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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.

We present APEX observations of HCN (9-8) and (4-3) lines toward a sample of hot cores. The spectral shapes of the main transitions are asymmetric and self-absorbed, as expected for high optical depth in a possibly infalling envelope. For spherical symmetry, the large column densities of these sources would mean that the central region is obscured by dust above a certain frequency. However, we detected the vibrationally excited satellite lines (v_2=1; J=9-8) at 797 GHz, which originate from the inner regions. This indicates that high-frequency ALMA observations of hot core centers will be feasible., Comment: 4 pages, 3 figures, to appear in "Science with ALMA: a new era for Astrophysics" Conference Proceedings (to be published by Springer "Astrophysics and Space Science (ApSS)")