Study of maser emission in 18 cm lines in the star formation region G 109.871+2.114 (Cep A)

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Abstract

The results of the study of the star formation region G 109.871+2.114 (Cep A) in OH lines by 18 cm are presented. Polarization observations (monitoring) were performed on a large radio telescope in Nançay (France) in 2007–2024. OH maser emission is highly variable. The structure of the spectrum and the flux density of the individual features are changing. However, the radial velocities of most features changed slightly. Short-term flares of emission ion from individual features were observed. Many features have strong circular polarization, reaching 100%, but weak linear polarization. A new features at –15.53 km/s and a short-term part at 1.58 km/s with high circular and low linear polarizations were detected in the 1667 MHz line. The spectral features of our monitoring were spatially identified with the maser spots on the Cohen, Argon and Fish maps. The magnitude of the monotonic decrease in splitting, and, consequently, the longitudinal magnetic field of three Zeeman pairs (–16.2L / –14.25R km/s and –6.94L / –0.82R in the 1665 MHz line and –15.76L / –14.2R in the 1667 MHz line). For the –13.95L / –11.60R pair no splitting change was detected in the 1665 MHz line. Broadband absorption and emission are observed in the satellite lines of 1612 and 1720 MHz, respectively. A Zeeman pair was also detected in the 1720 MHz line. The value of the positional angle χ is calculated for linearly polarized emission of most spectral details in both main lines of 1665 and 1667 MHz. It is shown that the magnetic field in the H II regions is oriented either along the external magnetic field or along the radio jets.

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About the authors

E. E. Lekht

Lomonosov Moscow State University

Author for correspondence.
Email: eelekht@mail.ru

Sternberg Astronomical Institute

Russian Federation, Moscow

N. T. Ashimbaeva

Lomonosov Moscow State University

Email: eelekht@mail.ru

Sternberg Astronomical Institute

Russian Federation, Moscow

V. V. Krasnov

P. N. Lebedev Physical Institute, Russian Academy of Sciences

Email: eelekht@mail.ru

Astrospace Center 

Russian Federation, Moscow

V. R. Shoutenkov

P. N. Lebedev Physical Institute, Russian Academy of Sciences

Email: eelekht@mail.ru

Pushchino Radio Astronomy Observatory, Astrospace Center

Russian Federation, Pushchino

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Supplementary files

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2. Fig. 1. Spectra of OH maser emission in the main line of 1665 MHz in the direction of Cep A at the epoch of 2.07.2009. The upper panel shows the spectra in right (bold line) and left (thin line) circular polarizations. The other panels show the spectra in linear polarization at the positions of the plane of polarization of the antenna (PA) of 0°, 45°, 90° and 135°.

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3. Fig. 2. The same as in Fig. 1, but in the era of 05/31/2020.

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4. Fig. 3. The same as in Fig. 1, but in the OH line 1667 MHz and in the epoch of 11/15/2007.

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5. Fig. 4. The same as in Fig. 1, but in the OH line 1667 MHz and in the epoch 08.10.2022.

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6. Fig. 5. Variability of the flux density of the main features in Cep A in the 1665 MHz line. The radial velocities of the features are indicated.

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7. Fig. 6. The same as in Fig. 5, but in the 1667 MHz line.

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8. Fig. 7. Variability of the position angle χ of the linearly polarized emission of the main features in Cep A in the 1665 MHz line. The radial velocities of the features are indicated. The graphs are approximated by second-degree polynomials and straight lines.

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9. Fig. 8. The same as in Fig. 7, but in the 1667 MHz line.

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10. Fig. 9. Evolution of the maser emission parameters in the OH 1665 MHz line in the range of radial velocities from –12.5 to –11.2 km/s.

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11. Fig. 10. Systematic decrease of splitting in the Zeeman quartet (see text) in the OH lines 1665 (a) and 1667 MHz (b). The maximum measurement errors are shown by straight line segments at the bottom left. Panel (c) shows the splitting ratio. All graphs are approximated by straight lines. The dotted line in panel (c) corresponds to the theoretical value of 1.67.

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12. Fig. 11. Systematic decrease of splitting in the Zeeman pair –5.43R / –3.10L km/s in the OH line 1667 MHz (a) and –6.94L / –0.82R km/s in the line 1665 MHz. The graphs are approximated by a second-degree polynomial and straight lines. The scale for the longitudinal magnetic field is shown on the right. The maximum measurement errors are shown by straight line segments at the bottom left.

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13. Fig. 12. Map of the location of OH maser spots in the 1665 MHz line in February 2001 according to [19]. The radial velocities of the features are indicated. The circles indicate maser spots for right-hand circular polarization, and the triangles for left-hand ones. The straight line segments indicate the vectors of the transverse magnetic field (this paper).

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14. Fig. 13. The same as in Fig. 12, but for the 1667 MHz line.

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15. Fig. 14. Spectra in the 1612 and 1720 MHz satellite lines. In the 1612 MHz line, the radiation is not polarized. To increase sensitivity, the spectrum (panel (a)) was obtained by averaging observations in four directions of linear polarization. Rc and Lc are right and left circular polarizations, respectively. The epochs of observations are indicated.

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16. Fig. 15. Superpositions of H2O spectra in different years obtained during monitoring on the RT-22 radio telescope in PRAO (Pushchino).

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