arXiv subjects

Paolo Ventura

Publications and source records attributed to Paolo Ventura.

At least 19 recordsLinked to original sources

arXiv2026

Linear Stability of the Lamb-Chaplygin Dipole

We describe the linearized dynamics near the Lamb-Chaplygin dipole, a classical traveling solution of the two-dimensional Euler equations. Exploiting the Hamiltonian structure of the system together with its symmetries, we identify all possible sources of linear instability. For general perturbations in $L^1\cap L^p$, $p>2$, growth can occur through two explicit mechanisms triggered by: {\rm (i)} a nonzero circulation on the core of the dipole, and {\rm (ii)} a nontrivial component along the generalized eigenvectors associated with the eigenvalue $0$. In particular, we completely classify the spectrum and the Jordan chains of the operator associated with the linear dynamics. Both mechanisms hint for a nonlinear dynamics that may drift along the symmetry-generated family of traveling dipoles without moving away from it.

math.AP↗

arXiv2026

The pollution from massive AGB stars favoured by strong hot bottom burning

Stars of intermediate mass (~4-8Msun) evolve to the stage of white dwarfs through the asymptotic giant branch (AGB) stage: stationary hydrogen shell burning and helium thermal pulses, wind mass loss and planetary nebula ejection. Almost the totality of the mass lost (the initial mass minus the remnant white dwarf mass) is heavily processed `hot bottom burning' (HBB), as plain convection reaches the outer edge of the H-burning shell. This phase has been subject of intense investigations in the latest 25 years, in connection to three main research subjects: 1) the chemical evolution of proton-capture elements cycled in these stars, and their intrinsic uncertainties due to the uncertainty in the description of the AGB models; 2) the role of AGBs in the formation of multiple populations in globular clusters; 3) the possible AGB role in the composition of hot gas with high N/O in some primordial galaxies, particularly in those hosting a massive black holes. We here summarize some main achievements and problems in these research fields.

astro-ph.SR↗

arXiv2026

The applicability of the JAGB method for measuring the distance of galaxies subject to different metal enrichment rates

The JAGB method has been proposed in recent years as a possible distance indicator for galaxies in the Local Group and beyond. However, the nature of the stars populating the J region, and the conditions required for the direct application of this method, still need to be clarified. We investigate the robustness of the JAGB method through a detailed theoretical analysis of the stars populating the J region of the (J-Ks, J) diagram. The main goal is to identify the properties of the corresponding J luminosity function (JLF) that are minimally affected by the previous evolutionary history of the host galaxy, particularly its metal enrichment history. We use a population synthesis approach based on AGB stellar evolution models coupled consistently with dust formation in the stellar wind. Synthetic stellar distributions in the (J-Ks, J) diagram and the related JLFs are calculated for different assumptions on the metallicity evolution of the interstellar medium, in order to study how the JLF depends on the efficiency of metal enrichment. We find that the JAGB population is dominated by stars formed between about 1 and 6 Gyr ago, while stars formed outside this interval contribute only marginally to the JAGB region. The shape of the JLF strongly depends on the metal enrichment history, and the position of the J-band peak varies by more than 0.3 mag among the different cases explored. Conversely, the mean J-band magnitude, MavJ, is much less sensitive to the previous history of the galaxy and therefore represents a more reliable distance indicator. For all the cases investigated we find MavJ = -6.2 +/- 0.05 mag. We also discuss the uncertainties related to the still poorly constrained mass-loss process experienced by low-mass stars during the red giant branch phase.

astro-ph.GA↗

arXiv2026

The role of mass loss in constraining quenching time in dwarf galaxies from AGB and RGB star counts

