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  • 1
    Electronic Resource
    Electronic Resource
    Small grains in nebulae: The case of IC 418 (1986)
    Springer
    Astrophysics and space science 122 (1986), S. 81-96 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present near infrared maps of the planetary nebula IC 418 taken at 5 arc sec spatial resolution. These show an extremely compact source structure at 1.65 μm, and very much broader and more elliptical emission distributions at 2.2 and 3.6 μm. The possibility that the near infrared continuum derives from small grains with low thermal capacity is discussed, and synthetic spectra are derived. These indicate a very much slower variation of flux with wavelength than would be deduced for grains at unitary temperature, and imply a near infrared continuum which is significantly cooler than maximum grain temperatures. Fits to the spectrum of IC 418 suggest that peak grain temperatures may approachT p0∼103 K (for εαλ−2), for instance, compared to the observed colour temperature of ∼650 K. Finally, the relevant grains are required to be refractory and, given the large C/O ratio in this nebula, probably consist of graphite. Their mass is also required to be small, and is probably of order ∼2×10−7 M ⊙. This compares with the probable total mass for large dust grains of ∼10−4 M ⊙. Although they are both small and warm, such grains would be expected to persist over the lifetime of the nebula, and may also be responsible for various NIR band emission features.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00654383
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  • 2
    Electronic Resource
    Electronic Resource
    Long slit spectroscopy of the nuclear zone of NGC 253 (1990)
    Springer
    Astrophysics and space science 170 (1990), S. 315-317 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have undertaken a study of the physical conditions of the lonized gas within the starburst region of the galaxy NGC 253, using long slit spectroscopy at high spatial and spectral resolution. Our aim was to identify the appropriate parameters describing the dynamics of the gas and the excitation conditions, in relation with the starburst process. Non-gaussian line profiles and line spliting are clearly observed, indicating a complicated dynamical structure; furthermore, the zones presenting non-circular motions and localized outflow of gas have been spatially sampled at the seeing limit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00652686
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  • 3
    Electronic Resource
    Electronic Resource
    Departures from axial symmetry, and starburst activity, in the bulge of NGC 4736 (1990)
    Springer
    Astrophysics and space science 170 (1990), S. 305-310 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Evidence for circumnuclear starbursts in spiral galaxies can take the form of a blue excess near the nucleus, but more frequently (because of dust extinction)Hi or radio continuum emission. Such bursts are quite common, even in objects with no interacting companions. One consequence of a burst is to sweep gas outwards from the inner zone, so further bursts (and perhaps the initial one) need to be re-supplied with gas. This can be effectedvia a non-axisymmetric gravitational potential, not necessarily a bar, in the nuclear bulge. Searching for these non-axisymmetric features, we have performed two-dimensional photometry on CCD maps inB, V, R, I, andZ bands, for a set of galaxies which have undergone starburst activity, and we present here a set of results from that photometric programme.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00652684
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  • 4
    Electronic Resource
    Electronic Resource
    Gradients of excitation in the nuclei of starburst galaxies (1989)
    Springer
    Astrophysics and space science 157 (1989), S. 165-171 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This article forms a part of a wider study of the nuclear and circum-nuclear zones of moderately active galaxies. The use of long-slit spectroscopy at high spectral and spatial resolution has enabled us to measure gradients of gaseous excitation and also velocity fields in a sample of four bright nuclear starburst galaxies selected from the sample of Balzano (1983). We find variations not only in the intensity but also in the ‘quality’ of the emission spectra (line ratios and line widths) of the interstellar gas between regions separated by relatively short distances. We stress the need for studies with at least the present degree of angular and spectral resolution if physical sense is to be made of the interstellar excitation regimes in external galaxies, as well as to investigate a possible evolutionary link between nuclei of intermediate activity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00637324
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  • 5
    Electronic Resource
    Electronic Resource
    Chemical abundances of the WR-ring nebulae NGC 2359 and RCW 78 (1989)
    Springer
