Compared photometric properties of stellar atmosphere models of cool stars T. Lejeune

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Compared photometric properties of stellar
atmosphere models of cool starsT. Lejeune
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Introduction

• The GAIA Galactic survey will observe more than
  1 billion of stars in our Galaxy, and will obtain
  photometry in 15 medium-band and 5 large-band
  filters with the challenging goal to determine
  the fundamental physic parameters (Teff, logg,
  Fe/H) across a very wide of stellar types.
• In order to test the capabilities of the
  photometric system and the performances of the
  classification algorithms, various stellar
  librairies of synthetic and/or empirical spectra
  are required .
• In this study, I present preliminary results of
  the compared photometric properties of themost
  recent grids of synthetic spectra available for
  cool stars (ATLAS, NMARCS, PHOENIX, BaSeL) in
  view of their application for GAIA photometry.
  For this purpose, new empirical temperature-color
  calibrations, established from globular cluster
  data for the metallicity range 2.0 Fe/H
  0.0, and applied to the calibration of the BaSeL
  3.1 spectral library (Westera et al. 2002), are
  used to confront model predictions with observed
  photometry.

available at http//www.astro.mat.uc.pt/BaSeL
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Grids of synthetic stellar spectra

• The table below gives the parameter space covered
  by the different models compared in this study.
  The BaSeL models are a compilation of different
  grids of models assembled into a unified stellar
  library of semi-empirically (BaSeL 2.2) and
  empirically (3.1) calibrated spectral energy
  distributions (see Lejeune et al. 1997, 1998, and
  Westera et al. 2001).

BaSeL2.2 Kurucz 1995 Allard Hauschildt
1995 Bessell et al. 1989 models BaSeL3.1
Kurucz 1995 Allard Hauschildt 1995 Scholz
1997 models
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The BaSeL 3.1 calibrations

• The BaSeL 2.2 spectral library (Lejeune et al.
  1997, 1998) was defined from a set of empirical
  (Fe/H0) and semi-empirical (Fe/H?0)
  color-temperature relations.
• The new BaSeL 3.1 calibrations are purely
  empirical color temperature relations defined in
  the metallicity range 2.0 Fe/H 0.0. They
  are based on a large amount of UBVRIJHKL
  photometric data compiled in the literature for
  the globular clusters (GC) 47Tuc (Fe/H-0.7),
  M5 (-1.10), M3 (-1.34), NGC6397 (-1.82) and M92
  (-2.16) (see Westera et al. 2001 for a complete
  list of the sources of data).
• From a given effective temperature scale, the
  calibrations are constructed, both for the giants
  and the dwarfs, from color-color relationships
  deduced from the GC photometry. In addition,
  Teff-logg relations are defined for each of the
  above metallicities from the data of GCs stars.
• The BaSeL 3.1 calibrations hence provide the only
  existing homogeneous set of empirical
  metallicity-dependent Teff-UBVRIJHKL
  transformations in the temperature range 2000 K
  to 50000 K between Fe/H-2.0 and 0.0.

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Comparisons in the two-color diagrams

• In Fig. 1.1 1.2, models from the different
  grids (BaSeL, ATLAS (Castelli), NMARCS, PHOENIX)
  are compared in the (B-V)/(V-I) color-color
  diagram for the same value of logg1. The lines
  connect the models with the same metallicity, for
  the range Fe/H-0.6, -0.3, 0.0 (thick line),
  0.3 and 0.6, and models with the same Teff
  (5500 K 3600 K), as indicated on each panel.
• Large differences exist between the different
  grids, specially in the low temperature regime
  (Teff lt 4000 K). Above 4000 K, the NMARCS models
  provide the best overall agreement with the
  ''empirically-calibrated'' BaSeL 3.1 models, both
  in B-V and V-I, and in particular for Fe/H0.0.
  In comparison, the PHOENIX models appear too blue
  in B-V, but agree very well with the V-I BaSeL
  3.1 colors, while the B-V model colors from the
  ATLAS grid are systematically redder below 4500
  K. Below 4000 K, very important deviations exists
  between all the models.
• Fig. 2 Same as Fig. 1 but for logg4.5. While
  large differences still exist below 4000 K, the
  PHOENIX models provide the best overall agreement
  with the BaSeL 3.1 calibrations.
• The BaSeL 2.2 grid is surimposed to the BaSeL
  3.1 models (WLBC01) as the dashed lines for
  comparison. Important differences can be seen
  between these two grids, specially in the
  isometallicity models, because of the change of
  semi-empirical to empirical calibrations at all
  metallicities in BaSeL 3.1.