The capability of reconstructing the past star formation history of dwarf elliptical galaxies out of the Local Volume relies on modelling bright stellar populations currently evolving through the red giant branch (RGB) and the asymptotic giant branch (AGB) phases. Recent studies proposed the use of the relative fractions of RGB and AGB stars populating specific boxes in the observational colour-magnitude plane to infer the epoch within which 90\% of the stellar population of the galaxy formed (T90). We aim at understanding the physical process of stellar evolution that are constrained by the relationship between the relative fraction of AGB and RGB stars of dwarf galaxies and the T90 epoch. We use updated stellar models that include the description of dust formation in the wind, to undertake a population synthesis approach, to allow monitoring the variation of the NAGB/NRGB ratio with time. The effects of some specific ingredients, such as the mass loss experienced by low-mass stars during the RGB phase, and the details of the time variation of the star formation rate, are extensively explored and tested against data. The mass lost by low-mass stars during the RGB evolution proves the most relevant ingredients affecting the time variation of NAGB/NRGB: at metallicities ~ 1/10 solar, a mass loss ~ 0.25Msun is required to reproduce the observations. This analysis allows to derive a relationship between NAGB/NRGB and T90, with a ~ 1 Gyr uncertainty on T90.

astro-ph.GA↗

arXiv2026

Instability of two-dimensional Taylor-Green Vortices

For a wide class of linear Hamiltonian operators we develop a general criterion that characterizes the unstable eigenvalues as the zeros of a holomorphic function given by the determinant of a finite-dimensional matrix. We apply the latter result to prove the spectral instability of the Taylor-Green vortex in two-dimensional ideal fluids. The linearized Euler operator at this steady state possesses different invariant subspaces, within which we apply our criterion to rule out or detect instabilities. We show linear stability of odd perturbations, for which the unstable spectrum can appear only on the real axis. We exclude this possibility by applying our stability criterion. Real instabilities, instead, exist and can be detected with the same criterion if we consider suitable rescalings of the Taylor-Green vortex. In the subspace of functions even in both variables, the problem is reduced to finding a single complex root of our stability function. We successfully locate this value by combining our general criterion with a rigorous computer-assisted argument. As a consequence, we fully characterize the unstable spectrum of the Taylor-Green vortex.

math.AP↗

arXiv2026

Large-scale time-series spectroscopy for stellar ages

To date, Galactic Astronomy has largely concerned itself with astrophysical processes, and with the locations, space motions and compositions of objects. Consider, for example, the elucidation of the components of the Galaxy over the past decades, its mapping as enabled by Gaia and its predecessors, the photometric and spectroscopic characterization of innumerable astrophysical objects in various wavelength ranges, both from the ground and from space, and the expanding discovery and characterization of exoplanets; all focused on the current, static Galaxy. This White Paper proposes a dedicated program to derive stellar ages from time-series spectroscopy to hasten the transformation of this static conception into a dynamical one with age-labeled objects and events.

astro-ph.IM↗

arXiv2025

Long-wave instability of periodic shear flows for the 2D Navier-Stokes equations

In 1959, Kolmogorov proposed to study the instability of the shear flow $(\sin(y),0)$ in the vanishing viscosity regime in tori $\mathbb{T}_α\times \mathbb{T}$. This question was later resolved by Meshalkin and Sinai. We extend the problem to general shear flows $(U(y),0)$ and show that every $U(y)$ exhibits long-wave instability whenever $\|\partial_y^{-1} U\|_{L^2} > ν$ and $α\ll ν$, with $ν$ being the kinematic viscosity. This instability mechanism confirms previous findings by Yudovich in 1966, supported also by several numerical results, and is established through two independent approaches: one via the construction of Kato's isomorphism and one via normal forms. Unlike in many other applications of the latter methods, both proofs deal with the presence of a delicate term in the linearized operator that becomes singular as $α$ approaches $0$.

math.AP↗

arXiv2025

s-process Enriched Post-AGB Star J003643.94-723722.1 in the SMC with an Extreme C/O Ratio and the First Precise Detection of Lead

