Astronomy & Astrophysics最新文献

{"title":"Formation of adducts of C6H with Na+, Mg+, and Al+ metal cations by radiative association","authors":"Niyazi Bulut,Octavio Roncero,María Mallo,Germán Molpeceres,Marcelino Agúndez,Maysa Yusef-Buey,José Cernicharo,Carlos Cabezas,Piotr S. Zuchowski","doi":"10.1051/0004-6361/202661307","DOIUrl":"https://doi.org/10.1051/0004-6361/202661307","url":null,"abstract":"The Mg-bearing cations MgC_4H+$, MgC_6H^+, MgC_3N^+, and MgC_5N^+ have been recently observed in the carbon-rich envelope IRC,+10216. These species are thought to form upon radiative association between Mg$^+ and the corresponding neutral radical. This work aims to determine the radiative association rate coefficients for the cations Al+$, Mg^+, and Na$^+ reacting with carbon chain radicals to estimate the relative importance of metal-bearing carbon cations for each of these metals. For this purpose we used statistical methods in combination with highly accurate ab initio calculations to obtain the radiative association rate coefficient. Moreover, anisotropic effects beyond the mono-dimensional capture model were considered, providing a more reliable description of the simulated processes. The radiative association rate coefficients to form Mg-, Al- and Na-C_6H+$, as well as MgC_4H^+, MgC_3N^+, and MgC_5N$^+, were calculated with the new restrictions considered in this work. These new rate coefficients are included in a chemical model of the C-rich AGB envelope IRC,+10216. The abundances of MgC_4H+$, MgC_6H^+, and MgC_5N^+ are consistent with observations, with differences no greater than one order of magnitude, corroborating radiative association as a plausible formation mechanism for these adducts. However, the calculated abundance of MgC_3N$^+ is about two orders of magnitude lower than observed, which indicates that either the chemistry of this species is significantly different from the others or the calculated rate coefficient is too low. The abundances obtained for Na-C_6H+$ and Al-C_6H$^+ are around two orders of magnitude lower than for Mg, making very difficult to detect the analogous metal-bearing cations in IRC,+10216.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"1 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149991","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":"Gas phase Elemental abundances in Molecular cloudS (GEMS). XII. First look into the para-to-ortho ratio of c-C_3D_2 towards pre-stellar cores","authors":"J. Ferrer Asensio,A. Fuente,E. Roueff,K. Furuya,B. Tercero,N. Sakai","doi":"10.1051/0004-6361/202661110","DOIUrl":"https://doi.org/10.1051/0004-6361/202661110","url":null,"abstract":"Symmetric molecules that contain several identical nuclei give rise to distinct isomers that are characterized by the total nuclear spin and produce separate ortho and para species that cannot be exchanged through radiative or inelastic transitions and whose relative populations can preserve information about formation conditions. In the interstellar medium, cyclopropenylidene (c-C_3H_2) and its doubly deuterated isotopologue provide a prime laboratory for studying spin isomer ratios, which have been measured for the main species but are yet unavailable for c-C_3D_2. We present new detections of both ortho and para transitions of c-C_3D_2 towards three sources in the Taurus molecular cloud from the GEMS Large program, and revisit previous detections towards four other cores from the literature in order to investigate the para-to-ortho ratio for doubly deuterated c-C_3D_2 for the first time. We computed the ortho and para c-C_3D_2 column densities towards all the sources using the non-local thermodynamic equilibrium (non-LTE) software RADEX thanks to the recent availability of dedicated collision rate coefficients. The total derived column densities of c-C_3D_2 for the new sources present a range of (2.5 - 5.3) times 10 11 $ cm^-2. The para-to-ortho ratio derived for B213-C16-1 and TMC1-C, 1.5^ +0.7 _ -0.6 and 1.3^ +1.3 _ -0.7 , respectively, are higher than the statistical ratio of 0.5, while for TMC1-NH3 the ratio is statistical within errors, 1.0^ +1.4 _ -0.7 . The para-to-ortho ratio for the revisited sources from the literature, L1544, TMC-1C, TMC-2, and L1517B are 0.4^ +0.1 _ -0.1 , 0.5^ +0.1 _ -0.1 , 0.6^ +0.2 _ -0.1 , and 1.4^ +0.7 _ -0.5 , respectively. In this case, the first three cores show para-to-ortho statistical values, and the latter a non-statistical one. Thus, from all the sources studied, four have statistical c-C_3D$_ 2 para-to-ortho ratios and three sources show higher non-statistical ratios. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule derived through a detailed radiative transfer tool. The observed larger-than-statistical c-C_3D_2 para-to-ortho ratio values for some of the sources may result from p- or D_ D2H+ 3 ^ + driven reactions. The actual ratio is not directly related to the ortho-to-para ratios of and c- . These findings provide an important reference point for future chemical models to investigate the formation pathways of c-C_3D_2. H2 C3H2","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"8 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149992","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":"Radio emission from radio-bright fast blue optical transients powered by trans-relativistic shocks in confined circumstellar material","authors":"Liang-Duan Liu,Jia-Sen Zhang,Zhao-Sheng Zhang,Yun-Wei Yu","doi":"10.1051/0004-6361/202660976","DOIUrl":"https://doi.org/10.1051/0004-6361/202660976","url":null,"abstract":"Fast blue optical transients (FBOTs) are luminous, rapidly evolving explosions whose radio emission provides a vital probe of the shock dynamics and the density structure of the circumstellar material (CSM). Most existing radio studies rely on epoch-by-epoch spectral fits; however, this approach lacks dynamical consistency and fails to reproduce the steep post-peak fading observed in several well-sampled events. We aim to develop a self-consistent physical framework for interpreting the radio emission of FBOTs and for constraining the immediate mass-loss history of their progenitors. We present a forward-shock synchrotron model for ejecta interacting with a dense but radially confined CSM described by a broken power-law density profile. By incorporating synchrotron self-absorption and external free-free absorption, we modeled the multi-frequency radio light curves and broadband spectra of a representative sample of FBOTs. We show that the diversity of FBOT radio morphologies can be naturally explained by shock interaction within a finite CSM shell. Absorption regulates the early-time evolution, while the rapid post-peak decline marks the forward shock's exit from the dense inner region into a tenuous environment. The inferred shock velocities are trans-relativistic (v ∼ 0.1--0.5c) and are associated with high mass-loading rates (dot M ∼ 10^ -4 --10^ -3 ,M_⊙, ^ yr -1 ), yet modest total CSM masses (M_ ̊m csm ∼ 10^ -4 --10^ -2 ,M_⊙). These properties point to brief episodes of enhanced mass loss occurring years to decades prior to explosion, rather than long-lived steady winds. Our framework provides a physically motivated interpretation of FBOT radio emission and a robust diagnostic for reconstructing the immediate mass-loss history of their progenitors.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"6 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149956","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":"Phakoseismology: Asteroseismology of gravitationally microlensed stars","authors":"Marcus L. Marcussen,Mikkel N. Lund","doi":"10.1051/0004-6361/202662677","DOIUrl":"https://doi.org/10.1051/0004-6361/202662677","url":null,"abstract":"Asteroseismology relies on disk-integrated photometry or radial velocity, where geometric cancellation suppresses oscillation modes of high spherical degree l (l This cancellation restricts observations to deeply penetrating, low-degree oscillations, leaving the outer stellar envelope inaccessible. We investigate whether a high-magnification gravitational microlensing event can momentarily circumvent this limitation by differentially magnifying the surface of an oscillating source star. We modelled the disk-integrated visibility of solar-like oscillations during microlensing events, weighting the stellar surface by limb darkening and by the spatially varying magnification. We computed the relative visibility of angular multiplets and the distribution of power among their azimuthal components, and constructed a likelihood model for recovering the three-dimensional orientation of the stellar spin axis from resolved rotational multiplets. When the projected lens--source separation becomes comparable to the angular radius of the source, the magnification gradient acts as a transient spatial filter that breaks the symmetry of disk integration, which normally causes high-degree modes to cancel out. The visibilities of high-degree modes then rise by up to three orders of magnitude. The same non-axisymmetric weighting makes the relative power of the azimuthal components depend on both the inclination and the sky-projected azimuth of the spin axis, imprinting the full spin orientation onto the power spectrum. With relative-power measurement uncertainties of 5%, we are able to recover the input orientation to σ_i=2.9^̧irc and σ_γ=11.5^̧irc. Microlensing can act as a transient spatial filter for unresolved stellar oscillations, doing more than amplifying distant stars: the magnification pattern itself becomes a tool for spatially selective asteroseismology. The effect is strongest for rare close encounters and is unlikely to support large samples. Moreover, the newly visible high-degree modes will overlap with rotationally split and, in evolved stars, mixed modes, making mode identification an observational challenge. Nevertheless, individual high-quality events may open a new window onto envelope physics and spin geometries for stars in the Galactic bulge and beyond.