Pub Date : 2026-02-25DOI: 10.1051/0004-6361/202557616
A. Guseva, L. Manchon, L. Petitdemange, C. Pinçon
Context. Despite significant progress in the observational characterization of stellar magnetic fields, the physical processes that govern their intensity and topology, which could certainly result from their formation history, remain poorly understood. During the pre-main-sequence (PMS) phase, the inner layers of these stars tend to contract, and a radiative core gradually develops. In contrast, the convective envelope is gradually braked through the magnetic interactions with the accretion disk and winds, thus slowly developing a differential rotation inside the star. It is likely during this PMS phase that the dynamo processes that efficiently generated strong dipolar magnetic fields through vigorous convective motions in protostars become highly perturbed, leading to the observed diversity in the magnetism on the main sequence.Aims. We aim to study the stability of dipolar magnetic fields inherited from the proto-stellar phase by considering the emergence of a large-scale radial differential rotation resulting from the combined actions of contraction and of the interactions with the surrounding medium.Methods. We performed 3D convective dynamo simulations of rotating spherical shells with an imposed differential rotation (shear) between the bottom and top boundaries. We used anelastic approximation, which allowed us to consider background density and gravity profiles and convective zone thicknesses close to those predicted in PMS low-mass stars by the 1D stellar evolution code Cesam2k20. We then carried out a parameter study by systematically varying the shear amplitude.Results. Radial differential rotation can induce dipole collapse leading to weaker and oscillatory magnetic fields. We highlight that the stability of dipolar dynamos mainly depends on the relative importance of shear measured by a shear-based Rossby number compared to the vigor of convective motions as measured by the convective Rossby number. We show that the stability criterion depends on the field strength and the size of the radiative core. Differential rotation seems to perturb the α2 dynamo mechanism responsible for dipolar magnetic fields by shearing poloidal field lines and affecting turbulent magnetic transport processes.Conclusions. The PMS phase can represent a critical period for the magnetic properties of stars, as the development of shear layers can perturb the stability of strong initial dipoles. Applying the stability criterion in PMS stellar evolution models, we qualitatively reproduced the trends observed in the magnetic topologies of low-mass stars when assuming an efficient internal angular momentum redistribution process. This suggests that stellar magnetic properties are intimately related to the PMS angular momentum evolution.
{"title":"Radial differential rotation leading to dipole collapse in pre-main-sequence stars","authors":"A. Guseva, L. Manchon, L. Petitdemange, C. Pinçon","doi":"10.1051/0004-6361/202557616","DOIUrl":"https://doi.org/10.1051/0004-6361/202557616","url":null,"abstract":"<i>Context.<i/> Despite significant progress in the observational characterization of stellar magnetic fields, the physical processes that govern their intensity and topology, which could certainly result from their formation history, remain poorly understood. During the pre-main-sequence (PMS) phase, the inner layers of these stars tend to contract, and a radiative core gradually develops. In contrast, the convective envelope is gradually braked through the magnetic interactions with the accretion disk and winds, thus slowly developing a differential rotation inside the star. It is likely during this PMS phase that the dynamo processes that efficiently generated strong dipolar magnetic fields through vigorous convective motions in protostars become highly perturbed, leading to the observed diversity in the magnetism on the main sequence.<i>Aims.<i/> We aim to study the stability of dipolar magnetic fields inherited from the proto-stellar phase by considering the emergence of a large-scale radial differential rotation resulting from the combined actions of contraction and of the interactions with the surrounding medium.<i>Methods.<i/> We performed 3D convective dynamo simulations of rotating spherical shells with an imposed differential rotation (shear) between the bottom and top boundaries. We used anelastic approximation, which allowed us to consider background density and gravity profiles and convective zone thicknesses close to those predicted in PMS low-mass stars by the 1D stellar evolution code Cesam2k20. We then carried out a parameter study by systematically varying the shear amplitude.<i>Results.<i/> Radial differential rotation can induce dipole collapse leading to weaker and oscillatory magnetic fields. We highlight that the stability of dipolar dynamos mainly depends on the relative importance of shear measured by a shear-based Rossby number compared to the vigor of convective motions as measured by the convective Rossby number. We show that the stability criterion depends on the field strength and the size of the radiative core. Differential rotation seems to perturb the <i>α<i/><sup>2<sup/> dynamo mechanism responsible for dipolar magnetic fields by shearing poloidal field lines and affecting turbulent magnetic transport processes.<i>Conclusions.<i/> The PMS phase can represent a critical period for the magnetic properties of stars, as the development of shear layers can perturb the stability of strong initial dipoles. Applying the stability criterion in PMS stellar evolution models, we qualitatively reproduced the trends observed in the magnetic topologies of low-mass stars when assuming an efficient internal angular momentum redistribution process. This suggests that stellar magnetic properties are intimately related to the PMS angular momentum evolution.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"552 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147280057","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-25DOI: 10.1051/0004-6361/202558502
