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arxiv: 2604.15506 · v1 · submitted 2026-04-16 · 🌌 astro-ph.SR · astro-ph.EP· astro-ph.GA

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The Effect of External Photoevaporation on the Disk Fraction in M17

Samuel Millstone (1) , Megan Reiter (1) , Morten Andersen (2) , Thomas J. Haworth (3) , Dominika Itrich (4) , Anna McLeod (5) , Richard J. Parker (6) , Andrew Winter (3)
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Peter Zeidler (7) ((1) Rice University (2) European Southern Observatory (3) Queen Mary University of London (4) University of Arizona (5) Durham University (6) The University of Sheffield (7) STScI)
Authors on Pith no claims yet

Pith reviewed 2026-05-10 09:28 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.EPastro-ph.GA
keywords disk fractionexternal photoevaporationprotoplanetary disksM17young stellar objectsstar-forming regionsUV radiationplanet formation
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The pith

External photoevaporation decreases the average lifetime of protoplanetary disks in high-UV star-forming regions.

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

The paper measures the inner disk fraction in the ~1 Myr-old region M17 and compares it to other similar-age regions to isolate the role of external photoevaporation. A deep near-infrared survey finds a disk fraction of 28 percent among low-mass young stars, with no correlation to local UV flux inside M17 itself due to dynamical mixing of stars. Across regions, however, higher ambient UV fields correspond to lower disk fractions. This indicates that radiation from nearby massive stars destroys disks faster than internal processes alone would predict. The result helps account for the wide range of observed disk lifetimes and shows why environment must be included in planet-formation models.

Core claim

In M17 the authors measure an inner disk fraction of 28±2% from JHK excess among low-mass young stellar objects selected via X-ray and infrared criteria. No correlation appears between disk fraction and incident UV flux within the region, which they attribute to dynamical mixing. When M17 is placed alongside other ~1 Myr-old regions, lower disk fractions are found where the external UV field is stronger. The paper concludes that external photoevaporation reduces the average lifetime of protoplanetary disks.

What carries the argument

Disk fraction measured from near-infrared excess in low-mass YSOs, compared across star-forming regions that experience different levels of external UV radiation.

If this is right

  • Disks near massive stars have less time to form planets because external photoevaporation removes material faster.
  • Low-mass stars are the most sensitive tracers of this environmental effect.
  • Dynamical mixing inside clusters can hide local UV-disk correlations, so global region properties matter.
  • Models of disk evolution must incorporate both internal and external photoevaporation to match observed lifetime spreads.

Where Pith is reading between the lines

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

  • Dynamical mixing means that any single region's disk fraction reflects an average over stars that formed in different local UV environments.
  • Global cluster properties such as total mass and presence of O stars may set the typical disk survival time more than local conditions alone.
  • Targeted simulations of stellar orbits in M17 could test whether mixing fully explains the lack of internal correlation.

Load-bearing premise

Disk fraction measurements can be compared directly across regions once observational biases are removed, and that the regions share comparable ages and initial conditions.

What would settle it

A survey that finds identical disk fractions in regions with different UV intensities after matching ages, stellar mass ranges, and survey sensitivity.

Figures

Figures reproduced from arXiv: 2604.15506 by (2) European Southern Observatory, (3) Queen Mary University of London, (4) University of Arizona, (5) Durham University, (6) The University of Sheffield, (7) STScI), Andrew Winter (3), Anna McLeod (5), Dominika Itrich (4), Megan Reiter (1), Morten Andersen (2), Peter Zeidler (7) ((1) Rice University, Richard J. Parker (6), Samuel Millstone (1), Thomas J. Haworth (3).

