Formation of a disk structure in the symbiotic binary AX Persei during its 2007–10 precursor-type activity^⋆

¹ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
² Crimean Astrophysical Observatory, Nauchny, Ukraine
³ ANS Collaboration, c/o Osservatorio Astronomico di Padova, Sede di Asiago, 36032 Asiago (VI), Italy
⁴ INAF Osservatorio Astronomico di Padova, Sede di Asiago, 36032 Asiago (VI), Italy
⁵ Dipartimento di Astronomia, Universita’ di Padova, Osservatorio Astrofisico, 36012 Asiago (VI), Italy

Received: 28 March 2011
Accepted: 12 September 2011

Abstract

Context. AX Per is an eclipsing symbiotic binary. During active phases, deep narrow minima are observed in its light curve, and the ionization structure in the binary changes significantly. From  ~2007.5, AX Per entered a new active phase.

Aims. We aim to derive the ionization structure in the binary and its changes during the recent active phase.

Methods. We used optical high- and low-resolution spectroscopy and UBVR_CI_C photometry. We modeled the SED in the optical and broad wings of the Hα line profile during the 2007–10 higher level of the AX Per activity.

Results. After 10 orbital cycles (~18.6 years), we again measured the eclipse of the hot component by its giant companion in the light curve. We derived a radius of 27 ± 2 R_⊙ for the eclipsed object and 115 ± 2 R_⊙ for the eclipsing cool giant. The new active phase was connected with a significant enhancement of the hot star wind. From quiescence to activity, the mass-loss rate increased from  ~9 × 10^-8 to  ~3 × 10^-6 M_⊙   yr^-1, respectively. The wind causes the emission of the He⁺⁺ zone, located in the vicinity of the hot star, and also is the reason for the fraction of the [O iii] zone at farther distances. Simultaneously, we identified a variable optically thick warm (T_eff ~ 6000 K) source that contributes markedly to the composite spectrum. The source was located at the hot star’s equator and has the form of a flared disk, whose outer rim simulates the warm photosphere.

Conclusions. The formation of the neutral disk-like zone around the accretor during the active phase was connected with its enhanced wind. It is probable that this connection represents a common origin of the warm pseudophotospheres that are indicated during the active phases of symbiotic stars.