Leibniz Institute for Astrophysics Potsdam

Abstract Bipolar light bridges (BLBs) are bright regions located between sunspot umbrae of opposite magnetic polarity. They are typically characterized by strong magnetic fields and intense flows, which are believed to be closely associated with major solar flares. Despite their importance, their fine structure, formation, and evolution remain poorly understood. In this work, we analyze the observations of a well-defined BLB obtained by the Goode Solar Telescope (GST) at the Big Bear Solar Observatory and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The high-resolution GST observations reveal that the BLB is composed of fine, penumbral filament-like structures with widths of approximately 100–150 km. The corresponding Doppler velocity maps present a stable pattern of spatially adjacent red- and blueshifted patches within the BLB throughout the 5.5 hr GST observation. HMI observations show that the BLB arises from the converging and shearing motions of sunspots with opposite polarities. Penumbral regions originating from different polarities gradually evolve and interact, eventually forming the BLB. The observed Doppler velocity pattern, characterized by red- and blueshifted patches, can be interpreted as a projection effect of the Evershed flow within the penumbrae. Therefore, we argue that the BLB is formed through the compression and stretching of penumbral structures from oppositely polarized sunspots.

Abstract Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning ∼90○ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo which results in steeper peaks. This arises from different instantaneous mass loss due to distinct central mass distributions of halos, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo.

The Sun is the most efficient particle accelerator in the solar system, capable of accelerating particles such as electrons and protons to relativistic energies. Solar Energetic Particles (SEPs) are known to be accelerated both at solar flare reconnection sites and by shocks driven by coronal mass ejections. One way to distinguish between these two SEP acceleration mechanisms is through their energy spectra, either fluence or peak intensity.While the spectral breaks commonly observed in solar energetic electron (SEE) spectra may represent signatures of the acceleration process, several transport-related effects have also been proposed as their origin. In this study, we analyse the energy spectra of intense SEE events measured by Solar Orbiter’s Energetic Particle Detector (EPD) between December 2020 and December 2022. EPD’s unprecedented energy resolution enables us to identify spectral features with greater detail than previously possible.We investigate the shape of SEE spectra by fitting them with a range of mathematical models. Our results are compared with previous studies, and we explore possible connections to transport-related effects. In addition, we examine potential correlations between spectral features and parameters such as radial distance or properties of the associated solar events.Our analysis reveals four distinct spectral shapes: single power-law, double power-law, and two types of triple power-law spectra, namely knee–knee (KK) and ankle–knee (AK) forms. No significant correlations with radial distance are found. However, the observed spectral shapes exhibit an ordering with respect to the longitudinal separation between the spacecraft and the associated solar flare.We conclude that multiple processes likely contribute to shaping SEE spectra. Our results suggest that the two breaks observed in KK triple power-law spectra arise from distinct physical effects, namely Langmuir-wave generation and pitch-angle scattering. Furthermore, the break in double power-law spectra may represent a merger of the first and second breaks seen in KK triple power-law spectra.

Context. Solar flares appear as a sudden local enhancement of the emission of electromagnetic waves from the radio to the γ -ray range on the Sun. This radiation is primarily generated by energetic electrons. In addition, these electrons play an important role, since they carry a substantial part of the energy released during a flare. Thus, the generation of highly energetic electrons during flares is one of the basic questions in flare physics. The flare is generally understood as a manifestation of magnetic reconnection in the corona. Several mechanisms of particle acceleration during flares are discussed. Observations and numerical simulations show that a strong magnetohydrodynamic (MHD) turbulence appears in the outflow region of the reconnection site. We consider this turbulence as an ensemble of large-amplitude steepened MHD waves. Aims. We studied the interaction of electrons with such a wave structure to determine whether the interaction accelerates the electrons. Methods. The properties of the large-amplitude steepened MHD waves are described in terms of simple MHD waves. The interaction of an electron with such a wave structure was treated with a mechanism equivalent to the well-known diffusive shock acceleration. Results. The multiple interactions of an electron with a large-amplitude steepened MHD waves lead to its continuous acceleration to high energies. The resulting differential electron flux consists of a thermal and a non-thermal component, the latter of which is governed by a power law. The spectral index depends on the density compression within the wave. The stronger the turbulence, the stronger the density compression, and the lower the spectral index. Thus, strong (e.g. GOES M- or X-class) flares are accompanied by strong MHD turbulence in the outflow region of the reconnection site and by a large number of highly relativistic electrons, which are required to account for the solar hard X- and γ -ray radiation. Conclusions. Strong MHD turbulence in the outflow region of the magnetic reconnection site is able to accelerate electrons to highly relativistic energies.

