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REVIEW 3 major objections 3 minor 67 references

Gaia DR2 distances to Collinder 419 and NGC 2264 and new astrometric orbits for HD 193 322 Aa,Ab and 15 Mon Aa,Ab

T0 review · 3 major / 3 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Using Gaia DR2, this paper measures distances of about 1.0 kpc and 719 pc for Collinder 419 and NGC 2264, and derives orbit-based total masses of 76 and 45 solar masses for their central O-star binaries.

desk verdict Solid Gaia distances, honest orbit work, but the 15 Mon period is provisional and the mass table should include distance systematics. read the letter →

arxiv 1908.02040 v1 pith:QB6DW3K7 submitted 2019-08-06 astro-ph.SR astro-ph.GA

J. Maíz Apellániz (CAB , INTA-CSIC) This is my paper · ORCID
classification astro-ph.SRastro-ph.GA
keywords astrometryvisualbinariesGaiaDR2parallaxzeropointCollinder419NGC2264massiveO-typestarsluckyimaging
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

Using Gaia DR2 parallaxes, proper motions, and photometry, the paper measures distances to two nearby clusters that contain O stars and derives new visual orbits for the bright multiple systems at their cores. The distances are $1006^{+37}_{-34}$ pc for Collinder 419 and $719\pm16$ pc for NGC 2264, with the uncertainty budget dominated by the spatial covariance of the Gaia parallaxes rather than by individual measurements. Combining these distances with decades of speckle, lucky-imaging, and interferometric positions, the paper obtains total masses of $76.1^{+9.9}_{-7.4}\,M_\odot$ for the three stars in HD 193 322 Aa,Ab and $45.1^{+3.6}_{-3.3}\,M_\odot$ for the two stars in 15 Mon Aa,Ab. If correct, these results replace older, partly discrepant distance estimates and provide two of the best mass anchors for massive O-type binaries.

What carries the argument

The load-bearing machinery is two methods. The distance method is supervised and multifilter: it starts from a spectroscopically chosen reference O star, removes Gaia sources with poor astrometry (RUWE > 1.4), contaminated photometry ($d_{\rm CC} > 0.2$ mag), or large external parallax uncertainty, then applies position, proper-motion, and isochrone-with-extinction-band cuts, and finally a normalized-parallax $\sigma$-clip; the surviving objects are combined in a weighted mean that explicitly includes the spatial covariance of the Gaia DR2 parallaxes, the mean is shifted by a single $+0.040$ mas zero-point correction with a further $0.010$ mas added to the uncertainty, and a Bayesian prior for young disk populations converts the parallax into a distance. The orbit method replaces the usual local $\chi^2$ fit with a global likelihood search: 128 seeded minimizations probe the whole seven-dimensional space, promising solutions are rounded onto a 255-point-per-dimension grid, and the algorithm expands through adjacent grid cells like an amoeba until no more above-threshold points appear; parameters are re-parametrized as periastron distance $d \equiv a\sin i$ and $\varpi \equiv \omega+\Omega$ to break the correlations that make the likelihood surface snake-shaped. The astrometric input is the Washington Double Star catalog combined with AstraLux lucky-imaging measurements, whose new calibration uses Gaia DR2 reference coordinates.

What would settle it

Take an independent geometric distance to either cluster—an improved water-maser parallax for NGC 2264, or a future Gaia data release with a per-star parallax zero point—and compare it with the quoted values; a disagreement larger than the combined uncertainties falsifies the single-offset assumption. The tightest check is the 15 Mon Aa,Ab mass: obtain resolved radial velocities of the two components, combine them with the visual orbit to get a purely dynamical mass, and compare with $45.1^{+3.6}_{-3.3}\,M_\odot$; because mass scales as distance cubed, any discrepancy also pinpoints the distance error.

