Mapping Stellar Evolution Timescales for Intermediate-Mass Stars Using Gaia DR3 Data

Introduction to the Study of Intermediate-Mass Stars

Intermediate-mass stars, typically ranging from 2 to 8 solar masses, play a crucial role in galactic evolution. Their lifecycles bridge the gap between low-mass stars, which evolve slowly and end as white dwarfs, and high-mass stars, which undergo rapid evolution and culminate in supernovae. Understanding their evolutionary pathways provides critical insights into nucleosynthesis, chemical enrichment, and the dynamical history of galaxies.

The Role of Gaia DR3 in Stellar Astrophysics

The third data release from the Gaia mission (DR3) has revolutionized stellar astrophysics by providing high-precision astrometric, photometric, and spectroscopic data for over 1.8 billion stars. Key contributions of Gaia DR3 include:

• Astrometric precision: Parallax measurements with uncertainties as low as 0.02 milliarcseconds for bright stars.
• Photometric data: High-accuracy G, BP, and RP band magnitudes enabling precise stellar classification.
• Radial velocities: Spectroscopic measurements for millions of stars, critical for kinematic studies.
• Stellar parameters: Derived quantities such as effective temperature, surface gravity, and metallicity.

Methodology for Mapping Stellar Evolution Timescales

Sample Selection from Gaia DR3

The selection of intermediate-mass stars from Gaia DR3 requires stringent criteria to ensure data quality and evolutionary phase accuracy:

• Stars with parallax-over-error ratios (ϖ/σ_ϖ) greater than 10 to minimize distance uncertainties.
• Effective temperatures (T_eff) between 5,000 K and 15,000 K, corresponding to spectral types A through early F.
• Surface gravities (log g) ranging from 3.5 to 4.5 dex, ensuring main-sequence or subgiant classification.
• Metallicity ([Fe/H]) constraints to avoid contamination from chemically peculiar stars.

Evolutionary Phase Identification

Intermediate-mass stars undergo distinct evolutionary phases, each with characteristic timescales:

• Main Sequence: Core hydrogen burning phase, lasting ~100 Myr to ~2 Gyr depending on mass.
• Subgiant Branch: Transition phase following hydrogen exhaustion in the core, lasting ~10-100 Myr.
• Red Giant Branch: Shell hydrogen burning phase with significant radius expansion, lasting ~10 Myr.
• Horizontal Branch/Clump: Core helium burning phase, lasting ~100 Myr.

Ages from Isochrone Fitting

To derive precise ages, stellar parameters from Gaia DR3 are compared with theoretical isochrones:

• The PARSEC (Padova and Trieste Stellar Evolution Code) models provide isochrones with varying metallicity and age.
• Bayesian inference methods, such as Markov Chain Monte Carlo (MCMC), are applied to derive posterior probability distributions for stellar ages.
• Systematic uncertainties from model physics (e.g., convection, rotation) are accounted for through comparative analysis with other codes like MESA (Modules for Experiments in Stellar Astrophysics).

Refining Nucleosynthesis Pathways

Chemical Abundance Trends

Gaia DR3's Radial Velocity Spectrometer (RVS) data enables the study of light and alpha-element abundances (e.g., Mg, Si, Ca) in intermediate-mass stars:

• The [α/Fe] ratio serves as a chronometer, distinguishing stars formed in different galactic epochs.
• Nitrogen enhancements ([N/Fe]) in evolved stars indicate the operation of the CNO cycle and mixing processes during the red giant phase.

S-Process Element Production

Intermediate-mass stars contribute to slow neutron-capture (s-process) nucleosynthesis during the asymptotic giant branch (AGB) phase:

• The first dredge-up mixes s-process elements like barium ([Ba/Fe]) to the surface.
• Gaia DR3's spectroscopic data allows tracking of s-process enrichment in stellar populations.

Challenges and Systematic Uncertainties

Despite its precision, Gaia DR3 data presents several challenges for evolutionary studies:

• Binary Contamination: Unresolved binaries can bias luminosity and mass estimates.
• Differential Reddening: Variations in interstellar extinction affect color-magnitude diagram placement.
• Model Physics: Uncertainties in convective overshooting and rotation alter theoretical evolutionary tracks.
• Metallicity Gradients: Galactic metallicity variations complicate age determinations for field stars.

Future Prospects with Upcoming Gaia Releases

The anticipated Gaia DR4 and later releases will further enhance stellar evolution studies:

• Improved radial velocities for faint stars will expand kinematic analyses.
• Additional elemental abundances (e.g., C, N, O) will refine nucleosynthesis models.
• Time-domain photometry will identify pulsating variables (e.g., δ Scuti stars) to probe interior structure.

Conclusion: Implications for Galactic Archaeology

The combination of Gaia DR3's astrometric precision and advanced stellar modeling enables unprecedented mapping of intermediate-mass star evolution. These results inform galactic chemical evolution models by constraining nucleosynthesis yields and star formation histories across cosmic time.