Astronomy & Astrophysics最新文献

{"title":"Mutual impedance experiments in a magnetized plasma","authors":"P. Dazzi, P. Henri, L. Bucciantini, F. Lavorenti, F. Califano, G. Wattieaux, O. Randriamboarison, K. Issautier","doi":"10.1051/0004-6361/202450312","DOIUrl":"https://doi.org/10.1051/0004-6361/202450312","url":null,"abstract":"<i>Context.<i/> A mutual impedance experiment is an active in situ space plasma diagnostic that is used to determine the electron density and temperature. Such parameters are inferred from the mutual impedance spectrum measured between a pair of electric antennas embedded in the plasma. This state-of-the-art plasma diagnostic technique is limited to unmagnetized plasmas; that is, ones with a plasma frequency much larger than the electron cyclotron frequency. This limit is not expected to be valid in the plasma environment surrounding magnetized planets such as Mercury and Jupiter that will be explored by the ESA JUICE and joint ESA/JAXA Bepi-Colombo missions.<i>Aims.<i/> The goal of this work is to extend the mutual impedance diagnostic technique to magnetized plasmas, focusing on measurements of the electron density and temperature, and to extend it to the electron temperature anisotropy.<i>Methods.<i/> To achieve this, we developed the first quantitative three-dimensional instrumental model for mutual impedance experiments in a magnetized plasma. This model is valid for arbitrary values of the electron temperature and magnetic field. Our model is based on the linearized Vlasov-Maxwell coupled system of equations. We numerically computed the electric potential generated and simultaneously measured by the mutual impedance experiment, in order to compute the mutual impedance spectrum in a magnetized plasma.<i>Results.<i/> First, we identify in the numerical mutual impedance spectra a number of local spectral signatures, associated with characteristic frequencies that can be used for plasma diagnostics. We show how the magnetic field strength and direction modify such spectral signatures. Second, we show that electron-neutral collision smooth out the spectrum, as long as the scattering-to-plasma frequency ratio is greater than 10<sup>−3<sup/> . Below such a value, mutual impedance experiments are insensitive to electron-neutral scattering and the plasma can be considered collisionless. Third, we show that the electron temperature directly controls the amplitude of the mutual impedance spectra, so that such behavior can be used as an electron temperature diagnostic. Fourth, we explore for the first time the possibility of diagnosing electron temperature anisotropies with mutual impedance experiments. We show how an electron temperature anisotropy significantly modifies the mutual impedance spectral signatures, as a result of the modified propagation of the electron Bernstein waves generated by the experiment.<i>Conclusions.<i/> The results of our model, in terms of plasma diagnostics, are discussed in terms of the propagation properties in a magnetized plasma of the electrostatic waves generated by the active mutual impedance experiment. The results of our model will significantly extend the plasma diagnostic capabilities of the current and future mutual impedance experiment such as the PWI/AM2P experiment on board BepiColombo and the RPWI/MIME expe","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"6 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143745299","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}

{"title":"A deep neural network approach to compact source removal","authors":"M. Madarász, G. Marton, I. Gezer, S. Lehner, J. Roquette, M. Audard, D. Hernandez, O. Dionatos","doi":"10.1051/0004-6361/202453262","DOIUrl":"https://doi.org/10.1051/0004-6361/202453262","url":null,"abstract":"<i>Context.<i/> Analyzing extended emission in photometric observations of star-forming regions requires maps free from compact foreground, embedded, and background sources, which can interfere with various techniques used to characterize the interstellar medium. Within the framework of the NEMESIS project, we applied machine-learning techniques to improve our understanding of the star formation timescales, which involves the unbiased analysis of the extended emission in these regions.<i>Aims.<i/> We present a deep learning-based method for separating the signals of compact sources and extended emission in photometric observations made by the <i>Herschel<i/> Space Observatory, facilitating the analysis of extended emission and improving the photometry of compact sources.<i>Methods.<i/> Central to our approach is a modified U-Net architecture with partial convolutional layers. This method enables effective source removal and background estimation across various flux densities, using a series of partial convolutional layers, batch normalization, and ReLU activation layers within blocks. Our training process utilized simulated sources injected into <i>Herschel<i/> images, with controlled flux densities against known backgrounds. A dynamic, signal-to-noise ratio (S/N)-based adaptive masking system was implemented to assess how prominently a compact source stands out from the surrounding background.<i>Results.<i/> The results demonstrate that our method can significantly improve the photometric accuracy in the presence of highly fluctuating backgrounds. Moreover, the approach can preserve all characteristics of the images, including the noise properties.<i>Conclusions.<i/> The presented approach allows users to analyze extended emission without the interference of disturbing point sources or perform more precise photometry of sources located in complex environments. We also provide a Python tool with tutorials and examples to help the community effectively utilize this method.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"33 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143745300","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}