F. Rincon
For more than 40 years the quest to understand how large-scale magnetic fields emerge from turbulent flows in rotating astrophysical systems, such as the Sun, has been a major focus of computational astrophysics research. Using a parameter scan and phenomenological analysis of maximally simplified three-dimensional cartesian magnetohydrodynamic simulations of large-scale non-linear helical turbulent dynamos, I present results in this Letter that strongly point to an asymptotic ultimate regime of the large-scale solar dynamo at large magnetic Reynolds numbers, Rm, involving helicity fluxes between hemispheres. I obtained corresponding numerical solutions at both Pm > 1 and Pm < 1, and show that they can currently only be achieved in clean, simplified numerical set-ups. The analysis further strongly suggests that all global simulations to date lie in non-asymptotic turbulent magnetohydrodynamic regimes highly sensitive to changes in kinetic and magnetic Reynolds numbers. Ideas are presented to attempt to reach the ultimate regime in such ’realistic’ global spherical models at a reasonable numerical cost. Overall, the results clarify the current state, and some hard limitations of the brute-force numerical modelling approach applied to this, and other similar astrophysical turbulence problems.
{"title":"Ultimate large-Rm regime of the solar dynamo","authors":"F. Rincon","doi":"10.1051/0004-6361/202558502","DOIUrl":"https://doi.org/10.1051/0004-6361/202558502","url":null,"abstract":"For more than 40 years the quest to understand how large-scale magnetic fields emerge from turbulent flows in rotating astrophysical systems, such as the Sun, has been a major focus of computational astrophysics research. Using a parameter scan and phenomenological analysis of maximally simplified three-dimensional cartesian magnetohydrodynamic simulations of large-scale non-linear helical turbulent dynamos, I present results in this Letter that strongly point to an asymptotic ultimate regime of the large-scale solar dynamo at large magnetic Reynolds numbers, <i>Rm<i/>, involving helicity fluxes between hemispheres. I obtained corresponding numerical solutions at both <i>Pm<i/> > 1 and <i>Pm<i/> < 1, and show that they can currently only be achieved in clean, simplified numerical set-ups. The analysis further strongly suggests that all global simulations to date lie in non-asymptotic turbulent magnetohydrodynamic regimes highly sensitive to changes in kinetic and magnetic Reynolds numbers. Ideas are presented to attempt to reach the ultimate regime in such ’realistic’ global spherical models at a reasonable numerical cost. Overall, the results clarify the current state, and some hard limitations of the brute-force numerical modelling approach applied to this, and other similar astrophysical turbulence problems.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"32 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147280058","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-25DOI: 10.1051/0004-6361/202558768
L. Pasquini, R. Saglia, F. Patat, L. Berni, D. Bossini, L. Magrini, H. Ludwig, M. T. Murphy, J. R. de Medeiros, J. Chanamé
Context. Wide binaries offer a unique opportunity to test gravity in the low-acceleration regime, where deviations from Newtonian dynamics may appear.Aims. We used high-resolution VLT–ESPRESSO archival spectra to study 26 wide binaries with projected separations > 13 000 AU. By combining precise radial velocities with Gaia proper motions and parallaxes, we tested whether these systems are consistent with Newtonian gravity in the low-acceleration regime.Methods. We used multiple radial-velocity measurements and stellar parameters to remove systems affected by unresolved triple or chance alignments as well as young systems. For the remaining binaries, we combined radial velocities (corrected for convective shift and gravitational redshift) with Gaia proper motions, parallaxes, and positions in a bid to find bound Newtonian orbital solutions.Results. Of the 26 initial systems, 14 were discarded: 12 due to radial-velocity variability indicating unresolved close binaries, 1 that hosts a faint Gaia companion, and 1 that is too young. Of the remaining 12, 9 can be fitted with a bound orbital solution, while the velocity differences of the other 3 are too large to be reconciled with any bound Newtonian orbit.Conclusions. For the three systems that cannot be fitted with a bound orbit, repeated radial-velocity observations allowed us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3D separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.
