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Supernova explosions attributed to the unseen companion in several binary systems identified by the Third Gaia Data Release (Gaia DR3) may be responsible for a number of well-known and well-studied features in the radio sky, including the Low-Latitude-Intermediate-Velocity Arch and the North-Celestial-Pole Loop. Slices from the Longitude-Latitude-Velocity data cube of the $\lambda$-21-cm galactic neutral hydrogen HI4PI survey (HI4PI Collaboration et al. 2016) show multiple signatures of an expanding shell. The source of this expansion, which includes the Low-Latitude-Intermediate-Velocity Arch on the approaching side, may be the neutron star candidate in the Gaia DR3 1093757200530267520 binary. If we make the simplifying assumptions that the expansion of the cavity is uniform and spherically symmetric, then the explosion took place about 700,000 years ago. The momentum is in reasonable agreement with recent model estimates for a supernova this old. The HI on the receding side of this cavity is interacting with the gas approaching us on the near side of a second cavity. The North-Celestial-Pole Loop appears to be located at the intersection of these two expanding features. The neutron star candidate in the Gaia DR3 1144019690966028928 binary may be (in part) responsible for this cavity. Explosions from other candidates may account for the observed elongation along the line of sight of this second cavity. We can use the primary star in these binaries to anchor the distances to the Low-Latitude-Intermediate-Velocity Arch and North-Celestial-Pole Loop, which are about 167 and about 220 pc, respectively., Comment: Published in The Astrophysical Journal

Gaussian decomposition of HI4PI profiles was done for two regions along high velocity Complex M, the first with corresponding H-alpha emission and the second without. In the former, we detected the Critical Ionization Velocity (CIV) signature, a line width of 25 km/s, of H-alpha. In low-velocity gas, the CIV effect ionizes the atoms and produces the signatures of He (34 km/s), CNO (13.4 km/s), and the metals (around 6 km/s). In anomalous-velocity gas, the CIV effect ionizes the ions and produces the signatures of ionized CNO (20 km/s) and-or H-alpha (25.5 km/s), as well as the CIV signatures of metals around 6 km/s. These distinct CIV signatures indicate that the physical conditions are different in each velocity regime. The particles in the low-velocity gas cloud are initially neutral, the temperatures can be cold (50-100 K), and the abundances can be cosmic. The densities need to be high enough to impose regular recombination. The anomalous-velocity gas, on the other hand, initially contains a significant population of ionized and excited particles. The temperature and abundance values may be similar to those that characterize low-density gas, but the densities must be low enough so that the plasma is essentially collision-less, similar to the conditions in the solar wind., Comment: 12 pages, 5 figures

56 UMa is a wide binary system that contains a chemically peculiar red giant and a faint companion. Due to its surface chemical abundances, the red giant was classified as a barium (Ba) star. This implies that the companion has to be a white dwarf, since Ba stars form when mass is transferred to them from an s-process rich Asymptotic Giant Branch (AGB) star. However, in the case of 56 UMa, the companion might be too massive to be the progeny of an AGB star that efficiently produced s-process elements such as barium. In this Letter, we revisit the orbital parameters of the system and perform a full spectral analysis with the goal of investigating the Ba-star classification of the giant and unravelling the nature of its faint companion. We combined radial-velocity and astrometric data to refine the orbital parameters of the system, including the orbital inclination and the companion mass. Then, we re-determined the stellar parameters of the giant and its chemical abundances using high-resolution HERMES spectra. Finally, we investigated the morphology of the interstellar gas in the vicinity of the system. The faint component in 56 UMa has a mass of $1.31 \pm 0.12$ M$_{\odot}$, which, together with the mixed s+r abundance profile of the red giant, confirms that the giant is not a standard barium star. Additionally, the clear identification of a cavity surrounding 56 UMa could indicate that a supernova explosion occurred about 10 5 years ago in the system, suggesting that the faint companion might be a neutron star. However, finding an evolutionary scenario that explains all the observables is not trivial, so we discuss different possible configurations of the system and their respective merits., Comment: Accepted for publication in A&AL

