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  • 1
    Electronic Resource
    Electronic Resource
    Evolution of dense galactic nuclei through dwarf star collisions (1974)
    [s.l.] : Nature Publishing Group
    Nature 248 (1974), S. 398-400 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Van den Bergh3 has pointed out that most collisions in the nuclei of 'normal' galaxies such as M32 involve two dwarf M stars or one M dwarf (dM) and one late-type giant. The reason is the high abundance of dM stars and the large cross section of giants. The evolution of a nucleus therefore depends ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/248398b0
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  • 2
    Electronic Resource
    Electronic Resource
    On rotating isothermal configurations in the post-Newtonian approximation of general relativity (1973)
    Springer
    Astrophysics and space science 24 (1973), S. 117-126 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The equations which govern the structure of a rotating, truncated isothermal sphere in the post-Newtonian approximation of general relativity are derived and solved numerically. Each model is parameterized by both a rotation and a relativity parameter. The density inside the configurations is tabulated and graphed as a function of both distance from the center and co-latitude. Relativistic gravitational effects are found to pull the models into states which are considerably more centrally condensed than one predicts classically. Rotation tends to flatten the isothermal configurations into oblate spheroids, though for even the largest rotation parameters the degree of flattening is only a few percent. The computed models may be similar to the cores of relativistic star clusters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00648679
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  • 3
    Electronic Resource
    Electronic Resource
    The flow of an infinite compressible fluid past a rigid, gravitating sphere and interacting degenerate star-red giant binaries (1980)
    Springer
    Astrophysics and space science 67 (1980), S. 427-443 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We compute two examples of the flow structure of an infinite medium flowing hypersonically past a non-accreting, gravitating, rigid sphere. The resulting flow depends strongly on the ratioA of kinetic energy at infinity to potential energy on the sphere's surface per unit mass.A=0.25 yields a flow rather like that past a hard, gravitationless sphere upstream, but with a closed shock downstream.A=0.028 yields a circulating eddy flow downstream of the sphere which causes the isodensity contours to be extended upstream. Application to a compact object immersed in a binary companion is discussed. We pictorially illustrate the fluid flow past a degenerate star starting to spiral into its giant companion. The accretion rates onto ‘hard’ gravitating objects can be many orders of magnitude less than the classical Hoyle-Lyttleton-Bondi rates unless cooling dominates the flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00642396
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  • 4
    Electronic Resource
    Electronic Resource
    The mass and angular momentum losses from spinars (1972)
    Springer
    Astrophysics and space science 16 (1972), S. 171-178 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Mestel's stellar wind theory is applied to estimate the mass and angular momentum losses for the recently proposed spinar model of quasars. If a spinar is uniformly rotating and has temperatures in the corotating regions of over a billion degrees, then it is found that all the rotational energy will be lost in 103 yr. This time scale is much shorter than the total life time of the spinar model which is about 106 yr. Hence it is suggested that the temperatures of the corotating regions of spinars must be less than a billion degrees.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00642732
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  • 5
    Electronic Resource
    Electronic Resource
    The variable $$\dot M$$ scenario for nova outbursts (1987)
    Springer
    Astrophysics and space science 131 (1987), S. 419-429 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract An evolutionary scenario for classical novae is proposed, which is intended to solve the discrepancies that exist between theory and observations:the space densities of classical novae deduced from surveys in the solar neighbourhood are lower by about two orders of magnitude than those predicted theoretically, and the mass transfer rates in nova binaries, as estimated from observed luminosities in quiescence, are higher than those allowed by the thermonuclear runaway model for nova outbursts. These discrepancies disappear if mass transfer (at a high rate) takes place for only a few hundred years before and a few hundred years after an eruption, but declines afterwards and remains off for most of the time between outbursts. We show that such a behavior is to be expected if one takes into account the variation of binary separation, due to mass ejection on the one hand and angular momentum losses on the other hand. One of the aspects of this scenario, on which we report in more detail, is the possibility of enhanced Roche-lobe overflow of the secondary, due to its expansion that results from irradiation by the high nova luminosity. We followed the evolution of a 0.5 M⊙ main sequence star illuminated by a changing flux, typical of a classical nova. The numerical results indicate that, in spite of the slight binary separation that may occur after eruption, mass loss from the irradiated and thus bloated secondary should continue for a few hundred years. Other aspects of the variable $$\dot M$$ scenario are briefly summarized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00668120
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  • 6
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    On the origins of enigmatic stellar populations in Local Group galactic nuclei (2016)
    Oxford University Press
    Details
    Publication Date: 2016-09-20
