Stellar Ages: A Code to Infer Properties of Stellar Populations

TL;DR

Stellar Ages addresses the challenge of dating stellar populations by jointly inferring per-star ages, metallicities, and extinctions using a Bayesian mixture model that combines single-star isochrone information with population-level CMD constraints. It introduces a discretized grid of $(t,[\mathrm{M/H}],\tilde{A}_V)$ with Dirichlet weights and a Gibbs sampler over latent per-star assignments, with a likelihood that averages over the IMF and extinction via quasi‑Monte Carlo. Validation on synthetic populations and application to stars around SN 2004dj yield a robust age distribution and a progenitor mass of $13.95^{+3.33}_{-1.96} M_{\odot}$, with a median age of $\log_{10}(\mathrm{Age}/\mathrm{yr}) = 7.19^{+0.10}_{-0.13}$. The approach demonstrates consistency with prior work and is adaptable to additional properties and instruments, offering a practical path to constraining SFHs and CCSN progenitors.

Abstract

We present a novel statistical algorithm, Stellar Ages, which currently infers the age, metallicity, and extinction posterior distributions of stellar populations from their magnitudes. While this paper focuses on these parameters, the framework is readily adaptable to include additional properties, such as rotation, in future work. Historical age-dating techniques either model individual stars or populations of stars, often sacrificing population context or precision for individual estimates. Stellar Ages does both, combining the strengths of these approaches to provide precise individual ages for stars while leveraging population-level constraints. We verify the algorithm's capabilities by determining the age of synthetic stellar populations and actual stellar populations surrounding a nearby supernova, SN 2004dj. In addition to inferring an age, we infer a progenitor mass consistent with direct observations of the precursor star. The median age inferred from the brightest nearby stars is $\log_{10}$(Age/yr) = $7.19^{+0.10}_{-0.13}$, and its corresponding progenitor mass is $13.95^{+3.33}_{-1.96}$ $\text{M}_{\odot}$.

Figures (12)

• Figure 1: Joint probability density functions for two magnitudes, given an age and metallicity. These two examples have a metallicity of $[M/H] = -0.20$, $\tilde{A}_{\text{V}} = 0.0$, and ages of $\log_{10}$(t/yr) = $7.00$ and $7.20$. These density functions represent the likelihoods of observing a star at the F814W and F606W bands of the ACS HRC imager. These joint probability density functions are a convolution of measurement uncertainties with stellar evolution models, initial mass function, and an extinction distribution.
• Figure 2: F606W vs F814W Magnitude-Magnitude plots for synthetic data. The dots are color-coded by the inferred most likely age of each star. The legend shows the corresponding inferred age for each color. From left to right, the plots represent simulated 50pc, 100pc, and 150pc number densities. The true values of these synthetic populations are $\log_{10}$(t/yr)= $7.0$ and $8.0$, $[M/H] = 0.0$ and $\tilde{A}_{\text{V}} = 0.0$
• Figure 3: Inferred age weights for the synthetic data found from Figure \ref{['fig:SynthCMDMostLikely']}, marginalized over metallicity and extinction. The panels correspond to the simulated 50pc, 100pc, 150pc datasets. The true values are $\log_{10}$(t/yr) = 7.00, 8.00, $\tilde{A}_{\text{V}} = 0.0, 0.0$ and $[M/H] = 0.00, 0.00$.
• Figure 4: Inferred median extinction distribution weights for the synthetic data found in Figure\ref{['fig:SynthCMDMostLikely']}, marginalized over age and metallicity. The panels present the same simulated 50pc, 100pc, and 150pc datasets as in \ref{['fig:SynthAgeViolin']}. The true extinctions values were $\tilde{A}_{\text{V}} = 0.0$.
• Figure 5: RA and DEC coordinates for all stars near SN2004dj that have sufficient quality. As in figures \ref{['fig:SynthCMDMostLikely']} through \ref{['fig:SynthAvViolin']}, the panels are organized by the circle radii surrounding SN2004dj. The left panel presents all stars within 50 pc, the middle panel shows all stars within 100 pc, and the right panel shows all within 150 pc. The sizes of the points reflect the relative magnitude of the star across all panels. The red star indicates the location of SN2004dj.