The Carina Project. V. The impact of NLTE effects on the iron content

M. Fabrizio (1), T. Merle (2), F. Thévenin (2), M. Nonino (4), G. Bono (1,4), P. B. Stetson (5,6), I. Ferraro (4), G. Iannicola (4), M. Monelli (7,8), A. R. Walker (9), R. Buonanno (1,10), F. Caputo (4), C. E. Corsi (4), M. Dall’Ora (11), S. Degl’Innocenti (12,13) P. François (14), R. Gilmozzi (15), M. Marconi (11), A. Pietrinferni (16), P.G. Prada Moroni (12,13), F. Primas (15), L. Pulone (4), V. Ripepi (11) and M. Romaniello (15) ((1)Dipartimento di Fisica, Universitá di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy; (2) Université de Nice Sophia-antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Lagrange, BP 4229, 06304 Nice,France; (3) INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 40131 Trieste, Italy; (4)INAF–Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, Rome, Italy; (5) Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada; (6) Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by AURA, Inc., under cooperative agreement with the NSF; (7) Instituto de Astrof´ısica de Canarias, Calle Via Lactea,
E38200 La Laguna, Tenerife, Spain; (8) Departamento de Astrof´ısica, Universidad de La Laguna, Tenerife, Spain; (9) Cerro Tololo Inter-American Observatory, National Optical
Astronomy Observatory, Casilla 603, La Serena, Chile (10) Agenzia Spaziale Italiana–Science Data Center, ASDC c/o ESRIN, via G. Galilei, 00044 Frascati, Italy (11) INAF–Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
(12) Dipartimento di Fisica, Univiversit`a di Pisa, Largo B. Pontecorvo 2, 56127 Pisa, Italy; (13) INFN, Sez. Pisa, via E. Fermi 2, 56127 Pisa, Italy; (14) Observatoire de Paris-Meudon, GEPI, 61 avenue de l’Observatoire, 75014 Paris, France; (15) European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany (16) INAF–Osservatorio Astronomico Collurania, via M. Maggini, 64100 Teramo, Italy)

We have performed accurate iron abundance measurements for 44 red giants (RGs) in the Carina dwarf spheroidal (dSph) galaxy. We used archival, high-resolution spectra (R~38,000) collected with UVES at ESO/VLT either in slit mode (5) or in fiber mode (39, FLAMES/GIRAFFE-UVES). The sample is more than a factor of four larger than any previous spectroscopic investigation of stars in dSphs based on high-resolution (R>38,000) spectra. We did not impose the ionization equilibrium between neutral and singly-ionized iron lines. The effective temperatures and the surface gravities were estimated by fitting stellar isochrones in the V, B-V color-magnitude diagram. To measure the iron abundance of individual lines we applied the LTE spectrum synthesis fitting method using MARCS model atmospheres of appropriate metallicity. We found evidence of NLTE effects between neutral and singly-ionized iron abundances. Assuming that the FeII abundances are minimally affected by NLTE effects, we corrected the FeI stellar abundances using a linear fit between FeI and FeII stellar abundance determinations.
We found that the Carina metallicity distribution based on the corrected FeI abundances (44 RGs) has a weighted mean metallicity of [Fe/H]=-1.80 and a weighted standard deviation of sigma=0.24 dex. The Carina metallicity distribution based on the FeII abundances (27 RGs) gives similar estimates ([Fe/H]=-1.72, sigma=0.24 dex). The current weighted mean metallicities are slightly more metal poor when compared with similar estimates available in the literature. Furthermore, if we restrict our analysis to stars with the most accurate iron abundances, ~20 FeI and at least three FeII measurements (15 stars), we found that the range in iron abundances covered by Carina RGs (~1 dex) agrees quite well with similar estimates based on high-resolution spectra.

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