arXiv subjects

José Eduardo Méndez-Delgado

Publications and source records attributed to José Eduardo Méndez-Delgado.

7 recordsLinked to original sources

arXiv2026

DIGging into HII regions: the role of ionizing photon leakage and spectral hardening in coupling HII regions and diffuse ionized gas

The optical emission-line ratios of star-forming galaxies are broadly understood in the framework of photoionization, yet some systematic trends remain unexplained. Using 50--100 pc resolution PHANGS-MUSE observations of 19 nearby galaxies, we investigate how escaping ionizing radiation couples HII regions to the diffuse ionized gas (DIG), analyzing thousands of HII regions and DIG sightlines. HII regions and the DIG form a continuous sequence across standard line-ratio diagnostic diagrams (e.g., BPT diagrams), where progressively fainter regions show elevated high- and low-ionization line ratios, and the DIG extends the same trends into the composite--LINER regime. Line-ratio variations with global specific star formation rate (sSFR) suggest progressively harder ionizing radiation with increasing star formation activity, contrary to expectations for emission increasingly dominated by older stellar populations. Line-of-sight mixing explains part of the behavior but cannot reproduce all line-ratio trends. We therefore develop a model in which ionizing photons escape HII regions and propagate through the diffuse ISM, undergoing geometric dilution, attenuation, and spectral hardening. The model predicts that progressively fainter HII regions are increasingly ionized by leaked radiation from neighboring luminous regions, implying radiative coupling between HII regions. Joint modeling of HII regions and the DIG reproduces the HII region luminosity distribution and DIG surface brightness for escape fractions of $\sim$0.4$-$0.6 and ionizing-photon mean free paths of $\sim$0.5$-$1 kpc, and qualitatively reproduces the observed line-ratio sequences and their dependence on sSFR. Our results suggest that ionizing photon leakage may provide an important non-local feedback channel linking compact star-forming sites to large-scale galaxy disks.

astro-ph.GA↗

arXiv2026

A Broken Clock Is Right Twice a Day: [Ce/Mg] Is Not a Universal Chemical Clock

The ratio of $s$-process to $α$-element enrichment ($[s/α]$) has been proposed as a ``chemical clock,'' or a means to estimate stellar ages. However, the age--$[s/α]$ relation varies with metallicity and location in the Galaxy, and the observed trends are not well predicted by galactic chemical evolution models. We quantify the age--[Ce/Mg] correlation across the Galactic disk in roughly 100,000 red giant stars observed with APOGEE in the SDSS-V Milky Way Mapper survey. We find that the slope of the correlation varies significantly with metallicity and guiding radius in the chemical thin disk. The trend is steepest in the outer disk and below Solar metallicity, while the most metal-rich stars and those in the inner disk show no correlation with age. In contrast, [Ce/Mg] patterns in the chemical thick disk are consistent across the Galaxy. Halo stars have higher [Ce/Mg] than the chemical thick disk, suggesting that asymptotic giant branch (AGB) enrichment is important even at low metallicity. Overall, patterns in [Ce/Mg] trace both the local star formation history and AGB nucleosynthesis. The complex interplay between [Ce/Mg], age, metallicity, and Galactic position means that [Ce/Mg] (and by extension $[s/α]$) is not a universal chemical clock.

astro-ph.GA↗

arXiv2026

A comparative study of O, Ne, Cl, and Ar in Hii regions and PNe of the Galactic disk: Temporal evolution of radial gradients?

(Abridged) We compare the radial abundance gradients of O, Ne, Cl, and Ar using a sample of 42 HII regions and 176 planetary nebulae (PNe) from the DESIRED catalogue in the Galactic disk, comprising the highest-quality observations currently available and presenting the first gradient analysis for this dataset. For all objects, two sets of chemical abundances were compiled: one derived from collisionally excited lines (CELs) and another incorporating the temperature fluctuation parameter (t^2). Oxygen abundances were corrected for dust depletion, and HII region results were compared with Cepheid stars, which trace the present-day interstellar medium. PNe distances were compiled from recent literature, excluding bulge or halo objects to ensure a disk-only sample. All gradients are statistically significant (p < 0.05), except for Cl and Ar in HII regions. The O/H gradients from HII regions and Cepheids are consistent when t^2 is included, underscoring the importance of accounting for temperature inhomogeneities in nebular analyses. The O and Ne gradients traced by older objects are flatter than the present-day gradient by -0.028 +- 0.008 dex kpc-1 on average from both RLs and CELs. This could indicate a temporal steepening of the Galactic abundance gradient; however, this behavior is not reproduced by chemical evolution models, suggesting additional physical processes are at play. The most plausible explanation is that our PNe sample has been strongly affected by radial migration. Under this interpretation, the PNe gradient cannot reliably trace past abundance gradients, but provides a valuable constraint on radial stellar migration, offering important input for chemo-dynamical models of the Galactic disk and for hydrodynamical simulations.

