Multi-Wavelength Observations of the High-Mass Star Forming Complexes W33 and DR 21

Abstract

High-mass stars play a key role in shaping the Universe. Stars with masses above 8 M_sun have a huge impact on the energy budget of galaxies, through their stellar winds, expanding HII regions, outflows, and supernova explosions. Although insight into the formation of stars is gained from low-mass star forming regions, the knowledge about high-mass star formation is still incomplete and has to be increased through new observations and theories. Observationally, different stages of star formation are distinguished. A cold molecular cloud collapses and fragments into smaller entities ("clumps" and "cores"). The cores contract further and slowly start to warm up their center where a protostar is formed. The protostar grows through accretion of material while at a certain point, hydrogen burning sets in. The evolving protostar starts to heat up its birth cloud, changing the chemistry in different layers around the star (hot core phase). An HII region is formed once the radiation of the protostar is energetic enough to ionize the surrounding material. With time, the radiation of the protostar destroys the birth cloud and the star becomes observable at optical wavelengths.<br /> In the course of this dissertation, the formation of high-mass stars was studied along the described evolutionary sequence on the basis of multi-wavelength observations of the high-mass star forming complexes W33 and DR 21. In W33, molecular clouds in several stages of star formation are detected, from quiescent cold clouds to highly active HII regions. Two radial velocity components with a difference of ~20 km/s were detected towards different parts of the complex. For a long time, this raised the question if W33 is a physically connected star forming complex or if the star forming regions with different radial velocities are located at different distances along the line-of-sight. Due to this peculiar velocity structure, the distance to W33 was not well known. As part of the dissertation, the trigonometric parallax distance of W33 was determined with Very Long Baseline Interferometry observations of water masers in the complex, yielding a distance of 2.4 kpc. Since the star forming regions with the different radial velocity components are located at similar distances, we conclude that W33 is physically connected. Furthermore, these observations yield the proper motions of the water masers from which we inferred the internal motions of the star forming regions and the motions of these regions within the W33 complex. <br /> Since the clouds in the W33 complex are physically linked and are in different stages of star formation, we conducted a chemical study of these clouds with single dish and interferometer observations at submillimeter wavelengths to gather information about the chemical composition on different scales along the evolutionary sequence. On larger scales, the number of detected molecules and their complexity increases from the prestellar phase to the HII region phase. On smaller scales, the clouds in the hot core phase show the highest chemical complexity and diversity. The observed molecules, some of them quite complex, were generated on the dust grains and then released into the gas phase as primary molecules or produced in the gas phase by the evaporated molecules as secondary molecules. In the HII region phase, almost no complex molecules are detected anymore and the spectrum resembles the spectra of the clouds in the protostellar phase before the excitation of a hot core. The complex molecules are either destroyed by photo-dissociation or their emission is not compact enough to be detected by the interferometer. <br /> With interferometer observations at radio wavelengths, we looked for hypercompact and ultracompact HII regions in the W33 Main cloud. We detected an ultracompact HII region and inferred the spectral type of the dominating star which ionizes the surrounding material. The ultracompact HII region has an arc-shape similar to cometary HII regions. Furthermore, water masers and a Class I methanol maser are detected in the W33 Main cloud. While the water masers are probably associated with an outflow in W33 Main, the methanol maser is located offset from any dust or molecular line emission and it is not clear what powers it. <br /> DR 21 contains two cometary HII regions, whose sizes classify them as ultracompact and compact HII regions. To study the velocity field of the two HII regions, we analyzed archival radio recombination line observations of DR 21. We detected two velocity components in the tails of both HII regions which indicate the presence of stellar winds. Stellar winds clear cavities around the stars and confine the ionized gas in thin shells around these cavities. The two velocity components probably originate from ionized gas at the near and the far side of these shells. The velocity distribution of both HII regions is best explained with a combination of bow shock and champagne flow models. The moving star produces a bow shock at the head of the cometary HII regions, increasing the velocity of the ionized gas compared to the systemic velocity of the neutral material. Increasing velocities of the ionized gas down both tails indicate the presence of a density gradient in the surrounding neutral material (champagne flow model). The density gradient in the southern HII region has been tentatively shown in molecular line observations of DR 21 but has to be confirmed with observations at higher spatial resolution.