arXiv subjects

Alessandro Bressan

Publications and source records attributed to Alessandro Bressan.

At least 19 recordsLinked to original sources

arXiv2026

Cosmography with DESI-DR1 Cosmic Chronometers: Direct H(z) measurements from Luminous Red Galaxy ages

Providing robust redshift estimates for almost 3 million luminous red galaxies (LRGs), the Dark Energy Spectroscopic Instrument (DESI) offers a unique opportunity to test the expansion rate of the Universe with independent approaches. We apply the cosmic chronometer method to derive new, independent constraints on the Hubble parameter at 0.3<z<1.2 from the differential age evolution of DESI LRGs. We select spectra applying spectroscopic cuts to ensure sample purity and remove contamination by star-forming objects, then build a robust sample of cosmic chronometers (CCs) by stacking to obtain stable, high signal-to-noise (S/N) spectra, which also serves as a democratic binning choice for the $t-z$ plane. Ages are estimated by measuring Lick indices on the stacked spectra and fitting them with a theoretical stellar population model. We obtain $t-z$ relations from which we derive $H(z)$ constraints via two independent approaches: a fit with a pivotal-redshift cosmography, and a direct estimate from the original CC method. The cosmographic fit yields posteriors for the kinematic parameters $\{H_{z_0}, q_{z_0}, j_{z_0}\}$ compatible with currently considered cosmologies, giving a precision-level estimate of $H(z)$. We provide the maximum-a-posteriori (MAP) $H(z)$ estimate, an array of the median confidence region in the $H-z$ plane, and its covariance matrix. We also leverage the redshift distributions of the $t-z$ relation for different velocity dispersion groups to obtain two independent local measurements using the discrete approximation $H(z) \approx -\Delta z/[\Delta t (1+z)]$; the one from the reddest envelope of CCs gives $H(z \approx 0.61) = 88.5^{+6.7}_{-12.6}$ (stat.) $\pm 8.1$ (syst.) km s$^{-1}$ Mpc$^{-1}$. Systematic uncertainties for both the cosmographic and discrete $H(z)$ measurements come from a comprehensive analysis of all methodological choices in the data treatment.

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arXiv2026

Bolometric correction for cosmologically redshifted stars with dust: an update to the YBC database

Observations from HST & JWST continue to reveal gravitationally magnified high-redshift star candidates, resulting in increasing demand for accurate stellar bolometric corrections to compare stellar models with observational data. We update YBC stellar bolometric correction database by incorporating bolometric corrections for cosmologically redshifted stars, called zYBC. The bolometric corrections are derived by redshifting stellar spectra from libraries, attenuated by extinction curves for both the dust in the host galaxy and the Milky Way, and followed by convolution with the transmission curves of photometric filters. Our methodology incorporates the effects due to the cosmological K-correction and the dust. Besides the spectral libraries in earlier YBC, we add NLTE-based spectral libraries for O, B stars, which are better suited for hot massive stars, particularly wind-included PoWR and CMFGEN models. The database supports key photometric systems for high-redshift studies, such as HST/WFC3, JWST/NIRCam, and CSST's MSC and MCI, and maintains the flexibility to incorporate additional photometric systems upon request. As examples, we present colors as functions of Teff at various redshifts for several photometric systems, which exhibit non-monotonic behaviors and demonstrate the necessity for a dedicated modelling. In particular, we find that the relations show larger dispersions at high redshift than the zero redshift case. This indicates that the stellar parameters of high redshift stars can be better determined than those of their local counterparts, given their redshifts reliably determined through other methods and their photometric data are of high enough quality for physical parameter determination through spectral-fitting. We also show the difference in the effect brought by the different amount of extinctions. zYBC represents a valuable resource for high-redshift star research.

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arXiv2026

The impact of stellar binaries and star cluster dynamics on pair-instability supernovae

Pair-instability supernovae (PISNe) are among the most luminous transients in the Universe. However, they have never been confidently observed. Solving this puzzle would have key implications for several astrophysical topics, including galaxy chemical enrichment, the interpretation of gravitational waves from binary black hole mergers, and the nature of red dropout sources seen by JWST. With this aim, we present the first in-depth study of PISN occurrence in binary stars, both in isolation and in dense star clusters. We employ the SEVN code, with PARSEC stellar tracks, to evolve a suite of 35 synthetic binary populations, including variations on formation channels, cluster properties, and upper limit of the stellar initial mass function. We find that binary interactions can boost the PISN rate by up to threefold, relative to single stars, whereas binary hardening can either enhance or suppress PISN production, depending on whether the progenitors are primordial or dynamically formed. Moreover, we showcase how our comprehensive framework for the cosmic PISN rate can be used to constrain uncertain aspects of stellar and galaxy evolution models, via comparison with observations, including the recipes for stellar-wind mass loss in very-massive stars, and the galaxy metallicity distribution throughout the Universe.

