Measuring the magnetic field of young stars using iSHELL observations: BP Tau and V347 Aur

Adwin Boogert; Bo Reipurth; Christian Flores; Michael S. Connelley

REVIEW 3 major objections 5 minor 1 cited by

Measuring the magnetic field of young stars using iSHELL observations: BP Tau and V347 Aur

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Fitting high-resolution near-infrared spectra with a magnetized radiative-transfer model recovers stellar parameters to within 91 K and measures the first magnetic field of the protostar V347 Aur at 1.36 kG.

desk verdict The first magnetic field measurement of the class I protostar V347 Aur is a real result, but the quoted error bars understate the systematic floor and the solar-calibrated line list needs an independent check at cool, low-gravity parameters. read the letter →

arxiv 1908.08583 v1 pith:UU4RAGQW submitted 2019-08-22 astro-ph.SR

Christian Flores , Michael S. Connelley , Bo Reipurth , Adwin Boogert This is my paper · ORCID

classification astro-ph.SR

keywords youngstellarobjectsmagneticfieldsZeemaneffectinfraredspectroscopypre-main-sequenceevolutionstarspotsTTauristarsprotostars

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

The paper establishes that high-resolution near-infrared K-band spectra can be modeled with a magnetized radiative-transfer code to recover the fundamental parameters of young stars in one self-consistent fit. The method is validated on nine main-sequence and giant stars plus the Sun, giving temperature uncertainties of 91 K and gravity uncertainties of 0.14 dex, with a magnetic detection limit of 0.31 kG. Applied to the well-studied class II star BP Tau, it recovers a surface field of 2.5 kG in agreement with earlier Zeeman studies, while its lower temperature points to starspots dominating the infrared. Applied to the class I protostar V347 Aur, it yields the first measurement of its magnetic field, 1.36 kG, with log g = 3.25. If correct, these results extend magnetic-field measurements to the earliest observable protostellar stage and show that starspot contamination must be handled when deriving masses and ages.

What carries the argument

The Zeeman effect is the load-bearing mechanism: in a magnetic field, a spectral line's components separate by an amount proportional to lambda squared times the Lande factor times the field strength, so line widths carry the magnetic signal. Six selected K-band windows include lines with high magnetic sensitivity, such as Ti lines, and lines with low sensitivity, such as CO rovibrational transitions, and all six are fitted simultaneously. The fitting chain combines a plane-parallel LTE magnetic synthesis code, model atmospheres, an empirically measured instrument profile, and a Markov chain Monte Carlo parameter search to recover temperature, gravity, field strength, rotation, microturbulence, veiling, and CO abundance together.

What would settle it

Fit the V347 Aur spectrum with a different atmospheric model grid or with line parameters that were not solar-calibrated; if the recovered field drops to the 0.31 kG detection limit, the 1.36 kG detection was an artifact of the calibration assumptions. Alternatively, obtain an independent spectropolarimetric or optical Zeeman measurement of V347 Aur; disagreement beyond the quoted uncertainties would falsify the transferability of the method.

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Extended reading notes

Core claim

The paper claims that the broad wavelength coverage of a high-resolution K-band spectrograph, combined with a magnetic radiative-transfer code that includes Zeeman broadening, is sufficient to determine the fundamental parameters of young stars in one consistent fit. Validated against nine main- and post-main-sequence stars and the Sun, the method returns effective temperatures with a scatter of 91 K and surface gravities with a scatter of 0.14 dex, and it sets a detection limit of 0.31 kG. Applied to the class II star BP Tau, it recovers a surface magnetic field of 2.5 kG, consistent with earlier optical Zeeman studies, but a temperature about 400 K cooler, which the authors attribute to starspot emission dominating the K band. Applied to the class I protostar V347 Aur, it yields the first measurement of that object's field, 1.36 kG, together with log g = 3.25, and it argues that dropping the magnetic term from the fit inflates rotation and turbulence estimates while degrading the fit.

Load-bearing premise

The solar-calibrated changes to 26 atomic line parameters are physically real and carry over to the cooler, lower-gravity photosphere of V347 Aur, rather than absorbing errors in the atmospheric models, radiative transfer, or instrument profile.

