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Publications (8)10.17 Total impact

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    ABSTRACT: We investigate the star formation history and chemical evolution of low surface brightness (LSB) disk galaxies by modelling their observed spectro-photometric and chemical properties using a galactic chemical and photometric evolution model incorporating a detailed metallicity depen dent set of stellar input data. For a large fraction of the LSB galaxies in our sample, observed properties are best explained by models incorporating an exponentially decreasing global star formation rate (SFR) ending at a present-day gas fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For some galaxies small amplitude star formation bursts are required to explain the contribution of the young (5-50 Myr old) stellar population to the galaxy integrated luminosity. This suggests that star formation has proceeded in a stochastic manner. The presence of an old stellar population in many late-type LSB galaxies suggests that LSB galaxies roughly follow the same evolutionary history as HSB galaxies, except at a much lower rate. In particular, our results imply that LSB galaxies do not form late, nor have a delayed onset of star formation, but simply evolve slowly.
    Astronomy and Astrophysics 04/2000; · 5.08 Impact Factor
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    L.B. van den Hoek, M.A.T. Groenewegen
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    ABSTRACT: We present theoretical yields of H, $^{4}$He, $^{12}$C, $^{13}$C, $^{14}$N, and $^{16}$O for stars with initial masses between 0.8 and 8 $M_{\odot}$ and initial metallicities $Z = 0.001$, 0.004, 0.008, 0.02, and 0.04. We use the evolutionary tracks of the Geneva group up to the early asymptotic giant branch (AGB) in combination with a synthetic thermal-pulsing AGB evolution model to follow in detail the chemical evolution and mass loss up to the end of the AGB including the first, second, and third dredge-up phases. Most of the relations used are metallicity dependent to make a realistic comparison with stars of different initial abundances. The effect of Hot Bottom Burning (HBB) is included in an approximate way. The free parameters in our calculations are the mass loss scaling parameter $\eta_{\rm AGB}$ for stars on the AGB (using a Reimers law), the minimum core mass for dredge-up $M_{\rm c}^{\rm min}$, and the third dredge-up efficiency $\lambda$. As derived from previous extensive modeling, $\eta_{\rm AGB}$ = 4, $M_{\rm c}^{\rm min}$ = 0.58 $M_{\odot}$, and $\lambda = 0.75$ including HBB are in best agreement with observations of AGB stars both in the Galactic disk and Magellanic Clouds. The influence of specific model assumptions and adopted parameter values on the resulting AGB yields is examined and compared with earlier theoretical work. We compare the abundances predicted during the final stages of the AGB with those observed in planetary nebulae in the Galactic disk and show that the model with the aforementioned parameters is in good agreement with the observations. The metallicity dependent yields of intermediate mass stars presented in this paper are well suited for use in galactic chemical evolution models.
    http://dx.doi.org/10.1051/aas:1997162. 11/1996;
  • L. B. van den Hoek, M. A. T. Groenewegen
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    ABSTRACT: We present theoretical yields of H, 4He, 12C,
    VizieR Online Data Catalog. 10/1996;
  • L. B. van den Hoek, T. de Jong
    01/1996;
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    L. B. van den Hoek, W. J. G. de Blok
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    ABSTRACT: We investigate the star formation history and chemical evolution of Low Surface Brightness (LSB) spiral galaxies by means of their observed spectro-photometric and chemical properties. We present preliminary results for Johnson-Cousins UBVRI magnitudes and stellar [O/H] abundance ratios using a galactic chemical evolution model incorporating a detailed metallicity dependent set of stellar input data covering all relevant stages of stellar evolution. Comparison of our model results with observations confirms the idea that LSB galaxies are relatively unevolved systems. However, we argue that recent and ongoing massive star formation plays an important role in determining the colours of many LSB spirals. We briefly discuss these results in the context of the spectral evolution of spiral galaxies in general.
