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i-Resonant States of Hydrogen Gas V. P. Grinina, *, N. A. Katyshevab, T. A. Ermolaevaa, c, and D. V. Dmitrieva, d a Pulkovo

Astronomical Observatory, Russian Academy of Sciences, St. Petersburg, 196140 Russia Sternberg Astronomical Institute, Moscow State University, Moscow, 119992 Russia c St. Petersburg State University, St. Petersburg, 119034 Russia d Crimean Astrophysical Observatory, Russian Academy of Sciences, Nauchnyi, Bakhchysarai region, Republic Crimea, 298409 Russia *e-mail: [email protected] b

Received January 23, 2020; revised April 13, 2020; accepted April 30, 2020

Abstract—The article considers the poorly studied states of hydrogen gas (called here “quasi-resonant”) with anomalously high intensity ratios of the lines in the Balmer series, I (Hα )/I (Hβ ). It is shown that such states arise in the case of large optical thicknesses of the emitting gas occurring at the frequencies of the Balmer lines, when the gas tends to a thermalization state. The influence on the formation of such states of basic model parameters as temperature, atomic density, and the characteristic size of the emitting region, as well as the presence of external radiation, is studied on simple models. It is shown that quasi-resonant states can exist in the magnetospheres of T Tauri stars at moderate accretion rates. DOI: 10.1134/S1063772920090012

1. INTRODUCTION As is known, in optically thick media, a photon emitted in a resonance line can undergo numerous scattering before it leaves the medium or “dies” as a result of transitions from an excited level under the action of electron impacts or other dissipative processes. Under these conditions, the main energy part of the line spectrum is concentrated mainly in resonance lines, which is often used in calculating the functions of gas cooling due to radiation losses. However, at very large optical thicknesses at the line frequencies, when the gas state approaches the state of local thermodynamic equilibrium (LTE), the situation changes and the resonance line ceases to be the main energy component of the line radiation of the plasma [1, 2]. In such cases, as calculations show, situations are possible in which the quanta in the lines of subordinate series can also be preserved during multiple scattering. These gas states shall be called quasi-resonant. Thus, a radiative transition from the third level of the hydrogen atom down is possible both with the formation of a photon pair in the lines Hα and L α and with the emission of one photon in the line Lβ . At large optical thicknesses, the photon in the line Lβ will be absorbed almost where it was emitted, and the hydrogen atom will again be at the third level. This situation can be repeated many times until the quantum in the line Hα leaves the medium or dies, e.g., as a result of a second-kind impact. As for the line Hβ , it

appears in transition 4–2. Therefore, the quantum survival probability in this line during scattering in the medium will be noticeably lower than unity due to the existence of the radiative transition 4–3 with th