arxiv: 2605.12486 · v1 · submitted 2026-05-12 · 🌌 astro-ph.GA · astro-ph.SR

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· Lean Theorem

Photometric determination of the mass accretion rates of pre-main sequence stars. IX. Recent star formation in the periphery of NGC 346

Christiaan Dik, Guido De Marchi

Pith reviewed 2026-05-13 03:03 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.SR

keywords pre-main sequence starsmass accretion ratesNGC 346Small Magellanic Cloudcircumstellar discsultraviolet radiationH-alpha emissionstar formation

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The pith

Mass accretion rates of young stars fall with proximity to hot massive stars, implying their ultraviolet light erodes gas discs.

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

The paper examines a quiet region outside the NGC 346 cluster in the Small Magellanic Cloud using Hubble images in visible and H-alpha bands. It identifies 137 pre-main sequence stars showing clear H-alpha excess and calculates their masses, ages around 16 million years, and accretion rates. The central result is that both the fraction of accreting stars and their accretion rates correlate with distance from a nearby group of hot massive stars. This pattern suggests the ultraviolet radiation from those massive stars is stripping material from the young stars' discs. The finding matters because it shows how feedback from massive stars can shape star formation even in regions without obvious clustering or nebulosity.

Core claim

In a peripheral region of NGC 346, the ratios of accreting to non-accreting pre-main sequence stars relative to older stars, together with the measured mass accretion rates, correlate with distance from a group of hot massive stars. The authors conclude that ultraviolet radiation from these massive stars erodes the circumstellar discs of the pre-main sequence stars, reducing accretion closer to them. The pre-main sequence population has a median age of roughly 16 Myr, average mass 0.8 solar masses, and median accretion rate 8 times 10 to the minus 9 solar masses per year, values comparable to those in other low-density SMC regions.

What carries the argument

The observed correlation of accretion indicators with distance to hot massive stars, interpreted as evidence for ultraviolet-driven disc erosion.

If this is right

Pre-main sequence stars nearer the hot stars exhibit lower accretion rates and fewer accreting members.
External ultraviolet radiation can erode discs even in low-density, non-clustered environments.
Accretion rates remain similar to those in other sparse SMC star-forming regions despite the lack of nebulosity.
Some identified pre-main sequence stars show near-infrared excess, confirming ongoing disc presence.
The region contains multiple stellar populations with ages from 10 Myr to 5 Gyr, indicating extended star formation history.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same ultraviolet feedback could shorten disc lifetimes enough to limit planet formation in the vicinity of massive stars.
Comparable distance-dependent patterns should appear in other galaxies containing groups of hot massive stars.
Future spectroscopy or infrared imaging could test whether disc gas content decreases measurably with proximity to the massive stars.
This process may help explain variations in disc survival across different star-forming environments.

Load-bearing premise

The H-alpha excess is produced only by accretion and the distance correlation reflects a direct causal effect from the massive stars rather than differences in age or selection.

What would settle it

Direct infrared measurements showing whether circumstellar disc sizes or gas masses are systematically smaller for pre-main sequence stars closer to the hot massive stars.

Figures

Figures reproduced from arXiv: 2605.12486 by Christiaan Dik, Guido De Marchi.

**Figure 1.** Figure 1: Colour image of the studied field south of NGC346, created by combining the HST V and I band images. with recent JWST observations in the region. Finally, a summary and conclusions are provided in Sect. 7. 2. Observations This work mainly uses observations collected with the HST, in the F555W band (hereafter ’V band’), F814W band (’I band’) and F656N band (’Hα band’), with exposure times of 1200 s, 400 s a… view at source ↗

**Figure 2.** Figure 2: Regions observed with HST and JWST and the rough extent of the cluster NGC 346 and the surrounding HII region N66. the HST and JWST fields — including the overlapping region — as well as the approximate extent of the NGC 346 cluster can be seen in [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗

**Figure 3.** Figure 3: Clearly visible is the MS, as well as a red-giants red [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

**Figure 3.** Figure 3: Colour-magnitude diagram showing the V magnitude as a function of the V − I colour of the stars in the studied region. Shown in orange are the (potentially) saturated stars that were recovered through Gaia. In brown are the objects with Hα excess emission as identified in section 4. The dashed lines represent isochrones for 10 Myr, 60 Myr, 400 Myr, and 5 Gyr. criterion requiring that the combined uncertai… view at source ↗

