithms used, and showed that we can accurately recover isophotal shape parameters down to 0.5 sec. From the isophotal parameters we can classify the galaxies in two subsamples: disky and non-disky galaxies. In three of these disky galaxies we found evidence for a nuclear stellar disk in the inner 1.5 sec. In addition these galaxies also have an outer disk, that seems to break up inside 2 sec - 3 sec. In the two galaxies for which there is kinematic evidence from the literature of a decoupled core, we found no indication for such subsystem from the isophotal shape analysis. In 80% of these early type galaxies there are indications for dust. For eight of these galaxies the dust has not been detected before.

Bosch, Frank C. Van Den↗

NASA NTRS1994

Hubble Space Telescope photometry of the central regions of Virgo cluster elliptical galaxies. 3: Brightness profiles

We have used the Planetary Camera on the Hubble Space Telescope (HST) to study the morphology and surface brightness parameters of a luminosity-limited sample of fourteen elliptical galaxies in the Virgo cluster. The total apparent blue magnitudes of the galaxies range between 9.4 and 13.4. In this paper, the core brightness profiles are presented, while the overall morphology and the isophotal shapes are discussed in two companion papers (Jaffe et al. (1994); van den Bosch et al. (1994)). We show that, in spite of the spherical aberration affecting the HST primary mirror, deconvolution techniques allow recovery of the brightness profile up to 0.2 arcsec from the center of the galaxies. We find that none of the galaxies has an isothermal core. On the basis of their morphological and photometrical properties, the galaxies can be divided in two physically distinct groups, referred to as Type I and Type II. All of the Type I galaxies are classified as E1 to E3 in the Revised Shapley Ames Catalog (Sandage & Tammann 1981), while Type II galaxies are classified as E5 to E7. The characteristics of Type II galaxies are explained by the presence of disks component on both the 1 arcsec and the 10 arcsec scales, while Type I galaxies correspond to the classical disk-free ellipticals.

Ferrarese, Laura↗

NASA NTRS1994

Far-ultraviolet photometry of the globular cluster omega Cen

We present far-ultraviolet images of the globular cluster omega Centauri obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 mission. A total of 1957 sources are detected at 1620 A to a limiting ultraviolet (UV) magnitude of 16.4 in the central 24 min diameter region of the field and a limit of 15.6 over the remainder of the 40 min diameter field. Over 1400 of these sources are matched with stars on a Stroemgren u band charge coupled devices (CCD) frame obtained with the Cerro Tololo Inter-American Observatory (CTIO) 0.9 m telescope to produce a (far-UV, u) color-magnitude diagram (CMD). Completeness of the sample and error estimates are determined by photometry of artificial stars added to the images. The horizontal branch (HB) of the CMD is heavily populated hotter than 9000 K. A large number of 'extreme HB' stars are found hotter than a conspicuous break in the HB at T(sub e) approximately 16000 K. There is also a significant population of stars above the HB, the brightest of which is 4 mag brighter than the HB. Most of the hotter of these appear to be 'AGB-manque' or 'Post-Early Asymptotic Giant Branch' stars. We compare the observations to recent theoretical evolutionary tracks for the zero-age HB and subsequent phases. The tracks match the data well, with the exception of the hotter HB stars, many of which fall below the zero-age horizontal branch. It is unclear as yet whether these are a special population or an artifact of errors in the models or photometry. We identify 33 stars with T(sub e) greater than or approximately = 50000 K, which are hotter than zero-age HB stars with envelope masses of 0.003 solar mass.

Whitney, Jonathan H.↗

NASA NTRS1994

Hubble Space Telescope imaging of super-star clusters in NGC 1569 and NGC 1705

We examine the structural properties of three super-star clusters in the nearby, H I-rich galaxies NGC 1569 and NGC 1705. The clusters, which have total absolute V magnitudes between -13.3 and -14.1, appear to be point sources on ground-based images but are partially resolved in new images obtained with the Hubble Space Telescope (HST) Planetary Camera. From deconvolved V- and I-band images we find that the three clusters have very compact cores with extended halos that are partially resolved into individual stars. Using new distances to the galaxies derived from color-magnitude diagrams for field stars, we find that the half-light radii are 2.2-3.4 pc. The cluster in NGC 1705 is barely resolved in the HST images. The clusters in NGC 1569, on the other hand, show significant substructure in their cores and ellipticities that are comparable to the flattenings seen in young clusters in the Large Magellanic Cloud (LMC). The clusters show internal (V-I) color gradients. The properties of these clusters are similar to R136, the core of the luminous star-forming complex 30 Doradus in the LMC, except that R136 has a lower luminosity and central surface brightness. The half-light surface brightness of the brightest cluster (NGC 1569 A) is 1.3 x 10(exp 6) L(sub v) solar/ sq cm, which is over 65 times higher than R136 and 1200 times higher than the mean rich LMC star cluster other than R136 after allowing for aging effects. The next brightest clusters in each of these galaxies are greater than or = 2 mag fainter. Thus, the super-star clusters represent an extreme but uncommon mode of star formation. In terms of luminosity and size, they appear to be good analogs of young globular clusters.

