Observations with Herschel: High-mass star formation and the search for NH+

Licentiatavhandling, 2014

The capture and interpretation of the electromagnetic radiation we receive from space is essential to our understanding of the cosmos. When the Earth's opaque atmosphere prevents the far-infrared radiation to reach ground based antennas, space observatories become crucial to collect that important piece of the large puzzle that otherwise would have been lost. In this thesis, we use observations of light hydrides in the far-infrared performed with the HerschelSpace Observatory and its sensitive instrument the Heterodyne Instrument for the Far-Intrared (HIFI). The goal is to increase our understanding of the physical and chemical conditions present in the interstellar medium (ISM)- the gas and dust between stars in galaxies - and how massive stars are able to form deep within the extremely large and ice cold gas clouds in the ISM. In Paper I, we present searches for a so far undetected key molecule in the interstellar nitrogen chemistry, NH^{+}, along with the anion NH^{-}_{2}. Despite the most sensitive searches up to date, no detections were made. The upper limits are, however, used to constrain chemical models of different cloud types in the ISM. High spectral resolution observations of the ammonia molecule in the far-infrared are used in Paper II to analyze the early phases of massive star formation. Due to the scarce number of massive stars forming in the Galaxy, typically at vast distances, several competing theories exist in massive star formation theory. Our multi-transitional observations of ammonia spectral lines allowed a detailed modeling of the accretion process in the massive star forming region G34.26+0.15 at a distance of 3.3 kpc. With the use of an accelerated lambda iteration code to model the observed line profiles, the radial distribution of the density, temperature and velocity field, as well as the ammonia abundance and ratio of its ortho and para symmetry forms, were deduced. The usage of seven rotational ammonia transitions probing different layers of the collapsing cloud allowed a detection of two cloud components moving inwards toward the central region of the massive star forming cloud. Our results do not agree with previous published results suggesting a freefall collapse of a single cloud, based on only a few observed transitions. The mass accretion rates are found to be high enough to overcome the expected radiation pressure from G34.26+0.15 and support the competitive accretion process in high-mass star formation theory.