Post-asymptotic giant branch (post-AGB) stars are exquisite tracers of s-process nucleosynthesis via their surface abundances. We present a comprehensive analysis of J003643.94$-$723722.1 (J003643), a single SMC post-AGB star, using high-resolution UVES/VLT spectra analysed with E-iSpec. We find C/O = 16.21 and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20 \mathrm{dex}$. We also report the first direct Pb detection in a post-AGB star from the Pb II 5608.853 A line, with $[\mathrm{Pb}/\mathrm{Fe}]=3.18\,\mathrm{dex}$. Comparison with post-AGB samples in the Galaxy and Magellanic Clouds reveals that J003643 has an unusually high C/O ratio. J003643's $[\mathrm{C}/\mathrm{Fe}]=1.33\pm0.14 \mathrm{dex}$ and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20\,\mathrm{dex}$ are consistent with third dredge-up enrichment, but its $[\mathrm{O}/\mathrm{Fe}]=-0.08\pm0.20\,\mathrm{dex}$ is low relative to objects of similar $[\mathrm{C}/\mathrm{Fe}]$ and $[\mathrm{Fe}/\mathrm{H}]$. Together with $[α/\mathrm{Fe}]\approx0\,\mathrm{dex}$ at $[\mathrm{Fe}/\mathrm{H}]\approx-1\,\mathrm{dex}$, consistent with SMC chemical evolution, this indicates the high C/O chiefly reflects oxygen deficiency rather than exceptional carbon enrichment. Additionally, we compare the full abundance pattern with yields from ATON, MONASH and FRUITY (the latter two with post-processing nucleosynthesis). Most elements are reproduced, but Pb is strongly underpredicted, highlighting a persistent gap in models of heavy-element production in AGB stars. The photospheric chemistry of J003643 adds to the growing diversity among post-AGB stars and underscores the complexity of AGB nucleosynthesis.

astro-ph.SR↗

arXiv2025

Infinitely many isolas of modulational instability for Stokes waves

This paper proves long-standing conjectures regarding the existence of infinitely many high-frequency modulational instability ``isolas" for a Stokes wave in arbitrary depth $ \mathtt{h} > 0 $, under longitudinal perturbations. We provide a complete characterization of the unstable spectral bands in the $L^2(\mathbb{R})$-spectrum of the water wave equations linearized around a Stokes wave of sufficiently small amplitude $ε$. The unstable spectrum is the union of isolated ``isolas" of elliptical shape, indexed by integers $ \mathtt{p}\geq 2 $, each with semiaxis of size $ |β_1^{(\mathtt{p})} (\mathtt{h})| ε^\mathtt{p}+ O(ε^{\mathtt{p}+2} )$. As first key achievement, we obtain an explicit formula for the coefficient $ β_1^{(\mathtt{p})} (\mathtt{h}) $ for any $ \mathtt{p} \geq 2 $, that remarkably depends solely on the maximal Taylor-Fourier coefficients of the Stokes wave. We provide simple expressions of the asymptotic expansion of such coefficients in the shallow-water limit $ \mathtt{h} \to 0^+ $, for any $ \mathtt{p} \geq 2 $. This allows to establish that the analytic function $β_1^{(\mathtt{p})}(\mathtt{h})$ is not zero for any $\mathtt{p} \geq 2$, by verifying that a combinatorial sum is not zero; this relies on a crucial combinatorial identity due to Koutschan, van Hoeij, and Zeilberger.

math.AP↗

arXiv2025

FEAST: probing the stellar population of the starburst dwarf galaxy NGC4449 with JWST/NIRCam

