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Polarimetry of stars at optical and near-infrared wavelengths is an invaluable tool for tracing interstellar dust and magnetic fields. Recent studies have demonstrated the power of combining stellar polarimetry with distances from the Gaia mission, in order to gain accurate, 3D information on the properties of the interstellar magnetic field and the dust distribution. However, access to optical polarization data is limited, as observations are conducted by different investigators, with different instruments, and are made available in many separate publications. To enable a more widespread accessibility of optical polarimetry for studies of the interstellar medium, we compile a new catalog of stellar polarization measurements. The data are gathered from 81 separate publications spanning two decades since the previous, widely used agglomeration of catalogs by C. Heiles. The compilation contains a total of 55,742 measurements of stellar polarization. We combine this database with stellar distances based on the Gaia Early Data Release 3, thereby providing polarization and distance data for 42,482 unique stars. We provide two separate data products: an extended catalog (containing all polarization measurements) and a unique source catalog (containing a subset of sources excluding duplicate measurements). We propose the use of a common tabular format for the publication of stellar polarization catalogs to facilitate accessibility and increase discoverability in the future.

Compact symmetric objects (CSOs) are a unique class of jetted active galactic nuclei defined by subkiloparsec radio emission, showing radio structure on both sides of the central engine. CSOs tend to exhibit little to no relativistic beaming, thereby allowing us to determine their physical characteristics, such as the magnetic field strength and particle energy density. Selected with a literature search, we describe very long baseline interferometry observations, imaging, and analyses of 167 CSO candidates. We identified 65 new bona fide CSOs, thus almost doubling the number of known bona fide CSOs to 144. With our greater breadth of sources, we confirm that edge-dimmed CSOs (CSO-1s) may represent a more diverse population than originally expected. We highlight a number of CSOs with complex morphologies, including candidates for supermassive binary black holes and CSOs that appear to have morphologies akin to wide-angle tail galaxies, which could perhaps indicate that some CSOs are experiencing a galactic merger.

Dust in the Interstellar Medium (ISM) is intimately linked to the life cycle of stars. Despite being of such fundamental importance to galaxy evolution, the dynamic behaviour and composition of the ISM are not yet fully understood. Observations of reddened Milky Way OB stars have revealed a strong UV extinction feature around 2175 Å and a precipitous extinction rise to the far UV along the lines of sight. Whilst the carrier(s) for this are at present still being debated, multiple laboratory studies suggest carbonate grains to be the key constituent. EREBUS is a mission concept being developed to study the composition of the ISM in both the Milky Way and Local Group Galaxies, primarily by mapping the spatial distribution of the UV extinction features. As these features are sensitive to the dust composition along the line of sight, EREBUS will provide a wealth of information about the spatial distribution and dynamic behaviour of the carrier(s). The mission proposes to deploy a satellite observatory equipped with a coarse UV spectrograph to map the extinction curve variability in the Milky Way in 3 dimensions and in the Local Group in 2 dimensions. In this paper, we discuss the scientific goals for the project, discuss a proposed observation strategy using an iterative process to develop a hierarchical map, and finally outline the instrument requirements and preliminary spacecraft architecture.

Magnetic fields may play a crucial role in setting the initial conditions of massive star and star cluster formation. To investigate this, we report SOFIA-HAWC+ 214 μm observations of polarized thermal dust emission and high-resolution GBT-Argus C18O(1-0) observations toward the massive Infrared Dark Cloud (IRDC) G28.37+0.07. Considering the local dispersion of B-field orientations, we produce a map of the B-field strength of the IRDC, which exhibits values between ∼0.03 and 1 mG based on a refined Davis–Chandrasekhar–Fermi method proposed by Skalidis & Tassis. Comparing to a map of inferred density, the IRDC exhibits a B–n relation with a power-law index of 0.51 ± 0.02, which is consistent with a scenario of magnetically regulated anisotropic collapse. Consideration of the mass-to-flux ratio map indicates that magnetic fields are dynamically important in most regions of the IRDC. A virial analysis of a sample of massive, dense cores in the IRDC, including evaluation of magnetic and kinetic internal and surface terms, indicates consistency with virial equilibrium, sub-Alfvénic conditions, and a dominant role for B-fields in regulating collapse. A clear alignment of magnetic field morphology with the direction of the steepest column density gradient is also detected. However, there is no preferred orientation of protostellar outflow directions with the B-field. Overall, these results indicate that magnetic fields play a crucial role in regulating massive star and star cluster formation, and therefore they need to be accounted for in theoretical models of these processes.

Magnetohydrodynamic (MHD) turbulence is a cross-field process relevant to many systems. A prerequisite for understanding these systems is to constrain the role of MHD turbulence, and in particular the energy exchange between kinetic and magnetic forms. The energetics of strongly magnetized and compressible turbulence has so far resisted attempts to understand them. Numerical simulations reveal that kinetic energy can be orders of magnitude larger than fluctuating magnetic energy. We solve this lack-of-balance puzzle by calculating the energetics of compressible and sub-Alfvénic turbulence based on the dynamics of coherent cylindrical fluid parcels. Using the MHD Lagrangian, we prove analytically that the bulk of the magnetic energy transferred to kinetic is the energy stored in the coupling between the ordered and fluctuating magnetic field. The analytical relations are in striking agreement with numerical data, up to second order terms.

