arXiv subjects

Stefano Menchiari

Publications and source records attributed to Stefano Menchiari.

13 recordsLinked to original sources

arXiv2026

A radio view on Gamma-Loud Protostars: Derivation of jet mechanical luminosity

Context. Gamma-Loud Protostars (GLPs) have been recently reported as Galactic hadronic accelerators whose acceleration site is situated in their protostellar jets. Theoretical and observational analysis situate radio cm luminosity as a thermal tracer of jet activity, presenting an unique opportunity to study jet properties as accelerators. Aims. We aim to develop a way to estimate the kinetic power of protostellar jets and compare their energetics with those extracted from the non-thermal gamma-ray side of GLPs. Methods. We combine theoretical and phenomenological relations to estimate the jet mechanical luminosity based on the radio cm luminosity. We relate the resulting values to the non-thermal contribution of GLPs, studying its behaviour and efficiency. Results. The derivation of the jet power successfully reproduces infrared and radio observations. The cosmic-ray energy correlates to the injected mechanical energy, implying an acceleration efficiency of 1--10%. Future radio observations are needed to find the definite accelerators and obtain reliable efficiencies.

astro-ph.HE↗

arXiv2026

Evidence for protostellar jets as a population of hadronic gamma-ray sources

Stars are born in darkness, deep within cold, dense molecular clouds where gravity drives the collapse of gas and dust, giving rise to protostars, the earliest stages of stellar evolution. Once considered purely thermal sources, these young systems are now emerging as sites of energetic non-thermal activity. While radio synchrotron jets hinted at the presence of relativistic electrons, direct confirmation of proton acceleration remained elusive. Here we report a statistically significant detection of gamma rays from a population of young stellar objects, revealing a Galactic class of Gamma-Loud Protostars. Observations point towards particle acceleration within protostellar jets, where gamma-ray emission arises from protons interacting with surrounding molecular clouds via pion decay. We find a correlation between cosmic-ray output and bolometric luminosity, suggesting that particle acceleration scales with the system's mechanical power. These findings open a new observational window into the role of non-thermal processes in protostellar evolution and suggest that gamma-ray studies of protostars can provide critical insights into accretion, ejection, and feedback in star formation. This previously overlooked emission traces the energetic feedback that young stars inject into their surroundings, shaping the conditions for subsequent star and planet formation.

astro-ph.HE↗

arXiv2026

Modelling Galactic neutrino emission: contributions from massive star clusters and interstellar cosmic rays

The recent detection of Galactic neutrinos by the IceCube Observatory constitutes a remarkable achievement for neutrino astrophysics. By means of model dependent analyses based on spatial and spectral templates, a purely diffuse neutrino flux was measured in which no individual source was resolved. We present here a novel theoretical computation about the expected neutrino emission from the Galactic Plane that, differently from previous models, includes both the contributions from cosmic-ray (CR) sea and hadronic sources, represented by star clusters and supernova remnants therein, which are to date believed to be the dominant sources of Galactic CR protons. For the modelling of sources, diffusive particle acceleration is considered at both the collective wind termination shock blown by member stars and at the supernova shocks. The predicted flux of very-high energy neutrinos from individual star clusters is found to be marginally detectable even by cubic kilometer scale detectors, such that their cumulative contribution is expected to appear as an unresolved diffuse component, on top of that guaranteed by the CR sea interacting with the gas along the Plane. The overall neutrino production of the Milky Way star cluster population is computed, based on multiple synthetic realizations of the cluster population reproducing local stellar observations. As a result, we obtain novel neutrino template maps and provide them to the community, to be tested in future neutrino analyses in order to constrain the role of star clusters for extreme CR acceleration and neutrino production. The normalization of our models is consistent with the IceCube best-fit of existing Galactic templates, suggesting that the unresolved contribution from cluster emission may be non-negligible.

astro-ph.HE↗

arXiv2026

SAETASS: Solver for Astroparticle Equation of Transport Analysis in Spherical Symmetry

