Browsing by Author "Zijlstra, AA"

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    A dwarf irregular galaxy at the edge of the local group: Stellar populations and distance of IC5152
    (1999) Zijlstra, AA; Minniti, D
    We have obtained V- and I-band photometry for about 700 stars in the field of the dwarf irregular galaxy IC 5152, previously considered a possible member of the Local Group. Deep VI color-magnitude diagrams are presented of the main body of this galaxy and of a nearby field. We infer a distance by comparison with the VI color-magnitude diagrams of similar galaxies and with theoretical isochrones. The distance modulus to IC 5152 is found to be m - M-o = 26.15 +/- 0.2, adopting E(V - I) = 0. This distance of D = 1.7 Mpc implies that IC 5152 is not a certain member of the Local Group. We also construct a deep optical luminosity function. By comparison with theoretical isochrones, we find a metallicity Z approximate to 0.002, which is lower than previous estimates. Using this metallicity and distance, IC 5152 now fits the metallicity-luminosity relation for dwarf galaxies. The youngest stars in the held studied have an age of similar to 10(7) Gyr, and there is a substantial population of stars with log t > 7.8-8 yr. The central region of IC 5152 is an active site of star formation, as found by comparing our optical photometry with Hubble Space Telescope ultraviolet images. Several candidate globular clusters are found, as well as a candidate for a potential nucleus.
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    Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the bulge
    (2004) Ruffle, PME; Zijlstra, AA; Walsh, JR; Gray, MD; Gesicki, K; Minniti, D; Comeron, F
    We present values for angular diameter, flux and extinction for 70 Galactic planetary nebulae observed using narrow-band filters. Angular diameters are derived using constant emissivity shell and photoionization line emission models. The mean of the results from these two models are presented as our best estimate. Contour plots of 36 fully resolved objects are included and the low-intensity contours often reveal an elliptical structure that is not always apparent from full width at half maximum measurements. Flux densities are determined, and for both Halpha and [O III] there is little evidence of any systematic differences between observed and catalogued values. Observed Halpha extinction values are determined using observed Halpha and catalogued radio fluxes. Halpha extinction values are also derived from catalogued Halpha and Hbeta flux values by means of an R-V dependent extinction law. R-V is then calculated in terms of observed extinction values and catalogued Halpha and Hbeta flux values. Comparing observed and catalogue extinction values for a subset of bulge objects, observed values tend to be lower than catalogue values calculated with R-V = 3.1. For the same subset we calculate (R-V) = 2.0, confirming that toward the bulge interstellar extinction is steeper than R-V = 3.1. For the inner Galaxy, a relation with the higher supernova rate is suggested, and that the low-density warm ionized medium is the site of the anomalous extinction. Low values of extinction are also derived using dust models with a turnover radius of 0.08 mum.
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    Deep Hubble Space Telescope STIS color-magnitude diagrams of the dwarf irregular galaxy WLM
    (2000) Rejkuba, M; Minniti, D; Gregg, MD; Zijlstra, AA; Alonso, MV; Goudfrooij, P
    We have obtained V- and I-band photometry for 1886 stars down to I = 27 and y = 28 in the held of the dwarf irregular galaxy WLM, using deep Space Telescope Imaging Spectrograph (STIS) CL- and LP-band images taken in the parallel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending blueward from the red giant clump, is an unambiguous signature of an old population. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of similar or equal to 1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram, we obtain an accurate distance modulus (m - M)(0) = 24.95 +/- 0.13 for WLM by using the Y magnitude of the horizontal branch, and by adopting E(V-I)= 0.03. The implications are that (1) WLM formed stars at high redshift and (2) the old population of WLM can be representative of a protogalactic fragment, related to those accreted to form the Milky Way halo.