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REVIEW 5 major objections 4 minor 28 references

Detection of the Long Period Variable Stars of And II Dwarf Satellite galaxy

T0 review · 5 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash

Pith's one-line read This paper compiles 825 candidate long-period variable stars in the And II dwarf galaxy and derives a distance modulus of 23.81 ± 0.10 mag.

desk verdict Useful first LPV catalog for And II, but the paper's own numbers don't yet line up (728 vs 825 LPVs; distance modulus 23.90–24.11 vs 23.81±0.10) and the catalog itself isn't included. read the letter →

arxiv 2412.00790 v1 pith:GK5FR3GR submitted 2024-12-01 astro-ph.GA

classification astro-ph.GA
keywords long-periodvariablestarsasymptoticgiantbranchAndromedaIIdwarfgalaxyspheroidalgalaxiestipofthereddistancemodulusStetsonvariabilityindexINTmonitoringsurvey
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

The paper aims to show that a homogeneous time-series survey of Andromeda's dwarf satellites can reveal the evolved stellar population of And II: it reports the discovery of hundreds of candidate long-period variable (LPV) stars, mostly asymptotic giant branch stars pulsating with amplitudes above 0.2 mag, and uses the tip of the red giant branch to measure the galaxy's distance. The immediate payoff is a catalog of 825 LPV candidates, although the paper's own Table 1 lists 728 for the same galaxy, and a distance modulus of 23.81 ± 0.10 mag, about 578 kpc, which matches earlier work. This matters because LPV and AGB stars are accessible tracers of star-formation history and mass loss in faint dwarf galaxies, and And II is one of the larger Andromeda satellites. The paper is the first in a series that will compare these populations across 55 dwarf galaxies and four globular clusters, with the goal of reconstructing how dwarf galaxies evolve under the influence of the Andromeda system.

What carries the argument

The argument is carried by the Stetson variability index computed from paired i- and V-band observations taken less than half the minimum LPV period, 60 days, apart: the index multiplies each star's standardized magnitude deviations, so coherent brightening or fading in both filters accumulates to a positive value while random photometric noise cancels. Candidates are kept if their estimated sinusoidal amplitude, $A = 2\sigma/0.701$, exceeds 0.2 mag. Distance comes from applying a Sobel edge-detection filter to the luminosity function of stars within two half-light radii, which locates the tip of the red giant branch; the tip magnitude then sets the distance modulus. The pipeline is anchored by Gaia DR3 cross-matching to remove Galactic foreground stars.

What would settle it

Cross-match the 825 candidate LPVs with Gaia DR3 astrometry and plot their positions on the survey images: if the candidates are spread uniformly across the WFC field instead of concentrating within And II's roughly 5.3 arcmin half-light radius, or if more than a few percent have parallax or proper motion typical of Milky Way foreground stars, then the selection is contaminated and the 825 count does not represent And II's AGB population.

Watch

Extended reading notes

Core claim

On its own terms, the paper's discovery is that And II hosts a rich population of long-period variable stars, 825 per the text though 728 per Table 1, identified from nine epochs of INT/WFC photometry in the i and V bands after removing foreground stars with Gaia DR3 and selecting variables with a Stetson variability index and an amplitude above 0.2 mag. The same photometric catalog yields a tip of the red giant branch at 20.40 ± 0.10 mag and a distance modulus of 23.81 ± 0.10 mag, about 578 kpc, in agreement with McConnachie et al. (2004). On this basis the paper claims that its approach, Sobel-filter edge detection on the luminosity function plus time-series variability selection, extends the known LPV census of Andromeda's dwarf satellites and supplies the raw material for future star-formation-history and dust-production studies.

Load-bearing premise

The load-bearing premise is that every star counted as a long-period variable is an intrinsic pulsating member of And II, not a Milky Way foreground star or a photometric artifact, and the selection has no stated threshold for the variability index and no quantified contamination rate after the Gaia cross-match.

Editorial extensions

If this is right

  • And II's large LPV sample gives the survey its statistically strongest target for studying AGB variability in a dwarf spheroidal galaxy.
  • The TRGB-based distance modulus of 23.81 ± 0.10 mag, about 578 kpc, places And II within the Andromeda system, consistent with McConnachie et al. (2004).
  • Because the photometry is complete to about 22 mag and roughly 50% complete at 23 mag, the catalog captures nearly the whole AGB and red-supergiant population of And II.
  • The same pipeline applied to the other monitored dwarfs and globular clusters will yield a homogeneous basis for comparing star-formation histories and dust production across the Local Group.

