TY  - GEN
ID  - heidok21903
Y1  - 2016///
TI  - Characterization of stellar and sub-stellar atmospheres
N2  - In this work I aim characterize a number of (sub)-stellar companions. When studying isolated brown
dwarfs, very low-mass stars, and directly imaged exoplanets with insignificant orbital motion, we have to
rely on theoretical models to determine basic parameters such as mass, age, and consequently, effective
temperature and surface gravity. While stellar and atmospheric models are rapidly evolving, we need a
powerful tool to test and calibrate them.
One way to test models is to compare theoretical isochrones produced by interior and atmospheric models
with observed sequences in open clusters. Open clusters contain many objects of the same chemical
composition and age, and spanning a range of masses. Using our own Lucky imaging observations and
literature data, I constructed a single-star sequence for the Hyades open cluster. I compare the obtained
sequence to a set of theoretical isochrones identifying systematic offsets and revealing probable issues in
the models.
However, there are many cases when it is impossible to test models before applying them to observations.
One example is applying atmospheric models for constraining parameters of the coolest known Y dwarf
WISE 0855-07. I demonstrate the limits of constraining effective temperature and the presence/absence
of water clouds that are introduced by unknown systematic effects in models and observations.
In the final chapter I introduce a novel method to take into account the above-mentioned systematics.
I construct a ?systematics vector? that allows us to reveal problematic wavelength ranges when fitting
atmospheric models to observed near-infrared spectra of brown dwarfs and directly imaged exoplanets.
This approach plays a crucial role when retrieving abundances of brown dwarfs and exoplanets, in particularly,
a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and
exoplanets. I show the way of constraining this parameter while eliminating systematics effects, which
significantly improves the reliability of a final result and our conclusions about formation history of certain
exoplanets and brown dwarfs.
A1  - Kopytova, Taisiya G.
AV  - public
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/21903/
ER  -