    12/1995;
  • M. A. T. Groenewegen, L. B. van den Hoek, T. de Jong
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    ABSTRACT: Based on a comparison of observations with new synthetic AGB evolution calculations we propose a revised evolutionary scenario for carbon stars in the solar neighbourhood. From observations we derive that the lowest initial mass from which carbon stars form is about 1.5Msun_. This constraint combined with four other constraints (the observed initial-final mass relation, the birth rate of carbon stars, the observed abundance ratios in planetary nebulae (PNe) and the number ratios C/M and S/C of AGB stars) are used to derive the following parameters for the synthetic AGB evolution model. Third dredge-up occurs for core masses above 0.58Msun_ and the dredge-up efficiency is λ=0.75. We consider a Reimers mass loss law (with a scaling factor η_AGB_) and the mass loss rate law recently proposed by Bloecker & Schoenberner (1993; with a scaling factor η_BS_). We find η_AGB_=4 and η_BS_=0.08. Both models fit the observations equally well. The model predicts that stars in the range 1.5Msun_<~M<~1.6Msun_ become carbon stars at their last thermal pulse (TP) on the AGB and live only a few 10^4^yr as carbon stars. More massive stars experience additional TPs as carbon stars (up to about 25 for a 3Msun_ star) and live up to 10^6^yr. For M>4Msun_ hot-bottom burning prevents the formation of carbon stars. For M<~2Msun_, M-stars skip the S-star phase when they become carbon stars. The average lifetime of the carbon star phase is ~3x10^5^yr. The carbon stars for which C/O ratios have been derived in the literature (with values <~1.5) are predominantly optical carbon stars with a 60μm excess. Yet, disk PNe are known with C/O ratios up to about 4. We predict that carbon stars with C/O ratios >1.5 are to be found among the infrared carbon stars. The model predicts that the probability that a carbon star has C/O>1.5 is about 30%, in reasonable agreement with the observed ratio of the surface density in the galactic plane of infrared carbon stars to all carbon stars. The infrared carbon stars are predicted to be (on average) more massive than the optical carbon stars. The fact that carbon stars with C/O>1.5 apparently never reach the optical carbon star phase (with a detached shell) is probably due to differences in evolution. If indeed infrared carbon stars are on average more massive (i.e. have larger core masses) than optical carbon stars, the interpulse period is shorter, and the increase in luminosity during the TP is smaller (due to the larger envelope mass). Both effects will decrease the likelihood of a detached shell to occur. We predict that two-thirds of all detached shells around optical carbon stars are oxygen-rich.
    Astronomy and Astrophysics 01/1995; · 5.08 Impact Factor
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    ABSTRACT: We present accurate CCD surface photometry in Johnson B and V and Cousins I for a complete, magnitude-limited sample of 56 elliptical galaxies from the RSA catalog. For each galaxy we have determined radial profiles of surface brightness, B-V and/or B-I colour index, ellipticity, position angle, and the third- and fourth order Fourier coefficients that describe the deviations of the B, V, and I isophotes from perfect ellipses, using a full two-dimensional fitting technique. The present sample contains 13 galaxies for which no previous isophote analysis has been published, and 26 without published colour gradients. The radial profiles of the ellipticity, position angle, and the third- and fourth-order Fourier coefficients are found to show considerable detail. The profiles are mostly similar in all passbands, except in cases where dust lanes or patches are present. In this respect, the higher-order Fourier coefficients turn out to be sensitive diagnostic tools for the presence of dust in elliptical galaxies. Isophotal deviations from ellipses on the level of 0.5-1% are found to be common in elliptical galaxies. As noted before by others, these deviations are due to structures that do not necessarily align with the apparent major or minor axes of the galaxies, advocating the use of both the cosine and sine higher-order terms in correlation studies. We show that fitting outer radial intensity profiles of elliptical galaxies is an excellent tool for determining the sky background for the surface photometry. The sky values determined from a power-law fit to the outer intensity profiles are found to be within 0.1% of the sky values at the corners of present-day large CCDs where the contribution of galaxy light is negligible. The average colour gradients for the sample galaxies in B-V and B-I are 0.06 and 0.14 mag arcsec^-2 per decade in radius, respectively. This compares well with colour gradients in elliptical galaxies found by others. The small uncertainty introduced by the sky background determination method applied here results in an average internal uncertainty of only ~ 0.01 mag for our colour gradients, which is significantly better than the results of previous studies. The profiles are compared extensively with results from other authors when available. The result is encouraging, especially for the ellipticity and position angle where we find mean RMS differences of 0.01 and 1.8 degrees, respectively. The surface brightness profiles generally agree to within +/- 0.05 mag.
    Astronomy and Astrophysics Supplement Series 04/1994; 104:179.
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    ABSTRACT: Not Available colour corrections; colour indices, ellipticity, position angle and their gradients, and effective higher order residual terms (table1, 4, 5 and 6 of the paper) (155 data files).
    VizieR Online Data Catalog. 11/1993;