Multi-wavelength interferometry of evolved stars using VLTI and VLBA

We report on our project of coordinated VLTI/VLBA observations of the atmospheres and circumstellar environments of evolved stars. We illustrate in general the potential of interferometric measurements to study stellar atmospheres and envelopes, and demonstrate in particular the advantages of a coordinated multi-wavelength approach including near/mid-infrared as well as radio interferometry. We have so far made use of VLTI observations of the near- and mid-infrared stellar sizes and of concurrent VLBA observations of the SiO maser emission. To date, this project includes studies of the Mira stars S Ori and RR Aql as well as of the supergiant AH Sco. These sources all show strong silicate emission features in their mid-infrared spectra. In addition, they each have relatively strong SiO maser emission. The results from our first epochs of S Ori measurements have recently been published and the main results are reviewed here. The S Ori maser ring is found to lie at a mean distance of about 2 stellar radii, a result that is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase. We discuss the status of our more recent S Ori, RR Aql, and AH Sco observations, and present an outlook on the continuation of our project.Comment: 9 pages, to appear in the proceedings of the ESO workshop "The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation", ESO Astrophysics Symposi

On the progenitor of binary neutron star merger GW170817

On 2017 August 17 the merger of two compact objects with masses consistent with two neutron stars was discovered through gravitational-wave (GW170817), gamma-ray (GRB 170817A), and optical (SSS17a/AT 2017gfo) observations. The optical source was associated with the early-type galaxy NGC 4993 at a distance of just ∼40 Mpc, consistent with the gravitational-wave measurement, and the merger was localized to be at a projected distance of ∼2 kpc away from the galaxy's center. We use this minimal set of facts and the mass posteriors of the two neutron stars to derive the first constraints on the progenitor of GW170817 at the time of the second supernova (SN). We generate simulated progenitor populations and follow the three-dimensional kinematic evolution from binary neutron star (BNS) birth to the merger time, accounting for pre-SN galactic motion, for considerably different input distributions of the progenitor mass, pre-SN semimajor axis, and SN-kick velocity. Though not considerably tight, we find these constraints to be comparable to those for Galactic BNS progenitors. The derived constraints are very strongly influenced by the requirement of keeping the binary bound after the second SN and having the merger occur relatively close to the center of the galaxy. These constraints are insensitive to the galaxy's star formation history, provided the stellar populations are older than 1 Gyr

Magnetic Fields with the Atacama Large Millimeter/Submillimeter Array

The Atacama Large Millimeter/submillimeter Array (ALMA) is one of the largest radio telescopes and is located at 5, 000 m altitude in the Atacama desert in Chile. Its unprecedented sensitivity at extremely high angular and spectral resolution in the (sub-)millimetre wavelength regime, allows for countless advances in astrophysics. One of the areas in which ALMA can make unique contributions, is in that of the study of astrophysical magnetic fields. ALMA is expected to map the magnetic field geometry, and in some cases strength, in a large number of star forming regions, around evolved stars and planetary nebulae, and in nearby galaxies. This chapter provides examples of the amount of improvement ALMA offers the study of magnetic fields based on the current state-of-the-art and shortly introduces new tools that will be available to analyse (sub-)millimetre polarimetric observations