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arXiv2026

Ages of the resolved stellar populations inside the JWST/MIRI bubbles in NGC 628

JWST images in the MIRI filters are characterized by prominent interstellar bubbles, most of which are expected to be created by mechanical energy injected into the interstellar medium by dying massive stars. In this work, we use resolved stellar populations (RSPs) in JWST/NIRCam images of NGC 628 from the JWST-FEAST dataset to determine the demography of stellar populations within these bubbles by comparing them with PARSEC+COLIBRI isochrones using a Bayesian framework. Our analysis reveals the presence of multiple stellar populations younger than 100 Myr within the bubbles, suggesting secondary star formation following the first generation of stars that formed the bubble. We find a clear preference for the most recently formed stars to lie closer to the bubble shell. In contrast, relatively older stars are distributed throughout the bubble interior, consistent with a scenario in which stars formed in dense shells are left behind as the shell expands. We also examine PHANGS-HST cluster candidates within large bubbles and find no convincing progenitor clusters responsible for the initial trigger, indicating that low-mass clusters or OB associations may be sufficient to drive the initial expansion. However, the cluster may dissolve as it co-evolves with the bubble, producing the dispersed stellar distributions observed in larger bubbles. We establish a fundamental plane relation for stellar feedback-driven bubbles that involves the stellar population mass, age, bubble size, gas density, and feedback efficiency, highlighting the ability of JWST/NIRCam CMDs to characterize stellar populations driving interstellar bubble expansion in nearby galaxies

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arXiv2026

The impact of envelope binding energies on the merger rate density of binary compact objects

The common envelope (CE) phase plays a key role in the formation of binary compact object systems. Its final outcome strongly depends on the envelope binding energy, but this quantity is often estimated using fitting formulas that are not fully consistent with the underlying stellar evolution models adopted in population-synthesis codes. Here, we investigate envelope binding energies across the most extensive stellar grid considered to date. Our stellar tracks, evolved with PARSEC v2.0, include hydrogen (H) -rich stars with metallicities ranging from $Z = 10^{-11}$ (Population III stars) to $Z = 0.03$, and initial masses between 2 and 2000 M$_\odot$, as well as pure-helium stars with masses from 0.36 to 350 M$_\odot$. We examine the sensitivity of the envelope binding energies to the selected core-envelope boundary definition and to different internal energy source contributions. For H-rich stars, we find that internal energy sources can alter the envelope binding energy by more than an order of magnitude, whereas the core boundary criteria play a secondary role. In contrast, for pure helium stars, the core-boundary criterion becomes the dominant factor. The envelope binding energies derived from different stellar tracks can show deviations of several orders of magnitude, with larger differences for more massive stars and higher metallicities.Finally, by implementing our new envelope binding energy prescriptions into the binary population synthesis code SEVN, we show that the predicted merger rate densities of compact binaries can differ by more than an order of magnitude compared to previous models. Our results highlight the importance of using envelope binding energies that are consistent with the underlying stellar evolution models and caution against extrapolating empirical fits beyond the considered parameter space.

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arXiv2026

Impact of initial mass function on the chemical evolution of high-redshift galaxies

Recent observations by the James Webb Space Telescope (JWST) have found evidence for an invariant relation between stellar mass, metallicity, and star formation rate up to $z\sim 8$ and its breakdown at higher redshifts. Understanding the underlying physics driving such correlations is thus crucial. Here, we explore the impact of the initial mass function (IMF) on the chemical evolution of high-redshift galaxies. Indeed, star formation and metal enrichment in galaxies are regulated by supernova (SN) explosions and metal yields from massive stars, which are sensitive to the high-mass end of the IMF. Using the semi-analytical galaxy evolution code \textsc{a-sloth}, we follow galactic baryon cycles along merger trees built from a high-resolution cosmological simulation. Stellar feedback is modeled with up-to-date stellar evolution tracks covering the full metallicity range ($Z \sim 10^{-11} - 0.03$) and a broad stellar mass range ($m_\star\sim2 - 600\ \rm M_\odot$), including metal yields from stellar winds, core-collapse SNe, (pulsational) pair-instability SNe, and Type Ia SNe. Assuming a Kroupa-like IMF with a varying upper mass limit $m_{\max}$, we find that only models with $m_{\max} \gtrsim 200\ \rm M_\odot$ can simultaneously reproduce the observed mass-metallicity-star formation rate relation and cosmic star formation history at $z\gtrsim 4$ owing to enhanced metal yields from pair-instability SNe. Our results confirm that very massive ($\gtrsim 200\ \rm M_\odot$) stars and pair-instability SNe play an important role in the star formation and chemical enrichment histories of high-$z$ galaxies. They also have profound implications for electromagnetic transients and gravitational-wave events.