It has not been shown so far whether the diffuse Galactic polarized emission at frequencies relevant for cosmic microwave background (CMB) studies originates from nearby or more distant regions of our Galaxy. This questions previous attempts that have been made to constrain magnetic field models at local and large scales. The scope of this work is to investigate and quantify the contribution of the dusty and magnetized local interstellar medium to the observed emission that is polarized by thermal dust. We used stars as distance candles and probed the line-of-sight submillimeter polarization properties by comparing the emission that is polarized by thermal dust at submillimeter wavelengths and the optical polarization caused by starlight. We provide statistically robust evidence that at high Galactic latitudes (\\(|b| 60^\\)), the \\(353\\) GHz polarized sky as observed by Planck is dominated by a close-by magnetized structure that extends between \\(200\\) and \\(300\\) pc and coincides with the shell of the Local Bubble. Our result will assist modeling the magnetic field of the Local Bubble and characterizing the CMB Galactic foregrounds.

The energy distribution is a fundamental property of magnetohydrodynamic (MHD) turbulence. In strongly magnetized turbulence energy imbalances can arise, quantified by the so-called residual energy: \\(E_r~=~(E_kin~ - ~E_mag)\\); \\(E_kin\\) and \\(E_mag\\) stand for the volume-averaged kinetic and magnetic energy, respectively. Numerical simulations of incompressible turbulence yield \\(E_r < 0\\), which is consistent with Solar wind observations, while in highly compressible turbulence simulations \\(E_r > \\) 0. Differences arise in the cascade of \\(E_r\\) between the two regimes. We explore the properties of \\(E_r\\) in weakly compressible MHD turbulence in the presence of an initially strong (guide) magnetic field. We study the influence of different driving mechanisms and field strengths on the cascade of \\(E_r\\). We run a suite of direct numerical simulations with the PENCIL code. All simulations are maintained through forcing in a quasi-static regime with sonic Mach numbers close to 0.1. We solely change the Alfvén Mach number, or equivalently the plasma beta (\\(\\)) of the simulations. We drive turbulence by either injecting velocity or magnetic fluctuations at large scales and study the power spectra of kinetic, magnetic, density, and \\(E_r\\). Magnetically-driven simulations show locally imbalanced Alfvénic fluctuations and a \\( k^-3/2\\) cascade, consistent with the dynamic alignment theory. Kinetically-driven simulations give rise to a \\( k^-1\\) scaling, consistent with interactions between Alfvén waves scattered by density inhomogeneities -- a hallmark of reflection-driven turbulence. Residual energy is positive with a spectral slope (\\(\\)) depending on \\(\\) as: for \\( = 4.0\\), \\(-2 -5/3\\), for \\( = 1.0\\), \\(-5/3 -3/2\\), and for \\( = 0.3\\), \\( -1\\).

The true 3-dimensional (3D) morphology of the Musca molecular cloud is a topic that has received significant attention lately. Given that Musca does not exhibit intense star-formation activity, unveiling its shape has the potential of also revealing crucial information regarding the physics that dictates the formation of the first generation of stars within molecular clouds. Here, we revisit the shape of Musca and we present a comprehensive array of evidence pointing towards a shape that is extended along the line-of-sight dimension: (a) 3D maps of differential extinction; (b) new non-local thermodynamic equilibrium radiative transfer simulations of CO rotational transitions from a sheet-like, magnetically-dominated simulated cloud; (c) an effective/critical density analysis of available CO observations; (d) indirect consequences that a filamentary structure would have had, from a theoretical star-formation perspective. We conclude that the full collection of observational evidence strongly suggests that Musca has a sheet-like geometry.

The magnetic field strength in interstellar clouds can be estimated indirectly by using the spread of dust polarization angles (\\( \\)). The method developed by Davis 1951 and by Chandrasekhar and Fermi 1953 (DCF) assumes that incompressible magnetohydrodynamic (MHD) fluctuations induce the observed dispersion of polarization angles, deriving \\(B 1/ \\) (or, \\( M_A\\), in terms of the Alfvénic Mach number). However, observations show that the interstellar medium (ISM) is highly compressible. Recently, Skalidis & Tassis 2021 (ST) relaxed the incompressibility assumption and derived instead \\(B 1/ \\) (\\( M_A^2\\)). We explored what the correct scaling is in compressible and magnetized turbulence with numerical simulations. We used 26 magnetized, ideal-MHD numerical simulations with different types of forcing. The range of \\(M_A\\) and sonic Mach numbers \\(M_s\\) explored are \\(0.1 M_A 2.0\\) and \\(0.5 M_s 20\\). We created synthetic polarization maps and tested the assumptions and accuracy of the two methods. The synthetic data have a remarkable consistency with the \\( M_A^2\\) scaling, which is inferred by ST, while the DCF scaling fails to follow the data. The ST method shows an accuracy better than \\(50\\%\\) over the entire range of \\(M_A\\) explored; DCF performs adequately only in the range of \\(M_A\\) for which it has been optimized through the use of a \"fudge factor\". For low \\(M_A\\), DCF is inaccurate by factors of tens. The assumptions of the ST method reflect better the physical reality in clouds with compressible and magnetized turbulence, and for this reason the method provides a much better estimate of the magnetic field strength over the DCF method.

Context. Calibration of optical polarimeters relies on the use of stars with negligible polarization (unpolarized standard stars) for determining the instrumental polarization zero-point. For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view - a process which is time-consuming. A more effective technique is to target fields containing multiple standard stars. While this method has been used for fields with highly polarized stars, there are no such sky regions with well-measured unpolarized standard stars. Aims. We aim to identify sky regions with tens of stars exhibiting negligible polarization, which are suitable for zero-point calibration of wide-field polarimeters. Methods. We selected stars in regions with extremely low reddening, located at high Galactic latitudes. We targeted four ~ 400 x 400 fields in the northern, and eight in the southern Equatorial hemisphere. Observations were carried out at the Skinakas Observatory and the South African Astronomical Observatory respectively. Results. We find two fields in the North and seven in the South with mean polarization lower than p < 0.1%. Conclusions. At least nine out of twelve fields can be used for zero-point calibration of wide-field polarimeters.