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REVIEW 4 major objections 7 minor 42 references

Chemical tagging of tidal tail star candidates of NGC 6362

T0 review · 4 major / 7 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Tidal tail stars of NGC 6362 fail chemical match to the cluster

desk verdict Solid abundance analysis for 24 RGB candidates, but the no-tidal-tail conclusion is a speculative leap beyond the sample. read the letter →

arxiv 2505.24522 v1 pith:EQNV4Z7C submitted 2025-05-30 astro-ph.GA

Andrés E. Piatti This is my paper
classification astro-ph.GA
keywords chemicaltaggingtidaltailsglobularclustersNGC6362stellarabundancesspectroscopypropermotionsGalacticfieldstars
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper tests whether 24 red giant stars previously proposed as tidal tail members of the globular cluster NGC 6362 really came from the cluster. Using high-resolution spectra, the author measured radial velocities and abundances of iron plus seven other elements and compared them with the cluster's known values. None of the stars matches the cluster's chemistry within the uncertainties, and their abundances resemble those of Milky Way thick-disk field stars. The author concludes that the candidates are field stars, that NGC 6362 likely has no detectable red-giant tidal tails, and that selecting tidal tail stars by similar proper motions is not a reliable approach.

What carries the argument

Chemical tagging, the use of element abundances ([Fe/H], Mg, Ca, Sc, Ti, Cr, Ni, Ba) as near-immutable fingerprints of a star's birth environment. The argument works because stellar surface abundances change little over a star's lifetime, so a candidate that shares the cluster's abundance pattern was almost certainly born with the cluster, while a mismatch indicates field contamination. The comparison relies on the cluster's mean abundances from high-resolution studies and on Milky Way disk and halo field-star compilations to classify the mismatched stars.

What would settle it

A search of the region beyond the Jacobi radius reaching below G=18 that finds red giants or main-sequence turnoff stars with [Fe/H] ≈ -1.07, [Mg/Fe] ≈ 0.54, [Ca/Fe] ≈ 0.26, and [Ba/Fe] ≈ 0.61 matching NGC 6362 within the uncertainties would directly contradict the conclusion that the cluster has no detectable tidal tails.

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Extended reading notes

Core claim

The paper establishes that the 24 red giant branch stars selected as the highest-ranked extra-tidal candidates of NGC 6362 do not share the cluster's mean chemical element abundances. Only two stars have iron abundances consistent with the cluster within the errors, and those two disagree in other elements; the rest span a metallicity range typical of Milky Way disk field stars. The measured radial velocities also lack a cluster-like distribution, with only about 9 percent within 1 sigma and about 60 percent within 3 sigma of the cluster mean. Because the observed sample statistically represents roughly 80 percent of the brighter red-giant candidates, the author concludes that none of the studied stars appear to be cluster-born and that the cluster likely has no detected tidal tails among red giants.

Load-bearing premise

The sample of 24 red giants is representative of any true tidal tail population of NGC 6362; if real tail stars are fainter than G=18, are main-sequence stars, or fell outside the 3-sigma proper-motion cut, the chemical null result would not disprove the tails.

Editorial extensions

If this is right

  • The 24 observed stars, representing ~80 percent of the red giant candidates brighter than G=18, are not chemically consistent with NGC 6362 membership.
  • NGC 6362 likely has no detectable red-giant tidal tails, and the candidate list is dominated by field stars along the line of sight.
  • Selecting tidal tail stars by matching mean cluster proper motions is unreliable; kinematic criteria should account for velocity dispersion or angular momentum.
  • The candidate stars belong mainly to the Milky Way thick disk, with some thin disk members and a minor halo component.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If true tidal tails exist but contain fainter red giants (G>18), main-sequence stars, or stars outside the 3-sigma proper-motion cut, this null result would not rule them out; a deeper, chemically tagged sample would be needed.
  • The same chemical-tagging test could be applied to other clusters with claimed tidal tails to estimate the field-contamination rate of kinematic selection.
  • Kinematic selection methods that track gradients in velocity dispersion along the tail, rather than mean motion matching, may rescue the detection of genuine tidal debris.
  • The conclusion that NGC 6362's chaotic orbit has washed out its tails suggests that inner-disk clusters generally may lose detectable tidal tails faster than outer-halo clusters.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 7 minor

Summary. The paper presents VLT/FLAMES GIRAFFE spectroscopy of 24 red giant stars that Kundu et al. (2019) catalogued as tidal-tail candidates of the globular cluster NGC 6362. The author measures radial velocities and abundances of Fe, Mg, Ca, Sc, Ti, Cr, Ni, and Ba using DAOSPEC equivalent widths and MOOG with Kurucz model atmospheres, validates the pipeline against archive GIRAFFE spectra of cluster red giants, and compares the derived abundances with cluster means from Massari et al. (2017). The main finding is that none of the 24 stars shares the cluster's multi-element abundance pattern, and most appear to be Milky Way field stars, mainly thick-disk members. From this the author concludes that the selected stars were not formed in NGC 6362 and speculates that NGC 6362 does not have detected tidal tails, also arguing that kinematic selection of tidal-tail stars is not as suitable as previously thought.

