5 research outputs found
On Filaments in Molecular Clouds and their Connection to Star Formation
In recent years, there have been many studies on the omnipresence and structures of filaments in star-forming regions, as well as the role of their fragmentation in the process of star formation. However, only a few comprehensive studies have analysed the evolution of filaments and their distribution with the Galactic disk where the filaments form self-consistently as part of large-scale molecular cloud evolution.
In this thesis, I study the effect of inclination on dust observations of filaments to evaluate whether the variations would enable the identification of further filaments in existing dust surveys. I address the early evolution of pc-scale filaments that form within individual clouds and focus on the questions how and when the filaments fragment, and how the fragmentation relates to typically used observables of the filaments.
I perform dust radiative transfer calculations on models of cylinders and reconstructions of observed star-forming regions. For evaluating the equilibrium state of filaments and the nature of their fragmentation I examine three simulated molecular clouds formed in kpc-scale numerical simulations modelling a self-gravitating, magnetised, stratified, supernova-driven interstellar medium.
I find that the observables of filaments in dust emission are on average on small scales influenced by inclination; yet the variations strongly depend on the structure of the object. The first fragments appear when the line masses of the simulated filaments lie well below the critical line mass of Ostriker’s isolated hydrostatic equilibrium solution. This indicate that, although the turbulence of the entire clouds is mostly driven by gravitational contraction, fragmentation does not occur do to gravitational instability, but is supported by colliding flow motions.
I conclude that there is no single quantity in my analysis that can uniquely trace the inclination and 3D structure of a filament based on dust observations alone. A simple model of an isolated, isothermal cylinder may not provide a good approach for fragmentation analysis, independently of the dominant driving source of the parental cloud
Line Profiles of Cores within Clusters. III. What is the most reliable tracer of core collapse in dense clusters?
Recent observational and theoretical investigations have emphasised the
importance of filamentary networks within molecular clouds as sites of star
formation. Since such environments are more complex than those of isolated
cores, it is essential to understand how the observed line profiles from
collapsing cores with non-spherical geometry are affected by filaments. In this
study, we investigate line profile asymmetries by performing radiative transfer
calculations on hydrodynamic models of three collapsing cores that are embedded
in filaments. We compare the results to those that are expected for isolated
cores. We model the five lowest rotational transition line (J = 1-0, 2-1, 3-2,
4-3, and 5-4) of both optically thick (HCN, HCO) as well as optically thin
(NH, HCO) molecules using constant abundance laws. We find
that less than 50% of simulated (1-0) transition lines show blue infall
asymmetries due to obscuration by the surrounding filament. However, the
fraction of collapsing cores that have a blue asymmetric emission line profile
rises to 90% when observed in the (4-3) transition. Since the densest gas
towards the collapsing core can excite higher rotational states, upper level
transitions are more likely to produce blue asymmetric emission profiles. We
conclude that even in irregular, embedded cores one can trace infalling gas
motions with blue asymmetric line profiles of optically thick lines by
observing higher transitions. The best tracer of collapse motions of our sample
is the (4-3) transition of HCN, but the (3-2) and (5-4) transitions of both HCN
and HCO are also good tracers.Comment: accepted by MNRAS; 13 pages, 16 figures, 6 table
How do velocity structure functions trace gas dynamics in simulated molecular clouds?
In Chira et al. (subm.), we investigate the time evolution of gas dynamics
within simulated molecular clouds, as well as how velocity structure functions
trace the dominating driving sources of turbulence. The molecular clouds are
formed self-consistently within kiloparsec-scale numerical simulations of the
interstellar medium that include self-gravity, magnetic fields, supernovae-
driven turbulence, and radiative heating and cooling.
Here, we provide the underlying data for the analysis and plots presented in
the paper submitted to Astronomy & Astrophysics. The simulations are run using
an implementation of the Flash code. We present data for each of the, in total,
160 timesteps in HDF5 format, the final velocity structure functions as
functions of lag and time.
Note that due to technical problems we are currently able to offer the raw
data for those simulations that resolve the local Jeans length with 4 cells
only. We will upload the higher Jeans-resolved data as soon as possible