Watch

Extended reading notes

Core claim

The central claim is that a careful, supervised combination of Gaia DR2 data yields robust parallaxes for Collinder 419 and NGC 2264, and that a global search of the seven-parameter orbital space gives reliable elements for the two central binaries. For Collinder 419 the paper obtains a distance of $1006^{+37}_{-34}$ pc, which it argues rules out the earlier $741\pm36$ pc CMD-based value of Roberts et al. (2010); for NGC 2264 it obtains $719\pm16$ pc, in agreement with the water-maser distance of Kamezaki et al. (2014). The new orbit for HD 193 322 Aa,Ab has $P = 44\pm1$ yr and $e = 0.58^{+0.03}_{-0.04}$, implying a total mass of $76.1^{+9.9}_{-7.4}\,M_\odot$ for its three components, about half the value the previous orbit implied at the old distance. The 15 Mon Aa,Ab orbit has $P = 108\pm12$ yr and $e = 0.770^{+0.023}_{-0.030}$, implying $45.1^{+3.6}_{-3.3}\,M_\odot$ for its two components, with a random uncertainty below ten percent. The paper also reports that the two NGC 2264 subclusters lie at the same distance but move with significantly different proper motions, and that a bright M-type star near Collinder 419 may be a runaway from a younger star-forming region behind the cluster.

Load-bearing premise

The load-bearing premise is that a single Gaia DR2 parallax zero-point correction of $+0.040$ mas applies to every member star in both clusters; the true zero point depends on magnitude, color, and sky position, and an error of $0.010$ mas would shift the Collinder 419 distance by about 1% and the derived binary masses by about 3%.

Editorial extensions

If this is right

  • If the distances stand, Collinder 419 is a ~1.0 kpc cluster rather than the ~0.74 kpc of the earlier CMD study, shifting the absolute magnitudes of all its members and halving the inferred HD 193 322 Aa,Ab mass to 76 $M_\odot$, a value consistent with three late-O/early-B stars.
  • The 15 Mon Aa,Ab total mass of $45.1^{+3.6}_{-3.3}\,M_\odot$ has a random uncertainty below ten percent, making it one of the best constrained masses for an O-type visual binary and a direct test for stellar evolution models at high mass.
  • The combination of Gaia DR2 with WDS and AstraLux data, together with the grid-searching orbit code, is a prescription for measuring orbits and masses of other massive binaries within ~1 kpc.
  • For NGC 2264 the two subclusters are at the same distance yet show a significant relative proper motion, indicating separate dynamical units within the cluster; the internal velocity dispersion within each subcluster dominates their proper-motion spreads.
  • For HD 193 322 Aa,Ab, the ephemerides imply the system will not complete its first observed revolution until the late 2020s, and the next periastron occurs around 2040, so continued astrometric monitoring will keep tightening the orbital parameters and mass.

Reading between the lines

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

  • Because the parallax zero point varies with magnitude, color, and position, a single-offset correction is a systematic risk; if future per-star zero points from later Gaia releases differ from +0.040 mas by 0.010 mas, the distances shift by roughly 1% and the derived masses by about 3% (mass $\propto$ distance cubed), so comparisons across clusters should carry that systematic in quadrature.
  • The period history of 15 Mon (published values from 23.6 to 190.5 years) shows how local fits on short arcs can lock onto wrong solutions; the same global-search approach used here could re-examine other long-period binaries whose published orbits disagree, since it maps the full likelihood surface instead of a local minimum.
  • If the approved spatially resolved spectroscopy of 15 Mon Aa and Ab succeeds, the new astrometric orbit plus double-lined radial velocities would yield individual component masses, converting the current total-mass measurement into a genuine calibration point for the upper main sequence.
  • The apparent counterclockwise relative motion between the NGC 2264 subclusters, combined with the cluster's young age and gas content, raises a testable dynamical question: the projected separation of about 4.5 pc is small compared with the distance uncertainties, and future radial-velocity or proper-motion data could reveal whether the two halves are bound or expanding apart.
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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