In order to model astrophysical environments characterized by radial stratification, such as supernova remnants or expanding superbubbles; correctly understanding the transport of non-thermal particles in astrophysical plasmas is essential. While large-scale Galactic propagation codes exist, they are often optimized for Cartesian or cylindrical geometries and lack the efficiency of one-dimensional spherically symmetric problems. In this work, we present SAETASS (Solver for Astroparticle Equation of Transport Analysis in Spherical Symmetry), a novel, open-source numerical tool designed to solve the time-dependent transport equation for astroparticles. The solver is built upon a conservative finite-volume framework that ensures exact particle conservation and numerical stability. To manage the interplay between diverse physical processes, SAETASS employs a modular operator-splitting architecture. Radial advection and continuous momentum losses are treated using a second-order, shock-capturing MUSCL-Hancock scheme, while the diffusive operator is integrated via an implicit, batched Crank-Nicolson algorithm. This approach allows for the robust handling of steep gradients, spatial discontinuities and regularity conditions at the origin. We rigorously validate the code through a suite of tests for pure advection, diffusion and losses. Finally, we demonstrate the solver's capabilities by modelling cosmic-ray proton transport in a real astrophysical scenario. Our results successfully recover established steady-state limits while revealing relevant pre-equilibrium temporal dynamics across Kolmogorov, Kraichnan and Bohm diffusion regimes. SAETASS provides the community with a lightweight, flexible tool for investigating particle acceleration and propagation in complex, radially dependent astrophysical environments.

astro-ph.HE↗

arXiv2026

Cosmic rays: constraints from future MeV detectors

Cosmic rays are charged energetic particles that permeate the interstellar medium. Their sizeable energy share and penetration power makes them essential players in the dynamical and chemical processes that rule Galactic evolution, such as the launching of outflows and the formation of star and planets. For these processes low-energy (MeV-GeV) CRs are particularly important, both because they are the most abundant and because they have the largest cross-section for ionization. The study of cosmic rays naturally connects with gamma-ray astronomy, as high-energy photons are the principal products of their interaction with the interstellar plasma. In this article, after reviewing our current understanding of Galactic cosmic rays as derived from direct measurements, we present the state of the art regarding Galactic cosmic rays covering their direct observables, their acceleration processes and models for their propagation in the Galactic Disk. We present then an excursus on the current state of gamma-ray observations, and propose new prospects for investigating the physical properties of Galactic cosmic rays, exploiting the observational capability of future MeV missions.

astro-ph.HE↗

arXiv2025

Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Fermi-LAT: constraints on the acceleration efficiency

Context. Young Massive Star Clusters, long considered as potentially important sources of galactic cosmic rays, have recently emerged as gamma-ray emitters up to very high energies. Aims. In order to quantify the contribution of this source class to the pool of Galactic CRs, we need to estimate the typical acceleration efficiency of these systems. Methods. We search for emission in the GeV band, as most of the energy is emitted in this band. We perform an analysis of Fermi-LAT data collected towards the M16 region, a star-forming region also known as the Eagle Nebula, which hosts the Young Massive Star Cluster NGC 6611. We model the acceleration at the stellar wind termination shock and the propagation through the wind-blown bubble to derive the energetics of the process and interpret the GeV observations. Results. We find significant GeV emission in correspondence of a molecular cloud associated to the Young Massive Star Cluster NGC 6611. We interpret this as hadronic emission associated to particle accelerated at the cluster wind termination shock and propagated through the low-density wind-excavated bubble to the cloud. Our modeling allows us to put firm constraints on the acceleration efficiency in NGC 6611, assessing it between $\sim$ 1 % and $\sim$ 4 %.

astro-ph.HE↗

arXiv2025

Gamma ray emission from embedded young massive star clusters unveiled by Fermi-LAT

Massive star clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNR) to reach the highest energy end of the Galactic CR spectrum. Thanks to fast mass losses through collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies. A handful of star clusters has been detected in gamma-rays confirming the idea that particle acceleration is taking place in these environments. Here we present a new analysis of Fermi-LAT data collected towards a few massive young star clusters and estimate the contribution of these types of sources to the bulk of CRs. We then briefly discuss the observational prospects for ASTRI and CTAO.