Editorial extensions

If this is right

Magnetic field measurements can be extended to class I protostars, the youngest pre-main-sequence phase, with a uniform parameter fit rather than assumed temperatures and gravities.
Because Zeeman broadening scales as wavelength squared, the near-infrared approach can detect weaker fields than optical studies, making it a route to a larger protostellar magnetic-field sample.
The 400 K difference between optical and infrared temperatures of BP Tau means single-temperature masses and ages of spotted young stars are uncertain by up to roughly a factor of two, depending on the evolutionary model.
Nonmagnetic fits to strongly magnetic young stars overestimate projected rotation and microturbulence, so magnetic terms cannot be ignored when measuring rotation in such objects.

Reading between the lines

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

If starspots systematically depress infrared temperatures, then masses derived from infrared-only fits to heavily spotted young stars are biased low unless spot filling factors are modeled explicitly.
The method's detection limit suggests that a survey of class I protostars using the same six spectral windows could map the distribution of fields at the earliest ages and test whether V347 Aur's relatively weak 1.36 kG field is typical.
A direct extension would be to fit the same spectra with two-temperature spotted models; the difference between the single-temperature and two-temperature recovered fields would indicate how much of the Zeeman broadening is masked by spot contrasts.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Short version: the genuinely new result is the first field measurement of V347 Aur, 1.36 kG, for a class I protostar. I think the detection is real, but the quoted error bars are smaller than the actual systematic uncertainty, and the solar-calibrated line list is the main thing to worry about.

The paper does real work. It measures iSHELL's spectral profile, recalibrates 26 K-band lines against solar spectra, validates on nine standard stars with 91 K and 0.14 dex scatter, recovers BP Tau's known 2.5 kG field, and runs a nonmagnetic control that forces v sin i and microturbulence to absorb some of the Zeeman broadening. That is a careful, honest setup. The authors flag the line-list caveat themselves in Appendix B in plain terms, which I credit. The citation pattern looks fine and engages the relevant YSO Zeeman literature.

The two soft spots are real and should be fixable. First, the 0.31 kG null-field floor is stated as a detection limit but never added to the B uncertainties. For V347 Aur that means the true uncertainty is closer to 0.3 kG than 0.05 kG, so the detection is roughly 4 sigma, not 20 sigma. Still detected, but the abstract's error bars overstate precision. Second, the line-list adjustments are calibrated on the Sun and include large van der Waals changes on exactly the Ti I and Ca I lines that carry the Zeeman signal. The standard-star sample does not cover the cool, low-gravity combination of V347 Aur (Teff ~ 3230 K, log g ~ 3.25), where van der Waals broadening is much stronger, so there is no direct check that the recalibration transfers. The nonmagnetic control helps, but it uses the same line list. This is a concern, not a fatal flaw; 1.36 kG is plausible and still above the floor even with the floor added in quadrature.

Minor point: the starspot interpretation for BP Tau's cool NIR temperature is reasonable but not uniquely established, and the authors say as much. The evolutionary-model discussion is illustrative and appropriately hedged.

This paper is for observers working on YSO magnetic fields and near-infrared parameter pipelines. It deserves a serious referee, though conditional acceptance is the right outcome: the B uncertainty should include the 0.31 kG systematic floor, and the line-list transfer should either be justified with a cool, low-gravity benchmark or discussed more cautiously.

Referee Report

3 major / 5 minor

Summary. The paper presents a method for deriving stellar parameters and surface magnetic fields of young stars by fitting iSHELL K-band spectra with the MoogStokes Zeeman-broadening code. The method is validated on nine main- and post-main-sequence stars plus the Sun, giving a temperature scatter of 91 K and a gravity scatter of 0.14 dex, and a null-field detection floor of 0.31 kG. Applied to BP Tau, the method recovers B = 2.5 kG, consistent with previous literature. For the class I source V347 Aur, the paper reports the first magnetic field measurement (B = 1.36 kG), along with Teff = 3233 K and log g = 3.25, and shows in a nonmagnetic control test that excluding B degrades the fit to the magnetically sensitive lines while increasing v sin(i) and microturbulence. The paper also discusses the influence of starspots on masses and ages derived from pre-main-sequence evolutionary tracks.