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arXiv2026

A homogeneous view of asymptotic giant branch carbon stars as seen by Gaia

Carbon stars on the asymptotic giant branch are major contributors to galactic dust enrichment, with gas mass-loss rates up to 1e-4 Msun/yr. We present a homogeneous spectral energy distribution analysis of the Gaia DR3 Golden Sample of carbon stars in the Milky Way and Magellanic Clouds. Our dataset includes 14,747 sources with multi-band photometry from Gaia, 2MASS, and WISE, combined with recent distance and extinction estimates. For a subsample of 2,494 Mira variables, we model multi-band light curves to derive accurate mean magnitudes. Stellar and circumstellar parameters are obtained by fitting observations with a large grid of synthetic spectra computed with the DUSTY radiative transfer code using COMARCS atmospheres. We derive effective temperature, optical depth, and gas mass-loss rate for each source. The distributions peak around Teff = 3150 K, with mass-loss rates spanning 1e-11 to 1e-4 Msun/yr and inner dust temperatures near 1000 K. We find a correlation between variability amplitude and mass-loss rate. This framework provides a statistically robust view of carbon stars across environments with different metallicities. Apparent environmental dependencies are influenced by luminosity distributions and selection effects rather than purely intrinsic metallicity differences. The combined Gaia and WISE selection limits the detection of both highly obscured and faint Magellanic Cloud sources, but the observed trends remain significant within the sampled populations.

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arXiv2025

Cosmography via stellar archaeology of low-redshift early-type galaxies from SDSS

Cosmic chronometers offer a model-independent way to trace the expansion history of the Universe via the dating of passively evolving objects. This enables testing the validity of cosmological models without concrete assumptions of their energy content. The main goal of this work is to derive model-independent constraints on the Hubble parameter up to $z \sim 0.4$ using stellar ages from the fitting of Lick index absorption lines in passively evolving galaxies. Contrary to recent related works that rely on finite differences to obtain a discrete measurement of the expansion of the Universe at an average redshift, our goal is to perform a cosmographic fit of $H(z)$ in terms of the Hubble constant ($H_0$) and the deceleration ($q_0$) and jerk ($j_0$) parameters. We carefully select spectra of massive and passively evolving galaxies from the SDSS Legacy Survey. After applying a stacking procedure to ensure a high signal-to-noise ratio, the strength of Lick indices is fit using two stellar population models (TMJ and Knowles) to derive stellar population parameters. A cosmographic fit to the stellar ages is performed, which in turn enables the sampling of the Hubble parameter within the considered redshift range. The baseline result comes from using the TMJ-modelled ages, and it yields a value of $H_0 = 70.0^{+4.1}_{-7.6} \text{ km s}^{-1} \text{ Mpc}^{-1}$ for the Hubble constant, where uncertainties refer only to the statistical treatment of the data. The sampling of the Hubble parameter at $0.05 < z < 0.35$ is competitive with discreet model-independent measurements from the literature. We finally draw attention to an unexpected oscillating pattern in a number of critical indices with respect to redshift, which translates into a similar behaviour in the $t-z$ relations. These features have never been discussed before, although they are present in previous measurements.

astro-ph.CO↗

arXiv2025

Enhanced Mass Loss of Very Massive Stars: Impact on the Evolution, Binary Processes, and Remnant Mass Spectrum

