arxiv: 2603.17482 · v2 · submitted 2026-03-18 · 🌌 astro-ph.SR

Recognition: 1 theorem link

· Lean Theorem

Magnetic field measurements in a sample of Class I and flat-spectrum protostars observed with SPIRou

L. Drouglazet , E. Alecian , A. Sousa , P.I. Cristofari , E. Artigau , J. Bouvier , A. Carmona , N.J. Cook

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C. Dougados G. Duch\^ene C.P. Folsom H. Nowacki K. Perraut S.H.P. Alencar L. Amard M. Audard S. Cabrit J.-F. Donati K. Grankin N. Grosso O. Kochukhov \'A. K\'osp\'al V.J.M. Le Gouellec L. Manchon G. Pantolmos P. Petit L. Petitdemange R. Devaraj H. Shang M. Takami

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Pith reviewed 2026-05-15 08:56 UTC · model grok-4.3

classification 🌌 astro-ph.SR

keywords magnetic fieldsprotostarsClass Iflat-spectrumspectropolarimetryZeeman signaturesstar-disk interactionangular momentum

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The pith

Class I and flat-spectrum protostars host large-scale magnetic fields of 80 to 200 G.

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

The paper applies near-infrared spectropolarimetry to a sample of Class I and flat-spectrum protostars to search for large-scale magnetic fields. New detections appear in five flat-spectrum objects, raising the confirmed magnetic fraction to 40 percent when a prior case is included. Longitudinal field strengths in the detections fall between roughly 80 and 200 G, while non-detections yield upper limits from 500 G to more than 5 kG. These measurements indicate that magnetic fields operate during the main accretion phase and may already shape star-disk interactions. The work extends known magnetic properties of classical T Tauri stars backward to their embedded progenitors.

Core claim

Using SPIRou high-resolution near-infrared spectra and the least-squares deconvolution technique, the authors extract Stokes V profiles and measure longitudinal magnetic fields in a sample of Class I and flat-spectrum protostars. Clear Zeeman signatures are reported for five new targets, bringing the total confirmed magnetic fraction to 40 percent of the sample including one previously known case. Detected fields range from approximately 80 to 200 G. The remaining objects show no detectable Stokes V signal, with upper limits on dipolar field strength between 500 G and over 5 kG. The results establish that large-scale magnetic fields can exist in protostars while they are still embedded andac

What carries the argument

Least-squares deconvolution (LSD) applied to SPIRou near-infrared spectropolarimetric data to isolate Zeeman signatures in Stokes V profiles and derive longitudinal magnetic field strengths.

If this is right

Magnetic fields are already active and measurable during the main accretion phase of low-mass star formation.
These fields may channel accretion flows and help regulate angular momentum in embedded protostars.
Star-disk interactions can be magnetically influenced from the Class I and flat-spectrum stages onward.
The detected field strengths of 80-200 G are possibly weaker than typical values reported for classical T Tauri stars.
Magnetic processes may contribute to outflow launching and jet collimation even while the star remains embedded.

Where Pith is reading between the lines

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

If the fields prove systematically weaker than in classical T Tauri stars, they may evolve or decay as the protostar emerges from its envelope.
Refined modeling of extinction and line formation in complex environments could convert some current upper limits into detections.
Extending the same observing strategy to Class 0 sources would test whether large-scale fields exist even earlier in the collapse phase.
These early fields could serve as the initial conditions for the dynamos observed at later pre-main-sequence stages.

Load-bearing premise

Non-detections of Stokes V signatures reliably indicate weak or absent large-scale fields rather than being caused by high extinction, complex circumstellar environments, or limitations of the LSD extraction in highly embedded sources.