Significance. If the calibration concerns are resolved, this is a valuable contribution: it extends Zeeman-broadening measurements to the K band with broad wavelength coverage, provides an externally consistent BP Tau benchmark, and delivers a new class I magnetic field measurement for V347 Aur. The empirical instrumental profile, the tabulated line-list modifications, the standard-star validation, and the nonmagnetic control test are explicit and reproducible steps that give the method a solid empirical base. The main risk is systematic rather than statistical: the solar-calibrated line parameters and the 0.31 kG null-field floor must be shown to transfer to V347 Aur's cool, low-gravity parameters before the reported 1.36 kG can be taken at face value.

major comments (3)

[Section 4.5 and Table 5] The quoted uncertainties for V347 Aur's magnetic field (B = 1.36+0.06-0.05 kG in Table 5) are posterior percentiles only and do not include the 0.31 kG null-field floor established in Section 4.5. Because that floor is the strongest field recovered on stars where a null field was expected, it should be added in quadrature as a systematic uncertainty, or the authors should demonstrate that it does not apply at V347 Aur's Teff = 3233 K and log g = 3.25. Even after adding the floor the detection remains significant, but the reported precision would be materially different and the paper should present the combined error budget.
[Appendix B, Table 8] The V347 Aur detection relies on the Zeeman broadening of Ti I and Ca I lines whose log(gf) and van der Waals constants were adjusted on the Sun (Table 8). Some VdW changes are large, up to about 1.2 dex (e.g., Ti I 22627.394 Å), and because van der Waals broadening is stronger in a cool, low-gravity atmosphere, an error absorbed into these constants at solar conditions could mimic or cancel the Zeeman width at V347 Aur's parameters. The authors themselves caution in Appendix B that the adjustments 'could be hiding defects associated with the stellar atmospheric models, the radiative transfer code, or even the measured instrument spectral profile.' I ask for a concrete external check: fit a standard star near V347 Aur's Teff and log g that was not used in the solar calibration, or explicitly propagate a plausible range of VdW errors into the B measurement, to quantify the resulting systematic uncertainty.
[Section 5.2 with Section 4] The statement that V347 Aur's parameters 'are contained within the range of stellar parameters we investigated in Section 4' is inaccurate for temperature: the coolest standard star in Table 4 is GJ 436 at 3401 K, while V347 Aur is 3233 K, and no standard star combines Teff below about 3400 K with log g around 3.2. The null-field floor of 0.31 kG in Section 4.5 was measured on warmer giants and on the Sun. The transfer of both the adjusted line list and the detection limit to V347 Aur is therefore an extrapolation and should be identified as such, or supported by additional anchor points.

minor comments (5)

[Section 3.1] The text says 'we adopted, for simplicity, a solar composition in all our models,' but Section 3.3 and Table 4 treat [M/H] as a free parameter; please clarify that 'solar composition' refers to element abundance ratios rather than a fixed metallicity.
[Equation (1) and surrounding text] There is a missing space before 'λ' in the sentence below Equation (1), and the wavelength unit is given as microns there while other parts of the paper use Ångströms; please harmonize the notation.
[Table 8] Please state the units and sign convention for the 'Waals' column and provide the origin of the default values, so that the modifications are fully reproducible.
[Section 6.1] The sentence 'if the covering factor ... iw known' contains a typo; it should read 'is known.'
[Table 4 note] The note says that actual errors are significantly larger than the tabulated formal uncertainties, but the paper does not state the final combined uncertainty for each standard star; consider adding a column or equation showing how sigma_Teff = 91 K, sigma_logg = 0.14, and the 0.31 kG magnetic floor combine with the MCMC errors.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation: the magnetic fields are free parameters in MoogStokes, validated against external standard-star benchmarks; the V347 Aur result is not forced by any fitted input.

