Astrophysique, Instrumentation et Modélisation

The properties and the spatial distribution of the large-scale structures of the Solar Corona (SC) determine the observed solar wind structure at 1 AU. Coronal Holes (CHs) are the primary source of the fast solar wind, which is the most geoeffective component of solar wind, and they appear as large dark patches in the Extreme Ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory (SDO) and the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SoHO). These observatories provide images of the SC at different wavelengths, which enables the identification of CH morphology and other large-scale structures along a given line of sight. It is crucial to understand the CH regions and their properties for effective space weather forecasting. This work is part of the WindTRUST project, with the primary goal of improving the reliability of solar wind models for space weather forecasting. Here, we aim to develop an automatic threshold-based CH detection tool for predictions across solar cycles 23, 24, and 25. We also plan to integrate this CH detection tool into a solar wind model validation pipeline, creating a fully automated validation system that provides a quantitative assessment of predictions. We categorized the large-scale features of the SC, such as active regions, solar flares, coronal mass ejections (CMEs), and filaments, based on their spatial distribution, phase of the solar cycle, and additional properties, including the GOES solar flare class. A Sequential Neural Network (NN) model was then trained by optimizing the architecture of the hidden layers to achieve higher predictive accuracy. The resulting model estimates the threshold required for integration into the Coronal Hole (CH) detection scheme, thereby enabling automated, consistent identification of CH boundaries in EUV images across solar cycles 23, 24, and 25. To interpret the performance of our NN model, we divided the predicted CH results into solar minimum and maximum cases across the solar cycles 23, 24, and 25. We also provide a comparison of our CH detection results with those obtained from other detection tools. Once we identify CH contours from our model, we validate them using a diagnostic test against CH contours from the Potential Field Source Surface (PFSS) model (non-MHD) and the WindPredict (WP) model (Polytropic and Alfven Wave) (MHD). Finally, we couple the CH detection tool with the validation pipeline to develop an automation tool for solar wind predictions.

Abstract The Ly α absorption profile in star-forming galaxies serves as a powerful tracer of the extended, dense neutral hydrogen in their surroundings during the Epoch of Reionization (EoR). We report a unique galaxy, A2744-z7DLA, at z ≈ 6.87, gravitationally lensed by the foreground galaxy cluster A2744, which exhibits both moderate Ly α emission and damped Ly α absorption, suggesting the presence of a dense neutral hydrogen environment. Our analysis suggests that the UV continuum turnover near Ly α is more likely shaped by a damped Ly α system rather than nebular continuum from the two-photon process. We analyze the physical properties of A2744-z7DLA with spectroscopic and photometric data from the JWST and the Hubble Space Telescope. The galaxy shows a compact morphology ( r e ∼ 0.3 kpc) and a broadened H α emission line, suggesting possible active galactic nucleus (AGN) activity. The broad component of H α has an FWHM of 2721 ± 200 km s −1 , corresponding to a black hole mass of M BH = 2.9 0 − 1.28 + 2.35 × 1 0 7 M ⊙ and a black hole to stellar mass ratio of log ( M BH / M ⊙ ) = − 1.3 1 − 0.36 + 0.37 . The Balmer decrement (H α /H β ) yields a dust attenuation of A V ≈ 1.15 ± 0.23. Furthermore, we perform spectral energy distribution fitting using both stellar and AGN models. The results show that the UV and optical wavelengths are dominated by star-forming regions, while the AGN component contributes primarily at longer wavelengths. This work provides new insights into the interplay between star formation, neutral gas, and potential AGN activity in galaxies during the EoR.

