[Paper Review] Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets

L. Girardi, G. Bertelli+6 more|CERN Bulletin|May 6, 2002

Stellar, planetary, and galactic studies65 references742 citations

TL;DR

This paper presents theoretical bolometric corrections and isochrones in multiple photometric systems—including Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey—using updated synthetic stellar spectra and a unified formalism for zero-point conversion. The key contribution is a comprehensive, publicly available database of absolute magnitudes and colors for stellar populations across diverse filter systems, enabling accurate modeling of resolved stellar populations beyond the local solar neighborhood.

ABSTRACT

We provide tables of theoretical isochrones in several photometric systems. To this aim, the following steps are followed: (1) First, we re-write the formalism for converting synthetic stellar spectra into tables of bolometric corrections. The resulting formulas can be applied to any photometric system, provided that the zero-points are specified by means of either ABmag, STmag, VEGAmag, or a standard star system that includes well-known spectrophotometric standards. Interstellar absorption can be considered in a self-consistent way. (2) We assemble an extended and updated library of stellar intrinsic spectra. It is mostly based on non-overshooting ATLAS9 models, suitably extended to both low and high effective temperatures. This offers an excellent coverage of the parameter space of Teff, logg, and [M/H]. We briefly discuss the main uncertainties and points still deserving more improvement. (3) From the spectral library, we derive tables of bolometric corrections for Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey systems (this latter consisting on the WFI, EMMI, and SOFI filter sets). (4) These tables are used to convert several sets of Padova isochrones into the corresponding absolute magnitudes and colours, thus providing a useful database for several astrophysical applications. All data files are made available in electronic form.

Motivation & Objective

• To provide theoretical bolometric corrections and isochrones for photometric systems beyond the Johnson-Cousins-Glass system, which are essential for interpreting resolved stellar population data.
• To address the lack of empirical calibrations for young, metal-poor, or super-metal-rich stars in distant galaxies and for new filter systems used in modern space and ground-based telescopes.
• To develop a self-consistent formalism for converting synthetic stellar spectra into magnitudes and colors across multiple photometric systems using standardized zero-points (ABmag, STmag, VEGAmag, or standard star systems).
• To update and extend the stellar spectral library with non-overshooting ATLAS9 models covering a broad range of $T_{\rm eff}$, $\log g$, and [M/H] for improved accuracy in isochrone predictions.
• To make all derived data—bolometric corrections, isochrones, and integrated colors—publicly accessible in electronic format for use in astrophysical research.

Proposed method

• A generalized formalism is derived to convert synthetic stellar spectra into bolometric corrections and color transformations, applicable to any photometric system with specified zero-point definitions (ABmag, STmag, VEGAmag, or standard star systems).
• An extended and updated library of intrinsic stellar spectra is assembled, primarily based on non-overshooting ATLAS9 models, extended to low and high $T_{\rm eff}$ to cover the full range of stellar parameters.
• Bolometric corrections are computed for five photometric systems: Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey (WFI, EMMI, SOFI).
• Theoretical Padova isochrones are transformed into absolute magnitudes and colors using the derived bolometric corrections, with consistent treatment of interstellar absorption and reddening.
• The resulting data are organized into electronic tables and made publicly available via a dedicated WWW site, with structured files for bolometric corrections, isochrones, and integrated colors of single-burst populations.
• The method ensures consistency across systems by using a common spectral library and zero-point definitions, enabling reliable comparison across different photometric systems.

Experimental results

Research questions

• RQ1 How can theoretical isochrones be accurately transformed into absolute magnitudes and colors across diverse photometric systems, including those not covered by empirical calibrations?
• RQ2 What is the impact of different filter transmission curves and zero-point systems (e.g., ABmag vs. VEGAmag) on the observed color-magnitude diagrams of stellar populations?
• RQ3 To what extent do new photometric systems like WFI and HST instruments produce significantly different CMDs compared to standard systems like Johnson-Cousins, and how can these differences be modeled?
• RQ4 How well do theoretical isochrones in the Washington system reproduce observed features such as the saturation of $C-T_1$ color in red giants, as seen in LMC data?
• RQ5 Can the same theoretical framework be applied consistently across multiple photometric systems to enable reliable population synthesis and age-metallicity analysis in extragalactic stellar populations?

Key findings

• Theoretical isochrones in the Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey systems are successfully computed using a consistent spectral library and formalism, enabling cross-system comparisons.
• The Washington photometric system offers excellent metallicity sensitivity via the $C-T_1$ color, which separates populations from main sequence to red giant phases, with $C-T_1$ saturating at ~3.4 for giants in both models and LMC observations.
• Significant differences exist between Johnson-Cousins and WFI filter systems: WFI's shorter-wavelength $B$ and $V$ filters produce a more modest separation in $B-V$ color, invalidating the direct use of Johnson-Cousins isochrones for WFI data in VEGAmag or ABmag systems.
• The ABmag system shifts isochrones to different magnitude and color intervals compared to VEGAmag, highlighting the importance of consistent zero-point definitions in photometric transformations.
• The database includes bolometric corrections for metallicities $[\text{Fe}/\text{H}] = -2.0$ to $+0.5$, with $T_{\rm eff}$ and $\log g$ coverage matching the regions shown in Fig. 1, ensuring broad applicability.
• All data—bolometric corrections, isochrones, and integrated colors—are publicly available in electronic form at http://pleiadi.pd.astro.it, with detailed documentation in readme.txt files.