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Laboratory analogue of a supersonic accretion column in a binary star system.

ABSTRACT: Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy-gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100-1,000?km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions.

SUBMITTER: Cross JE

PROVIDER: S-EPMC4910020 | biostudies-literature | 2016 Jun

REPOSITORIES: biostudies-literature

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Laboratory analogue of a supersonic accretion column in a binary star system.

Cross J E JE Gregori G G Foster J M JM Graham P P Bonnet-Bidaud J-M JM Busschaert C C Charpentier N N Danson C N CN Doyle H W HW Drake R P RP Fyrth J J Gumbrell E T ET Koenig M M Krauland C C Kuranz C C CC Loupias B B Michaut C C Mouchet M M Patankar S S Skidmore J J Spindloe C C Tubman E R ER Woolsey N N Yurchak R R Falize É É

Nature communications 20160613

Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy-gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100-1, ...[more]

PMID: 27291065

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Project description:Context:Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims:AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3?2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods:The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results:The estimated average mass-loss rate of W Aql is ? = 3.0×10-6 M? yr-1 and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10? of the CSE is asymmetric with arc-like structures at separations of 2-3? scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions:The CO(J = 3?2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of ~10? and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yrs) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

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Laboratory analogue of a supersonic accretion column in a binary star system.

Publications

Laboratory analogue of a supersonic accretion column in a binary star system.

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