| 1. |
- Davies, Melvyn B, et al.
(författare)
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The MODEST questions: Challenges and future directions in stellar cluster research
- 2006
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Ingår i: New Astronomy. - : Elsevier BV. - 1384-1092 .- 1384-1076. ; 12:3, s. 201-214
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Forskningsöversikt (refereegranskat)abstract
- We present a review of some of the current major challenges in stellar cluster research, including young clusters, globular clusters, and galactic nuclei. Topics considered include: primordial mass segregation and runaway mergers, expulsion of gas from clusters, the production of stellar exotica seen in some clusters (e.g., blue stragglers and extreme horizontal-branch stars), binary populations within clusters, the black-hole population within stellar clusters, the final parsec problem, stellar dynamics around a massive black hole, and stellar collisions. The Modest Questions posed here are the outcome of discussions which took place at the Modest-6A workshop held in Lund, Sweden, in December, 2005. Modest-6A was organised as part of the activities of the Modest Collaboration (see www.manybody.org for further details).
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| 2. |
- Johansson, Sveneric, et al.
(författare)
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Possibility of measuring the width of narrow FeII astrophysical laser lines in the vicinity of eta Carinae by means of Brown-Twiss-Townes heterodyne correlation interferometry
- 2005
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Ingår i: New Astronomy. - : Elsevier BV. - 1384-1092 .- 1384-1076. ; 10:5, s. 361-369
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Tidskriftsartikel (refereegranskat)abstract
- We consider the possibility of measuring the true width of the narrow Fe II optical lines observed in spectra of the Weigelt blobs in the vicinity of eta Carinae. The lines originate as a result of stimulated amplification of spontaneous emission of radiation in quantum transitions between energy levels showing inverted population [Johansson, S., Letokhov, V., 2002. JETP Lett. 75 (10) 495; Johansson, S., Letokhov, V., 2003. Phys. Rev. Lett. 90 (1) 01110-1; Johansson, S., Letokhov, V., 2004. A&A 428, 497]. The lines should have a subDoppler spectral width of 30-100 MHz, depending on the geometry of the lasing volume. To make measurements with a spectral resolution of R > 10(7) and an angular resolution better than 0.1 '', we suggest the use of the Brown-Twiss-Townes optical heterodyne intensity correlation interferometry. The estimates made of the SIN ratio for the optical heterodyne astrophysical laser experiment imply that it is feasible.
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| 3. |
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Peculiar temporal and spectral features in highly obscured HMXB pulsar IGR J16320-4751 using XMM-Newton
- 2023
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Ingår i: New Astronomy. - : Elsevier BV. - 1384-1076 .- 1384-1092. ; 98, s. 101942-
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Tidskriftsartikel (refereegranskat)abstract
- IGR J16320-4751 is a highly obscured HMXB source containing a very slow neutron star (P-spin similar to 1300 sec) orbiting its supergiant companion star with a period of similar to 9 days. It shows high column density (N-H similar to 2 - 5 x 10(23) cm(-2)) in the spectrum, and a large variation in flux along the orbit despite not being an eclipsing source. We report on some peculiar timing and spectral features from archival XMM-Newton observation of this source including 8 observations taken during a single orbit. The pulsar shows large timing variability in terms of average count rate from different observations, flaring activity, sudden changes in count rate, cessation of pulsation, and variable pulse profile even from observations taken a few days apart. We note that IGR J16320-4751 is among a small number of sources for which this temporary cessation of pulsation in the light curve has been observed. A time-resolved spectral analysis around the segment of missing pulse shows that variable absorption is deriving such a behavior in this source. Energy resolved pulse profiles in 6.2-6.6 keV band which has a partial contribution from Fe K-alpha photons, show strong pulsation. However, a more systematic analysis reveals a flat pulse profile from the contribution of Fe K-alpha photons in this band implying a symmetric distribution for the material responsible for this emission. Soft excess emission below 3 keV is seen in 6 out of 11 spectra of XMM-Newton observations.
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| 4. |
- Rajesh, S. R., et al.