astro-ph.HE↗

arXiv2024

Interpreting the LHAASO Galactic diffuse emission data

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration has obtained a measurement of the gamma-ray diffuse emission in the ultra-high energy range, $10-10^3$ TeV after masking the contribution of known sources. The measurement is provided in two Galactic regions and appears to be 2-3 times higher than the gamma-ray signal expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially implying that either additional emission sources exist or cosmic ray intensities have spatial variations. In this work, we calculate the hadronic gamma-ray diffuse emission outside the masks, considering a realistic gas distribution. We present a comprehensive calculation of the emission, which includes systematic uncertainties in the gas content of the Galactic disk, in the energy and spatial distribution of cosmic rays, as well as in the hadronic interaction cross-sections. Our results show that these factors mitigate the tension between data and predictions. The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below $\sim 50$ TeV, while at higher energies they are well described by our model. We argue that two plausible explanations for enhanced gamma-ray emission-unresolved sources and CR spectral hardening in the inner Galaxy-are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent.

astro-ph.HE↗

arXiv2024

Contribution of young massive star clusters to Galactic diffuse $\gamma$-ray emission

Context: Young massive stellar clusters (YMSCs) have emerged as potential $\gamma$-ray sources, after the recent association of a dozen YMSCs with extended $\gamma$-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the $\gamma$-ray detection of individual YMSCs rather challenging. As a result, the emission from most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse $\gamma$-ray radiation observed along the Galactic Plane. Aims: In this study, we estimate the possible contribution to the Galactic diffuse $\gamma$-ray emission from a synthetic population of YMSCs, and we compare it with observations obtained with different experiments, from 1 GeV to hundreds of TeV, in two regions of the Galactic Plane. Methods: As the population of galactic YMSCs is only known locally, we evaluate the contribution of $\gamma$-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We compute the $\gamma$-ray emission from each cluster assuming that the radiation is purely hadronic in nature and produced by cosmic rays accelerated at the cluster's collective wind termination shock. Results: We find that the $\gamma$-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy, and that an important role is played by the power injected by Wolf-Rayet stellar winds. The predicted $\gamma$-ray flux should be considered as a lower limit, given that our calculation neglects the contribution of supernovae exploding in YMSCs.

astro-ph.HE↗

arXiv2024

Probing hadronic gamma-ray and high-energy neutrino emission from Gaia DR2 star clusters

Young and massive stellar clusters are a potential source of galactic cosmic rays due to at least two acceleration mechanisms. Collective stellar winds from massive stars form a wind-blown bubble with a termination shock at which particle acceleration to PeV energies may be achieved. Furthermore, shock acceleration may occur at supernova remnants (SNRs) expanding inside the bubble. We apply a model of cosmic ray acceleration at both the collective wind termination shock and SNR shocks to the catalog of known stellar clusters derived from the Gaia DR2. Predictions for the secondary fluxes of gamma-ray and neutrino emission are derived based on hadro-nuclear interactions with the surrounding medium. We compare our modelling under baseline and optimistic scenarios to available data, finding consistent results. An anticipated flux range is provided for a shortlist of the most promising stellar clusters. Approximately 10 clusters may be detectable with future facilities, and 1-3 could be currently operating as PeVatrons. Among these, data from three gamma-ray detected clusters can be consistently described by our model. Several further as-yet-undetected stellar clusters offer promising targets for future gamma-ray observations, although the flux range allowed by our model can be broad (>~factor 10). The large angular size of the wind-blown bubble may pose a challenge, leading to low surface brightness emission, thus exacerbating the problem of source confusion. Nevertheless, we discuss how further work will help to constrain stellar clusters as PeVatron candidates.

astro-ph.HE↗

arXiv2024

Cygnus OB2 as a test case for particle acceleration in young massive star clusters

