Summary of Master Thesis

My Degree thesis is the first work relative to a research program between the I.Te.S.R.E./C.N.R. (Cortiglioni, Palazzi, Palumbo, Cioni Bologna-Italy) and the P.N. Lebedev Institute (Sorochenko, Tsivilev Moscow-Russia). The purpose is to map some galactic HII regions in relative abundance of He⁺ (y⁺) to obtain the total abundance of He and derive its primordial value. The RRLs observations in the Orion nebula HII region were made using the radiotelescope at 22 GHz from the radioastronomical station of Medicina (Bologna-Italy) and the radiotelescope at 36 GHz from the radioastronomical station of Puschino (Moscow-Russia). The characteristic is that both telescopes at these frequencies have the same beamwidth; this is important to observe a region with the same extension and with the same chemical and physical properties and this is necessary to determine how characteristic parameters of RRLs varies with frequency in order to integrate existing models. The HII region in the Orion nebula was chosen because even if it has been the subject of several observations an extensive map in y⁺ up to 8' from q1OriC did not exist. In addition from Medicina it was observed the HII region in the Rosette nebula at 8.3 GHz not well studied in RRLs. In particular observations at high frequencies are important in the construction of models. Together with observations from Medicina (October 1995 - January 1996) the preparation of this thesis requested reduction and analysis of radio spectroscopic data taken from Medicina and Puschino, together with continuum data taken from the radioastronomical station of Noto (Siracusa-Italy). The analysis of all data obtained needed a good knowledge of different instruments and observing methods. Part of the analysis was made in collaboration with Dr. A.P. Tsivilev (P.N. Lebedev Institute of Academic Science of Russia) during his visit to Te.S.R.E. Institute in June 1996. This experience gave me the opportunity to learn the theoretical-interpretative elements that represent a basic notion to study HII regions. From hundreds of spectra obtained from the Orion HII region I determined the characteristic parameters of RRLs (C66a, He66a, H66a) such as: FWHM, VLSR, TL and the 13 positions observed permitted to see how these parameters change inside the region. The mean value of the FWHM is in accordance with preceding observations and the growth with quantum number (n) is confirmed. This could be explained by considering that at low frequencies, i.e. at high n, widening effects on spectral lines are more efficient. The FWHM of the He+ line grows with radial distance from q1OriC but this is probably not a real growth because in more distant positions the signal to noise ratio does not permit to divide the C+ line. As for VLSR of both H+ and He+ lines the behavior with radial distance shows that Strömgren spheres of both elements do not coincide. This is important in the determination of the total abundance of helium that must be as accurate as possible for the corresponding determination of the primordial abundance. The difference between Strömgren spheres is the main parameter for the construction of models. The accuracy reached is 1% and results are in perfect agreement with those obtained from Puschino relative to the transitions H56a, He56a, C56a and with previous results, consequently I conclude, there is not a gradient in y+ at least for n less than 130. Characteristic parameters of RRLs were obtained also in the observation of Rosette HII region. The corresponding transitions observed were H92a and He92a. The value obtained for TL/TC give the possibility to develop a new model of the region together with the value obtained for y+ that looks greater than previous measurements at lower frequencies. Values of TeLTE and TeNON-LTE do not permit to define a gradient because only three positions have been observed. Finally it is important to note that with an integration time of 14 hours it was possible to reach a sensitivity of ~ 3 mK at the frequency of 8.3 GHz. The observing technique was position switching. In the ipothesis that with an integration time of 56 hours all observing and instruments conditions will not change the sensitivity would be ~ 1.5 mK which is the value necessary to observe 3He+in galactic HII regions. 3He is directly connected with the baryons to photons ratio and can put limits to the baryon density of the Universe.