LIN 358: a symbiotic binary accreting above the steady hydrogen fusion limit

Abstract

ABSTRACT Symbiotic binaries are long-period interacting binaries consisting of a white dwarf (WD) accreting material from a cool evolved giant star via stellar winds. In this paper, we study the symbiotic binary LIN 358 located in the Small Magellanic Cloud. We have observed LIN 358 with the integral field spectrograph WiFeS and obtained its line emission spectrum. With the help of the plasma simulation and spectral synthesis code cloudy, we have constructed a 2D photoionization model of LIN 358. From comparison with the observations, we have determined the colour temperature of the WD in LIN 358 to be 19 eV, its bolometric luminosity L = (1.02 ± 0.15) × 1038 erg s−1, and the mass-loss rate from the donor star to be 1.2 × 10−6 M⊙ yr−1. Assuming a solar H to He ratio in the wind material, a lower limit to the accreted mass fraction in LIN 358 is 0.31. The high mass accretion efficiency of a wind Roche lobe overflow implies that the WD is accreting above the upper boundary of stable hydrogen fusion and thus growing in mass with the maximal rate of ≈4 × 10−7 M⊙ yr−1. This causes the WD photosphere to expand, which explains its low colour temperature. Our calculations show that the circumstellar material in LIN 358 is nearly completely ionized except for a narrow cone around the donor star, and that the WD emission is freely escaping the system. However, due to its low colour temperature, this emission can be easily attenuated by even moderate amounts of neutral interstellar medium. We speculate that other symbiotic systems may be operating in a similar regime, thus explaining the paucity of observed systems.