Context . Classical Cepheids (DCEPs) are among the most important distance calibrators thanks to the correlation between their period and luminosity (PL relation), and play a crucial role in the calibration as the first rung of the extragalactic distance ladder. Given their typical age, they also constitute an optimal tracer of the young population in the Galactic disc. Aims . We aim to increase the number of available DCEPs with high-resolution spectroscopic metallicities, study the galactocentric radial gradients of several chemical elements, and analyse the spatial distribution of the Galactic young population of stars in the Milky Way disc. Methods . We performed a complete spectroscopical analysis of 136 spectra obtained from three different high-resolution spectrographs, for a total of 60 DCEPs. More than half have pulsational periods longer than 15 days, up to 70 days, doubling the number of stars in our sample with P > 15 d . We derived radial velocities, atmospheric parameters, and chemical abundances for up to 33 different species. Results . We present an updated list of trusted spectroscopic lines for the detection and estimation of chemical abundances. We used this new set to revisit the abundances already published in the context of the C-MetaLL (Cepheids-Metallicity in the Leavitt Law) survey and increase the number of available chemical species. For the first time (to our knowledge), we present the estimation of abundances for Cepheids for dysprosium (Dy, Z = 66), as well as a systematic estimation of erbium (Er, Z Z = 68), lutetium (Lu, Z = 71), and thorium (Th, Z = 90) abundances. Conclusions . We calculated a galactic radial gradient for [ Fe/H ] with a slope of −0.064 ± 0.002 dex kpc −1 , in good agreement with recent literature estimation. The other elements also exhibit a clear negative radial trend, with this effect diminishing and eventually disappearing for heavier neutron-capture elements. Depending on the proposed spiral arms model present in several literature sources, our most external stars agree on tracing either the Perseus, the Norma-Outer, or both the Outer and the association Outer-Scutum-Centaurus arms.

We present stellar kinematics of the MUSE Most Massive Galaxies (M3G) Survey, comprising 25 galaxies brighter than -25.7 mag in the K_s band and stellar masses above M_⊙. The galaxies are divided into brightest cluster galaxies (BCGs) and lower-ranked (in brightness) galaxies (non-BCGs) in three rich galaxy clusters within the core of the Shapley Supercluster. We find several velocity maps with rich kinematic structures, including multiple spin reversals within the region encompassing two central effective radii, typically associated with BCGs. Additionally, the majority of the BCGs show rotation around the major axis of the galaxy at least in one of the visible velocity components. These kinematic structures are possible only if galaxies have non-axisymmetric shapes and contain several orbital families with both prograde and retrograde rotations. We argue that such velocity maps provide further evidence that the evolution of very massive galaxies is influenced by, possibly repeated, gas-poor major merging. There are six fast rotators in the M3G sample, and they are all among non-BCGs and typically ranked below the third brightest galaxy in their clusters. Based on the properties of the h_3 Gauss-Hermite moment, fast rotation can be linked to the dominance of prograde rotating short-axis tubes in the orbital distribution. Slow rotators are BCGs or the second or sometimes third brightest galaxies, indicating that the galaxy mass (brightness) is not the only driver of low spin and that the location within the local environment also plays a role. Slow rotators, as evidenced from their multi-spin velocity maps, require more complex orbital structures. Furthermore, some BCGs show kinematic evidence for a secondary component at larger radii, which is likely not in equilibrium with the main galaxy and possibly made of stars accreted from other cluster galaxies. Multi-spin velocity maps, low angular momentum, and additional kinematic components highlight the differences in the evolutionary histories of BCGs (including second-ranked galaxies) and non-BCGs.

We performed high-spatial resolution spectropolarimetric observations of active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examine the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focus on the analysis of various spectral lines. In particular, we study the Si I 10827 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å photospheric lines, as well as the He I 10830 Å triplet. GRIS data revealed the presence of flare-related red- and blueshifted spectral line components, reaching Doppler velocities up to 90 km/s, and complex Si I profiles where the He i spectral line contribution is blueshifted. In contrast, the photospheric Ca i and Na i transitions remained unchanged, indicating that the flare did not modify the physical conditions of the lower photosphere. We combined that information with simultaneous imaging in the Ca ii H line and TiO band with the improved High-resolution Fast Imager (HiFI+), finding that the flare emission did not affect the inverse granulation or nearby plage, in agreement with the results from GRIS. We also complement the previous studies with a forward modelling computation, concluding that the He I spectral line emission reflects a complex response of the flaring chromosphere. Radiative excitation from coronal EUV irradiation, energy deposition by flare-accelerated electrons, and dynamic field-aligned plasma flows likely act together to produce the observed supersonic downflows and upflows. We plan to expand these findings through inversions of the He I 10830 Å triplet signals in the future.