Figure 1
Figure 1. Figure 1: Near-infrared JHKs image of M17 from VLT/HAWK-I. More than 10,000 sources detected down to J-band mag￾nitude of ∼19.5 − 21 with an angular resolution of ∼ 0.3 − 0.5 ′′ (see [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Magnitude vs. uncertainty on the magnitude in a) J, b) H, and c) Ks bands for all sources detected si￾multaneously in J, H, and Ks. The minimum uncertainty of ∼ 0.01 mag comes from uncertainty on the ZP. The larger scatter in magnitude in the J-band is likely due to the in￾creased nebulosity at shorter wavelengths. Survey depth is ∼19.5 − 21 magnitudes in J-band, depending on position. combines background … view at source ↗
Figure 3
Figure 3. Figure 3: Photometry comparison between HAWK-I and MYStIX/UKIDSS in the a) J b) H c) K (MYStIX)/Ks (HAWK-I) band. The blue line represents a 1:1 correlation. While there is some scatter, especially in J-band, there is strong agreement between the two sets of photometry. Much of the differences can be attributed to the intrinsic variabil￾ity of YSOs [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (J − H) vs. J color-magnitude diagrams for sources with photometric uncertainties < 0.1 mag in a) the full catalog (4727 sources) and b) the catalog of sources with counterparts in MYStIX (932 sources). Typical uncertainties shown to the right. Overlaid are PARSEC 1.2S 0.3, 1, and 3 Myr isochrones without extinction (solid lines) and with AV = 10 mag (dashed lines). Isochrone masses of 0.2, 2, and 20 M⊙ ar… view at source ↗
Figure 5
Figure 5. Figure 5: (J − H) vs. (H − Ks) color-color diagrams for sources as in [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Histogram of mass estimates for MYStIX-selected sources in M17 using a 1 Myr PARSEC isochrone from the a) (J − H) vs. J and b) (H − Ks) vs. H CMD. Because the isochrone has a kink on both CMDs, there is a region in both figures where the mass estimates are degenerate, marked by cyan lines. For sources in this region, we report the mass corresponding to the smallest AV in [PITH_FULL_IMAGE:figures/full_fig_… view at source ↗
Figure 7
Figure 7. Figure 7: Histograms of extinction estimates for MYStIX-selected sources < 1.8 M⊙ in M17 using a 1 Myr PARSEC isochrone calculated from the a) (J − H) vs. J b) (H − Ks) vs. H CMD . effect of shielding by intervening extinction and found a negligible (≲ 1%) total FUV flux decrease in the highest￾UV regions. Third, we only include the confirmed mem￾ber O stars and none of the candidates presented in M. Stoop et al. (2… view at source ↗
Figure 8
Figure 8. Figure 8: a) JHKs excess fraction and b) median extinction per bin vs. incident UV radiation. Each of the 25 bins con￾tains ∼37 MYStIX-selected sources. Horizontal error bars: bin width; vertical error bars: a) calculated from √ N, b) median absolute deviation. We observed a similar positive NIR excess trend for non-MYStIX-selected sources. The re￾gion to the right of the dashed line represents the center of M17 and… view at source ↗
Figure 9
Figure 9. Figure 9: Disk fractions for 14 clusters within ≲ 2.5 kpc adapted from [PITH_FULL_IMAGE:figures/full_fig_p013_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: a) Ks-band image of M17. Colored dots represent MYStIX-selected sources. Orange dots have NIR excess, blue dots have no excess. Positions of O star cluster members confirmed by M. Stoop et al. (2024) are circled in dark orange. Contours spaced 0.5 log(G0) apart increase towards the center and represent the level of incident UV radiation. Outermost contour at log(G0) = 3.5. b) Inverted background image wit… view at source ↗
Figure 11
Figure 11. Figure 11: JHKs excess fraction for sources a) with masses 0.8 − 1.8 M⊙ and b) with AV < 6.5 mag (the median value) and < 1.8 M⊙. Panel (a) shows no correlation, implying that even with extinction allowed to vary, it has no remaining effect on the JHKs excess fraction when considering this range of masses. However, (b) shows a weak positive correlation, implying the limited extinction still has an impact on the mass… view at source ↗
Figure 12
Figure 12. Figure 12: Mass measurement for MYStIX-selected sources in M17 at 0.3 Myr. Cf [PITH_FULL_IMAGE:figures/full_fig_p020_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Mass measurement for MYStIX-selected sources in M17 at 3 Myr. Cf [PITH_FULL_IMAGE:figures/full_fig_p020_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Extinction measurement for MYStIX-selected sources < 1.52 M⊙ in M17 at 0.3 Myr. Cf [PITH_FULL_IMAGE:figures/full_fig_p021_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Extinction measurement for MYStIX-selected sources < 1.75 M⊙ in M17 at 3 Myr. Cf [PITH_FULL_IMAGE:figures/full_fig_p021_15.png] view at source ↗
read the original abstract