{"title":"Wide binaries without viable bound Newtonian orbits","authors":"L. Pasquini, R. Saglia, F. Patat, L. Berni, D. Bossini, L. Magrini, H. Ludwig, M. T. Murphy, J. R. de Medeiros, J. Chanamé","doi":"10.1051/0004-6361/202558768","DOIUrl":"https://doi.org/10.1051/0004-6361/202558768","url":null,"abstract":"<i>Context.<i/> Wide binaries offer a unique opportunity to test gravity in the low-acceleration regime, where deviations from Newtonian dynamics may appear.<i>Aims.<i/> We used high-resolution VLT–ESPRESSO archival spectra to study 26 wide binaries with projected separations > 13 000 AU. By combining precise radial velocities with <i>Gaia<i/> proper motions and parallaxes, we tested whether these systems are consistent with Newtonian gravity in the low-acceleration regime.<i>Methods.<i/> We used multiple radial-velocity measurements and stellar parameters to remove systems affected by unresolved triple or chance alignments as well as young systems. For the remaining binaries, we combined radial velocities (corrected for convective shift and gravitational redshift) with <i>Gaia<i/> proper motions, parallaxes, and positions in a bid to find bound Newtonian orbital solutions.<i>Results.<i/> Of the 26 initial systems, 14 were discarded: 12 due to radial-velocity variability indicating unresolved close binaries, 1 that hosts a faint <i>Gaia<i/> companion, and 1 that is too young. Of the remaining 12, 9 can be fitted with a bound orbital solution, while the velocity differences of the other 3 are too large to be reconciled with any bound Newtonian orbit.<i>Conclusions.<i/> For the three systems that cannot be fitted with a bound orbit, repeated radial-velocity observations allowed us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3D separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"347 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147280056","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-25DOI: 10.1051/0004-6361/202558431
Elena Fernández-García, Juan E. Betancort-Rijo, Francisco Prada, Tomoaki Ishiyama, Anatoly Klypin, José Ruedas
We present a new theoretical framework for the halo mass function (HMF) that accurately predicts the abundance of dark matter halos over an exceptionally wide range of masses and redshifts, based on a generalised Press–Schechter model with triaxial collapse (GPS+). The HMF is formulated mainly as a function of the variance of the linear density field, with a weak explicit mass dependence and no explicit redshift dependence, which is able to naturally reproduce the correct normalisation and high-mass behaviour without requiring an empirical fitting. Using the UCHUUN-body simulation suite under Planck cosmology, combining six simulations with up to 300 realisations, we measured the HMF over 6.5 ≤ log(M200m/[h−1M⊙]) ≤ 16 and 0 ≤ z ≤ 20 with reduced cosmic variance. Over this full domain, we find that GPS+ matches the simulations to within 10–20%, performing similarly to the Sheth–Tormen model at z ≲ 2, but with substantially results at higher redshifts. In the latter case, the Sheth–Tormen model can deviate by 70–80%, while GPS+ will remain within ∼20%. Finally, we show that the halo mass definition is key: M200m yields a nearly universal, weakly redshift-dependent HMF, whereas adopting the evolving virial overdensity from (Bryan, G. L. & Norman, M. L. 1998, ApJ, 495, 80) ends up degrading the agreement at low redshifts and high masses.
