arXiv subjects

Miqaela K. Weller

Publications and source records attributed to Miqaela K. Weller.

11 recordsLinked to original sources

arXiv2026

Evidence for Enhanced Helium Enrichment from Asteroseismology

The helium abundance of a star has a dramatic effect on luminosity, internal structure, and evolutionary timescales, yet direct helium measurements are unavailable for most stars. In this work, we infer stellar helium abundances by comparing observed and predicted luminosities for a sample of subgiants with asteroseismic masses from the APOKASC sample and spectroscopic parameters from recent survey compilations. Stellar luminosities are predicted using grids of theoretical stellar evolution models and offsets are attributed to helium differences from the assumed model value. From this analysis, we see clear evidence of helium enrichment correlated with metallicity, $ΔY/ΔZ = 1.959 \pm 0.061$, with the dominant error source being systematic differences in stellar fundamental parameters. Our inferred enrichment slope exceeds the value implied by a solar-calibrated helium enrichment relation, suggesting that commonly adopted assumptions regarding helium enrichment may underestimate stellar helium abundances at high metallicity. We further show that adopting the revised enrichment law can alter predicted main-sequence lifetimes by up to $\sim$ 1 Gyr and, in some cases, modify stellar structure through changes in convective core development. These results demonstrate that uncertainties in helium enrichment propagate directly into stellar age determinations and highlight the importance of empirical helium constraints for stellar evolution models.

astro-ph.SR↗

arXiv2026

The LBT $Y_{\rm p}$ Project II: MODS Spectra, Physical Conditions, and Oxygen Abundances in Local Metal-Poor Nebulae

Empirically measuring the primordial He mass fraction, $Y_{\rm p}$, requires a significant number of low-metallicity nebulae with direct constraints on He/H and O/H abundances. This technique requires high-fidelity measurements of the gas-phase physical conditions, namely the electron temperature ($T_e$) and density ($n_e$). To this end, we present deep rest-optical spectroscopy for a sample of 62 low-metallicity ($\lesssim$ 20% solar O/H) galaxies acquired using the Multi-Object Double Spectrographs (MODS) on the Large Binocular Telescope (LBT) as part of the LBT $Y_{\rm p}$ Project. We discuss new fitting methods that recover the intensity of up to 61 H and He recombination lines, of which, up to 26 will be used to determine gas-phase He abundances, and we examine the emission line properties of the LBT $Y_{\rm p}$ Project sample. We assess different scaling relations in the low-metallicity interstellar medium (ISM), finding that $n_e$[Ar IV] measured in 31 targets is systematically larger than $n_e$[S II] or $n_e$[O II]. The larger densities are insufficient to significantly bias $T_e$[O III] or the O/H abundance. $T_e$[S III] and $T_e$[O III] are strongly correlated over a range of $\sim$10$^4$ K with very low scatter, and we calibrate new $T_e$[S III]-$T_e$[O III] scaling relations for use in other low-metallicity environments. We examine different $T_e$ measured in the low-ionization gas, finding significant scatter compared to $T_e$[O III]. The precision direct O/H derived in this analysis (median uncertainty $\sim$4%) are consistent with prior literature measurements, albeit with relatively large scatter. These data provide a key component necessary to empirically measure $Y_{\rm p}$ and the abundance patterns of other elements in the ISM.

astro-ph.GA↗

arXiv2026

Constraints on the Galactic Chemical Evolution of $^3\rm{He}$

We examine the galactic chemical evolution (GCE) of $^3\rm{He}$ in one-zone and multi-zone models, with particular attention to the stellar yields and GCE parameters that can reproduce both the protosolar $^3\rm{He}$ abundance and recent gas-phase $^3\rm{He}/^4\rm{He}$ measurements in the Orion nebula. Published stellar models indicate negligible net $^3\rm{He}$ production by massive stars, while the predicted yields from asymptotic giant branch (AGB) stars are metallicity-dependent and span a range of $\sim 2.5$ depending on the extra mixing processes incorporated in the stellar models. The dominant contribution to $^3\rm{He}$ production comes from $1-2\ M_\odot$ stars, making $^3\rm{He}$ evolution slow compared to other AGB elements and to Fe enrichment from Type Ia supernovae. We constrain our GCE models to reproduce the observed [O/H] in the interstellar medium, and our fiducial models adopt an empirically motivated IMF-averaged oxygen yield $y_{\rm O} \approx 1.2\ Z_{\rm O, \odot}$. Even with the lowest of the AGB $^3\rm{He}$ yields, based on stellar models with rotational and thermohaline mixing, our GCE models tend to overpredict the protosolar and Orion $^3\rm{He}$ abundances; they require a slow onset of star formation and low star formation efficiency to come close to the observed values. With a higher oxygen yield, calibration to observed [O/H] implies stronger outflows, making it easier to reproduce the observed $^3\rm{He}$. Alternatively, the true $^3\rm{He}$ yield could be lower than that predicted by existing stellar models, suggesting that mixing in red giants is not yet fully captured. Future $^3\rm{He}$ measurements that probe higher metallicity environments could help distinguish these possibilities.

