JAGB Stars: Carbon-rich AGB Standard Candles

• JAGB stars are intermediate-age, carbon-rich TP-AGB stars with low 12C/13C ratios and lithium enrichment, defined by their unique evolutionary and photometric properties.
• Their absolute magnitude calibration (M_J ≈ -6.20) is empirically established across the LMC, SMC, and Milky Way using detached eclipsing binary distances and robust photometry.
• The distinct near-infrared photometric signature of JAGB stars enables precise extragalactic distance measurements and provides independent constraints on the Hubble constant.

J-Branch Asymptotic Giant Branch (JAGB) stars, also frequently referred to as J-type carbon stars or simply “JAGB stars,” are a distinct, photometrically well-defined population of extremely red, intermediate-age, carbon-rich thermally pulsing asymptotic giant branch (TP-AGB) stars. Their unique combination of evolutionary, nucleosynthetic, and photometric properties yields a remarkably uniform luminosity in the near-infrared—enabling their foundational role as extragalactic standard candles. The statistical, empirical, and astrophysical basis for the JAGB method now underpins a new, independent rung on the cosmological distance ladder, offering competitive or complementary constraints to Cepheids and the tip of the red giant branch (TRGB) in distance and Hubble constant measurements.

1. Evolutionary Status, Nucleosynthesis, and Physical Properties

JAGB stars represent the TP-AGB evolutionary stage for intermediate-mass stars in the initial mass range ∼2–5 M$_\odot$, with typical ages of 0.3–1 Gyr (Madore et al., 2020, Madore et al., 2022). During this phase, these stars experience repeated thermal pulses and “third dredge-up” episodes that transport processed $^{12}$C from the He-intershell to the stellar envelope. When the photospheric C/O ratio exceeds unity, these objects acquire their characteristic carbon-star (C-star) status: their atmospheres become C-rich, dominated by C$_2$ and CN molecular bands, resulting in very red near-infrared colors (Madore et al., 2023, Liu et al., 2017).

Among carbon stars, JAGB (J-type carbon) stars are defined by low $^{12}$C/$^{13}$C ratios (typically $\sim$3–10), high $^{14}$N/$^{15}$N ($\gtrsim$440), lithium enrichment, and the absence of s-process element enhancements that typify N-type (mainstream) carbon stars. These traits indicate a unique nucleosynthetic pathway involving proton-ingestion events (PIEs) during early thermal pulses, especially when enhanced convective overshoot is present (Choplin et al., 14 Oct 2024, Liu et al., 2017). In such PIEs, protons are mixed from the envelope into the hot He-intershell, leading to non-standard mixing and nucleosynthesis (including CNO cycling products, high Li via the Cameron-Fowler mechanism, and suppression of the $^{13}$C pocket and hence s-process production) (Choplin et al., 14 Oct 2024).

JAGB stars are also directly implicated as the dominant stellar source of N-rich presolar silicon carbide (SiC) AB2 grains, based on isotopic and abundance matches: low $^{12}$C/$^{13}$C, high $^{14}$N/$^{15}$N, and near-solar heavy-element isotopic ratios (Liu et al., 2017).

2. Photometric and Color–Magnitude Diagram Characteristics

Operationally, JAGB stars are isolated using near-infrared color–magnitude diagrams (CMDs), notably $J$ versus $(J-K)$ plots. In the LMC and SMC, they occupy a narrow color interval—classically $1.30 \leq (J-K) \leq 2.00$ (mag)—appearing as a horizontal “plume” redward of the TRGB and the oxygen-rich AGB locus (Madore et al., 2020, Zgirski et al., 2021). Empirically, they cluster tightly at $J \sim 12.3$ in the LMC, corresponding to a nearly constant absolute magnitude. In contemporary JWST NIRCam studies, JAGB selection boxes in $(F150W-F277W)$ (JWST) or $(F814W-F110W)$ (HST/WFC3) are typically adopted: e.g., $1.0 < (F150W-F277W) < 1.5$ (JWST).

Key photometric criteria for robust selection:

Photometric System	Color Range	Reference Magnitude
Ground (2MASS)	$1.3 \leq (J-K) \leq 2.0$	$J$
HST/WFC3	$1.4 \leq (F814W-F110W) \leq 1.8$	$F110W$
JWST/NIRCam	$1.0 < (F150W-F277W) < 1.5$	$F150W$

The luminosity function of JAGB stars is symmetric and nearly Gaussian in the LMC and SMC, with an intrinsic single-epoch scatter $\sigma \simeq 0.27$ mag (decreased to $\sim$0.15 mag in multi-epoch averages) (Madore et al., 2020).