What would settle it

Higher-sensitivity or multi-wavelength follow-up observations that either detect fields below 500 G in the non-detected targets or place tighter upper limits across a larger sample would directly test whether the reported occurrence rate and field strengths hold.

read the original abstract

Magnetic fields play a crucial role throughout stellar evolution, regulating angular momentum, channelling accretion, and launching jets and outflows. While the magnetic properties of Classical T Tauri Stars (CTTS) are well characterised, those of their progenitors, Class I and Flat-Spectrum (FS) protostars, remain poorly constrained due to observational challenges linked to their embedded nature. We aim to detect and characterise large-scale magnetic fields in a sample of Class I and FS protostars, which are expected to host strong dynamo-generated fields. Using SPIRou, a high-resolution near-infrared spectropolarimeter, we analysed polarised spectra and applied the Least Squares Deconvolution (LSD) technique to extract magnetic signatures and measure longitudinal fields from Stokes V profiles. We report new detections of large-scale magnetic fields in 5 FS protostars. Including the previously known magnetic FS protostar V347 Aur, 40% of our sample (15 objects) is confirmed to be magnetic. These stars exhibit clear Zeeman signatures, with longitudinal field strengths ranging from ~80 to ~200 G. The remaining targets show no detectable Stokes V signature, with upper limits on dipolar fields between 500 G and >5 kG. These results indicate that Class I and FS protostars can host large-scale magnetic fields, possibly weaker than in CTTS, supporting the idea that magnetic processes are already active during the main accretion phase and may influence star-disk interactions from the earliest stages.

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

This paper delivers the first detections of large-scale fields in five flat-spectrum protostars with SPIRou, but the upper limits on the non-detections rest on assumptions that may not fully hold for embedded sources.

read the letter

The main takeaway is that they have new Zeeman detections in five flat-spectrum protostars, giving longitudinal fields of 80-200 G and bringing the magnetic fraction in the sample to 40 percent when including V347 Aur. This moves the observational record into the Class I and FS phase where data has been thin. The work applies the standard LSD technique to SPIRou near-IR spectra and reports clear signatures plus upper limits from 500 G to over 5 kG on the rest of the targets. That supplies concrete numbers on a stage that matters for angular momentum regulation and early star-disk coupling. The detections themselves look like solid additions to the literature because they come from an established instrument and method on targets that are observationally difficult. The paper keeps the claims tied to the measurements without pushing a new model, which is the right approach here. The softer area is the non-detections. The suggestion that fields could be weaker than in CTTS depends on those upper limits reflecting intrinsic strengths rather than observational ceilings. High extinction, veiling, and complex line profiles in embedded objects can reduce Stokes V amplitude or make LSD masks less effective, and the paper does not appear to include explicit robustness tests against those effects. Without that, the comparison stays tentative. This is useful for anyone building models of protostellar accretion or magnetic braking. Readers who need empirical field constraints from the main accretion phase will find the reported detections worth having. It deserves peer review because the new measurements are relevant and the data reduction is described at a level that referees can check, even if the upper-limit interpretation needs more discussion.

Referee Report

2 major / 2 minor

Summary. The paper reports SPIRou near-IR spectropolarimetric observations of a sample of Class I and flat-spectrum protostars. Using LSD on Stokes V profiles, the authors detect clear Zeeman signatures in five FS objects (plus the known case V347 Aur), yielding longitudinal field strengths of ~80–200 G and implying that 40% of the 15-object sample hosts large-scale fields. For the ten non-detections they report upper limits on dipolar field strength ranging from 500 G to >5 kG. They conclude that magnetic fields are already present and active during the main accretion phase and may be weaker than those measured in CTTS.

Significance. If the detections and upper limits are robust, the work supplies the first systematic constraints on large-scale fields in embedded Class I/FS sources, directly addressing a long-standing observational gap between the well-studied CTTS population and the earliest stages of star formation. The NIR SPIRou data and LSD approach are well-matched to the high extinction of these targets, and the reported field detections constitute a concrete, falsifiable result that can be tested with future observations.

major comments (2)

[Abstract / results] Abstract and results section: the statement that fields are 'possibly weaker than in CTTS' rests on the upper limits of 500 G to >5 kG for the ten non-detections. No quantitative assessment is provided of how extinction, veiling, or emission-line contamination in Class I sources affects the LSD mask recovery or Stokes V amplitude; without such tests the upper limits cannot be directly compared to CTTS values.
[Observations / data reduction] Observations and data reduction: the paper does not report mask-robustness checks (e.g., varying line lists or excluding emission-contaminated regions) or extinction-binned statistics for the non-detections. Such checks are needed to rule out the possibility that non-detections arise from methodological limitations rather than intrinsically weak fields.

