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REVIEW 3 major objections 5 minor 34 references

Precise astrometry of the quadruple protostar VLA1623–2417 shows component W is moving toward the A/B system at 1–2 km/s, ruling out an earlier ejection scenario.

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2026-08-02 02:06 UTC pith:HVP2WRUA

load-bearing objection A careful archival astrometry study that delivers the first proper motions for VLA1623-A and Aa/Ab and an order-of-magnitude improvement for B and W; the ejection-rule-out claim for W is probably right, but the W fit has a data-selection soft spot and one numeric inconsistency that need referee attention. the 3 major comments →

arxiv 2607.14428 v1 pith:HVP2WRUA submitted 2026-07-15 astro-ph.GA

Accurate proper motions of the protostellar system VLA1623-2417

Ricardo Hern\'andez Garnica (IRyA-UNAM) , Laurent Loinard (IRyA-UNAM , BHI-Harvard) , Carlos Carrasco-Gonz\'alez (IRyA-UNAM) , Jazm\'in Ord\'o\~nez-Toro (IRyA-UNAM) , Johanan Ram\'irez-Arellano (IRyA-UNAM) , Mar\'ia Jos\'e Maureira (MPE) , Isaac C. Radley (Leeds)
show 6 more authors
Eleonora Bianchi (INAF) Claire J. Chandler (NRAO) Luis F. Rodr\'iguez (IRyA-UNAM) Rosa M. Torres (CUCEI UdG) Aina Palau (IRyA-UNAM)
This is my paper
classification astro-ph.GA MSC 85A0485A35 PACS 97.10.Bt97.21.+a95.75.Mn
keywords proper motionsastrometryprotostarsVLA1623–2417multiple star systemsinterferometrycircumbinary diskjet
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper measures the three-dimensional motions of the four protostars in VLA1623–2417 over more than three decades using archival VLA and ALMA data, achieving order-of-magnitude improvements in proper-motion accuracy. The central result is that component W, previously suspected of being dynamically ejected from the A/B system, is actually moving toward A/B at 1–2 km/s. This inward motion rules out the ejection hypothesis and leaves two alternatives: W is either unbound and merely shares the cloud's velocity dispersion, or it is on a highly inclined, nearly edge-on orbit around the A/B pair. The paper also provides, for the first time, separate proper motions for the tight binary components Aa and Ab, showing orbital motion but with too short a time baseline to yield a reliable mass.

Core claim

The relative proper motions of the Class 0 protostar VLA1623–2417 W with respect to components A and B show that their projected separations are decreasing at a rate of about 1 to 2 km/s. This is inconsistent with scenarios in which W was dynamically ejected from the VLA1623–2417 system, because an ejected object would be moving apart from the system. Instead, the authors argue that W is either not gravitationally bound to the A/B system, or it is moving on a highly inclined orbit that makes its plane-of-sky motion point toward A/B. A highly eccentric orbit is regarded as unlikely because the system would repeatedly pass through unstable close configurations. The paper also reports the first

What carries the argument

The analysis rests on a careful astrometric reduction of 37 archival VLA and ALMA interferometric observations, plus published BIMA and SMA positions, spanning 34.5 years for B, 32.5 years for W, and about 11 years for Aa/Ab. Position errors are estimated by adding systematic, source- and frequency-dependent uncertainties that bring reduced chi-squared to unity. A key interpretive step is the identification of a low-frequency (cm) secondary peak west of W as a stationary one-sided jet shock; observations whose right-ascension resolution is worse than three times the W–shock separation are excluded from the W proper-motion fit to avoid contamination by free-free emission.

Load-bearing premise

The conclusion that W is moving inward depends on the interpretation that the low-frequency (cm) peak west of W is a stationary jet shock whose offset from the protostar is roughly constant, and that measurements with insufficient resolution to separate this peak were correctly excluded.

What would settle it

A future high-resolution observation that resolves the western peak and shows it to be moving away from W at a speed comparable to the suspected jet velocity, rather than remaining at a constant offset, would invalidate the stationarity assumption and could change the inferred proper motion of W.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • The ejection scenario for VLA1623–2417 W, previously proposed to explain its morphology and separation, is ruled out by the measured inward motion.
  • The absolute proper motions of VLA1623–2417 A, Aa, and Ab are now available for the first time, enabling future dynamical modeling of the A/B binary and the Aa/Ab pair.
  • The A/B system appears bound, with a total mass near 2 solar masses consistent with the relative motion of A and B.
  • If W is on a bound, inclined orbit, continued astrometric monitoring over decades could reveal orbital curvature and eventually yield a mass estimate for the whole system.
  • The detection of a one-sided jet from W suggests that even young, low-luminosity protostars can drive collimated outflows visible in cm free-free emission.

