3 research outputs found
Submm Observations of Massive Star Formation in the Giant Molecular Cloud NGC 6334 : Gas Kinematics with Radiative Transfer Models
Context. How massive stars (M>8 Ms) form and how they accrete gas is still an open research
field, but it is known that their influence on the interstellar medium (ISM) is immense. Star
formation involves the gravitational collapse of gas from scales of giant molecular clouds
(GMCs) down to dense hot molecular cores (HMCs). Thus, it is important to understand the
mass flows and kinematics in the ISM.
Aims. This dissertation focuses on the detailed study of the region NGC 6334, located in
the Galaxy at a distance of 1.7 kpc. It is aimed to trace the gas velocities in the filamentary,
massive star-forming region NGC 6334 at several scales and to explain its dynamics. For
that purpose, different scales are examined from 0.01–10 pc to collect information about the
density, molecular abundance, temperature and velocity, and consequently to gain insights about
the physio-chemical conditions of molecular clouds. The two embedded massive protostellar
clusters NGC 6334I and I(N), which are at different stages of development, were selected to
determine their infall velocities and mass accretion rates.
Methods. This astronomical source was surveyed by a combination of different observatories,
namely with the Submillimeter Array (SMA), the single-dish telescope Atacama Pathfinder
Experiment (APEX), and the Herschel Space Observatory (HSO). It was mapped with APEX in
carbon monoxide (13CO and C18O, J=2–1) at 220.4 GHz to study the filamentary structure and
turbulent kinematics on the largest scales of 10 pc. The spectral line profiles are decomposed
by Gaussian fitting and a dendrogram algorithm is applied to distinguish velocity-coherent
structures and to derive statistical properties. The velocity gradient method is used to derive
mass flow rates. The main filament was mapped with APEX in hydrogen cyanide (HCN) and
oxomethylium (HCO+, J=3–2) at 267.6 GHz to trace the dense gas. To reproduce the position-
velocity diagram (PVD), a cylindrical model with the radiative transfer code Line Modeling
Engine (LIME) is created with a collapsing velocity field. Both clusters NGC 6334I and I(N)
were observed with the interferometer SMA in HCN (J=4–3) at 354.5 GHz at the smallest
scales of 0.01 pc. The combination of interferometric and multi-frequency single-dish data gives
a wide range of rotational transitions, which probe the gas at different excitation conditions and
optical depths. The molecule HCN and its isotopologues H13CN/HC15N trace radii of a HMC
from 1.0–0.01 pc by a range of level energies (E=4–1067 K) and optical depths (tau=100–0.1).
The HMCs, which have a rich line spectra, are analyzed by using 1D (myXCLASS) and 3D
numerical radiative transfer codes (RADMC-3D and LIME) in and outside of local thermodynamic
equilibrium (LTE). Multiple components and the fragmentation of the clusters are modeled
with these tools. Together with the optimization package MAGIX, the data are compared and
reproduced with synthetic maps and spectra from these models.
Results. 1. The main filament shows a velocity gradient from the end toward its center, where
the most massive clumps accumulate at both ends, in accordance to predictions of a longitudinal
contraction. The 3D structure is determined by taking the inclination and curvature of the
filament into account, and the free-fall time is estimated to approximately 1 Myr; 2. The total gas mass is
2.3E5 Ms and the average temperature 20 K. The majority of the velocity gradients are aligned
with the magnetic field, which runs perpendicular to the filaments. The calculation of the average
Mach numbers yields a turbulence which is super-sonic (M_S=5.7) and sub-Alfvénic (M_A=0.86).
In general, the derived scaling relations are in agreement with Larson's relations. 3. The SMA
observations reveal multiple bipolar molecular outflows, blue asymmetric infall profiles, rotating
cores and an ultra compact (UC) HII region in NGC 6334I which affects the surrounding gas.
The average mass accretion rates are 1E-3 Ms/yr for the envelopes and 3E-4 Ms/yr for the
cores, where the latter are derived from modified Bondi-Hoyle models. The orientation of the
magnetic field is in NGC 6334I(N) consistent over all scales and most outflows are aligned
perpendicular to it; 4. In the line surveys of the HMCs, 20 different molecules are identified
with typical temperatures of 100 K. A cruel separation between the HMCs of the clusters is
determined on the basis of the relative abundances.
Conclusions. The combination of single-dish with interferometric data is helpful to constrain
the parameter space of a model. The envelope hinders the determination of infall velocities in
HMCs via line profiles. Systematic motions as a result of gravitational attraction are diffcult to
find because of the turbulent nature of the ISM. The magnetic field energy in NGC 6334 is as
important as the kinetic energy and regulates partly the direction of the inflowing gas and thus
the geometry and collapse of the molecular clouds. NGC 6334 is heavily affected by the HII
regions (produced by the OB stars), and the free-fall time and mass surface density suggest that
it classifies as a starburst system
Evolution of complex organic molecules in hot molecular cores: Synthetic spectra at (sub-)mm wavebands
Hot molecular cores (HMCs) are intermediate stages of high-mass star
formation and are also known for their rich emission line spectra at (sub-)mm
wavebands. The observed spectral feature of HMCs such as total number of
emission lines and associated line intensities are also found to vary with
evolutionary stages. We developed various 3D models for HMCs guided by the
evolutionary scenarios proposed by recent empirical and modeling studies. We
then investigated the spatio-temporal variation of temperature and molecular
abundances in HMCs by consistently coupling gas-grain chemical evolution with
radiative transfer calculations. We explored the effects of varying physical
conditions on molecular abundances including density distribution and
luminosity evolution of the central protostar(s). The time-dependent
temperature structure of the hot core models provides a realistic framework for
investigating the spatial variation of ice mantle evaporation as a function of
evolutionary timescales. With increasing protostellar luminosity, the water ice
evaporation font (100K) expands and the spatial distribution of gas phase
abundances of these COMs also spreads out. We simulated the synthetic spectra
for these models at different evolutionary timescales to compare with
observations. A qualitative comparison of the simulated and observed spectra
suggests that these self-consistent hot core models can reproduce the notable
trends in hot core spectral variation within the typical hot core timescales of
10 year. These models predict that the spatial distribution of various
emission line maps will also expand with evolutionary time. The model
predictions can be compared with high resolution observation that can probe
scales of a few thousand AU in high-mass star forming regions such as from
ALMA.[Abridged]Comment: accepted for publication in A&
The global velocity field of the filament in NGC 6334
Aims. Star formation involves the collapse of gas from the scale of giant molecular clouds down to dense cores. Our aim is to trace the velocities in the filamentary, massive star-forming region NGC 6334 and to explain its dynamics.
Methods. The main filament was mapped with the single-dish telescope APEX in HCO+ (J = 3–2) at 267.6 GHz to trace the dense gas. In order to reproduce the position−velocity diagram, we use a 3D radiative transfer code and create a model of a cylinder that undergoes a gravitational collapse toward its center.
Results. We find a velocity gradient in the filament from the end toward its center, with the highest masses being found at both ends. Similar velocities have been predicted by recent calculations of the gravitational collapse of a sheet or cylinder of gas, and the observed velocities are consistent with these predictions. The 3D structure is revealed by taking the inclination and curvature of the filament into account. The free-fall collapse timescale of the filamentary molecular cloud is estimated to be ~1 Myr