3. Absolute Magnitude Calibration and Metallicity Dependence

The cornerstone of the JAGB method is the empirical calibration of their absolute magnitude in the $J$ band.

• LMC Calibration: With the detached eclipsing binary (DEB) geometric distance to the LMC ($\mu_0=18.477\pm0.004$ stat $\pm0.026$ sys), and the mean $J$ magnitude of 3,341 LMC JAGB stars ($\langle m_J \rangle = 12.31\pm0.01$), the absolute magnitude is $M_J = -6.22\pm0.01($stat$)\pm0.03($sys$)$ (Madore et al., 2020).
• SMC Calibration: With $\mu_0=18.965\pm0.05$ (sys) and $\langle m_J \rangle = 12.81\pm0.01$, $M_J = -6.18\pm0.01($stat$)\pm0.05($sys$)$.
• Milky Way Calibration: Combining open clusters and field carbon stars yields $M_J = -6.19\pm0.04$ ($2209.08127$, $2110.04576$).

The adopted mean zero point is

$$M_J = -6.20 \pm 0.01~(\text{stat}) \pm 0.04~(\text{sys})~\text{mag}$$

with no statistically significant dependence on metallicity across LMC ([Fe/H]$\sim-0.4$), SMC ($-0.7$), and MW (solar) metallicities, as established by the lack of systematic zero-point shift across calibrators (Madore et al., 2022). At higher metallicity ($Z\sim0.02$, MW field), some recent Gaia DR3 studies report a marginal offset ($\sim$0.3–0.4 mag fainter in the MW than LMC/SMC), possibly reflecting sample selection, O-rich contamination, or unrecognized systematics (Magnus et al., 8 Oct 2024). Empirical evidence in M31 and LMC–SMC comparisons constrains metallicity sensitivity to $<0.05$ mag/dex across the Local Group (Freedman et al., 2020, Lee, 2023, Zgirski et al., 2021).

4. Methodology of Distance Determination

The JAGB method yields extragalactic distances via the equation

$\mu_0 = m_J - M_J - A_J$

where $m_J$ is the mean apparent J magnitude (or relevant NIR passband), $M_J$ is the calibrated absolute magnitude, and $A_J$ is the total line-of-sight extinction. In JWST applications, analogous formulae are used for filters such as F115W and F150W, with color cuts tailored to segregate the JAGB locus (Li et al., 9 Jan 2024, Lee et al., 6 Aug 2024, Lee et al., 2023).

Practical workflow:

1. Obtain deep NIR imaging (ground-based or space-based) sufficient to capture the JAGB population above the completeness limit.
2. Construct de-reddened CMD; select JAGB candidates via color and magnitude cuts (e.g., $1.3 < (J-K) < 2.0$).
3. Model the observed JAGB luminosity function (LF) using methods such as a Gaussian plus background, Lorentzian profile, or smoothed histograms (GLOESS).
4. Extract the mode, mean, or median of the LF as the reference apparent magnitude.
5. Apply the calibration formula to derive $\mu_0$; propagate statistical and systematic uncertainties through the error budget.

Robustness is enhanced by employing outer-disk fields (to minimize crowding, blending, and internal reddening) and cross-checking with independent indicators (TRGB, Cepheids). The use of “convergence algorithms” to objectively define unbiased regions is now standard in JWST large-sample studies (Lee et al., 6 Aug 2024, Lee et al., 2023).

5. Statistical, Systematic, and Population-Dependent Uncertainties

Statistical precision: With large samples ($N$), the error on the mean/mode falls as $\sigma/\sqrt{N}$ (e.g., $\sigma\simeq0.27$ mag single-epoch). For $N\sim200$, statistical errors can approach 0.02 mag ($\sim$1% in distance) (Madore et al., 2020, Zgirski et al., 2021).