Reading between the lines

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

  • Applied consistently to the other dwarfs in the survey, this catalog would let the LPV fraction be compared against galaxy mass, metallicity, and distance from Andromeda, a comparison the current paper only sets up.
  • Because the 0.2 mag amplitude cutoff is a selection threshold rather than a physical limit, the true LPV population of And II is likely larger; a higher-cadence follow-up could measure how many low-amplitude variables were missed.
  • Correlating the candidates with mid-infrared photometry would test the expected link between variability and dust production, since stars with larger amplitude or redder colors should show stronger mid-infrared excess.
  • Before using the numbers in comparisons, a reader would need to reconcile the paper's internal differences: 825 LPVs reported in the results versus 728 in Table 1, and a distance modulus of 23.81 ± 0.10 mag in the results versus 23.90–24.11 mag in the abstract.
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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

5 major / 4 minor

Summary. This manuscript reports time-series photometry of the Andromeda II dwarf spheroidal galaxy obtained with the Wide Field Camera on the 2.5 m Isaac Newton Telescope, with the goal of detecting long-period variable (LPV) stars and deriving the tip of the red giant branch (TRGB) and distance modulus. The paper states that 10,384 stars were identified in the And II field and that 825 LPV stars were detected (Section 4), with a TRGB magnitude of 20.40 ± 0.10 mag and a distance modulus of 23.81 ± 0.10 mag (~578 kpc). The abstract, however, gives a Sobel-filter distance modulus range of 23.90–24.11 mag, and Table 1 lists only 728 LPV candidates for And II. The paper also describes the use of Gaia DR3 cross-matching to remove foreground stars, a Stetson variability index for variability selection, and a 0.2 mag amplitude threshold, but it does not specify the numerical cutoff for the variability index, the number of epochs required, or a quantified contamination rate. The catalog itself is not provided in the manuscript.

Significance. If the reported LPV catalog and distance modulus are correct and reproducible, this work would be a useful contribution to studies of evolved stars in Local Group dwarf galaxies, complementing earlier surveys of And II and providing a foundation for star formation history and dust-production analyses. The paper uses established reduction and photometry tools (THELI, DAOPHOT/ALLFRAME), applies Gaia DR3 foreground subtraction, performs artificial-star completeness tests, and compares the derived TRGB with the literature (McConnachie et al. 2004). However, the internal inconsistencies in the central detection count and the distance modulus, together with an incompletely specified variability selection and the absence of the catalog, currently prevent the reader from validating the paper's quantitative claims.

major comments (5)
  1. [Table 1 and Section 4] The central detection count is internally inconsistent: Table 1 lists N_LPV = 728 for And II, while Section 4 states that 'the detected LPV stars (825) are notably higher than those found in other target galaxies.' The same catalog cannot contain both numbers. The authors must reconcile these values and explicitly state which number is the final LPV count used for the analysis and figure preparation.
  2. [Abstract and Section 4] The distance modulus in the abstract, 'Using the Sobel filter, we have calculated the distance modulus for this satellite galaxy, which ranges from 23.90 to 24.11 mag,' is inconsistent with the value in Section 4, 'The distance modulus computed for this galaxy stands at 23.81 ± 0.10 mag (~ 578 kpc).' These two claims refer to the same measured quantity yet do not overlap within the quoted uncertainties; the authors should clarify which value is the adopted result and explain the origin of the discrepancy.
  3. [Sections 3.3 and 3.4] The LPV selection is not reproducible as described. The Stetson variability index L is introduced but no numerical cutoff is given, and the amplitude threshold of 0.2 mag is stated without specifying the number of epochs, the required time baseline, the treatment of single-filter measurements, or the criteria for a star to be considered variable. In addition, no contamination rate is quantified after Gaia DR3 cross-matching. Without these details, the reader cannot assess the reliability of the LPV catalog or the significance of the total count.
  4. [Section 4 and Section 5] The spatial coverage of the photometry is described inconsistently: Section 4 says 'photometric measurements required the utilization of CCD1, CCD3, and CCD4 of WFC due to the considerable size of this galaxy,' while Section 5 states that the photometric catalogs were 'focused mainly on the area covered by CCD4 of the WFC.' The authors should clarify which area was actually used for the LPV detection and TRGB measurement, since this directly affects the interpretation of the reported stellar counts.
  5. [General (catalog availability)] The manuscript is presented as producing an LPV catalog, yet no machine-readable catalog, data table, or access link is included. The paper's central claims (LPV counts, light curves, CMD positions) cannot be independently checked without the catalog. The authors should either include the catalog as supplementary material or provide a persistent link to it.
minor comments (4)
  1. [Section 3.3] The phrase 'before LPV cognition' appears to be a typographical error; it should read 'before LPV detection' or 'before variability identification.'
  2. [Table 1] The table header contains formatting issues: 'P lummer' should be 'Plummer', and the column header 'NLP V' should be 'N_LPV'. Also, the lower uncertainty on the ellipticity of And XIII is printed as '−20', which is clearly a typo for '−0.20'.
  3. [Equation (1)] The amplitude estimate A = 2σ/0.701 assumes a sinusoidal light curve, but long-period variables are often non-sinusoidal. The authors should state this as an approximation and discuss any systematic uncertainty this introduces to the amplitude threshold.
  4. [Section 2] The text says observations were made in 'nine different periods' in the abstract and 'up to nine epochs' in Section 5, but the number of epochs per filter and per star is not quantified. Since the variability detection depends on epoch sampling, a typical or median number of epochs per star should be reported.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the LPV catalog and TRGB distance measurement are derived from the survey photometry and external calibrators, not from the claims they support; the internal number inconsistencies are a correctness concern, not circularity.

