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  • Other Sources  (4)
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  • 1
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    Interstellar absorption lines in the spectrum of supernova Evans in M83 (NGC 5236) (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is pointed out that supernovae in other galaxies create spontaneous opportunities to probe spectroscopically the gaseous material in galactic halos. Because of their brightness, supernovae offer an unparalleled chance to obtain spectra with high wavelength resolution and/or signal-to-noise ratio. The present investigation is concerned with observations of visual interstellar lines in the spectrum of a supernova discovered on July 3, 1983, and identified as Type I. The supernova was located 124 arc sec south and 122 arc sec west of the nucleus of M83. Evidence is obtained that for velocities near that of M83, the line of sight toward the supernova contains a complex array of absorption features of Ca II. In both Ca II and Na I, the absorptions associated with M83 are far more prominent than those halfway through the disk (and halo) of the Milky Way Galaxy. It is felt that many of the displaced velocity components in the M83 system arise from a gaseous halo.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 281; 585-592
    Format: text
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  • 2
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    Stellar model chromospheres. V - Alpha Centauri A /G2 V/ and Alpha Centauri B /K1 V/ (1976)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Models for the upper photospheres and lower chromospheres of Alpha Centauri A and B are derived from high-dispersion spectrograms of the Ca II K-line emission cores and damping wings. Effective temperatures, surface gravities, and ages consistent with the measured broadband colors, metallicities, and absolute magnitudes are estimated for the two stars. The spectrograms are calibrated by fitting the far-wing K profiles with synthetic fluxes based on radiative-equilibrium models, and the model atmospheres are obtained through a partial-redistribution analysis of the line cores and damping wings. These models are compared with those previously proposed for Procyon, Arcturus, and the sun. Many features in all the models are found to be quite similar, and some evidence suggests that Alpha Cen A may be significantly older and more evolved than the sun, even though both stars exhibit similar chromospheric properties.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 210; Nov. 15
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Spectroscopy of chromospheric lines of giants in the globular cluster (1994)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: Spectroscopic observations of chromospheric transitions (Mg II, H-alpha, and Ca II K) from two red giants (A31 and A59) in the globular cluster NGC 6572 were made with the Goddard High Resolution Spectrograph on the Hubble Space Telescope and the coude spectrograph of the 1.9 m telescope at the Mount Stromlo Observatory. These measurements give evidence for chromospheric activity and outward motions within the atmospheres. The surface flux of the Mg II emission is comparable to that in disk population giants of similar (B-V) color. The Mg II profiles are asymmetric, which is most likely caused by absorption in an expanding stellar atmosphere and/or by possible interstellar features. Notches are found in the core of the H-alpha line of A59, which are similar to those found in Cepheids. This suggests that shocks are present in the atmosphere of A59 and indicates that hydrodynamic phenomena are influencing the levvel of chromospheric emission and producing upper atmospheric motions which may lead to mass loss.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 421; 2; p. 542-549
    Format: text
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  • 4
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    Chromospheres of two red giants in NGC 6752 (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Two red giant stars, A31 and A59, in the globular cluster NGC 6752 exhibit Mg II (2800 A) emission with surface fluxes comparable to those observed among metal-deficient halo field giants, and among low-activity Population I giants. Optical echelle spectra of these cluster giants reveal emission in the core of the Ca II K (3933.7 A) line, and in the wing of the H-alpha (6562.8 A) profile. Asymmetries exist both in the emission profiles and the line cores. These observations demonstrate unequivocally the existence of chromospheres among old halo population giants, and the presence of mass outflow in their atmospheres. Maintenance of a relatively constant level of chromospheric activity on the red giant branch contrasts with the decay of magnetic dynamo activity exhibited by dwarf stars and younger giants. A purely hydrodynamic phenomenon may be responsible for heating the outer atmospheres of these stars, enhancing chromospheric emission, thus extending the atmospheres and facilitating mass loss.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 361; L9-L13
    Format: text
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