REVIEW
Fundamental parameters for 45 open clusters with Gaia DR2, an improved extinction correction and a metallicity gradient prior
Solar and Stellar Astrophysics · Submitted 24 Sep 2020 · 49 references
Pith's verdict: A useful catalog paper for 45 leftover open clusters with transparent methods, but the distance validation is partly circular and the 'substantial improvement' claim outruns the evidence.
T0 · AI review
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Referee report
The paper
The paper claims
This paper provides the first Gaia DR2-based fundamental parameters—memberships, mean proper motions and parallaxes, radial velocities for 12 clusters, and isochrone-fit distances, ages, extinctions, and metallicities—for 45 open clusters…
The reading
What carries the argument
The machinery has three parts. Membership comes from a maximum-likelihood Gaussian mixture in the three-dimensional space of proper motion components and parallax, using each star's full covariance matrix. Parameters come from an isochrone-fitting code that minimizes a likelihood comparing synthetic clusters from PARSEC 1.2S isochrones to Gaia DR2 $G_{BP}$/$G_{RP}$ colour-magnitude data, with the cross-entropy method selecting the best fits and bootstrapping error estimates. The novel piece is the extinction correction: a fourth-degree polynomial $k_m(A_V, G_{BP}-G_{RP})$ for the Gaia bands, constructed by convolving Kurucz model spectra with the Maíz Apellániz & Weiler (2018) passbands and the Fitzpatrick et al. (2019) extinction law, which removes a convergence failure seen in about 20% of the initial fits when extinction was high. Metallicity is constrained by a prior from the Galactic abundance gradient rather than fixed to solar, which the authors show reduces bias in $A_V$ and gives [Fe/H] values consistent with high-resolution spectroscopy to within a baseline error of 0.15 dex.
Extended reading notes
Core claim
The central claim is that, for 45 open clusters with no previous Gaia DR2-based membership estimate, the combination of a maximum-likelihood astrometric membership model and a cross-entropy isochrone fit yields reliable fundamental parameters: membership probabilities, mean proper motions and parallaxes, radial velocities for 12 clusters, and age, distance, extinction $A_V$, and [Fe/H] for all 45. The authors validate the updated fitting procedure—a full fourth-degree extinction polynomial for the Gaia $G$, $G_{BP}$, $G_{RP}$ passbands built from the Fitzpatrick et al. (2019) extinction law, plus a metallicity prior from the Galactic abundance gradient—against clusters with high-resolution spectroscopic metallicities, and find agreement with literature [Fe/H], with parallax distances, and with the better sources in the literature. They further claim these results represent a substantial improvement over previous published values and provide the first distance and age for Majaess 65 and the first age for Ruprecht 123, with DC 3 emerging as one of the oldest and most distant known open clusters.
Load-bearing premise
The load-bearing premise is that the PARSEC 1.2S stellar models and the Fitzpatrick et al. (2019) extinction law correctly describe every cluster's stars and line of sight, so any systematic bias in those models for certain masses, metallicities, or sightlines is inherited by every fitted distance, age, and extinction.
Editorial extensions
If this is right
- For all 45 clusters, the catalogue gives the first Gaia DR2-based mean proper motions, parallaxes, memberships, and isochrone-fit parameters, so future Galactic-structure work can include them.
- The updated extinction polynomial and metallicity-gradient prior can be applied to other cluster samples; the validation shows they avoid the systematic overestimate in $A_V$ that comes from assuming solar metallicity.
- Comparing parameters with the DAML and MWSC catalogues shows that a human-curated compilation can outperform a homogeneous automated catalogue once the latter's values are removed, and highlights cases where Gaia data revise old ages, such as NGC 133.
- The identification of known DAML clusters among newly reported discoveries implies that some claimed new clusters are re-detections, so cross-matching against existing catalogues is necessary.
- For the youngest clusters, the pre-main-sequence portions of the PARSEC isochrones match the main-sequence fits, indicating the models are usable down to about 4 Myr in Gaia bands, but results younger than roughly 10 Myr need visual confirmation.
Reading between the lines
- A natural next test is to apply the same pipeline to the remaining DAML clusters still lacking Gaia memberships; the success rate on these 45 difficult leftovers suggests a substantial fraction of the catalogue can be brought to the same standard.
- Because the paper's [Fe/H] values are strongly informed by the gradient prior, they should not be read as independent abundance measurements; the real payoff is that the prior absorbs a nuisance parameter and sharpens distances, ages, and extinction.
- The rediscovery result implies that automated cluster finders may overcount new clusters by a few percent; cross-matching in position, proper motion, and parallax space, not just position, should become standard.
- The fourth-degree extinction polynomial is a concrete reusable artefact: anyone fitting Gaia photometry with the same extinction law can adopt these coefficients, and comparing them with alternative extinction laws would test how much of the parameter improvement depends on that choice.
