Magnetic fields in close white dwarf binaries

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2020-09

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Universidad de Valparaíso

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item.page.issne

item.page.doiurl

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Instituto de Fisica y Astronomia

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Resumen

White dwarfs are the degenerate remnants of stars that are born with initial masses 8 − 10 Mο (e.g., Smartt et al. 2009 [63], Cummings et al.2019 [16]). Whereas the possibility that some of these stellar remnants possess strong magnetic fields was explored already in 1947 (Blackett et al. 1947 [9]), the observational confirmation occurred much later but still already 50 years ago (Kemp et al. 1970 [34]). It is now firmly established that a small fraction (2 − 10 per cent) of single white dwarfs exhibit magnetic fields of B :2 1 MG (Hollands et al. 2015, Ferrario et al. 2015 [18], Kawka et al. 2010 [65]), and there is evidence that weaker fields are equally or possibly even more common (Landstreet & Bagnulo 2019 [39]). Interestingly, the fraction of magnetic white dwarfs is different for white dwarfs in binary stars and seems to depends on the binary configuration. A relatively large fraction (∼ 30 per cent) of cataclysmic variables, i.e. mass transferring systems, contains a strongly magnetic white dwarf, while they are nearly absent (less than one per cent) among detached white dwarf plus M dwarf binaries. The origin of magnetic fields in white dwarfs is still debated, with current hypotheses including fossil fields (Angel et al. 1981 [4], Braithwaite & Spruit 2004 [10]), binary interactions either during a common envelope phase (Tout et al. 2008 [66]) or mergers (Garc´ıa-Berro et al. 2012 [24]), and processes internal to the white dwarf (Isern et al. 2017 [29]). In this thesis I investigate magnetic white dwarfs in binaries observationally as well as theoretically. Weakly magnetic white dwarfs in close binaries may produce synchrotron emission which should be detectable with ALMA. I analysed ALMA observations of two close white dwarf binary stars finding evidence for synchrotron emission in one of them, while the interpretation of the observations of the other ob- ject remains unclear. I combine my observational results with observations from the literature and theoretically investigate whether a crystallisation and rotation driven dynamo could be responsible for at least some of the detected magnetic fields in close binaries. I found that indeed crystallisation might be important for understanding magnetic fields in close white dwarf binary stars and can most likely produce stronger fields than was previously thought. However, the rotation and crystallisation driven dynamo cannot explain the entire observed population of magnetic white dwarfs. For single white dwarfs, the merger scenario is most likely much more important.

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ESTRELLAS ENANAS BLANCAS, CAMPO MAGNETICO SOLAR, MAGNETOHIDRODINAMICA

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