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"198 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149949","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":"4U 1624-490 as a possible intermediate-mass X-ray binary: Constraints from XRISM and NuSTAR spectroscopy","authors":"R. Iaria,A. Anitra,T. Di Salvo,A. Sanna,F. Barra,L. Burderi,C. Maraventano,S. Caserta,W. Leone,C. Miceli","doi":"10.1051/0004-6361/202660603","DOIUrl":"https://doi.org/10.1051/0004-6361/202660603","url":null,"abstract":"High-inclination dipping neutron-star X-ray binaries provide a unique view of accretion-flow structure because the line of sight intercepts the outer-disc absorber and ionised plasma above the disc. Their orbital variability is generally dominated by absorption, although disc reflection may also contribute. We investigate the orbital evolution of the ionised absorber and reflection component in 4U 1624-490 to constrain the kinematics and nature of its highly ionised plasma. We analysed two XRISM/Resolve observations with simultaneous NuSTAR coverage. The data were divided into seven orbital-phase intervals covering persistent and dipping states. We performed phase-resolved fits including neutral partial covering, a photoionised absorber, thermal Comptonisation, and ionised disc reflection. xxv and Fe, xxvi absorption features are detected over most of the orbit. The absorber column density, ionisation state, covering fraction, and projected velocity vary with orbital phase in agreement with previous results. It is systematically blueshifted, with line-of-sight velocities of approximately 85--440 -1 , consistent with a structured outflow or vertically extended plasma. Absorption-only models leave broad Fe--K residuals, whereas ionised reflection systematically improves the fits and varies in visibility along the orbit. The velocity modulation has a semi-amplitude of K=210 and, under an orbital interpretation, implies a mass function of f(M)=0.83 -1 The orbital spectral variability therefore requires the combined effects of ionised absorption, Comptonised emission, and disc reflection. Residual xxvi may indicate additional kinematic complexity close to the dip; nevertheless, the velocity modulation and its fitted parameters remain consistent when the two dip-adjacent measurements are excluded. If this modulation traces the neutron-star orbital motion, the inferred donor mass, M_2=2.66 is compatible with a Roche-lobe-filling B9 V--B9.5 V star. Under this interpretation, 4U 1624-490 may be an intermediate-mass X-ray binary rather than a classical low-mass one.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"52 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149958","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 existence of null points of flow and magnetic fields as prerequisites for the presence of astropauses and their co-location","authors":"Dieter H. Nickeler,Kuljeet Singh Saddal,Rodrigo Meneses Pacheco,Michaela Kraus","doi":"10.1051/0004-6361/202661467","DOIUrl":"https://doi.org/10.1051/0004-6361/202661467","url":null,"abstract":"Astropauses can be defined via magnetic or fluid separatrices. For the boundary layer between the stellar wind and the local interstellar medium, two separatrices can occur, the magnetopause and the hydropause. The question is whether they are identical. To address this, we investigate under which conditions the null points of the magnetic and velocity field, which define the corresponding separatrices, coincide spatially. We analysed the case of a non-monotonous pressure distribution driving stationary counterstreaming ideal hydrodynamic flows and demonstrate that the existence of an extremum of the pressure in the three-dimensional case together with a non-degeneracy condition on the velocity field implies that this point is also a stagnation point. Turning to ideal magnetohydrodynamics (MHD) and considering the regularity of all fields, we formulated the momentum equations, revealing their quasi-linearity. This allowed us to prove the occurrence of null points by examining the rank of the Jacobians of the vector fields. We found that for a given null point of either the velocity or the magnetic field at the pressure extremum, the pendant vector field has a co-located null if the Jacobian of this field is non-singular. We extended the analysis to resistive MHD and further to MHD with general non-ideal terms and proved that if at least one of the first-order momentum equations is ideal, the stagnation point and the magnetic null point are co-located. Even for general fluid theory, the two null points co-locate as long as the curl of the generalised non-ideal terms is zero at one of them or can be represented by a linear operator in the magnetic and velocity field. In general, it is not excluded that the velocity and magnetic nulls are dislocated. However, this would require specific properties of the curl of the non-ideal term, which would reduce the probability of a relaxed quasi-stationary configuration.