Summary of Materials science doctoral thesis: Rotational spectral line formation and radiative transfer in circumstellar envelopes

The research in this thesis follows two directions: The first, using high performance computing currently available, we studied atomic and molecular emissions in astrophysical environment. The radiative transfer model was applied to explain the characteristics of the recombination line masers Hnα in the envelope of the star MWC 349A.
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Summary of Materials science doctoral thesis: Rotational spectral line formation and radiative transfer in circumstellar envelopes MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY -------------------- TRAN NGOC HUNG ROTATIONAL SPECTRAL LINE FORMATION AND RADIATIVE TRANSFER IN CIRCUMSTELLAR ENVELOPES Specialization: Optics Numerical code: 944 01 10 SUMMARY OF MATERIALS SCIENCE DOCTORAL THESIS HANOI, 2021 The thesis was completed at: Department of Physics, Graduate University of Science and Technology, VAST Center of Engineering Physics, Institute of Physics, VAST Supervisors: Assoc. Prof. Dr. Dinh Van Trung Assoc. Prof. Dr. Pham Hong Minh Reviewer 1: .............................................................................................................................. .............................................................................................................................. Reviewer 2: .............................................................................................................................. .............................................................................................................................. Reviewer 3: ............................................................................................................................. .............................................................................................................................. The thesis will be defended at: .............................................................................................................................. .............................................................................................................................. Time:..................................................................................................................... The thesis could be found at: • National Library of Vietnam • Library of Graduate University Science and Technology • Library of Institute of Physics PREFACE The atomic and molecular emission in the radio region is the opportunity for us to explore the kinetic and physical properties of astrophysical environments. Strong and directional maser emissions allow high angular resolution up to milliseconds to study astro-objects at unprecedented details. In recent decades, advances in computational techniques have greatly assisted the analysis of astronomical data. Numerically modeling can accurately simulate the optical images or radio spectrum to help us understand the processes going on in the universe. Predictions from these numerical models serve as guidance for many studies. The research in this thesis follows two directions: The first, using high performance computing currently available, we studied atomic and molecular emissions in astrophysical environment. The radiative transfer model was applied to explain the characteristics of the recombination line masers Hnα in the envelope of the star MWC 349A. The seconds, we modeled a gaseous envelope of a binary system of stars by using numerically hydrodynamic calculation. The hydrodynamic simulation using the open-source PLUTO program models envelopes of asymptotic giant branches (AGBs) to study the morphology of spiral structures surrounding these stars. The thesis is presented in four chapters as follows: - Chapter 1. An overview of the astronomical environment and the theoretical basis in radio astronomy research. Two important basic theories presented are the radiative transfer theory and the hydrodynamic theory. - Chapter 2. Present the radiative transfer modeling process in a circumstellar and the work blockes in building simulation programs. Hydrodynamics in an envelope of a binary system has also been modeled by using the open-source program PLUTO. 1 - Chapter 3. Modeling the transmission of molecular radiation in a circumstellar medium and applying it to recombination line masers Hnα in the envelope of the star MWC 349A. - Chapter 4. Modeling hydrodynamics of binary systems for considering the influence of the orbital factors on the morphology of the envelope. This model is combined with the radiative transfer model to obtain images of molecular radiation that can be compared with the observed data. 2 Chapter 1. Overview circumstellar media, radiative transfer and structures of envelopes of evolved stars 1.1 Formation and development of stars Stars are formed from nebulae which are giant clouds of gases. If a gaseous cloud is massive enough that the gaseous pressure is not strong enough to balance gravity, it undergoes gravitational collapse. As a result of this collapse, new stars are formed in the core of the gaseous cloud. 1.2 Observations of circumstellar envelopes The advances in astronomical observations help us resolve many structures of circumstellar envelopes. The Hubble space telescope became a pioneer icon in the exploration of the universe by optical photography. Besides, many radio observatories around the world help collect a lot of important data. One of the famous observatories is the ALMA observatory which has had many important discoveries. 1.3 Atomic and molecular spectra observed in circumstellar envelopes 1.3.1 Hydrogen recombination lines Hydrogen gas around stars is ionized by UV radiation to create an ionized envelope known as the H II region. This ionization occurs concurrently with recombination process between protons and free electrons. The process will emit continuous bound-free r ...