full rationale

The derivation chain is self-contained. The magnetic field is a free parameter in the MoogStokes radiative transfer code (Section 3.1: 'MoogStokes assumes a radial and uniform magnetic field in the radiative transfer computation'), and the method is validated on nine main- and post-main-sequence stars with independent Teff and log(g) determinations plus the Sun before being applied to BP Tau and V347 Aur. BP Tau (B = 2.5 kG) is an external benchmark checked against Johns-Krull et al. (1999, 2007), not an input. The V347 Aur detection (B = 1.36 kG) is judged against the empirically defined null-field floor Blimit = 0.31 kG (Section 4.5), so the detection is not forced by a prior. The only internal calibration is the adjustment of 26 VALD3 line parameters against solar spectra (Appendix B); the authors explicitly caution that this 'could be hiding defects associated with the stellar atmospheric models, the radiative transfer code, or even the measured instrument spectral profile.' That is a transferability risk rather than circularity, because the Sun is an external calibrator and the YSO spectra are not part of that fit. Self-citations (e.g., Connelley & Greene 2010; Reipurth 2008) provide spectral classification and distance context and are not load-bearing for the magnetic field measurement. No equation reduces to its own input, and no fitted parameter is renamed as a prediction.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The central B measurements rest on the physical Zeeman effect, MARCS model atmospheres, and the MoogStokes uniform-field assumption. The only fitted inputs beyond the stellar parameters themselves are the solar-calibrated line-list modifications, the CO abundance scaling, and the veiling, none of which is independently validated for the YSO regime; the solar-calibrated modifications carry the largest risk because they directly affect the line profiles used for the magnetic measurement.

free parameters (3)

Modified line transition parameters (26 K-band lines) = See Table 8
log(gf) and van der Waals constants were manually adjusted until the synthesized solar spectrum matched iSHELL observations; the paper acknowledges these may absorb model defects, which would propagate into derived stellar parameters.
CO abundance scaling = Fit, not listed in Table 5
A parameterization of all processes affecting CO formation and destruction; fitted simultaneously for every star, effectively a fudge factor in the line fits.
IR K-band veiling r_K = BP Tau 1.08, V347 Aur 0.97
Fitted to account for circumstellar excess emission; partially degenerate with metallicity and affects line depths, though it is a physically motivated parameter.

assumptions (4)

standard math The Zeeman effect with effective Landé g factors from the adopted line lists describes the magnetic broadening in Stokes I (Eq. 1)
Physical basis for converting line-profile broadening into a magnetic field strength; standard atomic physics.
domain assumption MARCS 1D hydrostatic LTE model atmospheres with solar composition represent the photospheres of the YSOs
Adopted for all models, with metallicity fixed to [M/H] = 0 for the young stars (Section 5.1); deviations from solar abundances or LTE could bias the fits.
domain assumption MoogStokes' assumption of a radial, uniform magnetic field is sufficient to recover the average surface field strength
The models fit a single B value; real stellar fields are complex, as the paper itself notes, so the recovered value is an average that could depend on field geometry.
ad hoc to paper The solar-calibrated modified line list is valid for cool, low-gravity stars such as V347 Aur
The paper explicitly cautions in Appendix B that the modifications could hide defects in the atmospheric models, radiative transfer code, or instrument profile; applying them to very different stars is an unverified transfer.

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Cite this review

Pith. "Pith review of Measuring the magnetic field of young stars using iSHELL observations: BP Tau and V347 Aur." pith.science (2026). https://pith.science/paper/UU4RAGQW

@misc{pith2026190808583,
  author       = {Pith},
  title        = {Pith review of: Measuring the magnetic field of young stars using iSHELL observations: BP Tau and V347 Aur},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/UU4RAGQW}},
  note         = {Machine review of arXiv:1908.08583}
}

abstract

While it has been suggested that there is a connection between the magnetic properties and the internal structure of young stars, there have not been enough magnetic measurements to firmly establish such a correlation at the earliest ages. Here, we contribute to this endeavor by presenting stellar parameters and magnetic field strength measurements of BP Tau and V347 Aur, both stars observed with the near-infrared spectrograph iSHELL. We first test the accuracy of our method by fitting synthetic stellar spectra to a sample of nine main and post-main-sequence stars. We report uncertainties of $\sigma_{\rm Teff}$ = 91 K in temperature and $\sigma_{\rm log(g)}$ = 0.14 in gravity. We then apply the modeling technique to BP Tau and measure a surface magnetic field strength of $\langle \rm B \rangle $ = 2.5$^{+0.15}_{-0.16}$ kG, confirming literature results. For this star, however, we obtain a much lower temperature value than previous optical studies ($\Delta \rm T \sim 400$ K) and interpret this significant temperature difference as due to the relatively higher impact of starspots at near-infrared wavelengths than at optical wavelengths. We further apply this technique to the class I protostellar source V347 Aur and measure for the first time its magnetic field strength $\langle \rm B \rangle = $ 1.36$^{+0.06}_{-0.05}$ kG and its surface gravity log(g) = 3.25$^{+0.14}_{-0.14}$. Lastly, we combine our measurements with pre-main-sequence stellar evolutionary models and illustrate the effects produced by starspots on the retrieved masses and ages of young stars.