full rationale

The paper's central outputs—an LPV candidate catalog and a TRGB-based distance modulus—are not constructed from the quantities they claim to predict. Variable-star selection (Section 3.3) relies on a Welch-Stetson variability index computed from the time-series photometry, and Section 3.4 applies an amplitude cut A = 2σ/0.701; neither threshold is fitted to the reported LPV count, so the count does not reduce to the selection criterion by construction. The distance modulus is obtained by applying a Sobel filter to the observed luminosity function (Section 4, TRGB = 20.40 ± 0.10 mag) and is presented as agreeing with, not calibrated to, McConnachie et al. (2004). No uniqueness theorem or ansatz is imported from the authors' own prior work in a load-bearing way; earlier self-citations (e.g., Javadi et al.) provide astrophysical context for why AGB/LPV stars trace star formation, but the And II measurements stand on the present photometry. The notable internal inconsistencies (Table 1 lists 728 LPV stars vs. 825 in Section 4; the abstract's distance-modulus range 23.90–24.11 vs. Section 4's 23.81 ± 0.10) are manuscript-coherence or reproducibility concerns, as are the unstated Stetson-index threshold and the lack of a quantified contamination estimate, rather than circularity. For circularity purposes the derivation chain is self-contained; no step reduces by definition to its input.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central claims (LPV counts and distance modulus) rest on standard photometric calibration and variability selection. The main ledger entries are the unstated variability-index cutoff and the 0.2 mag amplitude threshold, both of which change the catalog size. No new physical entities are introduced.

free parameters (2)
  • Minimum variability amplitude threshold = 0.2 mag
    Stars with amplitudes below 0.2 mag are excluded because of 'uncertainty regarding the nature of stars with lower amplitudes' (Section 3.4). This threshold directly sets the size of the LPV candidate sample.
  • Stetson variability index cutoff = not stated
    Section 3.3 describes the Stetson index but never gives the cutoff used to label a star as variable. The reported counts (728 or 825) cannot be reproduced without this threshold.
assumptions (4)
  • standard math Light curves are sinusoidal, so amplitude A = 2σ/0.701 (Equation 1).
    The amplitude conversion assumes random sampling of a sine wave; this is an idealized model for LPV pulsations.
  • domain assumption Photometric transformations from Landolt standards to the instrumental system are valid.
    All magnitudes, TRGB, and distance modulus rely on the calibration described in Section 3.2.
  • domain assumption Completeness from artificial star tests (complete to 22 mag, 50% at 23 mag) represents the real crowded-field detection efficiency.
    Used to claim that nearly all AGB and RSG populations are detected (Section 3.2).
  • domain assumption Gaia DR3 cross-matching removes foreground stars without removing significant numbers of And II variables.
    Section 3.3 removes foreground stars via Gaia but provides no test of how many true members are lost or how many foreground variables remain.

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Cite this review

Pith. "Pith review of Detection of the Long Period Variable Stars of And II Dwarf Satellite galaxy." pith.science (2026). https://pith.science/paper/GK5FR3GR

@misc{pith2026241200790,
  author       = {Pith},
  title        = {Pith review of: Detection of the Long Period Variable Stars of And II Dwarf Satellite galaxy},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GK5FR3GR}},
  note         = {Machine review of arXiv:2412.00790}
}
read the original abstract

We conducted an extensive study of the spheroidal dwarf satellite galaxies around the Andromeda galaxy to produce an extensive catalog of LPV stars. The optical monitoring project consists of 55 dwarf galaxies and four globular clusters that are members of the Local Group. We have made observations of these galaxies using the WFC mounted on the 2.5 m INT in nine different periods, both in the i-band filter Sloan and in the filter V-band Harris. We aim to select AGB stars with brightness variations larger than 0.2 mag to investigate the evolutionary processes in these dwarf galaxies. The resulting catalog of LPV stars in Andromeda's satellite galaxies offers updated information on features like half-light radii, TRGB magnitudes, and distance moduli. This manuscript will review the results obtained for And II galaxy. Using the Sobel filter, we have calculated the distance modulus for this satellite galaxy, which ranges from 23.90 to 24.11 mag.

Figures

Figures reproduced from arXiv: 2412.00790 by the authors.

Figure 1
Figure 1. Satellite galaxies of Andromeda. The semi-major axis of the galaxies is represented proportionally [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Distribution of the LPVs, detection of the TRGB, CMD, and light-curve in the And II galaxy. [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

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Reference graph

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