Where Pith is reading between the lines

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

  • If the western low-frequency peak is actually the stellar position or a moving knot rather than a stationary jet shock, the W right-ascension proper motion could be biased, and the inferred inward motion toward A/B would need re-evaluation.
  • The paper's method of combining multi-frequency archival data with a stationary-shock assumption could be applied to other embedded protostars where free-free emission contaminates astrometry, potentially revealing more systems with one-sided jets.
  • A natural extension would be to use the full 30-year baseline to search for acceleration in the relative motion of W with respect to A/B, which would distinguish between an unbound line-of-sight chance alignment and a bound, inclined orbit.
  • The mass discrepancy for the Aa/Ab pair (1.0 vs 0.4 solar masses depending on orbit orientation) suggests that detailed dynamical stability studies of triple systems could break the degeneracy between aligned and misaligned orbits.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 5 minor

Summary. The paper presents an astrometric analysis of the quadruple protostellar system VLA1623-2417 using 37 archival VLA/ALMA observations plus literature BIMA/SMA data, spanning ~11-34.5 years. The authors derive absolute proper motions for Aa, Ab, A (unresolved), B, and W, and relative proper motions for all pairs. The central claim is that W is approaching the A/B system at 1-2 km/s, ruling out a dynamical ejection scenario; instead, W is either unbound or on a highly inclined orbit. The paper also fits the Aa/Ab orbit with Orbitize!, finding degenerate solutions with masses 1.08 and 0.42 Msun, and explicitly cautions that the astrometry alone does not yield a reliable mass. The manuscript includes full position tables, images, and chi^2 diagnostics.

Significance. If the result holds, this is an important contribution: it provides the first absolute proper motion for VLA1623-2417A, order-of-magnitude improvements for B and W, and a direct test of the ejection hypothesis for W. The paper is commendable for publishing complete data tables and for using two independent relative-PM estimators. The orbit modeling is appropriately hedged, and the treatment of systematic errors is transparent even if debatable. However, the W proper-motion analysis relies on an unproven assumption about a stationary jet shock, and the printed B-W tangential velocity is internally inconsistent; both issues need to be resolved before the central claim can be accepted.

major comments (3)
  1. [§3.1.3, Table A2, Appendix B] This is the main technical concern; the assumption is plausible but not yet falsified.
  2. [Table 5] This is a concrete internal inconsistency that affects a quoted headline number.
  3. [§3.1, first paragraph] This is a standards-of-evidence concern rather than a fatal flaw.
minor comments (5)
  1. [Abstract] Grammar: 'Together, these data provides' should be 'provide'.
  2. [§3.1.3] The sentence 'the offset is always observed toward the west' is overstated: double peaks are reported for only two epochs. Rephrase to 'in the two epochs where double peaks are detected, the offset is toward the west'.
  3. [Table 5 and throughout] The columns labeled χ^2 are presumably reduced chi^2; please label them explicitly as χ_reduced^2.
  4. [§4.4] The statement that it would take 'only about 4,000 yr' for A/B and W to coincide assumes the current separation and constant relative speed; this is correct for ~1.7 km/s but should be phrased as an order-of-magnitude estimate.
  5. [Figure B2 caption] The caption states 'the centroid of the emission corresponds to VLA1623-2417 W,' which is an interpretation; consider wording that distinguishes measured peak from inferred stellar position.

Circularity Check

0 steps flagged

No significant circularity: proper motions are direct fits to measured positions; the jet-shock assumption is an auxiliary data-selection choice, not a fitted prediction.

full rationale

The paper's central results are absolute proper motions obtained by linear least-squares fits to measured source positions after standard gain-calibrator and parallax corrections. Relative proper motions are derived two independent ways—subtraction of absolute proper motions and direct fits to angular offsets—and the two agree within 1σ (Table 5). No parameter is defined in terms of the target result: the only tuning is the quadrature addition of systematic errors to make reduced χ²≈1, which affects quoted uncertainties, not the fitted values. The western-peak/shock interpretation in Sec. 3.1.3 is an auxiliary assumption used to select which X-band epochs enter the W fit; the W proper motion itself is fitted from the protostar positions, not from the shock-peak offsets, and the same inward W–A/B trend is present in the 11-yr ALMA/K/Q data alone (Table 4 and Fig. C1). Self-citations (e.g., Hernández Garnica et al. 2024) are methodological and not load-bearing; no uniqueness theorem is invoked. The paper explicitly flags its own limitations on the Aa/Ab orbit coverage and mass estimate. No circular step can be exhibited by reduction of an equation to its input.

Axiom & Free-Parameter Ledger

3 free parameters · 6 axioms · 2 invented entities

The paper is fundamentally empirical; it does not derive constants from theory. Main inputs are calibrated data, an assumed distance/parallax, solar motion, a linear-motion model, and a jet-contamination model. The only hand-fitted quantities are error floors and selection thresholds.

