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  • 1
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    Bowen fluorescence in AM Herculis stars (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The study relates the observations of the Bowen fluorescence lines in AM Her and EF Eri, two AM Her systems. The implications of source variability and the checks performed in order to verify the accuracy of the data are reviewed. The line ratios for H I and He II are shown to deviate considerably from those expected; in this light, the H I and He II emission in AM Her is analyzed in detail. This approach makes it possible to put limits on line and continuum optical depths as well as temperatures. These limits along with the observed Bowen emission are used to construct a picture of the emission-line regions in the two sources, producing the values for the size, density, and temperature of a region. The derived value of the size is found to be significantly less than the size of the Roche lobe of the white dwarf, while the large range of densities is attributed to the exponential temperature dependence in the expression used for density.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 633-648
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  • 2
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    A test of the massive binary black hole hypothesis - Arp 102B (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The emission-line spectra of several AGN have broad peaks which are significantly displaced in velocity with respect to the host galaxy. An interpretation of this effect in terms of orbital motion of a binary black hole predicts periods of a few centuries. It is pointed out here that recent measurements of the masses and sizes of many low-luminosity AGN imply orbital periods much shorter than this. In particular, it is found that the elliptical galaxy Arp 102B is the most likely candidate for observation of radial velocity variations; its period is expected to be about 3 yr. The H-alpha line profile of Arp 102B has been measured for 5 yr without detecting any change in velocity, and it is thus found that a rather restrictive observational test of the massive binary black hole hypothesis already exists, albeit for this one object.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 331; 46-48
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  • 3
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    SN 1991T: Reflections of past glory (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We have obtained photometry and spectra of SN 1991T which extend more than 1000 days past maximum light, by far the longest of SN Ia has been followed. Although SN 1991T exhibited nearly photometric behavior in the first 400 days following maximum, by 600 days its decline had slowed, and by 950 days the supernova brightness was consistent with a constant apparent magnitude of m(sub B) = 21.30. Spectra near maximum showed minor variations on the SN Ia theme which grew less conspicuous during the exponential decline. At 270 days the nebular spectrum was composed of Fe and Co lines common to SN Ia. However, by 750 days past maximum light, these lines had shifted in wavelength, and were superposed on a strong blue continuum. The luminosity of SN 1991T at 950 days is more than (9.0 x 10(exp 38)(D/13 Mpc)(2) ergs/s with a rate of decline of less than 0.04 mag per 100 days. We show that this emission is likely to be light that was emitted by SN 1991T near maximum light which has reflected from foreground dust, much like the light echos observed around SN 1987A.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 434; 1; p. L19-L23
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  • 4
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    The expanding photosphere method applied to SN 1992am AT cz = 14 600 km/s (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We present photometry and spectroscopy of Supernova (SN) 1992am for five months following its discovery by the Calan Cerro-Tololo Inter-American Observatory (CTIO) SN search. These data show SN 1992am to be type II-P, displaying hydrogen in its spectrum and the typical shoulder in its light curve. The photometric data and the distance from our own analysis are used to construct the supernova's bolometric light curve. Using the bolometric light curve, we estimate SN 1992am ejected approximately 0.30 solar mass of Ni-56, an amount four times larger than that of other well studied SNe II. SN 1992am's; host galaxy lies at a redshift of cz = 14 600 km s(exp -1), making it one of the most distant SNe II discovered, and an important application of the Expanding Photsphere Method. Since z = 0.05 is large enough for redshift-dependent effects to matter, we develop the technique to derive luminosity distances with the Expanding Photosphere Method at any redshift, and apply this method to SN 1992am. The derived distance, D = 180(sub -25) (sup +30) Mpc, is independent of all other rungs in the extragalactic distance ladder. The redshift of SN 1992am's host galaxy is sufficiently large that uncertainties due to perturbations in the smooth Hubble flow should be smaller than 10%. The Hubble ratio derived from the distance and redshift of this single object is H(sub 0) = 81(sub -15) (sup +17) km s(exp -1) Mpc(exp -1). In the future, with more of these distant objects, we hope to establish an independent and statistically robust estimate of H(sub 0) based solely on type II supernovae.
