Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk

A. Brunthaler; A. Hafner; A. M. Jacob; D. Riquelme; H. Beuther; H. Wiesemeyer; J. D. Soler; J. R. Dawson; L. Anderson; M. Busch

REVIEW 3 major objections 4 minor 116 references

Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk

T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read New deep 18-cm OH observations show hydroxyl column density scales linearly with molecular hydrogen across diffuse and dense gas.

desk verdict A solid, transparent observational follow-up that pushes OH absorption to lower column densities with genuinely new data, but whose central slope and abundance claims are partly conditional on a constant CO-to-H2 conversion factor. read the letter →

arxiv 2506.06149 v2 pith:W72AUZZ6 submitted 2025-06-06 astro-ph.GA

M. R. Rugel , H. Beuther , J. D. Soler , P. Goldsmith , L. Anderson , A. Hafner , J. R. Dawson , Y. Wang

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S. Bihr H. Wiesemeyer R. Guesten M.-Y. Lee D. Riquelme A. M. Jacob W.-J. Kim M. Busch S. Khan A. Brunthaler

This is my paper

classification astro-ph.GA

keywords OH18cmlinesinterstellarmediumCO-darkgasmoleculartracersGalacticdiskhydroxylabundanceabsorptionspectroscopyW43-South

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 sets out to determine whether the 18-cm ground-state hyperfine transitions of hydroxyl (OH) can serve as a reliable tracer of molecular hydrogen, especially in gas where carbon monoxide emission is weak or absent. Using deep VLA absorption spectra toward four bright continuum sources, combined with [CII] 158 $\mu$m, HI 21 cm, and CO data, the authors measure OH column densities down to $3.7\times10^{13}\,\mathrm{cm^{-2}}$ and compare them with H$_2$ and total hydrogen column densities. They find $N_{\rm OH}$ is linearly proportional to $N_{\rm H_2}$ (power-law exponent $1.0\pm0.1$) and to total hydrogen column density $N_{\rm H}$ ($1.2\pm0.1$), with median abundances $X_{\rm OH,H_2} = 1.2\times10^{-7}$ and $X_{\rm OH,H} = 4.8\times10^{-8}$. The result matters because it would make OH absorption a quantitative tracer of molecular gas across diffuse and dense environments, while the detection of only one CO-dark feature out of 23 shows OH is not an exclusive tracer of CO-dark gas.

What carries the argument

The load-bearing object is the ground-state hyperfine-splitting quartet of OH at 18 cm: the 1612, 1665, 1667, and 1720 MHz transitions. The 1667 MHz line is used for column densities through $N_{\rm OH}/T_{\rm ex} = C_0/f \int \tau\,dv$ (with $C_0 = 2.24\times10^{14}\,\mathrm{cm^{-2}\,K^{-1}\,km^{-1}\,s}$) assuming $T_{\rm ex} = 5\,\mathrm{K}$ and filling factor $f = 1$. Molecular hydrogen columns are derived from $^{12}$CO emission with a fixed conversion factor $\alpha_{\rm CO} = 2\times10^{20}\,\mathrm{cm^{-2}}\,(\mathrm{K\,km\,s^{-1}})^{-1}$ and $R_{21/10} = 0.7$. The linear fits are made in log space with the York (1966) method, accounting for errors in both coordinates.

What would settle it

Measure $N_{\rm H_2}$ toward these sightlines independently of CO, e.g., from dust optical depth or HI self-absorption, and re-fit $N_{\rm OH}$ vs $N_{\rm H_2}$; if the slope departs from $1.0$ by more than the quoted uncertainties, the linear-abundance claim fails.

Watch

Extended reading notes

Core claim

The central claim is that OH 18 cm absorption traces molecular gas linearly across environments spanning column densities $N_{\rm H_2} \sim 7.9\times10^{19}$ to $4.7\times10^{22}\,\mathrm{cm^{-2}}$. Combining new observations with THOR data, the authors fit a power law $N_{\rm OH} \propto N_{\rm H_2}^{1.0\pm0.1}$ and $N_{\rm OH} \propto N_{\rm H}^{1.2\pm0.1}$, and quote median abundances $X_{\rm OH,H_2} = 1.2^{+0.3}_{-0.2}\times10^{-7}$ and $X_{\rm OH,H} = 4.8^{+0.9}_{-0.8}\times10^{-8}$. They interpret the single CO-free absorption component and several intermediate molecular gas fractions as evidence that OH is present in both diffuse and dense gas; they conclude OH can trace molecular gas but never exclusively CO-dark gas.

