Stellar-wind accretion and Raman-scattered O vi features in the symbiotic star AG Draconis

Kim

2022

ApJ

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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.

Section: Asymmetric Disk Distributionsupporting

confidence: 86%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Kim

2022

ApJ

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“…The medium for radiative transfer through Raman and Rayleigh scattering corresponds to a thick H I region that is easily found in the slow stellar wind from a red giant (Lee & Lee 1997;Lee et al 2019). No consideration on the thermal motion of neutral hydrogen is given because the variation of the cross section and the branching ratio in the scale of thermal speed is negligible (Chang et al 2015(Chang et al , 2018.…”

Section: Geometry : Scattering Regionmentioning

confidence: 99%

“…Raman-scattered O VI features in symbiotic stars exhibit complicated profiles with multiple peaks separated by ∼ 30 − 50 km s −1 indicative of the O VI emission regions with physical dimension of ∼ 1 au (e.g. Shore et al 2010;Lee et al 2019). Because Raman and Rayleigh scattering sufficiently off resonance shares the same scattering phase function as Thomson scattering (Schmid 1995;Yoo et al 2002;Chang et al 2017), strong linear polarization may develop in an anisotropic scattering geometry.…”

Section: Introductionmentioning

confidence: 99%

3D Grid-Based Monte Carlo Code for Radiative Transfer through Raman and Rayleigh Scattering with Atomic Hydrogen -- STaRS

Chang,

Lee

2020

Preprint

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Emission features formed through Raman scattering with atomic hydrogen provide unique and crucial information to probe the distribution and kinematics of a thick neutral region illuminated by a strong far UV emission source. We introduce a new 3 dimensional Monte-Carlo code in order to describe the radiative transfer of line photons that are subject to Raman and Rayleigh scattering with atomic hydrogen. In this code entitled "Sejong Radiative Transfer through Raman and Rayleigh Scattering (STaRS), each photon is traced until escape with a tag attached carrying information including the position, direction, wavelength, and polarization. The thick neutral scattering region is divided into numerous cells with each cell being characterized by its velocity and density, which ensures huge flexibility of the code in analyzing Raman-scattered features formed in a neutral region with complicated kinematics and density distribution. As a test of the code, we revisit the formation of Balmer wings through Raman scattering of far UV continuum near Lyβ and Lyγ in a static neutral region. An additional check is made to investigate Raman scattering of O VI in an expanding neutral medium. We find fairly good agreement of our results with previous works, demonstrating the capability of dealing with radiative transfer modeling that can be applied to spectropolarimetric imaging observations of various objects including symbiotic stars, young planetary nebulae, and active galactic nuclei.

Line Formation of Raman-scattered He ii λ 4851 in an Expanding Spherical H i Shell in Young Planetary Nebulae

Choi

Chang

et al. 2020

ApJ

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We investigate line formation of Raman-scattered He II at 4851Å in an expanding neutral spherical shell that surrounds a point-like He II source located at the center. A new grid-based Monte Carlo code is used to take into consideration the H I density variation along each photon path. In the case of a monochromatic He II emission source, the resultant line profiles are characterized by an asymmetric double peak structure with a tertiary peak and a significant red tail that may extend to line centers of He IIλ4859 and Hβ. The peak separation corresponds to the expansion velocity, which we consider is in the range 20−40 km s −1 in this work. Tertiary red peaks are formed as a result of multiple Rayleigh reflections at the inner surface of a hollow spherical shell of H I. Due to a sharp increase of scattering cross section near resonance, the overall Raman conversion efficiency is significantly enhanced as the expansion speed increases. In the case of a He II line source with a Gaussian line profile with a full width at half maximum of 30 − 70 km s −1 , we obtain distorted redward profiles due to increasing redward cross section of H I. A simple application to the young planetary nebula IC 5117 is consistent with a neutral shell expanding with a speed ∼ 30 km s −1 .