8 research outputs found
Structure of Hot Molecular Cores
High-mass stars form deeply embedded in dense molecular gas, which they heat up and ionize due to their high
energy output. During an early phase, the ionization is confined to small regions, and the stellar radiation is
absorbed by dust. The high temperatures lead to the evaporation of ice mantles around dust grains, and many
highly excited and complex molecules can be observed in these Hot Molecular Cores. At later stages, the whole
molecular cloud is ionized and disrupted, and a star cluster becomes visible.
This PhD thesis aims at constraining the distribution of density, temperature, molecular abundances, and ve-
locity field in Hot Molecular Cores. This key information for high-mass star formation and for astrochemistry is
obtained by sophisticated radiative transfer modeling of both single-dish and interferometric observations.
With the APEX telescope, 12 sources were observed in submillimeter lines of the HCN, HCO+, and CO
molecules, covering a wide range of excitations and optical depths. This was extended with the Herschel space
telescope, which observed HCN lines in SgrB2-M up to high (THz) frequencies and excitations. The line shapes
and intensities were modeled with the spherical radiative transfer code RATRAN, assuming a radial power-law
density distribution and central heating.
With the VLA radio interferometer, vibrationally excited HCN and ionized gas was mapped at a high resolution
of 0.1" (1000 AU) in G10.47+0.03, SgrB2-M and -N. The SMA interferometer was used to observe hundreds of molecular lines and dust emission in G10.47+0.03, reaching a frequency of 690 GHz and a best resolution of 0.3".
The data were modeled with the three-dimensional radiative transfer code RADMC-3D, which computes the dust
temperature from stellar heating.
Modeling using a power-law density structure reproduces most single-dish lines, but the high-resolution data
show a central flattening and a rapid radial decrease of the density, resembling a Plummer profile. Modeling of
the line shapes indicates small-scale clumpiness. Internal heating by high-mass stars is consistent with the data
and traced by vibrationally excited HCN around small regions of ionized gas. Diffusion of radiation due to the
high column densities lead to hundreds of solar masses of hot (>300 K) gas. The HCN abundance increases with
temperature, reaching high values on the order of 10^−5 relative to H2 in the hot gas. Large-scale infall is traced by asymmetric line shapes and is slower than free-fall, while at the same time central expansion motions are detected by blue-shifted absorption and a change of the asymmetry with higher excitation.
I conclude that Hot Molecular Cores are characterized by the beginning feedback from high-mass stars, while
gravitational infall is ongoing. The increased thermal, radiative, turbulent, and wind-driven pressure in the central region leads to expansion motions and to a central flattening of the density. High temperatures are reached through diffusion of radiation by dust
Dimethyl ether in its ground state, v=0, and lowest two torsionally excited states, v11=1 and v15=1, in the high-mass star-forming region G327.3-0.6
The goal of this paper is to determine the respective importance of solid
state vs. gas phase reactions for the formation of dimethyl ether. This is done
by a detailed analysis of the excitation properties of the ground state and the
torsionally excited states, v11=1 and v15=1, toward the high-mass star-forming
region G327.3-0.6. With the Atacama Pathfinder EXperiment 12 m submillimeter
telescope, we performed a spectral line survey. The observed spectrum is
modeled assuming local thermal equilibrium. CH3OCH3 has been detected in the
ground state, and in the torsionally excited states v11=1 and v15=1, for which
lines have been detected here for the first time. The emission is modeled with
an isothermal source structure as well as with a non-uniform spherical
structure. For non-uniform source models one abundance jump for dimethyl ether
is sufficient to fit the emission, but two components are needed for the
isothermal models. This suggests that dimethyl ether is present in an extended
region of the envelope and traces a non-uniform density and temperature
structure. Both types of models furthermore suggest that most dimethyl ether is
present in gas that is warmer than 100 K, but a smaller fraction of 5%-28% is
present at temperatures between 70 and 100 K. The dimethyl ether present in
this cooler gas is likely formed in the solid state, while gas phase formation
probably is dominant above 100 K. Finally, the v11=1 and v15=1 torsionally
excited states are easily excited under the density and temperature conditions
in G327.3-0.6 and will thus very likely be detectable in other hot cores as
well.Comment: 12 pages (excluding appendices), 8 figures, A&A in pres
Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
To investigate the accretion and feedback processes in massive star
formation, we analyze the shapes of emission lines from hot molecular cores,
whose asymmetries trace infall and expansion motions. The high-mass star
forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in
various lines of HCN and its isotopologues, complemented by APEX data. The
observations are compared to spherically symmetric, centrally heated models
with density power-law gradient and different velocity fields (infall or
infall+expansion), using the radiative transfer code RATRAN. The HCN line
profiles are asymmetric, with the emission peak shifting from blue to red with
increasing J and decreasing line opacity (HCN to HCN). This is most
evident in the HCN 12--11 line at 1062 GHz. These line shapes are reproduced by
a model whose velocity field changes from infall in the outer part to expansion
in the inner part. The qualitative reproduction of the HCN lines suggests that
infall dominates in the colder, outer regions, but expansion dominates in the
warmer, inner regions. We are thus witnessing the onset of feedback in massive
star formation, starting to reverse the infall and finally disrupting the whole
molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI
were critically important.Comment: A&A, HIFI special issue, accepte