We present new JWST/NIRCam observations of the starburst irregular galaxy NGC 4449, obtained in Cycle 1 as part of the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program, which we use to investigate its resolved stellar populations and their spatial distributions. NGC4449 NIR color-magnitude diagrams reveal a broad range of stellar populations, spanning different evolutionary phases, from young main sequence stars, to old red giant branch stars and asymptotic giant branch (AGB) stars. The analysis of their spatial distributions shows that younger (< 10 Myr) populations form an S-shaped distribution aligned with the galaxy's north-south axis, while stars aged 10 - 60 Myr show shifting concentrations from the north to the south, consistent with the possibility that external interactions or tidal effects may have triggered star formation in spatially distinct bursts. Clusters of comparable ages generally follow these distributions, suggesting that cluster and field stars form at the same pace in each galaxy region. Thanks to the unprecedented high-spatial resolution and sensitivity of the JWST data we recover a clear gap between Oxygen-rich and the carbon star branch of the AGB population and the presence of a massive AGB star "finger". The analysis of these stars can provide constraints on AGB evolution models and dust production in this galaxy. These results confirms NGC 4449 status as a compelling example of a local dwarf starburst galaxy undergoing complex and possibly external driven star formation and underscore the power of JWST in probing the full lifecycle of stars in nearby starburst systems.

astro-ph.GA↗

arXiv2025

A study of Andromeda to improve our knowledge on the evolution and dust production by AGB stars

We study the AGB population of the galaxy M31, based on available HST and Spitzer data, to characterize the individual sources in terms of mass, metallicity and formation epoch of the progenitors. Particular attention is dedicated to the derivation of the dust production rate of the stars, in the attempt of determining the global current dust production rate of the galaxy, divided between the silicates and the carbonaceous dust contributions. We use results from stellar evolution modelling complemented by the description of the dust formation process in the wind, to be used in a population synthesis approach, based on the star formation history and age-metallicity relationship obtained in previous investigations. The comparison between the results from synthetic modelling and the data available are used for the characterization of AGB stars in M31. We find that the bulk of the AGB population of M31 is composed by low-mass stars of different metallicity formed between 6 Gyr and 14 Gyr ago, with an additional, significant contribution from the progeny of 1.7-2.5Msun stars formed during the secondary peak in the star formation, which occurred between 1 and 2 Gyr ago. The dust production rate of the galaxy is mostly provided by carbon stars, whose contribution is of the order of 4x10^{-4} Msun/yr, completed by silicates production from massive AGB stars, occurring at a rate of 6x10^{-5} Msun/yr. The implications of the present results on the reliability of AGB modelling are also commented.

astro-ph.GA↗

arXiv2025

Optimizing train dispatching for the Union Pacific Railroad

Union Pacific (UP) is one of the largest transportation companies in the world, with over 50.000 kms of rail network covering 23 states in the United States. In 2017 Union Pacific embarked on a project that within 5 years would lead it to become the only rail operator in the world equipped with a technology capable of fully automating the real-time management and optimization of train traffic. In 2021 the main milestone of such project has been reached with the first deployment of the automated dispatching system we present here. To attack such large and complex problem, we decomposed it into distinct but interrelated functional components, and developed optimization models and methods to handle such components. The models communicate with each other through variables and constraints, and by a careful timing of invocations. In this paper we give an overview of the overall approach.

math.OC↗

arXiv2025

On higher order isolas of unstable Stokes waves

We overview the recent result [3, Theorem 1.1] about the high-frequency instability of Stokes waves subject to longitudinal perturbations. The spectral bands of unstable eigenvalues away from the origin form a sequence of {\it isolas} parameterized by an integer $ \mathtt{p} \geq 2 $ for any value of the depth $ \mathtt{h} > 0 $ such that an explicit analytic function $β_1^{(\mathtt{p})}(\mathtt{h}) $ is not zero. In [3] it is proved that the map $ \mathtt{h} \mapsto β_1^{(\mathtt{p})}(\mathtt{h}) $ is not identically zero for any $ \mathtt{p} \geq 2 $ by showing that $ \lim_{\mathtt{h} \to 0^+}β_1^{(\mathtt{p})}(\mathtt{h}) = - \infty $. In this manuscript we compute the asymptotic expansion of $β_1^{(\mathtt{p})}(\mathtt{h}) $ in the deep-water limit $ \mathtt{h} \to + \infty $ -- it vanishes exponentially fast to zero -- for $\mathtt{p}=2$, $3$, $4$.