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↗

arXiv2025

Duration and properties of the embedded phase of star formation in 37 nearby galaxies from PHANGS-JWST

Light reprocessed by dust grains emitting in the infrared allows the study of the physics at play in dusty, embedded regions, where ultraviolet and optical wavelengths are attenuated. Infrared telescopes such as JWST have made it possible to study the earliest feedback phases, when stars are shielded by cocoons of gas and dust. This phase is crucial for unravelling the effects of feedback from young stars, leading to their emergence and the dispersal of their host molecular clouds. Here we show that the transition from the embedded to the exposed phase of star formation is short (< 4 Myr) and sometimes almost absent (< 1 Myr), across a sample of 37 nearby star-forming galaxies, covering a wide range of morphologies from massive barred spirals to irregular dwarfs. The short duration of the dust-clearing timescales suggests a predominant role of pre-supernova feedback mechanisms in revealing newborn stars, confirming previous results on smaller samples and allowing, for the first time, a statistical analysis of their dependencies. We find that the timescales associated with mid-infrared emission at 21 μm, tracing a dust-embedded feedback phase, are controlled by a complex interplay between giant molecular cloud properties (masses and velocity dispersions) and galaxy morphology. We report relatively longer durations of the embedded phase of star formation in barred spiral galaxies, while this phase is significantly reduced in low-mass irregular dwarf galaxies. We discuss tentative trends with gas-phase metallicity, which may favor faster cloud dispersal at low metallicities.

astro-ph.GA↗

arXiv2024

On the chlorine abundance in H II regions

Chlorine (Cl) is a chemical element of the group of the halogens and is between the 17th and the 20th most abundant elements in the Solar System. It is thought to be produced from the capture of a proton or neutron by specific alpha-element isotopes during both hydrostatic and explosive oxygen burning, though some contribution may come from Type Ia supernovae. Cl lines are quite rare in stellar spectra, so most of the information available about its abundance comes from analyzing the emission lines of ionized nebulae, especially the collisionally excited lines of Cl2+ ([Cl III] λλ5518,5538). Our goal is to accurately determine the Cl abundance in H II regions, and gather more information about its nucleosynthetic origin. For this work we used a sample of observations that encompasses the deepest spectra of H II regions available in the literature, from both the Milky Way and other galaxies in the local Universe, covering a range of oxygen (O) abundances, 12+log(O/H), from 7.18 to 8.70. As a first step, we determine the most representative electron temperature of the zone of the nebulae where the Cl2+ ion lies. To this aim we used a grid of photoionization models and diagnostics valid for other ions, as that parameter cannot be determined directly through [Cl III] lines. We then computed the total Cl abundance using different sets of ionization correction factors to account for the contribution from unseen ionization stages.

astro-ph.GA↗

arXiv2023

The abundance discrepancy in ionized nebulae: which are the correct abundances?

Ionized nebulae are key to understanding the chemical composition and evolution of the Universe. Among these nebulae, H~{\sc ii} regions and planetary nebulae are particularly important as they provide insights into the present and past chemical composition of the interstellar medium, along with the nucleosynthetic processes involved in the chemical evolution of the gas. However, the heavy-element abundances derived from collisional excited lines (CELs) and recombination lines (RLs) do not align. This longstanding abundance-discrepancy problem calls into question our absolute abundance determinations. Which of the lines (if any) provides the correct heavy-element abundances? Recently, it has been shown that there are temperature inhomogeneities concentrated within the highly ionized gas of the H~{\sc ii} regions, causing the reported discrepancy. However, planetary nebulae do not exhibit the same trends as the H~{\sc ii} regions, suggesting a different origin for the abundance discrepancy. In this proceedings, we briefly discuss the state-of-the-art of the abundance discrepancy problem in both H~{\sc ii} regions and planetary nebulae.

astro-ph.GA↗