{"id":"c197f430-a5ab-464d-a84d-e927600f4194","arxiv_id":"1908.02992","paper_version":1,"verdict":"ACCEPT","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"New Mo and Ru abundance measurements in 209 and 162 disc stars show that galactic chemical evolution models underproduce these elements across the disc metallicity range.","lead":"This paper reports the first large set of molybdenum and ruthenium abundance measurements in Milky Way disc stars, covering metallicities from slightly below solar to slightly above. The data show that current models of galactic chemical evolution underproduce both elements, pointing to missing nucleosynthesis sources.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Zero-point of [Mo/Fe] and [Ru/Fe] rests on VALD gf plus an external solar reference; gf errors do not cancel, so a 0.1–0.2 dex offset could erase the claimed GCE underproduction.","rationale":"The reader's weakest_assumption targets LTE/NLTE effects, and that is a legitimate concern. However, the paper itself argues (Section 4 and Conclusions) that NLTE corrections should be negligible in the disc sample and would not affect the main finding, because the discrepancy is already visible in the disc. The unexamined zero-point issue is more directly load-bearing: the derived [X/Fe] values assume that VALD gf values place stellar and solar abundances on the same absolute scale, but the solar reference is external (Asplund 2009), so gf errors propagate into every [Mo/Fe] and [Ru/Fe] measurement. Since the claimed underproduction offsets (0.1–0.4 dex) are comparable to typical log gf uncertainties for transition-metal lines, a systematic gf offset could reconcile the models with observations, especially for Mo. The paper is a valuable observational contribution with careful parameter cross-checks, per-star errors, and three independent GCE model sets, and the abundance sample is clearly novel. But the central conclusion should be conditional on an absolute-scale calibration check. If the proposed solar-fit test shows a gf zero-point offset smaller than ~0.1 dex, the original ACCEPT verdict would stand; if larger, the quantitative statement of underproduction would need to be revised downward or qualified. I therefore recommend CONDITIONAL rather than outright rejection, because the concern is concrete and testable without invalidating the dataset as a whole.","tokens_in":35810,"tokens_out":13778,"duration_ms":145401,"concrete_test":"Fit the same Mo I 5506/5533 and Ru I 4080/4584/4757 lines in a high-quality solar spectrum (e.g., the Kurucz solar flux atlas) using the VALD gf, the same LTE synthesis code, and a Castelli & Kurucz solar model. Compare the derived solar log A(Mo) and log A(Ru) with the adopted Asplund et al. (2009) values (1.88 and 1.75). If the offsets exceed 0.10 dex, subtract them from all stellar [Mo/Fe] and [Ru/Fe] values and redo the comparison with the Prantzos et al., Travaglio et al., and OMEGA+ model curves. If the corrected underproduction falls below the combined systematic uncertainty (~0.15 dex), the central claim would need to be revised.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim (Section 5) that existing GCE models underproduce Mo and Ru in the Galactic disc depends on the zero-point of the observed [Mo/Fe] and [Ru/Fe] values. The stellar abundances are derived by LTE spectral synthesis using log gf values adopted from the VALD database, while the solar reference is the external Asplund et al. (2009) abundances. Because the Sun is not analyzed with the same lines and gf, systematic errors in the absolute oscillator strengths do not cancel in the solar-relative normalization emphasized in Section 4. A uniform gf error of 0.1–0.2 dex enters all stellar [X/Fe] values directly. The reported underproduction is 0.1–0.4 dex (Section 4, bottom panels of Figs. 7 and 8), so a gf zero-point error of this size could substantially reduce or eliminate the discrepancy, particularly for Mo where the best models fall only about 0.1 dex below the data. The error budget in Table 3 includes atmospheric parameter uncertainties and fitting errors but omits gf uncertainty, and no solar calibration of the adopted gf is reported. Thus the claimed underproduction is not securely established until the absolute abundance scale is validated.