Very massive stars (VMS) play a fundamental role in astrophysics due to their winds and supernovae (SN), and their role as massive black hole (BH) progenitors. However, their origin and evolution remain a significant challenge. Recent theoretical work and observations suggest that VMS approaching the Eddington limit may experience mass loss above the standard wind predictions. This study investigates how enhanced winds influence single and binary VMS evolution, observable properties, and resulting BH populations. New stellar wind prescriptions, sensitive to the Eddington parameter ($Γ_e$) and the luminosity-to-mass ratio, were implemented into the stellar evolution code PARSEC v2.0. These updated single-star tracks (100 - 600 M$_{\odot}$ at Z=0.006) were used to model the VMS population in the Tarantula Nebula and integrated into the SEVN binary evolution code. The $Γ_e$-enhanced single-star tracks match observed VMS properties better than standard models. Explaining the most massive star, R136a1, through a single-star origin suggests a zero-age main sequence (ZAMS) mass limit of $<$ 400 M$_{\odot}$ regardless of the wind recipe used. However, binary stellar mergers also offer a suitable origin for R136a1 and other observed VMS, potentially lowering the upper ZAMS mass limit by ~100 M$_{\odot}$. In binaries, enhanced winds inhibit main-sequence stellar mergers and limit BH production above the pair-instability mass gap's lower edge (~50 M$_{\odot}$). Binary BHs merging in a Hubble time with enhanced winds yield more primary BHs above 30 M$_{\odot}$ and enable secondary BHs between 30-40 solar masses, a range not found with standard stellar winds at LMC metallicity. This study highlights the crucial role that stellar winds and binary interactions play in VMS evolution and offers predictions relevant for interpreting VMS observations and gravitational wave source origins.

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arXiv2025

Strikingly high fraction of fast rotators in Magellanic Cloud star clusters

There has been growing evidence that the rich star clusters in the Magellanic Clouds contain significant fractions of rapidly rotating stars. In this work, we aim to constrain these fractions by studying the colour-magnitude diagrams of four star clusters, selected among those with the most striking signatures of fast rotators. Using isochrones derived from PARSEC v2.0 stellar tracks, we generate distinct stellar populations, each covering a limited interval of initial rotation rates $ω_\mathrm{i}$, referred to as 'Partial Models' (PMs). Using optimization algorithms and Monte Carlo Markov Chains, PMs are combined to create the final best-fitting model. In our analysis, we adopt two key assumptions: a uniform age and an isotropic distribution of stellar spin axes within each cluster. The solutions are allowed to explore the entire range of $ω_\mathrm{i}$, and different values of age, metallicity, distance and foreground extinction. We find that the rotational velocity distributions in all four clusters reveal a high fraction of stars with $ω_\mathrm{i}$ close to the break-up value, in all cases. Specifically, the fraction of stars with $ω_\mathrm{i}>0.7$ exceeds $80\%$ in the clusters NGC 419 of the Small Magellanic Cloud (SMC) and NGC 1831 and NGC 1866 of the Large Magellanic Cloud (LMC). For NGC 2203 of the LMC, this fraction is smaller, although it still exceeds $50\%$, confirming that also this cluster is mainly populated by fast-rotating stars.

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arXiv2025

StAGE: Stellar Archaeology-driven Galaxy Evolution

We build a semi-empirical framework of galaxy evolution (dubbed StAGE) firmly grounded on stellar archaeology. The latter provides data-driven prescriptions that, on a population statistical ground, allow to define the age and the star formation history for the progenitors of quiescent galaxies (QGs). We exploit StAGE to compute the cosmic star formation rate (SFR) density contributed by the progenitors of local QGs, and show it to remarkably agree with that estimated for high-$z$ dusty star-forming galaxies which are faint/dark in the NIR, so pointing toward a direct progenitor-descendant connection among these galaxy populations. Furthermore, we argue that by appropriately correcting the observed stellar mass density by the contribution of such NIR-dark progenitors, StAGE recovers a SFR density which is consistent with direct determinations from UV/IR/radio surveys, so substantially alleviating a longstanding tension. Relatedly, we also show how StAGE can provide the average mass and metal assembly history of QGs, and their redshift-dependent statistics. Focusing on the supermassive black holes (BHs) hosted by massive QGs, we exploit StAGE to reconstruct the average BH mass assembly history, the cosmic BH accretion rate density as a function of redshift, and the evolution of the Magorrian-like relationship between the relic stellar and BH masses. All in all, StAGE may constitute a valuable tool to understand via a data-driven, easily expandable, and computationally low-cost approach the co-evolution of QGs and of their hosted supermassive BHs across cosmic times.