Significance. If restricted to the 24 observed stars, the chemical non-membership result is convincing and useful: the study is a direct spectroscopic test of extra-tidal candidates, uses standard and documented reduction and analysis tools, includes an internal consistency check on star #116, and validates the metallicity scale against archive spectra, reproducing [Fe/H] = -1.08 +/- 0.03 dex in agreement with Mucciarelli et al. (2016). The comparison against independent literature values (Massari et al. 2017; Reddy et al.; Venn et al.; Nissen & Schuster) gives the analysis a low circularity burden. However, the broader claims that NGC 6362 has no detected tidal tails and that kinematic selection is generally unsuitable are not supported by the data, because the observed sample is only a proper-motion-selected, bright, red-giant subset of the original candidate list. These overreaches, together with an internal inconsistency about star #186, require revision before the paper can be accepted.

major comments (4)
  1. [Section 5, first bullet; Section 2] The inference 'NGC 6362 does not have detected tidal tails' is not supported by the data. As stated in Section 2, the sample statistically represents about 80% of the Kundu et al. (2019) red giant candidates brighter than G = 18.0 mag, and the Kundu et al. catalogue itself selects only red giant branch stars with proper motions within 3 sigma of the cluster mean. Stars fainter than G = 18, main-sequence or subgiant stars, and escaped stars with larger proper-motion dispersion are therefore excluded before spectroscopy. The paper itself cites Wan et al. (2021) and Piatti (2023) to argue that escaping stars should show a larger velocity dispersion, so the 3 sigma proper-motion cut may preferentially remove the most credible tidal-tail members. The null chemical result is solid for the 24 stars observed, but it does not falsify the existence of tidal tails; it only shows that this particular RGB subset is dominated by field stars. The conclusion should be rephrased accordingly, or the author should supply a completeness argument covering the full plausible tidal-tail population.
  2. [Section 4, Table 2, star #186] The text states that 'star #186 does not share with NGC 6362 any of the estimated chemical element abundances', but Table 2 lists no [X/Fe] measurements for star #186 at all. Moreover, the paper's own criterion says that stars #52 and #186 have [Fe/H] values within the uncertainties of the cluster mean ([Fe/H] = -1.00 +/- 0.15 versus cluster -1.07 +/- 0.01). The exclusion of #186 as a cluster member is therefore not justified by the data presented. Either provide the missing abundances or revise the statement to acknowledge that only [Fe/H] is available for this star and that it is formally consistent with the cluster value within the combined uncertainties.
  3. [Section 4, Table 2] The multi-element conclusion is based on a sparse abundance matrix. Several stars, including #53 and #186, have no [X/Fe] entries in Table 2, and many others have measurements for only one or two elements besides iron. The claim that 'none of the studied stars would seem to share the mean cluster chemical element abundances' is too strong if interpreted as a statement about all seven elements; for many stars only [Fe/H] and a subset of [X/Fe] ratios are available. The author should quantify how many elements were actually measured per star and phrase the conclusion in terms of the measured elements, rather than implying full multi-element coverage for every star.
  4. [Section 5, second bullet] The conclusion that 'kinematics properties would not seem to be as suitable as previously thought for searching globular cluster tidal tails' is overgeneralized. The experiment tests only one selection recipe: a proper-motion cut within 3 sigma of the cluster mean combined with a CMD position on the red giant branch. It does not test kinematic selection based on escape-velocity distributions, tidal-tail streaming motions, or orbit-integrated predictions, which are the approaches recommended by the papers cited in Section 4 (Malhan et al. 2021; Grillmair 2025; Grondin et al. 2024). The conclusion should be limited to the statement that this particular proper-motion-plus-CMD selection is not reliable for NGC 6362, rather than a general statement about the unsuitability of kinematic methods.
minor comments (7)
  1. [Abstract and Section 4] The abstract and Section 4 state that the selected stars 'do not have overall metallicities' similar to the cluster, but Table 1 shows that stars #52 and #186 have [Fe/H] values within the combined uncertainties. Please qualify the statement, for example by saying that the full abundance patterns are not shared even where [Fe/H] agrees.
  2. [Section 2] The magnitude limit G = 18.0 mag is introduced without stating the photometric band; please specify that it is the Gaia G band used in the Kundu et al. (2019) catalogue.
  3. [Section 3.2] There is a typo in 'systemtic errors'; it should read 'systematic errors'.
  4. [Table 2] The entry for star #48 in the [Ba/Fe] column appears as '– -', which is likely a formatting artifact; please clean up the table.
  5. [Section 4] The phrase 'at what extend criteria' should be 'to what extent criteria'.
  6. [Section 4 and Figure 3] The population classification into thick disk, thin disk, and halo is described qualitatively by eye from Figure 3. A quantitative classification, such as a likelihood or sigma-distance calculation relative to the comparison samples, would make the 'mainly thick disk' claim more reproducible.
  7. [Section 5, first bullet] The sentence 'This conclusion agrees with the recent outcome by Piatti (2024)...' essentially repeats the author's own earlier conclusion, and should be framed as a consistency check rather than independent supporting evidence.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity; the chemical-tagging result is an independent empirical comparison, with only auxiliary self-citations.