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"86 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149951","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":"Fast superposition T-matrix method for computing light scattering by structurally diverse aggregates","authors":"J. Markkanen","doi":"10.1051/0004-6361/202662300","DOIUrl":"https://doi.org/10.1051/0004-6361/202662300","url":null,"abstract":"The superposition T-matrix method is widely used to compute the scattering, absorption, and extinction of light by aggregated particles found in cometary comae, planetary atmospheres, and protoplanetary discs. However, commonly used solvers rapidly lose their efficiency with decreasing fractal dimension or with increasing porosity. We present a new open-source implementation of a fast superposition T-matrix method, FaSTMM2, which provides efficient solutions for a wide range of fractal particle aggregates composed of polydisperse monomers. The solver accelerates the superposition T-matrix method using a broadband multilevel fast multipole algorithm. We have implemented diagonal translation operators at super-wavelength levels based on the plane-wave expansion of spherical waves, while sub-wavelength levels are treated using low-frequency stable spherical-wave translators. In addition, we introduced an adaptive octree data structure that enables the efficient treatment of aggregates composed of polydisperse monomers. Numerical examples demonstrate that FaSTMM2 provides an efficient solution with controllable accuracy for a wide range of aggregates and compositions. In particular, it achieves speed-ups of several orders of magnitude for highly porous aggregates with a fractal dimension of around two.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"42 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149953","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 search for periodic active galactic nucleus variability in Gaia Data Release 3","authors":"P. Huijse,J. Davelaar,J. De Ridder,N. Jannsen,C. Aerts","doi":"10.1051/0004-6361/202660885","DOIUrl":"https://doi.org/10.1051/0004-6361/202660885","url":null,"abstract":"Supermassive black hole binaries (SMBHBs) are expected to produce periodic modulations in active galactic nuclei (AGN) light curves, but distinguishing such signals from stochastic red-noise variability remains a major challenge. We present the first systematic search for statistically significant AGN periodicities using the optical photometry from the Gaia space mission Data Release 3 (DR3), with the goal of identifying SMBHB candidates and establishing a methodological data analysis framework that can be scaled to the forthcoming Data Release 4 (DR4). We analysed Gaia light curves of 384,152 sources from the Gaia celestial reference frame (CRF3). Stochastic variability was modelled as a damped random walk Gaussian process, and empirical false alarm probabilities were derived by comparing observed Lomb--Scargle periodogram peaks against 100,000 synthetic red-noise realisations. Candidates from this first stage were then re-evaluated using full Markov chain Monte Carlo inference under both exponential and powered-exponential kernels. We find 13 sources satisfying our statistical criterion (p < α = 10^ -5 ) after both stages of filtering, all of which cover fewer than 2.5 cycles of the candidate period and are systematically concentrated in a region of the parameter space indicative of model misspecification. No reliable periodic SMBHB candidates are retained. The ∼950-day baseline of Gaia DR3 confines all detections to the few-cycle regime where red noise most convincingly mimics periodicity, a limitation that photometric precision alone cannot overcome. The longer baseline of Gaia DR4 will be essential to push beyond this regime. We offer our data analysis software pipeline in open access to the community.