Evolutionary stellar population synthesis model predictions for old and intermediate-aged stellar populations

Several model predictions for studying old and intermediate-aged stellar populations are provided here. The models are described in these papers:

Reference

Main model predictions

Vazdekis et al. 1996 (ApJS,107,306)

Broadband colours, mass-to-light ratios and absorption line-strengths at low spectral resolution for single burst (SSPs) stellar populations and full chemo-evolutionary models

Vazdekis 1999 (ApJ,513,224)

SSPs spectral energy distributions (SEDs) at moderately high resolution (FWHM=1.8Å) in the optical range

Blakeslee, Vazdekis & Ajhar 2001(MNRAS,320,193)

Broadband and WFPC2 HST filter  system colours and surface brightness fluctuations

Vazdekis et al. 2003 (MNRAS, 340,1317)

Near-IR SEDs around the CaII triplet region at resolution 1.5Å (FWHM) and Ca II triplet and Paschen line-strengths

Vazdekis et al. 2009 (in preparation) SSPs spectral energy distributions at resolution (FWHM=2.3Å) in the optical range based on MILES stellar library

The main model ingredients are:

·         Initial Mass Function. We adopt the IMF shapes described in Vazdekis et al. 1996 (i.e unimodal and bimodal) and the two IMFs of Kroupa 2001 (MNRAS,322,231):

o        Unimodal: a power-law function characterized by its slope as a free parameter. The standard Salpeter (1955) IMF is obtained when the slope value is 1.3.

o        Bimodal: similar to the unimodal IMF for stars with masses above 0.6 Mo, but decreasing the number of the stars with lower masses by means of a transition to a shallower slope. Its slope is the only free parameter (as in the unimodal case).

o        Kroupa (2001) universal: a multi-part power-law IMF, which is similar to the Salpeter (1955) IMF for stars of masses above 0.5 Mo, but with a decreasing contribution of lower masses by means of two flatter segments.

o        Kroupa (2001) revised: a multi-part power-law IMF, in which the systematic effects due to unresolved binaries on the single-star IMF have been taken into account.

 

·         Theoretical isochrones. We use the homogeneous set of scaled-solar isochrones of Girardi et al. (A&AS,141,371), whereas in Vazdekis et al. 1996 and Vazdekis 1999 we used the Bertelli et al. 1994 (A&AS,106,275) and the stellar tracks of Pols et al. 1995 (MNRAS,274,964) for the very low-mass stars. The Girardi et al. isochrones cover a wide range of ages and metallicities and include the latest stages of the stellar evolution through the thermally pulsing AGB regime to the point of complete envelope ejection (employing a synthetic prescription). It is worth to note that the largest metallicity covered is Z=0.03 (instead of Z=0.05, as in Bertelli et al set). Solar metallicity value is Z=0.019. The input physics of the isochrones have been updated with an improved version of the equation of state, the opacities of Alexander & Ferguson 1994 (ApJ,437,879) and a milder convective overshoot scheme with respect to the Bertelli et al. set.

 

·         Stellar photometric libraries are used to transform the theoretical parameters of the isochrones to magnitudes and colours. We use extensive empirical (not theoretical) stellar libraries to obtain each colour as a function of temperature, metallicity and gravity. We use the metallicity-dependent relations of Alonso, Arribas & Martinez-Roger (1996,1999) (A&A,117,227; A&AS,140,261) for dwarfs and giants respectively. This treatment for the giants is the most important difference with respect to the models of Vazdekis et al (1996), where we used the calibrations of Ridgway et al. 1980 (ApJ,235,126) and Johnson 1966 (ARA&A,4,193). The empirical (not the theoretical) compilation of Lejeune, Cuisinier & Buser (1997, 1998) (A&AS,125,229; A&AS,130,65) are used for the coolest and hottest dwarfs (Teff<4000K) and giants (Teff<3500K), respectively, for solar metallicity; a semi-empirical approach was applied to other metallicities on the basis of these relations and the model atmospheres of Bessell et al. (1989,1991) (A&AS,77,1; A&AS,89,335) and the library of Fluks et al. 1994 (A&AS,105,311). We use the metal-dependent bolometric corrections given by Alonso, Arribas & Martinez-Roger (1995,1999) (A&A,297,197; A&AS,140,261) for dwarfs and giants, respectively. For the Sun we adopt the bolometric correction -0.12, with a bolometric magnitude of 4.70.

 

·         Stellar spectral libraries. Extensive empirical stellar libraries are used to predict the spectral properties of the stellar populations. Two set of predictions are computed:

 

o        A number of absorption line-strengths computed on the basis of the, so-called, empirical fitting functions. These functions describe the strengths of, previously defined, spectral features in terms of the main atmospheric parameters. 25 absorption features at resolution (FWHM~9Å) are calculated on the instrumental dependent LICK/IDS SYSTEM using the fitting functions of Worthey et al. 1994 (ApJS,94,687) and Worthey & Ottaviani 1997 (ApJS,111,377). The break at 4000A is calculated using the fitting functions of Gorgas et al. 1999 (A&AS,139,29). Using the fitting functions of Cenarro et al. 2002 we compute the CaII triplet feature around ~8600A at resolution 1.5Å (FWHM) and flux-calibration response curve.

 

o        Spectral energy distributions (with flux-calibrated response curve) at  moderately high resolution are computed for the optical range at FWHM=1.8Å on the basis of the stellar spectral library of Jones 1999 (PhD thesis, Univ. North Carolina at Chapel Hill) and MILES (Sánchez-Blázquez et al. 2006) at FWHM=2.3Å, and for the near-IR around the CaII triplet feature at resolution 1.5Å (FWHM) using the stellar library of Cenarro et al. (2001).

The models provide predictions for:

·         SINGLE AGE SINGLE METALLICITY, STELLAR POPULATIONS, SSPs, i.e. Simple Stellar Populations or instantaneous bursts.

 

·         FULL CHEMO-EVOLUTIONARY POPULATION SYNTHESIS, following the evolution of a galaxy from an initial gas cloud to the present time. For the latter, no initial metallicity is assumed, since it is derived from the IMF, the Star Formation Rate and the Age.