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arXiv2025

Spatially-resolved TRGB JWST color-magnitude as a tool to measure fossil stellar metallicity gradients in disk galaxies: NGC 628

We use archival JWST/NIRCam images in the F115W, F150W, and F200W filters to measure the Tip of the Red Giant Branch (TRGB) magnitudes across the disk of the late-type spiral galaxy NGC 628. In this exploratory study, we demonstrate how the metallicity-dependence of TRGB magnitudes in the near-infrared (NIR) filters can be exploited by making use of the theoretical isochrones to determine metallicities of the fossil 10 Gyr old population over kiloparsec scales without being affected by the age-metallicity-reddening degeneracy. We obtain a smooth metallicity gradient that decreases from $Z$=0.003 in the central regions to $Z$=0.002 in the external parts, with a typical error on $Z$ of 0.0002. The extinction variation across the disk caused by the diffuse interstellar dust is spiky with a median value of $A_V$=0.12 mag. We propose that the large bubbles in the disks of galaxies offer dust-free lines of sight for measuring the TRGB magnitudes, and hence the distance to galaxies, to an accuracy that is as good as that of the halo populations. Using the Phantom Void, we obtain a TRGB distance modulus of 29.81$\pm0.05(\rm stat)\pm0.06(\rm sys)$ mag for NGC 628, which agrees well with the most recent PNLF measurement of 29.89$^{+0.06}_{-0.09}$ for this galaxy.

astro-ph.GA↗

arXiv2025

A new prescription for the spectral properties of population III stellar populations

We investigated various emission properties of extremely low metallicity stellar populations in the Epoch of Reionization (EoR), using the new GALSEVN model, which has shown promising agreement between spectral predictions and observations at lower redshifts and higher metallicities. We find that emission-line diagnostics previously proposed to discriminate between population III (Pop III) stars and other primordial ionizing sources are effective, but only for stellar-population ages below $\sim1$ Myr. We provide other key quantities relevant to modeling Pop III stellar populations in the EoR, such as the production efficiency of ionizing photons, which is critical for reionization studies, the production rate of Lyman-Werner photons, which can dissociate H$_2$ and influence the efficiency of star formation, and the rates of different types of supernovæ, offering insights into the timescales of chemical enrichment in metal-poor environments. We complement our study with a self-consistent investigation of the gravitational-wave signals generated by the mergers of binary black holes that formed through stellar evolution and their detectability. The results presented here provide valuable predictions for the study of the EoR, on the crucial role of low-metallicity stellar populations in reionization mechanisms and star formation, as well as meaningful insights into potential observational counterparts to direct detections of Pop III stars.

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arXiv2024

Evolution and final fates of low- and intermediate-mass stars

Stars are unique bodies of the Universe where self-gravity compress matter to such high temperature and density that several nuclear fusion reactions ignite, providing enough feedback against further compression for a time that can be even larger than the age of the universe. The main property of a star is its mass because it determines its structure, evolutionary history, age, and ultimate fate. Depending on this quantity, stars are broadly classified as low-mass stars, like our Sun, intermediate mass stars as the variable star Delta Cephei, and massive stars as Betelgeuse, a red supergiant star in Orion constellation. Here we will introduce the basic notions useful to understand stellar evolution of low- and intermediate- mass stars. This mass range (0.1 M$_{\odot}$ - 10.0 M$_{\odot}$) deserves special attention, as it contains most of the stars in the universe. This chapter will focus on how these stars form, the processes that drive their evolution, and key details regarding their structure. Finally, we will discuss the death of such stars, emphasizing the unique fates associated with low- and intermediate-mass stars.

astro-ph.SR↗

arXiv2024

AESOPUS 2.1: Low-Temperature Opacities Extended to High Pressure

We address the critical need for accurate Rosseland mean gas opacities in high-pressure environments, spanning temperatures from 100 K to 32000 K. Current opacity tables from Wichita State University and AESOPUS 2.0 are limited to $\log(R) \le 1$, where $R=ρ\, T_6^{-3}$ in units of $\mathrm{g}\,\mathrm{cm}^{-3}(10^6\mathrm{K})^{-3}$. This is insufficient for modeling very low-mass stars, brown dwarfs, and planets with atmospheres exhibiting higher densities and pressures ($\log(R) > 1$). Leveraging extensive databases such as ExoMol, ExoMolOP, MoLLIST, and HITEMP, we focus on expanding the AESOPUS opacity calculations to cover a broad range of pressure and density conditions ($-8 \leq \log(R) \leq +6$). We incorporate the thermal Doppler mechanism and micro-turbulence velocity. Pressure broadening effects on molecular transitions, leading to Lorentzian or Voigt profiles, are explored in the context of atmospheric profiles for exoplanets, brown dwarfs, and low-mass stars. We also delve into the impact of electron degeneracy and non-ideal effects such as ionization potential depression under high-density conditions, emphasizing its notable influence on Rosseland mean opacities at temperatures exceeding $10,000$ K. As a result, this study expands AESOPUS public web interface for customized gas chemical mixtures, promoting flexibility in opacity calculations based on specific research needs. Additionally, pre-computed opacity tables, inclusive of condensates, are provided. We present a preliminary application to evolutionary models for very low-mass stars.