**Figure 5.** Figure 5: Hertzsprung–Russell diagram of the PMS candidates in the studied region. Shown are the isochrones and evolutionary tracks from Tognelli et al. (2011). The blue points indicate bona fide PMS stars. tribution, at magnitudes V ≲ 19.5, we have another group of sources, which likely contain the aforementioned Be stars. 4.1. Properties of PMS stars Following De Marchi et al. (2010), the effective temperature Tef… view at source ↗

**Figure 6.** Figure 6: Histograms showing the distribution of age (top left), mass (top right), accretion luminosity (bottom left), and mass accretion rate (bottom right) determined for the 137 bona fide PMS stars. The blue histograms represent the stars older than 16 Myr and the orange histograms stars younger than 16 Myr. Dashed vertical lines indicate the median of the respective distribution. 4.2. Accretion luminosity and ma… view at source ↗

**Figure 7.** Figure 7: Distribution of PMS stars in the field, colour-coded for their relative mass accretion rate: in blue “fast” accretors (M˙ acc > 10−8M⊙yr−1 ), in dark grey “intermediate” accretors (10−8.3M⊙yr−1 < M˙ acc < 10−8 M⊙yr−1 ), and in red “slow” accretors (M˙ acc < 10−8.3M⊙yr−1 ). The most massive young (non-PMS) stars are plotted in orange, with their barycentre in yellow. Radial shells around this barycentre ar… view at source ↗

**Figure 8.** Figure 8: Fraction of PMS stars relative to the total number of stars (left); M˙ acc of the PMS stars averaged over each shell (right). Following De Marchi et al. (2010), we selected young massive stars with log(Teff/K) ≥ 4.4 and log(L/L⊙) ≥ 4, having adopted a distance of 62.5 kpc (Graczyk et al. 2020) and AV = 0.84 (see Section 3). We note that the values of Teff were derived from the V − I colours, using the mo… view at source ↗

**Figure 9.** Figure 9: Histograms of the number of massive stars (8 − 20 M⊙) in our region (red line) and in the ONC (black line). The dashed line represents the IMF of the ONC rescaled to match the number of objects under the red line. The best fitting standard Kroupa (2002) IMF is shown by the dot-dashed line. The area that we surveyed represents a good example of massive star formation in the field. It is known that a non-ne… view at source ↗

**Figure 10.** Figure 10: Colour-magnitude diagrams obtained with the JWST/NIRCam (left) and HST (right) photometry. In the left panel, light blue dots are JWST sources outside the HST–JWST overlapping region, and blue dots are sources only included in the field observed with JWST. In the right panel, blue marks the HST sources in the overlapping region. The orange points indicate the two bona fide PMS stars with reliable NIRCam … view at source ↗

**Figure 11.** Figure 11: A model spectral energy distribution showing the logarithm of the flux density as a function of wavelength, zoomed in on the IR tail. Shown in blue are the two overlapping PMS stars and their flux density at different wavelengths. All curves are anchored to the model at 9000 Å The detection of both Hα excess (from our HST photometry) and near-IR excess (from JWST) for these two objects constitutes strong… view at source ↗

read the original abstract

We studied the properties of star formation and the characteristics of young stars in a quiet region located beyond the outskirts of the prominent star-forming cluster NGC 346 in the Small Magellanic Cloud (SMC). Utilising observations from the Hubble Space Telescope across the broad V and I bands, as well as the narrow Halpha band, we identified populations with ages of roughly 10, 60, 400 Myr and of 5 Gyr through isochrone comparison. We successfully identified 137 bona fide pre-main sequence (PMS) candidates exhibiting Halpha excess with a significance level of 5 sigma, accompanied by an Halpha line emission equivalent width exceeding 20 \AA. Physical parameters for these PMS stars were determined, including mass, age, accretion luminosity, and mass accretion rate. Most PMS stars have an age around 16 Myr and an average mass of 0.80 \pm 0.16 M_sun. The median mass accretion rate for all 137 PMS stars is estimated to be about 8.0 x 10^(-9) M_sun/yr. While this rate is lower than that observed in the NGC 346 cluster itself, it is comparable with those measured for PMS stars in low-density star-forming regions in the SMC, despite the absence of apparent clustering and nebulosity. Furthermore, our analysis reveals that the ratios of accreting and non-accreting PMS stars to non-PMS stars and their mass accretion rate correlate with their distance from a group of hot massive stars in the vicinity. This suggests that the ultraviolet radiation emitted by these massive stars might erode the circumstellar discs of nearby PMS stars. Lastly, the overlap between our studied region and observations from the James Webb Space Telescope reveals that some of the identified PMS stars display near-infrared excess.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper gives concrete accretion rates for 137 PMS stars in a low-density SMC periphery and reports a correlation with distance to hot stars, but the UV-erosion reading rests on untested assumptions about H-alpha and gradients.