(författare)
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Time variability of viscosity parameter in differentially rotating discs
- 2014
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Ingår i: New astronomy. - : Elsevier BV. - 1384-1076 .- 1384-1092. ; 30, s. 38-45
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Tidskriftsartikel (refereegranskat)abstract
- We propose a mechanism to produce fluctuations in the viscosity parameter (a) in differentially rotating discs. We carried out a nonlinear analysis of a general accretion flow, where any perturbation on the background a was treated as a passive/slave variable in the sense of dynamical system theory. We demonstrate a complete physical picture of growth, saturation and final degradation of the perturbation as a result of the nonlinear nature of coupled system of equations. The strong dependence of this fluctuation on the radial location in the accretion disc and the base angular momentum distribution is demonstrated. The growth of fluctuations is shown to have a time scale comparable to the radial drift time and hence the physical significance is discussed. The fluctuation is found to be a power law in time in the growing phase and we briefly discuss its statistical significance.
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| 5. |
- Thompson, Rodger I., et al.
(författare)
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Alternative data reduction procedures for UVES: Wavelength calibration and spectrum addition
- 2009
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Ingår i: New Astronomy. - : Elsevier BV. - 1384-1076 .- 1384-1092. ; 14:4, s. 379-390
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Tidskriftsartikel (refereegranskat)abstract
- This paper addresses alternative procedures to the ESO supplied pipeline procedures for the reduction of UVES spectra of two quasar spectra to determine the value of the fundamental constant μ=Mp/Me at early times in the universe. The procedures utilize intermediate product images and spectra produced by the pipeline with alternative wavelength calibration and spectrum addition methods. Spectroscopic studies that require extreme wavelength precision need customized wavelength calibration procedures beyond that usually supplied by the standard data reduction pipelines. An example of such studies is the measurement of the values of the fundamental constants at early times in the universe. This article describes a wavelength calibration procedure for the UV–visual Echelle spectrometer on the very large telescope, however, it can be extended to other spectrometers as well. The procedure described here provides relative wavelength precision of better than 3×10-7 for the long-slit Thorium–Argon calibration lamp exposures. The gain in precision over the pipeline wavelength calibration is almost entirely due to a more exclusive selection of Th/Ar calibration lines.
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| 6. |
- Öztaş, Ahmet M., et al.
(författare)
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Re-evaluation of Ωk of the normalised Friedmann-Lemaître-Robertson-Walker model : Implications for Hubble constant determinations
- 2021
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Ingår i: New Astronomy. - : Elsevier. - 1384-1076 .- 1384-1092. ; 88
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Tidskriftsartikel (refereegranskat)abstract
- The description of spacetime is an fundamental problem of cosmology. We explain why the current assignments of spacetime geometries for Ωk of the Friedmann-Lemaître-Robertson-Walker (FLRW) model are probably incorrect and suggest more useful descriptions. We show that Ωk represents not only curvature but the influence of matter density on the extent of spacetime between massive objects. Recent analyses of supernovae type Ia (SNe Ia) and HII/GEHR data with the FLRW model present the best fits with a small value for Ωm and a large Ωk. These results are consistent with our Universe exhibiting sparse matter density and quasi-Euclidean geometry and the small Ωm value agrees with Big Bang nucleosynthesis calculations. We suggest the geometry of our current Universe is better described by a value for Ωk≈1 rather than 0. As an example we extend the FLRW model towards the Big Bang and discover a simple explanation of how matter creation developed into the currently geometrically flat Universe with sparse, homogeneous, isotropic matter and energy distributions. Assigning Ωk≈1 to describe quasi-Euclidean spacetime geometry is also useful for estimating H0 and should help resolve the “tension” surrounding current estimates by different investigators.
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| 7. |
- Wyse, RFG, et al.
(författare)
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Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift
- 2002
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Ingår i: New Astronomy. - 1384-1092. ; 7:7, s. 395-433
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Tidskriftsartikel (refereegranskat)abstract
- The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population-all of the stars being old and metal-poor-similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to similar to0.3 M-.. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.
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| 8. |
- Kriminski, SA, et al.
(författare)
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On the stability of thermonuclear detonation in supernovae events
- 1998
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Ingår i: NEW ASTRONOMY. - : ELSEVIER SCIENCE BV. - 1384-1092. ; 3:6, s. 363-377
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The stability of a plane stationary thermonuclear detonation in an exploding carbon white dwarf is investigated. It is shown that detonation is unstable in a wide range of densities of white dwarf matter from rho approximate to 2 X 10(7) g/cm(3) to rho =
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