REVIEW 3 major objections 6 minor 44 references
Going Bayesian on the ages of nearby young stellar systems I. The expansion rate method
T0 review · 3 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read A fully Bayesian expansion-rate estimator recovers ages of nearby young stellar associations within 10 percent, with 100 percent credibility on simulated systems.
desk verdict Useful Bayesian wrapper for expansion ages; the validation is real but the abstract sells it as method performance when the numbers are conditional on truth-centered priors. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the Bayesian hierarchical age model built on the Lindegren linear velocity field, where the expansion age τ is a sampled parameter at the top of the hierarchy. From τ the model forms the expansion scale κμ = (1.02271 τ)^{-1}, draws the three Cartesian expansion components from a common Normal or Student-t distribution with scale κσ (with a Gamma prior on the Student-t degrees of freedom), and feeds them into the linear velocity field. A Generalised Gamma (left) prior with hyper-parameters set from literature age estimates embeds prior knowledge while allowing independent mode and variance, and an Exponential prior on κσ plus Gamma/Exponential priors on position and velocity dispersions regularize the inference. Sampling the full posterior with the NUTS algorithm propagates data uncertainties and correlations directly into the age posterior, avoiding the error leaks of successive propagation steps.
What would settle it
Run the method on a real stellar association whose age is independently known from a non-dynamical technique, using a literature prior that is deliberately shifted away from that age; if the 95% HDI excludes the true age, the claimed credibility does not hold outside the simulation's favorable prior conditions.
Extended reading notes
Core claim
The central claim is that the expansion rate method, reformulated as a Bayesian hierarchical model, is a more robust and credible age estimator than the frequentist implementations currently used. The age τ is treated as a random variable drawn from a Generalised Gamma (left-skewed) or Truncated Normal prior, converted into an expansion scale κμ = (1.02271 τ)^{-1}, and then the three expansion components κX, κY, κZ are drawn from a common normal or Student-t distribution with that scale; drawing them jointly propagates their correlations into the age posterior. Validated on synthetic associations (10-40 Myr, 50-150 pc, 25-75 members, 25%-100% radial-velocity coverage), the method yields errors within ±10%, uncertainties below 20%, and 100% credibility in all tested configurations, whereas classical and robust frequentist estimators degrade with age, distance, and sparse radial-velocity coverage, with credibilities dropping below 50% in the 20 and 40 Myr cases. In simulated star-forming regions the method is less accurate, with errors up to 80% at 3 Myr and 400 pc, but credibility remains 100% and the error decreases rapidly with age. The paper argues this validates the Bayesian formulation as the preferred implementation of the expansion rate method within its applicability domain.
Load-bearing premise
The validation performance depends on the age prior being centered near the true age and on informed phase-space dispersion priors set to the true values; if the literature age prior used for a real system is biased or too narrow, the Bayesian estimator inherits that bias, and the headline '<10% error, 100% credibility' result is not guaranteed.
Editorial extensions
If this is right
- Users of Kalkayotl can now obtain expansion ages with fully propagated uncertainties and a built-in prior check comparing prior and posterior, rather than point estimates from separate regressions.
- The 100 percent credibility on simulated associations means the 95% HDI can be read as a reliable uncertainty interval, which the frequentist estimators fail to provide even at 10 Myr when radial-velocity coverage is incomplete.
- Doubling the number of members with sparse radial velocities beats halving membership with full radial velocities, so membership completeness should be prioritised in target selection.
- For the nearest and youngest star-forming regions (around 3 Myr, under 150 pc), the method should be used cautiously or not at all, since errors can reach 80%.
- The uncertainty estimate behaves predictably, decreasing with more members and more radial-velocity coverage, unlike the erratic uncertainties of the frequentist estimators at older ages.
Reading between the lines
- A natural next step is to combine this dynamical age with isochrone or lithium-depletion ages in a single joint hierarchical model, letting the data arbitrate the offsets between techniques; the paper notes this as its own future work.
- The asymmetry found under shifted priors (a +5 Myr shift biases the result more than a -5 Myr shift) suggests that using a literature age prior that is systematically too old is more dangerous than one that is too young; users could test this by running the model with both a central and a deliberately younger prior and comparing posteriors.
