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

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.

Optical emission line spectra of symbiotic binaries

“…Indeed, the X-ray emission observed from symbiotic systems is often heavily absorbed, but this is mostly due to the low colour temperature of the WD and the ISM absorption (Nielsen & Gilfanov 2015). With our simulations we can show that the CSM in symbiotic binaries is in fact mostly ionized and does not obscure the WD emission (see also Kuuttila et al 2021).…”

“…In order to study the emission line spectra of accreting symbiotic binaries, we have run a grid of simulations with the photoionization and spectral synthesis code C version 17.02 (Ferland et al 2017). The simulations were performed in a 2D configuration as introduced by Kuuttila et al (2021). In this method the ionizing source, i.e.…”

“…From our simulations we obtain the volume emissivity of each line, which is then integrated over the simulated volume to get the total line luminosity, as also explained in Kuuttila et al (2021). In Fig.…”

“…In some symbiotic binaries none of the common nebular forbidden emission lines are observed (e.g. LIN358; Kuuttila et al 2021) which is commonly ascribed to the high density of the gas surrounding the WD. But as shown in Fig.…”

“…Recently Kuuttila et al (2021) studied a symbiotic binary LIN 358 with a combination of optical spectroscopy and 2D photoionization simulations and measured all main parameters of the system, including the WD temperature, the mass accretion rate and the mass-loss rate from the giant donor star. Here we utilise this method to explore the parameter space of symbiotic binaries with 2D photoionization simulations.…”