The Influence of Stellar Objects onto the Interstellar Medium: Isotopic Compositions and Maser Lines

Abstract

This 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. </br> 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 performed 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). 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. 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. This is a consequence of accounting for the full hyperfine structure splitting of the C³³S lines. The average ³⁴S/³³S value, after corrections for opacity, is 4.35+-0.44, which is lower than the value of 5.61 in the solar system. 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. </br> 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 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 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. Furthermore, we recently detected linear and circular polarization of NH₃ masers in Sgr B2.