我们提出了一个新的光环质量函数（HMF）理论框架，该框架基于广义Press-Schechter三轴坍缩（GPS+）模型，准确预测了在异常宽的质量和红移范围内暗物质晕的丰度。HMF主要被表述为线性密度场方差的函数，具有弱的显式质量依赖，没有显式红移依赖，这能够自然地再现正确的归一化和高质量行为，而不需要经验拟合。利用普朗克宇宙学下的UCHUUN-body模拟套件，结合6个模拟和多达300个实现，我们测量了6.5≤log(M200m/[h−1 M⊙])≤16和0≤z≤20的HMF，并减小了宇宙方差。在这整个区域内，我们发现GPS+与模拟的匹配度在10-20%以内，在z > 2处的表现与Sheth-Tormen模型相似，但在更高的红移处有很大的结果。在后一种情况下，Sheth-Tormen模型可以偏离70-80%，而GPS+将保持在~ 20%以内。最后，我们表明晕的质量定义是关键：M200m产生一个几乎普遍的、弱红移依赖的HMF，而采用（Bryan， G. L. & Norman， M. L. 1998, ApJ, 495,80）的不断发展的病毒密度最终会降低低红移和高质量的一致性。
{"title":"A redshift-independent theoretical halo mass function validated with UCHUU simulations","authors":"Elena Fernández-García, Juan E. Betancort-Rijo, Francisco Prada, Tomoaki Ishiyama, Anatoly Klypin, José Ruedas","doi":"10.1051/0004-6361/202558431","DOIUrl":"https://doi.org/10.1051/0004-6361/202558431","url":null,"abstract":"We present a new theoretical framework for the halo mass function (HMF) that accurately predicts the abundance of dark matter halos over an exceptionally wide range of masses and redshifts, based on a generalised Press–Schechter model with triaxial collapse (GPS+). The HMF is formulated mainly as a function of the variance of the linear density field, with a weak explicit mass dependence and no explicit redshift dependence, which is able to naturally reproduce the correct normalisation and high-mass behaviour without requiring an empirical fitting. Using the UCHUU<i>N<i/>-body simulation suite under <i>Planck<i/> cosmology, combining six simulations with up to 300 realisations, we measured the HMF over 6.5 ≤ log(<i>M<i/><sub>200m<sub/>/[<i>h<i/><sup>−1<sup/> <i>M<i/><sub>⊙<sub/>]) ≤ 16 and 0 ≤ <i>z<i/> ≤ 20 with reduced cosmic variance. Over this full domain, we find that GPS+ matches the simulations to within 10–20%, performing similarly to the Sheth–Tormen model at <i>z<i/> ≲ 2, but with substantially results at higher redshifts. In the latter case, the Sheth–Tormen model can deviate by 70–80%, while GPS+ will remain within ∼20%. Finally, we show that the halo mass definition is key: <i>M<i/><sub>200m<sub/> yields a nearly universal, weakly redshift-dependent HMF, whereas adopting the evolving virial overdensity from (Bryan, G. L. & Norman, M. L. 1998, ApJ, 495, 80) ends up degrading the agreement at low redshifts and high masses.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"347 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147280061","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-24DOI: 10.1051/0004-6361/202558445
Andrés E. Piatti
It has recently been suggested that the magnitude of the interaction between galaxies could be measured from the level of kinematic disturbance of their outer regions with respect to the innermost ones. In this work, I proved that the outer northeastern region of the Small Magellanic Cloud (SMC), a relatively recent stellar structure with a tidal origin from the interaction with the Large Magellanic Cloud (LMC), is imprinted by a residual velocity pattern. I obtained mean radial velocities (RVs) of star clusters formed in situ from GEMINI GMOS spectra, which, added to derived mean proper motions and heliocentric distances, allowed me to compute their 3D space-velocity components. These space velocities are different from those that the clusters would have if they instead rotated with the galaxy in an orderly fashion; i.e., their residual velocities are larger than the upper limit for an object pertaining to the SMC’s main body rotation disk. The level of kinematic disturbance depends on the SMC rotation disk adopted; galaxy rotation disks traced using relatively old objects are discouraged. The resulting kinematic disturbance arises in younger and older stellar populations, so the epoch of close interaction between both Magellanic Clouds cannot be uncovered on the basis of the kinematics behavior of stellar populations in the outer SMC regions.