    Astrophysics and space science 157 (1989), S. 3-7 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report on extensive spectroscopic observations of the WR-ring nebulae NGC 2359 and RCW 78, respectively, excited by the WN5 stars HD 56925 and WN8 HD 117688. For the first object we have determined abundances for O/H, Ne/H, N/H, and He/H in many different positions, including the ionized gas in the bubble, inside the optical shell structure, and the outermost zones associated with the S 298 Hii region. We do not find any significant difference in the N/H and O/H abundances over the entire nebula. The O/H and N/H abundances expected are close to those for a normal Hii region located at similar distance. In the case of He/H we find indication of local enhancements which sum to the abundance of metal rich galactic Hii regions like M17. RCW 78 appears to show slight overabundances of He/H and N/H in the two observed positions. The ionising temperature for the WN central star (HD 56925) of NGC 2359 is determined from the observed Hii region spectrum giving a value of 50000 K, appropriate to its spectral type.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00637303
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  • 6
    Electronic Resource
    Electronic Resource
    A New Ionization Correction scheme for He+/H+ and the determination of the helium abundance inHii regions (1989)
    Springer
    Astrophysics and space science 157 (1989), S. 9-14 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have developed a New Ionization Correction Factor (ICF) scheme for He+ based on the definition of the radiation softness parameter η (Vilchez and Pagel, 1988) which is very sensitive to the effective temperature of the ionizing star(s) and, therefore, a good indicator of the He+−He0 ionization structure whilst remaining insensitive to other nebular parameters. The ICF(η) relationship is obtained making use of the Stasinska (1980, 1982) photoionization models. After comparison with previous semiempirical approaches in the literature and its application to a selected sample of abundances it is found that this ICF scheme seems a powerful tool for the determination of He abundances, particularly for objects of intermediate to low excitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00637304
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  • 7
    Electronic Resource
    Electronic Resource
    Abundance calibration and evolution of GiantHii Regions (1989)
    Springer
    Astrophysics and space science 157 (1989), S. 61-64 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a self-consistent method to determine abundance and evolutionary status for GiantHii Regions (GHR) as well as the nominal temperature of the ionizing cluster and the ionization parameter. We define the ionization parameter-free plane [η,R 23] (Vilchez and Pagel, 1988) where observations are compared with the prediction of photoionization models, in order to obtain the abundance and evolutionary status for a given GHR. This new method allows us to re-calibrate the empirical relationship (R 23, O/H) by Pagelet al. (1979) up to abundances three times solar. The effects of evolution of GHR on their observed spectra are explained in terms of the cooling down of the ionizing population and some evolutionary tracks are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00637311
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  • 8
    Electronic Resource
    Electronic Resource
    Evidence for high velocity gas in giantHii regions (1989)
    Springer
    Astrophysics and space science 156 (1989), S. 237-242 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Spectroscopy of selected areas of the two GiantHii Regions (GHR) NGC 5471 and NGC 604 shows convincing evidence for the existence of ionised gas at high velocities (≥1000 km s−1) from the analysis of the profile of the Balmer (in NGC 604) and the [Oiii] (in NGC 5471) lines. The association of these effects with the presence of WR clusters and possible SNR inside the complex are analysed and the results are compared with previous indications in the literature for 30 Dor (in front of R 136) as well as with recent evidence from high-dispersion spectroscopy for some GHRs in M101.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00646372
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  • 9
    Electronic Resource
    Electronic Resource
    NLTE analysis of massive ob stars in open clusters (1990)
    Springer
    Astrophysics and space science 169 (1990), S. 7-13 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a new method for deriving stellar masses and distances using the theory of radiatively driven winds as presented by Kudritzkiet al. (1989) and the stellar parameters derived from NLTE spectroscopic analysis. We illustrate the procedure with five stars observed in three open clusters and show that the method can be advantageous against the more usual of using the expression for the gravity. The comparison with evolutionary tracks indicates a possible discrepancy between the masses and helium abundances (when enhanced) derived from these tracks and those obtained by the other methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00640677
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  • 10
    Electronic Resource
    Electronic Resource
    Density studies of giant extragalacticHII regions (1990)
    Springer
    Astrophysics and space science 171 (1990), S. 203-205 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have studied the variation of the electron density with position in a selected group of bright giant extragalacticHII regions with the use of long slit spectrophotometry at high spectral and spatial resolution. Density values for the ionized gas of the regions have been obtained from the measurement of the [SII] λλ6717, 6731 ratio. We conclude that the regions are not homogeneous; the density varies with position, with the zones of high density frequently associated to high surface brightness.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00646847
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