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents new LTE abundances of molybdenum and ruthenium for 209 and 162 F-, G-, and K-type stars in the Galactic disc, derived from two Mo I and three Ru I lines. The authors report [Mo/Fe] and [Ru/Fe] trends over -1.0 < [Fe/H] < +0.3, compare these with literature data for halo stars, and juxtapose the combined dataset with published Galactic chemical evolution models (Prantzos et al. 2018, Travaglio et al. 2004, and OMEGA+ models). Their central claim, stated in Section 5, is that existing GCE models with standard s-process, r-process, and p-process sources underproduce Mo and Ru in the Galactic disc, implying additional nucleosynthesis contributions such as LEPP, the i-process, or rare stellar events.","tokens_in":35977,"tokens_out":4465,"duration_ms":48282,"significance":"If the abundance scale is secure, this is a valuable observational contribution: it provides the first extended disc sample for Mo and Ru, cross-checks stellar parameters against nine literature studies, gives per-star and systematic errors, and compares with several independent GCE model sets without fitting any model parameters to the new data. The conclusion that standard nucleosynthesis sources underproduce Mo and Ru is a falsifiable constraint on r-process sites, LEPP, and the i-process. The main caveat, discussed below, is that the absolute zero-point of the [Mo/Fe] and [Ru/Fe] measurements is not independently validated, and this zero-point enters directly into the claimed discrepancy.","major_comments":[{"comment":"The central underproduction claim depends on the absolute zero-point of [Mo/Fe] and [Ru/Fe]. The abundances are derived using log gf values from the VALD database and normalized to the external solar abundances of Asplund et al. (2009), but the Sun is not analyzed with the same lines and gf values in this paper. A uniform error of 0.1-0.2 dex in the adopted gf therefore does not cancel in the solar-relative normalization and shifts all stellar [Mo/Fe] and [Ru/Fe] values directly. The reported discrepancies are about 0.1 dex for Mo and 0.2-0.4 dex for Ru (bottom panels of Figs. 7 and 8), so a zero-point offset of this size could substantially reduce or remove the discrepancy, especially for Mo. The error budget in Table 3 includes atmospheric-parameter and fitting errors but omits gf uncertainty. Please add a solar analysis using the same lines and gf values, or an equivalent independent calibration, and propagate the resulting zero-point uncertainty into the GCE comparison.","section":"Section 3, Section 4, Table 3, Figs. 7-8"},{"comment":"The paper states that no NLTE calculations for Mo or Ru are currently available and argues that NLTE corrections should be negligible and 'leveled using our analysis relative to the Sun.' Because the paper does not actually analyze the Sun, the leveling argument is not demonstrated from the presented data. Neutral-species NLTE effects can be metallicity- and temperature-dependent, and if they vary by ~0.1 dex across the sample, the shape of the [X/Fe] trends and the magnitude of the underproduction could change. I request either a quantitative estimate of possible NLTE corrections for these lines or a clear statement, with supporting line-formation arguments, of why the corrections should be constant across the sample. This is important because the conclusion in Section 5 explicitly relies on the LTE-based discrepancy.","section":"Section 4 and Section 5"},{"comment":"The systematic error budget is estimated from only two stars, HD154345 and HD82106, which have similar parameters (Teff around 5500 K and 4800 K, both near solar metallicity). The sample spans Teff from about 4400 to 6200 K and [Fe/H] down to about -1.0, and line-formation sensitivity can vary across this range. A two-star estimate may not capture the systematic uncertainty for the cooler or more metal-poor stars. This does not invalidate the main claim, but the stated 'average error of 0.14 dex' should be presented as a lower bound, or the error analysis should be extended to a wider parameter range.","section":"Section 3.1, Table 3"}],"minor_comments":[{"comment":"The text says that for HD 22879 'our upper limit for [Ru/Fe] is consistent' with Hansen et al. (2014), but Table 4 lists ours as >0.51, which is a lower limit, not an upper limit. Please correct the wording or the symbol.","section":"Section 4, Table 4"},{"comment":"Table A2 shows individual atmospheric-parameter differences as large as Delta(Teff) = -380 K and Delta([Fe/H]) = -0.39 for HD 224930 when compared with Takeda (2007). The general statement of 'good agreement' should be qualified or these outliers discussed, since they are larger than the nominal parameter uncertainties.","section":"Table A2 and Section 2"},{"comment":"The caption describes the OMEGA+ curves as 'marked with black dot, dashed and solid line'; the wording