{"title":"Cosmic ray bubbles from nova super-remnants and their contribution to local cosmic ray spectra","authors":"Rubén López-Coto, David Green, Javier Méndez-Gallego, Emma de Oña Wilhelmi","doi":"10.1051/0004-6361/202451880","DOIUrl":"https://doi.org/10.1051/0004-6361/202451880","url":null,"abstract":"<i>Context.<i/> Several new phenomena have arisen in the area of study of the repeating thermonuclear explosions called novae. For example, recurrent novae have been proven to be efficient cosmic ray hadronic accelerators thanks to the recent observations of RS Ophiuchi by different γ-ray instruments. Novae have also been demonstrated to have the ability to carve large cavities into the interstellar medium (ISM), with parallels with the remnants of supernovae.<i>Aims.<i/> Our aims are to calculate the effects of novae on their surrounding media, and the distances over which these effects dominate over the average quantities that are measured in the ISM.<i>Methods.<i/> We calculated the filling factor of novae and their contribution to cosmic ray fluxes using cosmic ray propagation codes. To limit the atomic density of the ISM surrounding the region of RS Oph, we used <i>Fermi<i/>-LAT observations of the region.<i>Results.<i/> The filling factor of novae in the Galaxy is not significant under all assumptions done in the paper. They do not dominate over the local cosmic ray fluxes, even at the lowest energies, for distances larger than a few parsecs. The particle density of the ISM surrounding them is, however, very much modified, being lowered by more than one order of magnitude with respect to galactic averages, confirming estimates done using other observatories.<i>Conclusions.<i/> Even though at global galactic distances, novae do not seem to be dominating cosmic ray transport, they have the power to modify the conditions of their surrounding ISM over parsec distances.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"22 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143745489","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}

{"title":"A dusty magnetospheric stream as the physical mechanism responsible for stellar occultations: Interpretation of the TESS light curve of the pre-transitional disk system UX Tau A","authors":"Erick Nagel, Jerome Bouvier","doi":"10.1051/0004-6361/202453365","DOIUrl":"https://doi.org/10.1051/0004-6361/202453365","url":null,"abstract":"<i>Context<i/>. Recent observations of the object UX Tau A containing a pre-transitional disk suggest that the inner disk is misaligned and precessing with respect to the outer disk. These motions lead to a highly dynamic environment that changes the reservoir of dust feeding the star. One of the effects of this is seen in the features of the Transiting Exoplanet Survey Satellite (TESS) optical light curve (LC), resembling dips of variable depth changing within the timescale of the inner disk dust replenishment.<i>Aims<i/>. For this work we interpreted the TESS LC corresponding to a time window around the date a spectrum was taken with the James Webb Space Telescope (JWST). The spectrum was taken in the mid-infrared, clearly a range tracing the emission of dust. Compared with previous spectra, the most recent spectrum suggests a strong decrease in the amount of dust in the inner disk; the observed spectral energy distribution shows a very small infrared excess.<i>Methods<i/>. The physical modeled flux comes from stellar radiation occulted by a sheet of evaporating dust following the magnetospheric field (<i>B<i/><sub>⋆<sub/>) lines. A grid of stream configurations were taken where the gas component explains the JWST spectrum and the H<sub><i>α<i/><sub/> profiles.<i>Results<i/>. Our quest to find a reasonable interpretation of the LC requires a tuning of the values associated with the truncation radius, the inclination of the disk with respect to the line of sight and the maximum size of the dusty grains.<i>Conclusions<i/>. We conclude that the dust evaporation accretion flow is able to explain the typical depths of the LC features periodically changing with the stellar rotational period. We conclude that the dust evaporation accretion flow is able to explain the dips observed in the UX Tau A TESS light curve, most notably the large amplitude dips up to Δ<i>mag<i/> ∼ 0.7 mag, while the lower level variability events (Δ<i>mag<i/> ≤ 0.2 mag) in the LC could also be accounted for by the periodic modulation caused by a hot surface spot. We also suggest that winds and warps are unlikely mechanisms for UX Tau A’s variability.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"34 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143745688","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}