Load-bearing premise

The H$_2$ column densities rest on a single CO-to-H$_2$ conversion factor ($\alpha_{\rm CO} = 2\times10^{20}\,\mathrm{cm^{-2}}\,(\mathrm{K\,km\,s^{-1}})^{-1}$ and $R_{21/10} = 0.7$); if that factor varies with environment, especially in diffuse gas, the quoted slopes and abundances would shift.

Editorial extensions

If this is right

OH 18 cm absorption can be used as a quantitative column-density tracer for molecular gas across diffuse and dense ISM environments.
The single CO-dark detection out of 23 means future surveys need deeper integrations or brighter background sources to find CO-dark gas in absorption.
The linear $N_{\rm OH}$--$N_{\rm H_2}$ relation supports using OH as a complementary cross-check on CO-based H$_2$ masses.
The abundance values provide observational anchors for chemical models of diffuse molecular clouds.
The satellite-line offsets toward W43-South support the expanding-shell scenario for OH inversion features around HII regions.

Reading between the lines

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

If the linear relation holds at still lower columns, OH absorption could probe the atomic-to-molecular transition itself, where CO fails.
Testing $N_{\rm H_2}$ via dust optical depth or HCO$^+$ on the same sightlines would show whether the fixed $\alpha_{\rm CO}$ assumption is responsible for the scatter.
The apparent lack of Galactocentric abundance trend suggests OH formation is robust to variations in radiation field, which could make OH a stable tracer across the disk.

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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

Solid, careful follow-up paper. The genuinely new content is the data: factor-of-5 deeper VLA OH spectra at 0.1 km/s toward four THOR continuum sources, plus new SOFIA [CII] and APEX CO (2-1) for two of them. That lets the authors resolve narrow, weak OH components down to N_OH ~ 3.7e13 cm^-2 and extend the N_OH-N_H2 relation about an order of magnitude below the THOR sample. The one CO-dark component out of 23 and the marginal [CII] association toward G31.388-0.383 are modest but honestly presented. The W43-South satellite/main-line offset is a nice resolved observation, even if the interpretation leans on Hafner et al.

The analysis is standard and transparent: line-to-continuum optical depths, fixed T_ex=5 K and T_spin=100 K with stated factor-of-two uncertainties, orthogonal regression that accounts for errors in both variables. The derived OH abundances agree with Liszt & Lucas and other prior work. Credit where due: the paper states its caveats rather than burying them, and the choice to use 12CO rather than 13CO for N_H2 is explicitly motivated.

The soft spot is exactly what the stress-test flags. The headline slope m_H2=1.0±0.1 and the abundances rest on constant alpha_CO=2e20 and R21/10=0.7. The authors acknowledge a factor-two uncertainty, and they even note that alpha_CO may be underestimated at N_H2=0.8e20 and that CO-dark gas would steepen the relations. That admission is the load-bearing issue: a factor-of-two gradient in alpha_CO across the 2.5-dex range would shift the fitted slope by about 0.1, comparable to the quoted error, and a larger gradient would move both slope and abundances beyond the formal errors. So the linearity is partly an assumption about conversion constancy, not a direct measurement of N_OH against true H2 column. I do not think this kills the paper; the abundance range and the qualitative conclusion that OH traces molecular gas across environments survive. But the precision on the slope is softer than the quoted ±0.1. Also, "never be an exclusive tracer" is a strong sentence for four sightlines; the 1/23 CO-dark fraction is consistent with prior work, but target selection toward weak-CO sources deserves more quantification.

The [CII] column estimate depends on assumed T_kin and n, and the 18 km/s association is plausible but not unambiguous. Minor relative to the above.

Bottom line: useful, honest dataset with carefully caveated conclusions. It deserves a serious referee. The main revision is to reframe the slope as conditional on constant alpha_CO and R21/10, and to soften or better support the exclusivity claim. I would cite this for the OH abundance values and the new low-column detections. Send it to review; it is a conditional-accept paper, not a desk reject.