{"title":"Kinematics of young star clusters in the outer northeastern region of the Small Magellanic Cloud","authors":"Andrés E. Piatti","doi":"10.1051/0004-6361/202558445","DOIUrl":"https://doi.org/10.1051/0004-6361/202558445","url":null,"abstract":"It has recently been suggested that the magnitude of the interaction between galaxies could be measured from the level of kinematic disturbance of their outer regions with respect to the innermost ones. In this work, I proved that the outer northeastern region of the Small Magellanic Cloud (SMC), a relatively recent stellar structure with a tidal origin from the interaction with the Large Magellanic Cloud (LMC), is imprinted by a residual velocity pattern. I obtained mean radial velocities (RVs) of star clusters formed in situ from GEMINI GMOS spectra, which, added to derived mean proper motions and heliocentric distances, allowed me to compute their 3D space-velocity components. These space velocities are different from those that the clusters would have if they instead rotated with the galaxy in an orderly fashion; i.e., their residual velocities are larger than the upper limit for an object pertaining to the SMC’s main body rotation disk. The level of kinematic disturbance depends on the SMC rotation disk adopted; galaxy rotation disks traced using relatively old objects are discouraged. The resulting kinematic disturbance arises in younger and older stellar populations, so the epoch of close interaction between both Magellanic Clouds cannot be uncovered on the basis of the kinematics behavior of stellar populations in the outer SMC regions.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"25 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147280062","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-23DOI: 10.1051/0004-6361/202557973
C. Ventura, M. Tailo, P. Ventura, F. D’Antona, A. P. Milone, A. F. Marino, C. Fiumi
Context. The study of the globular cluster 47 Tuc offers an opportunity to shed new light on the debated issue of the presence of multiple populations in globular clusters, as recent results from HST photometry and high-resolution spectroscopy outlined star-to-star differences in the surface chemical composition.Aims. The goal of the present investigation is the interpretation of recent JWST data of the low main sequence of 47 Tuc, in order to explore the stellar to sub-stellar transition, derive the mass distribution of the individual sources, and disentangle stars from different populations.Methods. Stellar evolution modelling of low-mass stars of metallicity [Fe/H] = −0.78 and oxygen content [O/Fe] = +0.4 and [O/Fe] = 0 was used to simulate the evolution of the first and the second generation of the cluster. The comparison between the calculated sequences with the data points was used to characterise the individual objects, split the different stellar components, and infer the current mass function of the cluster.Results. The first generation of 47 Tuc harbours ~45% of the overall population of the cluster, the remaining 55% making up the second generation. The transition from the stellar to the sub-stellar domain is found at 0.074 M ⊙ and 0.07 M⊙ for the first and second generations, respectively. The mass function of both stellar generations is consistent with a Kroupa-like profile down to ~0.22 M⊙.
{"title":"The stellar to sub-stellar masses transition in 47 Tuc","authors":"C. Ventura, M. Tailo, P. Ventura, F. D’Antona, A. P. Milone, A. F. Marino, C. Fiumi","doi":"10.1051/0004-6361/202557973","DOIUrl":"https://doi.org/10.1051/0004-6361/202557973","url":null,"abstract":"<i>Context<i/>. The study of the globular cluster 47 Tuc offers an opportunity to shed new light on the debated issue of the presence of multiple populations in globular clusters, as recent results from HST photometry and high-resolution spectroscopy outlined star-to-star differences in the surface chemical composition.<i>Aims<i/>. The goal of the present investigation is the interpretation of recent JWST data of the low main sequence of 47 Tuc, in order to explore the stellar to sub-stellar transition, derive the mass distribution of the individual sources, and disentangle stars from different populations.<i>Methods<i/>. Stellar evolution modelling of low-mass stars of metallicity [Fe/H] = −0.78 and oxygen content [O/Fe] = +0.4 and [O/Fe] = 0 was used to simulate the evolution of the first and the second generation of the cluster. The comparison between the calculated sequences with the data points was used to characterise the individual objects, split the different stellar components, and infer the current mass function of the cluster.<i>Results<i/>. The first generation of 47 Tuc harbours ~45% of the overall population of the cluster, the remaining 55% making up the second generation. The transition from the stellar to the sub-stellar domain is found at 0.074 M<sub> ⊙<sub/> and 0.07 M<sub>⊙<sub/> for the first and second generations, respectively. The mass function of both stellar generations is consistent with a Kroupa-like profile down to ~0.22 M<sub>⊙<sub/>.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"8 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146778559","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-23DOI: 10.1051/0004-6361/202557618
Agnibha De Sarkar
Context. Extended gamma-ray sources surrounding middle-aged pulsars, primarily observed at teraelectronvolt energies, have been interpreted as pulsar halos, where relativistic e± diffuse into the interstellar medium and produce inverse-Compton (IC) emission. HESS J1813-126, associated with the energetic, radio-quiet gamma-ray pulsar PSR J1813-1246, has been suggested as a candidate pulsar halo, though its nature remains uncertain.Aims. We interpreted the high-energy emission of PSR J1813-1246 using the synchro-curvature (SC) radiation model and tested whether the gamma-ray spectral energy distribution (SED) of HESS J1813-126 can be explained as a pulsar halo powered by PSR J1813-1246.Methods. We explain the X-ray and gamma-ray SEDs of the pulsar using the SC framework. We further computed the transport and losses of e± injected by the pulsar through time-dependent diffusion-loss equations, exploring various common pulsar halo transport models. The resulting IC emission was compared with Fermi-LAT, H.E.S.S., HAWC, and LHAASO data. We present predictions for the surface brightness profiles (SBPs) and the aperture-dependent emission for the different transport models, providing key diagnostics for assessing the observability of HESS J1813-126 with current and future instruments.Resuls. The SC framework successfully reproduces the emission of PSR J1813-1246. The SED of HESS J1813-126 can be consistently reproduced within different pulsar halo frameworks, albeit with distinct predictions across different transport models. The corresponding SBP predictions and aperture-dependent emission offer testable signatures for future imaging atmospheric Cherenkov telescopes, which will be crucial for discriminating between the transport models. We further examined the link between the pulsar central engine and its extended halo by comparing the pair multiplicities in the magnetospheric and halo regions.