{"title":"Resolved gas temperatures and 12C/13C ratios in SVS13A from ALMA Observations of CH3CN and CH313CN","authors":"T.-H. Hsieh, J. E. Pineda, D. M. Segura-Cox, P. Caselli, M. J. Maureira, L. A. Busch, M. T. Valdivia-Mena, C. Gieser, Y. Lin, Y.-R. Chou, V. Lattanzi, S. Spezzano, A. Lopez-Sepulcre, R. Neri","doi":"10.1051/0004-6361/202453273","DOIUrl":"https://doi.org/10.1051/0004-6361/202453273","url":null,"abstract":"<i>Context.<i/> Multiple systems are common in field stars and the frequency is found to be higher in early evolutionary stages. Thus, the study of young multiple systems during the embedded stages is key to a comprehensive understanding of star formation. In particular, the way material accretes from the large-scale envelope into the inner region and how this flow interacts with the system physically and chemically has not been well characterized observationally to date.<i>Aims.<i/> We aim to provide a snapshot of the forming protobinary system SVS13A, consisting of VLA4A and VLA4B. This includes a clear picture of its kinematic structures, physical conditions, and chemical properties.<i>Methods.<i/> We conducted ALMA observations toward SVS13A targeting CH<sub>3<sub/>CN and CH<sub>3<sub/><sup>13<sup/>CN J=12-11 K-ladder line emission with a high spatial resolution of ∼30 astronomical units (au) at a spectral resolution of ∼0.08 km s<sup>−1<sup/> .<i>Results.<i/> We used local thermal equilibrium (LTE) radiative transfer models to fit the spectral features of the line emission. We found the two-layer LTE radiative model that includes dust absorption is essential to interpreting the CH<sub>3<sub/>CN and CH<sub>3<sub/><sup>13<sup/>CN line emission. We identified two major and four small kinematic components and derived their physical and chemical properties.<i>Conclusions.<i/> We identified a possible infalling signature toward the bursting secondary source VLA4A, which may be fed by an infalling streamer from the large-scale envelope. The mechanical heating in the binary system, as well as the infalling shocked gas, are likely to play a role in the thermal structure of the protobinary system. By accumulating mass from the streamer, it is plausible that the system experienced a gravitationally unstable phase before the accretion outburst. Finally, the derived CH<sub>3<sub/>CN/CH<sub>3<sub/><sup>13<sup/>CN ratio is lower than the canonical ratio in the ISM and varies between VLA4A and VLA4B.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"183 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143736842","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}

{"title":"Distinguishing between light curves of ellipsoidal variables with massive dark companions, contact binaries, and semidetached binaries using principal component analysis","authors":"Milan Pešta, Ondřej Pejcha","doi":"10.1051/0004-6361/202451907","DOIUrl":"https://doi.org/10.1051/0004-6361/202451907","url":null,"abstract":"Photometric methods for identifying dark companion binaries – binary systems hosting quiescent black holes (BHs) and neutron stars (NSs) – operate by detecting ellipsoidal variations caused by tidal interactions. The limitation of this approach is that contact and semidetached binaries can produce similarly looking light curves. In this work, we address the degeneracy of ellipsoidal light curves by studying the differences between synthetically generated light curves of dark companion, semidetached, and contact binary systems. We inject the light curves with various levels of uncorrelated and correlated Gaussian noise to simulate the effects of instrumental noise and stellar spots. Using principal component analysis (PCA) and Fourier decomposition, we construct low-dimensional representations of the light curves. We find that the first three to five PCA components are sufficient to explain 99% of variance in the data. The PCA representations are generally more informative than the Fourier representation for the same number of coefficients as measured by both the silhouette scores of the representations and the macro recalls of random forest classifiers trained on the representations. The random forest classifiers reach macro recalls from 0.97 in the complete absence of noise to 0.67 in the presence of spots and strong instrumental noise, indicating that the classes remain largely separable even under adverse conditions. We find that instrumental noise significantly impacts the class separation only when its standard deviation exceeds 10<sup>−3<sup/> mag, whereas the presence of spots can markedly reduce the class separation even when they contribute as little as 1% of the light curve amplitude. We discuss the application of our method to real ellipsoidal samples, and we show that we can increase the purity of a sample of dark companion candidates by a factor of up to 25 if we assume a prior purity of 1%, significantly improving the cost efficiency of follow-up observations.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"694 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143736896","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}