O'Connell, Robert W.↗

NASA NTRS1994

Ultraviolet imaging of the AGN+starburst galaxy NGC 1068

Images of the Seyfert 2 galaxy NGC 1068 were obtained at two ultraviolet wavelengths by the Ultraviolet Imaging Telescope (UIT). These data represent the first detailed UV imagery of a composite (active galactic nucleus + starburst) disk galaxy. NGC 1068 cotains multiple components at UV wavelengths: the central active galactic nucleus; a population of very luminous starburst knots; a bright oval inner disk; and a fainter, more circular halo. The most luminous knot, which is located approximately 750 pc from the nucleus at PA 315 deg, is approximately 80 times the luminosity of 30 Doradus and gives NGC 1068 a 'double nucleus' appearance in the UV. Significant extended emission is observed throughout the disk, unlike other disk galaxies so far observed in the UV. The radial brightness profile in both UV bandpasses generally follows an exponential decline to approximately 5 kpc. A faint halo extending to approximately 13 kpc is likely to be a galaxian-sized reflection nebula where ambient dust scatters the intense UV continuum from the inner galaxy. UV colors show a striking asymmetric morphology, which is correlated with the observed molecular CO emission.

Neff, Susan G.↗

NASA NTRS1994

UIT: Ultraviolet observations of the small Magellanic cloud

Ultraviolet Imaging Telescope (UIT) far-ultraviolet (FUV) and near-ultraviolet (NUV) images are presented for a 40' field at the southwest end of the SMC centered near the H alpha emission object Henize 19. Photometry is obtained for 1309 stars common to the FUV (lambda (sub eff) = 1620 A) and NUV (lambda (sub eff) = 2490 A) images. We use UIT photometry and imagery to derive a UV color-magnitude diagram for hot field stars; confirm the consistency of low-metal-abundance stellar evolutionary models with the observations; study dust distribution in the field, finding an increase in E(B--V) from the northeast to the southwest; confirm for a large number of stars an extinction curve, like that derived by Hutchings for the SMC, which rises steeply from NUV to FUV; and derive a mass function for SMC field stars and its lack of dependence on assumed extinction curve characteristics. The mass function has a slope within the accepted range (Gamma = -1.7 +/- 0.5) over a wide range of assumed extinction parameters, including variations of the size (+/- approximately 1.5 mag x E(B--V) in the FUV) seen in the Galaxy.

Cornett, Robert H.↗

NASA NTRS1994

Hubble Space Telescope imaging of the central star forming region in NGC 1140 (exp 1)

We present broadband images taken with the Hubble Space Telescope's Planetary Camera of the central supergiant H II region in the amorphous galaxy NGC 1140. These images allow observations to a resolution of about 13 pc at the galaxy, and they reveal that its central 1/2 kpc contains 6-7 blue, luminous, compact super star clusters, many of which would be comparable in luminosity to globular clusters at the same age. A blue arc-shaped structure near the center may be a grouping of less luminous, R136/NGC 2070-sized clusters or a sheet of OB stars. Additional somewhat less luminous and redder clusters are also found farther out from the center. If these clusters are older, they too could have had luminosities comparable to those of the central six clusters at a comparable age. Thus, we find that NGC 1140 is remarkable in the number of extreme clusters that it has formed recently in a relatively small area of the galaxy. Since NGC 1140 exhibits global characteristics that are consistent with a recent merger, these clusters are likely to be a product of that event. This galaxy adds to the number of cases where rapid star formation has evidently produced super star clusters.

