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  • Article: Star Formation History in the Small Magellanic Cloud: The Case of NGC 602
    M. Cignoni, E. Sabbi, A. Nota, M. Tosi, S. Degl'Innocenti, P. G. Prada Moroni, L. Angeretti, Lynn Redding Carlson, J. Gallagher, M. Meixner, M. Sirianni, and L. J. Smith
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    ABSTRACT: Deep Hubble Space Telescope/Advanced Camera for Surveys photometry of the young cluster NGC 602, located in the remote low-density "wing" of the Small Magellanic Cloud (SMC), reveals numerous pre-main-sequence (PMS) stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history (SFH) into recent times and constraining several stellar population properties, such as the present-day mass function (PDMF), the initial mass function, and the binary fraction. To better characterize the PMS population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the SMC (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the SFH in the region by comparing observed and synthetic color-magnitude diagrams. The derived PDMF for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching (0.3-0.7) × 10–3 M ☉ yr–1 in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.
    The Astronomical Journal 02/2009; 137(3):3668. · 4.03 Impact Factor
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    Article: Star formation history in the SMC: the case of NGC602
    M. Cignoni, E. Sabbi, A. Nota, M Tosi, S. Degl'Innocenti, P. G. Prada Moroni, L. Angeretti, Lynn Redding Carlson, J Gallagher, M. Meixner, M. Sirianni, L. J. Smith
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    ABSTRACT: Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 \times 10^{-3} M_\odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula. Comment: 24 pages. Accepted for publication in AJ
    01/2009;
  • Article: Starbursts in the Local Universe: New Hubble Space Telescope Advanced Camera for Surveys Observations of the Irregular Galaxy NGC 4449
    F. Annibali, A. Aloisi, J. Mack, M. Tosi, R. P. van der Marel, L. Angeretti, C. Leitherer, and M. Sirianni
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    ABSTRACT: We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V), and F814W (I) broadband filters, and in the F658N (Hα) narrowband filter. Our photometry includes 300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and 400,000 objects in the (V, I) CMD, down to I 27. A subsample of 200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the west side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.
    The Astronomical Journal 04/2008; 135(5):1900. · 4.03 Impact Factor
  • Chapter: New HST/ACS Data of the Starburst Irregular Galaxy NGC 4449
    Francesca Annibali, A. Aloisi, J. Mack, M. Tosi, R. P. van der Marel, L. Angeretti, C. Leitherer, M. Sirianni
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    ABSTRACT: We present B, V and I photometry with HST/ACS of the resolved stellar populations in the Magellanic starburst galaxy NGC 4449. Our photometry provides ≈ 300,000 objects in the (B, V) color-magnitude diagrams (CMDs) and ≈ 400,000 objects in the (V, I) CMDs. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. Our data clearly indicate the presence of an age gradient, with young and intermediate-age stars concentrated toward the galactic center, and old stars present everywhere. We derive the distance of NGC 4449 with an accuracy of 5% from the tip of the red giant branch: D = 3.82 ± 0.18 Mpc.
    12/2007: pages 269-270;
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    Article: The Stellar Mass Distribution in the Giant Star Forming Region NGC 346
    E. Sabbi, M. Sirianni, A. Nota, M Tosi, J Gallagher, L. J. Smith, L. Angeretti, M. Meixner, M. S. Oey, R. Walterbos, A. Pasquali
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    ABSTRACT: Deep F555W and F814W Hubble Space Telescope ACS images are the basis for a study of the present day mass function (PDMF) of NGC346, the largest active star forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Gamma=-1.43+/-0.18 in the mass range 0.8-60 Mo, in excellent agreement with the Salpeter Initial Mass Function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~3 Myr) of NGC346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content. Comment: 26 pages, 7 figures, 1 table, accepted for publication in AJ
    10/2007;
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