math.AP↗

arXiv2024

Planetary Nebulae of the Large Magellanic Cloud I: A multiwavelength analysis

Planetary nebulae (PNe) have three main components: a central star (CS), ionised gas and dust in the nebula. Each of them contains critical chemical fingerprints of their evolution, serving as tracers of the evolution, nucleosynthesis and dust production that occurred during the preceding asymptotic giant branch (AGB) phase. We aim to build a bridge to link the PN phase to the evolution of their progenitors, trying to better understand the dust production and mass-loss mechanism during the final AGB phase. Here, we present a comprehensive study of nine Large Magellanic Cloud (LMC) spherical or elliptical PNe whose observations from the ultraviolet (UV) through the infrared (IR) are available in the literature. We characterize nebulae and CSs, finding information as the amount of gas that makes up the nebula and the dust that surrounds the CS, necessary to reconstruct the evolutionary history of mass-loss and dust production. We compare the observed energy distribution of the selected PNe to that obtained from photoionization modeling, taking into account the presence of dust. The physical and chemical parameters of the central stars are then compared with the predictions from the evolutionary tracks. We characterized the source, assigning to each CS a progenitor, early-AGB mass. We estimated the mass of the nebula and the dust-to-gas ratio. For 5 objects, we find evidence for the presence of a near-IR bump, which would be connected to the presence of hot dust.

astro-ph.GA↗

arXiv2024

s-process Enriched Evolved Binaries in the Galaxy and the Magellanic Clouds

Post-asymptotic giant branch stars (post-AGB) in binary systems, with typical orbital periods between ~100 to ~1000 days, result from a poorly understood interaction that terminates their precursory AGB phase. The majority of these binaries display a photospheric anomaly called 'chemical depletion', thought to arise from an interaction between the circumbinary disc and the post-AGB star, leading to the reaccretion of pure gas onto the star, devoid of refractory elements due to dust formation. In this paper, we focus on a subset of chemically peculiar binary post-AGBs in the Galaxy and the Magellanic Clouds (MCs) whose high-resolution optical spectroscopic study revealed a carbon and s-process enrichment, contrary to the commonly observed photospheric chemical depletion. Using spectral energy distribution (SED) fitting and period-luminosity-colour (PLC) relation methods, we determine the luminosity of the targets ($2700-8300L_{\odot}$), which enables confirmation of their evolutionary phase and estimation of initial masses (as a function of metallicity) ($1-2.5M_{\odot}$). In conjunction with predictions from dedicated ATON stellar evolutionary models, our results indicate a predominant intrinsic enrichment of carbon and s-process elements in our binary post-AGB targets. We qualitatively rule out extrinsic enrichment and inherited s-process enrichment from the host galaxy as plausible explanations for the observed overabundances. Our chemically peculiar subset of intrinsic carbon and s-process enriched binary post-AGBs also hints at potential variation in the efficiency of chemical depletion between stars with C-rich and O-rich circumbinary disc chemistries. However, critical observational studies of circumbinary disc chemistry are necessary to address gaps in our current understanding of disc-binary interactions inducing chemical depletion in binary post-AGB systems.

astro-ph.SR↗

arXiv2024

The first measurements of carbon isotopic ratios in post-RGB stars: SZ Mon and DF Cyg. E-iSpec: A spectral analysis tool to derive elemental abundances and isotopic ratios for evolved stars