is ambiguous. Please specify which line style corresponds to the short-delay-time and delay-time-distribution setups as in the legend.","section":"Figure 7 caption"},{"comment":"The abstract states an average error of 0.14 dex, while Section 3.1 reports a range of 0.12 to 0.16 dex for Ru and Mo. Please define the abstract value as the average of the quoted range or give per-element average errors.","section":"Abstract and Section 3.1"},{"comment":"The adopted line list for the five Mo I and Ru I lines is not tabulated. For reproducibility, please provide the wavelengths, excitation potentials, and log gf values for the lines used, along with the line-list source version.","section":"Section 3"}],"recommendation":"major_revision","confidential_remarks":"The paper is a solid observational contribution and the GCE comparison is largely appropriate; the independent Prantzos et al. (2018) model already shows Mo underproduction, so the OMEGA+ co-authorship is not a circularity problem. The main obstacle is the unvalidated abundance zero-point, which is a standard and fixable issue for a major revision. I see no concerns about novelty or scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThis paper gives you the first extended sample of Mo and Ru in disc stars: 209 Mo and 162 Ru measurements across [Fe/H] from –1 to +0.3. That alone fills a genuine gap. The abundance analysis is careful for an LTE study: two or three weak lines per element, spectral synthesis with custom line lists, stellar parameters cross-checked against nine literature studies, per-star errors, a parameter-sensitivity table, and a full data table in the appendix. They also compare with three GCE model sets, including the independent Prantzos et al. (2018) models and their own OMEGA+ runs, and the discussion of what could fix the underproduction (LEPP, i-process, r-process site variations) is measured.\n\nThe soft spot is the zero-point. The stellar abundances are derived with VALD log gf values, but the solar reference is the external Asplund et al. (2009) abundance—not a solar spectrum analyzed with the same lines. So gf errors do not cancel in [Mo/Fe] or [Ru/Fe], contrary to what the 'leveled using our analysis relative to the Sun' passage in Section 4 implies. If the adopted gf values are uniformly off by 0.1–0.2 dex, the disc abundance trends shift by that amount. For Mo, where the best model sits only about 0.1 dex below the data, that could largely erase the claimed discrepancy. Table 3 does not include gf uncertainty. For Ru, the offset is larger (0.2–0.4 dex), so the qualitative conclusion likely survives. The same underproduction was already present in metal-poor star data and in Prantzos et al., so the paper is not standing on the disc data alone.\n\nMinor issues: the LTE assumption is unverified (no NLTE calculations for Mo or Ru exist), and the systematic error budget is estimated from only two stars. Both are acknowledged in the text, so they are not hidden.\n\nOverall, this is solid observational work worthy of a serious referee. The referee should ask for a solar calibration of the adopted gf values, or at least a quantitative estimate of the gf uncertainty; without that, the Mo conclusion should be softened. I would bring it to a reading group interested in neutron-capture abundances or GCE.\n\nRegards.","headline":"First extended disc sample of Mo and Ru is a useful observational step, but the claimed underproduction for Mo is sensitive to an unvalidated gf zero-point tied to an external solar reference.","tokens_in":36594,"tokens_out":3895,"would_cite":true,"duration_ms":42435,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"New measurements of molybdenum and ruthenium in 209 and 162 Galactic disc stars show that present chemical evolution models underproduce both elements at all disc metallicities, pointing to a missing nucleosynthesis source such as LEPP…","keywords":["molybdenum","ruthenium","neutron-capture elements","Galactic chemical evolution","Galactic disc","s-process","r-process","LEPP"],"falsifier":"Compute non-LTE corrections for the Mo I 5506/5533 Å and Ru I 4080/4584/4757 Å lines across the stellar parameter range of the sample ($-1.0 < \\mathrm{[Fe/H]} < +0.3$, $T_{\\mathrm{eff}} \\sim 4600$–$6200$ K) using updated atomic data, and re-derive the disc abundances; if the corrected $\\mathrm{[Mo/Fe]}$ and $\\mathrm{[Ru/Fe]}$ values at the metal-poor end shift up by more than about 0.2 dex, the model underproduction claimed here could be a measurement artifact rather than a missing nucleosynthesis