The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)^⋆

¹ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
e-mail: skopal@astro.sk
² Sternberg Astronomical Institute, Moscow State University, Universitetskij pr., 13, Moscow 119991, Russia
³ INAF – Osservatorio Astronomico di Padova, 36012 Asiago VI, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, 40129 Bologna, Italy
⁵ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁶ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁷ INFN – Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via Pietro Giuria 1, 10125 Turin, Italy
⁸ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina
⁹ Instituto de Astrofísica de La Plata, CONICET-UNLP, CCT La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina
¹⁰ Scientific Research Institute, Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
¹¹ Castanet Tolosan Observatory, 6 place Clemence Isaure, 31320 Castanet Tolosan, France
¹² Observatoire de l’Eridan et de la Chevelure de Bérénice, 02400 Epaux-Bézu, France
¹³ ANS Collaboration, c/o Astronomical Observatory, 36012 Asiago VI, Italy
¹⁴ Observatoire de la Tourbière, 38690 Chabons, France
¹⁵ Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia
¹⁶ Vihorlat Astronomical Observatory, Mierová 4, 066 01 Humenné, Slovakia
¹⁷ Sonneberg Observatory, Sternwartestr. 32, 96515 Sonneberg, Germany

Received: 27 November 2019
Accepted: 3 March 2020

Abstract

Context. The star V426 Sge (HBHA 1704-05), originally classified as an emission-line object and a semi-regular variable, brightened at 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 light minima, 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-XRT and UVOT, optical UBVR_CI_C 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 nova outburst that ceased around 1990. From approximately 1972, a wave-like orbitally related variation with a period of 493.4 ± 0.7 days developed in the LC. This was interrupted by a Z And-type outburst from the beginning of August 2018 to the middle of February 2019. At the maximum of the 2018 outburst, the burning white dwarf (WD) increased its temperature to ≳2 × 10⁵ K, generated a luminosity of ∼7 × 10³⁷ (d/3.3 kpc)² erg s⁻¹ and blew a wind at the rate of ∼3 × 10⁻⁶ M_⊙ yr⁻¹. Our spectral energy distribution models from the current quiescent phase reveal that the donor is a normal M4-5 III giant characterised with T_eff ∼ 3400 K, R_G ∼ 106 (d/3.3 kpc) R_⊙ and L_G ∼ 1350 (d/3.3 kpc)² L_⊙ and the accretor is a low-mass ∼0.5 M_⊙ WD.

Conclusions. During the transition from the symbiotic nova outburst to the quiescent phase, a pronounced sinusoidal variation along the orbit develops in the LC of most symbiotic novae. The following eventual outburst is of Z And-type, when the accretion by the WD temporarily exceeds the upper limit of the stable burning. At this point the system becomes a classical symbiotic star.