{"title":"Characterization of AF Lep b at high spectral resolution with VLT/HiRISE★","authors":"A. Denis, A. Vigan, J. Costes, G. Chauvin, A. Radcliffe, M. Ravet, W. Balmer, P. Palma-Bifani, S. Petrus, V. Parmentier, S. Martos, A. Simonnin, M. Bonnefoy, R. Cadet, T. Forveille, B. Charnay, F. Kiefer, A.-M. Lagrange, A. Chiavassa, T. Stolker, A. Lavail, N. Godoy, M. Janson, R. Pourcelot, P. Delorme, E. Rickman, D. Cont, A. Reiners, R. De Rosa, H. Anwand-Heerwart, Y. Charles, A. Costille, M. El Morsy, J. Garcia, M. Houllé, M. Lopez, G. Murray, E. Muslimov, G. P. P. L. Otten, J. Paufique, M. Phillips, U. Seemann, A. Viret, G. Zins","doi":"10.1051/0004-6361/202453108","DOIUrl":"https://doi.org/10.1051/0004-6361/202453108","url":null,"abstract":"<i>Context.<i/> Since the recent discovery of the directly imaged super-Jovian planet AF Lep b, several studies have been conducted to characterize its atmosphere and constrain its orbital parameters. AF Lep b has a measured dynamical mass of 3.68 ± 0.48 <i>M<i/><sub>Jup<sub/>, radius of 1.3 ± 0.15 <i>R<i/><sub>Jup<sub/>, nearly circular orbit in spin-orbit alignment with the host star, relatively high metallicity, and near-solar to super-solar C/O ratio. However, key parameters such as the rotational velocity and radial velocity have not been estimated thus far, as they require high-resolution spectroscopic data that are impossible to obtain with classical spectrographs.<i>Aims.<i/> AF Lep b was recently observed with the new HiRISE visitor instrument at the VLT, with the goal of obtaining high-resolution (R ≈ 140 000) spectroscopic observations to better constrain the orbital and atmospheric parameters of the young giant exoplanet.<i>Methods.<i/> We compared the extracted spectrum of AF Lep b to self-consistent atmospheric models using ForMoSA, a forward modeling tool based on Bayesian inference methods. We used our measurements of the planet’s radial velocity to offer new constraints on its orbit.<i>Results.<i/> From the forward modeling, we find a C/O ratio that aligns with previous low-resolution analyses and we confirm its supersolar metallicity. We also unambiguously confirm the presence of methane in the atmosphere of the companion. Based on all available relative astrometry and radial velocity measurements of the host star, we show that two distinct orbital populations are possible for the companion. We derived the radial velocity of AF Lep b to be 10.51 ± 1.03 km s<sup>−1<sup/> and show that this value is in good agreement with one of the two orbital solutions, allowing us to rule out an entire family of orbits. Additionally, assuming that the rotation and orbit are coplanar, the derived planet’s rotation rate is consistent with the observed trend of increasing spin velocity with higher planet mass.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"11 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143736845","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}

{"title":"Global Hall-magnetohydrodynamic simulations of transition disks","authors":"Eleftheria Sarafidou, Oliver Gressel, Barbara Ercolano","doi":"10.1051/0004-6361/202452121","DOIUrl":"https://doi.org/10.1051/0004-6361/202452121","url":null,"abstract":"<i>Context.<i/> Transition disks (TDs) are a type of protoplanetary disk characterized by a central dust and gas cavity. The processes behind how these cavities are formed and maintained, along with their observed high accretion rates of 10<sup>−8<sup/>−10<sup>−7<sup/> <i>M<i/><sub>⊙<sub/> yr<sup>−1<sup/>, continue to be subjects of active research.<i>Aims.<i/> This work aims to investigate how the inclusion of the Hall effect (HE) alongside Ohmic resistivity (OR) and ambipolar diffusion (AD) affects the structure of the TD. Of key interest is the dynamical evolution of the cavity and whether it can indeed produce transonic accretion, as predicted by theoretical models in order to account for the observed high accretion rates despite the inner disk’s low density.<i>Methods.<i/> We present our results of 2D axisymmetric global radiation magnetohydrodynamic (MHD) simulations of TDs for which all three non ideal MHD effects are accounted. We used the NIRVANA-III fluid code and initialized our model with a disk cavity reaching up to <i>R<i/> = 8 au with a density contrast of 10<sup>5<sup/> . We performed three runs, one with only OR and AD, and one for each of the two configurations that arise when additionally including the HE, that is, with the field aligned (anti-aligned) with respect to the rotation axis.<i>Results.<i/> For all three runs, our models maintain an intact inner cavity and an outer standard disk. MHD winds are launched both from the cavity and from the disk. Notably, when the HE is included, ring-like structures develop within the cavity. We moreover obtain accretion rates of 3−8 × 10<sup>−8<sup/> <i>M<i/><sub>⊙<sub/> yr<sup>−1<sup/>, comparable to typical values seen in full disks. Importantly, we clearly observe (tran)sonic accretion (<i>v<i/><sub>acc<sub/> ≳ <i>c<i/><sub>s<sub/>) in the cavity. Additionally, outward magnetic flux transport occurs in all three runs.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"103 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143737137","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}