In this paper, we focus on the scientific case of Cygnus OB2, a northern sky young massive stellar cluster (YMSC) located towards the Cygnus X star-forming complex. We consider a model that assumes cosmic ray acceleration occurring only at the termination shock of the collective wind of the YMSC and address the question of whether, and under what hypotheses, hadronic emission by the accelerated particles can account for the observations of Cygnus OB2 obtained by Fermi-LAT, HAWC and LHAASO. In order to do so, we carefully review the available information on this source, also confronting different estimates of the relevant parameters with ad hoc developed simulations. Once other model parameters are fixed, the spectral and spatial properties of the emission are found to be very sensitive to the unknown properties of the turbulent magnetic field. Comparison with the data shows that our suggested scenario is incompatible with Kolmogorov turbulence. Assuming Kraichnan or Bohm type turbulence spectra, the model accounts well for the Very High Energy (VHE) data, but fails to reproduce the centrally peaked morphology observed by Fermi-LAT, suggesting that additional effects might be important for lower energy $γ$-ray emission. We discuss how additional progress can be made with a more detailed and extended knowledge of the spectral and morphological properties of the emission.

astro-ph.HE↗

arXiv2023

Mass and wind luminosity of young Galactic open clusters in Gaia DR2

Context. Star clusters constitute a significant part of the stellar population in our Galaxy. The feedback processes they exert on the interstellar medium impact multiple physical processes from the chemical to the dynamical evolution of the Galaxy. In addition, young and massive stellar clusters might act as efficient particle accelerators and contribute to the production of cosmic rays. Aims. We aim at evaluating the wind luminosity driven by the young (< 30 Myr) Galactic open stellar clusters observed by the Gaia space mission. This is crucial for determining the energy channeled into accelerated particles. Methods. To do this, we developed a method relying on the number, magnitude, and line-of-sight extinction of the stars observed per cluster. Assuming that the stellar mass function follows a Kroupa mass distribution and accounting for the maximum stellar mass allowed by the age and mass of the parent cluster, we conservatively estimated the mass and wind luminosity of 387 local clusters within the second data release of Gaia. Results. We compared the results of our computation with recent estimates of young cluster masses. With respect to these, our sample is three times more abundant, particularly above a few thousand solar masses. This is of the utmost relevance for predicting the gamma-ray emission resulting from the interaction of accelerated particles. The cluster wind luminosity distribution we obtained extends up to 3x10^38 erg/s. This is a promising feature in terms of potential particle acceleration scenarios.

astro-ph.GA↗

arXiv2023

Probing star clusters as cosmic ray factories

Stellar clusters (SC) are fundamental building blocks of galaxies and are among the most studied astronomical objects in the Cosmos. The recent association of diffuse $γ$-ray emission detected by different experiments with a dozen young SCs suggests the presence of some process able to accelerate particles at least up to hundreds of TeV. In this Ph.D. thesis, we investigate the capability of young massive stellar clusters (YMSC) to produce cosmic rays under the assumption that particles are accelerated at the cluster wind termination shock. The study is divided into three parts. First, we focus on the specific case of Cygnus OB2. We model the observed $γ$-ray emission (in a pure hadronic scenario) assuming different models for particle propagation in the neighborhood of the cluster. We found that particles accelerated by Cygnus OB2 can account for both the $γ$-ray spectrum and the radial morphology at very high energy. In the second part, we compute the diffuse $γ$-ray emission expected by the unresolved population of Galactic YMSC. For this purpose, we build a synthetic population of YMSC based on the properties of local SC. Under the assumption of a pure hadronic emission, we found that YMSCs can significantly contribute to the observed diffuse $γ$-ray emission at a few TeV. The final part of the work is dedicated to understand the impact of CRs produced by SCs on the ionization rate of molecular clouds close to those SCs. We found that the ionization rate can significantly differ from the expected value in clouds located in the unperturbed interstellar medium. We show that the measured value of ionization rate, paired with $γ$-ray observations, can be used to constrain particle diffusion in the vicinity of the stellar cluster.

astro-ph.HE↗