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  • ASTROPHYSICS  (12)
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  • 1
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    Images in the rocket ultraviolet - Young clusters in H II regions of M83 (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: UV images of M83 at 1540 and 2360 A reveal 18 compact sources that are associated with H II regions. E(B - V) values were estimated individually from the observed UV and optical colors and the Galactic UV extinction curve, using theoretical flux distributions. The dereddened colors are consistent with ages up to 3 x 10 to the 6th yr. A maximum possible age of 6.5 x 10 to the 6th yr is obtained assuming foreground reddening only. The distribution of observed colors is consistent with the Galactic reddening curve but not with enhanced far-UV extinction, as in the LMC 30 Dor curve. The H-alpha fluxes suggest either that dust within the H II regions absorbs up to 70 percent of the Lyman continuum radiation or that a similar fraction of the H-alpha flux is below the surface brightness detection limit. Cluster mass estimates depend on the range of stellar masses present but are probably in the range 10,000-100,000 solar masses.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 363; 154-158
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  • 2
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    Images in the rocket ultraviolet - UV fluxes of M31 globular clusters (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Images obtained by a rocket-borne UV imaging telescope are used here to determine near-UV fluxes for 17 sources in M31 that are optical globular-cluster candidates and for the bright open cluster vdB0 in M31. Far-UV fluxes or flux limits are determined for the same clusters. The m(NUV)-V colors for M31 clusters are similar to those of Galactic clusters, except for the high-metallicity M31 cluster Bo 171. Four of the detected clusters have optical, m(NUV) - V, and m(FUV) - V colors indicating ages of about 100 million years. These four clusters are probably similar to the so-called 'blue globular' clusters of the LMC. The existence of young LMC-type blue globulars and the possible existence of middle-aged metal-rich globulars may indicate that M31 has continued to form globular clusters throughout its life.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 334; 657-664
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  • 3
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    Initial mass functions from ultraviolet stellar photometry: A comparison of Lucke and Hodge OB associations near 30 Doradus with the nearby field (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 425; 1; p. 122-126
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  • 4
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    30 Doradus - Ultraviolet and optical stellar photometry (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 413; 2; p. 604-610.
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  • 5
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    Ultraviolet Imaging Telescope images - Limits on recent star formation in Holmberg IX (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 402; 2; p. L45-L48.
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  • 6
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    An Ultraviolet Imaging Telescope study of the globular cluster M79 (NGC 1904) (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The Ultraviolet Imaging Telescope detects about 100 blue and extremely blue horizontal-branch (HB) stars in M79. Comparison of the ultraviolet color-magnitude diagram of M79 with stellar interior models shows that the distribution of total masses of the detected stars along with the zero-age HB includes the range of 0.495-0.62 solar mass, corresponding to envelope masses 0.01-0.13 solar mass. The total number of HB stars in M79 is estimated at 220 +/- 10. Also detected are two hot stars brighter than the horizontal branch by at least 1.5 mag at 1520 A. One of these stars (UIT 1) appears to be descending in luminosity along a pre-white dwarf track. The other (UIT 2) is most likely a post-HB star evolving toward the asymptotic giant branch.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 395; 1; p. L17-L20.
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  • 7
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    Simulated optical images of galaxies at z about 1 using ultraviolet images of nearby galaxies (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Ultraviolet sounding rocket images of several nearby galaxies are used to simulate the appearance in optical bandpasses of similar systems at redshifts z = 0.5-2.67, as observed by the WFPC on HST and by ground-based instruments. Since the morphology of galaxies is a strong function of the wavelength, the appearance of galaxies at large redshifts is subjected to a large k-correction effect. The strong dependence of monochromatic surface brightness on redshift also implies that observed morphology of distant systems will be crucially dependent on the limiting surface brightness set by the sky background. Although the angle subtended by sources depends only weakly on z, the fraction of a galaxy whose surface brightness is above the detection threshold varies strongly with z. Morphological distinctions of spiral from elliptical, spiral from irregular, and barred from unbarred types become more difficult, as does accurate evaluation of interactions or of the local environment.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 368; 12-27
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  • 8
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    Images in the rocket ultraviolet - Star formation and extinction in the M51 system (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: UV images of M51 with up to 15 arcsec resolution were obtained by a rocket-borne telescope. The bandwidth was 970 A with maximum response at 2650 A. The two most prominent features of M51 in the UV are the bright central region and a region of intense star formation about 2.5 NE of the nucleus toward the companion NGC 5195. This complex is the source of 20 percent of the total UV flux in M51. The companion is much less prominent in the UV than in optical bands. Spiral arms show much higher contrast in UV and U bands than in the R band. The nuclear region in the UV band shows clumped emission from probable star-forming regions, possibly associated with the inner Lindblad resonance. These regions are probably the UV counterparts of FIR sources discovered in an EW scan across the nucleus. UV/U colors of most bright H II regions with known extinction are consistent with O star spectra reddened by amounts estimated from radio and Balmer line measurements. However, some of the brightest H II regions have redder UV/U colors, probably indicating the presence of cooler stars which contribute in the U band. CO and IR observations indicate that the companion NGC 5195 may be a reddened starburst in spite of its faintness in the UV.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 352; 55-67
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  • 9
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    Ultraviolet photometry of OB associations in M31 (1993)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 414; 1; p. L9-L12.
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  • 10
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    Implications of Ultraviolet Imaging Telescope observations for star formation histories in NGC 1275 (1992)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We discuss UV imagery of NGC 1275 obtained using the Goddard Ultraviolet Imaging Telescope. We are able to study the UV morphology down to mu 249 of about 25 mag/sq arcsec. There are significant nonaxisymmetric structures in the UV continuum associated with the low-velocity filament system. Continuum from the high-velocity system may also be present. The large aperture UV colors indicate that although the mass function extends to about 5 solar masses, more massive objects are not present. This implies either a cessation of star formation during the last 50-100 Myr or a truncated initial mass function.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 395; 1; p. L49-L54.
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