Referee Report

3 major / 4 minor

Summary. The paper presents new, high-sensitivity VLA observations of the four 18 cm ground-state hyperfine-splitting transitions of OH in absorption toward four continuum sources, complemented by VLA HI 21 cm absorption, APEX CO (2–1) emission, and SOFIA upGREAT [CII] 158 μm and [OI] 63 μm spectra. The authors derive OH column densities from the 1667 MHz line (assuming T_ex = 5 K), HI column densities from 21 cm absorption (T_spin = 100 K), and H2 column densities primarily from 12CO emission using a fixed alpha_CO = 2e20 cm^-2 (K km/s)^-1 and R_21/10 = 0.7. Comparing N_OH with N_H2 and N_H for their new sample combined with THOR data, they fit power-law relations and report N_OH ∝ N_H2^1.0±0.1 and N_OH ∝ N_H^1.2±0.1, with median abundances X_OH,H2 = 1.2e-7 and X_OH,H = 4.8e-8. They detect only one OH absorption component out of 23 without a CO counterpart, and report a tentative [CII] association toward one sightline plus excitation anomalies in W43-South.

Significance. The observations are a clear improvement over the THOR survey in both sensitivity (factor of 5) and spectral resolution (0.1–0.2 km/s), allowing the detection of narrow, faint OH absorption features down to N_OH = 3.7e13 cm^-2. If the reported linear N_OH–N_H2 relation and abundances are robust, they provide useful constraints on OH chemistry and on the utility of OH absorption as a molecular-gas tracer across diffuse and dense environments. The paper is commendably transparent about its assumptions, and Table A.1 provides a complete set of measured integrals that will enable future modeling. However, the central quantitative claims—especially the slope m_H2 = 1.0±0.1—rest on the assumed constancy of the CO-to-H2 conversion, and the paper does not quantify how plausible environmental variations of alpha_CO would affect that slope.

major comments (3)

[Sect. 3.2 and 4.2, Fig. 3] The central result that N_OH is linearly proportional to N_H2 with slope m_H2 = 1.0±0.1 is derived from N_H2 values obtained with a fixed alpha_CO = 2e20 cm^-2 (K km/s)^-1 and a fixed R_21/10 = 0.7. The quoted ±0.1 uncertainty is the statistical fit uncertainty only; it does not include the systematic effect of a possible environmental dependence of alpha_CO or R_21/10. In the diffuse, low-N_H2 regime specifically targeted by this paper (N_H2 ~ 8e19–1e21 cm^-2), alpha_CO is expected to be larger than the adopted value, and the authors themselves note in Sect. 5 that alpha_CO may be significantly underestimated at N_H2 = 0.8e20 cm^-2 and that CO-dark gas would imply steeper relations. A factor-of-two gradient in alpha_CO across the ~2.5 dex range of the fit changes the fitted slope by ~0.1, equal to the quoted uncertainty, and a larger gradient would move both the slope and the abundances beyond the quoted errors. The authors' counterargument in Sect. 5—that no trend of abundance with Galactocentric radius is seen—does not directly constrain a trend with N_H2. I therefore recommend that the paper either quantify the systematic uncertainty on m_H2 arising from plausible alpha_CO and R_21/10 variations (e.g., by refitting with an alpha_CO that increases at low N_H2 or by using an independent H2 tracer such as dust), or explicitly state that the linear slope is conditional on a constant conversion factor.
[Sect. 4.2, Fig. 3, Sect. 6] The conclusion that N_OH is linearly correlated with the total hydrogen column density N_H with exponent m_H = 1.2±0.1 is conditional on an assumption about saturated HI absorption. For the 12 non-saturated data points alone, the Spearman rank correlation has r_s = 0.6 with p = 0.08, meaning the null hypothesis of no correlation cannot be rejected at the current sample size. The m_H = 1.2±0.1 value is obtained only after assuming that N_H2 dominates in the saturated cases, and this important caveat is not carried into the conclusion in Sect. 6, where the relation is stated without qualification. The authors should either rephrase this as a model-dependent inference supported by the saturation assumption, or add a clear statement of the significance of the correlation on the unsaturated subsample.
[Sect. 3.2 and Table A.1] The combined sample used for the power-law fits does not rely on a single, homogeneous N_H2 estimator. For most line-of-sight features, N_H2 is derived from 12CO emission with a constant alpha_CO, whereas for features associated with HII regions (e.g., the 97.8 and 102.8 km/s components toward G29.957−0.018, and the 99.0 km/s component toward G29.935−0.053) N_H2 is adopted from Rugel et al. (2018), who used 13CO(1–0) with an assumed excitation temperature, an optical-depth correction, and an isotope-ratio rescaling. The authors themselves find in Sect. 3.2 that 13CO-based N_H2 is systematically lower by a factor of two or more compared to 12CO-based values for most columns. Since the HII-region points occupy the high-N_H2 end of the fitted range, a systematic offset or nonlinearity in the 13CO-based calibration could bias the fitted slope. I ask the authors to test the robustness of m_H2 by fitting only the 12CO-based sample, or by demonstrating that the slope is unchanged when the 13CO-based points are excluded.