- If the method is applied to a real association whose literature age comes from a technique that is itself biased, the claimed credibility will not hold, so the prior-check plots are not a diagnostic ornament but the main safeguard.
- Testing on real associations with independently calibrated ages would be the decisive transfer check, since the paper's validation uses priors centered at or near the true age in the simulations.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a Bayesian hierarchical implementation of the expansion rate method for estimating the ages of nearby young stellar systems, released as a module of the open-source code Kalkayotl. The model extends an existing linear velocity field model with an age parameter whose prior encodes literature age estimates, and samples the posterior with NUTS within PyMC. The authors benchmark the method against classical and robust frequentist estimators on synthetic stellar associations and star-forming regions, reporting errors below 10% and 100% credibility for associations between 10 and 40 Myr and up to 150 pc, and larger but decreasing errors for star-forming regions.
Significance. If the performance claims held under realistic prior misspecification, this would be a valuable contribution: it provides an open-source, fully Bayesian expansion-age estimator with documented assumptions, an extensive synthetic-data validation, and a useful sensitivity analysis. The use of NUTS with explicit prior/posterior checks and the public code release are clear strengths. However, the headline validation is conditioned on priors centred on the true simulation values, which is exactly the condition under which a Bayesian method is least informative and under which the 100% credibility metric is expected. The sensitivity tests in the paper itself demonstrate prior-shift errors of 10-15% and an HDI that excludes the true age for a younger prior, so the abstract's unqualified '<10% error' claim overstates the method's robustness.
major comments (3)
- [Sect. 5.1 and Sect. 3.2.2] The headline validation result is conditioned on priors that already encode the true simulation values. The age prior is set to mu_tau=23 Myr, sigma_tau=8 Myr (the true age of the beta-Pictoris template), and the phase-space dispersion priors are selected as 'informed' using the generating values in Table 1. The paper's own sensitivity analysis in Sect. 4.2 shows that a +5 Myr shift in the prior yields 10-15% errors, and Fig. 11 shows that a prior centred at 10 Myr (true age 23 Myr) produces a posterior 95% HDI of [14.2, 22.9] Myr, which excludes the true age. Therefore the abstract's unqualified claim that 'errors are <10%' is a statement about performance conditional on an accurate external age estimate, not a property of the method under the conditions in which it will be used. I recommend either recalibrating the claim (e.g., reporting errors as a function of prior offset) or adding a validation protocol that uses priors drawn from a realistic, possibly biased, distribution of literature ages.
- [Sect. 4.1 and Sect. 6.1] The '100% credibility' metric is in large part produced by the prior rather than by the data. Whenever the prior is centred on the true age and its scale is comparable to the true scatter (sigma_tau=8 Myr at 23 Myr), the 95% HDI is almost guaranteed to contain the true value even with modest data; Sect. 4.3 explicitly notes that for 25-source systems the prior dominates. The comparison against frequentist estimators is therefore not apples-to-apples: the Bayesian estimator receives external age information that is centred on the answer, while the frequentist estimators do not. The claim that 'the Bayesian age estimator is more robust and credible than the commonly used frequentist ones' (abstract) is not established by this comparison; a fairer test would give the frequentist estimators access to the same external age information, or would evaluate both with no prior or with the same prior misspecification.
- [Sect. 6.2 and Fig. 11] The manuscript itself states that 'the most important source of bias in our method is the a priori information embedded in the age prior' and recommends using weakly informative priors. This is a sound caveat, but it directly contradicts the unqualified abstract claim. For a real stellar system, a literature age prior can be biased or too narrow, precisely the condition under which the method's posterior can exclude the true age (Fig. 11). The authors should either move the prior-sensitivity results into the abstract and conclusions, or restrict the performance claim to cases where the prior is known to be reliable.
minor comments (6)
- [Sect. 2.1] The phrase 'the signal-to-noise ration' should be 'the signal-to-noise ratio'.
- [Sect. 3.2.2, Table 2] The shape parameter p of the Generalised Gamma distribution is listed in the table but not defined in the text; please define p and explain the f* factor in the caption or in a footnote.