source.","tokens_in":35583,"feed_emoji":"🌌","tokens_out":6939,"duration_ms":65124,"temperature":0.7,"pith_summary":"This paper measures the abundance of the neutron-capture elements molybdenum (Mo) and ruthenium (Ru) in 209 and 162 stars of the Milky Way disc, the first large sample covering metallicities from $\\mathrm{[Fe/H]} = -1.0$ to $+0.3$. It compares these measurements with several Galactic chemical evolution (GCE) simulations that include the standard nucleosynthesis sources: the slow and weak s-process, the rapid r-process, and the p-process. The central finding is that every model considered underproduces Mo and Ru relative to the observed disc abundances, by roughly 0.1 to 0.4 dex. The paper concludes that the disc requires an additional, not-yet-included nucleosynthesis source, such as the lighter-element primary process (LEPP), the intermediate i-process, or rare stellar events. A sympathetic reader would care because this pinpoints a concrete gap in the current inventory of where heavy elements are made.","feed_headline":"Galactic models underproduce molybdenum and ruthenium","feed_subtitle":"New abundances from 200 disc stars show a missing nucleosynthesis source beyond the usual s- and r-processes.","key_machinery":"The observational machinery is LTE spectral synthesis of two Mo I lines (5506, 5533 Å) and three Ru I lines (4080, 4584, 4757 Å) in high-resolution, high-signal-to-noise disc-star spectra, yielding abundances for 209 and 162 stars respectively with an average error of 0.14 dex. The theoretical machinery is a suite of Galactic chemical evolution (GCE) simulations, including a two-zone open-source model, that fold in stellar yields from AGB stars, massive stars, supernovae, and r-process events. The comparison works by plotting $\\mathrm{[Mo/Fe]}$ and $\\mathrm{[Ru/Fe]}$ against $\\mathrm{[Fe/H]}$ and overlaying model evolution tracks, which exposes the systematic underproduction. The paper also uses the isotopic decomposition of Mo and Ru (p-only, s-only, and r-only isotopes) as a diagnostic for which processes would need to be added, notably LEPP, the lighter-element primary process, and the i-process, an intermediate neutron-capture process.","core_discovery":"The paper's central claim is that the canonical stellar sources of heavy elements—AGB stars making the main s-process, massive stars making the weak s-process, neutron-star mergers or magneto-rotational supernovae making the r-process, and explosive p-process sites—do not produce enough molybdenum and ruthenium to match what is observed in Galactic disc stars. Even the most Mo-rich and Ru-rich GCE simulations, including those that add a LEPP component or vary the timing of r-process enrichment, remain below the data at essentially all metallicities in the disc. The paper further shows that Mo and Ru do not correlate tightly with each other in disc stars, while Ru scatters more against Ba and Eu, suggesting that Ru is substantially produced by an extra source that is probably not an s-process source because Mo and Ru receive similar s-process contributions. The conclusion is that the origin of Mo and Ru remains open, and that new stellar sites or processes must be added to chemical evolution calculations.","pith_inferences":["A natural extension not explored in the paper is to apply non-LTE corrections to the same lines; if those corrections are metallicity-dependent, the slope of $\\mathrm{[Mo/Fe]}$ versus $\\mathrm{[Fe/H]}$ would change and might erase part of the claimed shortfall.","The paper's logic implies that isotopic abundance patterns in meteorites or presolar grains (for example anomalies in $^{95}$Mo or $^{97}$Mo) could discriminate between an i-process and a LEPP origin for the missing Mo, since each process leaves a different isotope fingerprint.","The same LTE spectral-synthesis approach is commonly used for the neighbouring first-peak elements Sr, Y, and Zr; if non-LTE effects matter for Mo I and Ru I, they may also bias those elements, so the missing-source puzzle could extend across the whole first peak.","A testable extension would be to measure Mo and Ru in dwarf galaxies with similar metallicity spread; because their star-formation histories differ from the disc, they would separate contributions from prompt massive-star and delayed merger r-process sources."],"forward_implications":["Current GCE simulations need an additional source of Mo and Ru beyond main and weak s-process, standard r-process, and p-process to match disc observations.","The decoupling of Ru from Mo, Ba, and Eu implies that the extra Ru source must produce ruthenium without proportionally enriching