Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lict..........m&link_type=abstract
Licentiate Thesis, Technical Report No. 292L, Chalmers University of Technology, Gothenburg, Sweden
Astronomy and Astrophysics
Astronomy
Hii Region, Masers, Stars: Formation, Circumstellar Matter, Techniques: Interferometric
Scientific paper
``Radio astronomy is the study of the universe by observing electromagnetic radiation after it has been amplified. The use of amplifiers that preserve the oscillatory character of radiation - the phase information - is the mark of Radio astronomy.'' Thus, the development of low noise amplifiers for microwave and millimeter wavelengths is a major part of Radio astronomy as important as the observations themselves. This technical report involves those two aspects of Radio astronomy, the observational and technical aspects. In the first part, observations of methanol masers in massive star forming regions using Very Long Baseline Interferometry (VLBI) are presented. The second part concerns the realization of low noise amplifiers using in a radio camera. Recent observations have confirmed that the methanol masers are powerful tools for probing the regions of massive star formation. The methanol masers fall in two distinct classes related to their location in the star forming regions. Class I methanol masers are observed offset far away from the UC HII region emission peak. They are certainly collisionally pumped and may occur in the interface between high velocity gas outflows and the ambient molecular material. Class II methanol masers coincide with the UC HII region emission. They may be radiatively pumped by FIR radiation from the dust grains and reside either in spherical layers surrounding the UC HII regions or in circumstellar discs. The maser spots are usually compact (~1-10 AU) and lie in region of physical conditions n(H)~104-108 cm-3 and T=100-1000 K. CH3OH may be produced by hydrogenation of CO on the surface of the icy mantles of the dust grains . The methanol is then injected in the molecular gas by evaporation of the ice (n(H)=106 cm-3, T=100-300 K). In this report we present VLBI observations of 6.7 and 12.2 GHz methanol masers in the star forming regions NGC7538, W75N and S252. Our results show the existence of two groups of masers in NGC7538. The first group exhibits a linear velocity gradient and forms a line in our VLBI map which is consistent with a rotating disc of masers seen edge-on around a massive star. The second group of masers are blueshifted with respect to the first group and lie in a conical region south of the inferred disc. We argue that these masers probably arise in an outflow emerging approximately perpendicularly to the disc. We find that the maser positions at 6.7 and 12.2 GHz are coincident and those of the second group correspond approximately to the absolute positions of NH3, H2CO and OH masers associated with the radio-continuum and infrared source NGC7538-IRS1. In addition we find similar evidence for circumstellar discs of masers in W75N and S252. For these three sources associated with ultra compact HII regions, from the linear velocity gradients and assuming that the central protostars are massive, we derive that the radii of the circumstellar discs are in the range 300-1200 AU which is typical of protoplanetary discs. In the second part, we present the design and realization of 4 GHz cryogenic low noise amplifiers used as IF amplifiers in a radio-camera receiver (SISYFOS project) which will be installed in the Onsala 20m millimeter wave telescope. The requirements of the SISYFOS project at cryogenic temperature are a minimum gain of 25 dB and noise equivalent temperature less than 10 K over the frequency range 3.4-4.6 GHz. Because of its low noise and high gain properties, the MGF4310E series super low noise HEMT from Mitsubishi have been selected to satisfy these requirements. We show that a very simple input design using a high impedance series line viewed as a series inductor provides good matching over a broad bandwidth while ensuring the stability of the amplifier. The minimum noise equivalent temperature and gain of the amplifier measured at cryogenic ambient temperature over the specified bandwidth are 7 K and 28 dB respectively.
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