- [Sect. 3.2.1, Eq. (1)] The origin of the conversion factor 1.02271 in Eq. (1) is not explained; a sentence stating the unit conversion between km/s/pc and Myr would help.
- [Sect. 4.1] The phrase 'probed to minimise' should be 'proved to minimise' or 'was found to minimise'.
- [Sect. 4.3] The sentence 'This effect is due to the effect of the prior' is redundant; consider rephrasing to 'This effect is due to the prior, which is centred on the true value and dominates in low-information datasets.'
- [Sect. 5.2] The phrase 'we advice caution' should be 'we advise caution'.
Circularity Check
The headline '<10% errors / 100% credibility' is validated with priors centered on the true age and true phase-space dispersions; the paper's own sensitivity tests show the prior dominates in the low-information cases included in that headline, so the validation partially reduces to the prior input.
-
fitted input called prediction
[Sect. 4.3 and Fig. 4; Abstract 'errors are <10%']
"We notice that the age error in systems with 25 sources is smaller than that of systems with 50 sources. This effect is due to the effect of the prior, which is centred in the true value and dominates under these low-information content datasets."
The validation grid includes 25-source systems, and the paper explicitly says their small errors come from the age prior being centered on the true value and dominating the posterior. The abstract presents <10% errors and 100% credibility as properties of the estimator, but in low-information datasets the posterior mean is essentially the prior mean; since the prior mean was set to the true age, the 'prediction' is statistically forced by the prior. Fig. 11 confirms this: a 10 Myr prior for a true 23 Myr system gives a 95% HDI [14.2, 22.9] Myr excluding the true age. Real literature priors are not guaranteed to be centered on the truth, so the unconditional headline is not supported.
-
other
[Sect. 5 opening; Table 1 informed phase-space dispersion values]
"In this section, we present the validation of our new Bayesian expansion rate method (Sect. 3) with its default configuration of prior distributions and hyper-parameters values (see Sect. 4; with the informed hyper-parameters for phase-space dispersion using values of Table 1) evaluated on the grid of synthetic systems presented in Sect. 2."
The validation supporting the headline uses informed phase-space dispersion hyper-parameters equal to the Table 1 values that generated the synthetic systems. The paper notes this option cannot be the default because it is system-specific. Thus the <10% errors measure the accuracy of an oracle dispersion input, not the default method's ability to infer ages from expansion data. For real systems these dispersions must be estimated, so the headline statistic is conditional on an input that is not available in application and is partly constructed by the validation setup.
full rationale
The paper is not circular in the sense of a self-citation chain or a derivation that is identical to its assumptions by algebra; the Bayesian hierarchical model, the NUTS sampling, and the comparison against frequentist estimators are genuine. However, the central validation claim is partially circular and conditional: the age prior is centered on the true age and the phase-space dispersion priors are set to the true generating values, and the paper explicitly states that in 25-source systems the prior dominates. The abstract's '<10% error / 100% credibility' statement is therefore a property of the oracle prior configuration, not of the expansion-rate likelihood alone. The paper is transparent about this in Sect. 4.2 and Fig. 11, where a shifted prior produces 10-15% errors and an HDI excluding the true age, and Sect. 6.2 warns that the prior is the most important source of bias. Because the headline is presented without that condition, the validation partially reduces to its own input; this is a partial circularity in the validation protocol rather than a fully self-referential derivation. Score 6 reflects one or more 'predictions' that are statistically forced by prior inputs.
Assumptions & free parameters
free parameters (5)
- Age prior hyperparameters (µτ, στ) =
µτ=23 Myr, στ=8 Myr for β-Pictoris template
- Informed phase-space dispersion prior hyperparameters =
σXYZ=(16.0,13.2,7.4) pc, σUVW=(1.5,0.5,0.7) km/s (β-Pic); (1,1,1) pc and km/s (Taurus)
- κσ prior scale (Exponential) =
0.01 km/s/pc
- GGL age prior shape parameters (p, f*) =
p=10 with scale factor f*; alternative p=1.19
- κν prior (Gamma α=2, β=1) =
mode at dof=1
assumptions (5)
- domain assumption Young stellar associations are expanding at a constant rate since birth (Assumptions 1-3).
