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Träfflista för sökning "L773:0035 8711 OR L773:1365 2966 "

Sökning: L773:0035 8711 OR L773:1365 2966

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1.
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2.
  • Adamo, Angela, et al. (författare)
  • Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess
  • 2010
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 407:2, s. 870-890
  • Tidskriftsartikel (refereegranskat)abstract
    • We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5Myr). A fraction of low-mass (below 104 Msolar), very young (1-3Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 Msolar, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ~38 per cent of the galactic star formation rate.
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3.
  • Aldenius, Maria, et al. (författare)
  • Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants
  • 2006
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:1, s. 444-452
  • Tidskriftsartikel (refereegranskat)abstract
    • The most precise method of investigating possible space-time variations of the fine-structure constant, alpha equivalent to (1/hc)(e(2)/4 pi epsilon(0)), using high-redshift quasar absorption lines is the many-multiplet (MM) method. For reliable results this method requires very accurate relative laboratory wavelengths for a number of UV resonance transitions from several different ionic species. For this purpose laboratory wavelengths and wavenumbers of 23 UV lines from Mg I, Mg II, Ti II, Cr II, Mn II, Fe II and Zn II have been measured using high-resolution Fourier transform (FT) spectrometry. The spectra of the different ions (except for one Fe II line, one Mg I line and the Ti II lines) are all measured simultaneously in the same FT spectrometry recording by using a composite hollow cathode as a light source. This decreases the relative uncertainties of all the wavelengths. In addition to any measurement uncertainty, the wavelength uncertainty is determined by that of the Ar II calibration lines, by possible pressure shifts and by illumination effects. The absolute wavenumbers have uncertainties of typically +/- 0.001 -+/- 0.002 cm(-1) (Delta lambda approximate to 0.06-0.1 m angstrom at 2500 angstrom), while the relative wavenumbers for strong, symmetric lines in the same spectral recording have uncertainties of +/- 0.0005 cm(-1) (Delta lambda approximate to 0.03 m angstrom at 2500 angstrom) or better, depending mostly on uncertainties in the line-fitting procedure. This high relative precision greatly reduces the potential for systematic effects in the MM method, while the new Ti II measurements now allow these transitions to be used in MM analyses.
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4.
  • Axelsson, Magnus, et al. (författare)
  • On the origin of black hole spin in high-mass black hole binaries : Cygnus X-1
  • 2011
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 412:4, s. 2260-2264
  • Tidskriftsartikel (refereegranskat)abstract
    • To date, there have been several detections of high-mass black hole binaries in both the Milky Way and other galaxies. For some of these, the spin parameter of the black hole has been estimated. As many of these systems are quite tight, a suggested origin of the spin is angular momentum imparted by the synchronous rotation of the black hole progenitor with its binary companion. Using Cygnus X-1, the best studied high-mass black hole binary, we investigate this possibility. We find that such an origin of the spin is not likely, and our results point rather to the spin being the result of processes during the collapse.
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5.
  • Bautista, M A, et al. (författare)
  • Excitation of Sr II lines in Eta Carinae
  • 2002
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 331:4, s. 875-879
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the nature of the peculiar [Sr II] and Sr II emission filament found in the ejecta of Eta Carinae. To this purpose we carry out abinitio calculations of radiative transition probabilities and electron impact excitation rate coefficients for Sr II. Then we build a multilevel model for the system which is used to investigate the physical condition of the filament and the nature of the observed allowed and forbidden Sr II optical emission. It is found that the observed spectrum is consistent with the lines being pumped by the continuum radiation field in a mostly neutral region with electron density near 107cm-3 . Under these conditions, the observed emission can be explained without the need for a large Sr overabundance.
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6.
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7.
  • Bautista, M. A., et al. (författare)
  • The abundance of iron-peak elements and the dust composition in eta Carinae: manganese
  • 2011
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 410:4, s. 2643-2652
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the chemical abundances of the strontium filament found in the ejecta of eta Carinae. In particular, we derive the abundances of iron-peak elements from the spectra of their singly ionized ions present in the optical/infrared (IR) spectra. In this paper we analyse the spectrum of Mn II using a new non-local thermodynamic equilibrium (non-LTE) model for this system. In constructing this models we carried out theoretical calculations of radiative transition rates and electron impact excitation rate coefficients. We find that relative to Ni the gas-phase abundance ratio of Mn is roughly solar, similar to the Cr abundance but in contrast to the large enhancements in the abundances of Sc and Ti. We interpret this result as an indication of non-equilibrium condensation in the ejecta of eta Carinae.