{"title":"The theoretical pulsation spectra of hot B subdwarfs","authors":"N. Guyot, V. Van Grootel, S. Charpinet, M. Farnir, M.-A. Dupret, P. Brassard","doi":"10.1051/0004-6361/202452423","DOIUrl":"https://doi.org/10.1051/0004-6361/202452423","url":null,"abstract":"<i>Context.<i/> The <i>Kepler<i/> and TESS space missions have revealed the rich gravity (<i>g<i/>-)mode pulsation spectra of many hot subdwarf B (sdB) stars in detail. These spectra exhibit complex behaviors, with some stars exhibiting trapped modes interposing in the asymptotic period sequences of regular period spacing, while others do not.<i>Aims.<i/> We aim to thoroughly compute theoretical <i>g<i/>-mode pulsation spectra, using our current sdB models, useful for future reference when comparing to observations. This also enables us to explore relationships with features of the internal structure of these stars. Such studies provide guidance in conducting future asteroseismic analyses of these pulsators and insights on how to interpret their outcomes.<i>Methods.<i/> We used our STELlar modeling from the Université de Montréal (STELUM) code to compute static (parametric) and evolutionary models of sdB stars, with different prescriptions for their chemical and thermal structures. We used our adiabatic PULSE code to compute the theoretical spectra of <i>g<i/>-mode pulsations for degrees of <i>ℓ<i/> = 1 to 4 and for periods between 1000 s and 15 000 s, amply covering the range of observed <i>g<i/>-modes in these stars.<i>Results.<i/> We show that <i>g<i/>-mode pulsation spectra and, in particular, the appearance of trapped modes are highly dependent on the chemical and thermal structures in the models as the star evolves, particularly in the region just above the He-burning core. Depending on the prescriptions and specific evolutionary stage, we observe mainly three types of spectra for mid to high radial-order <i>g<i/>-modes (the ones observed in sdB stars): “flat” spectra of nearly constant period spacing; spectra with deep minima of the period spacing interposing between modes with more regular spacing (which correspond to trapped modes); and spectra showing a “wavy” pattern in period spacing. For the two latter cases, we have identified the region where the modes are trapped in the star.<i>Conclusions.<i/> Detailed comparisons with observed <i>g<i/>-mode spectra ought to be carried out next to progress on this issue and constrain the internal structure of core-He burning stars via asteroseismology, in particular, for the region above the He-burning core.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"15 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143736901","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}

{"title":"An effective model for magnetic field amplification by the magnetorotational and parasitic instabilities","authors":"Miquel Miravet-Tenés, Martin E. Pessah","doi":"10.1051/0004-6361/202452953","DOIUrl":"https://doi.org/10.1051/0004-6361/202452953","url":null,"abstract":"The magnetorotational instability (MRI) is considered a leading mechanism for driving angular momentum transport in differentially rotating astrophysical flows, including accretion disks and protoneutron stars. This process is mediated by the exponential amplification of the magnetic field whose final amplitude is envisioned to be limited by secondary (parasitic) instabilities. In this paper, we investigated the saturation of the MRI via parasitic modes relaxing previous approximations. We carried out the first systematic analysis of the evolution of parasitic modes as they feed off the exponentially growing MRI while being advected by the background shear flow. We provide the most accurate calculation of the amplification factor to which the MRI can grow before the fastest parasitic modes reach a comparable amplitude. We find that this amplification factor is remarkably robust, depending only logarithmically on the initial amplitude of the parasitic modes, in reasonable agreement with numerical simulations. Based on these insights, and guided by numerical simulations, we provide a simple analytical expression for the amplification of magnetic fields responsible for MRI-driven angular momentum transport. Our effective model for magnetic field amplification may enable going beyond the standard prescription for viscous transport currently employed in numerical simulations when the MRI cannot be explicitly resolved.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"5 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143736902","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}