minor comments (4)

[Sect. 2.2] There is a duplicated word in the sentence describing the upGREAT instrument: "which observes simultaneously and and comprises" should read "which observes simultaneously and comprises".
[Sect. 4.2] The phrase "This is holds true if we assume a lower spin temperature for N_Hi" is grammatically incorrect; it should be "This holds true if we assume...".
[Throughout] The word "extra-galactic" is used several times (e.g., in the abstract and in Sect. 2.1); the standard astronomical spelling is "extragalactic".
[Sect. 4.4] In the sentence "...it is over-posed by potential absorption, baseline ripples, and Galactic emission," the word "over-posed" appears to be a typo; "overlapped" or "dominated" would convey the intended meaning.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the fitted OH–H2 and OH–H correlations are measured from the new data, not forced by the adopted calibrations or by prior self-citations.

full rationale

The paper's central derivation chain is self-contained with respect to the circularity definitions. N_OH is computed from the OH 1667 MHz optical depth via Eq. (1) with a fixed excitation temperature (Tex = 5 K) and a pre-factor C0 taken from independent Einstein coefficients (Turner 1966; Goss 1968; Stanimirović et al. 2003). N_H2 is computed in Sect. 3.2 from 12CO integrated emission using an external standard conversion factor alpha_CO = 2e20 cm^-2 (K km/s)^-1 and R21/10 = 0.7 (Bolatto et al. 2013; Sakamoto et al. 1997), not from OH. The reported power-law slopes m_H2 = 1.0 ± 0.1 and m_H = 1.2 ± 0.1 are obtained in Sect. 4.2 by fitting the measured column-density pairs with the York (1966) algorithm; they are outputs of a regression, not identities imposed by the column-density equations. The self-citations to THOR (Rugel et al. 2018) and the reuse of some N_H2 estimates from that work provide comparison data and extend the sample, but the paper explicitly rescales N_OH to a uniform excitation temperature and refits the relation rather than importing the earlier slope ('To improve on previous estimates in Rugel et al. (2018)...'). The acknowledged caveat that alpha_CO may be underestimated at low N_H2 and that CO-dark gas would imply slightly steeper relations is a calibration-uncertainty/robustness concern, not a circular reduction: the H2 column is not defined in terms of OH, nor is the OH column defined in terms of H2. The only CO-dark detection rate and abundance claims are empirical comparisons of independent tracers. No load-bearing argument reduces, by the paper's own equations, to a fitted parameter renamed as a prediction or to a self-citation chain. The one possible minor self-citation (adopting prior THOR column densities) is data reuse rather than a circular premise, so the appropriate score is at the low end of the 'no significant circularity' band.

Assumptions & free parameters 8 free parameters · 6 assumptions · 0 invented entities

The paper's quantitative claims rest on a chain of adopted calibration parameters (OH T_ex, HI T_spin, alpha_CO, R_21/10, f=1) rather than on new theory or fits; these are taken from prior literature and carry factor-of-two uncertainties that propagate into all abundances and slopes. No new physical entities are introduced. Because the parameters are external calibrations and not fitted to the target result, the circularity burden is low even though the absolute numbers are model dependent.

free parameters (8)

OH excitation temperature T_ex = 5 K
Assumed uniform for all OH absorption components in Eq. (1); scales N_OH and all abundance values. Authors assign a factor-of-two uncertainty.
HI spin temperature T_spin = 100 K
Adopted for absorbing atomic gas in Eq. (2); scales N_HI and total hydrogen column. Factor-of-two uncertainty acknowledged.
CO-to-H2 conversion factor alpha_CO = 2e20 cm^-2 (K km/s)^-1
Standard Milky Way value from Bolatto et al. (2013); directly sets N_H2 and therefore X_OH,H2 and the correlation slopes.
12CO(2-1)/(1-0) ratio R21/10 = 0.7
Used to rescale APEX CO(2-1) to CO(1-0) equivalent; affects N_H2.
OH filling factor f = 1
Assumed unity in Eq. (1); if absorbing gas is clumpy, N_OH would be underestimated.
Kinetic temperature for [CII] abundance = 100 K
Assumed in Eq. (3) for the single [CII] column density estimate toward G31.388-0.383.
Volume density for [CII] abundance = 10^3 cm^-3
Assumed in Eq. (3); affects derived N_C+ but not the central OH result.
13CO excitation temperature for HII-region features = 20 K
Adopted from Rugel et al. (2018) for features associated with HII regions; affects two features in the sample.