Figures

Figures reproduced from arXiv: 1908.08583 by the authors.

**Figure 1.** Sample of M dwarfs in order of descending temperature. The shaded green regions illustrate the six wavelength ranges we used to fit our models to the data. GJ 380 Teff = 3 970 K, GJ 411 Teff = 3 604 K, GJ 412A Teff = 3 579 K, GJ 526 Teff = 3 555 K, GJ 436Teff = 3 401 K (see [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗

**Figure 2.** Sample of main- and post-main-sequence stars in order of descending temperature. 14 Her Teff = 5 430 K, TYC1293- 2421-1 Teff = 5 059 K, EPIC211304446 Teff = 4 856 K, BD+004988 Teff = 4 500 K [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗

**Figure 3.** Comparison between the observed spectrum of the M-dwarf GJ380 (in red) and the best fit model (in black). All the panels in the figure have the same bandwidth. temperature and gravity ranges of low-mass young stars (Doppmann et al. 2005; Baraffe et al. 2015; Feiden 2016). The main differences between the evolved stars and the low-mass young stars are their rotational velocities, magnetic field strength values, and … view at source ↗

Figures from the paper (10 more)

**Figure 4.** Figure 4: Comparison between the observed spectrum of the red giant TYC1293-2421-1 (in red) and the best fit model (in black). summarize the stellar parameters we recovered using our modeling approach. 4.1. Gravity Measurements The first stellar parameter of the sample of standa…

**Figure 6.** Figure 6: Comparison between the literature effective temperatures vs. our derived effective temperatures for the sample of main- and post-main-sequence stars. The solid black line corresponds to a one-to-one correlation between the model and the literature parameters. The das…

**Figure 7.** Figure 7: Comparison between the literature metallicities [M/H] vs. our derived metallicities for the sample of mainand post-main-sequence stars. The solid black line corresponds to a one-to-one correlation between the model and the literature parameters. The dashed gray lines…

**Figure 8.** Figure 8: Comparison between the observed spectrum of BP Tau (in red) and our best-fit model (in black) from MoogStokes. The six green panels show the six wavelength regions we used to fit the spectrum of the young star. the temperatures and gravities of several class I sources.…

**Figure 9.** Figure 9: Same as [PITH_FULL_IMAGE:figures/full_fig_p015_9.png]

**Figure 10.** Figure 10: Comparing the position of BP Tau and the class I source V347 Aur in an HR diagram with different stellar evolutionary models. The first row shows the evolutionary models from Feiden (2016), while the second row shows the model from Baraffe et al. (2015). The positions…

**Figure 11.** Figure 11: Spectral resolution as a function of wavelength measured from individual arc-lamp lines in the K2 mode of iSHELL. We fit a Gaussian model to each arc line to determine the wavelength position and spectral resolution R. The red solid line corresponds to the median of t…

**Figure 12.** Figure 12: The iSHELL spectral profile in the K2 mode 0. 00375 and the 0. 0075 slit widths. The left (right) box shows the measured 0. 00375 (0. 0075) spectral profile in black solid line. The dashed orange line corresponds to the best fit with a Voigt (Voigt∗BoxCar) profile. Th…

**Figure 13.** Figure 13: Upper panel: comparison between the iSHELL solar observations (blue thick line), the MoogStokes model with the default VALD3 line transition parameters (dashed red line), and the MoogStokes model after we modified the line transition parameters (solid black line). The…

**Figure 14.** Figure 14: Same as [PITH_FULL_IMAGE:figures/full_fig_p022_14.png]

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Forward citations

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Pith tools

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[2] [2]

write newline

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[3] [3]

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2017

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