The role of MHD instabilities in the magnetospheric propeller outflow and emission in the nova-like variable star AE Aquarii
| dc.contributor.advisor | Meintjes, P. J. | |
| dc.contributor.author | Venter, Louis Albert | |
| dc.date.accessioned | 2016-01-13T10:03:13Z | |
| dc.date.available | 2016-01-13T10:03:13Z | |
| dc.date.copyright | 2007-02 | |
| dc.date.issued | 2007-02 | |
| dc.date.submitted | 2007-02 | |
| dc.description.abstract | English: AE Aquarii is a close binary consisting of a white dwarf primary star and a K4-5 red dwarf, the secondary. Mass is transferred from the Roche lobe filling secondary to the Roche lobe of the white dwarf. The white dwarf has a spin period Pspin 33 s and a fast corotating magnetosphere. The trajectory of the flow brings it to a closest approach rcl 1010 cm which is outside the corotation radius. Observational studies of the emisssion lines, in conjuction with the observed spin-down of the white dwarf, suggests that the bulk of the mass transfer is propelled from the system. The ejection of the flow is proposed to result from the interaction of the mass flow with the fast rotating magnetosphere. The interaction transfers angular momentum from the magnetosphere to the mass flow. The unique contribution of this study lies therein that this plasma-magnetosphere interaction is modelled as being driven by the Kelvin-Helmholtz (KH) instability, which is assumed to grow at the interface between the mass flow and the magnetosphere. The process can be quantified by evaluating the Poynting flux S, of the magnetospheric field at the radius of closest approach. The energy dissipation rate of the field across the surface of the stream A is PMHD = S x A 1034 erg s−1. Furthermore, if the mass transfer is ejected by the propeller at the escape velocity vesc 1550 km s−1, the energy carried by the outflow is Pout = 12 ˙M v2 esc 5 × 1033 erg s−1, where ˙M is the mass transfer rate. It is therefore plausible that the magnetospheric propeller is responsible for the ejection of the mass transfer. The ultimate energy source for the propeller is the spin of the white dwarf which has been shown to be losing rotational kinetic energy at the rate Pspin 1034 erg s−1. The KH driven magnetospheric propeller also results in the formation of magnetized plasmoids of energized electrons that emit synchrotron emission between infra-red and 1GHz radio frequencies as they are ejected from the system and expand. A large diffuse remnant, which emits in the MHz frequency range, is expected to form as the ejected bubbles coalesce outside the system. Furthermore, the KH instability triggers turbulence in the outflow, which eventually heats the gas and results in optical flares outside the white dwarf ’s Roche lobe. On its trajectory outwards, the outflow disrupts the magnetic field of the secondary and currents are induced that may heat plasma trapped in the field to X-ray emitting temperatures. This Joule heated plasma can account for the observed non-pulsed X-ray emission from AE Aqr. | en_ZA |
| dc.description.abstract | Afrikaans: AE Aquarii is ’n kompakte dubbelster, bestaande uit ’n wit dwerg primˆere ster en ’n K4-5 rooi dwerg sekondˆere ster. Massa word vanaf die Roche lob vullende rooi dwerg na die wit dwerg oorgedra. Die gesloteveldlyngebied van die sekondˆere ster se magneetveld omsluit die wit dwerg. Die wit dwerg het ’n spin periode Pspin 33 s en die magnetosfeer koroteer met die ster. Die baan van die massa vloei bring dit tot ’n minimum radiale afstand rcl 1010 cm vanaf die wit dwerg. Studies van die waargenome stralingslyne en die toenemende spin periode, dui daarop dat die grootste deel van die massa oordrag uit die sisteem gegooi word. Daar word veronderstel dat die interaksie tussen die massa vloei en die vinnig roterende magnetosfeer die proses dryf. Die interaksie dra draaimomentum vanaf die magnetosfeer na die massa-vloei oor. Die unieke bydrae van hierdie ondersoek lˆe daarin dat di´e interaksie deur middel van die Kelvin- Helmholtz (KH) onstabiliteit gemodelleer word. Die teenwoordigheid van die onstabiliteit word by die skeiding tussen die massavloei en die magnetosfeer verwag. Die proses kan gekwantifiseer word deur die Poynting vloed S, van die magneetveld by rcl te beskou, oftewel S = 1 4 vrelB2. Die energieoordragtempo van die veld oor die oppervlak van die stroom A, is PMHD = S × A 1034 erg s−1. Indien die massaoordrag teen die ontsnapsnelhied vesc 1550 km s−1 uitgegooi word, is die energie wat die uitvloei weg dra, Pout = 12 ˙M v2 esc 5 × 1033 erg s−1, waar ˙M die massa-oordragtempo is. Dit is dus moontlik dat die magnetosferiese propeller verantwoordelik is vir die uitgooi van die massa-oordrag. Die primˆere energiebron vir die propeller is die rotasie van die wit dwerg. Dit is aangetoon dat die ster rotasie kinetiese energie verloor teen die tempo Pspin 1034 erg s−1. Die KH-gedrewe magnetosferiese propeller het ook tot gevolg dat gemagnetiseerde plasmaborrels wat sinkrotron straling van IR tot GHz radio frekwensies straal, gevorm word. ’n Groot diffuse gasnewel, wat in die honderde MHz frekwensies straal, kan ook uit die ontsnapte borrels vorm. Die KH onstabiliteit inisieer verder ook turbulensie in die uitvloei wat tot die verhitting van die gas en optiese uitbarstings buite die wit dwerg se Roche lob lei. Die uitvloei versteur ook die magneetveld van die rooi ster en induseer elektriese strome wat die plasma tot X-straal temperature verhit. Hierdie Joule verhitte plasma kan verantwoordelik wees vir die waargenome nie-gepulseerde X-strale van AE Aqr. | af |
| dc.identifier.uri | http://hdl.handle.net/11660/2189 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Thesis Ph.D. (Physics))--University of the Free State, 2007 | en_ZA |
| dc.subject | Cataclysmic variable stars | en_ZA |
| dc.subject | Solar radio emission | en_ZA |
| dc.subject | Stars -- Magnetic fields | en_ZA |
| dc.subject | Magnetospheric propeller | en_ZA |
| dc.subject | KH instability | en_ZA |
| dc.subject | Turbulence | en_ZA |
| dc.subject | Non-thermal radio emission | en_ZA |
| dc.subject | Thermal optical and X-ray emission | en_ZA |
| dc.subject | Magnetic cataclysmic variable | en_ZA |
| dc.subject | AE Aquarii | en_ZA |
| dc.title | The role of MHD instabilities in the magnetospheric propeller outflow and emission in the nova-like variable star AE Aquarii | en_ZA |
| dc.type | Thesis | en_ZA |