- domain assumption Phase-space distribution is multivariate Gaussian or a Gaussian mixture with mutual Mahalanobis distance below two (Assumption 4).
- domain assumption Gaia-like observational uncertainties are Gaussian and correctly modeled by PyGaia.
- standard math The Lindegren linear velocity field model with rotation and shear terms is adequate.
- domain assumption Unresolved binary astrometry corresponds to the center of mass and their radial velocities are unusable (Assumption 5).
Cite this review
Pith. "Pith review of Going Bayesian on the ages of nearby young stellar systems I. The expansion rate method." pith.science (2026). https://pith.science/paper/RXLDSIXA
@misc{pith2026250605110,
author = {Pith},
title = {Pith review of: Going Bayesian on the ages of nearby young stellar systems I. The expansion rate method},
year = {2026},
howpublished = {\url{https://pith.science/paper/RXLDSIXA}},
note = {Machine review of arXiv:2506.05110}
}
read the original abstract
Context. Determining the ages of young stellar systems is fundamental to test and validate current star-formation theories. Aims. We aim at developing a Bayesian version of the expansion rate method that incorporates the a priori knowledge on the stellar system's age and solves some of the caveats of the traditional frequentist approach. Methods. We upgrade an existing Bayesian hierarchical model with additional parameter hierarchies that include, amongst others, the system's age. For this later, we propose prior distributions inspired by literature works. Results. We validate our method on a set of extensive simulations mimicking the properties of real stellar systems. In stellar associations between 10 and 40 Myr and up to 150 pc the errors are <10%. In star forming regions up to 400 pc, the error can be as large as 80% at 3 Myr but it rapidly decreases with increasing age. Conclusions. The Bayesian expansion rate methodology that we present here offers several advantages over the traditional frequentist version. In particular, the Bayesian age estimator is more robust and credible than the commonly used the frequentist ones. This new Bayesian expansion rate method is made publicly available as a module of the free and open-source code Kalkayotl.
Figures
Figures from the paper (7 more)
Reference graph
Works this paper leans on
-
[1]
, " * write output.state after.block = add.period write newline
ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all := #1 ...
-
[2]
write newline
" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....
-
[3]
Bailer-Jones , C. A. L. 2015, , 127, 994
2015
-
[4]
Bailer-Jones , C. A. L. 2023, , 166, 269
work page 2023
-
[5]
Bailer-Jones , C. A. L., Rybizki , J., Fouesneau , M., Demleitner , M., & Andrae , R. 2021, , 161, 147
2021
-
[6]
Bailer-Jones , C. A. L., Rybizki , J., Fouesneau , M., Mantelet , G., & Andrae , R. 2018, , 156, 58
2018
-
[7]
Barrado y Navascu \'e s , D., Stauffer , J. R., & Patten , B. M. 1999, , 522, L53
work page 1999
-
[8]
Belikov , A. N., Kharchenko , N. V., Piskunov , A. E., Schilbach , E., & Scholz , R. D. 2002, , 387, 117
work page 2002
Show all 44 references
-
[9]
1952, , 11, 414
Blaauw , A. 1952, , 11, 414
1952
-
[10]
1964, , 2, 213
Blaauw , A. 1964, , 2, 213
1964
-
[11]
Brown , A. G. A., Dekker , G., & de Zeeuw , P. T. 1997, , 285, 479
1997