Systematics: Dominant sources include:

• Calibration zero-point: inherited from DEB distances ($\pm$0.03–0.04 mag).
• Photometric zero-points and extinction maps ($\lesssim$0.02 mag).
• Sample selection: contamination by O-rich AGBs (especially in higher-Z systems) if color cuts are too blue; use $(J-K)\geq1.5$ in high-metallicity environments (Magnus et al., 8 Oct 2024).
• Blending/crowding: significant in crowded inner disks, mitigated by outer-disk selection (Lee et al., 2023, Lee et al., 6 Aug 2024).
• Luminosity Function (LF) asymmetry: Skewed LFs can cause estimator dependence (mode vs mean vs median), with inter-method differences up to 0.2 mag reported in certain JWST samples (Li et al., 9 Jan 2024, Li et al., 7 Feb 2025).

Intrinsic dispersion: After correcting for photometric and environmental systematics, internal JAGB luminosity function width is $\sim$0.15–0.35 mag (multi- vs single-epoch, galaxy dependent) (Madore et al., 2020, Madore et al., 2021).

6. Astrophysical and Cosmological Applications

JAGB stars provide a high-luminosity, abundant standard candle, accessible out to at least 20 Mpc with JWST/NIRCam and up to $\sim$100 Mpc in optimal conditions (Lee et al., 6 Aug 2024, Lee et al., 2023, Lee et al., 2022). Their applications include:

• Extragalactic Distance Scale: JAGB-based distances are now available for dozens of galaxies out to 40 Mpc (Li et al., 7 Feb 2025). Results agree with Cepheid and TRGB distances at the $\sim$1–3% level, with mean offsets $\lesssim$0.03 mag.
• SN Ia Calibration and Hubble Constant: JAGB distances to SN Ia hosts yield H$_0$ in the range $67.8\pm2.7$ to $74.7\pm3.1$ km s$^{-1}$ Mpc$^{-1}$, with systematic floors limited by absolute calibration, field-to-field variations in NGC 4258 (maser anchor), and LF estimator choice (Lee et al., 6 Aug 2024, Li et al., 7 Feb 2025, Li et al., 9 Jan 2024).
• Astrophysical constraints: JAGB stars serve as sensitive probes of non-standard mixing, PIEs, and internal nucleosynthesis, and their association with SiC AB2 grains connects AGB evolution directly to presolar grain chemistry (Choplin et al., 14 Oct 2024, Liu et al., 2017).

Systematic Limitations and Current Debates:

• Variations in absolute magnitude across NGC 4258 fields at the $\sim$0.11 mag level (primarily in the mode of the LF) create a dominant systematics floor (Li et al., 7 Feb 2025).
• At high metallicity, empirical offsets of $\sim$0.3–0.4 mag relative to LMC/SMC calibrations may indicate incomplete understanding of O-rich contamination, population differences, or currently unmodeled evolutionary effects (Magnus et al., 8 Oct 2024).
• Skewness in the JAGB LF (up to $\sim$0.48) introduces significant estimator-dependent uncertainty; no universal correction or standardization currently exists (Li et al., 7 Feb 2025, Li et al., 9 Jan 2024).
• Theoretical models suggest PIEs are required for the observed abundance patterns, but only a subset of 2–3 M$_\odot$ AGBs produce J-type carbon stars, and additional mixing (thermohaline/rotation) may be needed to reconcile all isotopic and abundance constraints (Choplin et al., 14 Oct 2024).

7. Prospects and Future Directions

Future improvements in JAGB-based distance measurements and astrophysics rely on:

• Expanded calibration via additional anchors (e.g., LMC, SMC, new MW calibrators, and direct parallax samples as Gaia data improve).
• Comprehensive mapping of systematics related to LF shape, field-to-field variation, and estimator sensitivity, and search for empirical standardizations using large SN Ia host samples (Li et al., 7 Feb 2025).
• Improved understanding of possible metallicity dependence and O-rich contamination at near-solar Z (Magnus et al., 8 Oct 2024, Lee, 2023).
• Potential synergy with other distance indicators (Cepheid, TRGB, Mira) in joint analysis, especially as JWST and Roman extend reach and sensitivity.
• Utilization of high-resolution, multi-band IR imaging to push the method to higher-redshift supernova hosts and to more extreme environments.

JAGB stars have established themselves as a powerful, independently calibrated standard candle. Their combination of empirical luminosity constancy, accessibility in NIR, relative insensitivity to metallicity, and clear evolutionary scenario offers a robust, scalable route to precision cosmology and astrophysics (Madore et al., 2020, Li et al., 7 Feb 2025, Choplin et al., 14 Oct 2024, Lee et al., 6 Aug 2024).