molybdenum, which rules out a simple scaled s-process enhancement.","At $\\mathrm{[Fe/H]} < -0.2$, the timing of r-process enrichment (short delay versus a $t^{-1}$ delay distribution) changes predicted Mo and Ru levels, so these elements become new constraints on the delay times of neutron-star mergers.","The large scatter in $\\mathrm{[Mo/Eu]}$ and $\\mathrm{[Mo/Fe]}$ at low metallicity supports enrichment by rare, stochastic events rather than by numerous ordinary supernovae, motivating inhomogeneous GCE studies.","Mo and Ru abundances in disc stars can serve as a test bed for proposed sites such as the i-process in post-AGB stars or rare stellar events, because each makes a distinct isotopic signature."],"supporting_citations":[{"why":"Earlier metal-poor Mo and Ru abundances that define the low-metallicity baseline and the line-analysis technique for Mo I.","marker":"Hansen, Andersen & Christlieb (2014)"},{"why":"GCE model that includes a LEPP contribution; its Mo and Ru predictions are among the comparison curves the paper tests.","marker":"Travaglio et al. (2004)"},{"why":"Modern GCE predictions for Mo based on rotating massive-star and AGB yields; the paper reproduces and extends their finding of Mo underproduction.","marker":"Prantzos et al. (2018)"},{"why":"GCE results using a variant of the Travaglio chemical evolution model, providing a comparison set that underproduces Mo.","marker":"Bisterzo et al. (2017)"},{"why":"Provides the OMEGA+ two-zone GCE code used to compute new Mo and Ru evolution tracks with different r-process delay-time setups.","marker":"Côté et al. (2018c)"},{"why":"Shows the intermediate i-process can efficiently produce $^{95}$Mo and $^{97}$Mo, supporting the paper's suggestion of a missing non-s-process source.","marker":"Côté et al. (2018a)"},{"why":"Large compilation of low-metallicity Mo and Ru abundances used to extend the observed trend into the halo regime.","marker":"Roederer et al. (2014)"},{"why":"Adopted solar Mo and Ru reference abundances that set the zero point for $\\mathrm{[Mo/Fe]}$ and $\\mathrm{[Ru/Fe]}$.","marker":"Asplund et al. (2009)"}],"fun_headline_variants":["Missing source for molybdenum and ruthenium","Disc stars reveal underproduction of Mo and Ru","Mo and Ru: nucleosynthesis mystery in Milky Way","GCE models fall short for Mo and Ru abundances","New data show extra process for Mo and Ru"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that the Mo I and Ru I lines are formed under local thermodynamic equilibrium (LTE), so that the missing non-LTE corrections are small and cancel when the abundances are measured relative to the Sun; if those corrections grow with metallicity, the reported underproduction could shrink or disappear.","fun_headline_variants_meta":{"raw":{"variants":["Missing source for molybdenum and ruthenium","Disc stars reveal underproduction of Mo and Ru","Mo and Ru: nucleosynthesis mystery in Milky Way","GCE models fall short for Mo and Ru abundances","New data show extra process for Mo and Ru"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000221,"raw_usage":{"total_tokens":1507,"prompt_tokens":1062,"completion_tokens":445,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":678,"completion_tokens_details":{"reasoning_tokens":371}},"tokens_in":678,"tokens_out":445,"duration_ms":4481,"temperature":1.0,"reasoning_tokens":371,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T14:27:12.140581+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Compute non-LTE corrections for the Mo I 5506/5533 Å and Ru I 4080/4584/4757 Å lines across the stellar parameter range of the sample ($-1.0 < \\mathrm{[Fe/H]} < +0.3$, $T_{\\mathrm{eff}} \\sim 4600$–$6200$ K) using updated atomic data, and re-derive the disc abundances; if the corrected $\\mathrm{[Mo/Fe]}$ and $\\mathrm{[Ru/Fe]}$ values at the metal-poor end shift up by more than about 0.2 dex, the model underproduction claimed here could be a measurement artifact rather than a missing nucleosynthesis source.","supporting_citations":[{"cited_title":"J., Andersen A","cited_arxiv_id":null,"evidence_quote":"Earlier metal-poor Mo and Ru abundances that define the low-metallicity baseline and the line-analysis technique for Mo I."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"GCE model that includes a LEPP contribution; its Mo and Ru predictions are among the comparison curves the paper tests."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Modern GCE predictions for Mo based on rotating massive-star and AGB yields; the paper reproduces and extends their finding of Mo underproduction."}],"review_version":1}