Filaments in Herschel molecular cloud images are found to exhibit a "characteristic width". This finding is in tension with spatial power spectra of the data, which show no indication of this characteristic scale. We demonstrate that this discrepancy is a result of the methodology adopted for measuring filament widths. First, we perform the previously used analysis technique on artificial scale-free data, and obtain a peaked width distribution of filament-like structures. Next, we repeat the analysis on three Herschel maps and reproduce the narrow distribution of widths found in previous studies − when considering the average width of each filament. However, the distribution of widths measured at all points along a filament spine is broader than the distribution of mean filament widths, indicating that the narrow spread (interpreted as a "characteristic" width) results from averaging. Furthermore, the width is found to vary significantly from one end of a filament to the other. Therefore, the previously identified peak at 0.1 pc cannot be understood as representing the typical width of filaments. We find an alternative explanation by modelling the observed width distribution as a truncated power-law distribution, sampled with uncertainties. The position of the peak is connected to the lower truncation scale and is likely set by the choice of parameters used in measuring filament widths. We conclude that a "characteristic" width of filaments is not supported by the available data.

In diffuse molecular clouds, possible precursors of star-forming clouds, the effect of the magnetic field is unclear. In this work we compare the orientations of filamentary structures in the Polaris Flare, as seen through dust emission by Herschel, to the planeof-the-sky magnetic field orientation (B pos ) as revealed by stellar optical polarimetry with RoboPol. Dust structures in this translucent cloud show a strong preference for alignment with B pos . 70% of field orientations are consistent with those of the filaments (within 30• ). We explore the spatial variation of the relative orientations and find it to be uncorrelated with the dust emission intensity and correlated to the dispersion of polarization angles. Concentrating in the area around the highest column density filament, and in the region with the most uniform field, we infer the B pos strength to be 24 − 120 µG. Assuming that the magnetic field can be decomposed into a turbulent and an ordered component, we find a turbulent-to-ordered ratio of 0.2 − 0.8, implying that the magnetic field is dynamically important, at least in these two areas. We discuss implications on the 3D field properties, as well as on the distance estimate of the cloud.

Low mass X-ray binaries exhibit ionized emission from an extended disk atmosphere that surrounds the accretion disk. However, its nature and geometry is still unclear. In this work we present a spectral analysis of the extended atmosphere of EXO 0748-676 using high-resolution spectra from archival XMM-Newton observations. We model the RGS spectrum that is obtained during the eclipses. This enables us to model the emission lines that come only from the extended atmosphere of the source, and study its physical structure and properties. The RGS spectrum reveals a series of emission lines consistent with transitions of O viii, O vii, Ne ix and N vii. We perform both Gaussian line fitting and photoionization modelling. Our results suggest that there are two photoionization gas components, out of pressure equilibrium with respect to each other. One with ionization parameter of log ξ ∼ 2.5 and a large opening angle, and one with log ξ ∼ 1.3. The second component is possibly covering a smaller fraction of the source. From the density diagnostics of the O vii triplet using photoionization modelling, we detect a rather high density plasma of > 10 13 cm −3 for the lower ionization component. This latter component also displays an inflow velocity. We propose a scenario where the high ionization component constitutes an extended upper atmosphere of the accretion disk. The lower ionization component may instead be a clumpy gas created from the impact of the accretion stream with the disk.

Understanding the chemistry of the interstellar medium (ISM) is fundamental for the comprehension of Galactic and stellar evolution. X-rays provide an excellent way to study the dust chemical composition and crystallinity along different sight lines in the Galaxy. In this work, we study the dust grain chemistry in the diffuse regions of the ISM in the soft X-ray band (<1 keV). We use newly calculated X-ray dust extinction cross sections obtained from laboratory data in order to investigate the oxygen K and iron L shell absorption. We explore the XMM−Newton and Chandra spectra of five low-mass X-ray binaries (LMXBs) located in the Galactic plane and model the gas and dust features of oxygen and iron simultaneously. The dust samples used for this study include silicates with different Mg:Fe ratios, sulfides, iron oxides, and metallic iron. Most dust samples are in both amorphous and crystalline lattice configuration. We computed the extinction cross sections using Mie scattering approximation and assuming a power-law dust size distribution. We find that the Mg-rich amorphous pyroxene (Mg 0.75 Fe 0.25 SiO 3 ) represents the largest fraction of dust towards most of the X-ray sources, namely about 70% on average. Additionally, we find that ∼ 15% of the dust column density in our lines of sight is in metallic Fe. We do not find strong evidence for ferromagnetic compounds, such as Fe 3 O 4 or iron sulfides (FeS, FeS 2 ). Our study confirms that iron is heavily depleted from the gas phase into solids; more than 90% of iron is in dust. The depletion of neutral oxygen is mild, namely of between 10% and 20% depending on the line of sight.