-
[12]
2022, , 664, A31
Casamiquela , L., Olivares , J., Tarricq , Y., et al. 2022, , 664, A31
2022
-
[13]
Fischler, M. A. & Bolles, R. C. 1981, Commun. ACM, 24, 381–395
1981
-
[14]
E., Malo , L., et al
Gagn \'e , J., Mamajek , E. E., Malo , L., et al. 2018, , 856, 23
2018
-
[15]
J., Barrado , D., Bouy , H., et al
Galindo-Guil , F. J., Barrado , D., Bouy , H., et al. 2022, , 664, A70
2022
-
[16]
Galli , P. A. B., Loinard , L., Bouy , H., et al. 2019, , 630, A137
2019
-
[17]
Galli , P. A. B., Miret-Roig , N., Bouy , H., Olivares , J., & Barrado , D. 2023, , 520, 6245
2023
-
[18]
B., Stern , H
Gelman , A., Carlin , J. B., Stern , H. S., et al. 2014, Bayesian Data Analysis
2014
-
[19]
G., & Su, Y.-S
Gelman, A., Jakulin, A., Pittau, M. G., & Su, Y.-S. 2008, The Annals of Applied Statistics, 2, 1360
2008
-
[20]
Hoffman , M. D. & Gelman , A. 2011, arXiv e-prints, arXiv:1111.4246
2011 arXiv
-
[21]
D., Jackson , R
Jeffries , R. D., Jackson , R. J., Wright , N. J., et al. 2023, , 523, 802
2023
-
[22]
Juárez, M. A. & Steel, M. F. J. 2010, Journal of Business & Economic Statistics, 28, 52
2010
-
[23]
A., Gaidos , E., van Saders , J., Feiden , G
Lee , R. A., Gaidos , E., van Saders , J., Feiden , G. A., & Gagn \'e , J. 2024, [ [arXiv] 2312.15792 ]
2024 arXiv
-
[24]
2000, , 356, 1119
Lindegren , L., Madsen , S., & Dravins , D. 2000, , 356, 1119
2000
-
[25]
Luhman , K. L. 2022, , 164, 151
2022
-
[26]
Luhman , K. L. 2023, , 165, 269
2023
-
[27]
Mamajek , E. E. & Bell , C. P. M. 2014, , 445, 2169
2014
-
[28]
Miret-Roig , N., Galli , P. A. B., Brandner , W., et al. 2020, , 642, A179
2020
-
[29]
Miret-Roig , N., Galli , P. A. B., Olivares , J., et al. 2022, , 667, A163
2022
-
[30]
Morton , T. D. 2015, isochrones: Stellar model grid package , Astrophysics Source Code Library, record ascl:1503.010
2015
-
[31]
Z., & Berihuete , A
Olivares , J., Bouy , H., Dorn-Wallenstein , T. Z., & Berihuete , A. 2025, , 693, A12
2025
-
[32]
2023 a , , 671, A1
Olivares , J., Bouy , H., Miret-Roig , N., et al. 2023 a , , 671, A1
2023
-
[33]
Olivares , J., Lodieu , N., B \'e jar , V. J. S., et al. 2023 b , , 675, A28
2023
-
[34]
M., et al
Olivares , J., Moraux , E., Sarro , L. M., et al. 2018, , 612, A70
2018
-
[35]
M., Bouy , H., et al
Olivares , J., Sarro , L. M., Bouy , H., et al. 2020, , 644, A7
2020
-
[36]
2023, PeerJ Computer Science, 9, e1516
Oriol, A.-P., Virgile, A., Colin, C., et al. 2023, PeerJ Computer Science, 9, e1516
2023
-
[37]
2011, Journal of Machine Learning Research, 12, 2825
Pedregosa, F., Varoquaux, G., Gramfort, A., et al. 2011, Journal of Machine Learning Research, 12, 2825
2011
-
[38]
E., Alves , J., et al
Ratzenb \"o ck , S., Gro schedl , J. E., Alves , J., et al. 2023, , 678, A71
2023
-
[39]
Stacy, E. W. 1962, The Annals of Mathematical Statistics, 33, 1187
1962
-
[40]
R., Liebert , J., & Giampapa , M
Stauffer , J. R., Liebert , J., & Giampapa , M. 1995, , 109, 298
1995
-
[41]
H., Scott , J., et al
von Hippel , T., Jefferys , W. H., Scott , J., et al. 2006, , 645, 1436
2006
-
[42]
J., Jeffries , R
Wright , N. J., Jeffries , R. D., Jackson , R. J., et al. 2024, , 533, 705
2024
-
[43]
Wright , N. J. & Mamajek , E. E. 2018, , 476, 381
2018
-
[44]
Zari , E., Brown , A. G. A., & de Zeeuw , P. T. 2019, , 628, A123
2019
Reviewed August 7, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.