Main Colloquium
Dr. Simon Ellingsen	CANCELED

University of Tasmania, Australia

Methanol is one of the most common molecules in the interstellar medium and I will give an overview of how methanol masers are being used to study the structure of the Milky Way and the physics of starbursts. Astrometric measurements of interstellar masers represent the most accurate way of determining the scale and structure of the Milky Way. Over the last 15 years significant progress has been made in measuring the number and location of Milky Way spiral arms in the northern hemisphere. Collecting equivalent data for the southern hemisphere has been very slow, primarily due to differences in available infrastructure. I will present some new results for 6.7 GHz methanol masers from the southern hemisphere made as part of the Sprals project. I will highlight recent upgrades to hardware and development of new calibration techniques which have enabled this and their application in other astrometry and astrophysics applications. A decade ago the first extragalactic class I (collisionally excited) methanol masers were detected in the nearby starburst galaxy NGC253. This are not scaled-up or extreme versions of Galactic class I methanol masers, but appear to trace regions where there are very large volumes of molecular gas undergoing low-velocity shocks. I will explain the prospects for using this new type of extragalactic maser to improve our understanding of early-stage starbursts. +++ Please note that this Main Colloquium takes place on a Wednesday to accommodate the speaker +++

	Special Colloquium
Dr. Armin Rest	SCHEDULED

STScI Baltimore

High-cadence rolling surveys, which continuously monitor patches of the sky (e.g., PS1, ZTF, ATLAS), have become standard among extra-galactic transient searches over the last two decades. These surveys have given us large statistical samples of supernovae (SNe) and other transients in the low-z range up to redshifts of 1. With JWST, going deeper and redder, we can now expand transient astronomy into the early, high-redshift universe, which offers us a direct approach for probing both the first stars and the epoch of reionization, as well as insights into rare high energy physics and explosion mechanisms (e.g., Pair-Instability SNe and Superluminous SNe). I will discuss our recent discovery of dozens of SNe up to z~5 with the JADES and COSMOS project, their spectroscopic follow-up, and what we can expect in the future with both JWST and Roman.

	Promotionskolloquium
Yaoting Yan	SCHEDULED

MPIfR

My thesis investigates the influence of stellar objects onto the interstellar medium (ISM), following two lines of research, determining isotope ratios over much of the Milky Way and observing interstellar ammonia masers. Isotope abundance ratios provide a powerful tool to probe stellar nucleosynthesis, to evaluate the composition of stellar ejecta and to constrain the chemical evolution of the Milky Way. Molecular maser lines are signposts of star formation, probing the excitation and kinematics of very compact regions in the close environment of young stellar objects and providing useful targets for trigonometric parallax measurements. In the first part of this talk, I will present our observations of the J = 2-1 and 3-2 rotational transitions of various rare isotopologic variants of carbon monosulfide (CS), namely C33S, C34S, C36S, 13CS, 13C33S, and 13C34S, toward a large sample of 110 high-mass star-forming regions (HMSFRs) with the IRAM 30-meter telescope. With accurate distances obtained from trigonometric parallaxes, we confirm the previously identified 12C/13C and 32S/34S gradients as a function of Galactocentric distance. There is no 34S/33S gradient, but ratios are well below the values commonly reported in earlier publications. For the first time, we report positive gradients of 32S/33S, 34S/36S, 33S/36S and 32S/36S in our Galaxy. The Galactic 12C/13C gradients derived based on measurements of CN, C18O, and H2CO are in agreement with our results from C34S and indicate that chemical fractionation has little effect on 12C/13C ratios. The measured 34S/33S ratios as a function of Galactocentric radius indicate that 33S has a nucleosynthesis origin similar to that 34S. Interstellar 34S/33S values near the solar neighborhood suggest that the solar system ratio is, as perhaps also the 18O/17O ratio, peculiar. Our measurements support that 36S is a purely secondary nucleus; however, we note that data for lines containing this isotope are still sparse, particularly in the inner Galaxy. The predicted 12C/13C ratios from the latest Galactic chemical evolution (GCE) models are in good agreement with our results, while our 32S/34S and 32S/36S ratios show larger differences at larger Galactocentric distances. 32S/33S ratios show an offset across the entire inner 12 kpc of the Milky Way. All of this can serve as a guideline for further refinements of GCE models. The rest parts of this talk I will focus on ammonia (NH3) masers in our Galaxy. With the Effelsberg 100-m telescope, we discovered widespread non-metastable NH3 maser emission toward 17 high mass star forming regions (HMSFRs) in the Milky Way. This doubles the number of known non-metastable ammonia masers in our Galaxy. These maser lines arise from energy levels between 342 K up and 1449 K above the ground state and this probe the hot dense immediate neighborhoods of newly formed stars. With our higher angular resolution interferometric measurements from the Karl G. Jansky Very Large Array (JVLA), we determined detailed locations for maser spots emitted in multiple non-stable transitions toward a variety of regions. We greatly increase the number of detections in the Galaxy in all the lines targeted. The detected maser spots are not resolved by our JVLA observations. Lower limits to the brightness temperature are >400 K and reach values up to several 10^5 K, manifesting the lines' maser nature. In view of the masers' velocity differences with respect to adjacent hot molecular cores and/or ultra-compact (UC) H II regions, it is argued that all the measured ammonia maser lines may be associated with shocks caused either by outflows or by the expansion of UCH II regions.

	Special Colloquium
Dr. Anaëlle Maury	SCHEDULED

CEA Saclay

TBD