astro-ph.GA↗

arXiv2026

The LBT $Y_{\rm p}$ Project I: An Improved Determination of the Primordial Helium Abundance -- Project Description, Sample Selection, Observations, and Methodology

Extremely low metallicity HII regions have been observed with the goal of determining the primordial helium abundance ($Y_{\rm p}$). $Y_{\rm p}$, combined with standard big bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but $Y_{\rm p}$ must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of $Y_{\rm p}$. The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual HII regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a four-year period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of $Y_{\rm p}$ with a precision of $\sim$ 0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of $\sim$ 3%.

astro-ph.CO↗

arXiv2026

The LBT $Y_{\rm p}$ Project III: LUCI Spectra of Metal-Poor Nebulae

Accurately determining the elemental abundances of a low metallicity nebula strongly depends on measuring the density (n$_e$) and temperature (T$_e$) of the gas. Because these two parameters are inherently degenerate when derived solely from H and He recombination lines, we rely on the density-sensitive HeI $λ$10830 line to assist in resolving this issue, especially for accurate He abundances. To facilitate this, we present near-IR (NIR) LUCI spectra of 48 low-metallicity targets from the Large Binocular Telescope (LBT) and homogeneously reduce them using Pypeit as part of the LBT $Y_{\rm p}$ Project. IR spectra require special care, and we wavelength calibrate by-hand using the bright OH emission lines, carefully apply proper telluric corrections, and co-add the spectra of LUCI1 and LUCI2 on a resampled grid to ensure accurate results. We use a Gaussian profile to fit the emission lines and measure the fluxes relative to Paschen-gamma (P$γ$), resulting in HeI $λ$10830 to P$γ$ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR HeI $λ$10830 fluxes in low metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of $\widetildeσ = 0.08$, reduce the overall uncertainties in helium abundance estimates for individual targets. The increased size of the high-quality sample enables searching for systematic uncertainties and improves the reliability of the helium abundance determinations used to infer the primordial helium abundance ($Y_{\rm p}$).

astro-ph.GA↗

arXiv2026

The LBT Y$_\mathrm{p}$ Project IV: A New Value of the Primordial Helium Abundance

We present a new determination of the primordial helium abundance based on new, high-quality Large Binocular Telescope (LBT) observations of 54 metal-poor H II regions. These regions have been observed and analyzed uniformly. We also describe a number of updates to our methodology, including updated helium emissivities. Enabled by the large, high-quality dataset, we examine our sample targets for potential systematic errors, which could bias their results. We perform a standard 95% confidence level $χ^2$ cut and find that a significantly larger fraction (47/54 = 87%) of our sample qualifies than for previous datasets. We also screen for quality and reliability, flagging targets which may introduce significant systematic errors, producing a dataset of 41 targets. In a significant breakthrough for the field, that dataset includes 15 high SNR targets with low metallicity (O/H < 4 $\times$ 10$^{-5}$). Due to this low-metallicity dataset, for the first time, a weighted average for determining the primordial helium abundance (Y$_\mathrm{p}$) is well-justified and produces a robust result. By weighted average of our 15 low-metallicity targets, we determine Y$_\mathrm{p}$ = 0.2458 $\pm$ 0.0013. This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the BBN result, Y$_\mathrm{p}$ = 0.2467 $\pm$ 0.0002, based on the Planck determination of the baryon density.

astro-ph.CO↗

arXiv2026

The LBT $Y_{\rm p}$ Project V: Cosmological Implications of a New Determination of Primordial $^4$He

The primordial abundance of $^4$He plays a central role in big-bang nucleosynthesis (BBN) and in the cosmic microwave background (CMB). The LBT $Y_{\rm p}$ Project's new measurement of the primordial $^4$He mass fraction $Y_{\rm p} =0.2458 \pm 0.0013$ is the most precise determination to date. In this paper, we combine our new $Y_{\rm p}$ value with the latest primordial deuterium measurement, and assess the consequences for cosmology. For Standard BBN, where the number of light neutrino species is fixed at $N_ν=3$, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining $Y_{\rm p}$ , D/H, BBN, and the CMB, gives the cosmic baryon-to-photon ratio $η= (6.120 \pm 0.038) \times 10^{-10}$, corresponding to a baryon density parameter $Ω_{\rm B} h^2 = 0.02236 \pm 0.00014$. We then allow $N_ν$ to vary and thus measure relativistic species present during nucleosynthesis. We find $η= (6.101 \pm 0.044) \times 10^{-10}$ or $Ω_{\rm B} h^2= 0.02229 \pm 0. 00016$, and $N_ν= 2.925 \pm 0.082$, and for $N_ν\ge 3$, $ΔN_ν= N_ν-3 \le 0.125$ (95\% CL) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics, and with the standard cosmology that links BBN at $\sim 1 \ \rm sec$ and the CMB at $\sim 400,000$ yr.

astro-ph.CO↗

arXiv2025

Metals versus Non-metals: Chemical Evolution of Hydrogen and Helium Isotopes in the Milky Way

