|
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.