Formation of the Asymmetric Accretion Disk from Stellar Wind Accretion in an S-type Symbiotic Star

Abstract

The accretion process in a typical S-type symbiotic star, targeting AG Draconis, is investigated through 3D hydrodynamical simulations using the FLASH code. Regardless of the wind velocity of the giant star, an accretion disk surrounding the white dwarf is always formed. In models where the wind is faster than the orbital velocity of the white dwarf, the disk size and accretion rate are consistent with the predictions under Bondi–Hoyle–Lyttleton (BHL) conditions. In slower-wind models, unlike the BHL predictions, the disk size does not grow, and the accretion rate increases to a considerably higher level, up to >20% of the mass-loss rate of the giant star. The accretion disk in our fiducial model is characterized by a flared disk with a radius of 0.16 au and a scale height of 0.03 au. The disk mass of ∼5 × 10−8 M ⊙ is asymmetrically distributed, with the density peak toward the giant star being about 50% higher than the density minimum in the disk. Two inflowing spiral features are clearly identified, and their relevance to the azimuthal asymmetry of the disk is pointed out. The flow in the accretion disk is found to be sub-Keplerian, at about 90% of the Keplerian speed, which indicates a caveat of overestimating the O vi emission region from the spectroscopy of Raman-scattered O vi features at 6825 and 7082 Å.

The Emergence of a Neutral Wind Region in the Orbital Plane of Symbiotic Binaries during Their Outbursts

“…2. The N H obs values measured around the orbit will serve as a benchmark for testing the theoretical modeling of the wind morphology of wide interacting binaries containing an evolved giant (e.g., Bermúdez-Bustamante et al 2020;El Mellah et al 2020;Lee et al 2022). In particular, modeling the morphology of a massive slow wind blowing from red giants in S-type SySts with terminal velocity of a few times 10 km s −1 (e.g., Dumm et al 1999), at rates of a few times (10 −7 -10 −6 ) M e yr −1 (e.g., Seaquist et al 1993;Shagatova et al 2016), which is disrupted by the accreting WD, in a similar way to what has been done for massive X-ray binaries (see references in Section 3.3), should lead to a better understanding of the mass transfer problem in symbiotic binaries.…”

The 2020 Eclipse of R Aquarii in the Near-infrared: Mid-eclipse Observations of Disk Distress

“…The accretion flow in these systems is close to Keplerian. However, as noted by Lee et al (2022), SySt accretion involves the capture of slow stellar wind from a giant donor and is different from that in geometrically thin, cataclysmic, variable accretion disks.…”