Star formation drives changes in the compositions of galaxies, fusing H and He into heavier nuclei. This paper investigates the differences in abundance evolution between metal and non-metal isotopes using recent models of Galactic chemical evolution appropriate for the thin disk epoch. A strong degeneracy arises between metal yields from stellar populations and the mean Galactocentric radial velocity of the interstellar medium (ISM). Similar metallicities arise when increases (decreases) in metal yields are combined with increases (decreases) to the gas flow velocity. A similar degeneracy exists between metal yields and the rate of gas ejection from the ISM. We demonstrate that this degeneracy can be confidently broken with precise measurements of the hydrogen (D/H) and helium ($^3$He/$^4$He) isotope ratios in the Galactic ISM. At fixed O/H, higher metal yields lead to higher D/H and lower $^3$He/$^4$He. Measurements available to date are not sufficiently precise or numerous to draw confident conclusions. A detailed inventory of non-metal isotopes in the Milky Way would provide critical empirical constraints for stellar and galactic astrophysics, as well as a new test of Big Bang Nucleosynthesis. We forecast that only $\sim$4 additional measurements of $^3$He/$^4$He within $\sim$$3$ kpc of the Sun are required to measure the primordial $^3$He/$^4$He ratio at $\sim$30\% precision. In parallel, empirical benchmarks on metal yields also have the power to inform stellar models, since absolute yield calculations carry factor of $\sim$$2-3$ uncertainties related to various complex processes (e.g., rotational mixing, convection, mass loss, failed supernovae).

astro-ph.GA↗

arXiv2024

The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars are Metal-Rich

Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of $\sim$9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor an equilibrium scenario, in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the interstellar medium for long, which causes the ratio of star formation to accretion, $\dotΣ_\star / \dotΣ_\text{in}$, to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on $\sim$Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining $\dotΣ_\star / \dotΣ_\text{in}$ with increasing Galactocentric radius.

astro-ph.GA↗

arXiv2024

Modeling the Galactic Chemical Evolution of Helium

We examine the galactic chemical evolution (GCE) of $^4$He in one-zone and multi-zone models, with particular attention to theoretical predictions and empirical constraints on IMF-averaged yields. Published models of massive star winds and core collapse supernovae span a factor of 2 -- 3 in the IMF-averaged $^4$He yield, $y\mathrm{_{He}^{CC}}$. Published models of intermediate mass, asymptotic giant branch (AGB) stars show better agreement on the IMF-averaged yield, $y\mathrm{_{He}^{AGB}}$, and they predict that more than half of this yield comes from stars with $M=4-8 M_\odot$, making AGB $^4$He enrichment rapid compared to Fe enrichment from Type Ia supernovae. Although our GCE models include many potentially complicating effects, the short enrichment time delay and mild metallicity dependence of the predicted yields makes the results quite simple: across a wide range of metallicity and age, the non-primordial $^4$He mass fraction $ΔY = Y-Y_{\mathrm{P}}$ is proportional to the abundance of promptly produced $α$-elements, like oxygen, with $ΔY/Z_{\mathrm{O}} \approx (y\mathrm{_{He}^{CC}}+y\mathrm{_{He}^{AGB}})/y\mathrm{_{O}^{CC}}$. Reproducing solar abundances with our fiducial choice of the oxygen yield $y\mathrm{_{O}^{CC}}=0.0071$ implies $y\mathrm{_{He}^{CC}}+y\mathrm{_{He}^{AGB}} \approx 0.022$, i.e., $0.022M_\odot$ of net $^4$He production per solar mass of star formation. Our GCE models with this yield normalization are consistent with most available observations, though the implied $y\mathrm{_{He}^{CC}}$ is low compared to most of the published massive star models. More precise measurements of $ΔY$ in stars and gas across a wide range of metallicity and [$α$/Fe] ratio could test our models more stringently, either confirming the simple picture suggested by our calculations or revealing surprises in the evolution of the second most abundant element.

astro-ph.GA↗

arXiv2022

Mapping Progenitors of Binary Black Holes and Neutron Stars with Binary Population Synthesis

The first directly observed gravitational wave event, GW150914, featuring the merger of two massive black holes, highlighted the need to determine how these systems of compact remnant binaries are formed. We use the binary population synthesis code COSMIC (Compact Object Synthesis and Monte Carlo Investigation Code) to predict the types of massive stars that will show significant radial velocity variations, indicative of a potential compact object (i.e. a black hole or neutron star) orbiting the star. We "observe" the binaries generated in the populations with a similar number of epochs and RV accuracy as planned for the Milky Way Mapper. In this analysis, we are especially interested in systems where a compact remnant is orbiting a massive O or B star as these systems survived the first supernova and neutron star kick. We test the ability of the Milky Way Mapper observing strategy to distinguish among different mass loss and kick prescriptions. We find that Wolf-Rayet stars or hot subdwarfs in binaries could be detectable (i.e. luminous, high delta RV max), viable progenitors of such objects, while the different prescriptions primarily affect the number of sources.

astro-ph.HE↗