{"title":"Investigating the emission signatures of pulsar halo candidate HESS J1813-126","authors":"Agnibha De Sarkar","doi":"10.1051/0004-6361/202557618","DOIUrl":"https://doi.org/10.1051/0004-6361/202557618","url":null,"abstract":"<i>Context.<i/> Extended gamma-ray sources surrounding middle-aged pulsars, primarily observed at teraelectronvolt energies, have been interpreted as pulsar halos, where relativistic <i>e<i/><sup>±<sup/> diffuse into the interstellar medium and produce inverse-Compton (IC) emission. HESS J1813-126, associated with the energetic, radio-quiet gamma-ray pulsar PSR J1813-1246, has been suggested as a candidate pulsar halo, though its nature remains uncertain.<i>Aims.<i/> We interpreted the high-energy emission of PSR J1813-1246 using the synchro-curvature (SC) radiation model and tested whether the gamma-ray spectral energy distribution (SED) of HESS J1813-126 can be explained as a pulsar halo powered by PSR J1813-1246.<i>Methods.<i/> We explain the X-ray and gamma-ray SEDs of the pulsar using the SC framework. We further computed the transport and losses of <i>e<i/><sup>±<sup/> injected by the pulsar through time-dependent diffusion-loss equations, exploring various common pulsar halo transport models. The resulting IC emission was compared with <i>Fermi<i/>-LAT, H.E.S.S., HAWC, and LHAASO data. We present predictions for the surface brightness profiles (SBPs) and the aperture-dependent emission for the different transport models, providing key diagnostics for assessing the observability of HESS J1813-126 with current and future instruments.<i>Resuls.<i/> The SC framework successfully reproduces the emission of PSR J1813-1246. The SED of HESS J1813-126 can be consistently reproduced within different pulsar halo frameworks, albeit with distinct predictions across different transport models. The corresponding SBP predictions and aperture-dependent emission offer testable signatures for future imaging atmospheric Cherenkov telescopes, which will be crucial for discriminating between the transport models. We further examined the link between the pulsar central engine and its extended halo by comparing the pair multiplicities in the magnetospheric and halo regions.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"128 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146778546","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-23DOI: 10.1051/0004-6361/202555290
Beatrice Nyiransengiyumva, Mirjana Pović, Pheneas Nkundabakura, Tom Mutabazi, Antoine Mahoro
Context. The bi-modality in the distribution of galaxies usually obtained from colour-colour or colour-stellar mass (absolute magnitude) diagrams has been studied to determine the difference between galaxies in the blue cloud and in the red sequence, as well as to define the green valley region. As a transition region, green valley galaxies can offer clues about the morphological transformation of galaxies from late-type to early-type. Therefore, the selection of green valley samples is of fundamental importance.Aims. In this work, for the first time, we evaluate the selection effects of the most frequently applied green valley selection criteria. The aim is to understand how these criteria affect the identification of green valley galaxies, their properties, and their impact on galaxy evolution studies.Methods. Using the SDSS optical and GALEX ultraviolet data at redshift z < 0.1, we selected the eight most commonly used criteria based on colours (without and with Gaussian fittings), specific star formation rate, and star formation rate versus stellar mass. We then studied the properties of the green valley galaxies (e.g. their stellar mass, star formation rate, specific star formation rate, intrinsic brightness, and morphological and spectroscopic types) for each selection criterion.Results. We found that when using different criteria, we selected different types of galaxies. UV-optical colour-based criteria tend to select more massive galaxies, with lower star formation rates and a higher fractions of composite and elliptical galaxies than when using pure optical colours. Our results also show that the colour-based criteria are the most sensitive to galaxy properties, rapidly changing the selection of green valley galaxies.Conclusions. Whenever possible, we suggest avoiding the green valley colour-based selection and using other methods or a combination of several, such as the star formation rate versus stellar mass or specific star formation rate.