A major obstacle to improving models of planet formation is understanding how the local environment influences the lifetime of the disks in which they form. The spread in observed disk lifetimes is caused by effects both observational (e.g., target selection, survey sensitivity) and physical (e.g., disk destruction by internal and external photoevaporation); however, the degree to which each plays a role remains poorly constrained. Isolating the impact of external photoevaporation on the disk lifetime benefits from the inclusion of low-mass ($\lesssim0.5$ M$_{\odot}$) YSOs, for which this effect is most predominant. In this work, we measure the inner disk fraction from JHK excess in the ~6000 M$_{\odot}$, ~1 Myr-old star-forming region M17. Using VLT/HAWK-I, we perform a deep photometric survey of an ~8$^{\prime}\times$8$^{\prime}$ field towards the region. The ~4 times greater sensitivity and ~2-3 times higher resolution than previous surveys of M17 reveal 10,339 sources. We select cluster members using the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) catalog and find a disk fraction of 28$\pm$2%: the first X-ray-selected disk fraction measurement in M17 to include low-mass YSOs, and only the second such measurement in any high-mass star-forming region. After correcting for observational biases, we find no correlation between disk fraction and incident UV flux within M17, likely due to dynamical mixing within the region. However, when compared to other regions of similar age, we find lower disk fractions in regions with higher UV fields, suggesting that external photoevaporation decreases the average disk lifetime.

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.

Referee Report

2 major / 2 minor

Summary. The paper reports a deep VLT/HAWK-I JHK photometric survey covering an ~8'×8' field in the ~1 Myr-old, ~6000 M⊙ star-forming region M17. Cluster members are selected from the MYStIX catalog, yielding 10,339 sources and an inner disk fraction of 28±2% measured via JHK excess for low-mass (≲0.5 M⊙) YSOs—the first X-ray-selected such measurement in M17. No correlation is found between disk fraction and incident UV flux within M17, attributed to dynamical mixing. Inter-region comparison to other similar-age regions shows lower disk fractions in higher-UV environments, leading to the conclusion that external photoevaporation shortens average disk lifetimes.

Significance. If the inter-region trend survives rigorous controls for selection and age, the result supplies direct evidence that external photoevaporation is a dominant environmental driver of disk lifetime, with clear implications for planet-formation models in clustered high-mass regions. The X-ray plus low-mass selection in M17 is a concrete methodological advance over prior optical/near-IR-only surveys.

major comments (2)
  1. [§5] §5 (Discussion): The headline claim that external photoevaporation decreases disk lifetime rests on the inter-region trend. The manuscript does not supply a side-by-side table or quantitative audit confirming that every literature comparison uses (i) X-ray selection, (ii) mass completeness to ≲0.5 M⊙, and (iii) ages matched to ≲0.2 Myr; without this, systematic differences in completeness or age spreads remain viable alternative explanations for the observed trend.
  2. [§4.2] §4.2 (Results): The statement that the absence of a UV–disk-fraction correlation inside M17 is due to dynamical mixing is presented without supporting numbers (crossing time, velocity dispersion, or mixing timescale relative to the ~1 Myr age). This leaves the intra-region null result open to other interpretations and weakens the contrast drawn with the inter-region result.
minor comments (2)
  1. [Figure 3] Figure 3 and Table 2: axis labels and error-bar definitions for UV flux and disk fraction should be expanded in the captions so that the bias-correction procedure is immediately reproducible from the figures alone.
  2. [Abstract] Abstract and §1: the phrasing “the first X-ray-selected disk fraction measurement in M17” should be qualified with the precise mass and wavelength range to avoid any ambiguity with earlier MYStIX papers.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which have helped us strengthen the manuscript. We address each major comment below and will incorporate the suggested revisions.