Hunter, Deidre A.↗

NASA NTRS1994

UIT: Ultraviolet surface photometry of the spiral galaxy M74 (NGC 628)

Ultraviolet photometry, obtained from Ultraviolet Imaging Telescope (UIT) images at 1520 A (far-UV; magnitudes m(152)) and 2490 A (near-UV; magnitudes m(249)), of the spiral galaxy M74 (NGC 628) is compared with H-alpha, R, V, and B surface photometry and with models. M74's surface brightness profiles have a central peak with an exponential falloff; the exponential scale lengths of the profiles increase with decreasing wavelength for the broad-band images. The slope of the continuum-subtracted H-alpha profile is intermediate between those of far-UV and near-UV profiles, consistent with the related origins of H-alpha and UV emission in extreme Population I material. M74's color profiles all become bluer with increasing radius. The (m(152) - m(249)) color as measured by UIT averages near 0.0 (the color of an A0 star) over the central 20 sec radius and decreases from approximately -0.2 to approximately -0.4 from 20 sec to 200 sec. The spiral arms are the dominant component of the surface photometry colors; interarm regions are slightly redder. In the UV, M74's nuclear region resembles its disk/spiral arm material in colors and morphology, unlike galaxies such as M81. No UV 'bulge' is apparent. The m(152) - m(249) colors and models of M74's central region clearly demonstrate that there is no significant population of O or B stars present in the central 10 sec. M74's UV morphology and (m(152) - m(249)) color profiles are similar to those of M33, although M74 is approximately 0.5 mag redder. M81 has a smooth UV bulge which is much redder than the nuclear regions of M74 and M33. M74 is approximately 0.4 mag bluer than M81 in its outer disk, although M81 has bright UV sources only in spiral arms more than 5 kpc from its center. We investigate possible explanations for the color profiles of the galaxies and the differences among the galaxies: abundances; reddening due to internal dust; interplanetary magnetic field (IMF) variations, and the history of formation of the dominant generations of stars. Abundance and IMF variations do not produce large enough m(152) - m(249) or UV - V color differences. Comparing model UV/optical colors with those of M74 shows that M74's disk has undergone significant star formation over the past 500 Myr, and that either the star-formation history or the extinction varies systematically across M74's disk. Comparison of M74, M33, and M81 (UV - V) colors shows that M74 colors range from the bluest of M33's colors to the bluest of M81's. The failure of reddening models to cover the range of colors, and the known abundance range in such material, leads to the conclusion that star-formation history varies significantly as a function of radius in these galaxies, and that such variation is required to explain the range of colors observed in M74, M33, and M81.

Cornett, Robert H.↗

NASA NTRS1994

Initial mass functions from ultraviolet stellar photometry: A comparison of Lucke and Hodge OB associations near 30 Doradus with the nearby field

UV stellar photometry is presented for 1563 stars within a 40 minutes circular field in the Large Magellanic Cloud (LMC), excluding the 10 min x 10 min field centered on R136 investigated earlier by Hill et al. (1993). Magnitudes are computed from images obtained by the Ultraviolet Imaging Telescope (UIT) in bands centered at 1615 A and 2558 A. Stellar masses and extinctions are estimated for the stars in associations using the evolutionary models of Schaerer et al. (1993), assuming the age is 4 Myr and that the local LMC extinction follows the Fitzpatrick (1985) 30 Dor extinction curve. The estimated slope of the initial mass function (IMF) for massive stars (greater than 15 solar mass) within the Lucke and Hodge (LH) associations is Gamma = -1.08 +/- 0.2. Initial masses and extinctions for stars not within LH associations are estimated assuming that the stellar age is either 4 Myr or half the stellar lifetime, whichever is larger. The estimated slope of the IMF for massive stars not within LH associations is Gamma = -1.74 +/- 0.3 (assuming continuous star formation), compared with Gamma = -1.35, and Gamma = -1.7 +/- 0.5, obtained for the Galaxy by Salpeter (1955) and Scalo (1986), respectively, and Gamma = -1.6 obtained for massive stars in the Galaxy by Garmany, Conti, & Chiosi (1982). The shallower slope of the association IMF suggests that not only is the star formation rate higher in associations, but that the local conditions favor the formation of higher mass stars there. We make no corrections for binaries or incompleteness.

Hill, Jesse K.↗

NASA NTRS1994

An ultraviolet-visible investigation of the globular cluster NGC 1851

Two-color ultraviolet images of the globular cluster NGC 1851 were obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 Spacelab mission. A total of 133 stars are detected at 2490 A and 74 stars at 1520 A. An ultraviolet color-magnitude diagram based on the 46 well-photometered stars that appear in both images is presented. Thirty-nine of the 45 horizontal branch (HB) stars fall below the zero-age horizontal branch (ZAHB) of Z = 0.001 and Y = 0.23 predicted by Sweigart by as much as 0.6 mag if the interstellar reddening to the cluster is E(B - V) = 0.02. Supporting ground-based V and B observations, however, show excellent correlation with the same model ZAHB. A newly detected hot subdwarf star with T(sub eff) is approximately 26,000 K appears to be an extreme horizontal branch star. The measured flux for this star also falls approximately 0.6 mag below the position on the color-magnitude diagrams (CMD) predicted by parameters derived from ground-based spectroscopy by Landsman. The far-ultraviolet image is dominated by the ultraviolet-bright member star UV 5, which contributes 30% of the total flux at 1520 A. The UIT photometry is consistent with the classification of UV 5 as a post-asymptotic giant branch (PAGB) star. To a limit of 16.5 mag at 1520 A we find no ultraviolet counterpart within 6 minutes of the position of the X-ray source MX 0513-40.