21-cm HI4PI survey data are used to study the anomalous-velocity hydrogen gas associated with high-velocity cloud Complex M. These high-sensitivity, high-resolution, high-dynamic-range data show that many of the individual features, including MI, MIIa, and MIIb, are components of a long, arched filament that extends from about (l, b) = (105{\deg}, 53{\deg}) to (l, b) = (196{\deg}, 55{\deg}). Maps at different velocities, results from Gaussian analysis, and observations of associated high-energy emission make a compelling case that the MI cloud and the arched filament are physically interacting. If this is the case, we can use the distance to MI, 150 pc as reported by Schmelz & Verschuur (2022), to set the distance to Complex M. The estimated mass of Complex M is then about 120 solar masses and the energy implied using the observed line-of-sight velocity, -85 km/s, is 8.4 x 10^48 ergs. Integrating over 4{\pi} steradians, the total energy for a spherically symmetrical explosion is estimated to be 1.9 x 10^50 ergs, well within the energy budget of a typical supernova.

The high-velocity, neutral hydrogen feature known as MI may be the result of a supernova that took place about 100,000 years ago at a distance of 163 pc. Low-velocity HI data show a clear cavity, a structure indicative of regions evacuated by old exploding stars, centered on the spatial coordinates of MI, (l,b) = (165o, 65.o5). The invisible companion of the yellow giant star, 56 Ursae Majoris, may be the remains of the supernova that evacuated the cavity and blasted MI itself outward at 120 km/s. The mass and energy of MI are easily in line with what is expected from a supernova. The X-rays seen by ROSAT are consistent with an origin in the resulting bow shock. Ironically, this scenario for MI only came together because we were exploring low-velocity gas in the direction of high-velocity clouds., Comment: 6 pages, 4 figures

Following an established protocol of science, that results must be reproducible, we examine the Gaussian fits to Galactic 21-cm emission profiles obtained by two seemingly complementary methods: the semi-automated approach based on the method used by Verschuur (2004) and the automated technique of Nidever et al. (2008). Both methods use data from the Leiden/Argentine/Bonn all-sky survey. The appeal of an automated routine is great, if for no other reason than the time saved over semi-automated fits. The pitfalls, however, are often unanticipated, and the most important aspect of any algorithm is the reproducibility of the results. The comparisons led to the identification of four problems with the Nidever et al. (2008) analysis: (1) different methods of calculating the reduced chi-squared measuring the goodness of fit; (2) an ultra-broad component found bridging the gap between low and intermediate velocity gas; (3) the lack of an imposed spatial coherence allowing different components to appear and disappear in profiles separated by a fraction of a beam width; and (4) multiple, fundamentally different solutions for the profiles at both the North and South Galactic Poles. A two-step method would improve the algorithm, where an automated fit is followed by a quality-assurance, visual inspection. Confirming evidence emerges from this study of a pervasive component with a line width of order 34 km/s, which may be explained by the Critical Ionization Velocity (CIV) of helium. Since the Nidever et al. (2008) paper contains the only result in the refereed literature that contradicts the CIV model, it is important to understand the flaws in the analysis that let to this contradiction., Comment: 15 pages, 10 figures

Observations obtained from IRAS for a 120 sq deg region at the boundary of a large expanding shell in Eridanus are presented. It is found that the separation between the structures in the H I and IR emission is not due to a transition of the gas into molecular hydrogen at the location of the IR filaments. The molecular hydrogen column density of these filaments is at least two orders of magnitude below what is typically seen in high-latitude molecular clouds. As the H I and dust in this region are associated with a large expanding shell (or superbubble), the action of shocks and radiation fields may be responsible for the separations of gas and dust. Unless the gas and dust structures are resolved and lie in a favorable viewing geometry, the effect tends to be hidden. The results indicate that it is not always evident, from observations with spatial resolutions coarser than a few arcminutes, how interstellar gas and dust are related.

If HI clouds exist in pressure equilibrium in an environment where gas pressure is a function of z-distance and if HI cloud density is a function of z-distance, it can be shown that a quantity called the Virial Measure is a function of z-distance. The Virial Measure is that distance at which a cloud would be in gravitational equilibrium if its internal kinetic temperature is indicated by profile linewidth. The Virial Measure is derived from observed cloud parameters and has been calibrated for clouds of known distance so that it can be used to determine the distance to other HI clouds. The magnitude of various terms in the virial equation can thus be derived for several hundred HI clouds. It is demonstrated that the strength of the interstellar magnetic field is a function of z-distance.