Dusty post-red giant branch (post-RGB) stars are low- and intermediate-mass stars where the RGB evolution was prematurely terminated by a poorly understood binary interaction. These binary stars are considered to be low-luminosity analogues of post-asymptotic giant branch (post-AGB) binary stars. In this study, we investigated the chemical composition of two dusty post-RGB binary stars, SZ Mon and DF Cyg, using multi-wavelength spectroscopic data from HERMES/Mercator (optical) and the APOGEE survey (near-infrared). Owing to challenges posed by existing spectral analysis tools for the study of evolved stars with complex atmospheres, we developed E-iSpec: a dedicated spectral analysis tool for evolved stars, to consistently determine atmospheric parameters, elemental abundances, and carbon isotopic ratios. Our abundance analysis revealed that observed depletion patterns and estimated depletion efficiencies resemble those found in post-AGB binary stars. However, the onset of chemical depletion in post-RGB targets occurs at higher condensation temperatures ($T_{\rm turn-off, post-RGB}\approx1400$ K), than in most post-AGB stars ($T_{\rm turn-off, post-AGB}\approx1100$ K). Additionally, our study resulted in the first estimates of carbon isotopic ratios for post-RGB stars ($^{12}$C/$^{13}$C$_{\rm SZ Mon}=8\pm4$, $^{12}$C/$^{13}$C$_{\rm DF Cyg}=12\pm3$). We found that the observationally derived CNO abundances and the carbon isotopic ratios of our post-RGB binary targets are in good agreement with theoretical predictions from the ATON single star evolutionary models involving first dredge-up and moderately-deep extra mixing. This agreement emphasises that in post-RGB binary targets, the observed CNO abundances reflect the chemical composition expected from single star nucleosynthesis (i.e., convective and non-convective mixing processes) occurring during the RGB phase before it is terminated.

astro-ph.SR↗

arXiv2024

First isola of modulational instability of Stokes waves in deep water

We prove high-frequency modulational instability of small-amplitude Stokes waves in deep water under longitudinal perturbations, providing the first isola of unstable eigenvalues branching off from $\mathtt{i}\frac34$. Unlike the finite depth case this is a degenerate problem and the real part of the unstable eigenvalues has a much smaller size than in finite depth. By a symplectic version of Kato theory we reduce to search the eigenvalues of a $2\times 2$ Hamiltonian and reversible matrix which has eigenvalues with non-zero real part if and only if a certain analytic function is not identically zero. In deep water we prove that the Taylor coefficients up to order three of this function vanish, but not the fourth-order one.

math.AP↗

arXiv2023

MgAl burning chain in Omega Centauri

In this study, we report the results of Fe, Mg, Al, and Si abundances analysis for a sample of 439 stars in Omega Centauri, using high-resolution spectra obtained with the VLT/FLAMES multi-object spectrograph. Our analysis reveals the presence of four distinct Fe populations, with the main peak occurring at low metallicity, consistent with previous literature findings. We observe a discrete and pronounced Mg-Al anti-correlation, which exhibits variations in shape and extension as a function of metallicity. Specifically, this anti-correlation is present in stars with metallicities lower than approximately -1.3 dex, while it becomes less evident or absent for higher [Fe/H] values. Additionally, we detect (anti-) correlations between Mg and Si, and between Al and Si, whose extensions also vary with metallicity, similar to the Mg-Al anti-correlation. These results suggest that the MgAl cycle plays a crucial role in the formation of multiple populations in Omega Centauri, with the presence of all (anti-) correlations at metallicities lower than -1.3 dex providing evidence for the burning of Mg at very high temperatures (> 10^8 K), at least in the metal-poor regime. Furthermore, we observe a clear trend of stars with [Al/Fe] > +0.5 dex as a function of metallicity, confirming for the first time the existence of the two channels of Al production and destruction. This evidence can help to provide further constraints on the potential nature of the polluters responsible for the observed chemical anomalies in this stellar system. Finally, we find that the two most metal-poor populations identified in our sample are compatible with null or very small metallicity dispersion and we discuss how this result fit into a scenario where Omega Centauri is the remnant of a disrupted nucleated dwarf galaxy.

astro-ph.SR↗