Master Colloquium
Lennart Böhm	SCHEDULED

MPIfR

With the origin of chlorine presumed to be in core-collapse supernovae, stellar models predict the ratio of its two stable isotopes 35Cl/37Cl, between 1 and 4, but observations of Cl-bearing molecules have been limited due to their high-lying transitions. I will present observations of the HCl (1–0) line at 625 GHz, carried out using the SEPIA660 receiver on the APEX telescope. We detected both isotopes of HCl toward 27 Galactic sources, spanning a range of galactocentric radii, doubling the number of sources toward which it has previously been detected. Toward 11 sources we see pure emission with hints of an outflow wing while the remaining sources display complex profiles with both emission and absorption. In addition, the HCl detected in absorption toward NGC 4945, the first detection of this species in a nearby galaxy. For the Galactic sources, we obtained an isotopic ratio between 2.0 and 2.6 with an average value of 2.2+/-0.2. Further, we performed a radiative transfer analysis using RADEX with recently computed collisional rate coefficients between HCl and H2, which constrained HCl-bearing gas to trace warm, dense gas in the core and hot, translucent gas in the outflow.

	Special Colloquium
Dr. Felix Thiel	SCHEDULED

Queen University Canada

Very Long Baseline Interferometry (VLBI) is a technique in radio astronomy commonly used to image radio galaxies and their central black hole shadows at extremely high resolution. The resolution of ground-based VLBI however is limited both in baseline and observing frequency by the size of the Earth and molecular absorption of the atmosphere resepectively. While space missions are extremely costly, balloon-borne VLBI would give access to frequencies beyond those of the Event Horizon Telescope (EHT) at a fraction of the cost. As a first proof of concept for balloon-borne VLBI, the Balloon-borne VLBI Experiment (BVEX) will launch as part of the Canadian Space Agency (CSA) Stratos program from Timmins, Ontario, Canada in August 2025 and will operate above 99% of the Earth's atmosphere. This experimet consists of a K-band 22 GHz radio telescope and receiver, where the singals will be correlated with a large ground-based telescope. In this talk I will give an overview of the telescope, receiver and backend as well as technical challenges such as position tracking and timing reference stability that arise when attempting VLBI from the stratosphere. To wrap up, I will talk about how lessons learned from BVEX will help in designing a mm/sub-mm VLBI mission in the future, which could improve the uv-coverage of VLBI networks such as the EHT or next-generation EHT (ngEHT).