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Discovery of widespread non-metastable ammonia masers in 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. With the 100-m Effelsberg 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 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 105 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) HII 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 UCHII regions.
References: Yan et al. 2022, A&A, 659, A5; Yan et al. 2022, A&A, 666, L15; Yan et al. 2024, A&A, 686, A205; Yaoting Yan, 2024. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Direct measurements of carbon and sulfur isotope ratios in the Milky Way
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. With the IRAM 30-meter telescope, we measured the 12C/13C, 32S/34S, 32S/33S, 32S/36S, 34S/33S, 34S/36S, and 33S/36S abundance ratios based on isotopologues of CS, thus avoiding significant saturation effects. 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. It should be noted, however, that 12C/13C ratios near the Galactic center are higher than expected by linearly extrapolating data from the Galactic disk. 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.
References: Yan et al. 2023, A&A, 670, A98; Yaoting Yan, 2024. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.