read the letter

The main thing to know is that this work applies standard photometric H-alpha selection to a previously unexamined outer field near NGC 346 and comes up with a median accretion rate of 8e-9 solar masses per year for 137 candidates at roughly 16 Myr. It also flags that the fraction of accreting stars and their rates appear to fall with distance from a group of hot massive stars, which the authors link to possible disk erosion by UV radiation. That correlation is the fresh data point for this specific region, and the rates line up with other sparse SMC fields while sitting below the denser cluster values. They add a quick note on JWST overlap showing some near-IR excess, which helps confirm disks are still present. The photometry and isochrone fitting are done in the usual way, and the sample size plus the direct comparison to other SMC work make the numbers usable for disk-evolution models. The soft spots sit in the interpretation. At 16 Myr, H-alpha excess with EW >20 Å can come from chromospheric activity as well as accretion, especially at SMC metallicity, and the paper does not show tests that separate the two. They identify older populations in the field but do not check whether the PMS subsample or the detection completeness stays flat across the radial bins around the hot stars. If age or mass gradients exist, the distance trend could be selection or evolutionary rather than causal photoevaporation. No error budgets or completeness maps are described. This is incremental work in the ongoing series on photometric accretion rates. Readers who track environmental effects on disks in low-metallicity settings will find the sample and median rate useful even if the mechanism stays tentative. The data and method are solid enough to merit a serious referee, who can press on the gradient and activity checks. I would send it to review.

Referee Report

3 major / 2 minor

Summary. The paper uses HST V, I, and Hα photometry to identify 137 PMS candidates in a peripheral region of NGC 346 (SMC) via 5σ Hα excess and EW > 20 Å, derives their masses (~0.8 M⊙), ages (~16 Myr median), and mass accretion rates (median 8×10^{-9} M⊙ yr^{-1}), and reports that the ratios of accreting/non-accreting PMS to non-PMS stars plus the accretion rates correlate with distance from nearby hot massive stars, interpreted as UV photoevaporation eroding discs. Multiple older populations (60 Myr, 400 Myr, 5 Gyr) are also identified via isochrones, and some PMS stars show NIR excess in overlapping JWST data.

Significance. If the distance correlations prove causal after controlling for age and selection effects, the work would supply concrete, comparable statistics (137 stars, explicit 5σ + EW>20 Å cuts, median rate) on environmental disc erosion in a low-density, low-metallicity setting, extending the photometric accretion series to the SMC periphery and offering a falsifiable test of UV feedback models. The direct use of observed photometry and standard isochrones avoids circularity in the rate derivation itself.

major comments (3)

[Abstract / PMS candidate selection] Abstract and PMS selection: the 5σ Hα excess plus EW>20 Å cut is taken as pure accretion, yet at the reported ~16 Myr median age and SMC metallicity chromospheric activity can produce comparable EWs; no quantitative comparison to non-accreting stars or activity models is supplied, which is load-bearing for both the accretion-rate values and the UV-erosion interpretation of the distance trend.
[Results / distance correlation] Distance-correlation analysis: the reported trends in accreting/non-accreting ratios and median Ṁ_acc versus distance to the hot-star group are presented without completeness maps, detection-efficiency tests, or age/mass distribution checks across radial bins, leaving open the possibility that gradients in age or selection (rather than photoevaporation) drive the signal.
[Abstract] Abstract: the median accretion rate 8.0×10^{-9} M⊙ yr^{-1} is stated without error budget, uncertainty from distance/extinction/isochrone choice, or robustness tests against the free parameters (5σ threshold, 20 Å cutoff), undermining assessment of the correlation's statistical significance.

minor comments (2)

[Abstract] The abstract lists ages as 'roughly 10, 60, 400 Myr and of 5 Gyr'; clarify the precise isochrone ages adopted for the PMS subsample versus the older populations.
[Discussion / JWST overlap] The JWST NIR-excess statement is qualitative; specify how many of the 137 PMS stars show excess, at which bands, and any selection criteria applied.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments on our manuscript. We address each of the major comments below and indicate the revisions we will make to improve the paper.

read point-by-point responses

Referee: [Abstract / PMS candidate selection] Abstract and PMS selection: the 5σ Hα excess plus EW>20 Å cut is taken as pure accretion, yet at the reported ~16 Myr median age and SMC metallicity chromospheric activity can produce comparable EWs; no quantitative comparison to non-accreting stars or activity models is supplied, which is load-bearing for both the accretion-rate values and the UV-erosion interpretation of the distance trend.

