The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)статья
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Аннотация:Context. The star V426 Sge (HBHA 1704-05), originally classified as an emission-line object and a semi-regular variable, brightenedat the beginning of August 2018, showing signatures of a symbiotic star outburst.Aims. We aim to confirm the nature of V426 Sge as a classical symbiotic star, determine the photometric ephemeris of the lightminima, and suggest the path from its 1968 symbiotic nova outburst to the following 2018 Z And-type outburst.Methods. We re-constructed an historical light curve (LC) of V426 Sge from approximately the year 1900, and used original low-(R ~ 500–1500; 330–880 nm) and high-resolution (R ~ 11 000–34 000; 360–760 nm) spectroscopy complemented with Swift-XRTand UVOT, optical UBVRCIC and near-infrared JHKL photometry obtained during the 2018 outburst and the following quiescence.Results. The historical LC reveals no symbiotic-like activity from ~1900 to 1967. In 1968, V426 Sge experienced a symbiotic novaoutburst that ceased around 1990. From approximately 1972, a wave-like orbitally related variation with a period of 493.4 +\- 0.7 daysdeveloped in the LC. This was interrupted by a Z And-type outburst from the beginning of August 2018 to the middle of February2019. At the maximum of the 2018 outburst, the burning white dwarf (WD) increased its temperature to > 2 x 105 K, generateda luminosity of ~7x10^37 (d=3.3 kpc)^2 erg s^-1 and blew a wind at the rate of ~3x 10^-6 M_sum yr^-1. Our spectral energy distributionmodels from the current quiescent phase reveal that the donor is a normal M4-5 III giant characterised with T_eff ~ 3400 K, RG ~ 106 (d/3.3 kpc) R_sun and LG_G ~ 1350 (d/3.3 kpc)^2 L_sun and the accretor is a low-mass ~0.5 M_sun WD.Conclusions. During the transition from the symbiotic nova outburst to the quiescent phase, a pronounced sinusoidal variation alongthe orbit develops in the LC of most symbiotic novae. The following eventual outburst is of Z And-type, when the accretion by theWD temporarily exceeds the upper limit of the stable burning. At this point the system becomes a classical symbiotic star.