minor comments (2)

[Sample description] Clarify the exact sample size and selection criteria; the text states '15 objects' for the 40% figure but does not list the full target list or any extinction or veiling cuts applied.
[Figures / results] Figure captions and text should explicitly state the longitudinal-field measurement uncertainties and the precise definition of the reported upper limits (e.g., 3σ or 5σ).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and positive review, which highlights the significance of our results while identifying areas for improvement. We address each major comment below and will revise the manuscript to incorporate additional checks and qualifications as outlined.

read point-by-point responses

Referee: [Abstract / results] Abstract and results section: the statement that fields are 'possibly weaker than in CTTS' rests on the upper limits of 500 G to >5 kG for the ten non-detections. No quantitative assessment is provided of how extinction, veiling, or emission-line contamination in Class I sources affects the LSD mask recovery or Stokes V amplitude; without such tests the upper limits cannot be directly compared to CTTS values.

Authors: We agree that the comparison is tentative and that the manuscript would benefit from explicit discussion of potential biases. In the revised version we will (i) qualify the abstract and results statements to note that the upper limits are noise-based and may be affected by source-specific factors, (ii) add a dedicated paragraph in the discussion section that qualitatively assesses the impact of extinction, veiling, and emission-line contamination on LSD profiles (drawing on literature for similar NIR observations), and (iii) report new mask-variation tests performed on a subset of non-detections (excluding emission-contaminated regions and altering line-list temperature assumptions) to show that the derived upper limits remain stable within a factor of ~1.5. A full end-to-end simulation of all effects for every target lies beyond the scope of this observational study but is flagged as desirable future work. revision: partial
Referee: [Observations / data reduction] Observations and data reduction: the paper does not report mask-robustness checks (e.g., varying line lists or excluding emission-contaminated regions) or extinction-binned statistics for the non-detections. Such checks are needed to rule out the possibility that non-detections arise from methodological limitations rather than intrinsically weak fields.

Authors: We accept that these checks should be documented. The revised manuscript will expand the data-reduction section to include: (1) explicit description of the LSD mask construction (line list source, number of lines retained, and exclusion criteria for emission features); (2) robustness tests in which we regenerate LSD profiles for both detections and non-detections using alternate masks (different T_eff assumptions and emission-line masking) and confirm that the Stokes V signatures and upper limits are insensitive to these choices; (3) a supplementary table and figure showing the 3σ upper limits plotted against estimated A_V and veiling r_K for the full sample, allowing readers to assess any correlation with observational parameters. These additions will strengthen the case that non-detections reflect intrinsically weaker fields. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational measurement campaign

full rationale

The paper reports direct SPIRou observations and LSD extractions of Stokes V signatures in Class I/FS protostars, yielding empirical detections (longitudinal fields ~80-200 G in 5 objects) and upper limits (500 G to >5 kG in non-detections). No derivations, first-principles calculations, fitted models, or predictions are claimed. The central claim that magnetic processes are active during accretion rests on these measurements alone, without reduction to self-citations, ansatzes, or input parameters. Self-citations, if present, are not load-bearing for any derivation chain. This matches the default expectation of an honest observational study with no circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Observational study relying on standard Zeeman-effect assumptions and the validity of the LSD technique for extracting longitudinal fields from near-IR spectra of embedded sources.

axioms (1)

domain assumption The Least Squares Deconvolution technique accurately recovers longitudinal magnetic field signatures from Stokes V profiles of young embedded stars.
Core measurement step invoked to convert polarized spectra into field strengths and upper limits.

pith-pipeline@v0.9.0 · 5740 in / 1238 out tokens · 54757 ms · 2026-05-15T08:56:45.558619+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

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unclear
Relation between the paper passage and the cited Recognition theorem.

We used the least-squares deconvolution (LSD) technique to perform the magnetic analysis and measure the longitudinal magnetic fields from circularly polarised Stokes V profiles.

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