[Paper Review] The Pisa pre-main sequence tracks and isochrones. A database covering a wide range of Z, Y, mass, and age values

E. Tognelli, P. G. Prada Moroni+1 more|arXiv (Cornell University)|Jul 12, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy115 references164 citations

TL;DR

This paper presents a comprehensive database of pre-main sequence stellar tracks and isochrones computed with an updated FRANEC evolutionary code, incorporating state-of-the-art physics including improved equation of state and low-temperature opacities. The models span a wide range of metallicities (Z = 0.0002–0.03), masses (0.2–7.0 M⊙), ages (1–100 Myr), and helium abundances, with three mixing-length parameters (α = 1.2, 1.68, 1.9) and two initial deuterium abundances for Z ≥ 0.008, offering high-precision tools for age and mass determination of young stars with close agreement to observational data.

ABSTRACT

In recent years new observations of pre-main sequence stars (pre-MS) with Z <= Zsun have been made available. To take full advantage of the continuously growing amount of data of pre-MS stars in different environments, we need to develop updated pre-MS models for a wide range of metallicity to assign reliable ages and masses to the observed stars. We present updated evolutionary pre-MS models and isochrones for a fine grid of mass, age, metallicity, and helium values. We use a standard and well-tested stellar evolutionary code (i.e. FRANEC), that adopts outer boundary conditions from detailed and realistic atmosphere models. In this code, we incorporate additional improvements to the physical inputs related to the equation of state and the low temperature radiative opacities essential to computing low-mass stellar models. We make available via internet a large database of pre-MS tracks and isochrones for a wide range of chemical compositions (Z = 0.0002 - 0.03), masses (M = 0.2 - 7.0 Msun), and ages (1 - 100 Myr) for a solar-calibrated mixing length parameter α(i.e. 1.68). For each chemical composition, additional models were computed with two different mixing length values, namely α= 1.2 and 1.9. Moreover, for Z >= 0.008, we also provided models with two different initial deuterium abundances. The characteristics of the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.

Motivation & Objective

• To address the growing need for accurate pre-main sequence (pre-MS) models to interpret increasingly detailed observations of young stellar systems.
• To provide a fine-grained, widely applicable database of pre-MS evolutionary tracks and isochrones covering diverse metallicities, masses, ages, and chemical compositions.
• To improve age and mass determination of young stars by incorporating updated physical inputs such as the equation of state and low-temperature radiative opacities.
• To enable precise star formation history and initial mass function analysis in young clusters and associations across the Milky Way and Magellanic Clouds.
• To reduce theoretical uncertainties in pre-MS evolution by systematically testing different mixing-length parameters and deuterium abundances.

Proposed method

• The models are computed using the FRANEC stellar evolution code, which is well-tested and incorporates realistic outer boundary conditions from detailed atmosphere models.
• Key physical inputs include an updated equation of state, improved low-temperature radiative and conductive opacities, and nuclear reaction rates from the latest compilations.
• The code uses a solar-calibrated mixing-length parameter (α = 1.68), with additional models computed at α = 1.2 and α = 1.9 to assess convective treatment sensitivity.
• For Z ≥ 0.008, models are computed with two initial deuterium abundances: X_D = 4×10⁻⁵ (typical for Population II) and X_D = 2×10⁻⁵ (more appropriate for Population I).
• The database includes 19 metallicities (Z = 0.0002 to 0.03), three helium abundances per Z, 43 tracks per composition (for M = 0.2–7.0 M⊙), and 36 isochrones per composition (ages 1–100 Myr, with 1 Myr spacing up to 20 Myr).
• All models are made publicly available via the web at http://astro.df.unipi.it/stellar-models/, with data in the (log T_eff, log L/L⊙) plane and planned future availability in photometric systems.

Experimental results

Research questions

• RQ1 How do updated physical inputs such as the equation of state and low-temperature opacities affect the location and shape of pre-MS tracks in the Hertzsprung-Russell diagram?
• RQ2 To what extent do variations in the mixing-length parameter (α) influence the predicted evolution of low-mass pre-MS stars?
• RQ3 How do different initial deuterium abundances affect the luminosity and temperature evolution of pre-MS stars at low metallicities?
• RQ4 How well do the new theoretical models reproduce observed properties of young stellar systems, such as those in the Small and Large Magellanic Clouds?
• RQ5 What is the impact of chemical composition (Z and Y) on the age and mass determination of pre-MS stars in young clusters?

Key findings

• The database includes 19 metallicities (Z = 0.0002 to 0.03), three helium abundances per Z, and 43 evolutionary tracks per composition across 0.2–7.0 M⊙, with three α values (1.2, 1.68, 1.9).
• For Z ≥ 0.008, two initial deuterium abundances (X_D = 4×10⁻⁵ and 2×10⁻⁵) are used to account for Population I and II differences.
• The models show close agreement with observational data from young clusters and associations, including NGC602 in the Small Magellanic Cloud, validating their reliability for age and mass inference.
• Theoretical tracks and isochrones are available in the (log T_eff, log L/L⊙) plane and will soon be accessible in multiple photometric systems.
• The database is publicly accessible at http://astro.df.unipi.it/stellar-models/, with detailed model files listing time, luminosity, effective temperature, core conditions, and energy source contributions.
• Systematic uncertainties in pre-MS evolution are critically discussed, with the largest uncertainties arising at low masses (M < 0.5 M⊙), especially for low-metallicity stars.