    Description: We consider the origins of enigmatic stellar populations in four Local Group galactic nuclei, specifically the Milky Way, M31, M32 and M33. These are centrally concentrated blue stars, found in three out of the four nuclear star clusters (NSCs) considered here. Their origins are unknown, but could include blue straggler (BS) stars, extended horizontal branch stars and young recently formed stars. Here, we calculate order-of-magnitude estimates for various collision rates, as a function of the host NSC environment and distance from the cluster centre. These rates are sufficiently high that BSs, formed via collisions between main-sequence (MS) stars, could contribute non-negligibly (~1–10 per cent in mass) to every surface brightness profile, with the exception of the Milky Way. Stellar evolution models show that the envelopes of red giant branch (RGB) stars must be nearly completely stripped to significantly affect their photometric appearance, which requires multiple collisions. Hence, the collision rates for individual RGB stars are only sufficiently high in the inner 0.1 pc of M31 and M32 for RGB destruction to occur. Collisions between white dwarfs and MS stars, which should ablate the stars, could offer a steady and significant supply of gas in every NSC in our sample. The gas could either fragment to form new stars, or accrete on to old MS stars already present. Thus, collisional processes could contribute significantly to the observed blue excesses in M31 and M33; future studies should be aimed at better constraining theoretical predictions to compliment existing and future observational data.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The first transition Wolf-Rayet WN/C star in M31 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-12-02
    Description: Three decades of searches have revealed 154 Wolf–Rayet (WR) stars in M31, with 62 of WC type, 92 of WN type and zero of transition-type WN/C or WC/N. In apparent contrast, about two per cent of the WR stars in the Galaxy, the LMC and M33 simultaneously display strong lines of carbon and nitrogen, i.e. they are transition-type WN/C or WC/N stars. We report here the serendipitous discovery of M31 WR 84-1, the first transition star in M31, located at RA = 00 h 43 m 43 ${^{\rm s}_{.}}$ 61 Dec. = +41°45'27 ${^{\prime\prime}_{.}}$ 95 (J2000). We present its spectrum, classify it as WN5/WC6, and compare it with other known transition stars. The star is unresolved in Hubble Space Telescope narrow-band and broad-band images, while its spectrum displays strong, narrow emission lines of hydrogen, [N ii ], [S ii ] and [O iii ]; this indicates a nebula surrounding the star. The radial velocity of the nebular lines is consistent with that of gas at the same position in the disc of M31. The metallicity at the 11.8 kpc galactocentric distance of M31 WR 84-1 is approximately solar, consistent with other known transition stars. We suggest that modest numbers of reddened WR stars remain to be found in M31.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope He II narrow-band observations (2014)
    Oxford University Press
    Details
    Publication Date: 2014-05-04
    Description: We present Hubble Space Telescope ( HST ) imaging observations of the site of the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about 1 yr prior to the event, in a narrow band centred on the He ii 4686 Å emission line. In a ‘single-degenerate’ progenitor scenario, the hard photon flux from an accreting white dwarf (WD), burning hydrogen on its surface over ~1 Myr should, in principle, create a He iii Strömgren sphere or shell surrounding the WD. Depending on the WD luminosity, the interstellar density, and the velocity of an outflow from the WD, the He iii region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of He ii 4686 Å line emission in the HST data. Using simulations, we set 2 upper limits on the He ii 4686 Å luminosity of L He ii 〈 3.4 x 10 34 erg s –1 for a point source, corresponding to an emission region of radius r  〈 1.8 pc. The upper limit for an extended source is L He ii 〈 1.7 x 10 35  erg s –1 , corresponding to an extended region with r  ~ 11 pc. The largest detectable shell, given an interstellar-medium density of 1 cm –3 , has a radius of ~6 pc. Our results argue against the presence, within the ~10 5  yr prior to the explosion, of a supersoft X-ray source of luminosity L bol 3 x 10 37  erg s –1 , or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2–6 pc.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Observational signatures of SNIa progenitors, as predicted by models (2014)
    Oxford University Press
    Details
    Publication Date: 2014-11-24
    Description: A definitive determination of the progenitors of Type Ia supernovae (SNIa) has been a conundrum for decades. The single degenerate scenario – a white dwarf (WD) in a semi-detached binary system accreting mass from its secondary – is a plausible path; however, no simulation to date has shown that such an outcome is possible. In this study, we allowed a WD with a near Chandrasekhar mass of 1.4 M to evolve over tens of thousands of nova cycles, accumulating mass secularly while undergoing periodic nova eruptions. We present the mass accretion limits within which an SNIa can possibly occur. The results showed, for each parameter combination within the permitted limits, tens of thousands of virtually identical nova cycles where the accreted mass exceeded the ejected mass, i.e. the WD grew slowly but steadily in mass. Finally, the WD became unstable, the maximal temperature rose by nearly two orders of magnitude, heavy element production was enhanced by orders of magnitude and the nuclear and neutrino luminosities became enormous. We also found that this mechanism leading to WD collapse is robust, with WDs in the range 1.0–1.38 M , and an accretion rate of 5 x 10 –7 M yr –1 , all growing steadily in mass. These simulations of the onset of an SNIa event make observationally testable predictions about the light curves of pre-SN stars, and about the chemistry of SNIa ejecta.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    First detection and characterization of symbiotic stars in M31 (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-20
    Description: Symbiotic binaries are putative progenitors of Type Ia supernovae. The census of Galactic symbiotic binaries is so incomplete that we cannot reliably estimate the total population of these stars, and use it to check whether that number is consistent with the observed Type Ia supernova rate in spiral galaxies. We have thus begun a survey of the nearest counterpart of our own Galaxy, namely M31, where a relatively complete census of symbiotic stars is achievable. We report the first detections and spectrographic characterizations of 35 symbiotic binaries in M31, and compare these stars with the symbiotic population in the Milky Way. These newly detected M31 symbiotic binaries are remarkably similar to Galactic symbiotics, though we are clearly only sampling (in this feasibility study) the most luminous symbiotics in M31. We have also found, in M31, the symbiotic star (M31SyS J004233.17+412720.7) with the highest ionization level known amongst all symbiotics. An optical outburst of the M31 symbiotic star M31SyS J004322.50+413940.9 was probably a nova-like outburst, the first symbiotic outburst detected outside the Milky Way and Magellanic Clouds.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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