3 major / 3 minor

Summary. This paper presents two methodological applications of Gaia DR2 and ground-based lucky imaging. First, a supervised membership/distance algorithm combining Gaia astrometry (positions, proper motions, parallaxes) and photometry with quality cuts (RUWE, dCC) is applied to two clusters containing O stars. It yields distances of 1006+37-34 pc for Collinder 419 and 719±16 pc for NGC 2264, with uncertainties dominated by spatial covariance of Gaia parallaxes, and it shows the two NGC 2264 subclusters are at the same distance but have different proper motions. Second, a global grid-search orbit-fitting code for visual binaries is applied to WDS and AstraLux data, giving an eccentric 44-yr orbit for HD 193 322 Aa,Ab with total mass 76.1+9.9-7.4 Msun, and an eccentric 108±12-yr orbit for 15 Mon Aa,Ab with total mass 45.1+3.6-3.3 Msun.

Significance. If the results hold, the paper replaces uncertain photometric and Hipparcos distances for two nearby O-star clusters with Gaia-based distances and provides two new dynamical mass anchors for massive binaries. The distance pipeline is a careful treatment of correlated Gaia parallaxes, quality filtering, and robustness Monte Carlos, and the orbit code is a genuine improvement over local chi-squared fitting because it maps the full seven-dimensional likelihood and handles multimodality and non-ellipsoidal uncertainties. A notable strength is the explicit discussion of the distance covariance term, which dominates the distance errors, and the systematic parallax zero-point term. The main scientific value, however, rests on the 15 Mon orbit, where the observed arc is short relative to the period; that result needs additional support before the mass can be considered a reliable anchor.

major comments (3)
  1. [Section 4, Table 6, Abstract] The headline mass uncertainties in Table 6 and the Abstract exclude the distance contribution, as stated at the start of Section 4 ("do not include the uncertainties associated with distance"). Since the mass scales as distance cubed, the 3-4% distance uncertainty for Collinder 419 adds roughly a 10% systematic to the 76.1 Msun mass, and the 2% distance uncertainty for NGC 2264 adds roughly 7% to the 45.1 Msun mass. These are comparable to or larger than the quoted random uncertainties. The text mentions this in Sections 4.1 and 4.2, but the Abstract presents the masses without that caveat. Please propagate the distance uncertainty into the quoted mass uncertainties (or, at minimum, state the combined uncertainty prominently in the Abstract).
  2. [Section 4.2, Fig. 6] The 15 Mon Aa,Ab orbit is based on an arc spanning 1988.17-2018.91 (Table 5), which is less than one-third of the 108-yr best-fit period, and the paper correctly notes the system will not return to its first observed epoch until near the end of the century. Published periods range from 23.6 to 190.5 yr (Table 1). The statement that \"the likelihood around 190 a is very low\" relies on the adopted weights for the many WDS data points whose uncertainties are not directly measured but assigned and iterated in Section 2.2.2. With such a short arc, systematic errors in a few early epochs could plausibly move the solution between period families. Please add robustness tests: re-fit after excluding the early McAlister/Gies points, after excluding the 2018 AstraLux epochs, and after varying the weights assigned to the unmeasured-uncertainty points by plausible factors, and report whether the 108-yr period family and the resulting mass remain the preferred solution.
  3. [Section 2.2.2] The iterative weighting scheme for measurements lacking published uncertainties is described only qualitatively (``I also select initial values for their uncertainties'' followed by iteration). To make the orbit results reproducible and to allow an assessment of the influence of the weighting on the final solution, please specify the initial weight choices, the iteration rule, the convergence criterion, and the final assigned uncertainties (or at least a summary of how they changed from the initial values). This is directly relevant to the robustness of the period and mass claims, especially for 15 Mon.
minor comments (3)
  1. [Section 2.1, Eq. (2)] The parallax zero-point is applied as a single constant +0.040 mas with an added 0.010 mas systematic, despite the paper noting that the zero point depends on magnitude, color, and position. Given that the observed cluster members span a range of G magnitudes, please state explicitly whether 0.040±0.010 is meant to cover the full range across the sample, or whether the systematic uncertainty would need to be larger to account for the magnitude-dependent zero-point spread.
  2. [Section 3.3] The word "Montecarlo" appears twice; should be "Monte Carlo".
  3. [Section 3 heading] The heading reads "Distances to and membership of to Collinder 419 and NGC 2264"; the second "to" is extraneous.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: distances, orbits, and masses are derived from external astrometry and independent calibrations.