Abstract We introduce a new photometric catalog of RR Lyrae (RRL) variables (∼300,000) mainly based on data available in public datasets. We also present the largest and most homogeneous spectroscopic dataset of RRLs and blue horizontal branch (BHB) stars ever collected. This includes radial velocity measurements (∼16,000) and iron abundances (Δ S method for 8140 RRLs, plus 547 from literature). Elemental abundances based on high-resolution spectra are provided for 487 RRLs and 64 BHB stars. We identified candidate RRLs associated with the main Galactic components and their iron distribution function (IDF) becomes more metal rich when moving from the halo ([Fe/H] = −1.56) to the thick disk (TCD; [Fe/H] = −1.47) and thin disk (TND; [Fe/H] = −0.73). Furthermore, halo RRLs and RRLs in retrograde orbits are α enhanced ([ α /Fe]=0.27, σ = 0.18), while TCD RRLs are either α enhanced ([Fe/H] ≤ −1.0) or α poor ([Fe/H] > −1.0), and TND RRLs are mainly α poor ([ α /Fe] = −0.01, σ = 0.20). We also identified RRLs associated with the main stellar streams—Gaia–Sausage–Enceladus (GSE); Sequoia, Helmi, and Sagittarius—and we found that their IDFs are quite similar to halo RRLs. However, GSE RRLs lack the metal-poor/metal-rich tails and their α -element distribution is quite compact. The iron radial gradient in Galactocentric distance for TND, TCD, and halo RRLs is negative and it decreases from −0.026, to −0.010, and to −0.002 dex kpc −1 . The iron radial gradient based on dry halo (halo without substructures) RRLs is, within the errors, equal to the global halo. We also found a strong similarity between iron and [ α /Fe] radial gradients of Milky Way RRLs and M31 globular clusters throughout the full range of galactocentric distances covered by the two samples.

Context . The stability of a differentially-rotating fluid subject to its own gravity is a problem with applications across wide areas of astrophysics, from protoplanetary discs to entire galaxies. The shearing box formalism offers a conceptually simple framework for studying differential rotation in the local approximation. Aims . Aimed at self-gravitating, and importantly, vertically stratified protoplanetary discs, we develop two novel methods for solving Poisson’s equation in the framework of the shearing box with vertical vacuum boundary conditions (BCs). Methods . Both approaches naturally make use of multi-dimensional fast Fourier transforms (FFTs) for computational efficiency. While the first one exploits the linearity properties of the Poisson equation, the second, which is slightly more accurate, consists of finding the adequate discrete Green’s function (in Fourier space) adapted to the problem at hand. To this end, we have derived, in Fourier space, an analytical Green’s function satisfying the shear-periodic BCs in the plane as well as vacuum BCs, vertically. Results . Our spectral method demonstrates excellent accuracy, even with a modest number of grid points, and exhibits third-order convergence. It has been implemented in the NIRVANA-III code, where it exhibits good scalability up to 4096 CPU cores, consuming less than 6% of the total runtime. This was achieved through the use of P3DFFT , a fast Fourier Transform library that employs pencil decomposition, overcoming the scalability limitations inherent in libraries using slab decomposition. Conclusions . We have introduced two novel spectral Poisson solvers that guarantee high accuracy, performance, and intrinsically support vertical vacuum BCs in the shearing box framework. Our solvers enable high-resolution local studies involving self-gravity, such as magnetohydrodynamic (MHD) simulations of gravito-turbulence and/or gravitational fragmentation.

Aims. The secondaries of massive binary systems can be found as runaway stars after being ejected due to the supernova (SN) of the more massive component. We search for such stars inside supernova remnants (SNRs), where a recent SN is guaranteed to have occurred and the runaway star is expected nearby. In this paper, we present the massive runaway star HD 254577 as the pre-SN binary companion to the progenitor of the SN remnant IC 443 and the neutron star (NS) CXOU J61705.3+222127. Methods. We performed spectroscopic observations of the runaway star and specified its atmospheric parameters. We also used archival spectroscopic data on neighboring stars. Together with precise Gaia DR3 astrometry and photometry, we identified the possible birth origin of the runaway star. By isochrone fitting, we likewise determined its progenitor mass. From Gaia DR3 proper motions, we specified the possible explosion sites and calculated the NS velocity. Results. HD 254577 is a hot and evolved star with an effective temperature of 24 000 ± 1000 K (B0.5II) and a surface gravity of log( g [cm/s 2 ]) = 2.75 ± 0.25. It is probably a single star with a peculiar 3D velocity of 31.3 −0.9 +1.2 km s −1 , lying at a heliocentric distance of 1701 −54 +55 pc. The cometary tail of the NS implies that it is moving away from the same site as the runaway star. From the flight trajectories, we calculated typical NS pulsar velocities of 254 − 539 km s −1 at a distance of 1.7 kpc for 10 − 20 kyr ages. Together with the blue-shifted interstellar medium lines in its spectrum, HD 254577 must be the pre-SN binary companion to the progenitor of IC 443. By identifying the pre-SN companion and the possible parent cluster, we show that the progenitor zero-age main-sequence mass is high (31 − 64 M ⊙ ), favoring the jet scenario previously proposed. The SNR distance is precisely determined as 1701 −54 +55 pc. We also discuss the expansion dynamics of the SNR due to the highly off-centered explosion site. Our calculated pre-SN binary parameters might not favor a strongly interacting binary.