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Temperature-color calibrations

• Figs 3 and 4 compare the Teff-(U-B) and
  Teff-(B-V) calibrations predicted by each grid of
  models with the empirical calibrations adopted in
  BaSeL 3.1 for Fe/H0.0 (thick lines) and
  Fe/H-2.0 (thin lines), both for dwarfs (Figs
  3.1 and 3.2) and giants (Figs 4.1 and 4.2). Also
  displayed are the Alonso et al.'s empirical
  calibrations (Alonso et al. 1998, 1999), when
  available, for comparison. For each sequence of
  giants and dwarfs, the models have been
  interpolated in their original grid at the
  (Teff,logg) values adopted for the BaSeL 3.1
  calibrations (see WLBC01).
• Figs. 3.1 3.2 in B-V, and for Teff gt 4000 K,
  the theoretical calibrations predicted by all the
  models agree very well with the BaSeL 3.1
  empirical relations, both for Fe/H0.0 and
  -2.0. The agreement is even better than in Figs 2
  because of the logg values adopted from the
  empirical calibrations. Below 4000 K, the
  Kurucz/Castelli model calibration are in very
  good agreement with the empirical values (until
  Teff 3500 K), while the NMARCS Teff-(B-V)
  calibrations disagree significantly with the
  empirical scales. In U-B, the models are
  systematically redder than the empirical values,
  by 0.1 mag (at Fe/H0.0) or the
  Kurucz/Castelli, and more than 0.15 mag for the
  PHOENIX colors in the range 7000 K 4000 K.
• The deviations are less important at low
  metallicity
• Figs. 4.1 4.2 same as Figs 3.1 and 3.2 but for
  the giants. Large differences are still found
  below 4000 K in B-V, but the NMARCS giant models
  provide a closer match to the empirical relations
  than the NextGen models.

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M giants calibrations

• Fig. 5 shows the Teff-(B-V) and the Teff-(V-K)
  relations (both theoretical and empirical), for
  solar-metallicity, compared with observations of
  M giant field stars. The data are fro Fluks et
  al. (1994).
• The BaSeL calibrations are defined from a fit of
  the Fluks et al's data, and extrapoled until
  Teff2500 K (see LCB97).
• For Teff-(V-K) (lower panel), there is an
  excellent agreement between both the empirical
  (Ridgway 1980, Alonso et al. 1999) and the
  theoretical calibrations for Teff gt 3800 K. For
  lower temperatures, the PHOENIX models are still
  in very good agreement with the observed data,
  and appear slightly bluer (by 0.5 mag) than the
  BaSeL calibration.
• For Teff-(B-V) (upper panel), the best agreement
  with the BaSeL calibration is provided by the
  Castelli and the NMARCS models for Teff gt 3800
  K. The PHOENIX models show a very large
  discrepancy (D(B-V) 0.5 mag) at low temperature
  (Teff lt 3600 K). Note that the Alonso et al.
  (B-V)-Teff is not valid for Teff lt 4000 K.

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Conclusions

• In this study, we have examined the photometric
  properties of the recent grids of model
  atmosphere spectra for cool stars, in order to
  assess their reliabilty for the tests and the
  performance of the GAIA photometric system.
• UBVI model colors predictions from the PHOENIX
  (NextGen98), NMARCS (Bessell 1998), ATLAS
  (Kurucz/Castelli 97) grids have been compared in
  the range 7000 K ? Teff ? 3000 K with the new
  BaSeL 3.1 set of empirical photometric
  calibrations for Fe/H -2.0 to 0.5.
• In a general way, we find that the
  Kurucz/Castelli model colors agree well or very
  well with the empirical color-temperature
  relations at all metallicities and over the whole
  range of Teffs (gt 3500 K). Significant deviations
  are found for the Teff-(U-B) relation at solar
  metallicity.
• Below 4000 K, important discrepancies are
  observed in UBV between the PHOENIX and the
  NMARCS model predictions with respect to the
  BaSeL 3.1 empirical relations, although some
  uncertainties still exist in the empirical data
  at these low temperatures.
• These deviations appear to be less important for
  the PHOENIX model colors of dwarfs, and for
  NMARCS models of giants.

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References

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