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"39 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149993","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":"Spatio-temporal progression of star formation in the Carina OB1 association","authors":"C. Göppl,T. Preibisch","doi":"10.1051/0004-6361/202558189","DOIUrl":"https://doi.org/10.1051/0004-6361/202558189","url":null,"abstract":"The Carina OB1 association, harboring the famous Carina Nebula near its center, is the most massive, well-studied OB association in our Galaxy. Containing several massive star clusters with different ages, it shows a complex spatio-temporal structure. We explore the formation history and kinematic evolution of the eight most massive clusters in Car OB1. We determine the cluster ages, stellar populations, the initial massive star content, and the number of supernova explosions since the time of each cluster’s formation. Using DR3 astrometry, we determine the 3D configuration of the association and perform a traceback analysis of the cluster positions over the last $ ≈ 13$ Myr. Gaia The traceback analysis places the birth location of the oldest ($ ≃ 13$;Myr) cluster NGC 3293 in the Carina spiral arm. Approximately four Myr later, a series of ≃ 11 supernova explosions started in NGC 3293. We construct a possible scenario in which the expanding bubble driven by these supernova explosions interacted with the southern part of the original cloud complex and led to the sequential formation of the younger clusters in the central Carina Nebula. Our scenario suggests that the sequence of $ ≈ 16 ± 4$ supernova explosions in the association during the last 10;Myr played an important role in creating the massive clusters in Car OB1. However, the supernova feedback does not dominate the large-scale expansion of the association; the general expansion rather seems to reflect the initial kinematics of the original unbound cloud complex.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"10 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149150021","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":"Nature of small-scale Ca ii H brightenings associated with weak fields in a quiet-Sun region","authors":"Xiang Li,H. N. Smitha,S. K. Solanki,T. L. Riethmüller,L. P. Chitta,S. Jafarzadeh","doi":"10.1051/0004-6361/202660393","DOIUrl":"https://doi.org/10.1051/0004-6361/202660393","url":null,"abstract":"The chromospheric emission observed in the Ca ii H&K resonance lines is generally positively correlated with photospheric magnetic fields. However, a fraction of strong Ca ii H&K brightenings are found to occur over regions of low photospheric magnetic flux density. We wish to understand the origins of these chromospheric brightenings that are associated with weak fields in quiet-Sun regions. We used high-resolution and high-cadence observations in the Ca ii H line taken by the Sunrise Filter Imager (SuFI) along with the photospheric magnetograms recorded by the Imaging Magnetograph eXperiment (IMaX). These instruments were on board the first flight of the balloon-borne observatory Sunrise . We also used the atmospheric physical parameters inverted from the IMaX data by the code called Stokes-Profiles-INversion-O-Routines (SPINOR). We identified 217 Ca ii H brightenings associated with weak fields, 111 of which are located at the peripheries of strong-field regions in the magnetic canopies formed by the expansion of the kilogauss flux tube. The other brightenings are associated with either weak unipolar magnetic patches (25 brightenings) or with interactions between weak mixed-polarity magnetic fields (75 brightenings), or they lie in locations without clearly identifiable magnetic features (6 brightenings). For the latter two situations, the Ca ii H intensity is found to vary with a period of 3--5 minutes. This is accompanied by variations in the line-of-sight velocity, the temperature at łogτ = -2.5, and the magnetic flux with a similar period. About half of the strong Ca ii H brightenings associated with weak fields are located in magnetic canopies overlying very weak photospheric fields. These canopies are produced by the expansion of kilogauss flux tubes. About 10-12% of the brightenings are associated with small magnetic bright points where the magnetic field is probably not spatially resolved so that it appears weak, but it likely reaches kilogauss strength in reality. The remainder appear to be Ca ii H grains, which are overwhelmingly found to be associated with magnetic flux emergence or cancellation events, however. We conclude that although Ca ii grains are mainly driven by (magneto-)acoustic shock waves, the observations indicate that interactions between magnetic fields with opposite polarities also contribute to the formation of the brightest peaks.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"16 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149089769","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}