assumptions (6)

domain assumption Line-to-continuum ratios trace optical depth via F_line/F_cont = e^-tau, with negligible OH and HI emission after continuum subtraction.
Invoked in Sect. 3.1; requires absorbing gas to be in front of the background continuum source and emission to be resolved out by the interferometer.
domain assumption OH column density formula N_OH = C0 T_ex ∫ tau dv with filling factor 1 applies to all components.
Eq. (1); standard for optically thin OH, but requires constant T_ex and no clumping.
domain assumption CO emission traces molecular hydrogen with a constant conversion factor across the sampled environments.
Sect. 3.2; the basis for N_H2 and all molecular abundances.
domain assumption The 12CO(2-1)/(1-0) ratio is 0.7 in all observed clouds.
Sect. 3.2, used to homogenize APEX CO(2-1) data with CO(1-0) surveys.
domain assumption HI spin temperature of 100 K represents the cold neutral medium along these sightlines.
Sect. 3.1, Eq. (2).
ad hoc to paper The chosen velocity integration intervals isolate individual kinematic components.
Table A.1 defines 23 intervals by inspection; different choices could change component blending.

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Cite this review

Pith. "Pith review of Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk." pith.science (2026). https://pith.science/paper/W72AUZZ6

@misc{pith2026250606149,
  author       = {Pith},
  title        = {Pith review of: Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/W72AUZZ6}},
  note         = {Machine review of arXiv:2506.06149}
}

read the original abstract

The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new absorption line observations of OH at 18-cm wavelength towards four continuum sources. We compare these to the [CII] line at 1.9 THz obtained with SOFIA, observations of the neutral atomic hydrogen 21 cm line with the VLA, and CO lines obtained with APEX. We trace OH over a large range of molecular hydrogen column densities, and derive OH abundances with respect to molecular and total hydrogen column densities. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features. We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions. A potential association of [CII] 158 mu m emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears in agreement with previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it will never be an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions.

Figures

Figures reproduced from arXiv: 2506.06149 by the authors.

**Figure 1.** Top: THOR 1.4 GHz continuum emission in the Galactic coordinate frame (Wang et al. 2020). The sources discussed here are located at the center of the yellow circles. Bottom: VLA 1.4 GHz continuum towards the four positions analyzed in this work (blue crosses) in equatorial coordinates. The images are smoothed to an angular resolution of 18′′ . references therein), which also enables star formation (Glover & Clark 20… view at source ↗

**Figure 2.** OH and H i absorption towards the extragalactic source G31.388−0.383. The first five panels show optical depths of the four OH lines and H i, as derived from the line-to-continuum ratio. For the H i absorption, blue triangles indicate saturated channels. Emission spectra of the three CO isotopologues are shown below that. The data is taken with APEX and from the GRS (Jackson et al. 2006) survey. The bottom most pane… view at source ↗

**Figure 3.** OH column density vs. the column density of H i (left), H2 (middle), and the column density of the total number of hydrogen nuclei (right). Features associated with line-of-sight absorption are shown in blue, features associated with H ii regions in black (Rugel et al. 2018). The middle panel shows 𝑁OH vs. 𝑁𝐻2 . Lower limits in case of saturated H i absorption in the left and right plots are indicated by triangles p… view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p007_4.png]

**Figure 5.** Figure 5: Distribution of molecular gas fraction in this work (green) in comparison to the sample from THOR (blue). Many of the features included in both samples are upper limits on the molecular gas fraction, as the H i absorption saturate. We hence highlight the sub-sample of …

**Figure 6.** Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_6.png]

**Figure 7.** Figure 7: Distribution of VLA 18 cm and GLIMPSE 8 𝜇m continuum emission (top row Benjamin et al. 2003) in G29.935−0.053, with contours of ATLASGAL 870 𝜇m emission (black; at 0.5, 1.0, 1.5 and 2 Jy beam−1 Schuller et al. 2009) and VLA 18 cm continuum (white; in steps of 0.1 Jy b…

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

, " * write output.state after.block = add.period write newline

ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all := #1 ...

[2] [2]

write newline

" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....

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