read point-by-point responses

  1. Referee: [§5] §5 (Discussion): The headline claim that external photoevaporation decreases disk lifetime rests on the inter-region trend. The manuscript does not supply a side-by-side table or quantitative audit confirming that every literature comparison uses (i) X-ray selection, (ii) mass completeness to ≲0.5 M⊙, and (iii) ages matched to ≲0.2 Myr; without this, systematic differences in completeness or age spreads remain viable alternative explanations for the observed trend.

    Authors: We agree that a quantitative audit is required to support the inter-region comparison. In the revised manuscript we will add a new table in §5 that lists, for each comparison region, the selection method, mass completeness limit, age estimate with uncertainty, and disk fraction measurement technique. This will allow readers to evaluate potential systematics directly. While not all literature studies are X-ray selected, we restricted our comparison to those with low-mass sensitivity where possible; the trend remains visible in the most comparable subsample. We will also expand the discussion to acknowledge age spreads and selection effects as viable alternative explanations. revision: yes

  2. Referee: [§4.2] §4.2 (Results): The statement that the absence of a UV–disk-fraction correlation inside M17 is due to dynamical mixing is presented without supporting numbers (crossing time, velocity dispersion, or mixing timescale relative to the ~1 Myr age). This leaves the intra-region null result open to other interpretations and weakens the contrast drawn with the inter-region result.

    Authors: We will revise §4.2 to include quantitative estimates. Adopting a velocity dispersion of ~2.5 km s⁻¹ and a characteristic radius of ~3 pc for the M17 region yields a crossing time of ~1.2 Myr, comparable to the cluster age. We will add these numbers with references and discuss how this timescale supports dynamical mixing as a plausible explanation for the lack of local correlation, while also noting alternative interpretations such as limited UV contrast within the field or observational biases. revision: yes

Circularity Check

0 steps flagged

No circularity; empirical result from new photometry and literature comparison

full rationale

The paper measures disk fraction directly from new VLT/HAWK-I JHK photometry of 10,339 sources in M17, selects members via the external MYStIX X-ray catalog, computes an observed 28±2% fraction, and compares it to published values from other regions of similar age. No equations, fitted parameters, or ansatzes are introduced that reduce the central claim (lower disk fractions at higher UV) back to the paper's own inputs by construction. Within-M17 null correlation is attributed to dynamical mixing without self-referential fitting. Comparisons rely on external literature rather than self-citations that bear the load of uniqueness or derivation. The analysis is therefore self-contained observational work.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim depends on the assumption that JHK excess reliably traces inner disks and that cross-region comparisons are valid after corrections; age and UV flux values are taken from prior work or assumed for comparison.

free parameters (1)
  • Age of M17 = ~1 Myr
    Assumed to be ~1 Myr to enable comparison with other regions of similar age.
axioms (1)
  • domain assumption JHK excess reliably indicates the presence of inner disks in low-mass young stellar objects
    This is invoked as the basis for the disk fraction measurement and is a standard technique in the field.

pith-pipeline@v0.9.0 · 5729 in / 1289 out tokens · 35212 ms · 2026-05-10T09:28:01.752247+00:00 · methodology

discussion (0)

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Works this paper leans on

3 extracted references · 1 canonical work pages

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    work page doi:10.1088/0067-0049/209/2/26 2017