    Keywords: ASTROPHYSICS
    Type: The Astronomical Journal (ISSN 0004-6256); 107; 4; p. 1444-1452
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  • 5
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    The photoionization mechanism of LINERs - Stellar and nonstellar (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We present high quality spectroscopic observations of a sample of 14 LINERs. Starlight removal is achieved by the subtraction of a suitable absorption-line 'template' galaxy, allowing accurate measurements of emission lines. We use these line fluxes to examine the possible excitation mechanisms of LINERs. We suggest that LINERs with weak forbidden O I 6300-A emission may be H II regions photoionized by unusually hot O-type stars. LINERs with forbidden O I/H-alpha approximately greater than 1/6 may be powered by photoionization from a nonstellar continuum. This is supported by the detection of broad H-alpha emission, a correlation between line width and critical density, and pointlike X-ray emission in several of these objects.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 205; 1; p. 19-27.
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  • 6
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    Kinematic evidence for a relativistic Keplerian disk - Arp 102B (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Line profiles were calculated for a Keplerian disk with allowance made for relativistic effects. Excellent agreement was found between the resulting double peaked asymmetric profiles and the H-alpha line of Arp 102B; this yielded an accurate determination of several disk parameters. It is believed that the line profile of Arp 102B shows the most convincing direct kinematic evidence for rotation in any AGN.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 339; 742-751
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  • 7
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    SN 1961V - An extragalactic ETA Carinae analog (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Spectra of the site of the unique type V supernova SN 1961V in NGC 1058 and of two nearby H II regions have been obtained. Broad H-alpha emission with a luminosity of 2 x 10 to the 36th ergs/s is detected at the position, so that SN 1961V becomes the first historical extragalactic object classified as a supernova to be optically recovered. The characteristics and origin of the high-excitation H II regions of the site are discussed. It is argued that SN 1961V was not a supernova, but an exaggerated Eta Carinae-type outburst of a very massive, evolved star near the end of core hydrogen burning.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 342; 908-916
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  • 8
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    A gravitational lens candidate discovered with the Hubble Space Telescope (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Evidence is reported for gravitational lensing of the high-redshift (z = 3.8) quasar 1208 + 101, observed as part of the Snapshot survey with the HST Planetary Camera. An HST V image taken on gyroscopes resolves the quasar into three point-source components, with the two fainter images having separations of 0.1 and 0.5 arcsec from the central bright component. A radio observation of the quasar with the VLA at 2 cm shows that, like most quasars of this redhsift, 1208 + 101 is radio quiet. Based on positional information alone, the probability that the observed optical components are chance superpositions of Galactic stars is small, but not negligible. Analysis of a combined ground-based spectrum of all three components, using the relative brightnesses of the HST image, supports the lensing hypothesis. If all the components are lensed images of the quasar, the observed configuration cannot be reproduced by simple lens models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 386; L1-L3
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  • 9
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    A Hubble Space Telescope ultraviolet spectrum of SN 1993J (1994)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We obtained a Hubble Space Telescope (HST) ultraviolet spectrum of the Type II supernova SN 1993J in M81 on 1993 April 15. The approximately 1650-2900 A region is smoother than observed for SN 1987A and SN 1992A and lacks strong P Cygni absorptions caused by iron peak element lines. Synthetic spectra calculated using a parameterized Local Thermodynamic Equilibrium (LTE) procedure and a simple model atmosphere do not fit the UV region. Radio observations suggest that SN 1993J is embedded in a thick circumstellar envelope. The UV spectra of other supernovae that are believed to have thick circumstellar envelopes also have approximately 1650-2900 A regions lacking in strong P Cygni absorptions. Interaction of supernova ejecta and circumstellar matter may cause the smooth UV spectrum. If so, UV observations of supernovae will provide insight into the circumstellar environment of the supernova progenitors.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 (ISSN 0004-637X); 421; p. L27-L30
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  • 10
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    Spectra of low-mass helium star models and the type Ic supernova SN 1987M (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The question of whether the low-mass helium star models reproduce the helium-poor Type Ic spectrum of SN 1987M is investigated by comparing the theoretical spectrum from the 3.3-solar-mass model of Shigeyama et al. (1990) to observations of SN 1987M reported by Filippenko et al. (1990). The results indicate a number of positive correlations between the observed and the computed spectra. In both types of spectra, Ca II, O I, and Mg I emission lines dominate at late times, and, if extensive composition mixing is imposed, good fits can be made to the shapes of the emission features and good agreement can be found with the observed ratio of the forbidden and IR triplet blends of Ca II. The former indicates that the 3.3-solar-mass model predicts the proper density/velocity structure, and the latter indicates that the computed temperature structure is consistent with the observation though the lines are too strong relative to the continuum. This result is roughly independent of the He abundance. The computed and observed spectra are consistent with a small Ni-56 production in contrast to Type Ia thermonuclear events whose spectra are dominated by Fe-group lines at late times.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 411; 1; p. 313-322.
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