free parameters (3)
  • Per-source/per-band systematic position uncertainty = ~10-50 mas
    Added in quadrature to IMFIT errors; exact values chosen so that final linear fits have reduced chi^2 about 1 (Section 3.1). This sets the size of quoted proper-motion errors.
  • W X-band resolution exclusion threshold = 3x separation between W and western peak
    Hand-chosen cutoff: observations with RA resolution worse than 3 times the 0.33 arcsec W-shock separation were excluded from the W proper-motion fit (Section 3.1.3).
  • Orbitize! prior ranges = mass 0-2 M_sun; semi-major axis 20-75 au; phase 0.9-0.1 for second fit
    Chosen priors for the Aa/Ab orbit fits. The second solution uses a phase prior forcing periastron, and the reported masses (1.08 and 0.42 M_sun) depend on these choices (Section 4.2).
axioms (6)
  • domain assumption A linear and uniform proper-motion model describes each component's motion over the observed window.
    Used for all absolute PM fits (Section 3.1, Figure 2). Orbital curvature over long baselines could bias, though most components show consistency.
  • domain assumption All components are at the same distance d = 137.3 +/- 1.2 pc and share the reflex solar motion (U,V,W)_sun = (11.1,12.24,7.25) km/s.
    Converts mas/yr to km/s and separates peculiar from reflex motion (Sections 1 and 3.1).
  • domain assumption Gain calibrator catalog corrections and the adopted parallax 7.28 +/- 0.06 mas register positions to the barycentric frame.
    Basis of all absolute astrometry; errors in calibrator catalogs propagate directly.
  • ad hoc to paper The western low-frequency peak near W is a stationary jet shock, not the protostar or a moving knot.
    Used to identify and exclude contaminated X-band epochs and to explain the RA scatter of W (Section 3.1.3, Appendix B).
  • ad hoc to paper Free-free emission from jets contaminates low-frequency astrometry of A and W and is the cause of scatter.
    Justifies excluding C/X/Ku A-band data and selected X-band W data (Sections 3.1.1 and 3.1.3).
  • domain assumption Keplerian orbital mechanics and orbitize! priors are adequate for the Aa/Ab system.
    Used for orbit fitting; authors acknowledge degeneracy and unreliable mass (Section 4.2).
invented entities (2)
  • One-sided free-free jet from VLA1623-2417 W no independent evidence
    purpose: Explains low-frequency excess and the western secondary peak used to exclude data; also proposed as a physical jet.
    No independent detection outside the same morphology/spectral-index arguments (Radley et al. 2025); deep radio imaging would be a falsifiable test (Section 4.1).
  • Stationary western shock feature no independent evidence
    purpose: Defines a fixed astrometric reference to identify contaminated epochs.
    Assumed stationary based on constant separation in two epochs; if it moves, the W proper motion is biased (Section 3.1.3).

pith-pipeline@v1.3.0-alltime-deepseek · 25268 in / 16990 out tokens · 164118 ms · 2026-08-02T02:06:23.447777+00:00 · methodology

0 comments
read the original abstract

We present a detailed astrometric analysis of the quadruple protostellar system VLA1623-2417, deriving accurate absolute proper motions for components A, B, and W, as well as, for the first time, individual motions for the compact binary components of A (Aa and Ab). Our study combines 37 archival interferometric observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) at centimeter and millimeter wavelengths, supplemented with additional data from the Submillimeter Array (SMA) and the Berkeley-Illinois-Maryland Association (BIMA) millimeter array taken from the literature. Together, these data provides time baselines of ~34.5 years for component B, ~32.5 years for component W, and ~11 years for the individual components Aa and Ab. The relative proper motions of Aa/Ab indicate significant orbital motions, but cover too small a fraction of the orbit to provide a reliable mass estimate. The relative proper motions of W with respect to components A and B indicate that their projected separations are decreasing at a rate of 1 to 2 km/s. These inward motions are inconsistent with scenarios in which component W has been dynamically ejected from the VLA1623-2417 system. Instead, we argue that W is either not bound to the A/B components or is moving on a highly inclined orbit.

Figures

Figures reproduced from arXiv: 2607.14428 by Aina Palau (IRyA-UNAM), BHI-Harvard), Carlos Carrasco-Gonz\'alez (IRyA-UNAM), Claire J. Chandler (NRAO), Eleonora Bianchi (INAF), Isaac C. Radley (Leeds), Jazm\'in Ord\'o\~nez-Toro (IRyA-UNAM), Johanan Ram\'irez-Arellano (IRyA-UNAM), Laurent Loinard (IRyA-UNAM, Luis F. Rodr\'iguez (IRyA-UNAM), Mar\'ia Jos\'e Maureira (MPE), Ricardo Hern\'andez Garnica (IRyA-UNAM), Rosa M. Torres (CUCEI, UdG).

Figure 1
Figure 1. Figure 1: Image of the VLA 1623–2417 system at 100 GHz obtained with ALMA in 2023.62 (see [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2 [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Best fits to the relative positions of Aa and Ab, obtained using the Orbitize! package for the two possible solutions discussed in the text. In each case, the left panel displays the orbits projected onto the plane of the sky, while the right panels show the separation and position angle as functions of time. The red orbit corresponds to the parameters listed in [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Illustration of the plausible orbits of the Aa/Ab system discussed in the text. The background color image shows the sub-millimeter emission from ALMA and traces the dust emission in the system. Sources Aa and Ab are clearly seen, as well as their highly perturbed circumbinary disk. The orbit corresponding to a higher mass (red ellipse) is reasonably well aligned with the orientation of the circumbinary di… view at source ↗

discussion (0)

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