Parise, Ronald A.↗

NASA NTRS1993

Ultraviolet photometry of OB associations in M31

The study obtains near-UV and FUV magnitudes for 76 massive stars in 24 OB associations in the central and southern portions of M31 from images obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro 1 spacelab mission. A comparison is made with the previous UIT photometry of 30 stars in the giant association NGC 206. Extinctions are estimated from the relation between E(B - V) and the distance from the center of M31 derived by Hodge and Lee (1988) from topical CCD stellar photometry. From evolutionary models, lower limits to the maximum stellar mass are estimated at about 60-100 solar masses in NGC 206, A29, A61, A63, A130, and A132. For other associations, the limits are in the range of about 20-55 solar masses.

Hill, Jesse K.↗

NASA NTRS1993

30 Doradus - Ultraviolet and optical stellar photometry

Ultraviolet Imaging Telescope (UIT) UV magnitudes in four bands, together with optical B magnitudes, are presented for up to 314 early-type stars located in a 9.7 x 9.7 arcmin field centered on R136. The magnitudes have an rms uncertainty estimated at 0.10 mag from a comparison between the UIT magnitudes and the IUE spectra. Spectral types and E(B-V) color excesses are estimated. The mean color excesses following the two extinction curves agree well with the predictions of the two-component extinction model of Fitzpatrick and Savage (1984). However, the degree of nebular extinction is found to vary systematically by large amounts over the 30 Dor field. The minimum of nebular extinction in the central parts of the nebula suggests that dust has been expelled from this region by stellar winds. It is suggested that the form of the UV extinction curve can be understood as a consequence of the evolutionary state of the stellar population responsible for making the dust grains.

Hill, Jesse K.↗

NASA NTRS1993

Far-infrared emission from the intracluster medium

IRAS images of 56 clusters of galaxies are examined for evidence of diffuse, extended FIR emission at 60 and 100 microns. Of these 56 clusters, five show evidence of FIR excess, but at low levels of significance. For A496 and MKW 1, this excess is attributable to the presence of a strong point source at or near the position of the central cD galaxy. In A2344, the observed 100-micron excess is the result of the presence of a foreground cirrus concentration which coincides with the cluster. Examination of the sample as a whole indicates that clusters exhibit excess FIR emission at the 2-sigma level for apertures 4 and 10 arcmin in diameter. FIR luminosities range from about 1-0 exp 9 solar luminosities for the smallest aperture to about 10 exp 14 solar luminosities for the entire cluster.

Wise, Michael W.↗

NASA NTRS1993

Ultraviolet Imaging Telescope images - Limits on recent star formation in Holmberg IX

Ultraviolet Imaging Telescope near-UV and far-UV images of Holmberg IX show no OB associations. Relative fluxes measured for the galaxy as a whole in the UV and optical B and V bands are consistent with models in which about 0.6-0.7 of the V flux is from an old population of age 10 Gyr, while the remainder is from stars of age 20-200 Myr, which also contribute 0.80 of the near-UV flux and 0.99 of the far-UV flux. Individual stars measured in B and V appear to be evolved stars of mass about 12 solar masses and age about 20 Myr belonging to the youngest population in the galaxy.

Hill, Jesse K.↗

NASA NTRS1992

The ultraviolet color gradient in the late-type spiral galaxy M33

The ultraviolet surface brightness and color distributions for the late-type spiral galaxy M33 are derived from images at 1520 and 2490 A that were obtained by the Ultraviolet Imaging Telescope (UIT) on the Astro Spacelab mission. Although the surface brightness shows a general decline with radius, the dominant spiral arms cause significant deviations from an exponential fit. Colors in elliptical annuli become 0.2-0.3 mag bluer with increasing radius. Our measures of individual H II regions and sections of spiral arms follow this same trend. Interarm regions are redder than the arms. A metallicity gradient affecting only the colors of stars is inadequate in accounting for the observed color gradient. A plausible explanation invokes a combination of an LMC-type reddening curve and a radial gradient in the internal reddening in M33.

Landsman, Wayne B.↗