Context . Stellar activity remains one of the main limitations in the detection of Earth-like planets using radial velocity (RV) measurements. The Sun, as the only star for which surface features can be spatially resolved, offers a unique testbed for studying the impact of active regions on RV and photometric variability. Aims . Using SOAPv4 (Spot Oscillation And Planet), we modelled solar RV and photometric variability induced by spots and faculae over long timescales. Our goal is to verify whether present-day, state-of-the-art models of the cross-correlation function correctly reproduce the observed variability. Moreover, we aim to assess how the choice of input data and identification technique influences the agreement between simulated and observed signals. Methods . To simulate solar RV and photometric time series, we first identified active regions in SDO images. This was done using mathematical morphological transforms applied to SDO/HMI and AIA images. Mathematical morphological identification was validated against other state-of-the-art identification methods. Using these inputs, we ran SOAPv4 to simulate solar RVs and photometry, and we validated the results with HARPS-N RV observations, as well as with VIRGO/SPM photometric measurements. Results . The simulations that use mathematical morphological identification achieved the best match with the observed RV time series, yielding residuals with a measured standard deviation of ~0.91 m/s. Other state-of-the-art methods produced higher filling factors and, consequently, larger discrepancies. The photometric simulations reproduced the overall variability trends. Conclusions . We demonstrate that mathematical morphological transforms accurately identify solar active regions. Using these inputs, SOAPv4 reproduces the observed solar RV variability with a measured standard deviation of the residuals of ~0.91 m/s. Photometric simulations capture the overall variability trends, confirming that SOAP can reliably model the impact of both spots and faculae on solar RVs and photometry.

Context. It is well known that star-forming galaxies (SFGs) exhibit a tight correlation between their radio and infrared emissions, commonly referred to as the infrared-radio correlation (IRRC). Recent empirical studies have reported a dependence of the IRRC on the galaxy stellar mass, in which more massive galaxies tend to show lower infrared-to-radio ratios ( q IR ) with respect to less massive galaxies. One possible, yet unexplored, explanation is a residual contamination of the radio emission from active galactic nuclei (AGNs), not captured through “radio-excess” diagnostics. Aims. To investigate this hypothesis, we aim to statistically quantify the contribution of AGN emission to the radio luminosities of SFGs located within the scatter of the IRRC. Methods. Our Very Large Baseline Array (VLBA) AGN-sCAN program has targeted 500 galaxies that follow the q IR distribution of the IRRC, i.e., with no prior evidence for radio-excess AGN emission based on low-resolution (∼arcsec) VLA radio imaging. Our VLBA 1.4 GHz observations reach a 5 σ sensitivity limit of 25 μJy/beam, corresponding to a radio-brightness temperature of T b ∼ 10 5 K. This classification serves as a robust AGN diagnostic, regardless of the host galaxy’s star formation rate. Results. We detect four VLBA sources in the deepest regions, which are also the faintest VLBI-detected AGNs in SFGs to date. The effective AGN detection rate is 9%, when considering a control sample matched in mass and sensitivity, which is in good agreement with the extrapolation of previous radio AGN number counts. Despite the non-negligible AGN flux contamination (∼30%) in our individual VLBA detections, we find that the peak of the q IR distribution is completely unaffected by this correction. Although we cannot rule out a high incidence of radio-silent AGNs at (sub)μJy levels among the VLBA non-detections, we derive a conservative upper limit of < 0.1 dex of their cumulative impact on the q IR distribution. We conclude that residual AGN contamination from non-radio-excess AGNs is unlikely to be the primary driver of the M ★ – dependent IRRC.

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.