full rationale

The paper's central results are two Gaia DR2 cluster distances, obtained as the zero-point-corrected weighted mean of the parallaxes of member stars selected by position, proper motion, color-magnitude, and quality filters (Sec. 2.1, Eq. 2), and two visual-binary orbits fit to WDS and AstraLux relative astrometry (Sec. 2.2). The distances are not used to define the membership filter: the CMD isochrone is anchored to the reference star's observed photometry and to extinction parameters from Maíz Apellániz & Barbá (2018), not to the target distance, and the final distance is the measured parallax, not the isochrone. The orbit parameters (P, a, e, etc.) come from a global likelihood search over the data in Tables 4 and 5; those data are independent of the derived masses. The masses follow from Kepler's third law using the fitted P and a and the separately derived distances, so no equation defines an output as its own input. Self-citations (photometric sensitivity curves, extinction laws, the Bayesian distance prior, the earlier AstraLux data paper) are general calibrations or observations with stated assumptions that do not contain the target distances or masses; they are thus independent support, not circular inputs. The acknowledged short observed arc for 15 Mon (less than one-third of the fitted 108-yr period, Sec. 4.2) is a robustness concern and is correctly flagged by the author; it does not make the derivation circular.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

The central claims rest on a small set of modeling choices: a constant Gaia parallax zero point, a spatial covariance model, a distance prior, and the two-body Keplerian assumption. No new physical entities are introduced. The free parameters are the zero point and the hand-set cluster filter thresholds, with the isochrone age as a secondary choice.

free parameters (3)
  • Gaia DR2 parallax zero-point correction = 0.040 mas
    Added to the group parallax in Eq. 2; chosen as a literature average, but directly shifts the derived distances and, through the distance cubed, the masses.
  • Isochrone age = 3.2 Ma (Collinder 419), 1 Ma (NGC 2264)
    Hand-selected representative ages for clusters with O stars; sets the CMD membership band. Not fitted to parallax data; its effect on the distance is not explicitly tested.
  • Membership filter thresholds = Table 3 (RUWE<1.4, dCC<0.2, sigma_pi<0.1, r, r_mu, color range)
    Supervised choices; robustness tested by Monte Carlo over reasonable ranges, producing less than 1 microas change in group parallax, so they do not materially affect the central values.
assumptions (5)
  • standard math Kepler's third law relates orbital period, angular semi-major axis, distance, and total mass.
    Used in Section 4 to convert fitted P and a into MAa,Ab.
  • ad hoc to paper The Gaia DR2 parallax zero point is constant at 0.040 mas for all member stars in both clusters.
    Eq. 2 applies a single correction; literature reports a range of 0.029 to 0.050 mas, so this is a simplified model.
  • domain assumption The spatial covariance of Gaia DR2 parallaxes is described by the Lindegren et al. (2018) recipe and Campillay et al. (2019) Eq. 5.
    Section 2.1 states the covariance term dominates the distance uncertainties; if the model is wrong, the quoted errors are wrong.
  • domain assumption The young-disk distance prior of Maiz Apellaniz (2001, 2005) is appropriate for these clusters.
    Used to convert corrected parallax to distance; the prior is general, not tuned to these clusters.
  • domain assumption The relative motion of each visual binary is a single Keplerian orbit with negligible perturbation from other components, such as HD 193 322 B or the inner Ab1,Ab2 pair.
    The orbit fit in Section 4 assumes a two-body model; the paper does not model or test the effect of the known companions.