astro-ph.SR↗

arXiv2024

The cosmic rate of Pair-Instability Supernovae

Pair-instability supernovae (PISNe) have crucial implications for many astrophysical topics, including the search for very massive stars, the black hole mass spectrum, and galaxy chemical enrichment. To this end, we need to understand where PISNe are across cosmic time, and what are their favourable galactic environments. We present a new determination of the PISN rate as a function of redshift, obtained by combining up-to-date stellar evolution tracks from the PARSEC and FRANEC codes, with an up-to-date semi-empirical determination of the star formation rate and metallicity evolution of star-forming galaxies throughout cosmic history. We find the PISN rate to exhibit a huge dependence on the model assumptions, including the criterion to identify stars unstable to pair production, and the upper limit of the stellar initial mass function. Remarkably, the interplay between the maximum metallicity at which stars explode as PISNe, and the dispersion of the galaxy metallicity distribution, dominates the uncertainties, causing a $\sim$ seven-orders-of-magnitude PISN rate range. Furthermore, we show a comparison with the core-collapse supernova rate, and study the properties of the favourable PISN host galaxies. According to our results, the main contribution to the PISN rate comes from metallicities between $\sim 10^{-3}$ and $10^{-2}$, against the common assumption that views very-low-metallicity, Population III stars as exclusive or dominant PISN progenitors. The strong dependencies we find offer the opportunity to constrain stellar and galaxy evolution models based on possible future (or the lack of) PISN observations.

astro-ph.HE↗

arXiv2024

GalaPy, the highly optimised C++/Python spectral modelling tool for galaxies -- I. Library presentation and photometric fitting

Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe. To address these challenges, we have developed GalaPy, a new library for modelling and fitting SEDs of galaxies from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for attenuation, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. GalaPy has a hybrid implementation that combines the high performance of compiled C++ with the flexibility of Python, and exploits an object-oriented design. It generates models on the fly without relying on templates, and exploits fully Bayesian parameter space sampling. In this first work, we introduce the project and showcase the photometric SED fitting tools already available to users. The library is available on the Python Package Index (PyPI) and comes with extensive online documentation and tutorials.

astro-ph.GA↗

arXiv2024

The JWST Resolved Stellar Populations Early Release Science Program V. DOLPHOT Stellar Photometry for NIRCam and NIRISS

We present NIRCam and NIRISS modules for DOLPHOT, a widely-used crowded field stellar photometry package. We describe details of the modules including pixel masking, astrometric alignment, star finding, photometry, catalog creation, and artificial star tests (ASTs). We tested these modules using NIRCam and NIRISS images of M92 (a Milky Way globular cluster), Draco II (an ultra-faint dwarf galaxy), and WLM (a star-forming dwarf galaxy). DOLPHOT's photometry is highly precise and the color-magnitude diagrams are deeper and have better definition than anticipated during original program design in 2017. The primary systematic uncertainties in DOLPHOT's photometry arise from mismatches in the model and observed point spread functions (PSFs) and aperture corrections, each contributing $\lesssim0.01$ mag to the photometric error budget. Version 1.2 of WebbPSF models, which include charge diffusion and interpixel capacitance effects, significantly reduced PSF-related uncertainties. We also observed minor ($\lesssim0.05$ mag) chip-to-chip variations in NIRCam's zero points, which will be addressed by the JWST flux calibration program. Globular cluster observations are crucial for photometric calibration. Temporal variations in the photometry are generally $\lesssim0.01$ mag, although rare large misalignment events can introduce errors up to 0.08 mag. We provide recommended DOLPHOT parameters, guidelines for photometric reduction, and advice for improved observing strategies. Our ERS DOLPHOT data products are available on MAST, complemented by comprehensive online documentation and tutorials for using DOLPHOT with JWST imaging data.

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