full rationale

The paper's central result is that spectroscopic abundances of 24 candidate tidal-tail stars do not match NGC 6362's literature abundances. This is not circular: the candidate [Fe/H] and [X/Fe] values are measured from FLAMES/GIRAFFE spectra with MOOG, while the cluster reference values come from an independent high-resolution study (Massari et al. 2017), and the pipeline is validated against archival spectra of NGC 6362 red giants, recovering [Fe/H] = -1.08 +/- 0.03. The comparison field-star populations (Reddy et al., Venn et al., Nissen & Schuster) are external compilations. Self-citations to Piatti (2017, 2024) enter only as auxiliary inputs: the Piatti (2017) completeness expression supports the 80% representativeness claim, and Piatti (2024) provides reddening values and an independent density map used to reinforce the speculative no-tidal-tail conclusion. Neither defines the measured abundances, and the chemical non-membership of the observed stars does not reduce to these citations. The extrapolation from 24 stars to 'no detected tidal tails' is a statistical/sample-completeness inference, not a circular one; if anything it is a correctness risk because fainter, main-sequence, or higher-proper-motion tail stars are excluded by the Kundu et al. selection. No equation in the paper redefines its input as an output, and no fitted parameter is relabeled as a prediction.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

No new physical entities are introduced. The main inputs are standard stellar atmosphere models and literature abundance references. The free parameters and assumptions are those typical of abundance analyses; none are tuned to force the conclusion.

free parameters (1)
  • Assumed red giant mass M* = 0.75 M_sun = 0.75 solar masses
    Chosen by hand in Sec. 3.2 to compute log g from the photometric distance and bolometric correction. The derived abundances depend weakly on this choice, but it is an unmeasured input.
assumptions (4)
  • domain assumption 1D LTE model atmospheres (Kurucz 2005) with MOOG line analysis give accurate abundances for cool red giants
    Standard assumption in stellar spectroscopy; invoked in Sec. 3.2 when interpolating Kurucz models and running MOOG.
  • domain assumption Chemical abundances of red giant branch stars are not significantly altered by stellar evolution, so they preserve the birth composition
    Stated in Sec. 4: 'the abundance of chemical elements remains almost unchanged along the stellar lifetimes'.
  • domain assumption The mean cluster abundances from Massari et al. (2017) are representative of all NGC 6362 red giants, including potential tidal tail stars
    Used in Sec. 4 as the chemical reference. Small internal spread of 0.03 dex in [Fe/H] is cited from Marino et al. (2019), supporting this.
  • domain assumption Gaia DR2 proper motions and CMD selection in Kundu et al. (2019) correctly identify the candidate set
    The candidate list is taken from Kundu et al. (2019) as described in Secs. 1 and 2; the paper questions the kinematic criterion but uses the list as the starting point.

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Cite this review

Pith. "Pith review of Chemical tagging of tidal tail star candidates of NGC 6362." pith.science (2026). https://pith.science/paper/EQNV4Z7C

@misc{pith2026250524522,
  author       = {Pith},
  title        = {Pith review of: Chemical tagging of tidal tail star candidates of NGC 6362},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EQNV4Z7C}},
  note         = {Machine review of arXiv:2505.24522}
}
read the original abstract

The inner Milky Way disk globular cluster NGC~6362 appears to exhibit tidal tails composed of stars that have proper motions and positions in the color-magnitude diagram similar to those of cluster stars. Because recent results seem also to show that these stars are distributed across the regions least affected by interstellar absorption and reproduce the observed composite star field density map, we carried out a detailed spectroscopic analysis of a number of chemical element abundances of tidal tail star candidates in order to investigate the relationship of them with NGC~6362. From European Southern Observatory's VLT@FLAMES spectra we found that the red giant branch stars selected as cluster's tidal tail stars do not have overall metallicities nor abundances of Mg, Ca, Sc, Ti, Cr, Ni and Ba similar to the cluster's ones. Moreover, they are mainly alike to stars that belong to the Milky Way thick disk, some of them could be part of the thin disk and a minor percentage could belong to the Milky Way halo star population. On the other hand, since the resulting radial velocities do not exhibit a distribution function similar to that of cluster's stars, we concluded that looking for kinematic properties similar to those of the cluster would not seem to be an approach for selecting cluster's tidal tail stars as suitable as previously thought.

Figures

Figures reproduced from arXiv: 2505.24522 by the authors.

Figure 1
Figure 1. Spatial distribution of Kundu et al. (2019)’s tidal tail stars (left panel), and their location in the [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. [Fe/H] vs RV relationship for the studied stars (open circles, Table 1) and for NGC 6362 (red [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗
Figure 3
Figure 3. [X/Fe] vs [Fe/H] relationships for the studied stars. Small magenta, gray and pink dots represent [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗

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