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8.
  • Bautista, M A, et al. (författare)
  • [Ti II] and [Ni II] emission from the strontium filament of eta Carinae
  • 2006
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:4, s. 1991-2003
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the nature of the [Ti II] and [Ni II] emission from the so-called strontium filament found in the ejecta of eta Carinae. To this purpose, we employ multilevel models of the Ti II and Ni II systems, which are used to investigate the physical condition of the filament and the excitation mechanisms of the observed lines. For the Ti II ion, for which no atomic data were previously available, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. It is found that the observed spectrum is consistent with the lines being excited in a mostly neutral region with an electron density of the order of 10(7) cm(-3) and a temperature around 6000 K. In analysing three observations with different slit orientations recorded between 2000 March and 2001 November, we find line ratios that change among various observations, in a way consistent with changes of up to an order of magnitude in the strength of the continuum radiation field. These changes result from different samplings of the extended filament due to the different slit orientations used for each observation, and yield clues on the spatial extent and optical depth of the filament. The observed emission indicates a large Ti/Ni abundance ratio relative to solar abundances. It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas fractionation processes and does not reflect the absolute Ti/Ni ratio in the ejecta of eta Carinae. We study the condensation chemistry of Ti, Ni and Fe within the filament and suggest that the observed gas phase overabundance of Ti is likely the result of selective photoevaporation of Ti-bearing grains. Some mechanisms for such a scenario are proposed.
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9.
  • Beer, Martin E, et al. (författare)
  • Red giant depletion in globular cluster cores
  • 2004
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 348:2, s. 679-686
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the observed depletion of red giants in the cores ofpost-core-collapse globular clusters. In particular, the evolutionaryscenario we consider is a binary consisting of two low-mass stars whichundergoes two common-envelope phases. The first common-envelope phaseoccurs when the primary is a red giant resulting in a helium white dwarfand main-sequence star in a detached binary. The second common-envelopephase occurs shortly after the secondary becomes a red giant. During thesecond common-envelope phase, the degenerate helium cores mergeresulting in a core mass greater than the helium burning limit and theformation of a horizontal branch star. We show that stellar encountersenhance this evolutionary route in post-core-collapse clusters. Theseencounters increase the population of binary secondaries which wouldhave evolved on to the red giant branch in the recent past.
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10.
  • Bensby, Thomas, et al. (författare)
  • The origin and chemical evolution of carbon in the Galactic thin and thick discs
  • 2006
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 367:3, s. 1181-1193
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to trace the origin and evolution of carbon in the Galactic disc, we have determined carbon abundances in 51 nearby F and G dwarf stars. The sample is divided into two kinematically distinct subsamples with 35 and 16 stars that are representative of the Galactic thin and thick discs, respectively. The analysis is based on spectral synthesis of the forbidden [C I] line at 872.7 nm using spectra of very high resolution (R approximate to 220 000) and high signal-to-noise ratio (S/N greater than or similar to 300) that were obtained with the Coude Echelle Spectrograph (CES) spectrograph by the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile. We find that [C/Fe] versus [Fe/H] trends for the thin and thick discs are totally merged and flat for subsolar metallicities. The thin disc that extends to higher metallicities than the thick disc shows a shallow decline in [C/Fe] from [Fe/H] approximate to 0 and up to [Fe/H]approximate to+0.4. The [C/O] versus [O/H] trends are well separated between the two discs (due to differences in the oxygen abundances) and bear a great resemblance to the [Fe/O] versus [O/H] trends. Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick discs have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment [i.e. SN Ia and asymptotic giant branch (AGB) stars, respectively]. We further note that there exist other observational data in the literature that favour massive stars as the main sources for carbon. In order to match our carbon trends, we believe that the carbon yields from massive stars then must be very dependent on metallicity for the C, Fe and Y trends to be so finely tuned in the two disc populations. Such metallicity-dependent yields are no longer supported by the new stellar models in the recent literature. For the Galaxy, we hence conclude that the carbon enrichment at metallicities typical of the disc is mainly due to low- and intermediate-mass stars, while massive stars are still the main carbon contributor at low metallicities (halo and metal-poor thick disc).
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