Acoustic modes are excited by turbulent convection in the outer convective envelope of solar-like stars. Observational results from asteroseismic studies have shown that 44 % of solar-like stars do not present detectable stochastically excited acoustic modes. This phenomenon appears to be related to their rotation rate and magnetic activity. Indeed, rotation locally modifies the properties of convection, which in turn influences the stochastic excitation of stellar oscillation modes. In a first paper, we showed that uniform rotation tends to diminish the mode amplitudes significantly. However, convective envelopes in solar-type stars differentially rotate, and the rotation rate difference between mid-latitudes and the equator can reach up to 60%, as shown by recent asteroseismic works. In this paper, we examine the impact of differential rotation on the stochastic excitation of acoustic modes in solar-like stars. We provide theoretical predictions for the excitation of acoustic modes in a differentially rotating solar-like star. We used rotating mixing length theory to model the local influence of differential rotation on convection. We then estimated the resulting impact on power injection by turbulent convection into oscillation modes numerically, using a combination of the MESA stellar structure and evolution code and GYRE stellar pulsation code. We show that the power injected in acoustic modes differs by up to 30% for stars with the same mean rotation rate (i.e. 5 Ω_⊙) but a distinct differential rotation rate. The excitation of axisymmetric acoustic modes is further inhibited in the anti-solar differential rotation regime, where the poles of stars rotate faster than their equators compared to the uniform rotation case. This could hinder mode detection in such configurations. In contrast, in solar differential rotation regimes, where the equator rotates faster than the poles, acoustic mode excitation is less inhibited compared to the uniform rotation case. We also studied the influence of the azimuthal order and show that the amplitudes can differ by up to 30% for modes with the same order, ell, and a different azimuthal order, m. The trends for sectoral modes (with $| m | =l$) is the opposite than the one observed for the axisymmetric modes. This study permits a first prediction of the excitation of acoustic modes as a function of the differential rotation in solar-like pulsators. On the one hand, efficient mode excitation may enable the detection of rotational frequency splittings. On the other hand, weaker-than-expected or stronger-than-expected excitation compared to predictions based on uniform rotation could provide valuable hints into differential rotation. The results are crucial to interpret data from past and ongoing asteroseismic space missions, such as /K2 and TESS, and to prepare for PLATO. Kepler

We investigate the redshift evolution and group richness dependence of the quiescent galaxy fraction and red sequence (RS) parameters in COSMOS galaxy groups, spanning a wide redshift range, from z=0 to z=3.7. We analyzed the deep and well-characterized sample of groups recently detected with the AMICO algorithm in the COSMOS(-Web) field. Our study of the quiescent galaxy population is based on a machine-learning classification tool based on rest-frame magnitudes. The algorithm learns from several traditional methods to estimate the probability of a galaxy being quiescent, achieving high precision and recall. Starting from this classification, we computed quiescent galaxy fractions within groups via two methods: one based on the membership probabilities provided by AMICO, which rely on an analytical model, and another using a model-independent technique. We then detected the RS by estimating the ridgeline position using probability-weighted photometric data, followed by σ clipping to remove outliers. This analysis was performed using both rest-frame magnitudes and observer-frame magnitudes with rest-frame matching. We compared the results from both approaches and investigated the redshift and richness dependence of the RS parameters. We found that the quiescent galaxy population in groups builds up steadily from z = 1.5-2 across all richnesses, with faster and earlier growth in the richest groups. The first galaxies settle onto the RS ridgeline by z ∼ 2, consistent with current evolutionary scenarios. Notably, we reported a rare protocluster core hosting quiescent galaxies at z = 3.4, potentially one of the most distant early RSs observed. Extending our study to X-ray properties, we found that X-ray faint groups have, on average, lower quiescent fractions than X-ray bright ones, likely reflecting their typical location in filaments where pre-processing is lower. Leveraging the broad wavelength coverage of COSMOS2025, we traced RS evolution using observed and rest-frame colors over ∼ 12 Gyr, finding no significant trends in either the slope or the scatter of the ridgeline.

The most recently formed young stellar objects (YSOs) in active star-forming regions are excellent tracers of their parent cloud motion. Their positions and dynamics provide insights into cluster formation and constrain kinematic decoupling timescales between stars and gas. However, because of their strong extinction and young age, embedded YSOs are mainly visible at infrared wavelengths, and are thus absent from astrometric surveys such as . We measured the proper motions of 6,769 sources toward the NGC 2024 cluster in the Flame Nebula (d using optically visible stars shows excellent agreement, with uncertainties on the same order of magnitude. Gaia pc) using multi-epoch near-infrared observations from three ESO public surveys: VISIONS, VHS, and the VISTA/VIRCAM science verification program. Cross-validation of our results with Gaia -1 $ with mean measurement uncertainties of 0.22 mas,yr^-1, or 0.44 km,s^-1. This is the first homogeneous proper motion measurement of this quality for more than half of these stars. For Class I and flat-spectrum sources, our results provide a $>13-fold increase in available proper motion measurements. We analyzed the positional and kinematic differences between YSO classes and confirmed a previously reported inside-out age segregation from younger to older stars, likely driven by an outward movement of older stars. No evidence of prolonged hierarchical assembly was found. Instead, the results support a rapid ($<1$ Myr) cluster collapse into a centrally concentrated system. This scenario also accounts for the observed slightly higher 1D velocity dispersion of Class I sources relative to Class flat objects. YSO radial velocities generally align with the gas velocities measured from the molecular transitions of 12 CO(3-2)$, HNC$(1-0)$, HCN$(1-0)$, and show a weaker correlation with N_2H$^+(1-0). Some Class II and III objects already appear to be decoupling.

