The arrival of the Square Kilometer Array (SKA) will revitalise all aspects of Very Long Baseline Interferometry (VLBI) astronomy at the lower frequencies. In the last decade there have been huge strides towards routinely achieving high precision VLBI astrometry at frequencies dominated by the tropospheric contributions, most notably at 22GHz, using advanced phase referencing techniques. Nevertheless to increase the capability for high precision astrometric measurements at low radio frequencies (<8GHz) an effective calibration strategy of the systematic ionospheric propagation effects that is widely applicable is required. Observations at low frequencies are dominated by distinct direction dependent ionospheric propagation errors, which place a very tight limit on the angular separation of a suitable phase referencing calibrator. The MultiView technique holds the key to the compensation of atmospheric spatial-structure errors, by using observations of multiple calibrators and 2-D interpolation. In this paper we present the first demonstration of the power of MultiView using three calibrators, several degrees from the target, along with a comparative study of the astrometric accuracy between MultiView and phase-referencing techniques. MultiView calibration provides an order of magnitude improvement in astrometry with respect to conventional phase referencing, achieving ca. 100 micro-arcseconds astrometry errors in a single epoch of observations, effectively reaching the thermal noise limit. MultiView will achieve its full potential with the enhanced sensitivity and multibeam capabilities of SKA and the pathfinders, which will enable simultaneous observations of the target and calibrators. Our demonstration indicates that the 10 micro-arcseconds goal of astrometry at ~1.6GHz using VLBI with SKA is feasible using the MultiView technique.

In the hierarchical structure formation model, galaxies grow through various merging events. Numerical simulations indicate that the mergers can enhance the activity of the central supermassive black holes in the galaxies. Pfeifle et al. (2019a) identified a system of three interacting galaxies, J0849$+$1114, and provided multiwavelength evidence of all three galaxies containing active galactic nuclei. The system has substantial radio emission, and with high-resolution radio interferometric observation we aimed to investigate its origin, whether it is related to star formation or to one or more of the active galactic nuclei in the system. We performed high-resolution continuum observation of J0849$+$1114 with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected one compact radio emitting source at the position of the easternmost nucleus. Its high brightness temperature and radio power indicate that the radio emission originates from a radio-emitting active galactic nucleus. Additionally, we found that significant amount of flux density is contained in $\sim 100$ milliarcsec-scale feature related to the active nucleus.

Water fountains are evolved stars showing early stages of collimated mass loss during transition from the asymptotic giant branch, providing valuable insight into the formation of asymmetric planetary nebulae. We report the results of multi-epoch VLBI observations, which determine the spatial and three-dimensional kinematic structure of H2O masers associated with the water fountain IRAS 18113-2503. The masers trace three pairs of high-velocity (~150-300 km/s) bipolar bow shocks on a scale of 0.18'' (~2000 au). The expansion velocities of the bow shocks exhibit an exponential decrease as a function of distance from the central star, which can be explained by an episodic, jet-driven outflow decelerating due to drag forces in a circumstellar envelope. Using our model, we estimate an initial ejection velocity ~840 km/s, a period for the ejections ~10 yr, with the youngest being ~12 yr old, and an average envelope density within the H2O maser region n(H2) ~ 10^6 cm^(-3). We hypothesize that IRAS 18113-2503 hosts a binary central star with a separation of ~10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.

Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from TESS will record pulsation signatures from bright white dwarfs over the entire sky. We aim to demonstrate the sensitivity of TESS data to measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can constrain about their stellar structures. We present a case study of the pulsating DBV WD 0158$-$160 that was observed as TIC 257459955 with the 2-minute cadence for 20.3 days in TESS Sector 3. We measure the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies are compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models from LPCODE, and the structural models from WDEC. We detect and measure the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158$-$160 to $\sim0.01 \mu$Hz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole ($\ell=1$) modes with a mean period spacing of $38.1\pm1.0$ s. The global best-fit seismic models from both codes have effective temperatures that are $\gtrsim3000$ K hotter than archival spectroscopic values of $24{,}100-25{,}500$ K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints from Gaia astrometry.

We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Lambda CDM theory, or from disk instabilities, as suggested by its boxy/peanut shape, or both? We have obtained spectra for 28,000 stars at a spectral resolution of R = 11,000. From these spectra, we have determined stellar parameters and distances to an accuracy of < 1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], -2.8 < [Fe/H] < +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H] (Ness et al. 2012a), we propose that the stars with [Fe/H] > -0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] < -0.5) with the thick disk, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disk and inner halo. For the bulge stars with [Fe/H] > -0.5, we find two discrete populations; (i) stars with [Fe/H] ~ -0.25 which provide a roughly constant fraction of the stars in the latitude interval b = -5 deg to -10 deg, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] ~ +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disk. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. [abridged]