An analysis of the J-method from the perspective of the AGB evolution

TL;DR

The paper investigates the J-region populations in the $(J-K,J)$ CMD of the LMC and SMC to assess the reliability of the JAGB distance indicator. Using population synthesis that couples AGB evolution with dust-formation modelling and observed SFHs/AMRs, the authors predict how J-region stars populate the CMD and compare with data, identifying these sources as recently formed C-stars prior to heavy carbon buildup. They find that J-region stars predominantly come from $ m 1-3\,M_\odot$ progenitors formed 0.3–5 Gyr ago, with the LMC-JLF peaking near $M_J\sim -6.35$ mag and the SMC-JLF shifted to fainter magnitudes due to its older, lower-mass population; RGB mass loss plays a crucial role in shaping the faint end of the JLF. The work supports the utility of the J region for distance measurements under certain SFH/AMR conditions and provides insights into AGB dust production and evolution relevant for JWST-era studies.

Abstract

The JAGB method has been proposed as a distance indicator for Local Group galaxies. We investigate the populations of the J region in the (J-K,J) colour-magnitude plane of the Large and Small Magellanic Clouds (LMC and SMC), aiming to distinguish general features of the J luminosity function (JLF) from those sensitive to the stellar population of each galaxy. Using a population synthesis approach based on stellar evolution and dust formation modelling, we predict the distribution of stars within the J region and compare it with observations. Stars in this region are identified as recently formed C-stars that have not yet accumulated large amounts of carbon. Typically, 2--3 $M_\odot$ stars remain longer in the J region, while lower-mass stars evolve faster. The JLF of the LMC, peaked at the expected magnitudes for these stars, confirms this picture. In the SMC, the J flux distribution is shifted to higher magnitudes, indicating an older population with lower-mass progenitors.

Figures (8)

• Figure 1: Left: Time variation of the luminosity (black line, scale on the left) and the J magnitude (green, scale on the right) of a model star of initial mass $\rm 2.5~M_{\odot}$ and metallicity $\rm Z=0.008$. The black arrow indicates the start of the C-star phase. Right: Time evolution of the surface $\rm C/O$ and the DPR of the same model star shown in the left panel. Times are counted since the beginning of the TP-AGB phase. Grey-shaded regions indicate the phase during which $\rm 1.5 < J-K < 2$ mag.
• Figure 2: Left: Time variation of the luminosity (black line, scale on the left) and the J magnitude (magenta, scale on the right) of a model star of initial mass $\rm 4~M_{\odot}$ and metallicity $\rm Z=0.008$. Right: Time evolution of the surface carbon (black line), nitrogen (blue), oxygen (red) (black scale on the right) and the DPR (green scale on the right) of the same model star shown in the left panel. Times are counted since the beginning of the TP-AGB phase. Grey-shaded regions indicate the phase during which $\rm 1.5 < J-K < 2$ mag.
• Figure 3: Evolutionary tracks of model stars of metallicity $\rm Z=8\times 10^{-3}$ on the (J-K, J) colour-magnitude diagram. The different points along the tracks refer to some selected evolutionary stages taken during the AGB phase, chosen in order to well represent the excursion of the tracks on the observational plane. The grey shaded region indicate the box chosen by magnus24 for the J region. The masses given for $\rm M<1.5~M_{\odot}$ refer to the values attained at the TRGB.
• Figure 4: Duration of the evolutionary phase spent by model stars of different metallicity in the box delimiting the J region of the $\rm (J-K,J)$ plane, according to the definition by magnus24, as a function of the stellar mass (left panel) and age (right). The masses of $\rm M < 1.5~M_{\odot}$ stars reported in the abscissa of the left panel refer to the values at the start of the core helium burning phase.
• Figure 5: Left: The J luminosity function of the LMC AGB stars populating the J region of the $\rm (J-K,J)$ plane, obtained by means of population synthesis, is shown in black, and compared with the JLF by magnus24 (red line). Right: The distribution of the masses of the stars in the J region of the $\rm (J-K,J)$ plane, which correspond to the JLF shown in the left panel. The masses of $\rm M < 1.5~M_{\odot}$ stars refer to the values at the start of the core helium burning phase.