Context. Galaxies fly inside galaxy clusters and ram pressure by the intracluster medium (ICM) can remove a large amount of the interstellar medium (ISM) from the galaxy, and deposit the gas in the ICM. The ISM decoupled from the host galaxy leaves a long trail following the moving galaxy. Such long trails track galaxy motions and they can be detected with sensitive data in H α . Aims. We study the H α tail trailing NGC 4569 in the Virgo cluster. Methods. The initial discovery was made with the deep H α imaging data with CFHT from the VESTIGE project. The follow-up spectroscopic observations were carried out with APO / DIS , MMT / Binospec , and CFHT / SITELLE . Results. Aside from the known 80 kpc H α tail downstream of NGC 4569, the deep H α imaging data allow the H α tail detected to at least 230 kpc from the galaxy. More importantly, we confirmed the H α clumps implied from the imaging data with spectroscopic data. The H α clumps show a smooth radial velocity gradient across ∼1300 km/s, eventually reaching the velocity of the cluster. Conclusions. For the first time, this discovery offers a view of the full deceleration process of the stripped ISM. This discovery also showcases the potential with wide-field H α survey on galaxy clusters to discover intracluster optical emission-line clouds originated from cluster galaxies. These clouds provide kinematic tracers to the infall history of cluster galaxies and the turbulence in the ICM. They also serve as excellent multi-phase objects for studying the classical cloud-crushing problem and other important physical processes.

Context. HR6819 is the first system with a puffed-up low-mass stripped star + a classical Be star whose nature has been confirmed by optical interferometry. The system exhibits the most extreme mass ratio (15.7 ± 1.1), the lowest stripped star mass (0.270 ± 0.056 M ⊙ ), and one of the shortest orbital periods (40.3266 ± 0.0016 days) compared to similar observed binaries. As a post mass transfer candidate, HR6819 offers a unique opportunity to test the physics of binary interaction, especially the fraction of mass accreted by the Be progenitor (the efficiency of the mass transfer) required to produce the system’s extreme mass ratio. Aims. This work aims to reconstruct the possible evolutionary history of HR6819 in the context of stable mass transfer via Roche lobe overflow. We want to explore how the tight constraints on the system’s total mass, mass ratio and orbital period are limiting the range of possible progenitors of the system. Methods. Based on analytical expectations for the orbital evolution, we build grids of MESA simulations designed to match the present-day orbital period and mass ratio of the system, with different mass transfer efficiencies from fully conservative to 50% efficient. Results. We show that evolution via stable mass transfer cannot explain the combined extreme current mass ratio and tight orbital period of the system. There is a limit on how extreme the post-mass transfer mass ratio of the progenitor binary can be at a fixed detachment period, and this limit is dependent on the efficiency of the mass transfer episode: the less efficient the mass transfer episode, the less extreme the mass ratio at detachment. Even in the case of fully conservative mass transfer, the most extreme mass ratio we can produce with binary evolution simulations is q ∼ 11.5 at P ∼ 40 days, which is significantly below the observed value. We also show that the reported luminosities for each component significantly exceed the value expected from their mass. In particular, based on simple stripped star models, we find that the luminosity of the bloated stripped star requires a star with a mass of ∼0.7 M ⊙ , which is over twice the measured value. Conclusions. Our work shows that the post-interaction properties of HR6819, especially its extreme mass ratio and orbital period, cannot be produced by stable mass transfer under standard assumptions.