Original abstract
Reliable fundamental parameters of open clusters such as distance, age and extinction are key to our understanding of Galactic structure and stellar evolution. In this work we use {\it Gaia} DR2 to investigate 45 open clusters listed in the \emph{New catalogue of optically visible open clusters and candidates} (DAML) but with no previous astrometric membership estimation based on {\it Gaia} DR2. In the process of selecting targets for this study we found that some clusters reported as new discoveries in recent papers based on {\it Gaia} DR2 were already known clusters listed in DAML. Cluster memberships were determined using a maximum likelihood method applied to {\it Gaia} DR2 astrometry. This has allowed us to estimate mean proper motions and mean parallaxes for all investigated clusters. Mean radial velocities were also determined for 12 clusters, 7 of which had no previous published values. We have improved our isochrone fitting code to account for interstellar extinction using an updated extinction polynomial for the {\it Gaia} DR2 photometric band-passes and the Galactic abundance gradient as a prior for metallicity. The updated procedure was validated with a sample of clusters with high quality $[Fe/H]$ determinations. We then did a critical review of the literature and verified that our cluster parameter determinations represent a substantial improvement over previous values.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Reliable fundamental parameters of open clusters such as distance, age and extinction are key to our understanding of Galactic structure and stellar evolution. In this work we use {\it Gaia} DR2 to investigate 45 open clusters listed in the \emph{New catalogue of optically visible open clusters and candidates} (DAML) but with no previous astrometric membership estimation based on {\it Gaia} DR2. In the process of selecting targets for this study we found that some clusters reported as new discoveries in recent papers based on {\it Gaia} DR2 were already known clusters listed in DAML. Cluster memberships were determined using a maximum likelihood method applied to {\it Gaia} DR2 astrometry. This has allowed us to estimate mean proper motions and mean parallaxes for all investigated clusters. Mean radial velocities were also determined for 12 clusters, 7 of which had no previous published values. We have improved our isochrone fitting code to account for interstellar extinction using an updated extinction polynomial for the {\it Gaia} DR2 photometric band-passes and the Galactic abundance gradient as a prior for metallicity. The updated procedure was validated with a sample of clusters with high quality $[Fe/H]$ determinations. We then did a critical review of the literature and verified that our cluster parameter determinations represent a substantial improvement over previous values.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Reliable fundamental parameters of open clusters such as distance, age and extinction are key to our understanding of Galactic structure and stellar evolution. | background 1.00 | 0.04 | 0.05 | asserted 1.00 |
| s1 | In this work we use {\it Gaia} DR2 to investigate 45 open clusters listed in the \emph{New catalogue of optically visible open clusters and candidates} (DAML) but with no previous astrometric membership estimation based on {\it Gaia} DR2. | contribution 0.93 | 0.85 | 0.04 | asserted 0.98 |
| s2 | In the process of selecting targets for this study we found that some clusters reported as new discoveries in recent papers based on {\it Gaia} DR2 were already known clusters listed in DAML. | result 0.88 | 0.95 | 0.04 | asserted 1.00 |
| s3 | Cluster memberships were determined using a maximum likelihood method applied to {\it Gaia} DR2 astrometry. | method 1.00 | 0.94 | 0.03 | asserted 1.00 |
| s4 | This has allowed us to estimate mean proper motions and mean parallaxes for all investigated clusters. | result 0.98 | 0.96 | 0.06 | asserted 1.00 |
| s5 | Mean radial velocities were also determined for 12 clusters, 7 of which had no previous published values. | result 0.96 | 0.97 | 0.06 | asserted 1.00 |
| s6 | We have improved our isochrone fitting code to account for interstellar extinction using an updated extinction polynomial for the {\it Gaia} DR2 photometric band-passes and the Galactic abundance gradient as a prior for metallicity. | method 0.86 | 0.96 | 0.07 | asserted 1.00 |
| s7 | The updated procedure was validated with a sample of clusters with high quality $[Fe/H]$ determinations. | method 0.84 | 0.93 | 0.06 | asserted 1.00 |
| s8 | We then did a critical review of the literature and verified that our cluster parameter determinations represent a substantial improvement over previous values. | result 0.55 | 0.93 | 0.62 | asserted 1.00 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:2009.11834 by the authors.
References
How the literature uses it
What this paper does with the arXiv papers it cites
What it cites · 5 sentences without a confident label
Compares with 1 paper
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For IC 1442 improved central coordinates are $α=22^h16^m04^s$; $δ=+53^o59^'29^''$ in J2000, similar to the value estimated by [ref] .
Photometric study of the young open clusters IC 1442, King 21, and Trumpler 7 · Results · 0.99
Cites as background 1 paper
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A summary of various past, pre-Gaia, efforts to compile homogeneous OC parameters is given in [ref] and a review of pre-Gaia results of Galactic structure with OCs can be found in [ref] .
Observational properties of the open cluster system of the Milky Way and what they tell us about our Galaxy · Introduction · 1.00
Discussion
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