Authors: We recognize the potential contribution of chromospheric activity to Hα emission in stars of this age and metallicity. Our selection criteria follow established practices in the photometric accretion series for identifying accreting PMS stars. In the revised version, we will include a quantitative comparison of the Hα equivalent widths of our PMS candidates against those of non-accreting stars in the same field and discuss relevant activity models for low-metallicity environments. This addition will better justify the accretion interpretation and support the distance correlation analysis. revision: yes
Referee: [Results / distance correlation] Distance-correlation analysis: the reported trends in accreting/non-accreting ratios and median Ṁ_acc versus distance to the hot-star group are presented without completeness maps, detection-efficiency tests, or age/mass distribution checks across radial bins, leaving open the possibility that gradients in age or selection (rather than photoevaporation) drive the signal.

Authors: The referee correctly identifies a gap in our analysis. We will add completeness maps derived from artificial star experiments, evaluate detection efficiencies in different radial bins, and examine the distributions of ages and masses as a function of distance from the hot stars. These checks will allow us to assess whether the observed trends are robust against selection effects or age gradients. If confirmed, we will strengthen the discussion of UV photoevaporation; if not, we will revise the interpretation accordingly. revision: yes
Referee: [Abstract] Abstract: the median accretion rate 8.0×10^{-9} M⊙ yr^{-1} is stated without error budget, uncertainty from distance/extinction/isochrone choice, or robustness tests against the free parameters (5σ threshold, 20 Å cutoff), undermining assessment of the correlation's statistical significance.

Authors: We agree that providing an error budget and robustness tests will enhance the manuscript. We will expand the abstract to include uncertainties and add a dedicated subsection detailing the error sources (distance, extinction, isochrone choice) and performing sensitivity tests by varying the selection thresholds. This will better contextualize the median rate and the significance of the reported correlations. revision: yes

Circularity Check

0 steps flagged

No significant circularity; results from direct photometry, isochrones, and standard conversions

full rationale

The paper derives PMS identifications, ages, masses, and accretion rates directly from HST V, I, and Hα photometry using 5σ excess plus EW > 20 Å criteria, standard isochrone fitting for parameters, and conventional L_acc-to-Ṁ_acc conversions. The reported radial correlations are observational statistics computed from these independently measured quantities, with no equations or claims that reduce the outputs (e.g., median Ṁ_acc or ratios) to fitted inputs from the same dataset by construction. Self-citations to the series for methodology are present but not load-bearing for the central claims, which remain falsifiable against external data and benchmarks.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central results rest on standard stellar-evolution isochrones, a fixed 5-sigma detection threshold, and the assumption that H-alpha excess traces accretion luminosity via an unstated conversion formula. No new physical entities are introduced.

free parameters (2)

5-sigma significance threshold
Used to select bona-fide H-alpha excess sources; value chosen by authors.
Equivalent-width cutoff of 20 Å
Additional filter to confirm accretion; conventional but still a free choice.

axioms (2)

domain assumption Standard pre-main-sequence isochrones accurately map observed colors and magnitudes to mass and age.
Invoked when assigning ages ~16 Myr and masses 0.80 Msun.
domain assumption H-alpha excess above the chosen threshold is produced by accretion rather than chromospheric activity.
Central to identifying the 137 PMS candidates and deriving accretion rates.

pith-pipeline@v0.9.0 · 5646 in / 1638 out tokens · 99468 ms · 2026-05-13T03:03:14.044794+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear
the ratios of accreting and non-accreting PMS stars to non-PMS stars and their mass accretion rate correlate with their distance from a group of hot massive stars in the vicinity. This suggests that the ultraviolet radiation emitted by these massive stars might erode the circumstellar discs of nearby PMS stars.
IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear
Most PMS stars have an age around 16 Myr and an average mass of 0.80 ± 0.16 M⊙. The median mass accretion rate for all 137 PMS stars is estimated to be about 8.0 × 10^{-9} M⊙ yr^{-1}.

Reference graph

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