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Cite this review

Pith. "Pith review of Gaia DR2 distances to Collinder 419 and NGC 2264 and new astrometric orbits for HD 193 322 Aa,Ab and 15 Mon Aa,Ab." pith.science (2026). https://pith.science/paper/QB6DW3K7

@misc{pith2026190802040,
  author       = {Pith},
  title        = {Pith review of: Gaia DR2 distances to Collinder 419 and NGC 2264 and new astrometric orbits for HD 193 322 Aa,Ab and 15 Mon Aa,Ab},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QB6DW3K7}},
  note         = {Machine review of arXiv:1908.02040}
}
read the original abstract

[ABRIDGED] CONTEXT: Gaia DR2 has opened a trove of astrometric and photometric data for Galactic clusters close to the Sun. Lucky imaging has been an operational technique to measure the positions of visual binary systems for a decade and a half, enough to apply its results to the calculation of orbits of some massive multiple systems within ~1 kpc of the Sun. AIMS: We are measuring distances to Galactic stellar groups containing O stars and I start with two of them: Collinder 419 in Cygnus and NGC 2264 in Monoceros. I also aim to derive new astrometric orbits for the Aa,Ab components for the main ionizing sources for both clusters: HD 193 322 and 15 Mon, respectively. METHODS: First, I present a method that uses Gaia DR2 photometry, positions, proper motions, and parallaxes to obtain the membership and distance of a stellar group and apply it to Collinder 419 and NGC 2264. Second, I present a new code that calculates astrometric orbits by searching the whole 7-parameter orbit space and apply it to HD 193 322 Aa,Ab and 15 Mon Aa,Ab using as input literature data from the Washington Double Star Catalog (WDS) and the AstraLux measurements recently presented by Ma\'iz Apell\'aniz et al. (2019). RESULTS: I obtain Gaia DR2 distances of 1006+37-34 pc for Collinder 419 and 719+-16 pc for NGC 2264, with the main contribution to the uncertainties coming from the spatial covariance of the parallaxes. The two NGC 2264 subclusters are at the same distance (within the uncertainties) and they show a significant relative proper motion. The distances are shown to be robust. HD 193 322 Aa,Ab follows an eccentric (e = 0.58+0.03-0.04) orbit with a period of 44+-1 a and the three stars it contains have a total mass of 76.1+9.9-7.4 M_Sol. The orbit of 15 Mon Aa,Ab is even more eccentric (e = 0.770+0.023-0.030), with a period of 108+-12 a and a total mass of 45.1+3.6-3.3 M_Sol for its two stars.

Figures

Figures reproduced from arXiv: 1908.02040 by the authors.

Figure 1
Figure 1. Collinder 419 Gaia DR2 distances and membership results. Top row (left to right): source density diagram, DSS2 Blue image, and 2MASS J image. Middle row (left to right): proper motions, color-parallax, and magnitude-parallax diagrams. Bottom row (left to right): color-magnitude diagram, parallax histogram, and normalized-parallax histogram. In all diagrams a heat-type scale (increasing as white-yellow-orange-red-bla… view at source ↗
Figure 2
Figure 2. Same as [PITH_FULL_IMAGE:figures/full_fig_p013_2.png] view at source ↗
Figure 3
Figure 3. GOSSS spectrograms of HDE 228 911 and HDE 228 882. 14 [PITH_FULL_IMAGE:figures/full_fig_p014_3.png] view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: Plane-of-the-sky orbit plots for HD 193 322 Aa,Ab (left) and 15 Mon Aa,Ab (right). The mode (highest likelihood) orbit is shown as a thick black line, with a star marking the orbital center, a dashed line the line of nodes, and a cross the periastron. In both cases the…
Figure 5
Figure 5. Figure 5: Likelihood plots of every parameter pair for the orbital fitting of HD 193 322 Aa,Ab. The levels plotted range between 0.1% and 100% of the maximum collapsed (from the other five parameters) likelihood in each case using a linear scale. The cross marks the projection o…
Figure 6
Figure 6. Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p017_6.png]
Figure 7
Figure 7. Figure 7: Normalized separation (top) and position angle (bottom) residue plots for HD 193 322 Aa,Ab (left) and 15 Mon Aa,Ab (right). The vertical position shows the residue position in the normalized O-C sense (data-fit divided by the individual observed uncertainties) and the …

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