{"title":"The impact of selection criteria on the properties of green valley galaxies","authors":"Beatrice Nyiransengiyumva, Mirjana Pović, Pheneas Nkundabakura, Tom Mutabazi, Antoine Mahoro","doi":"10.1051/0004-6361/202555290","DOIUrl":"https://doi.org/10.1051/0004-6361/202555290","url":null,"abstract":"<i>Context.<i/> The bi-modality in the distribution of galaxies usually obtained from colour-colour or colour-stellar mass (absolute magnitude) diagrams has been studied to determine the difference between galaxies in the blue cloud and in the red sequence, as well as to define the green valley region. As a transition region, green valley galaxies can offer clues about the morphological transformation of galaxies from late-type to early-type. Therefore, the selection of green valley samples is of fundamental importance.<i>Aims.<i/> In this work, for the first time, we evaluate the selection effects of the most frequently applied green valley selection criteria. The aim is to understand how these criteria affect the identification of green valley galaxies, their properties, and their impact on galaxy evolution studies.<i>Methods.<i/> Using the SDSS optical and GALEX ultraviolet data at redshift <i>z<i/> < 0.1, we selected the eight most commonly used criteria based on colours (without and with Gaussian fittings), specific star formation rate, and star formation rate versus stellar mass. We then studied the properties of the green valley galaxies (e.g. their stellar mass, star formation rate, specific star formation rate, intrinsic brightness, and morphological and spectroscopic types) for each selection criterion.<i>Results.<i/> We found that when using different criteria, we selected different types of galaxies. UV-optical colour-based criteria tend to select more massive galaxies, with lower star formation rates and a higher fractions of composite and elliptical galaxies than when using pure optical colours. Our results also show that the colour-based criteria are the most sensitive to galaxy properties, rapidly changing the selection of green valley galaxies.<i>Conclusions.<i/> Whenever possible, we suggest avoiding the green valley colour-based selection and using other methods or a combination of several, such as the star formation rate versus stellar mass or specific star formation rate.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"1 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146778801","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-20DOI: 10.1051/0004-6361/202557300
Asako Sato, Anaëlle Maury, Josep M. Girart, Andrea Bracco, Patrick Hennebelle, Qizhou Zhang, Valeska Valdivia
Context. The pristine stages of disk formation during the protostellar phase and the processes of dust evolution in young disks remain largely unconstrained. Dust thermal emission at millimeter wavelengths and its polarization offer key insights into the physical processes and spatial distribution of material at the envelope-disk interface, as well as on early dust evolution around solar-type protostars.Aims. We characterize the dust properties and polarization mechanisms from envelope to disk scales in two young stellar objects (YSOs), IRAS 04166+2706 (K04166) and IRAS 04169+2702 (K04169) embedded in the same Taurus filament. We explore the nature of the dust polarization across scales and investigate their stage of evolution.Methods. We present deep (∼1.4 mm and ∼3 mm) polarimetric observations sampling the dust emission from 25 au to 3000 au using the Atacama Large Millimeter/submillimeter Array (ALMA). We modeled Stokes I emission to characterize disk and envelope contributions, while the polarization properties were analyzed to identify the dominant polarization mechanisms. Finally, in this work, we discuss the physical properties of both sources across scales.Results. K04166 shows extended Stokes I and polarized emission tracing a tentative hourglass magnetic field morphology in its envelope. In the inner envelope and disk (<100 au), the properties of the polarized emission change, suggesting the presence of a toroidal magnetic field around the disk or the presence of large grains in the inner envelope. K04169 exhibits compact Stokes I and polarized emission, consistent with self-scattering from the disk. Both disks are extremely compact, yet K04166 retains a substantial envelope while the one of K04169 is largely dissipated.Conclusions. Our multiscale ALMA polarimetric observations reveal a transition from magnetically aligned grains in envelopes to self-scattering in disks within the transition region of 20–50 au. These results provide important clues on dust grain growth and magnetic field morphology at the disk-envelope scales. The two sources, separated by <0.5 pc and embedded in a common filament, display striking differences, indicating that K04166 is a young embedded object with a substantial envelope threaded by relatively organized magnetic fields. Meanwhile, K04169 is more evolved, likely to be a young T-Tauri star. However, in both disks, the presence of large grains already suggest a scenario of early dust evolution in disks of the Class 0 stage.
{"title":"Dissecting the dust distribution and polarization around two B213 young stellar objects with ALMA","authors":"Asako Sato, Anaëlle Maury, Josep M. Girart, Andrea Bracco, Patrick Hennebelle, Qizhou Zhang, Valeska Valdivia","doi":"10.1051/0004-6361/202557300","DOIUrl":"https://doi.org/10.1051/0004-6361/202557300","url":null,"abstract":"<i>Context<i/>. The pristine stages of disk formation during the protostellar phase and the processes of dust evolution in young disks remain largely unconstrained. Dust thermal emission at millimeter wavelengths and its polarization offer key insights into the physical processes and spatial distribution of material at the envelope-disk interface, as well as on early dust evolution around solar-type protostars.<i>Aims<i/>. We characterize the dust properties and polarization mechanisms from envelope to disk scales in two young stellar objects (YSOs), IRAS 04166+2706 (K04166) and IRAS 04169+2702 (K04169) embedded in the same Taurus filament. We explore the nature of the dust polarization across scales and investigate their stage of evolution.<i>Methods<i/>. We present deep (∼1.4 mm and ∼3 mm) polarimetric observations sampling the dust emission from 25 au to 3000 au using the Atacama Large Millimeter/submillimeter Array (ALMA). We modeled Stokes <i>I<i/> emission to characterize disk and envelope contributions, while the polarization properties were analyzed to identify the dominant polarization mechanisms. Finally, in this work, we discuss the physical properties of both sources across scales.<i>Results<i/>. K04166 shows extended Stokes <i>I<i/> and polarized emission tracing a tentative hourglass magnetic field morphology in its envelope. In the inner envelope and disk (<i><<i/>100 au), the properties of the polarized emission change, suggesting the presence of a toroidal magnetic field around the disk or the presence of large grains in the inner envelope. K04169 exhibits compact Stokes <i>I<i/> and polarized emission, consistent with self-scattering from the disk. Both disks are extremely compact, yet K04166 retains a substantial envelope while the one of K04169 is largely dissipated.<i>Conclusions<i/>. Our multiscale ALMA polarimetric observations reveal a transition from magnetically aligned grains in envelopes to self-scattering in disks within the transition region of 20–50 au. These results provide important clues on dust grain growth and magnetic field morphology at the disk-envelope scales. The two sources, separated by <i><<i/>0.5 pc and embedded in a common filament, display striking differences, indicating that K04166 is a young embedded object with a substantial envelope threaded by relatively organized magnetic fields. Meanwhile, K04169 is more evolved, likely to be a young T-Tauri star. However, in both disks, the presence of large grains already suggest a scenario of early dust evolution in disks of the Class 0 stage.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"2 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146778545","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-20DOI: 10.1051/0004-6361/202557635
Gabrielle L. Taylor, Stefan J. Wagner, Alicja Wierzcholska, Michael Zacharias
Aims. The blazars Mrk 421 and Mrk 501 have shown multi-wavelength variability on all observed timescales, and have been well studied at high energies on short timescales. We aim to characterise the long-term temporal behaviour of these blazars at synchrotron energies, namely in the optical, UV, and X-ray, in order to assess current models of these objects and their processes.Methods. Including amongst the longest light curves ever studied for these sources, we investigated 20 years of data (2005–2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index versus flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions.Results. Mrk 421 and Mrk 501 both showed intriguing variability at all studied wavelengths; this variability has been found to be energy-dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray data of Mrk 501 (host frame timescale of ∼390 days, > 3σ) but not in the UV or X-ray data of Mrk 421, or in the UV data of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.
{"title":"Not so Swift: 20 years of multi-wavelength observations of Mrk 421 and Mrk 501","authors":"Gabrielle L. Taylor, Stefan J. Wagner, Alicja Wierzcholska, Michael Zacharias","doi":"10.1051/0004-6361/202557635","DOIUrl":"https://doi.org/10.1051/0004-6361/202557635","url":null,"abstract":"<i>Aims.<i/> The blazars Mrk 421 and Mrk 501 have shown multi-wavelength variability on all observed timescales, and have been well studied at high energies on short timescales. We aim to characterise the long-term temporal behaviour of these blazars at synchrotron energies, namely in the optical, UV, and X-ray, in order to assess current models of these objects and their processes.<i>Methods.<i/> Including amongst the longest light curves ever studied for these sources, we investigated 20 years of data (2005–2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index versus flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions.<i>Results.<i/> Mrk 421 and Mrk 501 both showed intriguing variability at all studied wavelengths; this variability has been found to be energy-dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray data of Mrk 501 (host frame timescale of ∼390 days, > 3<i>σ<i/>) but not in the UV or X-ray data of Mrk 421, or in the UV data of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"4 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146778544","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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